1
|
Budé LMI, Steensma BR, Zivkovic I, Raaijmakers AJE. The coax monopole antenna: A flexible end-fed antenna for ultrahigh field transmit/receive arrays. Magn Reson Med 2024; 92:361-373. [PMID: 38376359 DOI: 10.1002/mrm.30036] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2023] [Revised: 01/15/2024] [Accepted: 01/15/2024] [Indexed: 02/21/2024]
Abstract
PURPOSE The coax monopole antenna is presented for body imaging at 7 T. The antenna is fed at one end, eliminating the possibility of cable-coil coupling and simplifying cable routing. Additionally, its flexibility improves loading to the subject. METHODS Like the coax dipole antenna, an interruption in the shield of the coaxial cable allows the current to extend to the outside of the shield, generating a B1 + field. Matching is achieved using a single inductor at the distal side, and a cable trap enforces the desired antenna length. Finite difference time domain simulations are employed to optimize the design parameters. Phantom measurements are conducted to determine the antenna's B1 + efficiency and to find the S-parameters in straight and bent positions. Eight-channel simulations and measurements are performed for prostate imaging. RESULTS The optimal configuration is a length of 360 mm with a gap position of 40 mm. Simulation data show higher B1 + levels for the coax monopole (20% in the prostate), albeit with a 5% lower specific absorbance rate efficiency, compared to the fractionated dipole antenna. The S11 of the coax monopole exhibits remarkable robustness to loading changes. In vivo prostate imaging demonstrates B1 + levels of 10-14 μT with an input power of 8 × 800 W, which is comparable to the fractionated dipole antenna. High-quality images and acceptable coupling levels were achieved. CONCLUSION The coax monopole is a novel, flexible antenna for body imaging at 7 T. Its simple design incorporates a single inductor at the distal side to achieve matching, and one-sided feeding greatly simplifies cable routing.
Collapse
Affiliation(s)
- Lyanne M I Budé
- Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
- Department of Electrical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| | - Bart R Steensma
- Division of Imaging and Oncology, UMC Utrecht, Utrecht, The Netherlands
| | - Irena Zivkovic
- Department of Electrical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| | - Alexander J E Raaijmakers
- Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
- Division of Imaging and Oncology, UMC Utrecht, Utrecht, The Netherlands
| |
Collapse
|
2
|
Harrevelt SD, Meliado EFM, van Lier ALHMW, Reesink D, Meijer RP, Pluim JPW, Raaijmakers AJE. Deep learning based correction of RF field induced inhomogeneities for T2w prostate imaging at 7 T. NMR Biomed 2023; 36:e5019. [PMID: 37622473 DOI: 10.1002/nbm.5019] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/18/2022] [Revised: 07/03/2023] [Accepted: 07/06/2023] [Indexed: 08/26/2023]
Abstract
At ultrahigh field strengths images of the body are hampered by B1 -field inhomogeneities. These present themselves as inhomogeneous signal intensity and contrast, which is regarded as a "bias field" to the ideal image. Current bias field correction methods, such as the N4 algorithm, assume a low frequency bias field, which is not sufficiently valid for T2w images at 7 T. In this work we propose a deep learning based bias field correction method to address this issue for T2w prostate images at 7 T. By combining simulated B1 -field distributions of a multi-transmit setup at 7 T with T2w prostate images at 1.5 T, we generated artificial 7 T images for which the homogeneous counterpart was available. Using these paired data, we trained a neural network to correct the bias field. We predicted either a homogeneous image (t-Image neural network) or the bias field (t-Biasf neural network). In addition, we experimented with the single-channel images of the receive array and the corresponding sum of magnitudes of this array as the input image. Testing was carried out on four datasets: the test split of the synthetic training dataset, volunteer and patient images at 7 T, and patient images at 3 T. For the test split, the performance was evaluated using the structural similarity index measure, Wasserstein distance, and root mean squared error. For all other test data, the features Homogeneity and Energy derived from the gray level co-occurrence matrix (GLCM) were used to quantify the improvement. For each test dataset, the proposed method was compared with the current gold standard: the N4 algorithm. Additionally, a questionnaire was filled out by two clinical experts to assess the homogeneity and contrast preservation of the 7 T datasets. All four proposed neural networks were able to substantially reduce the B1 -field induced inhomogeneities in T2w 7 T prostate images. By visual inspection, the images clearly look more homogeneous, which is confirmed by the increase in Homogeneity and Energy in the GLCM, and the questionnaire scores from two clinical experts. Occasionally, changes in contrast within the prostate were observed, although much less for the t-Biasf network than for the t-Image network. Further, results on the 3 T dataset demonstrate that the proposed learning based approach is on par with the N4 algorithm. The results demonstrate that the trained networks were capable of reducing the B1 -field induced inhomogeneities for prostate imaging at 7 T. The quantitative evaluation showed that all proposed learning based correction techniques outperformed the N4 algorithm. Of the investigated methods, the single-channel t-Biasf neural network proves most reliable for bias field correction.
Collapse
Affiliation(s)
- Seb D Harrevelt
- Department of Biomedical Engineering, Technische Universiteit Eindhoven, Eindhoven, The Netherlands
| | | | | | - Daan Reesink
- Department of Oncological Urology, UMC Utrecht, Utrecht, The Netherlands
| | - Richard P Meijer
- Department of Oncological Urology, UMC Utrecht, Utrecht, The Netherlands
| | - Josien P W Pluim
- Department of Biomedical Engineering, Technische Universiteit Eindhoven, Eindhoven, The Netherlands
| | - Alexander J E Raaijmakers
- Department of Biomedical Engineering, Technische Universiteit Eindhoven, Eindhoven, The Netherlands
- Department of Radiotherapy, UMC Utrecht, Utrecht, The Netherlands
| |
Collapse
|
3
|
Steensma BR, Sadeghi-Tarakameh A, Meliadò EF, van den Berg CAT, Klomp DWJ, Luijten PR, Metzger GJ, Eryaman Y, Raaijmakers AJE. Tier-based formalism for safety assessment of custom-built radio-frequency transmit coils. NMR Biomed 2023; 36:e4874. [PMID: 36368912 PMCID: PMC10411033 DOI: 10.1002/nbm.4874] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/08/2022] [Revised: 10/13/2022] [Accepted: 11/08/2022] [Indexed: 06/16/2023]
Abstract
The purpose of this work is to propose a tier-based formalism for safety assessment of custom-built radio-frequency (RF) coils that balances validation effort with the effort put in determinating the safety factor. The formalism has three tier levels. Higher tiers require increased effort when validating electromagnetic simulation results but allow for less conservative safety factors. In addition, we propose a new method to calculate modeling uncertainty between simulations and measurements and a new method to propagate uncertainties in the simulation into a safety factor that minimizes the risk of underestimating the peak specific absorption rate (SAR). The new safety assessment procedure was completed for all tier levels for an eight-channel dipole array for prostate imaging at 7 T and an eight-channel dipole array for head imaging at 10.5 T, using data from two different research sites. For the 7 T body array, the validation procedure resulted in a modeling uncertainty of 77% between measured and simulated local SAR distributions. For a situation where RF shimming is performed on the prostate, average power limits of 2.4 and 4.5 W/channel were found for tiers 2 and 3, respectively. When the worst-case peak SAR among all phase settings was calculated, power limits of 1.4 and 2.7 W/channel were found for tiers 2 and 3, respectively. For the 10.5 T head array, a modeling uncertainty of 21% was found based on B1 + mapping. For the tier 2 validation, a power limit of 2.6 W/channel was calculated. The demonstrated tier system provides a strategy for evaluating modeling inaccuracy, allowing for the rapid translation of novel coil designs with conservative safety factors and the implementation of less conservative safety factors for frequently used coil arrays at the expense of increased validation effort.
Collapse
Affiliation(s)
- Bart Romke Steensma
- Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
| | | | - Ettore Flavio Meliadò
- Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
| | | | - Dennis W J Klomp
- Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Peter R Luijten
- Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Gregory J Metzger
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | - Yigitcan Eryaman
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | - Alexander J E Raaijmakers
- Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
- Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| |
Collapse
|
4
|
Harrevelt SD, Roos THM, Klomp DWJ, Steensma BR, Raaijmakers AJE. Simulation-based evaluation of SAR and flip angle homogeneity for five transmit head arrays at 14 T. MAGMA 2023; 36:245-255. [PMID: 37000320 PMCID: PMC10140109 DOI: 10.1007/s10334-023-01067-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Subscribe] [Scholar Register] [Received: 10/04/2022] [Revised: 01/13/2023] [Accepted: 01/31/2023] [Indexed: 04/01/2023]
Abstract
INTRODUCTION Various research sites are pursuing 14 T MRI systems. However, both local SAR and RF transmit field inhomogeneity will increase. The aim of this simulation study is to investigate the trade-offs between peak local SAR and flip angle uniformity for five transmit coil array designs at 14 T in comparison to 7 T. METHODS Investigated coil array designs are: 8 dipole antennas (8D), 16 dipole antennas (16D), 8 loop coils (8D), 16 loop coils (16L), 8 dipoles/8 loop coils (8D8L) and for reference 8 dipoles at 7 T. Both RF shimming and kT-points were investigated by plotting L-curves of peak SAR levels vs flip angle homogeneity. RESULTS For RF shimming, the 16L array performs best. For kT-points, superior flip angle homogeneity is achieved at the expense of more power deposition, and the dipole arrays outperform the loop coil arrays. DISCUSSION AND CONCLUSION For most arrays and regular imaging, the constraint on head SAR is reached before constraints on peak local SAR are violated. Furthermore, the different drive vectors in kT-points alleviate strong peaks in local SAR. Flip angle inhomogeneity can be alleviated by kT-points at the expense of larger power deposition. For kT-points, the dipole arrays seem to outperform loop coil arrays.
Collapse
Affiliation(s)
- Seb D Harrevelt
- Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.
| | - Thomas H M Roos
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Dennis W J Klomp
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Bart R Steensma
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Alexander J E Raaijmakers
- Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| |
Collapse
|
5
|
Kikken MWI, Steensma BR, van den Berg CAT, Raaijmakers AJE. Multi-echo MR thermometry in the upper leg at 7 T using near-harmonic 2D reconstruction for initialization. Magn Reson Med 2023; 89:2347-2360. [PMID: 36688273 DOI: 10.1002/mrm.29591] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/09/2022] [Revised: 01/02/2023] [Accepted: 01/03/2023] [Indexed: 01/24/2023]
Abstract
PURPOSE The aim of this work is the development of a thermometry method to measure temperature increases in vivo, with a precision and accuracy sufficient for validation against thermal simulations. Such an MR thermometry model would be a valuable tool to get an indication on one of the major safety concerns in MR imaging: the tissue heating occurring due to radiofrequency (RF) exposure. To prevent excessive temperature rise, RF power deposition, expressed as specific absorption rate, cannot exceed predefined thresholds. Using these thresholds, MRI has demonstrated an extensive history of safe usage. Nevertheless, MR thermometry would be a valuable tool to address some of the unmet needs in the area of RF safety assessment, such as validation of specific absorption rate and thermal simulations, investigation of local peak temperatures during scanning, or temperature-based safety guidelines. METHODS The harmonic initialized model-based multi-echo approach is proposed. The method combines a previously published model-based multi-echo water/fat separated approach with an also previously published near-harmonic 2D reconstruction method. The method is tested on the human thigh with a multi-transmit array at 7 T, in three volunteers, and for several RF shims. RESULTS Precision and accuracy are improved considerably compared to a previous fat-referenced method (precision: 0.09 vs. 0.19°C). Comparison of measured temperature rise distributions to subject-specific simulated counterparts show good relative agreement for multiple RF shim settings. CONCLUSION The high precision shows promising potential for validation purposes and other RF safety applications.
Collapse
Affiliation(s)
- Mathijs W I Kikken
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Bart R Steensma
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Cornelis A T van den Berg
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.,Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Alexander J E Raaijmakers
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.,Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| |
Collapse
|
6
|
van Leeuwen CC, Steensma BR, Klomp DWJ, van den Berg CAT, Raaijmakers AJE. The Coax Dipole: A fully flexible coaxial cable dipole antenna with flattened current distribution for body imaging at 7 Tesla. Magn Reson Med 2021; 87:528-540. [PMID: 34411327 PMCID: PMC9292881 DOI: 10.1002/mrm.28983] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2021] [Revised: 07/14/2021] [Accepted: 08/04/2021] [Indexed: 01/26/2023]
Abstract
Purpose The coax dipole antenna, a flexible antenna for body imaging at 7T is presented. Similar to the high impedance coil, this coaxial cable antenna is fed on the central conductor and through gaps in the shield, the current passes to the outside of the antenna to generate B1 field. This could achieve more favorable current distributions and better adaptation to the body curvature. Methods Finite difference time domain (FDTD) simulations are performed to optimize the positions of the gaps in the shield for a flat current profile. Lumped inductors are added to each end to reduce losses. The performance of a single antenna is compared to a fractionated dipole using B1 maps and MR thermometry. Finally, an array of eight coax dipoles is evaluated in simulations and used for in‐vivo scanning. Results An optimal configuration is found with gaps located at 10 cm from the center and inductor values of 28 nH. In comparison to the fractionated dipole antenna, in single antenna phantom measurements the coax dipole achieves similar B1 amplitude with 18% lower peak temperature. In simulations, the eight‐channel array of coax dipoles improved B1 homogeneity by 18%, along with small improvements in transmit efficiency and specific absorption rate (SAR). MRI measurements on three volunteers show more consistent performance for the coax dipoles. Conclusion The coax dipole is a novel antenna design with a flattened current distribution resulting in beneficial properties. Also, the flexible design of the coax dipoles allows better adaptation to the body curvature and can potentially be used for a wide range of imaging targets.
Collapse
Affiliation(s)
- Carel C van Leeuwen
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Bart R Steensma
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Dennis W J Klomp
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | | | - Alexander J E Raaijmakers
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.,Biomedical Engineering Department, Eindhoven University of Technology, Eindhoven, The Netherlands
| |
Collapse
|
7
|
Steensma BR, Meliadò EF, Luijten P, Klomp DWJ, van den Berg CAT, Raaijmakers AJE. SAR and temperature distributions in a database of realistic human models for 7 T cardiac imaging. NMR Biomed 2021; 34:e4525. [PMID: 33955061 PMCID: PMC8244032 DOI: 10.1002/nbm.4525] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/15/2020] [Revised: 03/23/2021] [Accepted: 03/27/2021] [Indexed: 06/12/2023]
Abstract
PURPOSE To investigate inter-subject variability of B1+ , SAR and temperature rise in a database of human models using a local transmit array for 7 T cardiac imaging. METHODS Dixon images were acquired of 14 subjects and segmented in dielectric models with an eight-channel local transmit array positioned around the torso for cardiac imaging. EM simulations were done to calculate SAR distributions. Based on the SAR distributions, temperature simulations were performed for exposure times of 6 min and 30 min. Peak local SAR and temperature rise levels were calculated for different RF shim settings. A statistical analysis of the resulting peak local SAR and temperature rise levels was performed to arrive at safe power limits. RESULTS For RF shim vectors with random phase and uniformly distributed power, a safe average power limit of 35.7 W was determined (first level controlled mode). When RF amplitude and phase shimming was performed on the heart, a safe average power limit of 35.0 W was found. According to Pennes' model, our numerical study suggests a very low probability of exceeding the absolute local temperature limit of 40 °C for a total exposure time of 6 min and a peak local SAR of 20 W/kg. For a 30 min exposure time at 20 W/kg, it was shown that the absolute temperature limit can be exceeded in the case where perfusion does not change with temperature. CONCLUSION Safe power constraints were found for 7 T cardiac imaging with an eight-channel local transmit array, while considering the inter-subject variability of B1+ , SAR and temperature rise.
Collapse
Affiliation(s)
- Bart R. Steensma
- Center for Image SciencesUniversity Medical Center UtrechtUtrechtThe Netherlands
| | - Ettore F. Meliadò
- Center for Image SciencesUniversity Medical Center UtrechtUtrechtThe Netherlands
- Tesla Dynamic CoilsZaltbommelThe Netherlands
| | - Peter Luijten
- Center for Image SciencesUniversity Medical Center UtrechtUtrechtThe Netherlands
| | - Dennis W. J. Klomp
- Center for Image SciencesUniversity Medical Center UtrechtUtrechtThe Netherlands
| | | | | |
Collapse
|
8
|
Stijnman PRS, Erturk MA, van den Berg CAT, Raaijmakers AJE. A single setup approach for the MRI-based measurement and validation of the transfer function of elongated medical implants. Magn Reson Med 2021; 86:2751-2765. [PMID: 34036617 PMCID: PMC8596675 DOI: 10.1002/mrm.28840] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/22/2020] [Revised: 04/23/2021] [Accepted: 04/23/2021] [Indexed: 12/15/2022]
Abstract
Purpose To propose a single setup using the MRI to both measure and validate the transfer function (TF) of linear implants. Conventionally, the TF of an implant is measured in one bench setup and validated using another. Methods It has been shown that the TF can be measured using MRI. To validate this measurement, the implant is exposed to different incident electric fields, while the temperature increase at the tip is monitored. For a good validation, the incident electric fields that the implant is exposed to should be orthogonal. We perform a simulation study on six different methods that change the incident electric field. Afterward, a TF measurement and validation study using the best method from the simulations is performed. This is done with fiberoptic temperature probes at 1.5 T for four linear implant structures using the proposed single setup. Results The simulation study showed that positioning local transmit coils at different locations along the lead trajectory has a similar validation quality compared with changing the implant trajectory (ie, the conventional validation method). For the validation study that was performed, an R2 ≥ 0.91 was found for the four investigated leads. Conclusion A single setup to both measure and validate the transfer function using local transmit coils has been shown to work. The benefits of using the proposed validation method are that there is only one setup required instead of two and the implant trajectory is not varied; therefore, the relative distance between the leap tip and the temperature probe is constant.
Collapse
Affiliation(s)
- Peter R S Stijnman
- Computational Imaging Group for MRI diagnostics and therapy, Center for Image Sciences UMC Utrecht, Utrecht, the Netherlands.,Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, Brabant, the Netherlands
| | - M Arcan Erturk
- Restorative Therapies Group, Implantables R&D, Medtronic PLC, Minneapolis, Minnesota, USA
| | - Cornelis A T van den Berg
- Computational Imaging Group for MRI diagnostics and therapy, Center for Image Sciences UMC Utrecht, Utrecht, the Netherlands
| | - Alexander J E Raaijmakers
- Computational Imaging Group for MRI diagnostics and therapy, Center for Image Sciences UMC Utrecht, Utrecht, the Netherlands.,Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, Brabant, the Netherlands
| |
Collapse
|
9
|
Bourfiss M, Steensma BR, Te Riele ASJM, Leiner T, Velthuis BK, Raaijmakers AJE. Feature-tracking cardiac magnetic resonance of the right ventricle: Effect of field strength, resolution and imaging sequence. Eur J Radiol 2021; 138:109671. [PMID: 33773860 DOI: 10.1016/j.ejrad.2021.109671] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2021] [Accepted: 03/18/2021] [Indexed: 11/28/2022]
Affiliation(s)
- Mimount Bourfiss
- Department of Cardiology, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands.
| | - Bart R Steensma
- Department of Radiology, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - Anneline S J M Te Riele
- Department of Cardiology, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - Tim Leiner
- Department of Radiology, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - Birgitta K Velthuis
- Department of Radiology, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - Alexander J E Raaijmakers
- Department of Radiology, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands; Eindhoven University of Technology, Department of Biomedical Engineering, Den Dolech 2, 5612AZ, Eindhoven, the Netherlands
| |
Collapse
|
10
|
Tokaya JP, van den Berg CAT, Luijten PR, Raaijmakers AJE. Explaining RF induced current patterns on implantable medical devices during MRI using the transfer matrix. Med Phys 2021; 48:132-141. [PMID: 32383157 PMCID: PMC7898303 DOI: 10.1002/mp.14225] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2019] [Revised: 04/08/2020] [Accepted: 04/27/2020] [Indexed: 11/06/2022] Open
Abstract
PURPOSE In this work a simulation study is performed to gain insights in the patterns of induced radiofrequency (RF) currents for various implant-like structures at 1.5 T. The previously introduced transfer matrix (TM) is used to determine why certain current patterns have a tendency to naturally occur. This can benefit current safety assessment techniques and may enable the identification of critical exposure conditions. THEORY AND METHODS The induced current on an elongated implant can be determined by multiplication of the incident electric field along the implant with its TM. The eigenmode spectrum of the TMs for various lengths and various types of implants are determined. The eigenvector with the highest eigenvalue describes the incident electric field pattern that induces the highest current which in turn will lead to highest heating. Subsequently, a statistical probability analysis is performed using a wide range of potential incident electric field distributions in a representative human subject model during a 1.5 T MR exam which are determined by means of electromagnetic FDTD simulations. These incident electric field distributions and the resulting induced current patterns are projected onto eigenvectors of the TM to determine which eigenmodes of the implant dominate the current patterns. RESULTS The eigenvectors of the TM of bare and insulated wires resemble sinusoidal harmonics of a string fixed at both ends similar to the natural-current distribution on thin antennas(1). The currents on implants shorter than 20 cm are generally dominated by the first harmonic (similar to half a sine wave). This is firstly because for these implant lengths (relative to the RF wavelength), the first eigenvalue is more than three times bigger than the second showing the ability of an implant to accommodate one eigenmode better than another. Secondly, the incident electric fields have a high likelihood (≳95,7%) to project predominantly on this first eigenmode. CONCLUSION The eigenmode spectrum of the TM of an implant provides insight into the expected shape of induced current distributions and worst-case exposure conditions. For short implants, the first eigenvector is dominant. In addition, realistic incident electric field distributions project more heavily on this eigenvector. Both effects together cause significant currents to always resemble the dominant eigenmode of the TM for short implants at 1.5 T.
Collapse
Affiliation(s)
- Janot P. Tokaya
- Department of RadiotherapyUniversity Medical Center UtrechtP.O. Box 85500Utrecht3508 GANetherlands
| | | | - Peter R. Luijten
- Department of RadiologyUniversity Medical Center UtrechtP.O. Box 85500Utrecht3508 GANetherlands
| | - Alexander J. E. Raaijmakers
- Department of RadiotherapyUniversity Medical Center UtrechtP.O. Box 85500Utrecht3508 GANetherlands
- Department of Biomedical EngineeringEindhoven University of TechnologyEindhoven
| |
Collapse
|
11
|
Meliadò EF, Sbrizzi A, van den Berg CAT, Luijten PR, Raaijmakers AJE. Real-time assessment of potential peak local specific absorption rate value without phase monitoring: Trigonometric maximization method for worst-case local specific absorption rate determination. Magn Reson Med 2020; 85:3420-3433. [PMID: 33350525 PMCID: PMC7986921 DOI: 10.1002/mrm.28635] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/16/2020] [Revised: 11/12/2020] [Accepted: 11/16/2020] [Indexed: 12/16/2022]
Abstract
Purpose Multi‐transmit MRI systems are typically equipped with dedicated hardware to sample the reflected/lost power in the transmit channels. After extensive calibration, the amplitude and phase of the signal at the feed of each array element can be accurately determined. However, determining the phase is more difficult and monitoring errors can lead to a hazardous peak local specific absorption rate (pSAR10g) underestimation. For this purpose, methods were published for online maximum potential pSAR10g estimation without relying on phase monitoring, but these methods produce considerable overestimation. We present a trigonometric maximization method to determine the actual worst‐case pSAR10g without any overestimation. Theory and Method The proposed method takes advantage of the sinusoidal relation between the SAR10g in each voxel and the phases of input signals, to return the maximum achievable SAR10g in a few iterations. The method is applied to determine the worst‐case pSAR10g for three multi‐transmit array configurations at 7T: (1) body array with eight fractionated dipoles; (2) head array with eight fractionated dipoles; (3) head array with eight rectangular loops. The obtained worst‐case pSAR10g values are compared with the pSAR10g values determined with a commonly used method and with a more efficient method based on reference‐phases. Results For each voxel, the maximum achievable SAR10g is determined in less than 0.1 ms. Compared to the reference‐phases‐based method, the proposed method reduces the mean overestimation of the actual pSAR10g up to 52%, while never underestimating the true pSAR10g. Conclusion The proposed method can widely improve the performance of parallel transmission MRI systems without phase monitoring.
Collapse
Affiliation(s)
- Ettore Flavio Meliadò
- Department of Radiology, University Medical Center Utrecht, Utrecht, the Netherlands.,Computational Imaging Group for MR diagnostics & therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, the Netherlands.,Tesla Dynamic Coils BV, Zaltbommel, the Netherlands
| | - Alessandro Sbrizzi
- Department of Radiology, University Medical Center Utrecht, Utrecht, the Netherlands.,Computational Imaging Group for MR diagnostics & therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, the Netherlands
| | - Cornelis A T van den Berg
- Computational Imaging Group for MR diagnostics & therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, the Netherlands.,Department of Radiotherapy, Division of Imaging & Oncology, University Medical Center Utrecht, Utrecht, the Netherlands
| | - Peter R Luijten
- Department of Radiology, University Medical Center Utrecht, Utrecht, the Netherlands
| | - Alexander J E Raaijmakers
- Department of Radiology, University Medical Center Utrecht, Utrecht, the Netherlands.,Computational Imaging Group for MR diagnostics & therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, the Netherlands.,Biomedical Image Analysis, Department Biomedical Engineering, Eindhoven University of Technology, Eindhoven, the Netherlands
| |
Collapse
|
12
|
Annink KV, van der Aa NE, Dudink J, Alderliesten T, Groenendaal F, Lequin M, Jansen FE, Rhebergen KS, Luijten P, Hendrikse J, Hoogduin HJM, Huijing ER, Versteeg E, Visser F, Raaijmakers AJE, Wiegers EC, Klomp DWJ, Wijnen JP, Benders MJNL. Introduction of Ultra-High-Field MR Imaging in Infants: Preparations and Feasibility. AJNR Am J Neuroradiol 2020; 41:1532-1537. [PMID: 32732273 DOI: 10.3174/ajnr.a6702] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2019] [Accepted: 05/19/2020] [Indexed: 12/13/2022]
Abstract
BACKGROUND AND PURPOSE Cerebral MR imaging in infants is usually performed with a field strength of up to 3T. In adults, a growing number of studies have shown added diagnostic value of 7T MR imaging. 7T MR imaging might be of additional value in infants with unexplained seizures, for example. The aim of this study was to investigate the feasibility of 7T MR imaging in infants. We provide information about the safety preparations and show the first MR images of infants at 7T. MATERIALS AND METHODS Specific absorption rate levels during 7T were simulated in Sim4life using infant and adult models. A newly developed acoustic hood was used to guarantee hearing protection. Acoustic noise damping of this hood was measured and compared with the 3T Nordell hood and no hood. In this prospective pilot study, clinically stable infants, between term-equivalent age and the corrected age of 3 months, underwent 7T MR imaging immediately after their standard 3T MR imaging. The 7T scan protocols were developed and optimized while scanning this cohort. RESULTS Global and peak specific absorption rate levels in the infant model in the centered position and 50-mm feet direction did not exceed the levels in the adult model. Hearing protection was guaranteed with the new hood. Twelve infants were scanned. No MR imaging-related adverse events occurred. It was feasible to obtain good-quality imaging at 7T for MRA, MRV, SWI, single-shot T2WI, and MR spectroscopy. T1WI had lower quality at 7T. CONCLUSIONS 7T MR imaging is feasible in infants, and good-quality scans could be obtained.
Collapse
Affiliation(s)
- K V Annink
- From the Departments of Neonatology (K.V.A., N.E.v.d.A., J.D., T.A., F.G., M.J.N.L.B.), and Paediatric Neurology (F.E.J.), University Medical Center Utrecht Brain Center
| | - N E van der Aa
- From the Departments of Neonatology (K.V.A., N.E.v.d.A., J.D., T.A., F.G., M.J.N.L.B.), and Paediatric Neurology (F.E.J.), University Medical Center Utrecht Brain Center
| | - J Dudink
- From the Departments of Neonatology (K.V.A., N.E.v.d.A., J.D., T.A., F.G., M.J.N.L.B.), and Paediatric Neurology (F.E.J.), University Medical Center Utrecht Brain Center
| | - T Alderliesten
- From the Departments of Neonatology (K.V.A., N.E.v.d.A., J.D., T.A., F.G., M.J.N.L.B.), and Paediatric Neurology (F.E.J.), University Medical Center Utrecht Brain Center
| | - F Groenendaal
- From the Departments of Neonatology (K.V.A., N.E.v.d.A., J.D., T.A., F.G., M.J.N.L.B.), and Paediatric Neurology (F.E.J.), University Medical Center Utrecht Brain Center
| | - M Lequin
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - F E Jansen
- From the Departments of Neonatology (K.V.A., N.E.v.d.A., J.D., T.A., F.G., M.J.N.L.B.), and Paediatric Neurology (F.E.J.), University Medical Center Utrecht Brain Center
| | - K S Rhebergen
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - P Luijten
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - J Hendrikse
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - H J M Hoogduin
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - E R Huijing
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - E Versteeg
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - F Visser
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - A J E Raaijmakers
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - E C Wiegers
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - D W J Klomp
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - J P Wijnen
- the Departments of Radiology (M.L., P.L., J.H., H.J.M.H., E.R.H., E.V., F.V., A.J.E.R., E.C.W., D.W.J.K., J.P.W.), and Otorhinolaryngology and Head and Neck Surgery (K.S.R.), University Medical Center Utrecht, University Utrecht, Utrecht, the Netherlands
| | - M J N L Benders
- From the Departments of Neonatology (K.V.A., N.E.v.d.A., J.D., T.A., F.G., M.J.N.L.B.), and Paediatric Neurology (F.E.J.), University Medical Center Utrecht Brain Center
| |
Collapse
|
13
|
Meliadò EF, Sbrizzi A, van den Berg CAT, Steensma BR, Luijten PR, Raaijmakers AJE. Conditional safety margins for less conservative peak local SAR assessment: A probabilistic approach. Magn Reson Med 2020; 84:3379-3395. [PMID: 32492249 PMCID: PMC7540599 DOI: 10.1002/mrm.28335] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2020] [Revised: 04/28/2020] [Accepted: 05/01/2020] [Indexed: 12/17/2022]
Abstract
Purpose The introduction of a linear safety factor to address peak local specific absorption rate (pSAR10g) uncertainties (eg, intersubject variation, modeling inaccuracies) bears one considerable drawback: It often results in over‐conservative scanning constraints. We present a more efficient approach to define a variable safety margin based on the conditional probability density function of the effectively obtained pSAR10g value, given the estimated pSAR10g value. Methods The conditional probability density function can be estimated from previously simulated data. A representative set of true and estimated pSAR10g samples was generated by means of our database of 23 subject‐specific models with an 8‐fractionated dipole array for prostate imaging at 7 T. The conditional probability density function was calculated for each possible estimated pSAR10g value and used to determine the corresponding safety margin with an arbitrary low probability of underestimation. This approach was applied to five state‐of‐the‐art local SAR estimation methods, namely: (1) using just the generic body model “Duke”; (2) using our model library to assess the maximum pSAR10g value over all models; (3) using the most representative “local SAR model”; (4) using the five most representative local SAR models; and (5) using a recently developed deep learning–based method. Results Compared with the more conventional safety factor, the conditional safety‐margin approach results in lower (up to 30%) mean overestimation for all investigated local SAR estimation methods. Conclusion The proposed probabilistic approach for pSAR10g correction allows more accurate local SAR assessment with much lower overestimation, while a predefined level of underestimation is accepted (eg, 0.1%).
Collapse
Affiliation(s)
- Ettore Flavio Meliadò
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands.,Tesla Dynamic Coils, Zaltbommel, The Netherlands
| | - Alessandro Sbrizzi
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Cornelis A T van den Berg
- Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands.,Department of Radiotherapy, Division of Imaging & Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Bart R Steensma
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Peter R Luijten
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Alexander J E Raaijmakers
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands.,Biomedical Image Analysis, Department Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| |
Collapse
|
14
|
Stijnman PRS, Tokaya JP, van Gemert J, Luijten PR, Pluim JPW, Brink WM, Remis RF, van den Berg CAT, Raaijmakers AJE. Accelerating implant RF safety assessment using a low-rank inverse update method. Magn Reson Med 2019; 83:1796-1809. [PMID: 31566265 PMCID: PMC7003873 DOI: 10.1002/mrm.28023] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2019] [Revised: 09/04/2019] [Accepted: 09/07/2019] [Indexed: 11/17/2022]
Abstract
Purpose Patients who have medical metallic implants, e.g. orthopaedic implants and pacemakers, often cannot undergo an MRI exam. One of the largest risks is tissue heating due to the radio frequency (RF) fields. The RF safety assessment of implants is computationally demanding. This is due to the large dimensions of the transmit coil compared to the very detailed geometry of an implant. Methods In this work, we explore a faster computational method for the RF safety assessment of implants that exploits the small geometry. The method requires the RF field without an implant as a basis and calculates the perturbation that the implant induces. The inputs for this method are the incident fields and a library matrix that contains the RF field response of every edge an implant can occupy. Through a low‐rank inverse update, using the Sherman–Woodbury–Morrison matrix identity, the EM response of arbitrary implants can be computed within seconds. We compare the solution from full‐wave simulations with the results from the presented method, for two implant geometries. Results From the comparison, we found that the resulting electric and magnetic fields are numerically equivalent (maximum error of 1.35%). However, the computation was between 171 to 2478 times faster than the corresponding GPU accelerated full‐wave simulation. Conclusions The presented method enables for rapid and efficient evaluation of the RF fields near implants and might enable situation‐specific scanning conditions.
Collapse
Affiliation(s)
- Peter R S Stijnman
- Computational Imaging Group for MRI diagnostics and therapy, Centre for Image Sciences UMC Utrecht, Utrecht, The Netherlands.,Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| | - Janot P Tokaya
- Computational Imaging Group for MRI diagnostics and therapy, Centre for Image Sciences UMC Utrecht, Utrecht, The Netherlands
| | - Jeroen van Gemert
- Circuit & Systems Group of the Electrical Engineering, Delft University of Technology, Delft, The Netherlands.,C. J. Gorter Center for High Field MRI, Leiden University Medical Center, Leiden, The Netherlands
| | - Peter R Luijten
- Department of Radiology, UMC Utrecht, Utrecht, The Netherlands
| | - Josien P W Pluim
- Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| | - Wyger M Brink
- C. J. Gorter Center for High Field MRI, Leiden University Medical Center, Leiden, The Netherlands
| | - Rob F Remis
- Circuit & Systems Group of the Electrical Engineering, Delft University of Technology, Delft, The Netherlands
| | - Cornelis A T van den Berg
- Computational Imaging Group for MRI diagnostics and therapy, Centre for Image Sciences UMC Utrecht, Utrecht, The Netherlands
| | - Alexander J E Raaijmakers
- Computational Imaging Group for MRI diagnostics and therapy, Centre for Image Sciences UMC Utrecht, Utrecht, The Netherlands.,Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| |
Collapse
|
15
|
Meliadò EF, Raaijmakers AJE, Sbrizzi A, Steensma BR, Maspero M, Savenije MHF, Luijten PR, van den Berg CAT. A deep learning method for image-based subject-specific local SAR assessment. Magn Reson Med 2019; 83:695-711. [PMID: 31483521 PMCID: PMC6899474 DOI: 10.1002/mrm.27948] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2019] [Revised: 07/23/2019] [Accepted: 07/24/2019] [Indexed: 12/31/2022]
Abstract
Purpose Local specific absorption rate (SAR) cannot be measured and is usually evaluated by offline numerical simulations using generic body models that of course will differ from the patient's anatomy. An additional safety margin is needed to include this intersubject variability. In this work, we present a deep learning–based method for image‐based subject‐specific local SAR assessment. We propose to train a convolutional neural network to learn a “surrogate SAR model” to map the relation between subject‐specific B1+ maps and the corresponding local SAR. Method Our database of 23 subject‐specific models with an 8–transmit channel body array for prostate imaging at 7 T was used to build 5750 training samples. These synthetic complex B1+ maps and local SAR distributions were used to train a conditional generative adversarial network. Extra penalization for local SAR underestimation errors was included in the loss function. In silico and in vivo validation were performed. Results In silico cross‐validation shows a good qualitative and quantitative match between predicted and ground‐truth local SAR distributions. The peak local SAR estimation error distribution shows a mean overestimation error of 15% with 13% probability of underestimation. The higher accuracy of the proposed method allows the use of less conservative safety factors compared with standard procedures. In vivo validation shows that the method is applicable with realistic measurement data with impressively good qualitative and quantitative agreement to simulations. Conclusion The proposed deep learning method allows online image‐based subject‐specific local SAR assessment. It greatly reduces the uncertainty in current state‐of‐the‐art SAR assessment methods, reducing the time in the examination protocol by almost 25%.
Collapse
Affiliation(s)
- E F Meliadò
- Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Netherlands.,Tesla Dynamic Coils, Zaltbommel, Netherlands
| | - A J E Raaijmakers
- Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Netherlands.,Biomedical Image Analysis, Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, Netherlands
| | - A Sbrizzi
- Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Netherlands
| | - B R Steensma
- Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Netherlands
| | - M Maspero
- Division of Imaging & Oncology, Department of Radiotherapy, University Medical Center Utrecht, Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Netherlands
| | - M H F Savenije
- Division of Imaging & Oncology, Department of Radiotherapy, University Medical Center Utrecht, Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Netherlands
| | - P R Luijten
- Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands
| | - C A T van den Berg
- Division of Imaging & Oncology, Department of Radiotherapy, University Medical Center Utrecht, Netherlands.,Computational Imaging Group for MR Diagnostics & Therapy, Center for Image Sciences, University Medical Center Utrecht, Netherlands
| |
Collapse
|
16
|
Navest RJM, Mandija S, Andreychenko A, Raaijmakers AJE, Lagendijk JJW, van den Berg CAT. Understanding the physical relations governing the noise navigator. Magn Reson Med 2019; 82:2236-2247. [PMID: 31317566 PMCID: PMC6771522 DOI: 10.1002/mrm.27906] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2019] [Revised: 05/30/2019] [Accepted: 06/24/2019] [Indexed: 11/28/2022]
Abstract
Purpose The noise navigator is a passive way to detect physiological motion occurring in a patient through thermal noise modulations measured by standard clinical radiofrequency receive coils. The aim is to gain a deeper understanding of the potential and applications of physiologically induced thermal noise modulations. Methods Numerical electromagnetic simulations and MR measurements were performed to investigate the relative contribution of tissue displacement versus modulation of the dielectric lung properties over the respiratory cycle, the impact of coil diameter and position with respect to the body. Furthermore, the spatial motion sensitivity of specific noise covariance matrix elements of a receive array was investigated. Results The influence of dielectric lung property variations on the noise variance is negligible compared to tissue displacement. Coil size affected the thermal noise variance modulation, but the location of the coil with respect to the body had a larger impact. The modulation depth of a 15 cm diameter stationary coil approximately 3 cm away from the chest (i.e. radiotherapy setup) was 39.7% compared to 4.2% for a coil of the same size on the chest, moving along with respiratory motion. A combination of particular noise covariance matrix elements creates a specific spatial sensitivity for motion. Conclusions The insight gained on the physical relations governing the noise navigator will allow for optimized use and development of new applications. An optimized combination of elements from the noise covariance matrix offer new ways of performing, e.g. motion tracking.
Collapse
Affiliation(s)
- R J M Navest
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands.,Computational Imaging Group for MRI Diagnostics & Therapy, Centre for Image Sciences, University Medical Center Utrecht, Utrecht, Netherlands
| | - S Mandija
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands.,Computational Imaging Group for MRI Diagnostics & Therapy, Centre for Image Sciences, University Medical Center Utrecht, Utrecht, Netherlands
| | - A Andreychenko
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands.,ITMO University, St. Petersburg, Russian Federation.,Department of Healthcare, Research and Practical Clinical Center of Diagnostics and Telemedicine Technologies of the Moscow, Moscow, Russian Federation
| | - A J E Raaijmakers
- Computational Imaging Group for MRI Diagnostics & Therapy, Centre for Image Sciences, University Medical Center Utrecht, Utrecht, Netherlands.,Department of Radiology, University Medical Center Utrecht, Utrecht, Netherlands.,Deptartment of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, Netherlands
| | - J J W Lagendijk
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands
| | - C A T van den Berg
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands.,Computational Imaging Group for MRI Diagnostics & Therapy, Centre for Image Sciences, University Medical Center Utrecht, Utrecht, Netherlands
| |
Collapse
|
17
|
Bourfiss M, Steensma BR, Te Riele ASJM, Leiner T, Velthuis BK, Raaijmakers AJE. P160Feature tracking cardiac magnetic resonance of the right ventricle: effect of resolution, field strength and imaging sequence. Eur Heart J Cardiovasc Imaging 2019. [DOI: 10.1093/ehjci/jez117.023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Affiliation(s)
- M Bourfiss
- University Medical Center Utrecht, Cardiology, Utrecht, Netherlands (The)
| | - B R Steensma
- University Medical Center Utrecht, Radiology, Utrecht, Netherlands (The)
| | - A S J M Te Riele
- University Medical Center Utrecht, Cardiology, Utrecht, Netherlands (The)
| | - T Leiner
- University Medical Center Utrecht, Radiology, Utrecht, Netherlands (The)
| | - B K Velthuis
- University Medical Center Utrecht, Radiology, Utrecht, Netherlands (The)
| | - A J E Raaijmakers
- University Medical Center Utrecht, Radiology, Utrecht, Netherlands (The)
| |
Collapse
|
18
|
Meliadò EF, van den Berg CAT, Luijten PR, Raaijmakers AJE. Intersubject specific absorption rate variability analysis through construction of 23 realistic body models for prostate imaging at 7T. Magn Reson Med 2018; 81:2106-2119. [PMID: 30414210 DOI: 10.1002/mrm.27518] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/23/2018] [Revised: 07/16/2018] [Accepted: 08/11/2018] [Indexed: 01/28/2023]
Abstract
PURPOSE For ultrahigh field (UHF) MRI, the expected local specific absorption rate (SAR) distribution is usually calculated by numerical simulations using a limited number of generic body models and adding a safety margin to take into account intersubject variability. Assessment of this variability with a large model database would be desirable. In this study, a procedure to create such a database with accurate subject-specific models is presented. Using 23 models, intersubject variability is investigated for prostate imaging at 7T with an 8-channel fractionated dipole antenna array with 16 receive loops. METHOD From Dixon images of a volunteer acquired at 1.5T with a mockup array in place, an accurate dielectric model is built. Following this procedure, 23 subject-specific models for local SAR assessment at 7T were created enabling an extensive analysis of the intersubject B1 + and peak local SAR variability. RESULTS For the investigated setup, the maximum possible peak local SAR ranges from 2.6 to 4.6 W/kg for 8 × 1 W input power. The expected peak local SAR values represent a Gaussian distribution ( μ / σ = 2.29 / 0.29 W/kg) with realistic prostate-shimmed phase settings and a gamma distribution Γ(24,0.09) with multidimensional radiofrequency pulses. Prostate-shimmed phase settings are similar for all models. Using 1 generic phase setting, average B1 + reduction is 7%. Using only 1 model, the required safety margin for intersubject variability is 1.6 to 1.8. CONCLUSION The presented procedure allows for the creation of a customized model database. The results provide valuable insights into B1 + and local SAR variability. Recommended power thresholds per channel are 3.1 W with phase shimming on prostate or 2.6 W for multidimensional pulses.
Collapse
Affiliation(s)
- Ettore F Meliadò
- Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands.,MR Code BV, Zaltbommel, The Netherlands
| | | | - Peter R Luijten
- Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands
| | - Alexander J E Raaijmakers
- Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands.,Biomedical Image Analysis, Department Biomedical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands
| |
Collapse
|
19
|
Steensma BR, Luttje M, Voogt IJ, Klomp DWJ, Luijten PR, van den Berg CAT, Raaijmakers AJE. Comparing signal-to-noise ratio for prostate imaging at 7T and 3T. J Magn Reson Imaging 2018; 49:1446-1455. [PMID: 30350388 PMCID: PMC6587835 DOI: 10.1002/jmri.26527] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2018] [Revised: 09/12/2018] [Accepted: 09/12/2018] [Indexed: 12/16/2022] Open
Abstract
Background In MRI, the signal‐to‐noise ratio (SNR) theoretically increases with B0 field strength. However, because of attenuation of the radiofrequency (RF) fields at 7T, it is not certain if this SNR gain can be realized for prostate imaging. Purpose/Hypothesis To investigate the SNR gain in prostate imaging at 7T as compared with 3T. It is expected that SNR will improve for prostate imaging at 7T compared with 3T. Study Type Prospective. Subjects Four healthy volunteers and one prostate cancer patient. Field Strength/Sequence All subjects were scanned at 3T and at 7T using optimal coil setups for both field strengths. For all volunteers, proton density‐weighted images were acquired for SNR analysis and actual flip angle imaging (AFI) B1+| maps were acquired for correction of measured SNR values. In the patient, a T2‐weighted (T2w) image was acquired at 3T and at 7T. Assessment SNR was calculated in the prostate region for all volunteers. SNR was normalized for flip angle, receiver bandwidth, and voxel volume. SNR was also calculated for different sensitivity encoding (SENSE) acceleration factors. Statistical Testing SNR values are represented as the arithmetic mean of SNR values in the prostate. Estimated SNR in the T2w image is calculated as the arithmetic mean of the signal intensity (SI) divided by the standard deviation of the SI in a specified zone. Tumor‐to‐tissue contrast is calculated as (SItumor+SIzone)/( SItumor‐SIzone). Results An increase in SNR ranging from 1.7‐fold to 2.8‐fold was measured in the prostate at 7T in comparison to 3T for four volunteers. At 7T, it is possible to achieve a 4‐fold SENSE acceleration in the left‐right direction with similar SNR to a nonaccelerated 3T image. T2w imaging was done at 3T and 7T in one patient, where improved tumor‐to‐tissue contrast was demonstrated at 7T. Data Conclusion SNR improves for prostate imaging at 7T as compared with 3T. Level of Evidence: 2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:1446–1455.
Collapse
Affiliation(s)
- Bart R Steensma
- University Medical Center Utrecht, Department of Radiology, Utrecht, The Netherlands
| | - Mariska Luttje
- University Medical Center Utrecht, Department of Radiology, Utrecht, The Netherlands
| | - Ingmar J Voogt
- University Medical Center Utrecht, Department of Radiology, Utrecht, The Netherlands
| | - Dennis W J Klomp
- University Medical Center Utrecht, Department of Radiology, Utrecht, The Netherlands
| | - Peter R Luijten
- University Medical Center Utrecht, Department of Radiology, Utrecht, The Netherlands
| | | | - Alexander J E Raaijmakers
- University Medical Center Utrecht, Department of Radiology, Utrecht, The Netherlands.,Eindhoven University of Technology, Department of Biomedical Engineering, Utrecht, The Netherlands
| |
Collapse
|
20
|
van Rijssel MJ, Pluim JPW, Luijten PR, Gilhuijs KGA, Raaijmakers AJE, Klomp DWJ. Estimating B 1+ in the breast at 7 T using a generic template. NMR Biomed 2018; 31:e3911. [PMID: 29570887 PMCID: PMC5947628 DOI: 10.1002/nbm.3911] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 06/26/2017] [Revised: 01/30/2018] [Accepted: 01/30/2018] [Indexed: 06/08/2023]
Abstract
Dynamic contrast-enhanced MRI is the workhorse of breast MRI, where the diagnosis of lesions is largely based on the enhancement curve shape. However, this curve shape is biased by RF transmit (B1+ ) field inhomogeneities. B1+ field information is required in order to correct these. The use of a generic, coil-specific B1+ template is proposed and tested. Finite-difference time-domain simulations for B1+ were performed for healthy female volunteers with a wide range of breast anatomies. A generic B1+ template was constructed by averaging simulations based on four volunteers. Three-dimensional B1+ maps were acquired in 15 other volunteers. Root mean square error (RMSE) metrics were calculated between individual simulations and the template, and between individual measurements and the template. The agreement between the proposed template approach and a B1+ mapping method was compared against the agreement between acquisition and reacquisition using the same mapping protocol. RMSE values (% of nominal flip angle) comparing individual simulations with the template were in the range 2.00-4.01%, with mean 2.68%. RMSE values comparing individual measurements with the template were in the range8.1-16%, with mean 11.7%. The agreement between the proposed template approach and a B1+ mapping method was only slightly worse than the agreement between two consecutive acquisitions using the same mapping protocol in one volunteer: the range of agreement increased from ±16% of the nominal angle for repeated measurement to ±22% for the B1+ template. With local RF transmit coils, intersubject differences in B1+ fields of the breast are comparable to the accuracy of B1+ mapping methods, even at 7 T. Consequently, a single generic B1+ template suits subjects over a wide range of breast anatomies, eliminating the need for a time-consuming B1+ mapping protocol.
Collapse
|
21
|
Tokaya JP, Raaijmakers AJE, Luijten PR, van den Berg CAT. MRI-based, wireless determination of the transfer function of a linear implant: Introduction of the transfer matrix. Magn Reson Med 2018; 80:2771-2784. [PMID: 29687916 PMCID: PMC6220769 DOI: 10.1002/mrm.27218] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/09/2017] [Revised: 03/23/2018] [Accepted: 03/24/2018] [Indexed: 12/17/2022]
Abstract
PURPOSE We introduce the transfer matrix (TM) that makes MR-based wireless determination of transfer functions (TFs) possible. TFs are implant specific measures for RF-safety assessment of linear implants. The TF relates an incident tangential electric field on an implant to a scattered electric field at its tip that generally governs local heating. The TM extends this concept and relates an incident tangential electric field to a current distribution in the implant therewith characterizing the RF response along the entire implant. The TM is exploited to measure TFs with MRI without hardware alterations. THEORY AND METHODS A model of rightward and leftward propagating attenuated waves undergoing multiple reflections is used to derive an analytical expression for the TM. This allows parameterization of the TM of generic implants, e.g., (partially) insulated single wires, in a homogeneous medium in a few unknowns that simultaneously describe the TF. These unknowns can be determined with MRI making it possible to measure the TM and, therefore, also the TF. RESULTS The TM is able to predict an induced current due to an incident electric field and can be accurately parameterized with a limited number of unknowns. Using this description the TF is determined accurately (with a Pearson correlation coefficient R ≥ 0.9 between measurements and simulations) from MRI acquisitions. CONCLUSION The TM enables measuring of TFs with MRI of the tested generic implant models. The MR-based method does not need hardware alterations and is wireless hence making TF determination in more realistic scenarios conceivable.
Collapse
Affiliation(s)
- Janot P Tokaya
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands
| | | | - Peter R Luijten
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | | |
Collapse
|
22
|
van Kalleveen IML, Hoogendam JP, Raaijmakers AJE, Visser F, Arteaga de Castro CS, Verheijen RHM, Luijten PR, Zweemer RP, Veldhuis WB, Klomp DWJ. Boosting the SNR by adding a receive-only endorectal monopole to an external antenna array for high-resolution, T 2 -weighted imaging of early-stage cervical cancer with 7-T MRI. NMR Biomed 2017; 30:e3750. [PMID: 28574604 DOI: 10.1002/nbm.3750] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/27/2016] [Revised: 04/18/2017] [Accepted: 04/19/2017] [Indexed: 06/07/2023]
Abstract
The aim of this study was to investigate the signal-to-noise ratio (SNR) gain in early-stage cervical cancer at ultrahigh-field MRI (e.g. 7 T) using a combination of multiple external antennas and a single endorectal antenna. In particular, we used an endorectal monopole antenna to increase the SNR in cervical magnetic resonance imaging (MRI). This should allow high-resolution, T2 -weighted imaging and magnetic resonance spectroscopy (MRS) for metabolic staging, which could facilitate the local tumor status assessment. In a prospective feasibility study, five healthy female volunteers and six patients with histologically proven stage IB1-IIB cervical cancer were scanned at 7 T. We used seven external fractionated dipole antennas for transmit-receive (transceive) and an endorectally placed monopole antenna for reception only. A region of interest, containing both normal cervix and tumor tissue, was selected for the SNR measurement. Separated signal and noise measurements were obtained in the region of the cervix for each element and in the near field of the monopole antenna (radius < 30 mm) to calculate the SNR gain of the endorectal antenna in each patient. We obtained high-resolution, T2 -weighted images with a voxel size of 0.7 × 0.8 × 3.0 mm3 . In four cases with optimal placement of the endorectal antenna (verified on the T2 -weighted images), a mean gain of 2.2 in SNR was obtained at the overall cervix and tumor tissue area. Within a radius of 30 mm from the monopole antenna, a mean SNR gain of 3.7 was achieved in the four optimal cases. Overlap between the two different regions of the SNR calculations was around 24%. We have demonstrated that the use of an endorectal monopole antenna substantially increases the SNR of 7-T MRI at the cervical anatomy. Combined with the intrinsically high SNR of ultrahigh-field MRI, this gain may be employed to obtain metabolic information using MRS and to enhance spatial resolutions to assess tumor invasion.
Collapse
Affiliation(s)
| | - J P Hoogendam
- Department of Gynaecological Oncology, UMC Utrecht Cancer Centre, the Netherlands
| | | | - F Visser
- Department of Radiology, UMC Utrecht, the Netherlands
| | | | - R H M Verheijen
- Department of Gynaecological Oncology, UMC Utrecht Cancer Centre, the Netherlands
| | - P R Luijten
- Department of Radiology, UMC Utrecht, the Netherlands
| | - R P Zweemer
- Department of Gynaecological Oncology, UMC Utrecht Cancer Centre, the Netherlands
| | - W B Veldhuis
- Department of Radiology, UMC Utrecht, the Netherlands
| | - D W J Klomp
- Department of Radiology, UMC Utrecht, the Netherlands
| |
Collapse
|
23
|
Tokaya JP, Raaijmakers AJE, Luijten PR, Bakker JF, van den Berg CAT. MRI-based transfer function determination for the assessment of implant safety. Magn Reson Med 2017; 78:2449-2459. [PMID: 28164362 DOI: 10.1002/mrm.26613] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2016] [Revised: 12/16/2016] [Accepted: 12/28/2016] [Indexed: 11/11/2022]
Abstract
PURPOSE We introduce a new MR-based method to determine the transfer function (TF) for radiofrequency (RF) safety assessment of active implantable medical devices. Transfer functions are implant-specific measures that relate the incident tangential electric field on an (elongated) implant to a scattered electric field at its tip. The proposed method allows for TF determination with a high spatial resolution in relatively fast measurements without requiring dedicated bench setups from MRI images. THEORY AND METHODS The principle of reciprocity is used in conjunction with the potential to measure currents with MRI to determine TF. Low-flip angle 3D dual gradient echo MRI data are acquired with an implant as transceive antenna, which requires minimal hardware adaptations. The implant-specific TF is determined from the acquired MRI data, with two different postprocessing methods for comparison. RESULTS TFs of linear and helical implants can be determined accurately (with a Pearson correlation coefficient R ≥ 0.7 between measurements and simulations, and a difference in field at the tip ΔEtip ≤ 19%) from relatively quick (t < 20 minutes) MRI acquisitions with (several) millimeter spatial resolution. CONCLUSION Transfer function determination with MRI for RF safety assessment of implantable medical devices is possible. The proposed MR-based method allows for TF determination in more realistic exposure scenarios and solid media. Magn Reson Med 78:2449-2459, 2017. © 2017 International Society for Magnetic Resonance in Medicine.
Collapse
Affiliation(s)
- J P Tokaya
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands
| | - A J E Raaijmakers
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.,Biomedical Image Analysis, Eindhoven University of Technology, Eindhoven, The Netherlands
| | - P R Luijten
- Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands
| | - J F Bakker
- Medtronic Eindhoven Design Center, Eindhoven, The Netherlands
| | - C A T van den Berg
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands
| |
Collapse
|
24
|
Raaijmakers AJE, Luijten PR, van den Berg CAT. Dipole antennas for ultrahigh-field body imaging: a comparison with loop coils. NMR Biomed 2016; 29:1122-1130. [PMID: 26278544 DOI: 10.1002/nbm.3356] [Citation(s) in RCA: 55] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/01/2014] [Revised: 04/10/2015] [Accepted: 06/08/2015] [Indexed: 06/04/2023]
Abstract
Although the potential of dipole antennas for ultrahigh-field (UHF) MRI is largely recognized, they are still relatively unknown to the larger part of the MRI community. This article intends to provide electromagnetic insight into the general operating principles of dipole antennas by numerical simulations. The major part focuses on a comparison study of dipole antennas and loop coils at frequencies of 128, 298 and 400 MHz. This study shows that dipole antennas are only efficient radiofrequency (RF) coils in the presence of a dielectric and/or conducting load. In addition, the conservative electric fields (E-fields) at the ends of a dipole are negligible in comparison with the induced E-fields in the center. Like loop coils, long dipole antennas perform better than short dipoles for deeply located imaging targets and vice versa. When the optimal element is chosen for each depth, loop coils have higher B1 (+) efficiency for shallow depths, whereas dipole antennas have higher B1 (+) efficiency for large depths. The cross-over point depth decreases with increasing frequency: 11.6, 6.2 and 5.0 cm for 128, 298 and 400 MHz, respectively. For single elements, loop coils demonstrate a better B1 (+) /√SARmax ratio for any target depth and any frequency. However, one example study shows that, in an array setup with loop coil overlap for decoupling, this relationship is not straightforward. The overlapping loop coils may generate increased specific absorption rate (SAR) levels under the overlapping parts of the loops, depending on the drive phase settings. Copyright © 2015 John Wiley & Sons, Ltd.
Collapse
Affiliation(s)
| | - P R Luijten
- UMC Utrecht, Department of Radiology, Utrecht, the Netherlands
| | | |
Collapse
|
25
|
Hurshkainen AA, Derzhavskaya TA, Glybovski SB, Voogt IJ, Melchakova IV, van den Berg CAT, Raaijmakers AJE. Element decoupling of 7T dipole body arrays by EBG metasurface structures: Experimental verification. J Magn Reson 2016; 269:87-96. [PMID: 27262656 DOI: 10.1016/j.jmr.2016.05.017] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/12/2015] [Revised: 05/07/2016] [Accepted: 05/25/2016] [Indexed: 06/05/2023]
Abstract
Metasurfaces are artificial electromagnetic boundaries or interfaces usually implemented as two-dimensional periodic structures with subwavelength periodicity and engineered properties of constituent unit cells. The electromagnetic bandgap (EBG) effect in metasurfaces prevents all surface modes from propagating in a certain frequency band. While metasurfaces provide a number of important applications in microwave antennas and antenna arrays, their features are also highly suitable for MRI applications. In this work we perform a proof-of-principle experiment to study finite structures based on mushroom-type EBG metasurfaces and employ them for suppression of inter-element coupling in dipole transceive array coils for body imaging at 7T. We firstly show experimentally that employment of mushroom structures leads to reduction of coupling between adjacent closely-spaced dipole antenna elements of a 7T transceive body array, which reduces scattering losses in neighboring channels. The studied setup consists of two active fractionated dipole antennas previously designed by the authors for body imaging at 7T. These are placed on top of a body-mimicking phantom and equipped with the manufactured finite-size periodic structure tuned to have EBG properties at the Larmor frequency of 298MHz. To improve the detection range of the B1+ field distribution of the top elements, four additional elements were positioned along the bottom side of the phantom. Bench measurements of a scattering matrix showed that coupling between the two top elements can be considerably reduced depending on the distance to the EBG structure. On the other hand, the measurements performed on a 7T MRI machine indicated redistribution of the B1+ field due to interaction between the dipoles with the structure. When the structure is located just over two closely spaced dipoles, one can reach a very high isolation improvement of -14dB accompanied by a strong field redistribution. In contrast, when put at a certain height over the antennas the structure provides a moderate isolation improvement together with a slight increase of B1+ level. This study provides a tool for the decoupling of dipole antennas in ultrahigh field transceive arrays, possibly resulting in denser element placement and/or larger subject-element spacing.
Collapse
Affiliation(s)
- Anna A Hurshkainen
- Department of Nanophotonics and Metamaterials, ITMO University, 197101 St. Petersburg, Russia.
| | - Tatyana A Derzhavskaya
- Department of Nanophotonics and Metamaterials, ITMO University, 197101 St. Petersburg, Russia.
| | - Stanislav B Glybovski
- Department of Nanophotonics and Metamaterials, ITMO University, 197101 St. Petersburg, Russia.
| | - Ingmar J Voogt
- University Medical Center Utrecht, 3584 CX, Netherlands.
| | - Irina V Melchakova
- Department of Nanophotonics and Metamaterials, ITMO University, 197101 St. Petersburg, Russia.
| | | | | |
Collapse
|
26
|
Simonis FFJ, Raaijmakers AJE, Lagendijk JJW, van den Berg CAT. Validating subject-specific RF and thermal simulations in the calf muscle using MR-based temperature measurements. Magn Reson Med 2016; 77:1691-1700. [PMID: 27120403 DOI: 10.1002/mrm.26244] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/05/2016] [Revised: 03/01/2016] [Accepted: 03/25/2016] [Indexed: 02/05/2023]
Abstract
PURPOSE Ongoing discussions occur to translate the safety restrictions on MR scanners from specific absorption rate (SAR) to thermal dose. Therefore, this research focuses on the accuracy of thermal simulations in human subjects during an MR exam, which is fundamental information in that debate. METHODS Radiofrequency (RF) heating experiments were performed on the calves of 13 healthy subjects using a dedicated transmit-receive coil while monitoring the temperature with proton resonance frequency shift (PRFS) thermometry. Subject-specific models and one generic model were used for electromagnetic and thermal simulations using Pennes' bioheat equation, with the blood equilibration constant equaling zero. The simulations were subsequently compared with the experimental results. RESULTS The mean B1+ equaled 15 µT in the center slice of all volunteers, and 95% of the voxels had errors smaller than 2.8 µT between the simulation and measurement. The intersubject variation in RF power to achieve the required B1+ was 11%. The resulting intersubject variation in median temperature rise was 14%. Thermal simulations underestimated the median temperature increase on average, with 34% in subject-specific models and 28% in the generic model. CONCLUSIONS Although thermal measures are directly coupled to tissue damage and therefore suitable for RF safety assessment, insecurities in the applied thermal modeling limit their estimation accuracy. Magn Reson Med 77:1691-1700, 2017. © 2016 International Society for Magnetic Resonance in Medicine.
Collapse
Affiliation(s)
- F F J Simonis
- Department of Radiotherapy, Imaging Division, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - A J E Raaijmakers
- Department of Radiology, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - J J W Lagendijk
- Department of Radiotherapy, Imaging Division, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - C A T van den Berg
- Department of Radiotherapy, Imaging Division, University Medical Center Utrecht, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| |
Collapse
|
27
|
de Boer A, Hoogduin JM, Blankestijn PJ, Li X, Luijten PR, Metzger GJ, Raaijmakers AJE, Umutlu L, Visser F, Leiner T. 7 T renal MRI: challenges and promises. MAGMA 2016; 29:417-33. [PMID: 27008461 PMCID: PMC4891364 DOI: 10.1007/s10334-016-0538-3] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/16/2015] [Revised: 02/02/2016] [Accepted: 02/15/2016] [Indexed: 01/07/2023]
Abstract
The progression to 7 Tesla (7 T) magnetic resonance imaging (MRI) yields promises of substantial increase in signal-to-noise (SNR) ratio. This increase can be traded off to increase image spatial resolution or to decrease acquisition time. However, renal 7 T MRI remains challenging due to inhomogeneity of the radiofrequency field and due to specific absorption rate (SAR) constraints. A number of studies has been published in the field of renal 7 T imaging. While the focus initially was on anatomic imaging and renal MR angiography, later studies have explored renal functional imaging. Although anatomic imaging remains somewhat limited by inhomogeneous excitation and SAR constraints, functional imaging results are promising. The increased SNR at 7 T has been particularly advantageous for blood oxygen level-dependent and arterial spin labelling MRI, as well as sodium MR imaging, thanks to changes in field-strength-dependent magnetic properties. Here, we provide an overview of the currently available literature on renal 7 T MRI. In addition, we provide a brief overview of challenges and opportunities in renal 7 T MR imaging.
Collapse
Affiliation(s)
- Anneloes de Boer
- Department of Radiology, University Medical Centre Utrecht, Post box 85500, 3508 GA, Utrecht, The Netherlands
| | - Johannes M Hoogduin
- Department of Radiology, University Medical Centre Utrecht, Post box 85500, 3508 GA, Utrecht, The Netherlands.
| | - Peter J Blankestijn
- Department of Nephrology, University Medical Centre Utrecht, Utrecht, The Netherlands
| | - Xiufeng Li
- Department of Radiology, Centre for Magnetic Resonance Research, University of Minnesota Medical School, Minneapolis, MN, USA
| | - Peter R Luijten
- Department of Radiology, University Medical Centre Utrecht, Post box 85500, 3508 GA, Utrecht, The Netherlands
| | - Gregory J Metzger
- Department of Radiology, Centre for Magnetic Resonance Research, University of Minnesota Medical School, Minneapolis, MN, USA
| | - Alexander J E Raaijmakers
- Department of Radiology, University Medical Centre Utrecht, Post box 85500, 3508 GA, Utrecht, The Netherlands
| | - Lale Umutlu
- Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, Essen, Germany
| | - Fredy Visser
- Department of Radiology, University Medical Centre Utrecht, Post box 85500, 3508 GA, Utrecht, The Netherlands.,Philips Healthcare, Best, The Netherlands
| | - Tim Leiner
- Department of Radiology, University Medical Centre Utrecht, Post box 85500, 3508 GA, Utrecht, The Netherlands
| |
Collapse
|
28
|
Slobozhanyuk AP, Poddubny AN, Raaijmakers AJE, van den Berg CAT, Kozachenko AV, Dubrovina IA, Melchakova IV, Kivshar YS, Belov PA. Enhancement of Magnetic Resonance Imaging with Metasurfaces. Adv Mater 2016; 28:1832-8. [PMID: 26754827 DOI: 10.1002/adma.201504270] [Citation(s) in RCA: 58] [Impact Index Per Article: 7.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/31/2015] [Revised: 10/28/2015] [Indexed: 05/12/2023]
Abstract
It is revealed that the unique properties of ultrathin metasurface resonators can improve magnetic resonance imaging dramatically. A metasurface formed when an array of metallic wires is placed inside a scanner under the studied object and a substantial enhancement of the radio-frequency magnetic field is achieved by means of subwavelength manipulation with the metasurface, also allowing improved image resolution.
Collapse
Affiliation(s)
- Alexey P Slobozhanyuk
- Department of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, 197101, Russia
- Nonlinear Physics Center, Australian National University, Canberra, ACT, 0200, Australia
| | - Alexander N Poddubny
- Department of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, 197101, Russia
- Ioffe Physical-Technical Institute of the Russian Academy of Sciences, St. Petersburg, 194021, Russia
| | - Alexander J E Raaijmakers
- Department of Radiotherapy, University Medical Center Utrecht, P.O. Box 85500, 3508, GA, Utrecht, The Netherlands
| | - Cornelis A T van den Berg
- Department of Radiotherapy, University Medical Center Utrecht, P.O. Box 85500, 3508, GA, Utrecht, The Netherlands
| | - Alexander V Kozachenko
- Department of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, 197101, Russia
| | - Irina A Dubrovina
- Institute of Experimental Medicine, Russian Academy of Medical Sciences, St. Petersburg, 197376, Russia
| | - Irina V Melchakova
- Department of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, 197101, Russia
| | - Yuri S Kivshar
- Department of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, 197101, Russia
- Nonlinear Physics Center, Australian National University, Canberra, ACT, 0200, Australia
| | - Pavel A Belov
- Department of Nanophotonics and Metamaterials, ITMO University, St. Petersburg, 197101, Russia
| |
Collapse
|
29
|
Ertürk MA, Raaijmakers AJE, Adriany G, Uğurbil K, Metzger GJ. A 16-channel combined loop-dipole transceiver array for 7 Tesla body MRI. Magn Reson Med 2016; 77:884-894. [PMID: 26887533 DOI: 10.1002/mrm.26153] [Citation(s) in RCA: 111] [Impact Index Per Article: 13.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2015] [Revised: 12/21/2015] [Accepted: 01/17/2016] [Indexed: 12/17/2022]
Abstract
PURPOSE To develop a 16-channel transceive body imaging array at 7.0 T with improved transmit, receive, and specific absorption rate (SAR) performance by combining both loop and dipole elements and using their respective and complementary near and far field characteristics. METHODS A 16-channel radiofrequency (RF) coil array consisting of eight loop-dipole blocks (16LD) was designed and constructed. Transmit and receive performance was quantitatively investigated in phantom and human model simulations, and experiments on five healthy volunteers inside the prostate. Comparisons were made with 16-channel microstrip line (16ML) and 10-channel fractionated dipole antenna (10DA) arrays. The 16LD was used to acquire anatomic and functional images of the prostate, kidneys, and heart. RESULTS The 16LD provided > 14% improvements in the signal-to-noise ratio (SNR), peak B1+, B1+ transmit, and SAR efficiencies over the 16ML and 10DA in simulations inside the prostate. Experimentally, the 16LD had > 20% higher SNR and B1+ transmit efficiency compared with other arrays, and achieved up to 51.8% higher peak B1+ compared with 10DA. CONCLUSION Combining loop and dipole elements provided a body imaging array with high channel count and density while limiting inter-element coupling. The 16LD improved both near and far-field performance compared with existing 7.0T body arrays and provided high-quality MRI of the prostate kidneys and heart. Magn Reson Med 77:884-894, 2017. © 2016 International Society for Magnetic Resonance in Medicine.
Collapse
Affiliation(s)
- M Arcan Ertürk
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | | | - Gregor Adriany
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | - Kâmil Uğurbil
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | - Gregory J Metzger
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| |
Collapse
|
30
|
van Gorp JS, Seevinck PR, Andreychenko A, Raaijmakers AJE, Luijten PR, Viergever MA, Koopman M, Boer VO, Klomp DWJ. (19)F MRSI of capecitabine in the liver at 7 T using broadband transmit-receive antennas and dual-band RF pulses. NMR Biomed 2015; 28:1433-1442. [PMID: 26373355 DOI: 10.1002/nbm.3390] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/29/2015] [Revised: 08/07/2015] [Accepted: 08/10/2015] [Indexed: 06/05/2023]
Abstract
Capecitabine (Cap) is an often prescribed chemotherapeutic agent, successfully used to cure some patients from cancer or reduce tumor burden for palliative care. However, the efficacy of the drug is limited, it is not known in advance who will respond to the drug and it can come with severe toxicity. (19)F Magnetic Resonance Spectroscopy (MRS) and Magnetic Resonance Spectroscopic Imaging (MRSI) have been used to non-invasively study Cap metabolism in vivo to find a marker for personalized treatment. In vivo detection, however, is hampered by low concentrations and the use of radiofrequency (RF) surface coils limiting spatial coverage. In this work, the use of a 7T MR system with radiative multi-channel transmit-receive antennas was investigated with the aim of maximizing the sensitivity and spatial coverage of (19)F detection protocols. The antennas were broadband optimized to facilitate both the (1)H (298 MHz) and (19)F (280 MHz) frequencies for accurate shimming, imaging and signal combination. B1(+) simulations, phantom and noise measurements showed that more than 90% of the theoretical maximum sensitivity could be obtained when using B1(+) and B1(-) information provided at the (1)H frequency for the optimization of B1(+) and B1(-) at the (19)F frequency. Furthermore, to overcome the limits in maximum available RF power, whilst ensuring simultaneous excitation of all detectable conversion products of Cap, a dual-band RF pulse was designed and evaluated. Finally, (19)F MRS(I) measurements were performed to detect (19)F metabolites in vitro and in vivo. In two patients, at 10 h (patient 1) and 1 h (patient 2) after Cap intake, (19)F metabolites were detected in the liver and the surrounding organs, illustrating the potential of the set-up for in vivo detection of metabolic rates and drug distribution in the body.
Collapse
Affiliation(s)
- Jetse S van Gorp
- University Medical Center Utrecht, Image Sciences Institute, Utrecht, the Netherlands
| | - Peter R Seevinck
- University Medical Center Utrecht, Image Sciences Institute, Utrecht, the Netherlands
| | - Anna Andreychenko
- University Medical Center Utrecht, Radiotherapy, Utrecht, the Netherlands
| | | | - Peter R Luijten
- University Medical Center Utrecht, Radiology, Utrecht, the Netherlands
| | - Max A Viergever
- University Medical Center Utrecht, Image Sciences Institute, Utrecht, the Netherlands
| | - Miriam Koopman
- University Medical Center Utrecht, Medical Oncology, Utrecht, the Netherlands
| | - Vincent O Boer
- Hvidovre Hospital, Danish Research Center for Magnetic Resonance, Hvidovre, Denmark
| | - Dennis W J Klomp
- University Medical Center Utrecht, Radiology, Utrecht, the Netherlands
| |
Collapse
|
31
|
Bluemink JJ, Raaijmakers AJE, Koning W, Andreychenko A, Rivera DS, Luijten PR, Klomp DWJ, van den Berg CAT. Dielectric waveguides for ultrahigh field magnetic resonance imaging. Magn Reson Med 2015; 76:1314-24. [DOI: 10.1002/mrm.26007] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2014] [Revised: 09/11/2015] [Accepted: 09/12/2015] [Indexed: 11/09/2022]
Affiliation(s)
- Johanna J. Bluemink
- UMC Utrecht Cancer Center / UMC Utrecht Center for Imaging Sciences, Department of Radiotherapy; University Medical Center Utrecht; The Netherlands
| | - Alexander J. E. Raaijmakers
- UMC Utrecht Cancer Center / UMC Utrecht Center for Imaging Sciences, Department of Radiotherapy; University Medical Center Utrecht; The Netherlands
| | - Wouter Koning
- UMC Utrecht Cancer Center / UMC Utrecht Center for Imaging Sciences, Department of Radiotherapy; University Medical Center Utrecht; The Netherlands
| | - Anna Andreychenko
- UMC Utrecht Cancer Center / UMC Utrecht Center for Imaging Sciences, Department of Radiotherapy; University Medical Center Utrecht; The Netherlands
| | - Debra S. Rivera
- UMC Utrecht Cancer Center / UMC Utrecht Center for Imaging Sciences, Department of Radiotherapy; University Medical Center Utrecht; The Netherlands
| | - Peter R. Luijten
- UMC Utrecht Cancer Center / UMC Utrecht Center for Imaging Sciences, Department of Radiotherapy; University Medical Center Utrecht; The Netherlands
| | - Dennis W. J. Klomp
- UMC Utrecht Cancer Center / UMC Utrecht Center for Imaging Sciences, Department of Radiotherapy; University Medical Center Utrecht; The Netherlands
| | - Cornelis A. T. van den Berg
- UMC Utrecht Cancer Center / UMC Utrecht Center for Imaging Sciences, Department of Radiotherapy; University Medical Center Utrecht; The Netherlands
| |
Collapse
|
32
|
Restivo MC, van den Berg CAT, van Lier ALHMW, Polders DL, Raaijmakers AJE, Luijten PR, Hoogduin H. Local specific absorption rate in brain tumors at 7 tesla. Magn Reson Med 2015; 75:381-9. [PMID: 25752920 DOI: 10.1002/mrm.25653] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2014] [Revised: 01/20/2015] [Accepted: 01/20/2015] [Indexed: 11/06/2022]
Abstract
PURPOSE MR safety at 7 Tesla relies on accurate numerical simulations of transmit electromagnetic fields to fully assess local specific absorption rate (SAR) safety. Numerical simulations for SAR safety are currently performed using models of healthy patients. These simulations might not be useful for estimating SAR in patients who have large lesions with potentially abnormal dielectric properties, e.g., brain tumors. THEORY AND METHODS In this study, brain tumor patient models are constructed based on scans of four patients with high grade brain tumors. Dielectric properties for the modeled tumors are assigned based on electrical properties tomography data for the same patients. Simulations were performed to determine SAR. RESULTS Local SAR increases in the tumors by as much as 30%. However, the location of the maximum 10-gram averaged SAR typically occurs outside of the tumor, and thus does not increase. In the worst case, if the tumor model is moved to the location of maximum electric field intensity, then we do observe an increase in the estimated peak 10-gram SAR directly related to the tumor. CONCLUSION Peak local SAR estimation made on the results of a healthy patient model simulation may underestimate the true peak local SAR in a brain tumor patient.
Collapse
Affiliation(s)
- Matthew C Restivo
- Imaging Division, University Medical Center Utrecht, Utrecht, Netherlands
| | | | | | - Daniël L Polders
- Imaging Division, University Medical Center Utrecht, Utrecht, Netherlands
| | | | - Peter R Luijten
- Imaging Division, University Medical Center Utrecht, Utrecht, Netherlands
| | - Hans Hoogduin
- Imaging Division, University Medical Center Utrecht, Utrecht, Netherlands
| |
Collapse
|
33
|
van der Velden TA, Italiaander M, van der Kemp WJM, Raaijmakers AJE, Schmitz AMT, Luijten PR, Boer VO, Klomp DWJ. Radiofrequency configuration to facilitate bilateral breast (31) P MR spectroscopic imaging and high-resolution MRI at 7 Tesla. Magn Reson Med 2014; 74:1803-10. [PMID: 25521345 DOI: 10.1002/mrm.25573] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/13/2014] [Revised: 11/10/2014] [Accepted: 11/14/2014] [Indexed: 12/24/2022]
Abstract
PURPOSE High-resolution MRI combined with phospholipid detection may improve breast cancer grading. Currently, configurations are optimized for either high-resolution imaging or (31) P spectroscopy. To be able to perform both imaging as well as spectroscopy in a single session, we integrated a (1) H receiver array into a (1) H-(31) P transceiver at 7T. To ensure negligible signal loss due to coupling between elements, we investigated the use of a floating decoupling loop to enable bilateral MRI and (31) P MRS. METHODS Two quadrature double-tuned radiofrequency coils were designed for bilateral breast MR with active detuning at the (1) H frequency. The two coils were placed adjacent to each other and decoupled for both frequencies with a single resonant floating loop. Sensitivity of the bilateral configuration, facilitating space for a 26-element (1) H receive array, was compared with a transceiver configuration. RESULTS The floating loop was able to decouple the elements over 20 dB for both frequencies. Enlargement of the elements, to provide space for the receivers, and the addition of detuning electronics altered the (31) P sensitivity by 0.4 dB. CONCLUSION Dynamic contrast-enhanced scans of 0.7 mm isotropic, diffusion-weighted imaging, and (31) P MR spectroscopic imaging can be acquired at 7T in a single session as demonstrated in a patient with invasive ductal carcinoma.
Collapse
Affiliation(s)
- Tijl A van der Velden
- University Medical Centre Utrecht, department of Radiology, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | | | - Wybe J M van der Kemp
- University Medical Centre Utrecht, department of Radiology, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - Alexander J E Raaijmakers
- University Medical Centre Utrecht, department of Radiology, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - A M Th Schmitz
- University Medical Centre Utrecht, department of Radiology, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - Peter R Luijten
- University Medical Centre Utrecht, department of Radiology, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - Vincent O Boer
- University Medical Centre Utrecht, department of Radiology, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| | - Dennis W J Klomp
- University Medical Centre Utrecht, department of Radiology, Heidelberglaan 100, 3584CX, Utrecht, the Netherlands
| |
Collapse
|
34
|
Sbrizzi A, Raaijmakers AJE, Hoogduin H, Lagendijk JJW, Luijten PR, van den Berg CAT. Transmit and receive RF fields determination from a single low-tip-angle gradient-echo scan by scaling of SVD data. Magn Reson Med 2013; 72:248-59. [PMID: 24022840 DOI: 10.1002/mrm.24912] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/18/2013] [Revised: 07/01/2013] [Accepted: 07/16/2013] [Indexed: 11/08/2022]
Abstract
PURPOSE A new method, called Transmit and Receive Patterns from Low-Tip-angle gradient-Echo Images (TRIPLET), is described which simultaneously maps the B1+ and B1- fields of a transmit/receive radiofrequency coil array. The input data are low-tip-angle gradient-echo images, which can be acquired in a relatively short scanning time. THEORY AND METHODS For each voxel in the field of view, a matrix can be assembled with the low-tip-angle gradient-echo image values of the radiofrequency coil array. Applying the singular value decomposition to those matrices, datasets are obtained which show a high resemblance with the true B1+ and B1- fields. These datasets are a voxel-wise scaled version of the true radiofrequency maps. The channel independent scaling parameters can be found by implicitly forcing the reconstructed fields to be solutions of the Maxwell equations. This is achieved by introducing a multipole expansion consisting of Bessel/Fourier functions. RESULTS Two FDTD simulated radiofrequency fields for two coil array combinations at 7 T and a measured, in vivo dataset at 7 T are investigated to illustrate the singular value decomposition analysis of the low-tip-angle gradient-echo images and to show how the B1+ and B1- fields can be reconstructed by Transmit and Receive Patterns from Low-Tip-angle gradient-Echo Images. CONCLUSION The Transmit and Receive Patterns from Low-Tip-angle gradient-Echo Images algorithm can convert the datasets from singular value decomposition analysis of low-tip-angle gradient-echo images to true B1+ and B1- fields.
Collapse
Affiliation(s)
- Alessandro Sbrizzi
- Imaging Division, University Medical Center Utrecht, Utrecht, The Netherlands
| | | | | | | | | | | |
Collapse
|
35
|
Andreychenko A, Bluemink JJ, Raaijmakers AJE, Lagendijk JJW, Luijten PR, van den Berg CAT. Improved RF performance of travelling wave MR with a high permittivity dielectric lining of the bore. Magn Reson Med 2012; 70:885-94. [PMID: 23044511 DOI: 10.1002/mrm.24512] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2012] [Revised: 08/15/2012] [Accepted: 09/08/2012] [Indexed: 12/14/2022]
Abstract
Application of travelling wave MR to human body imaging is restricted by the limited peak power of the available RF amplifiers. Nevertheless, travelling wave MR advantages like a large field of view excitation and distant location of transmit elements would be desirable for whole body MRI. In this work, improvement of the B1+ efficiency of travelling wave MR is demonstrated. High permittivity dielectric lining placed next to the scanner bore wall effectively reduces attenuation of the travelling wave in the longitudinal direction and at the same time directs the radial power flow toward the load. First, this is shown with an analytical model of a metallic cylindrical waveguide with the dielectric lining next to the wall and loaded with a cylindrical phantom. Simulations and experiments also reveal an increase of B1+ efficiency in the center of the bore for travelling wave MR with a dielectric lining. Phantom experiments show up to a 2-fold gain in B1+ with the dielectric lining. This corresponds to a 4-fold increase in power efficiency of travelling wave MR. In vivo experiments demonstrate an 8-fold signal-to-noise ratio gain with the dielectric lining. Overall, it is shown that dielectric lining is a constructive method to improve efficacy of travelling wave MR.
Collapse
Affiliation(s)
- A Andreychenko
- Department of Radiology, University Medical Center Utrecht, The Netherlands; Department of Radiotherapy, University Medical Center Utrecht, The Netherlands
| | | | | | | | | | | |
Collapse
|
36
|
de Greef M, Ipek O, Raaijmakers AJE, Crezee J, van den Berg CAT. Specific absorption rate intersubject variability in 7T parallel transmit MRI of the head. Magn Reson Med 2012; 69:1476-85. [PMID: 22760930 DOI: 10.1002/mrm.24378] [Citation(s) in RCA: 55] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2012] [Revised: 04/30/2012] [Accepted: 05/24/2012] [Indexed: 11/08/2022]
Abstract
Patient-specific radiofrequency shimming in high-field MRI strengthens the need for online, patient-specific specific absorption rate (SAR) monitoring. Numerical simulation is currently most effective for this purpose but may require a patient-specific dielectric model. To investigate whether a generic model may be combined with a safety factor to account for variation within the population, generic SAR behavior is studied for 7T MRI of the head. For six detailed head models, radiofrequency fields were simulated for an eight-channel parallel transmit array. SAR behavior is studied through comparison of the eigenvalues/eigenvectors of the local Q-matrices. Furthermore, numerical radiofrequency shimming experiments without and with SAR constraints were performed where SAR during optimization was evaluated on a generic model. In both cases, the ability of different generic models to predict actual SAR levels was evaluated. The largest eigenvalue distribution is comparable between models. Radiofrequency shimming without constraints improves the |B +1| homogeneity while the SAR increases substantially. Imposing constraints on SAR during optimization, estimating SAR on a generic model, was effective. A safety factor of 1.4 was found to be sufficient. Generic SAR behavior makes a generic head model a practical alternative to patient-specific models and allows effective |B +1| shimming with SAR constraints.
Collapse
Affiliation(s)
- Martijn de Greef
- Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
| | | | | | | | | |
Collapse
|
37
|
Ipek O, Raaijmakers AJE, Klomp DWJ, Lagendijk JJW, Luijten PR, van den Berg CAT. Characterization of transceive surface element designs for 7 tesla magnetic resonance imaging of the prostate: radiative antenna and microstrip. Phys Med Biol 2011; 57:343-55. [PMID: 22170777 DOI: 10.1088/0031-9155/57/2/343] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Ultra-high field magnetic resonance (≥7 tesla) imaging (MRI) faces challenges with respect to efficient spin excitation and signal reception from deeply situated organs. Traditional radio frequency surface coil designs relying on near-field coupling are suboptimal at high field strengths. Better signal penetration can be obtained by designing a radiative antenna in which the energy flux is directed to the target location. In this paper, two different radiative antenna designs are investigated to be used as transceive elements, which employ different dielectric permittivities for the antenna substrate. Their transmit and receive performances in terms of B(+)(1), local SAR (specific absorption rate) and SNR (signal-to-noise ratio) were compared using extensive electromagnetic simulations and MRI measurements with traditional surface microstrip coils. Both simulations and measurements demonstrated that the radiative element shows twofold gain in B(+)(1) and SNR at 10 cm depth, and additionally a comparable SAR peak value. In terms of transmit performance, the radiative antenna with a dielectric permittivity of 37 showed a 24% more favorable local SAR(10g avg)/(B(+)(1))(2) ratio than the radiative antenna with a dielectric permittivity of 90. In receive, the radiative element with a dielectric permittivity of 90 resulted in a 20% higher SNR for shallow depths, but for larger depths this difference diminished compared to the radiative element with a dielectric permittivity of 37. Therefore, to image deep anatomical regions effectively, the radiative antenna with a dielectric permittivity of 37 is favorable.
Collapse
Affiliation(s)
- O Ipek
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, the Netherlands.
| | | | | | | | | | | |
Collapse
|
38
|
Abstract
A new hybrid imaging-treatment modality, the MRI-Linac, involves the irradiation of the patient in the presence of a strong magnetic field. This field acts on the charged particles, responsible for depositing dose, through the Lorentz force. These conditions require a dose calculation engine capable of taking into consideration the effect of the magnetic field on the dose distribution during the planning stage. Also in the case of a change in anatomy at the time of treatment, a fast online replanning tool is desirable. It is improbable that analytical solutions such as pencil beam calculations can be efficiently adapted for dose calculations within a magnetic field. Monte Carlo simulations have therefore been used for the computations but the calculation speed is generally too slow to allow online replanning. In this work, GPUMCD, a fast graphics processing unit (GPU)-based Monte Carlo dose calculation platform, was benchmarked with a new feature that allows dose calculations within a magnetic field. As a proof of concept, this new feature is validated against experimental measurements. GPUMCD was found to accurately reproduce experimental dose distributions according to a 2%-2 mm gamma analysis in two cases with large magnetic field-induced dose effects: a depth-dose phantom with an air cavity and a lateral-dose phantom surrounded by air. Furthermore, execution times of less than 15 s were achieved for one beam in a prostate case phantom for a 2% statistical uncertainty while less than 20 s were required for a seven-beam plan. These results indicate that GPUMCD is an interesting candidate, being fast and accurate, for dose calculations for the hybrid MRI-Linac modality.
Collapse
Affiliation(s)
- S Hissoiny
- École Polytechnique de Montréal, Département de génie informatique et génie logiciel, 2500 Chemin de Polytechnique, Montréal, Québec H3T 1J4, Canada.
| | | | | | | | | |
Collapse
|
39
|
van den Bergen B, Klomp DWJ, Raaijmakers AJE, de Castro CA, Boer VO, Kroeze H, Luijten PR, Lagendijk JJW, van den Berg CAT. Uniform prostate imaging and spectroscopy at 7 T: comparison between a microstrip array and an endorectal coil. NMR Biomed 2011; 24:358-365. [PMID: 20960577 DOI: 10.1002/nbm.1599] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/11/2009] [Revised: 06/21/2010] [Accepted: 07/09/2010] [Indexed: 05/30/2023]
Abstract
An endorectal coil and an eight-element microstrip array were compared for prostate imaging at 7 T. An extensive radiofrequency safety assessment was performed with the use of finite difference time domain simulations to determine safe scan parameters. These simulations showed that the endorectal coil can deliver substantially more B(1)(+) to the prostate than can the microstrip array within the specific absorption rate safety guidelines. However, the B(1)(+) field of the endorectal coil is very inhomogeneous, which makes the use of adiabatic pulses compulsory for T(1) - or T(2) -weighted imaging. As a consequence, a full prostate examination is only possible in a feasible amount of time when the microstrip array is used for T(1) - and T(2) -weighted imaging, whereas the endorectal coil is required for spectroscopic imaging. The pulse parameters were optimised within the specific absorption rate guidelines and thereafter used to provide a good illustration of the possibilities of prostate imaging at 7 T.
Collapse
Affiliation(s)
- Bob van den Bergen
- University Medical Centre Utrecht, Department of Radiotherapy, the Netherlands.
| | | | | | | | | | | | | | | | | |
Collapse
|
40
|
Raaymakers BW, Lagendijk JJW, Overweg J, Kok JGM, Raaijmakers AJE, Kerkhof EM, van der Put RW, Meijsing I, Crijns SPM, Benedosso F, van Vulpen M, de Graaff CHW, Allen J, Brown KJ. Integrating a 1.5 T MRI scanner with a 6 MV accelerator: proof of concept. Phys Med Biol 2009. [PMID: 19451689 DOI: 10.1088/0031‐9155/54/12/n01] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
At the UMC Utrecht, The Netherlands, we have constructed a prototype MRI accelerator. The prototype is a modified 6 MV Elekta (Crawley, UK) accelerator next to a modified 1.5 T Philips Achieva (Best, The Netherlands) MRI system. From the initial design onwards, modifications to both systems were aimed to yield simultaneous and unhampered operation of the MRI and the accelerator. Indeed, the simultaneous operation is shown by performing diagnostic quality 1.5 T MRI with the radiation beam on. No degradation of the performance of either system was found. The integrated 1.5 T MRI system and radiotherapy accelerator allow simultaneous irradiation and MR imaging. The full diagnostic imaging capacities of the MRI can be used; dedicated sequences for MRI-guided radiotherapy treatments will be developed. This proof of concept opens the door towards a clinical prototype to start testing MRI-guided radiation therapy (MRIgRT) in the clinic.
Collapse
Affiliation(s)
- B W Raaymakers
- Department of Radiotherapy, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
| | | | | | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
41
|
Raaymakers BW, Lagendijk JJW, Overweg J, Kok JGM, Raaijmakers AJE, Kerkhof EM, van der Put RW, Meijsing I, Crijns SPM, Benedosso F, van Vulpen M, de Graaff CHW, Allen J, Brown KJ. Integrating a 1.5 T MRI scanner with a 6 MV accelerator: proof of concept. Phys Med Biol 2009; 54:N229-37. [PMID: 19451689 DOI: 10.1088/0031-9155/54/12/n01] [Citation(s) in RCA: 439] [Impact Index Per Article: 29.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
At the UMC Utrecht, The Netherlands, we have constructed a prototype MRI accelerator. The prototype is a modified 6 MV Elekta (Crawley, UK) accelerator next to a modified 1.5 T Philips Achieva (Best, The Netherlands) MRI system. From the initial design onwards, modifications to both systems were aimed to yield simultaneous and unhampered operation of the MRI and the accelerator. Indeed, the simultaneous operation is shown by performing diagnostic quality 1.5 T MRI with the radiation beam on. No degradation of the performance of either system was found. The integrated 1.5 T MRI system and radiotherapy accelerator allow simultaneous irradiation and MR imaging. The full diagnostic imaging capacities of the MRI can be used; dedicated sequences for MRI-guided radiotherapy treatments will be developed. This proof of concept opens the door towards a clinical prototype to start testing MRI-guided radiation therapy (MRIgRT) in the clinic.
Collapse
Affiliation(s)
- B W Raaymakers
- Department of Radiotherapy, University Medical Center Utrecht, Heidelberglaan 100, 3584 CX, Utrecht, The Netherlands.
| | | | | | | | | | | | | | | | | | | | | | | | | | | |
Collapse
|
42
|
Meijsing I, Raaymakers BW, Raaijmakers AJE, Kok JGM, Hogeweg L, Liu B, Lagendijk JJW. Dosimetry for the MRI accelerator: the impact of a magnetic field on the response of a Farmer NE2571 ionization chamber. Phys Med Biol 2009; 54:2993-3002. [DOI: 10.1088/0031-9155/54/10/002] [Citation(s) in RCA: 121] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
|
43
|
Abstract
Many methods exist to improve treatment outcome in radiotherapy. Two of these are image-guided radiotherapy (IGRT) and proton therapy. IGRT aims at a more precise delivery of the radiation, while proton therapy is able to achieve more conformal dose distributions. In order to maximally exploit the sharp dose gradients from proton therapy it has to be combined with soft-tissue based IGRT. MRI-guided photon therapy (currently under development) offers unequalled soft-tissue contrast and real-time image guidance. A hybrid MRI proton therapy system would combine these advantages with the advantageous dose steering capacity of proton therapy. This paper addresses a first technical feasibility issue of this concept, namely the impact of a 0.5 T magnetic field on the dose distribution from a 90 MeV proton beam. In contrast to photon therapy, for MR-guided proton therapy the impact of the magnetic field on the dose distribution is very small. At tissue-air interfaces no effect of the magnetic field on the dose distribution can be detected. This is due to the low-energy of the secondary electrons released by the heavy protons.
Collapse
Affiliation(s)
- B W Raaymakers
- Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
| | | | | |
Collapse
|
44
|
Raaijmakers AJE, Raaymakers BW, Lagendijk JJW. SU-GG-T-530: MR-Guided Radiotherapy: Magnetic Field Dose Effects. Med Phys 2008. [DOI: 10.1118/1.2962279] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
|
45
|
Raaymakers BW, Lagendijk JJW, Raaijmakers AJE, Kerkhof EM, van der Put R, Kok JGM, van Dijk I, van Vulpen M, Overweg J, Brown K. SU-GG-J-60: Constructing a 6 MV Radiotherapy Accelerator with Integrated 1.5T MRI functionality: Status Report. Med Phys 2008. [DOI: 10.1118/1.2961610] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
|
46
|
Raaymakers BW, Raaijmakers AJE, Lagendijk JJW. SU-GG-T-376: Feasibility of MRI Guided Proton Therapy: Magnetic Field Dose Effects. Med Phys 2008. [DOI: 10.1118/1.2962128] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
|
47
|
Raaijmakers AJE, Raaymakers BW, van Dijk I, Kok JGM, Lagendijk JJW. SU-GG-T-283: Dosimetry for Hybrid 1.5 T MRI Radiotherapy System: Impact of the Magnetic Field On a NE2571 Ionisation Chamber. Med Phys 2008. [DOI: 10.1118/1.2962035] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
|
48
|
Raaijmakers AJE, Raaymakers BW, Lagendijk JJW. Magnetic-field-induced dose effects in MR-guided radiotherapy systems: dependence on the magnetic field strength. Phys Med Biol 2008; 53:909-23. [DOI: 10.1088/0031-9155/53/4/006] [Citation(s) in RCA: 196] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
|
49
|
Lagendijk JJW, Raaymakers BW, Raaijmakers AJE, Overweg J, Brown KJ, Kerkhof EM, van der Put RW, Hårdemark B, van Vulpen M, van der Heide UA. MRI/linac integration. Radiother Oncol 2007; 86:25-9. [PMID: 18023488 DOI: 10.1016/j.radonc.2007.10.034] [Citation(s) in RCA: 355] [Impact Index Per Article: 20.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2007] [Revised: 10/23/2007] [Accepted: 10/23/2007] [Indexed: 11/16/2022]
Abstract
PURPOSE/OBJECTIVES In radiotherapy the healthy tissue involvement still poses serious dose limitations. This results in sub-optimal tumour dose and complications. Daily image guided radiotherapy (IGRT) is the key development in radiation oncology to solve this problem. MRI yields superb soft-tissue visualization and provides several imaging modalities for identification of movements, function and physiology. Integrating MRI functionality with an accelerator can make these capacities available for high precision, real time IGRT. DESIGN AND RESULTS The system being built at the University Medical Center Utrecht is a 1.5T MRI scanner, with diagnostic imaging functionality and quality, integrated with a 6MV radiotherapy accelerator. The realization of a prototype of this hybrid system is a joint effort between the Radiotherapy Department of the University of Utrecht, the Netherlands, Elekta, Crawley, U.K., and Philips Research, Hamburg, Germany. Basically, the design is a 1.5 T Philips Achieva MRI scanner with a Magnex closed bore magnet surrounded by a single energy (6 MV) Elekta accelerator. Monte Carlo simulations are used to investigate the radiation beam properties of the hybrid system, dosimetry equipment and for the construction of patient specific dose deposition kernels in the presence of a magnetic field. The latter are used to evaluate the IMRT capability of the integrated MRI linac. CONCLUSIONS A prototype hybrid MRI/linac for on-line MRI guidance of radiotherapy (MRIgRT) is under construction. The aim of the system is to deliver the radiation dose with mm precision based on diagnostic quality MR images.
Collapse
Affiliation(s)
- Jan J W Lagendijk
- Department of Radiotherapy, University Medical Center Utrecht, Heidelberglaan, The Netherlands.
| | | | | | | | | | | | | | | | | | | |
Collapse
|
50
|
Raaijmakers AJE, Hårdemark B, Raaymakers BW, Raaijmakers CPJ, Lagendijk JJW. Dose optimization for the MRI-accelerator: IMRT in the presence of a magnetic field. Phys Med Biol 2007; 52:7045-54. [DOI: 10.1088/0031-9155/52/23/018] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
|