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Hossain S, Taracila V, Robb FJ, Moore J, Winkler SA. Design of a volumetric cylindrical coil-tuned at 298 MHz for 7 T imaging. INSTRUMENTATION SCIENCE & TECHNOLOGY 2023; 52:433-455. [PMID: 39100769 PMCID: PMC11293480 DOI: 10.1080/10739149.2023.2286376] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 08/06/2024]
Abstract
The concept of a 2D cylindrical High Pass Ladder (2D c-HPL) is used in the development of this ultra high radio frequency (UHRF) volumetric head coil for 7T tuned at the Larmor frequency of 298 MHz. The architecture of the 2D c-HPL helps to overcome the challenges associated with non-uniform magnetic field distribution. The prototype consists of an individual resonating array of inductance-capacitance (LC) elements and each component is tuned to the precisef o frequency. The tuning of the (i) inductance, (ii) capacitance, (iii) mesh size, and (iv) coupling coefficient play critical roles to attain the desired Larmor frequency. For this proof-of-concept, the prototype of a volumetric head coil consists of a cylindrical array size of 4 ×6, with individual LC components of inductance magnitude, 98 nH and four fixed value capacitors and one tunable capacitor that allowed to achieve the desired precession frequency,f r = 298 M H z . The model was tested for three differentf o values of 269 MHz, 275 MHz and 286 MHz. The mutual coupling and the eigenfrequencies were compared through bench testing and dispersion equation. The experimental data were in good agreement (< 5%) with the theoretical eigenfrequencies from the dispersion relation. The theoretical eigenfrequencies and the experimental eigenfrequencies are in good agreement for eigenmodes (1,2), (1,3), (2,2), (2,3) and (4,3).
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Affiliation(s)
- Shadeeb Hossain
- Department of Radiology, Weill Cornell Medicine, NY 10021, USA
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Petzold J, Schmitter S, Silemek B, Winter L, Speck O, Ittermann B, Seifert F. Towards an integrated radiofrequency safety concept for implant carriers in MRI based on sensor-equipped implants and parallel transmission. NMR IN BIOMEDICINE 2023; 36:e4900. [PMID: 36624556 DOI: 10.1002/nbm.4900] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/31/2022] [Revised: 10/11/2022] [Accepted: 01/04/2023] [Indexed: 06/15/2023]
Abstract
To protect implant carriers in MRI from excessive radiofrequency (RF) heating it has previously been suggested to assess that hazard via sensors on the implant. Other work recommended parallel transmission (pTx) to actively mitigate implant-related heating. Here, both ideas are integrated into one comprehensive safety concept where native pTx safety (without implant) is ensured by state-of-the-art field simulations and the implant-specific hazard is quantified in situ using physical sensors. The concept is demonstrated by electromagnetic simulations performed on a human voxel model with a simplified spinal-cord implant in an eight-channel pTx body coil at 3 T . To integrate implant and native safety, the sensor signal must be calibrated in terms of an established safety metric (e.g., specific absorption rate [SAR]). Virtual experiments show that E -field and implant-current sensors are well suited for this purpose, while temperature sensors require some caution, and B 1 probes are inadequate. Based on an implant sensor matrix Q s , constructed in situ from sensor readings, and precomputed native SAR limits, a vector space of safe RF excitations is determined where both global (native) and local (implant-related) safety requirements are satisfied. Within this safe-excitation subspace, the solution with the best image quality in terms of B 1 + magnitude and homogeneity is then found by a straightforward optimization algorithm. In the investigated example, the optimized pTx shim provides a 3-fold higher mean B 1 + magnitude compared with circularly polarized excitation for a maximum implant-related temperature increase ∆ T imp ≤ 1 K . To date, sensor-equipped implants interfaced to a pTx scanner exist as demonstrator items in research labs, but commercial devices are not yet within sight. This paper aims to demonstrate the significant benefits of such an approach and how this could impact implant-related RF safety in MRI. Today, the responsibility for safe implant scanning lies with the implant manufacturer and the MRI operator; within the sensor concept, the MRI manufacturer would assume much of the operator's current responsibility.
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Affiliation(s)
- Johannes Petzold
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
- Biomedical Magnetic Resonance, Otto von Guericke University Magdeburg, Magdeburg, Germany
| | - Sebastian Schmitter
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Berk Silemek
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Lukas Winter
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Oliver Speck
- Biomedical Magnetic Resonance, Otto von Guericke University Magdeburg, Magdeburg, Germany
| | - Bernd Ittermann
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Frank Seifert
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
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Gokyar S, Robb FJL, Kainz W, Chaudhari A, Winkler SA. MRSaiFE: An AI-based Approach Towards the Real-Time Prediction of Specific Absorption Rate. IEEE ACCESS : PRACTICAL INNOVATIONS, OPEN SOLUTIONS 2021; 9:140824-140834. [PMID: 34722096 PMCID: PMC8553142 DOI: 10.1109/access.2021.3118290] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/03/2023]
Abstract
The purpose of this study is to investigate feasibility of estimating the specific absorption rate (SAR) in MRI in real time. To this goal, SAR maps are predicted from 3T- and 7T-simulated magnetic resonance (MR) images in 10 realistic human body models via a convolutional neural network. Two-dimensional (2-D) U-Net architectures with varying contraction layers and different convolutional filters were designed to estimate the SAR distribution in realistic body models. Sim4Life (ZMT, Switzerland) was used to create simulated anatomical images and SAR maps at 3T and 7T imaging frequencies for Duke, Ella, Charlie, and Pregnant Women (at 3, 7, and 9 month gestational stages) body models. Mean squared error (MSE) was used as the cost function and the structural similarity index (SSIM) was reported. A 2-D U-Net with 4 contracting (and 4 expanding) layers and 64 convolutional filters at the initial stage showed the best compromise to estimate SAR distributions. Adam optimizer outperformed stochastic gradient descent (SGD) for all cases with an average SSIM of 90.5∓3.6 % and an average MSE of 0.7∓0.6% for head images at 7T, and an SSIM of >85.1∓6.2 % and an MSE of 0.4∓0.4% for 3T body imaging. Algorithms estimated the SAR maps for 224×224 slices under 30 ms. The proposed methodology shows promise to predict real-time SAR in clinical imaging settings without using extra mapping techniques or patient-specific calibrations.
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Affiliation(s)
- Sayim Gokyar
- Department of Radiology, Weill Cornell Medicine, New York City, NY 10065 USA
| | - Fraser J L Robb
- GE Healthcare Coils, 1515 Danner Drive, Aurora, OH 44202 USA
| | - Wolfgang Kainz
- Center for Devices and Radiological Health, U.S. Food and Drug Administration, Silver Spring, MD, USA
| | - Akshay Chaudhari
- Integrative Biomedical Imaging Informatics at Stanford (IBIIS), James H. Clark Center, 318 Campus Drive, S255 Stanford, CA 94305 USA
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Mao X, Bari S, Love DJ, Rispoli JV. Global and peak local specific absorption rate control on parallel transmit systems using k-means SAR compression model. Magn Reson Med 2020; 85:1093-1103. [PMID: 32810320 DOI: 10.1002/mrm.28456] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/30/2020] [Revised: 07/09/2020] [Accepted: 07/10/2020] [Indexed: 12/29/2022]
Abstract
PURPOSE To improve the specific absorption rate (SAR) compression model capability in parallel transmission (pTx) MRI systems. METHODS A k-means clustering method is proposed to group voxels with similar SAR behaviors in the scanned object, providing a controlled upper-bounded estimation of peak local SARs. This k-means compression model and the conventional virtual observation point (VOP) model were tested in a pTx MRI framework. The pTx pulse design with different SAR controlling schemes was simulated using a numerical human head model and an eight-channel 7T coil array. Multiple criteria (including RF power, global and peak local SARs, and excitation accuracy) were compared for the performance testing. RESULTS The k-means compression model generated a narrower overestimation bound, leading to a more accurate local SAR estimation. Among different pTx pulse design approaches, the k-means compression model showed the best trade-off between the SAR and excitation accuracy. CONCLUSIONS The developed SAR compression model is advantageous for pTx framework given the narrower overestimation bound and control over the compression ratio. Results also illustrate that a moderate increase of maximum RF power can be useful for reducing the maximum local SAR deposition.
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Affiliation(s)
- Xianglun Mao
- School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, USA
| | - Sumra Bari
- School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, USA
| | - David J Love
- School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, USA
| | - Joseph V Rispoli
- School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN, USA.,Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN, USA
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Milshteyn E, Guryev G, Torrado-Carvajal A, Adalsteinsson E, White JK, Wald LL, Guerin B. Individualized SAR calculations using computer vision-based MR segmentation and a fast electromagnetic solver. Magn Reson Med 2020; 85:429-443. [PMID: 32643152 DOI: 10.1002/mrm.28398] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2019] [Revised: 04/28/2020] [Accepted: 06/05/2020] [Indexed: 11/06/2022]
Abstract
PURPOSE We propose a fast, patient-specific workflow for on-line specific absorption rate (SAR) supervision. An individualized electromagnetic model is created while the subject is on the table, followed by rapid SAR estimates for that individual. Our goal is an improved correspondence between the patient and model, reducing reliance on general anatomical body models. METHODS A 3D fat-water 3T acquisition (~2 minutes) is automatically segmented using a computer vision algorithm (~1 minute) into what we found to be the most important electromagnetic tissue classes: air, bone, fat, and soft tissues. We then compute the individual's EM field exposure and global and local SAR matrices using a fast electromagnetic integral equation solver. We assess the approach in 10 volunteers and compare to the SAR seen in a standard generic body model (Duke). RESULTS The on-the-table workflow averaged 7'44″. Simulation of the simplified Duke models confirmed that only air, bone, fat, and soft tissue classes are needed to estimate global and local SAR with an error of 6.7% and 2.7%, respectively, compared to the full model. In contrast, our volunteers showed a 16.0% and 20.3% population variability in global and local SAR, respectively, which was mostly underestimated by the Duke model. CONCLUSION Timely construction and deployment of a patient-specific model is computationally feasible. The benefit of resolving the population heterogeneity compared favorably to the modest modeling error incurred. This suggests that individualized SAR estimates can improve electromagnetic safety in MRI and possibly reduce conservative safety margins that account for patient-model mismatch, especially in non-standard patients.
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Affiliation(s)
- Eugene Milshteyn
- Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, USA.,Harvard Medical School, Boston, MA, USA
| | - Georgy Guryev
- Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Angel Torrado-Carvajal
- Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, USA.,Harvard Medical School, Boston, MA, USA.,Medical Image Analysis and Biometry Laboratory, Universidad Rey Juan Carlos, Madrid, Spain
| | - Elfar Adalsteinsson
- Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA.,Institute for Medical Engineering and Science, Massachusetts Institute of Technology, Cambridge, MA, USA.,Harvard-MIT Division of Health Sciences Technology, Cambridge, MA, USA
| | - Jacob K White
- Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Lawrence L Wald
- Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, USA.,Harvard Medical School, Boston, MA, USA.,Harvard-MIT Division of Health Sciences Technology, Cambridge, MA, USA
| | - Bastien Guerin
- Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Charlestown, MA, USA.,Harvard Medical School, Boston, MA, USA
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Winter L, Seifert F, Zilberti L, Murbach M, Ittermann B. MRI‐Related Heating of Implants and Devices: A Review. J Magn Reson Imaging 2020; 53:1646-1665. [DOI: 10.1002/jmri.27194] [Citation(s) in RCA: 35] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2019] [Revised: 04/29/2020] [Accepted: 04/30/2020] [Indexed: 12/11/2022] Open
Affiliation(s)
- Lukas Winter
- Physikalisch‐Technische Bundesanstalt (PTB) Braunschweig and Berlin Germany
| | - Frank Seifert
- Physikalisch‐Technische Bundesanstalt (PTB) Braunschweig and Berlin Germany
| | - Luca Zilberti
- Istituto Nazionale di Ricerca Metrologica Torino Italy
| | - Manuel Murbach
- ZMT Zurich MedTech AG Zurich Switzerland
- Institute for Molecular Instrumentation and Imaging (i3M) Universidad Politécnica de Valencia (UPV) Valencia Spain
| | - Bernd Ittermann
- Physikalisch‐Technische Bundesanstalt (PTB) Braunschweig and Berlin Germany
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Pendse M, Stara R, Khalighi MM, Rutt B. IMPULSE: A scalable algorithm for design of minimum specific absorption rate parallel transmit RF pulses. Magn Reson Med 2019; 81:2808-2822. [PMID: 30426583 PMCID: PMC6372346 DOI: 10.1002/mrm.27589] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2018] [Revised: 09/14/2018] [Accepted: 10/10/2018] [Indexed: 12/27/2022]
Abstract
PURPOSE Managing local specific absorption rate (SAR) in parallel transmission requires ensuring that the peak SAR over a large number of voxels (> 10 5 ) is below the regulatory limit. The safety risk to the patient depends on cumulative (not instantaneous) SAR thus making a joint design of all RF pulses in a sequence desirable. We propose the Iterative Minimization Procedure with Uncompressed Local SAR Estimate (IMPULSE), an efficient optimization formulation and algorithm that can handle uncompressed SAR matrices and optimize pulses for all slices jointly within a practical time frame. THEORY AND METHODS IMPULSE optimizes parallel transmit pulses for small-tip-angle slice selective excitation to minimize a single cost function incorporating multiple quantities (local SAR, global SAR, and per-channel power) averaged over the entire multislice scan subject to a strict constraint on excitation accuracy. Pulses for an 8-channel 7T head coil were designed with IMPULSE and compared with pulses designed using generic optimization algorithms and VOPs to assess the computation time and SAR performance benefits. RESULTS IMPULSE achieves lower SAR and shorter computation time compared with a VOP approach. Compared with the generic sequential quadratic programming algorithm, computation time is reduced by a factor of 5-6 by using IMPULSE. Using as many as 6 million local SAR terms, up to 120 slices can be designed jointly with IMPULSE within 45 s. CONCLUSIONS IMPULSE can handle significantly larger number of SAR matrices and slices than conventional optimization algorithms, enabling the use of uncompressed or partially compressed SAR matrices to design pulses for a multislice scan in a practical time frame.
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Affiliation(s)
- Mihir Pendse
- Stanford University Department of Radiology, 1201 Welch Road Stanford, CA, 94305-5105, USA
| | - Riccardo Stara
- Stanford University Department of Radiology, 1201 Welch Road Stanford, CA, 94305-5105, USA
| | | | - Brian Rutt
- Stanford University Department of Radiology, 1201 Welch Road Stanford, CA, 94305-5105, USA
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Dixit N, Stang PP, Pauly JM, Scott GC. Thermo-Acoustic Ultrasound for Detection of RF-Induced Device Lead Heating in MRI. IEEE TRANSACTIONS ON MEDICAL IMAGING 2018; 37:536-546. [PMID: 29053449 PMCID: PMC5942199 DOI: 10.1109/tmi.2017.2764425] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Patients who have implanted medical devices with long conductive leads are often restricted from receiving MRI scans due to the danger of RF-induced heating near the lead tips. Phantom studies have shown that this heating varies significantly on a case-by-case basis, indicating that many patients with implanted devices can receive clinically useful MRI scans without harm. However, the difficulty of predicting RF-induced lead tip heating prior to scanning prevents numerous implant recipients from being scanned. Here, we demonstrate that thermo-acoustic ultrasound (TAUS) has the potential to be utilized for a pre-scan procedure assessing the risk of RF-induced lead tip heating in MRI. A system was developed to detect TAUS signals by four different TAUS acquisition methods. We then integrated this system with an MRI scanner and detected a peak in RF power absorption near the tip of a model lead when transmitting from the scanner's body coil. We also developed and experimentally validated simulations to characterize the thermo-acoustic signal generated near lead tips. These results indicate that TAUS is a promising method for assessing RF implant safety, and with further development, a TAUS pre-scan could allow many more patients to have access to MRI scans of significant clinical value.
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Winkler SA, Picot PA, Thornton MM, Rutt BK. Direct SAR mapping by thermoacoustic imaging: A feasibility study. Magn Reson Med 2017; 78:1599-1606. [PMID: 27779779 PMCID: PMC5405009 DOI: 10.1002/mrm.26517] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2016] [Revised: 09/01/2016] [Accepted: 09/27/2016] [Indexed: 11/09/2022]
Abstract
PURPOSE To develop a new method capable of directly measuring specific absorption rate (SAR) deposited in tissue using the thermoacoustic signal induced by short radiofrequency (RF) pulse excitation. THEORY A detailed model based on the thermoacoustic wave generation and propagation is presented. METHODS We propose a new concept for direct measurement of SAR, to be used as a safety assessment/monitoring tool for MRI. The concept involves the use of short bursts of RF energy and the measurement of the resulting thermoacoustic excitation pattern by an array of ultrasound transducers, followed by image reconstruction to yield the 3D SAR distribution. We developed a simulation framework to model this thermoacoustic SAR mapping concept and verified the concept in vitro. RESULTS Simulations show good agreement between reconstructed and original SAR distributions with an error of 4.2, 7.2, and 8.4% of the mean SAR values in axial, sagittal, and coronal planes and support the feasibility of direct experimental mapping of SAR distributions in vivo. The in vitro experiments show good agreement with theory (r2 = 0.52). CONCLUSIONS A novel thermoacoustic method for in vivo mapping of local SAR patterns in MRI has been proposed and verified in simulation and in a phantom experiment. Magn Reson Med 78:1599-1606, 2017. © 2016 International Society for Magnetic Resonance in Medicine.
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Affiliation(s)
- Simone A. Winkler
- Department of Radiology, Stanford University, Stanford, California, USA
| | | | | | - Brian K. Rutt
- Department of Radiology, Stanford University, Stanford, California, USA
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