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Meerbothe TG, Meliado EF, Stijnman PRS, van den Berg CAT, Mandija S. A database for MR-based electrical properties tomography with in silico brain data-ADEPT. Magn Reson Med 2024; 91:1190-1199. [PMID: 37876351 DOI: 10.1002/mrm.29904] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2023] [Revised: 10/06/2023] [Accepted: 10/06/2023] [Indexed: 10/26/2023]
Abstract
PURPOSE Several reconstruction methods for MR-based electrical properties tomography (EPT) have been developed. However, the lack of common data makes it difficult to objectively compare their performances. This is, however, a necessary precursor for standardizing and introducing this technique in the clinical setting. To enable objective comparison of the performances of reconstruction methods and provide common data for their training and testing, we created ADEPT, a database of simulated data for brain MR-EPT reconstructions. METHODS ADEPT is a database containing in silico data for brain EPT reconstructions. This database was created from 25 different brain models, with and without tumors. Rigid geometric augmentations were applied, and different electrical properties were assigned to white matter, gray matter, CSF, and tumors to generate 120 different brain models. These models were used as input for finite-difference time-domain simulations in Sim4Life, used to compute the electromagnetic fields needed for MR-EPT reconstructions. RESULTS Electromagnetic fields from 84 healthy and 36 tumor brain models were simulated. The simulated fields relevant for MR-EPT reconstructions (transmit and receive RF fields and transceive phase) and their ground-truth electrical properties are made publicly available through ADEPT. Additionally, nonattainable fields such as the total magnetic field and the electric field are available upon request. CONCLUSION ADEPT will serve as reference database for objective comparisons of reconstruction methods and will be a first step toward standardization of MR-EPT reconstructions. Furthermore, it provides a large amount of data that can be exploited to train data-driven methods. It can be accessed from https://doi.org/10.34894/V0HBJ8.
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Affiliation(s)
- T G Meerbothe
- Department of Radiotherapy, Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
- Computational Imaging Group for MR Therapy and Diagnostics, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands
| | - E F Meliado
- Department of Radiotherapy, Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
- Computational Imaging Group for MR Therapy and Diagnostics, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands
| | - P R S Stijnman
- Department of Radiotherapy, Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
- Computational Imaging Group for MR Therapy and Diagnostics, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands
| | - C A T van den Berg
- Department of Radiotherapy, Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
- Computational Imaging Group for MR Therapy and Diagnostics, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands
| | - S Mandija
- Department of Radiotherapy, Division of Imaging and Oncology, University Medical Center Utrecht, Utrecht, The Netherlands
- Computational Imaging Group for MR Therapy and Diagnostics, Center for Image Sciences, University Medical Center Utrecht, Utrecht, The Netherlands
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Jeong H, Andersson J, Hess A, Jezzard P. Effect of subject-specific head morphometry on specific absorption rate estimates in parallel-transmit MRI at 7 T. Magn Reson Med 2023; 89:2376-2390. [PMID: 36656151 PMCID: PMC10952207 DOI: 10.1002/mrm.29589] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/07/2022] [Revised: 12/02/2022] [Accepted: 12/31/2022] [Indexed: 01/20/2023]
Abstract
PURPOSE To assess the accuracy of morphing an established reference electromagnetic head model to a subject-specific morphometry for the estimation of specific absorption rate (SAR) in 7T parallel-transmit (pTx) MRI. METHODS Synthetic T1 -weighted MR images were created from three high-resolution open-source electromagnetic head voxel models. The accuracy of morphing a "reference" (multimodal image-based detailed anatomical [MIDA]) electromagnetic model into a different subject's native space (Duke and Ella) was compared. Both linear and nonlinear registration methods were evaluated. Maximum 10-g averaged SAR was estimated for circularly polarized mode and for 5000 random RF shim sets in an eight-channel transmit head coil, and comparison made between the morphed MIDA electromagnetic models and the native Duke and Ella electromagnetic models, respectively. RESULTS The averaged error in maximum 10-g averaged SAR estimation across pTx MRI shim sets between the MIDA and the Duke target model was reduced from 17.5% with only rigid-body registration, to 11.8% when affine linear registration was used, and further reduced to 10.7% when nonlinear registration was used. The corresponding figures for the Ella model were 16.7%, 11.2%, and 10.1%. CONCLUSION We found that morphometry accounts for up to half of the subject-specific differences in pTx SAR. Both linear and nonlinear morphing of an electromagnetic model into a target subject improved SAR agreement by better matching head size, morphometry, and position. However, differences remained, likely arising from details in tissue composition estimation. Thus, the uncertainty of the head morphometry and tissue composition may need to be considered separately to achieve personalized SAR estimation.
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Affiliation(s)
- Hongbae Jeong
- Wellcome Centre for Integrative Neuroimaging, FMRIB Division, Nuffield Department of Clinical NeurosciencesUniversity of OxfordOxfordUK
- Athinoula A. Martinos Center for Biomedical Imaging, Department of RadiologyMassachusetts General HospitalBostonMassachusettsUSA
| | - Jesper Andersson
- Wellcome Centre for Integrative Neuroimaging, FMRIB Division, Nuffield Department of Clinical NeurosciencesUniversity of OxfordOxfordUK
| | - Aaron Hess
- Wellcome Centre for Integrative Neuroimaging, FMRIB Division, Nuffield Department of Clinical NeurosciencesUniversity of OxfordOxfordUK
- Centre for Clinical Magnetic Resonance Research, Department of Cardiovascular MedicineUniversity of OxfordOxfordUK
- British Heart Foundation Centre for Research ExcellenceOxfordUK
| | - Peter Jezzard
- Wellcome Centre for Integrative Neuroimaging, FMRIB Division, Nuffield Department of Clinical NeurosciencesUniversity of OxfordOxfordUK
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Ding B, Dragonu I, Rua C, Carlin JD, Halai AD, Liebig P, Heidemann R, Correia MM, Rodgers CT. Parallel transmit (pTx) with online pulse design for task-based fMRI at 7 T. Magn Reson Imaging 2022; 93:163-174. [PMID: 35863691 DOI: 10.1016/j.mri.2022.07.003] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2021] [Revised: 06/17/2022] [Accepted: 07/11/2022] [Indexed: 10/31/2022]
Abstract
PURPOSE Parallel transmission (pTx) is an approach to improve image uniformity for ultra-high field imaging. In this study, we modified an echo planar imaging (EPI) sequence to design subject-specific pTx pulses online. We compared its performance against EPI with conventional circularly polarised (CP) pulses. METHODS We compared the pTx-EPI and CP-EPI sequences in a short EPI acquisition protocol and for two different functional paradigms in six healthy volunteers (2 female, aged 23-36 years, mean age 29.2 years). We chose two paradigms that are typically affected by signal dropout at 7 T: a visual objects localiser to determine face/scene selective brain regions and a semantic-processing task. RESULTS Across all subjects, pTx-EPI improved whole-brain mean temporal signal-to-noise ratio (tSNR) by 11.0% compared to CP-EPI. We also compared the ability of pTx-EPI and CP-EPI to detect functional activation for three contrasts over the two paradigms: face > object and scene > object for the visual objects localiser and semantic association > pattern matching for the semantic-processing paradigm. Across all three contrasts, pTx-EPI showed higher median z-scores and detected more active voxels in relevant areas, as determined from previous 3 T studies. CONCLUSION We have demonstrated a workflow for EPI acquisitions with online per-subject pulse calculations. We saw improved performance in both tSNR and functional acquisitions from pTx-EPI. Thus, we believe that online calculation pTx-EPI is robust enough for future fMRI studies, especially where activation is expected in brain areas liable to significant signal dropout.
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Affiliation(s)
- Belinda Ding
- Wolfson Brain Imaging Centre, University of Cambridge, UK.
| | | | - Catarina Rua
- Wolfson Brain Imaging Centre, University of Cambridge, UK; Department of Clinical Neurosciences, University of Cambridge, UK; Invicro, Invicro London, UK
| | | | - Ajay D Halai
- MRC Cognition and Brain Science Unit, Cambridge, UK
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Hardy BM, Banik R, Yan X, Anderson AW. Bench to bore ramifications of inter-subject head differences on RF shimming and specific absorption rates at 7T. Magn Reson Imaging 2022; 92:187-196. [PMID: 35842192 DOI: 10.1016/j.mri.2022.07.009] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2021] [Revised: 06/30/2022] [Accepted: 07/11/2022] [Indexed: 11/20/2022]
Abstract
PURPOSE This study shows how inter-subject variation over a dataset of 72 head models results in specific absorption rate (SAR) and B1+ field homogeneity differences using common shim scenarios. METHODS MR-CT datasets were used to segment 71 head models into 10 tissue compartments. These head models were affixed to the shoulders and neck of the virtual family Duke model and placed within an 8 channel transmit surface-loop array to simulate the electromagnetic fields of a 7T imaging experiment. Radio frequency (RF) shimming using the Gerchberg-Saxton algorithm and Circularly Polarized shim weights over the entire brain and select slices of each model was simulated. Various SAR metrics and B1+ maps were calculated to demonstrate the contribution of head variation to transmit inhomogeneity and SAR variability. RESULTS With varying head geometries the loading for each transmit loop changes as evidenced by changes in S-parameters. The varying shim conditions and head geometries are shown to affect excitation uniformity, spatial distributions of local SAR, and SAR averaging over different pulse sequences. The Gerchberg-Saxton RF shimming algorithm outperforms circularly polarized shimming for all head models. Peak local SAR within the coil most often occurs nearest the coil on the periphery of the body. Shim conditions vary the spatial distribution of SAR. CONCLUSION The work gives further support to the need for fast and more subject specific SAR calculations to maintain safety. Local SAR10g is shown to vary spatially given shim conditions, subject geometry and composition, and position within the coil.
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Affiliation(s)
- Benjamin M Hardy
- Vanderbilt University Institute of Imaging Science, 1161 21st Avenue South, Nashville, TN 37232, USA; Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Science Center, Nashville, TN 37232, USA.
| | - Rana Banik
- Vanderbilt University Institute of Imaging Science, 1161 21st Avenue South, Nashville, TN 37232, USA; Department of Biomedical Engineering, Vanderbilt University, PMB 351631, 2301 Vanderbilt Place, Nashville, TN 37235, USA.
| | - Xinqiang Yan
- Vanderbilt University Institute of Imaging Science, 1161 21st Avenue South, Nashville, TN 37232, USA; Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Medical Center North, 1161 21st Ave. South, Nashville, TN 37232, USA.
| | - Adam W Anderson
- Vanderbilt University Institute of Imaging Science, 1161 21st Avenue South, Nashville, TN 37232, USA; Department of Biomedical Engineering, Vanderbilt University, PMB 351631, 2301 Vanderbilt Place, Nashville, TN 37235, USA; Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, Medical Center North, 1161 21st Ave. South, Nashville, TN 37232, USA.
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Brink WM, Yousefi S, Bhatnagar P, Remis RF, Staring M, Webb AG. Personalized local SAR prediction for parallel transmit neuroimaging at 7T from a single T1-weighted dataset. Magn Reson Med 2022; 88:464-475. [PMID: 35344602 PMCID: PMC9314883 DOI: 10.1002/mrm.29215] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/20/2021] [Revised: 01/20/2022] [Accepted: 02/13/2022] [Indexed: 11/26/2022]
Abstract
Purpose Parallel RF transmission (PTx) is one of the key technologies enabling high quality imaging at ultra‐high fields (≥7T). Compliance with regulatory limits on the local specific absorption rate (SAR) typically involves over‐conservative safety margins to account for intersubject variability, which negatively affect the utilization of ultra‐high field MR. In this work, we present a method to generate a subject‐specific body model from a single T1‐weighted dataset for personalized local SAR prediction in PTx neuroimaging at 7T. Methods Multi‐contrast data were acquired at 7T (N = 10) to establish ground truth segmentations in eight tissue types. A 2.5D convolutional neural network was trained using the T1‐weighted data as input in a leave‐one‐out cross‐validation study. The segmentation accuracy was evaluated through local SAR simulations in a quadrature birdcage as well as a PTx coil model. Results The network‐generated segmentations reached Dice coefficients of 86.7% ± 6.7% (mean ± SD) and showed to successfully address the severe intensity bias and contrast variations typical to 7T. Errors in peak local SAR obtained were below 3.0% in the quadrature birdcage. Results obtained in the PTx configuration indicated that a safety margin of 6.3% ensures conservative local SAR estimates in 95% of the random RF shims, compared to an average overestimation of 34% in the generic “one‐size‐fits‐all” approach. Conclusion A subject‐specific body model can be automatically generated from a single T1‐weighted dataset by means of deep learning, providing the necessary inputs for accurate and personalized local SAR predictions in PTx neuroimaging at 7T.
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Affiliation(s)
- Wyger M Brink
- C.J. Gorter Center for High Field MRI, Department of Radiology, Leiden University Medical Center, Leiden, the Netherlands
| | - Sahar Yousefi
- C.J. Gorter Center for High Field MRI, Department of Radiology, Leiden University Medical Center, Leiden, the Netherlands.,Division of Image Processing, Department of Radiology, Leiden University Medical Center, Leiden, the Netherlands
| | - Prernna Bhatnagar
- C.J. Gorter Center for High Field MRI, Department of Radiology, Leiden University Medical Center, Leiden, the Netherlands.,Circuits and Systems Group, Department of Microelectronics, Delft University of Technology, Delft, the Netherlands
| | - Rob F Remis
- Circuits and Systems Group, Department of Microelectronics, Delft University of Technology, Delft, the Netherlands
| | - Marius Staring
- Division of Image Processing, Department of Radiology, Leiden University Medical Center, Leiden, the Netherlands
| | - Andrew G Webb
- C.J. Gorter Center for High Field MRI, Department of Radiology, Leiden University Medical Center, Leiden, the Netherlands
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Malik SJ, Hand JW, Satnarine R, Price AN, Hajnal JV. Specific absorption rate and temperature in neonate models resulting from exposure to a 7T head coil. Magn Reson Med 2021; 86:1299-1313. [PMID: 33811667 DOI: 10.1002/mrm.28784] [Citation(s) in RCA: 10] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2020] [Revised: 02/11/2021] [Accepted: 03/06/2021] [Indexed: 11/07/2022]
Abstract
PURPOSE To investigate safe limits for neonatal imaging using a 7T head coil, including both specific absorption rate (SAR) and temperature predictions. METHODS Head-centered neonate models were simulated using finite-difference time domain-based electromagnetic and thermal solvers. The effects of higher water content of neonatal tissues compared with adults, position shifts, and thermal insulation were also considered. An adult model was simulated for comparison. RESULTS Maximum and average SAR are both elevated in the neonate when compared with an adult model. When normalized to B1+ , the SAR experienced by a neonate is greater than an adult by approximately a factor of 2; when normalized to net forward power (forward-reflected), this increases to a factor of 2.5-3.0; and when normalized to absorbed power, approximately a factor of 4. Use of age-adjusted dielectric properties significantly increases the predicted SAR, compared with using adult tissue properties for the neonates. Thermal simulations predict that change in core temperature/maximum temperature remain compliant with International Electrotechnical Commission limits when a thermally insulated neonate is exposed at the SAR limit for up to an hour. CONCLUSION This study of two neonate models cannot quantify the variability expected within a larger population. Likewise, the use of age-adjusted dielectric properties have a significant effect, but while their use is well motivated by literature, there is uncertainty in the true dielectric properties of neonatal tissue. Nevertheless, the main finding is that unlike at lower field strengths, operational limits for 7T neonatal MRI using an adult head coil should be more conservative than limits for use on adults.
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Affiliation(s)
- Shaihan J Malik
- Department of Biomedical Engineering, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom.,Center for the Developing Brain, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom
| | - Jeffrey W Hand
- Department of Biomedical Engineering, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom
| | - Ryan Satnarine
- Department of Biomedical Engineering, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom
| | - Anthony N Price
- Department of Biomedical Engineering, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom.,Center for the Developing Brain, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom
| | - Joseph V Hajnal
- Department of Biomedical Engineering, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom.,Center for the Developing Brain, School of Biomedical Engineering and Imaging Sciences, King's College London, St. Thomas' Hospital, London, United Kingdom
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Abstract
After introduction of the first human 7 tesla (7T) system in 1999, 7T MR systems have been employed as one of the most advanced platforms for human MR research for more than 20 years. Currently, two 7T MR models are approved for clinical use in the U.S.A. The approval facilitated introduction of the 7T system, summing up to around 100 worldwide. The approval in Japan is much awaited. As a clinical MR scanner, the 7T MR system is drawing attention in terms of safety.Several large-sized studies on bioeffects have been reported for vertigo, dizziness, motion disturbances, nausea, and others. Such effects might also be found in MR workers and researchers. Frequency and severity of reported bioeffects will be presented and discussed, including their variances. The high resonance frequency and shorter RF wavelength of 7T increase the concern about the safety. Homogeneous RF pulse excitation is difficult even for the brain, and a multi-channel parallel transmit (pTx) system is considered mandatory. However, pTx may create a hot spot, which makes the estimation of specific absorption rate (SAR) to be difficult. The stronger magnetic field of 7T causes a large force of displacement and heating on metallic implants or devices, and the scan of patients with them should not be conducted at 7T. However, there are some opinions that such patients might be scanned even at 7T, if certain criteria are met. This article provides a brief review on the effect of the static magnetic field on humans (MR subjects, workers, and researchers) and neurons, in addition to scan sound, SAR, and metal implants and devices. Understanding and avoiding adverse effects will contribute to the reduction in safety risks and the prevention of incidents.
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Affiliation(s)
- Tomohisa Okada
- Human Brain Research Center, Graduate School of Medicine, Kyoto University
| | - Thai Akasaka
- Human Brain Research Center, Graduate School of Medicine, Kyoto University
| | - Dinh Hd Thuy
- Human Brain Research Center, Graduate School of Medicine, Kyoto University
| | - Tadashi Isa
- Human Brain Research Center, Graduate School of Medicine, Kyoto University
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