Browsing by Subject "Ultra-high field"

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Improving radiofrequency power and specific absorption rate management with bumped transmit elements in ultra-high field MRI
(Wiley, 2020) Sadeghi-Tarakameh, Alireza; Adriany, G.; Metzger, G. J.; Lagore, R. L.; Jungst, S.; DelaBarre, L.; Van de Moortele, P. F.; Uğurbil, K.; Atalar, Ergin; Eryaman, Y.
Purpose: In this study, we investigate a strategy to reduce the local specific absorption rate (SAR) while keeping constant inside the region of interest (ROI) at the ultra‐high field (B0 ≥ 7T) MRI. Methods: Locally raising the resonance structure under the discontinuity (i.e., creating a bump) increases the distance between the accumulated charges and the tissue. As a result, it reduces the electric field and local SAR generated by these charges inside the tissue. The at a point that is sufficiently far from the coil, however, is not affected by this modification. In this study, three different resonant elements (i.e., loop coil, snake antenna, and fractionated dipole [FD]) are investigated. For experimental validation, a bumped FD is further investigated at 10.5T. After the validation, the transmit performances of eight‐channel arrays of each element are compared through electromagnetic (EM) simulations. Results: Introducing a bump reduced the peak 10g‐averaged SAR by 21, 26, 23% for the loop and snake antenna at 7T, and FD at 10.5T, respectively. In addition, eight‐channel bumped FD array at 10.5T had a 27% lower peak 10g‐averaged SAR in a realistic human body simulation (i.e., prostate imaging) compared to an eight‐channel FD array. Conclusion: In this study, we investigated a simple design strategy based on adding bumps to a resonant element to reduce the local SAR while maintaining inside an ROI. As an example, we modified an FD and performed EM simulations and phantom experiments with a 10.5T scanner. Results show that the peak 10g‐averaged SAR can be reduced more than 25%.
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In vivo human head MRI at 10.5T: a radiofrequency safety study and preliminary imaging results
(Wiley, 2020) Sadeghi-Tarakameh, Alireza; DelaBarre, L.; Lagore, R. L.; Torrado-Carvajal, A.; Wu, X.; Grant, A.; Adriany, G.; Metzger, G. J.; Van de Moortele, P.-F.; Uğurbil, K.; Atalar, Ergin; Eryaman, Y.
Purpose: The purpose of this study is to safely acquire the first human head images at 10.5T. Methods: To ensure safety of subjects, we validated the electromagnetic simulation model of our coil. We obtained quantitative agreement between simulated and experimental and specific absorption rate (SAR). Using the validated coil model, we calculated radiofrequency power levels to safely image human subjects. We conducted all experiments and imaging sessions in a controlled radiofrequency safety lab and the whole‐body 10.5T scanner in the Center for Magnetic Resonance Research. Results: Quantitative agreement between the simulated and experimental results was obtained including S‐parameters, maps, and SAR. We calculated peak 10 g average SAR using 4 different realistic human body models for a quadrature excitation and demonstrated that the peak 10 g SAR variation between subjects was less than 30%. We calculated safe power limits based on this set and used those limits to acquire T2‐ and ‐weighted images of human subjects at 10.5T. Conclusions: In this study, we acquired the first in vivo human head images at 10.5T using an 8‐channel transmit/receive coil. We implemented and expanded a previously proposed workflow to validate the electromagnetic simulation model of the 8‐channel transmit/receive coil. Using the validated coil model, we calculated radiofrequency power levels to safely image human subjects.