Comparison between simulated decoupling regimes for specific absorption rate prediction in parallel transmit MRI

Abstract

Purpose: The use of EM modelling is critical for SAR characterisation in parallel transmission MRI. RF arrays that include decoupling networks can be difficult to characterise accurately in simulation. A practical method of simplifying modelling is to exclude the decoupling networks and model each transmit element in isolation. Results from this type of model can be related to a real device by applying 'active decoupling' to the real device to suppress residual coupling when in use. Here we compare this approach with a full model that includes decoupling networks.Methods: EM simulations for a variety of adult male voxel models placed within an 8-channel TEM array tuned for 3T operation were run with and without decoupling networks included. The resulting EM fields and SAR estimates were compared using basic normalisation, and simulated active decoupling. Results:Modelling the transmit elements independently leads to variations which have significantly different SAR estimates of ~20% on average compared with the full model if not normalised appropriately. After "active decoupling", SAR was still generally seen to be overestimated by ~7% with independent channel modelling; despite having similar B 1 + fields distributions. Conclusion:Modelling transmission elements independently may lead to substantially incorrect SAR estimates if the corresponding MRI system is not run in an analogous manner.

In vivo human head MRI at 10.5T: A radiofrequency safety study and preliminary imaging results

“…In previous studies, safety factors/margins were proposed based on intersubject variation of peak local SAR as well as variation with respect to EM properties of tissues and model translation . In this work, we used a safety buffer based on the RF loss in the pathway from the RFPAs to the feed point of the dipoles, which was measured as 3.9 dB.…”

“…In this work, we used a safety buffer based on the RF loss in the pathway from the RFPAs to the feed point of the dipoles, which was measured as 3.9 dB. Considering the intersubject variability, this 3.9 dB safety buffer along with the calculated safe power limit based on our 4‐model worst‐case scenario ensures safe use of the CP mode at 10.5T in vivo imaging.…”

A numerical and experimental study of RF shimming in the presence of hip prostheses using adaptive SAR at 3 T

“…Two 1V voltage sources were positioned 90° apart on 1 end ring, and driven successively by a Gaussian pulse until a −40 dB convergence was reached, using the finite‐difference time domain solver. The coil was matched to 50 Ω with a reflection coefficient better than −20 dB, and decoupling between the 2 channels was better than −18 dB, using an in‐house implementation of the cosimulation method based on the work of Beqiri et al The B 1 fields and E‐fields from each individual channel were exported to MATLAB for processing.…”

Extended RF shimming: Sequence‐level parallel transmission optimization applied to steady‐state free precession MRI of the heart

“…The coil model was comprised of conductive elements modeled as lossy copper metal and all lumped elements were replaced with 50 Ω sources. This enabled the model to be tuned, matched and de‐coupled using circuit co‐simulation, which was implemented in Matlab (The Mathworks, Natick, MA, USA); the application of this method to our specific transmit coil is described in detail in Beqiri et al The NORMAN voxel model, which has a BMI of 23.5, was placed heart‐centered in the coil (Figure A) for this tuning and matching process, in a similar manner to that performed in the physical coil . Another simulation was produced using the same coil model with an enlarged version of the NORMAN voxel model (Figure B), which was stretched in the anterior–posterior and left–right directions in order to emulate a larger male with a BMI of 31.…”