Abstract
Safety limits for the permitted specific absorption rate (SAR) place restrictions on pulse sequence design, especially at ultrahigh fields (≥ 7 tesla). Due to intersubject variability, the SAR is usually conservatively estimated based on standard human models that include an applied safety margin to ensure safe operation. One approach to reducing the restrictions is to create more accurate subject-specific models from their segmented MR images. This study uses electromagnetic simulations to investigate the minimum number of tissue groups required to accurately determine SAR in the human head. Tissue types from a fully characterized electromagnetic human model with 47 tissue types in the head and neck region were grouped into different tissue clusters based on the conductivities, permittivities, and mass densities of the tissues. Electromagnetic simulations of the head model inside a parallel transmit head coil at 7 tesla were used to determine the minimum number of required tissue clusters to accurately determine the subject-specific SAR. The identified tissue clusters were then evaluated using 2 additional well-characterized electromagnetic human models. A minimum of 4-clusters-plus-air was found to be required for accurate SAR estimation. These tissue clusters are centered around gray matter, fat, cortical bone, and cerebrospinal fluid. For all 3 simulated models, the parallel transmit maximum 10g SAR was consistently determined to within an error of <12% relative to the full 47-tissue model. A minimum of 4-clusters-plus-air are required to produce accurate personalized SAR simulations of the human head when using parallel transmit at 7 tesla.
Highlights
The energy deposition in human tissue permitted from RF pulses in MRI is limited by the energy deposition per unit mass, known as the specific absorption rate or SAR
Note that the maximum 10g SAR for CP mode is much lower than the worst-case 10g SAR for all possible B1 shims (0.140 W/kg vs. 1.580 W/kg)
Based on simulation results for 3T, they concluded that the additional modelling of CSF is not important for accurate SAR simulation at lower field strengths, which we found to be different when operating using 7T pTx
Summary
The energy deposition in human tissue permitted from RF pulses in MRI is limited by the energy deposition per unit mass, known as the specific absorption rate or SAR. Using computational human body models, it is possible to estimate SAR using electromagnetic simulations, such as with a finite difference time domain method.[2,3] Those simulations determine, among other parameters, the electric field E(r) at each spatial location r in the model. This can be used to calculate the SAR at each location through: SAR (r) = 1 V
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