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Calculation of SAR for Transmit Coil Arrays
Weihua Mao1, Zhangwei Wang, Michael B Smith
1Center for NMR Research, H066, Department of Radiology, The Pennsylvania State University College of Medicine, 500 University Drive, Hershey, PA 17033.
Summary
Evaluating transmit arrays for MRI safety requires assessing numerous B(1) field distributions. Superposition of electric field calculations can predict specific absorption rate (SAR) but accuracy depends on the source model used.
Area of Science:
- Magnetic Resonance Imaging (MRI) safety
- Electromagnetic field modeling
Background:
- Transmit coil arrays enable flexible B(1) field shaping for MRI.
- Altering B(1) fields impacts electric fields and specific absorption rate (SAR) distributions, complicating safety assessments.
- Infinite possible field distributions pose a challenge for evaluating MRI transmit array safety.
Purpose of the Study:
- To evaluate the superposition method for predicting SAR in MRI transmit arrays.
- To compare the accuracy of superposition versus simultaneous drive simulations for SAR prediction.
- To investigate the impact of B(1) field shimming on maximum local SAR levels.
Main Methods:
- Simulated electric field calculations for individual coils.
- Superposition of individual electric field calculations to predict total SAR.
- Comparison with full numerical calculations using simultaneous coil drive.
- Evaluation using both current and voltage source models.
- Analysis of maximum local SAR for different B(1) field shimming strategies.
Main Results:
- Superposition of electric field calculations shows good agreement with simultaneous drive simulations when using current sources.
- Agreement is reduced when using voltage sources, indicating the influence of coil coupling.
- Shimming B(1) field homogeneity in a small region can lead to significantly elevated maximum local SAR levels.
Conclusions:
- The superposition method is a viable approach for estimating SAR in MRI transmit arrays, particularly with current source models.
- Careful consideration of source models is crucial for accurate SAR prediction using superposition.
- Optimizing B(1) field homogeneity without considering SAR can compromise patient safety due to high local SAR.
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