Calculation of SAR for transmit coil arrays

Transmit coil arrays allowing independent control of individual coil drives facilitate adjustment of the B₁ field distribution, but when the B₁ field distribution is changed the electric field and SAR distributions are also altered. This makes safety evaluation of the transmit array a challenging problem because there are potentially an infinite number of possible field distributions in the sample. Local SAR levels can be estimated with numerical calculations, but it is not practical to perform separate full numerical calculations for every current distribution of Interest. Here we evaluate superposition of separate electric field calculations - one for each coil - for predicting SAR in a full numerical calculation where all coils are driven simultaneously. It is important to perform such an evaluation because the effects of coil coupling may alter the result. It is shown that while there is good agreement between the superimposed and simultaneous drive results when using current sources in the simulations, the agreement is not as good when voltage sources are used. Finally, we compare maximum local SAR levels for B₁ field distributions that are either unshimmed or shimmed over one of three regions of interest. When B₁ field homogeneity is improved in a small region of interest without regard for SAR, the maximum local SAR can become very high.