Local and whole-body SAR in UHF body imaging: Implications for SAR matrix compression

Transmit arrays for body imaging have characteristics of both volume and local transmit coils. This study evaluates two specific absorption rate (SAR) aspects, local and whole-body SAR, of arrays for body imaging at 7 T and also for a 3 T birdcage.

Simulations were performed for six antenna arrays at 7 T and one 3 T birdcage. Local SAR matrices and the whole-body SAR matrix were computed and evaluated with random shims. A set of reduced local SAR matrices was determined by removing all matrices dominated by the whole-body SAR matrix.

The results indicate that all RF transmit coils for body imaging in this study are constrained by the local SAR limit. The ratio between local and whole-body SAR is nevertheless smaller for arrays with large FOV, as these arrays also expose a larger part of the human body. By using the whole-body SAR matrix, the number of local SAR matrices can be reduced (e.g., 33.3% matrices remained for an 8-channel local array and 89.7% for a 30-channel remote array; 12.1% for the 3 T birdcage).

For transmit antenna arrays used for body imaging at 7 T as well as for the 3 T birdcage, all evaluated cases show that the local SAR limit was reached before reaching the whole-body SAR limit. Nevertheless, the whole-body SAR matrix can be used to reduce the number of local SAR matrices, which is important to reduce memory and computing time for a virtual observation points (VOP) compression. This step can be included as a pre-compression prior to a VOP compression.