SAR Reduction in Parallel Transmission by k-Space Dependent RF Pulse Selection

Parallel transmission has gained considerable interest during the last few years [1,2]. The use of multiple individual RF transmit coils have been introduced to overcome B1 homogeneity limitations and to improve multi-dimensional RF pulses by shortening their duration. Of special concern in all these transmit applications is the specific absorption rate (SAR), which has to be kept below certain limits to avoid excessive patient heating. Different approaches have been discussed to reduce SAR in parallel transmission. Thus, the degree of freedom in RF pulse design allows selecting solutions with minimal SAR [2] e.g. via regularization techniques that help to enforce low SAR [1,3]. Furthermore, the interplay between k-space trajectory and RF waveform can be used (VERSE [4]) for SAR reduction. The optimal RF pulse thus obtained is then used for the corresponding MR experiment. In that respect the parallel transmission RF pulse is optimized almost independently from the MR signal sampling process.