Investigation of alternative RF power limit control methods for 0.5T, 1.5T, and 3T parallel transmission cardiac imaging: A simulation study

Abstract

Purpose: To investigate safety and performance aspects of parallel-transmit (pTx) RF control-modes for a body coil at $B_0\le 3T$ .

Methods: Electromagnetic simulations of 11 human voxel models in cardiac imaging position were conducted for $B_0=0.5T$ , $1.5T$ and $3T$ and a body coil with a configurable number of transmit channels (1, 2, 4, 8, 16). Three safety modes were considered: the 'SAR-controlled mode' (SCM), where specific absorption rate (SAR) is limited directly, a 'phase agnostic SAR-controlled mode' (PASCM), where phase information is neglected, and a 'power-controlled mode' (PCM), where the voltage amplitude for each channel is limited. For either mode, safety limits were established based on a set of 'anchor' simulations and then evaluated in 'target' simulations on previously unseen models. The comparison allowed to derive safety factors accounting for varying patient anatomies. All control modes were compared in terms of the $B_1^{+}$ amplitude and homogeneity they permit under their respective safety requirements.

Results: Large safety factors (approximately five) are needed if only one or two anchor models are investigated but they shrink with increasing number of anchors. The achievable $B_1^{+}$ is highest for SCM but this advantage is reduced when the safety factor is included. PCM appears to be more robust against variations of subjects. PASCM performance is mostly in between SCM and PCM. Compared to standard circularly polarized (CP) excitation, pTx offers minor $B_1^{+}$ improvements if local SAR limits are always enforced.

Conclusion: PTx body coils can safely be used at $B_0\le 3T$ . Uncertainties in patient anatomy must be accounted for, however, by simulating many models.

Keywords: RF safety; electromagnetic simulation; parallel transmission; patient model uncertainty; position uncertainty; safety factor.