Comparison of tight-fitting 7T parallel-transmit head array designs using excitation uniformity and local specific absorption rate metrics

Abstract

Purpose: We model the performance of parallel transmission (pTx) arrays with8, 16, 24, and 32 channels and varying loop sizes built on a close-fitting helmetfor brain imaging at 7 T and compare their local specific absorption rate (SAR)and flip-angle performances to that of birdcage coil (used as a baseline) andcylindrical 8-channel and 16-channel pTx coils (single-row and dual-row). Methods: We use the co-simulation approach along with MATLAB scriptingfor batch-mode simulation of the coils. For each coil, we extracted B 1+ mapsand SAR matrices, which we compressed using the virtual observation pointsalgorithm, and designed slice-selective RF shimming pTx pulses with multiplelocal SAR and peak power constraints to generate L-curves in the transverse,coronal, and sagittal orientations. Results: Helmet designs outperformed cylindrical pTx arrays at a constant num-ber of channels in the flip-angle uniformity at a constant local SAR metric: up to29% for 8-channel arrays, and up to 34% for 16-channel arrays, depending on theslice orientation. For all helmet arrays, increasing the loop diameter led to betterlocal SAR versus flip-angle uniformity tradeoffs, although this effect was morepronounced for the 8-channel and 16-channel systems than the 24-channel and32-channel systems, as the former have more limited degrees of freedom andtherefore benefit more from loop-size optimization. Conclusion: Helmet pTx arrays significantly outperformed cylindrical arrayswith the same number of channels in local SAR and flip-angle uniformitymetrics. This improvement was especially pronounced for non-transverse sliceexcitations. Loop diameter optimization for helmets appears to favor large loops,compatible with nearest-neighbor decoupling by overlap.