Accelerated B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping and robust parallel transmit pulse design for heart and prostate...

To investigate the impact of reduced k-space sampling on $B_1^{+}$ mapping and the resulting impact on phase shimming and dynamic/universal parallel transmit (pTx) RF pulse design.

Channel-wise 3D $B_1^{+}$ maps were measured at 7 T in 35 and 23 healthy subjects for the heart and prostate region, respectively. With these $B_1^{+}$ maps, universal phase shims optimizing homogeneity and $B_1^{+}$ efficiency were designed for heart and prostate imaging. In addition, universal 4kT-point pulses were designed for the heart. Subsequently, individual phase shims and individual 4kT-pulses were designed based on $B_1^{+}$ maps with different acceleration factors and tested on the original maps. The performance of the pulses was compared by evaluating their coefficients of variation (CoV), $B_1^{+}$ efficiencies and specific energy doses (SED). Furthermore, validation measurements were carried out for one heart and one prostate subject.

For both organs, the universal phase shims showed significantly higher $B_1^{+}$ efficiencies and lower CoVs compared to the vendor provided default shim, but could still be improved with individual phase shims based on accelerated $B_1^{+}$ maps (acquisition time = 30 s). In the heart, the universal 4kT-pulse achieved significantly lower CoVs than tailored phase shims. Tailored 4kT-pulses based on accelerated $B_1^{+}$ maps resulted in even further reduced CoVs or a 2.5-fold reduction in SED at the same CoVs as the universal 4kT-pulse.

Accelerated $B_1^{+}$ maps can be used for the design of tailored pTx pulses for prostate and cardiac imaging at 7 T, which further improve homogeneity, $B_1^{+}$ efficiency, or SED compared to universal pulses.