Accelerated mapping and robust parallel transmit pulse design for heart and prostate imaging at 7 T.

Abstract

To investigate the impact of reduced k-space sampling on mapping and the resulting impact on phase shimming and dynamic/universal parallel transmit (pTx) RF pulse design. Channel-wise 3D maps were measured at 7 T in 35 and 23 healthy subjects for the heart and prostate region, respectively. With these maps, universal phase shims optimizing homogeneity and 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 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), 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 efficiencies and lower CoVs compared to the vendor provided default shim, but could still be improved with individual phase shims based on accelerated 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 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 maps can be used for the design of tailored pTx pulses for prostate and cardiac imaging at 7 T, which further improve homogeneity, efficiency, or SED compared to universal pulses.