In Vivo 3D Liver Imaging at 7 T Using kT‐Point pTx Pulses and a 32‐Tx‐Channel Whole‐Body Radiofrequency Coil Array

At 7 T, a main challenge is the flip angle variations arising from the spatially inhomogeneous B₁ ⁺ transmit profiles of the RF coils. To address this problem, static pTx with local pTx body arrays has been used. However, for larger organs, such as the liver, local arrays provide insufficient homogeneity, and often dynamic pTx is needed. In this study, the benefits of a 32-Tx-channel remote array are shown. Relative channel-wise 3D B₁ ⁺ maps are acquired in free breathing in 11:38 min with a radial phase encoding (RPE) acquisition scheme in five healthy volunteers at 7 T. For each subject, an ROI is drawn manually covering the whole liver. The necessary number of k_T-point pTx pulses is analyzed by comparing L-curves that depict the tradeoff between FA homogeneity and RF power for a large range of regularization parameters. For all subjects, no FA dropouts remain, and CV values under 17% can be achieved with magnitude and phase shimming. Two to three k_T-points showed a good tradeoff between FA homogeneity and RF power. To analyze different channel numbers, a fixed phase between the channel-wise B₁ ⁺ maps is set to achieve 8, 16, or 20 B₁ ⁺ maps for shimming. The higher channel number showed superior shimming performance for static and dynamic pTx. Comparing 32 versus 8 individual channels, a 1.3- to 1.6-fold improvement in homogeneity can be found for the same RF power using phase shimming. With 32 Tx channels, static pTx shim and two k_T-points were applied in vivo in free breathing with a 3D RPE GRE sequence with 1.4-mm isotropic resolution. The in vivo signal magnitudes of the small FA acquisitions showed good agreement with the predicted relative B₁ ⁺ maps while neglecting B₀ nonuniformity. In summary, this study shows the advantages of using a remote 32-Tx channel array for in vivo liver imaging at 7 T.