Whole-liver flip-angle shimming at 7 T using parallel-transmit k -point pulses and Fourier phase-encoded DREAM mapping.

PURPOSE

To obtain homogeneous signal throughout the human liver at 7 T. Flip angle (FA) shimming in 7T whole-liver imaging was performed through parallel-transmit k -point pulses based on subject-specific multichannel absolute maps from Fourier phase-encoded dual refocusing echo acquisition mode (PE-DREAM).

METHODS

The optimal number of Fourier phase-encoding steps for PE-DREAM mapping was determined for a 7T eight-channel parallel-transmission system. FA shimming experiments were performed in the liver of 7 healthy subjects with varying body mass index. In these subjects, first shimming and Fourier PE-DREAM mapping were performed. Subsequently, three small-flip-angle 3D gradient-echo scans were acquired, comparing a circularly polarized (CP) mode, a phase shim, and a k -point pulse. Resulting homogeneity was assessed and compared with estimated FA maps and distributions.

RESULTS

Fourier PE-DREAM with 13 phase-encoding steps resulted in a good tradeoff between accuracy and scan time. Lower coefficient of variation values (average [min-max] across subjects) of the estimated FA in the volume of interest were observed using k -points (7.4 [6.6%-8.0%]), compared with phase shimming (18.8 [12.9%-23.4%], p < 0.001) and CP (43.2 [39.4%-47.1%], p < 0.001). k -points delivered whole-liver images with the nominal FA and the highest degree of homogeneity. CP and phase shimming resulted in either inaccurate or imprecise FA distributions. Here, locations having suboptimal FA in the estimated FA maps corresponded to liver areas suffering from inconsistent signal intensity and T -weighting in the gradient-echo scans.

CONCLUSION

Homogeneous whole-liver 3D gradient-echo acquisitions at 7 T can be obtained with eight-channel k -point pulses calculated based on subject-specific multichannel absolute Fourier PE-DREAM maps.