Comparison of 31P B1 maps (in the axial plane) within the rectangular bottle phantom (left panel). (A) The configuration of uHDC blocks (blue bars) placed around the phantom. The B1 maps (the scale bar is an arbitrary unit, a.u.) with the uHDC blocks (B, E) shows large enhancement of B1 efficiency compared to without the uHDC blocks (C, F). The B1 ratio maps (D, G) represent B1 with uHDC over B1 without uHDC, indicating significant enhancement that reached over 200% improvement in some image areas.