Figure 6: Snapshots of ideal current patterns yielding the optimal transmit efficiency, for two sizes of the two-dimensional excitation ROI and different main magnetic field strengths.

The density of ideal current patterns at ωt = 0 is shown for the spherical sample with radius a = 10 cm, for circular disk ROI’s of size r_2D = 0.3a and r_2D = 0.6a. The corresponding 2D ROI’s are shown in the schematics of the first column. The shape and density of the ideal current patterns differ significantly as the 2D ROI size changes. Specifically, at r_2D = 0.3a, they resemble two compressed distributed loops of elliptical shape on opposite sides of the sphere precessing at the Larmor frequency. For the larger 2D ROI, the eccentricity of the elliptical shape also increases, and the ideal current patterns become more localized around the excitation plane. As the magnetic field strength increases, the ideal current patterns become more complex, with wave and propagation delay affecting their shape.