Fig. 3.

a: Radio-frequency (RF) and slice-selection gradient waveforms superimposed on the same graph (RF waveforms are scaled to arbitrary units). RF waveforms are Hanning-windowed sinc pulses with 7 lobes (8 zero-crossings). The green RF pulse is perfectly centered with respect to the slice-selection gradient lobe. As a result, it creates a perfectly refocused slice profile with no through-plane phase variation. The red RF pulse however is shifted by 500 us and is therefore not completely refocused by the refocusing lobe of the gradient waveform. As a result, this pulse creates a slice profile with a linear phase variation in the slice direction. Note that the duration of the slice selection gradient is long enough to allow playing the time-shifted pulse without cropping it, which is crucial for maintaining the quality of the slice profile across time-shifts. b: Global z-shims obtained by time-shifting the RF waveforms played on different transmit channels by the same amount. Two RF drives are considered: Birdcage mode and RF-shimming (equivalent to 1-spoke). In both cases, the RF amplitudes and phases are kept constant for all the “steps” of the z-shim process (one step corresponds to one time-shift). The right-most images of the BC mode and RF-shimming image rows are composite images obtained by taking the maximum intensity of all z-shim steps for each pixel.