Fig. 7.

Phase variation across the slice and fraction of signal retained for pseudo birdcage mode (pBC), 1-spoke and 2-spoke SA-STA pulses (SA-STA pulses were optimized for magnetization uniformization and through-plane signal recovery, but not the pBC mode pulses). These through-slice phase profiles show that optimization of the sub-pulse time-shifts (1-spoke and 2-spoke results in rows 3 & 4) results in a phase pattern that varies more slowly at locations with strong through-plane B0 gradient compared to the birdcage mode excitation (row 2). The fraction of signal retained reveals that, in order to create uniform magnetization profiles, the SA-STA algorithm creates through-plane signal losses in low-B0 regions and then increases the signal in these regions by “over-flipping” the spins. As a consequence, SA-STA excitations create uniform magnetization (which is a combination of the flip-angle and spin dephasing) profiles but non-uniform flip-angle weighting.