New FOCI pulses with reduced radiofrequency power requirements

Abstract

Purpose: To propose new frequency offset corrected inversion (FOCI) pulses with significantly reduced radiofrequency (RF) power deposition for spin echo imaging by incorporating the variable-rate selective excitation (VERSE) schemes into the pulse design. Materials and Methods: Two schemes are proposed to design the new FOCI pulses with dramatically reduced peak RF power requirements. In scheme A, the time-dilation function is derived from a predefined adiabaticity factor modulation function. In scheme B, the time-dilation function is predefined, while the adiabaticity factor is conserved. Results: The new FOCI pulses are shown to be able to operate at reduced specific absorption rate (SAR), specifically at the same peak RF power as that of a five- or sevenlobe sinc inversion pulse of the same duration. Using the new FOCI pulse, significant gain in sensitivity was observed in in vivo spin-echo echo-planar imaging, which was attributed to the improved refocusing slice profile. Conclusion: The new FOCI pulses can replace the 180° five- or seven-lobe sinc pulses in spin-echo imaging with the same peak RF power requirement and significantly improved slice profile.