The equivalence between off-resonance and on-resonance pulse sequences and its application to steady-state free precession with diffusion in inhomogeneous fields

摘要

We show that the spin dynamics of any pulse sequence with off-resonant pulses is identifical to that of a modified sequence with on-resonant pulses, including relaxation and diffusion effects. This equivalence applies to pulse sequences with arbitrary offset frequency δω_0 which may exceed the RF field strength ω_1. Using this approach, we examine steady-state free precession (SSFP) in grossly inhomogeneous fields. We show explicitly that the magnitude of the magnetization for each mode tat an offset frequency δω_0 is equal to that for SSFP with on-resonance pulses of rescaled amplitude, with the same dependence on relaxation times and difffusion coefficient. The rescaling depends on offset frequency and RF field strength. The theoretical results have been tested experimentally and excellent agreement is found.