Selective Adiabatic Refocusing Pulse Train for Nonlinear Phase Dispersion and Flip Angle Error Compensation

Introduction: Adiabatic full passage (AFP) pulses are well known for their immunity to the B1 inhomogeneity [1], and have been applied in many areas of magnetic resonance spectroscopy (MRS) and MRI, such as signal sensitivity enhancement and sharp volume boundary definition [2-4]. However, the application of selective AFP refocusing pulses has been limited by the inherent nonlinear phase dispersion and off-resonance effect. To address this problem, extensive research work has been conducted for improving signal sensitivity [5-7]. Nonlinear phase dispersion produced by a selective 90° (HS1) pulse can be approximately expressed as φ1= πν/a, where ν is the resonance frequency of spins and a is the frequency accelerator [5,7]. If the same pulse sweeps reversely, the nonlinear phase dispersion can be approximated as φ2 = -φ1 = -πν/a. Thus, the net nonlinear phase dispersion generated by two selective 90° (HS1) pulses with alternate frequency sweep (AFS) directions is zero. Due to the AFS mechanism of the two pulses, the off-resonance induced flip-angle error of the first 90° (HS1) pulse can be compensated by the second one with AFS. In this study, we designed and implemented an adiabatic refocusing pulse train that compensates the nonlinear phase dispersion and off-resonance induced flip-angle errors across the 3D volume. The AFS-AFP pulse train also excises spin-locking along the static field (B0) direction.