Ensuring both velocity and spatial responses robust to B0/B1+ field inhomogeneities for velocity-selective arterial spin labeling through dynamic phase-cycling

Abstract

Purpose: To evaluate both velocity and spatial responses of velocity-selective arterial spin labeling (VS-ASL), using velocity-insensitive and velocity-compensated waveforms for control modules, as well as a novel dynamic phase-cycling approach, at different B₀ / $B_1^{+}$ field inhomogeneities.

Methods: In the presence of imperfect refocusing, the mechanism of phase-cycling the refocusing pulses through four dynamics was first theoretically analyzed with the conventional velocity-selective saturation (VSS) pulse train. Numerical simulations were then deployed to compare the performance of the Fourier-transform based velocity-selective inversion (FT-VSI) with these three different schemes in terms of both velocity and spatial responses under various B₀ / $B_1^{+}$ conditions. Phantom and human brain scans were performed to evaluate the three methods at $B_1^{+}$ scales of 0.8, 1.0, and 1.2.

Results: The simulations of FT-VSI showed that, under nonuniform B₀ / $B_1^{+}$ conditions, the scheme with velocity-insensitive control was susceptible to DC bias of the static spins as systematic error, while the scheme with velocity-compensated control had deteriorated velocity-selective labeling profiles and, thus, reduced labeling efficiency. Through numerical simulation, phantom scans, and brain perfusion measurements, the dynamic phase-cycling method demonstrated considerable improvements over these issues.

Conclusion: The proposed dynamic phase-cycling approach was demonstrated for the velocity-selective label and control modules with both velocity and spatial responses robust to a wide range of B₀ and $B_1^{+}$ field inhomogeneities.

Keywords: ${\mathrm{B}}_1^{+}$ field inhomogeneity; B0 field inhomogeneity; arterial spin labeling; cerebral blood flow; velocity-selective inversion.