Ensuring both velocity and spatial responses robust to  field inhomogeneities for velocity-selective arterial spin labeling through dynamic phase-cycling.
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 /  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 /  conditions. Phantom and human brain scans were performed to evaluate the three methods at  scales of 0.8, 1.0, and 1.2.


RESULTS
The simulations of FT-VSI showed that, under nonuniform B0 /  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  field inhomogeneities.
['Liu|Dapeng|D|0000-0002-4432-3202', 'Li|Wenbo|W|0000-0002-0199-1534', 'Xu|Feng|F|0000-0001-7958-3787', 'Zhu|Dan|D|0000-0002-0940-1519', 'Shin|Taehoon|T|0000-0003-1780-573X', 'Qin|Qin|Q|0000-0002-6432-2944']
D001158:Arteries D001783:Blood Flow Velocity D002560:Cerebrovascular Circulation D005583:Fourier Analysis D006801:Humans D018810:Magnetic Resonance Angiography D019047:Phantoms, Imaging D013113:Spin Labels
arterial spin labeling velocity-selective inversion B0 field inhomogeneity cerebral blood flow