Perfusion imaging with pulsed arterial spin labeling techniques, like flow-sensitive alternating inversion recovery (FAIR), may suffer from inflow of fresh, i.e., unlabeled, spins. Inflow of fresh spins is caused by the arrival of unlabeled spins in the image slice and can lead to underestimation of the perfusion if not taken into account. In this study it was shown that a decrease in B1 field strength toward the edge of the transmit coil and the consequent reduction in the inversion efficiency leads to a narrowing of the arterial delivery function and a reduction in FAIR signal. Increasing the B1 amplitude of the adiabatic inversion pulse from 2.3 to 5.7 times its minimum amplitude requirement resulted in an observed increase of 40 to 80% in the rat brain FAIR signal at inflow times longer than 0.65 s. For coils with limited dimensions and significant B1 inhomogeneity over the perfusion labeling slab, the application of an excessively large B1 amplitude in combination with adiabatic inversion is recommended to optimize the FAIR perfusion contrast.
Schepers, J., Osch, van, M. J. P., Bartels, L. W., Heukels, S. N., Viergever, M. A., & Nicolaij, K. (2005). The effect of B1 field inhomogeneity and the nonselective inversion profile on the kinetics of FAIR-based perfusion MRI. Magnetic Resonance in Medicine, 53(6), 1355-1362. https://doi.org/10.1002/mrm.20480