A comparison study between the saturation-recovery-T₁ and CASL MRI methods for quantitative CBF imaging

The saturation-recovery (SR)-T₁ MRI method for quantitatively imaging cerebral blood flow (CBF) change (ΔCBF) concurrently with the blood oxygenation level dependence (BOLD) alteration has been recently developed and validated by simultaneous measurement of relative CBF change using laser Doppler flowmetry (LDF) in rats at 9.4T. In this study, ΔCBF induced by mildly transient hypercapnia and measured by the SR-T₁ MRI method was rigorously compared with an established perfusion MRI method—continuous arterial spin labeling (CASL) approach in normal and preclinical middle cerebral artery occlusion (MCAo) rat models. The results show an excellent agreement between ΔCBF values measured with these two imaging methods. Moreover, the intrinsic longitudinal relaxation rate (R₁ ^int) was experimentally determined in vivo in normal rat brains at 9.4T by comparing two independent measures of the apparent longitudinal relaxation rate (R₁ ^app) and CBF measured by the CSAL approach across a wide range of perfusion. In turn, the R₁ ^int constant can be employed to calculate the CBF value based on the R₁ ^app measurement in healthy brain. This comparison study validates the fundamental relationship for linking brain tissue water R₁ ^app and cerebral perfusion, demonstrates the feasibility of imaging and quantifying both CBF and its change using the SR-T₁ MRI method in vivo.