Regional variation of cerebral blood flow and arterial transit time in the normal and hypoperfused rat brain measured using continuous arterial spin labeling MRI

Continuous arterial spin labeling ( CASL) is a noninvasive magnetic resonance ( MR) method for measuring cerebral perfusion. In its most widely used form, CASL incorporates a postlabeling delay to minimize the sensitivity of the technique to transit time effects, which otherwise corrupt cerebral blood flow ( CBF) quantification. For this delay to work effectively, it must be longer than the longest transit time present in the system. In this work, CASL measurements were made in four coronal slices in the rat brain using a range of postlabeling delays. By doing this, direct estimation of both CBF and arterial transit time (delta(a)) was possible. These measurements were performed in the normal brain and during hypoperfusion induced by occlusion of the common carotid arteries. It was found that, in the normal rat brain, significant regional variation exists for both CBF and da. Mean values of CBF and da in the selected gray matter regions of interest were 233 mL/100 g min and 266 ms, respectively, with the latter ranging from 100 to 500 ms. Therefore, use of a 500-ms postlabeling delay is suitable for any location in the normal rat brain. After common carotid artery occlusion, CBF decreased and da increased by regionally dependent amounts. In the sensory cortex, da increased to a mean value of 740 ms, significantly greater than 500 ms. These results highlight the importance of either (a) determining da as part of the CASL measurement or (b) knowing the approximate range of values da is likely to take for a given application, so that the parameters of the CASL sequence can be chosen appropriately.