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Fig 4. CBF errors due to prolonged arrival time in ischemic stroke with ASL. A, A contrast-enhanced MR angiogram shows right internal carotid artery occlusion. B, Diffusion-weighted image demonstrates abnormality of the right putamen, caudate, and a small area in the right frontal operculum. C, A PASL image of CBF demonstrates punctate high signal intensity (arrows) surrounding an area of apparent low flow in the right parietal lobe. The high signal intensity is probably due to delayed arrival of the label via collateral flow which is still within large vessels rather than in the parenchyma. The low measured CBF centrally is due to decay of the label before it enters the tissue. Normal flow was seen in this region on Xe-CT (images not shown). D, Follow-up CT 2 days later demonstrates infarction of the right caudate and putamen (arrow) but preserved gray-white contrast in the right parietal region that demonstrates low signal intensity on the ASL CBF map. CBF underestimation in the setting of collateral flow is a fundamental difficulty with the use of standard ASL in this patient population.
