Recently, several implementations of arterial spin labeling (ASL) techniques have been developed for producing MRI images sensitive to local tissue perfusion. For quantitation of perfusion, both pulsed and continuous labeling methods potentially suffer from a number of systematic errors. In this study, a general kinetic model for the ASL signal is described that can be used to assess these errors. With appropriate assumptions, the general model reduces to models that have been used previously to analyze ASL data, but the general model also provides a way to analyze the errors that result if these assumptions are not accurate. The model was used for an initial assessment of systematic errors due to the effects of variable transit delays from the tagging band to the imaging voxel, the effects of capillary/tissue exchange of water on the relaxation of the tag, and the effects of incomplete water extraction. In preliminary experiments with a human subject, the model provided a good description of pulsed ASL data during a simple sensorimotor activation task.

Our purpose was to use whole brain echo planar magnetic resonance imaging (MRI) to identify and characterize diffusion abnormalities in acute cerebral ischemia. We studied 40 patients as early as 3 hours after onset of signs and symptoms of cerebral ischemia. Diffusion-weighted imaging (DWI) of the entire brain could be completed in 3 seconds or, using seven different diffusion sensitivities (maximum b = 1,271 sec/mm2), in 48 seconds. Measurements and synthetic maps were made of apparent diffusion coefficients (ADC), a physiological parameter that characterizes the self-diffusion of water in tissue. Early ischemic lesions were identified with DWI as hyperintense regions of decreased ADC in all patients who subsequently developed infarction, before changes were evident on conventional MRI in cases studied earlier than 6 hours after onset of ischemic symptoms. Lesions as small as 4 mm in diameter were identified. The extent of lesions within white matter was best defined by controlling for the anisotropic effect of axonal orientation. The mean ADC (+/- SD) for control regions in the 36 patients was 9.15 (+/- 2.91) x 10(-4) mm2/sec. Mean ADC of ischemic regions was 56% of control values at 6 hours or less and stayed significantly reduced for 3 to 4 days after onset of ischemia. The relative ADC increased progressively over time to be pseudonormalized at 5 to 10 days and elevated in the chronic state, making the distinction of acute lesions adjacent to chronic infarcts readily apparent. DWI with echo planar imaging measures a unique physiological parameter that is sensitive to ischemic changes before conventional MRI. Its potential role in the quantitative study of human stroke pathophysiology and therapeutics is the subject of further investigation.

These MR indexes may allow the identification and quantification of viable but ischemically threatened cerebral tissue amenable to therapeutic interventions in the hyperacute care of stroke patients.

The EPISTAR technique is a rapid, noninvasive means for creating qualitative maps of CBF.