Noncontrast CT is commonly used to diagnose acute stroke because of its ability to detect acute hemorrhage, although it is less sensitive for early detection of ischemic stroke. The American Academy of Neurology (AAN) sponsored a systemic literature review to address the utility of diffusion- and perfusion-weighted MRI in diagnosis and prognosis for acute ischemic stroke.

 The AAN now recommends diffusion-weighted MRI (DWI) over CT for diagnosis of acute ischemic stroke (AAN Level A recommendation), estimating its sensitivity at 80%-90% in patients presenting to the emergency department with possible stroke. DWI volume at baseline was graded as useful to predict baseline stroke severity and final lesion volume in anterior-circulation stroke syndromes (AAN Level B recommendation). However, the report did not address the diagnostic accuracy of DWI for hemorrhagic stroke. Prior to treatment with tissue plasminogen activator, acute hemorrhage must be ruled out. Noncontrast CT may continue to be needed until there is more evidence demonstrating the efficacy of MRI for this indication.

Perfusion-weighted MRI (PWI) was graded as possibly useful to predict clinical stroke severity (AAN Level C recommendation), but there was insufficient evidence to evaluate PWI for diagnosing acute ischemic stroke (AAN Level U recommendation).

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