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CT scans taken during the initial hours of a suspected stroke cannot rule out stroke
Noncontrast cranial computed tomography (CT) is not very good at identifying acute stroke in the first hours after symptom onset, concludes a review of the subject by Richelle J. Cooper, M.D., and David L. Schriger, M.D., of the University of California, Los Angeles. However, it is the imaging method of choice for suspected stroke victims because it is readily available in most emergency departments (EDs) and can be performed quickly. CT is also the best way to identify intracranial hemorrhage, a contraindication to clot-busting (thrombolytic) therapy, which should be given within 3 hours after stroke onset.
Unfortunately, emergency medicine, radiology, and neurology physicians only correctly identify these hemorrhages 73 to 87 percent of the time. Large infarcts near the middle cerebral artery are also relative contraindications to thrombolytic therapy, but again physicians typically identify just 67 to 93 percent of these infarcts. Also, the classic neurologic changes associated with stroke, such as ischemic infarction, may not be visible on a CT scan until more than 24 hours after symptom onset. Some subtle signs of stroke, such as cerebral edema, do appear. Although these signs are helpful when present, their absence in no way rules out acute stroke, note the authors, whose work is supported by the Agency for Healthcare Research and Quality (National Research Service Award fellowship F32 HS00134).
Magnetic resonance imaging (MRI) can identify acute stroke more predictably and sooner after stroke symptom onset than CT scans. However, MRI is more time consuming than CT scans and is not readily available around the clock at many hospitals. Also, MRI images are more susceptible to movement artifact, and some patients are precluded from MRI either because they are too large for the scanner or have intracranial surgical clips or a pacemaker. As more specific therapies for stroke become available, MRI and MR angiography appear promising both for identifying acute strokes and for selecting among treatments.
See "How accurate is a CT scan in identifying acute strokes?" by Drs. Cooper and Schriger, in the November 1999 Western Journal of Medicine 171, pp. 356-357.
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