Andrew Wilner, MD, Neurology, 11:42AM Nov 12, 2010
According to the American Academy of Neurology's recent evidence based guideline (Schellinger et al. 2010), diffusion-weighted imaging (DWI) magnetic resonance imaging (MRI) is more accurate than noncontrast computerized axial tomography (CT) for the diagnosis of acute ischemic stroke (Level A). This conclusion was based largely on a Class I study that unequivocally demonstrated superiority of MRI (DWI and gradient echo) compared to CT (p<0.0001). The lack of sensitivity for early stroke on CT will come as no surprise to the many physicians who have cared for patients presenting with a dense hemiparesis, aphasia and a "negative" CT scan.
In the emergency room at my hospital, patients with suspected acute stroke are immediately evaluated with a noncontrast CT as part of a "Code BAT" protocol to help determine eligibility for intravenous tissue plasminogen activator (tPA). CT scanning can be done within minutes and requires minimal patient cooperation. If the CT scan reveals an absence of hemorrhage, tumor, or other contraindication, the patient can proceed through the remaining gauntlet of criteria before receiving tPA.
MRI on the other hand, requires the patient to remain motionless for optimal images. Patients must be screened ahead of time for metal implants, and an ever increasing number of patients are ineligible for MRI because of implanted cardiac pacemakers and/or defibrillators. Many patients also have relative contraindications such as cochlear implants, hemostatic clips, insulin pumps, and nerve stimulators.
In addition to the findings on the physical and neurological examination, neuroimaging can help with prognosis. DWI, which measures the net movement of water in tissue due to random molecular motion, reveals hyperintense ischemic tissue damage within minutes to hours after acute stroke. For anterior circulation ischemic strokes, baseline DWI volumes probably predict final infarct volumes (Level B). For vertebrobasilar artery territory strokes, they are less precise. In conjunction with T1 weighted images and apparent diffusion coefficient (ADC) maps, DWI can differentiate acute from less acute lesions. To the extent that infarct size correlates with clinical outcome, DWI may also assist with long term prognosis in anterior circulation stroke syndromes (Level C). The value of perfusion-weighted imaging (PWI) in diagnosing acute ischemic stroke is less clear (Level U)
Improved diagnostic imaging will help eliminate false negative CT scans and provide an earlier, more certain diagnosis of acute ischemic stroke. False-negatives are less common with DWI, but may also occur. In addition, more sensitive imaging will decrease the misdiagnosis of acute ischemic stroke in patients with conversion disorder, postictal paresis, Bell's palsy, or other conditions that may be mistaken for stroke in the ER. Misdiagnosis is particularly worrisome because some of these patients may receive tPA, a drug with considerable potential morbidity. MRI with DWI may be preferable to noncontrast CT for cooperative patients when the diagnosis of acute ischemic stroke is in question.
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