Background: The ideal imaging tool for acute stroke should detect all forms of stroke and provide accurate and immediate results. Computed tomography (CT) is the modality most commonly used in the initial evaluation of acute stroke. Although it is sensitive for detecting intra-cranial bleeding, CT is less sensitive for other forms of stroke and has substantial inter-rater variability in interpretation. The advantages of magnetic resonance imaging (MRI) include improved assessment of acute ischemic injury because of a variety of causes and better inter-rater reliability. Chalela and colleagues compared CT with MRI in the emergency assessment of acute stroke.

The Study: The prospective, blind, single-site study evaluated consecutive patients who were referred to a U.S. hospital between September 2000 and February 2002 for assessment of a possible acute stroke. The decision to choose one imaging method over the other was made by the emergency department physician. Randomization was not attempted because of concerns about delay in the urgent assessment and treatment of patients with stroke. Patients who did not have either CT or MRI performed were excluded from the analysis. If possible, MRI was performed first, and the second investigation was initiated within 120 minutes. Four experts, who were unaware of clinical information, independently interpreted each scan. Initial imaging findings were correlated with the final neurologic diagnosis established for each patient.

Results: The average age of the 356 participants was 76 years. The median time from symptom onset to testing was 367 minutes for MRI and 390 minutes for CT. MRI was performed before CT in 304 patients (85 percent). Overall, 217 patients had a final diagnosis of acute stroke. Acute ischemic stroke was diagnosed in 190 patients, and acute intracranial hemorrhage was diagnosed in 27. The sensitivity of MRI was significantly higher than CT for all patients with acute stroke or acute ischemic stroke. Unlike MRI, CT did not detect any of the 17 percent of false-negative results for acute ischemic stroke. The two modalities were comparable for acute intracranial hemorrhage; however, MRI was superior at detecting chronic hemorrhage and all intraparenchymal hemorrhages, excluding chronic microbleeds. The sensitivities of MRI and CT relative to the final diagnosis were 83 and 26 percent, respectively. Compared with the final diagnosis of acute stroke, the accuracy of MRI was 89 percent, and CT was 54 percent.

Conclusion: MRI is more effective than CT in the initial diagnosis of acute stroke. Most of the superiority of MRI was attributed to its ability to detect acute ischemic stroke. The authors suggest that their results are widely generalizable because the study was performed in a community hospital. They recommend MRI as the initial imaging modality in suspected acute stroke, and they suggest that MRI should be rapidly available in more centers to prevent delay in the initiation of therapy for acute stroke.