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CT is currently the routine imaging modality for evaluating patients with acute stroke; the technique is particularly sensitive to intracranial hemorrhage. Multimodal MRI has emerged as an invaluable procedure for assessing ischemic stroke, but its value for detecting hemorrhagic stroke is not clear.To compare the usefulness of CT and MRI for the assessment of acute stroke in the emergency setting.Investigators at a single US community hospital conducted this prospective study between September 2000 and February 2002. The study included consecutive patients with suspected acute stroke, irrespective of symptom severity. Patients with symptoms indicative of subarachnoid hemorrhage and those who were too ill to undergo MRI were excluded. All participants underwent multimodal MRI scanning (gradient-echo MRI and diffusion-weighted imaging [DWI]; performed after a median of 367 min [range 36 min to 8 days] after symptom onset) and noncontrast CT scanning (performed after a median of 390 min [range 36 min to 8 days] after symptom onset) in a nonrandomized order. Four readers, who had no other patient information, interpreted the scans independently of each other. A diagnosis of ischemic stroke was given to all patients with imaging evidence of brain infarction, even if their deficits were transient.The main outcome variables were the accuracies of MRI and CT at diagnosing ischemic or hemorrhagic stroke.The study enrolled a total of 356 patients (median age 76 years, range 21-100 years), 217 (61%) of whom had a final diagnosis of acute stroke on the basis of clinical and imaging information. There were 190 ischemic strokes (53% of the study population), with a median NIH Stroke Scale (NIHSS) score of 3, and 27 hemorrhagic strokes (8% of the study population). MRI was better than CT at detecting any acute stroke (detection frequency 52% vs 17%) and acute ischemic stroke (46% vs 10%; P<0.0001 for both comparisons). Among the 90 patients who were scanned within 3 h of symptom onset, MRI identified almost half of all cases with acute ischemic stroke, whereas CT identified less than 10% of those cases. MRI was also superior to CT at detecting chronic intracranial hemorrhage (P<0.0001), but the two imaging techniques did not differ significantly in terms of detecting acute intracranial hemorrhage. MRI had a sensitivity of 83% (95% CI 78-88%) and a specificity of 97% (95% CI 92-99%) for detecting any acute stroke, compared with a sensitivity of 26% (95% CI 20-32%) and a specificity of 98% (95% CI 93-99%) with CT. In the subset of patients who were scanned within 3 h of symptom onset, the respective sensitivity and specificity values were 76% (95% CI 64-86%) and 96% (95% CI 79-100%) with MRI, and 27% (95% CI 17-40%) and 100% (95% CI 85-100%) with CT.MRI was more sensitive than, and as specific as, CT at detecting acute ischemic and hemorrhagic strokes within 3 h of symptom onset and beyond.