EVIDENCE-BASED ANSWER:
It depends on whether the stroke is schemic or hemorrhagic. For early detection of ischemic stroke, magnetic resonance imaging (MRI) using diffusion-weighted imaging (DWI) is highly sensitive and specific, whereas computed tomography (CT) is less sensitive but about as specific (strength of recommendation [SOR]: B, a meta-analysis of lower quality RCTs). MRI using DWI and CT are probably comparable for detecting acute hemorrhagic stroke (SOR: B, a cohort study).
When thrombolysis is being considered and hemorrhage must be ruled out rapidly, either test is acceptable if it can be performed and interpreted within 45 minutes of patient arrival, although MRI typically costs about twice as much as CT (SOR: C, expert opinion).
EVIDENCE SUMMARY
Clinical A Cochrane review identified 7 studies that compared MRI with CT for detecting ischemic stroke in a total of 226 patients, average age 65 years, with stroke-like symptoms.1 Investigators performed imaging within 12 hours of symptom onset in all patients, including those whose final diagnosis was transient ischemic attack (TIA). They identified 161 patients with ischemic stroke based on a combination of imaging and clinical examination. MRI with DWI was more sensitive than CT (0.99; 95% confidence interval [CI], 0.23-1.00 vs 0.39; 95% CI, 0.16-0.69); both techniques had comparable specificity (0.92; 95% CI, 0.83-0.97 and 1.00; 95% CI, 0.94-1.00, respectively).
Many issues could have affected the ischemic stroke analysis: All studies included some retrospective data collection; in all but one study, the MRI was performed a mean of one hour after the CT; and in 4 studies, the physicians reading the scans weren’t blinded to the clinical outcome. The Cochrane authors also found evidence of “prescreening” that appeared to select for patients with middle-cerebral artery infarcts. They concluded that the reliability and generalizability of the results “were questionable.”
MRI and CT have similar sensitivity and specificity for hemorrhagic stroke
A prospective cohort study of 27 patients (mean age 76 years) who had an acute hemorrhagic stroke that was imaged using both MRI with DWI and CT within 3 hours of symptom onset found that both imaging studies had comparable sensitivity (0.81; 95% CI, 0.61-0.93 vs 0.89; 95% CI, 0.70-0.97, respectively) and specificity (1.0; 95% CI, 0.98-1.0 for both).2