False-negative diffusion-weighted MR findings in acute ischemic stroke

BACKGROUND AND PURPOSE: Lesions associated with acute stroke are often miss ed by diffusion-weighted imaging (DWI), suggesting that the sensitivity of this technique for detecting acute ischemic stroke may not be as high as in itially thought. Our aim was to estimate the rate of false-negative DWI stu dies in patients with persistent neurologic deficit due to an ischemic stro ke and to identify which stroke lesions are most likely to be missed by DWI . METHODS: We reviewed MR images obtained within 48 hours after stroke onset in 139 patients admitted for symptoms consistent with ischemic stroke in wh om the deficit lasted more than 24 hours. Cases of negative initial DWI fin dings with an ischemic lesion visible on followup MR studies and a final di agnosis of arterial ischemic stroke were analyzed in terms of delay between onset of symptoms and initial DWI (MR latency), size and vascular distribu tion of the lesions, and relationship to findings in patients with positive initial DWI results. RESULTS: We found eight cases (5.8%) of false-negative initial DWI studies, of which four were positive on initial fluid-attenuated inversion recovery (FLAIR) imaging. Follow-up FLAIR/DWI showed a hyperintensity matching clin ical presentation in all eight patients. The mean size of the lesion was 0. 19 +/- 0.16 cm(3). False-negative studies occurred more often in cases of s troke in the posterior (19%) than in the anterior (2%) circulation or when DWI was obtained within 24 hours after symptom onset. Of the six: false-neg ative vertebrobasilar stroke lesions, five were located in the brain stem. In all, 31% of patients with vertebrobasilar ischemic stroke had a false-ne gative initial DWI study during the first 24 hours. CONCLUSION: A false-negative DWI study is not uncommon during the first 24 hours of ischemic stroke. Vertebrobasilar stroke should therefore not be ru led out on the basis of early negative DWI, especially when symptoms persis t and are suggestive of this diagnosis.