MRI Mismatch may Identify Stroke Patients within Critical Treatment Window

A number of MRI techniques have been 

considered as a means for identifying 

elapsed time from stroke onset. (MRI 

angiogram courtesy of Wikipedia.)

A mismatch in the images produced by two different MRI techniques of patients experiencing a stroke are likely to indicate that patients are candidates for intravenous thrombolysis, a new study shows.

The study, led by Dr. Götz Thomalla, of the Universitätsklinikum Hamburg-Eppendorf, Germany, adds to the growing evidence that diffusion-weighted (DWI) MRI and fluid-attenuated inversion recovery (FLAIR) MRI may be useful for identifying patients who are within the 4.5 hour window when thrombolytic therapy is beneficial. The study is online ahead of print publication in October, The Lancet Neurology. (Link to published site)

"Patients with an acute ischemic lesion detected with DWI but not with FLAIR imaging are likely to be within a time window for which thrombolysis is safe and effective," Thomalla and co-authors wrote. “These findings lend support to the use of DWI-FLAIR mismatch for selection of patients in a future randomized trial of thrombolysis in patients with unknown time of symptom onset.”

Because their study involved patients with known time of symptom onset it does not provide evidence for the effectiveness and safety of MRI-based thrombolysis in patients with an unknown time of symptom onset. Current guidelines exclude stroke patients with unknown time of onset from administration of thrombolysis.

Researchers have long been interested in using multiple MRI techniques for identifying the amount of elapsed time from symptom onset to presentation as a way to select candidates for thrombolytic therapy. DWI-FLAIR mismatch has been an attractive method because DWI can detect changes in water diffusion within three minutes of ischemia onset, and FLAIR can detect a net increase in water within one to four hours after symptom onset.

In the current study, the researchers analyzed clinical and MRI data from 543 patients presenting with acute stroke and a known time of symptom onset between Jan. 1, 2001 and May 31, 2009 at clinics in Berlin and Hamburg, Germany. All had a DWI and a FLAIR MRI within 12 hours of the onset of symptoms, and had an average National Institutes of Health Stroke Scale score of 8. Two neurologists, who were blinded to the clinical data, judged the MRI data for visibility of acute ischemic lesions on DWI, FLAIR and DWI-FLAIR mismatch images. Diagnosis was by consensus.

Overall acute ischemic lesions were identified on DWI in 516 patients (95 percent) and on FLAIR in 271 patients (50 percent). DWI-FLAIR mismatch identified patients within 4.5 hours of symptom onset with 78 percent specificity (72 to 84) and 83 percent positive predictive value (79 to 88). However, sensitivity was 62 percent (95% CI 57–67), and negative predictive value was 54 percent (48 to 60).

In an editorial accompanying the article in the journal, Dr. Michael D. Hill, and Dr. Richard Frayne, of the University of Calgary in Canada wrote that the study sets the stage for validating DWI-FLAIR mismatch as a biomarker identifying candidates for thrombolysis.

“Validation would allow this biomarker to be used as a participant selection method for randomized trials of thrombolytic stroke therapy in patients with stroke on awakening or with unwitnessed stroke onset,” they wrote. “Such trials are on the drawing board and it will be these kinds of imaging biomarkers – which are simple, practical, and easily implementable at many sites—that will allow relevant candidates to be selected for enrolment in these trials.” 

By Michael O’Leary, contributing writer, Health Imaging Hub