MRIs pinpoint time of stroke

Prompt scans could render more patients eligible for clot-busting therapy

MRI scans of stroke patients can indicate when the stroke occurred, a revelation that could allow more aggressive treatment to limit brain damage, French researchers report online November 2 in Radiology.

For a person arriving at a hospital with a stroke, the clock is ticking. When a clot obstructs an artery in the brain, millions of neurons are lost with each passing minute as tissue is starved of blood and oxygen. A clot-busting drug called tPA, or tissue plasminogen activator, can often dissolve the clot and free up the vessel. But the drug is generally considered safe to administer only in the first three to 4½ hours after a stroke begins (SN: 10/25/08, p. 16).

Stoke patients typically get a CT scan, which enables doctors to discern whether the stroke results from a blood clot or, less commonly, from a hemorrhage, which shows up as a dark mass on the CT, says neurologist Andrew Barreto of the University of Texas Medical School at Houston. MRI, or magnetic resonance imaging, is used much less often and usually only at large medical centers.

Unfortunately, a CT scan cannot pinpoint when a stroke began. Neither can many patients, either because they can’t recall exactly when their symptoms first appeared or because they woke up already in the throes of a stroke. In such cases, doctors “guesstimate” the stroke’s onset, Barreto says, but hesitate to give tPA if too many hours might have passed. Giving tPA too late won’t help tissue that’s already dead and risks causing a brain bleed. After the tPA window closes there is little doctors can do but monitor the patient.

MRI offers a more precise look into the brain of a stroke patient than a CT scan. In the new study, physicians Catherine Oppenheim and Mina Petkova of the University of Paris Descartes examined MRIs of 130 patients, average age 65, who had been admitted to Sainte-Anne Hospital in Paris from May 2006 to October 2008 with clot-based strokes that had documented onset times. About half had undergone an MRI within three hours of onset, while the others were imaged three to 12 hours after stroke symptoms started.

The doctors examined the MRIs without knowing when each was done. They applied three standard tests to what they observed, all measurements that assessed the extent of dead tissue in the brain resulting from a clot. One measurement, called fluid-attenuated inversion recovery, clearly distinguished between MRIs taken during the first three hours and those taken later. That measurement was accurate in about 90 percent of cases, whereas the other tests were less exact. “MRI could be used as a surrogate marker of stroke [duration] when the onset time of the stroke is unknown,” Oppenheim and her colleagues conclude.  

“These data look provocative,” Barreto says. “If a CT scan shows no bleeding but subtle changes, you don’t always know what to do with the patient. That’s where an MRI is superior.” The MRI measurement that worked best in this study might reveal whether it’s advisable to give tPA six hours or more after onset, especially in people who awaken with a stroke, he says. Although an MRI can take 30 to 45 minutes to complete, Baretto says, that delay might be worthwhile if the readings expand the group of patients who could benefit from tPA. “It’s not perfect,” he says, “but it’s really good compared with other tests.”

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