MRI After Stroke May Extend Clot Buster Limit

Prior studies have shown that the tPA clot buster is effective up to three hours after the onset of stroke symptoms. In the new study, researchers at the university of North Carolina (UNC-CH, Chapel Hill, USA; www.med.unc.edu) and Washington University (St. Louis, MO, USA) used MRI to look at the brains of seven stroke victims at two different times: three to six hours after the onset of symptoms and three to five days later. Because the brain is roughly symmetrical, they could compare oxygen usage in tissue at the site affected by a blood clot to the equivalent site on the opposite side of the brain.

The researchers determined that if, at the first time point, the blocked tissue was using less than about 35% of the oxygen used by its counterpart on the other side of the brain, the tissue did not survive to the second time point. These results suggested that brain regions using more than about 35% may represent salvageable tissue. The next step is to investigate extending the tPA window beyond three hours after the start of symptoms for patients with salvageable tissue.

MRI has several advantages for this application, say the researchers. For example, positron emission tomography (PET) imaging utilizes an expensive radioactive isotope and can be painful. Almost every community hospital has MR imaging and it is noninvasive.

"If one of the blood vessels is blocked, brain tissue goes through a process before it's going to die. This takes time,” said Weili Lin, Ph.D., professor of radiology and biomedical engineering at UNC-CH School of Medicine and leader of the research team. "It depends on how much cerebral blood flow is being reduced, and on how long the tissue is under attack.” Thus, depending on the extent of injury, tissue that gets limited blood flow could survive for much more than three hours.


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