Feb. 21, 2001 A new use for magnetic resonance imaging may reduce brain damage from stroke and save lives, according to a study led by a scientist at the University of North Carolina at Chapel Hill. Weili Lin, PhD, associate professor of radiology and biomedical engineering at UNC-CH School of Medicine said MR imaging can measure oxygen use in injured brain tissue after a stroke caused by a brain artery blockage. This may help doctors determine whether injured tissue could be saved using the potent clot-busting drug, tissue plasminogen activator (tPA).
The research will be presented February 14 at the 26th International Stroke Conference in Fort Lauderdale, Fla.
Previous studies have shown that the clot buster is effective up to three hours after the onset of stroke symptoms. But tPA could help some stroke victims well after this three-hour deadline has passed, Lin said.
"If one of the blood vessels is blocked, brain tissue goes through a process before it's going to die. This process takes time," Lin said. "It depends on how much cerebral blood flow is being reduced, and on how long the tissue is under attack." Thus, depending on extent of injury, tissue that gets limited blood flow could survive for much more than three hours. Lin and his collaborators at UNC and Washington University in St. Louis used MR imaging to look at the brains of seven stroke victims at two time points: three to six hours after the onset of symptoms, and three to five days later. Because the brain is roughly symmetrical, the researchers 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, called the contralateral region.
They determined that if, at the first time point, the blocked tissue was using less than about 35 percent of the oxygen its counterpart on the other side of the brain used, the tissue did not survive to the second time point. "These results raise the possibility that brain regions [using more than about 35 percent] of the contralateral hemisphere may represent salvageable tissue," the study authors note.
Lin hopes to use the results of this study to look into extending the window for using tPA beyond three hours after the start of symptoms for patients with salvageable tissue. "If we can extend beyond three hours, I think that would open up a lot of opportunities for patients to receive adequate treatments," he said.
The UNC scientist said that when he was with Washington University in St. Louis, only about 10 percent of stroke victims make it to the hospital within the three-hour window, even after several years of community education about stroke symptoms. A stroke victim may not immediately recognize he or she is having a stroke, Lin noted. "Some patients, for example, may have a headache and start feeling a little weakness. They might take a nap, expecting to feel better when they wake up. They don't know that's a symptom of stroke. So they won't come into the hospital until later in the process, when they are so weak they cannot even stand up. This may be well after three hours have passed," he said.
MR imaging has several advantages over other systems for looking at oxygen usage in the brain. Almost every community hospital has MR imaging. Another imaging tool called positron emission tomography, or PET, can be very painful to use, and it utilizes a radioactive isotope that is very expensive to make and can damage the patient's health if it is used repeatedly, Lin explained. UNC's Magnetic Resonance Imaging Research Laboratory recently became the fourth facility in the United States to acquire a new high-powered MR imaging system for research into Alzheimer's disease, Parkinson's disease, brain mapping, and other areas. This study was supported by a grant from the National Institute of Neurological Disorders and Stroke, National Institutes of Health.
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