Magnetic resonance imaging, or MRIs, might be able to help physicians figure out if clot-busting drugs will help patients who present with unclear-onset or “wake-up strokes,” a new study finds.

Researchers in South Korea used MRIs to screen 430 patients who were either uncertain of when their stroke symptoms began or who woke up while a stroke was in progress. This is an important factor as clot-busting drugs are most effective at preventing disability if administered within four-and-a-half hours of the onset of symptoms.

“Wake-up, or unclear-onset, strokes account for a quarter of all ischemic strokes but have been automatically excluded from clot-busting techniques because the onset time cannot be known,” said lead study author Dong-Wha Kang, M.D., from the Ulsan College of Medicine in Seoul, South Korea. “Our study shows that such patients could also be treated safely and effectively.” Findings were presented at the American Stroke Association’s International Stroke Conference 2011, which started Wednesday and ends today in Los Angeles.