LONDON – The CT scan, the most common method of diagnosing a stroke in emergency rooms, catches only about one in four cases – far fewer than an MRI scan, which also was better at spotting the type of stroke, a U.S. government-funded study showed.

The study led some experts, writing in the medical journal The Lancet, to say MRI scans should replace CT scans as the standard of care. The journal published the stroke study in today’s issue.

“This mantle should now be passed to magnetic resonance imaging,” wrote Dr. Geoffrey Donnan and colleagues at the University of Melbourne in Australia in an accompanying commentary.

But others argue it’s not such a clear choice. MRI results take more time, which can prove deadly to a stroke patient, they contend.

“The time delay between MRI and CT may be around 15 to 20 minutes,” said Dr. Joseph Broderick, chairman of neurology at the University of Cincinnati College of Medicine. “And in an emergency, 15 to 20 minutes can make a big difference.”

Chalela and colleagues examined 356 patients, of whom 217 were ultimately diagnosed with an acute stroke. Patients were scanned both by CT and MRI machines. CT scans are a type of X-ray, whereas magnetic resonance imaging uses powerful magnets instead of radiation to produce an image.

The scans were independently interpreted by four experts, who had no other patient information. Based on the MRI scans, experts accurately diagnosed acute strokes 83 percent of the time. Using the CT scans, however, they were right just 26 percent of the time.

MRI scans were also more precise in spotting the cause of the stroke – a blood clot or bleeding in the brain.