The most common method of diagnosing a stroke in emergency rooms catches only about one out of every four cases — far fewer than an...

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LONDON — The most common method of diagnosing a stroke in emergency rooms catches only about one out of every 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 today’s issue of the medical journal The Lancet, to declare that MRI scans should replace CT X-rays as the standard.

“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.

However, others argue it’s not such a clear-cut choice. Getting results from an MRI scan takes more time, a delay that can prove deadly to a stroke patient, these doctors contend.

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“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.”

Broderick had no role in the study, which was led by Dr. Julio Chalela of the Medical University of South Carolina. Chalela was with the U.S. National Institutes of Health when the study was conducted.

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 only 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. The vast majority of strokes are caused by clots. In patients scanned within three hours of symptoms, MRIs detected strokes caused by clots in 41 of 90 patients, while CT scans picked up only six of the 90 patients.

The first few hours after a stroke are critical, since clot-busting drugs must be given within three hours to have a real impact. If they are given to the wrong patients, however, death or severe disability can result.

Though CT scans may lose out to MRI scans on accuracy, on issues such as time and money, CT scans are far ahead. Widely available in emergency rooms in all developed countries, CT machines are compact pieces of equipment that produce images in as little as two minutes. In comparison, MRI machines are large, coffin-like structures that require patients to lie still for up to 30 minutes. They are also unsuitable for patients with pacemakers or other metal objects in their bodies, or those who may be pregnant.

MRI scans also cost significantly more than CT scans and require specialized technicians to operate them and to read the scans.

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