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Strokes often missed with traditional scans, study says

Strokes often missed with traditional scans, study says

The test most commonly used to diagnose strokes in emergency rooms catches only about one out of every four cases _ far fewer than a rival imaging technique, new research suggests.
Results of a U.S. government-funded study show that the more accurate magnetic resonance imaging, or MRI, scans should replace traditional computed tomography, or CT X-rays as the standard of care, a stroke specialist contends in an editorial published with the study in Friday's issue of the medical journal The Lancet.
But in the race against time to diagnose and treat a stroke, waiting for an MRI could prove deadly, others believe.
"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 at the U.S. National Institute of Health in Bethesda, Maryland when the study was conducted.
Chalela and colleagues examined 356 patients, of whom 217 were ultimately diagnosed with an acute stroke. Researchers looked at a consecutive series of patients referred to the hospital's stroke team, who were all suspected of having an acute stroke.
Patients were scanned both by CT and MRI machines. CT scans are a type of X-ray, whereas MRIs use 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 differentiating between the two types of strokes _ those caused by blood clots or bleeding in the brain. The vast majority of strokes are caused by clots. In patients scanned within three hours of their symptom onset, MRI scans detected strokes caused by clots in 41 of 90 patients, while CT scans only picked up 6 of the 90 patients.
The first few hours following a stroke are critical, since clot-busting drugs must be given within three hours if they are to have a real impact. If they are given to the wrong patients, however, death or severe disability can result.
Strokes are the second leading cause of death worldwide, and account for approximately 5.5 million deaths each year.
Though CT scans have long been the diagnostic method of choice in emergency situations, Dr. Geoffrey A. Donnan and colleagues write in an accompanying commentary that "this mantle should now be passed to magnetic resonance imaging." MRI scans should be adopted as the new standard of care, according to the commentary, which also criticizes governments and health care systems for their poor track record of assessing new technologies.
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, metal objects, or who may be pregnant.
MRI scans also cost significantly more than CT scans, and require specialized technicians to be operated. Interpreting the scans demands radiological expertise as well, since the subtleties of MRI imaging are not as easy to grasp as the more obvious pictures produced by CT scans.
"The superiority of MRI in detecting stroke in ideal conditions is unquestioned," said Dr. Lee Schwamm, an associate professor of neurology at Harvard Medical School. Schwamm was not connected to the study. But combined with a consideration of patient symptoms, he says that CT scans are just as effective in diagnosing patients in emergencies.
MRI scans may offer more detailed information, but such information hasn't yet been proven to make a difference in patient outcomes. Experts say studies are needed to determine if MRI scans might save more lives than CT scans in emergencies.
Schwamm likens the comparison between the two techniques to the difference between FM and AM radio. "FM radio is better because it's high-definition, and is great if you're listening to classical music," he said. "But sometimes all you need is the weather and the news, so AM is just fine."
Doctors say the time needed to perform MRI scans is a major strike against them. CT machines are usually conveniently located next door to emergency rooms, whereas MRI machines are generally on another floor of the hospital.
In emergency situations, CT scans, available within minutes, allow doctors to make decisions more rapidly than if they were forced to wait for MRI results. CT scans can also be interpreted by attending physicians, rather than specialist radiologists who might not be immediately available. "The accuracy difference between MRI and CT doesn't tell the whole story," said Schwamm. When doctors are up against the clock, CT scans are often good enough, according to Schwamm.
In the same Lancet issue, an all-stroke special, two other studies found that stroke patients treated in a stroke-unit care have a better chance of recovery than if they were treated in a conventional hospital ward.
Alteplase, also known as TPA, the only approved clot-busting stroke drug, is safe to give patients in regular hospitals as part of routine stroke treatment, provided the correct medical guidelines are in place.


Updated : 2021-06-16 17:12 GMT+08:00