Although computed tomography scans have been the go-to test to diagnose strokes, a new report published in Neurology recommends using diffusion magnetic resonance imaging instead.

The window for treatment of ischemic strokes - caused by lack of blood flow in the brain - is merely hours. And according to these new findings, diffusion MRI is considered more useful than a CT scan for diagnosing acute ischemic stroke within 12 hours of a person's first stroke symptom. The findings have become the new guideline from the American Academy of Neurology.

"[The researchers are] saying MRI is definitely better if you have a choice," said Dr. Ralph Sacco, chairman of neurology at University of Miami and spokesman for AAN. "I'm not saying a CT scan has no role...If a CT scan can be obtained more rapidly [than an MRI], go with it."
The AAN looked at past studies, mainly from the mid-2000s, in order to give the best possible guideline regarding acute ischemic stroke testing.

The best test for that job appears to be diffusion-weighted imaging MRI, according to the report - accurately detecting stroke 83 percent of the time versus 26 percent of the time by CT, according to one reviewed study.

DWI measures molecular water motion in the tissue, showing where water diffusion is restricted, thus indicating brain damage has occurred.

"DWI looks for early changes in the brain, suggesting brain injury," Sacco told DOTmed News. "For example, within the first three to six hours of a spell, often a CT scan within that period will be negative, while a DWI is very sensitive and can already pick up areas of the brain that are injured."

But the new guideline doesn't mean to suggest patients should avoid CT scans, explained Sacco. In some cases, MRI scans are not readily available, and a CT scan can help start the process of diagnosis and treatment. If a patient's CT scan is negative, clinicians often follow up with an MRI. And sometimes, an MRI is used as follow up even if a CT scan is positive because MRI could more accurately show the size and severity of the stroke.

Twenty-four hours after stroke symptoms appear, CT scans are very accurate in diagnosis - about equal to an MRI scan, said Sacco.

"This guideline gives doctors clear direction in using MRI first, ultimately helping people get an acute stroke diagnosis and treatment faster," Dr. Peter Schellinger, lead researcher from the Johannes Wesling Clinical Center in Germany, said in a statement. "MRI can be added later if more information is needed. Otherwise MRI should be used first."

"Both imaging techniques are still important, but there are significant advantages of MRI," said Sacco.

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