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Physicians Update

 
Summer 2006: Neuroscience

Advances in Imaging Improve Stroke Diagnosis

In the last decade, magnetic resonance imaging (MRI) has revolutionized the diagnosis of stroke. Prior to the use of MRI, physicians based their stroke diagnoses largely on physical symptoms together with negative findings on computed tomography (CT), explains Jeffry R. Alger, Ph.D., medical biophysicist and a researcher in the UCLA Ahmanson-Lovelace Brain Mapping Center. While CT was used to rule out the less common form of stroke caused by bleeding, it provided no positive sign of blood vessel blockage, the more common cause.

Two hurdles hindered the use of MRI for diagnosing stroke. First, it was difficult to get patients just hours out from a stroke into imaging studies to prove the value of MRI because they were being evaluated and treated. Second, the imaging procedure was lengthy, limiting the amount of information obtained. That is changing, particularly at UCLA. MRI can be vital in the assessment and care of stroke patients, says Dr. Alger, whose research focuses on the development of novel in vivo imaging techniques that use MRI and magnetic resonance spectroscopy for study of the brain and the nervous system.

“At UCLA, our MRI protocols are very specific in defining whether a patient has had a stroke. We can differentiate hemorrhagic stroke from ischemic stroke, and these procedures sometimes even allow us to see the blood clot itself,” Dr. Alger notes. “By using certain MRI procedures that measure functional parameters, we can tell how severe the stroke is, which usually is related to how long the patient has been suffering. Stroke onset is not always clear from clinical measurements, particularly if a patient wakes up with symptoms of stroke. We can’t know when during the night the stroke began. There are now MRI procedures that show indications of how old a stroke is. That has a lot to do with how the patient is then managed.”

Using MRI in stroke diagnosis and assessment came about by scientific accident. About 17 years ago, Michael Moseley, Ph.D., a researcher at the University of California, San Francisco, decided to try a new type of MRI called diffusion-weighted MRI in an animal model of stroke, just to see what he could see. Dr. Alger says the procedure delineated the tissue involved in the brain attack much better than any tool then in existence. This marked the beginning of the development of a battery of MRI procedures that helps in the diagnosis of stroke. “We now have a number of MRI stroke assessment protocols at UCLA that can be completed in 25 minutes,” Dr. Alger says. “These procedures clearly define whether a patient has had a stroke, which vessels are involved and how severe the stroke is. That information is quite useful in defining what the patient is suffering from, and very helpful in deciding how to manage individual patients. These procedures provide a tremendous amount of information.”

UCLA’s Clinical Image Processing Service, under the direction of diagnostic neuroradiologist Pablo Villablanca, M.D., exclusively processes three-dimensional image data. “Within five to 10 seconds, we can create a complete data set of three-dimensional renderings from MRIs or CT angiograms. That information can be used for diagnosis, triage and for treatment planning,” he explains. “We can also fuse images—a CT image to an MR image, for example—to superimpose two different data sets to gain new perspectives about the disease.” Today, four MRI procedures are used at UCLA to evaluate stroke patients:

Diffusion MRI measures the rate at which water molecules move around in brain tissue. Where blood flow is interrupted by stroke, the molecules slow down, allowing the physician to see how large the stroke is and what part of the brain is affected. This procedure is being tested to see if it can provide an early sign that the patient might improve when given new drugs or treatment procedures.

Gradient echo MRI identifies blood in brain tissue. Recent UCLA studies have shown that it can identify strokes caused by bleeding. The use of gradient echo MRI to identify bleeding in the brain and blood clot within an artery that is blocking flow provides information that a conventional MRI cannot capture.

MR angiography or MRA allows clinicians and researchers to visualize blood vessels and the blood flowing through them to identify narrowed or blocked segments.

Perfusion MRI measures the amount of blood flowing through individual parts of the brain in the small blood vessels that feed the cells. Researchers use this procedure to identify whether various areas of the brain are being properly supplied with oxygen-rich blood.

Because MRI shows how severe a stroke is and when it happened, researchers may be able to determine, for example, that patients with less severe or newer strokes may benefit from a particular therapy, while those with more severe or older strokes do not. In this way, MRI could help patients avoid undergoing costly treatments that in the end would not help them. “That is the future—doing therapeutic trials to improve understanding of individual patients so we can personalize their therapies,” Dr. Alger says. “Cost is important as well. Some therapies are expensive. If we can be certain a therapy won’t work because of the individual characteristics of a patient, we can lower costs as well as save patients from unnecessary treatments.”

For example, one new stroke treatment, clot-busting drugs, appears to work only within a very narrow time window— three hours after initiation of the stroke. There’s interest in using MRI to define a group of patients who are outside that narrow time window, but perhaps could still benefit from clot-busting therapy. The biggest complication of clot-busting drugs is bleeding, but researchers can now monitor patients for that using MRI. “Maybe some patients outside that narrow window after stroke could benefit,” Dr. Alger says. “We don’t know this now.”

In another UCLA-based clinical trial funded by the National Institutes of Health, researchers are using MRI to characterize patients who might benefit from a new corkscrew-like device used to pull out the clot. For patients who are unable to undergo MRI due to severe claustrophobia or implanted devices that are MR incompatible, CT-based stroke protocols can be employed. “Three complementary studies— a noncontrast CT scan of the brain, a CT angiogram of the neck and brain and CT perfusion of the brain—provide similar information asrevealed through multimodal MRI,” Dr. Villablanca notes. “The high special resolution of these techniques provides extremely detailed images of the brain vessels and may give information about vessels and aneurysms that is not available by MRI. CT perfusion provides information about blood flow and volume in cubic millimeter per volume of brain tissue, thereby allowing the stroke neurologist to gain a deeper understanding of tissue ischemia.”

The knowledge base derived from imaging is expanding exponentially, and is driving clinical research. “To me, this has happened phenomenally fast,” Dr. Alger says. “The work we are doing here is going to improve the way we treat patients and, hopefully, enhance their recovery.” 





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