Until recently, people who had a stroke had no choice but to live with the debilitating consequences. There wasn't much that doctors could do for them.
“During my residency [in the 1970s], when people came in with a stroke, you gave them supportive care, and whatever happened to them happened,” says Charles Strother, M.D., an Emeritus Professor of Neuroradiology at the University of Wisconsin-Madison.
But modern imaging devices provide doctors with a way to see what's going on in your brain and, even more important, do something about it to help minimize brain injury and resulting disability.
“Imaging is crucial for our understanding of where in the brain a stroke has occurred, and what type of stroke it is,” says Ralph Sacco, M.D., Chair of Neurology at the University of Miami Miller School of Medicine.
IS IT ISCHEMIC OR HEMORRHAGIC?
When you arrive at the emergency room with symptoms of a stroke — for instance, slurred speech or paralysis on one side of the body — doctors must first determine whether the stroke is ischemic (caused by a clot obstructing the flow of blood to the brain) or hemorrhagic (caused by a burst vessel that leaks blood into surrounding tissue).
Administered within three hours of the onset of symptoms of an ischemic stroke, tPA (tissue plasminogen activator) helps to dissolve a blood clot, and, in some cases, stroke patients can walk out of the hospital the next day with no signs of paralysis or other stroke-related problems.
But if given to someone having a hemorrhagic stroke, tPA will cause more bleeding in the brain, resulting in greater disability and possibly death.
For this reason, doctors will likely order a CT scan to see if there is bleeding in the brain. CT (Computed Tomography) imaging produces multiple X-rays of your brain that can be viewed individually or assembled by a computer into a vivid 3-dimensional image.
A CT scan uses ionizing radiation to produce images — the same kind a dentist uses to make X-rays of your teeth. A CT scan of the brain, however, requires as much radiation as about 200 panoramic dental X-rays, but that's still a relatively small amount. We absorb natural background radiation all the time from cosmic rays, radon in the earth and other sources. A typical CT scan exposes a person to less than one year's worth of normal background radiation.
Still, Americans are exposed to seven times more radiation than they were in 1980, with the greatest increase coming from medical imaging. For this reason, the American Heart Association (AHA) recently issued a treatment guideline that advises doctors to use CT scans only after “thoughtful consideration of the potential benefit to the patient,” and after every effort has been made to minimize radiation exposure.
ARE REPEATED SCANS DANGEROUS?
“We don't know the exact risk of cancer related to radiation and imaging,” says Thomas C. Gerber, M.D., an Associate Professor of Medicine and Radiology at the Mayo Clinic College of Medicine in Jacksonville, FL, and lead author of the AHA advisory. “All we know is derived from survivors of atomic explosions at Hiroshima and Nagasaki, and they received full-body exposure to high levels of radiation. Imaging exposes specific parts of the body such as the brain or the heart to much lower levels of radiation. Some medical physicists say the potential risk of cancer at small doses is negligible.”
While most hospitals have a CT scanner readily available for diagnosing stroke patients, MRI (Magnetic Resonance Imaging) also produces highly detailed images without exposing the patient to radiation. MRI uses powerful magnetic fields instead of ionizing radiation to create detailed images of the brain.
“The main advantages of CT scanning are availability and speed,” said Robert J. Adams, M.D., M.S., Professor of Neuroscience and Director of the Stroke Center at the Medical University of South Carolina. “But MR imaging offers an infinite number of ways to look at different aspects of the brain, which may provide useful information such as how old the stroke is, or if an old stroke was associated with bleeding.”
“But I would not tell patients to choose a [stroke] center based on the availability of MRI, even though MRI can be very helpful,” he said. “Patients need to call 9-1-1 as soon as they experience the first symptoms of stroke, and get to the closest stroke center, regardless of the availability of MRI.”
CT and MRI scanners also can be used to create detailed images of blood vessels in the brain, often revealing the precise location and size of a clot. CTA (computerized tomographic angiography, which involves radiation, requires the injection of a contrast agent that will make the vessels stand out vividly on a CT scan. Magnetic Resonance Angiography (MRA), which uses magnetic waves instead of radiation, can do the same but without the injection. Such scans provide clear pictures of the condition of blood vessels, whether they're clogged by a clot or bulging from an aneurysm that is on the verge of bursting.
Brain scans help doctors with intra-arterial delivery of tPA, which involves guiding a catheter through a blood vessel from the groin to the brain to dissolve a clot. Alternatively, a catheter outfitted with a corkscrew-like tip or with a suction tip can grab the clot and pull it out. For an aneurysm, a catheter can be used to insert a thin wire into the bulge in the artery, filling it up and relieving pressure caused by pulsing blood.
A perfusion scan often administered after a stroke uses CT or MRI to measure the amount of blood that passes through various brain regions around the site of a blocked artery to reveal which areas of the brain are struggling because of impaired circulation. “We call that a brain at risk,” says Sacco. “In the area surrounding the stroke, you'll find low perfusion — or low blood flow — but that area of the brain might be saved,” he explains.
DTI (Diffusion Tensor Imaging), an experimental imaging technique, uses a standard MRI scanner to detect the movement of water in the brain to map the location of the delicate fiber tracts that link one region of the brain to another. By revealing damage to these tracts, DTI can explain certain post-stroke symptoms, such as muscle weakness, movement disorders (spasticity) or speech problems (aphasia). This information can help rehab, speech and occupational therapists devise effective programs to maximize recovery. Research suggests that DTI may also be useful in predicting whether a patient will overcome speech problems and to what extent.
“DTI allows you to say, here's the pathway that's been impacted by the stroke,” explains Strother. “Then we might be able to design a rehabilitation plan that would specifically address that function. Even in older people there's a lot of plasticity and redundancy in the brain that we can take advantage of, and the more we understand, the better off we'll be.”
DTI and other new technologies may one day eliminate the need to use radiation to obtain clear pictures of the brain, but right now CT remains an invaluable tool.
“MRI will, of course, advance,” says Strother, “but there are efforts underway to reduce the dose required to obtain a good CT. I think that both will be used, depending on which does a task the best.”
SCANS AT A GLANCE
Various scanning techniques help doctors see the type of stroke a patient has suffered, the part of the brain affected and the degree of injury — as well as treat the cause of the stroke, in some cases. Here's what you need to know about these imaging tools:
Magnetic Resonance Imaging (MRI)
HOW IT WORKS: Creates detailed images of the brain using magnetic fields instead of radiation.
WHAT IT'S USED FOR: To distinguish a stroke caused by a blood clot from a stroke caused by bleeding in the brain. Because they are so detailed, MRI images also can reveal the location and extent of even small blockages and tissue damage.
DRAWBACKS: MRI scanners are a newer and more expensive technology, so they're not as widely available as CT scanners.
COST: Medicare reimburses $528.
Computed Tomography Angiography (CTA)
HOW IT WORKS: A contrast agent is injected intravenously, which enables the CT scanner to reveal the arteries in sharp detail.
WHAT IT'S USED FOR: To reveal the precise location of a blocked or bleeding artery, and expose aneurysms or other irregularities.
DRAWBACKS: Injecting the contrast agent is an invasive procedure that may produce a reaction in the patient.
COST: Medicare reimburses $518.
Computed Tomography (CT)
HOW IT WORKS: Creates vivid X-ray images of brain slices using ionizing radiation that can reveal damage.
WHAT IT'S USED FOR: To distinguish a stroke caused by a blood clot from a stroke caused by bleeding in the brain.
DRAWBACKS: Involves exposure to ionizing (X-ray) radiation, which may increase a patient's risk of cancer slightly.
COST: Medicare reimburses $218.
Diffusion-weighted MRI (DWI)
HOW IT WORKS: The MRI scanner is used to detect water movement in the brain, which reveals areas around the stroke site that have not died, but are suffering from low blood flow.
WHAT IT'S USED FOR: DWI can detect a stroke very early, thereby enabling doctors to get a head start on saving “tissue at risk.”
DRAWBACKS: A newer technology not yet widely available in hospitals.
COST: Medicare has not established reimbursement schedule.
Diffusion Tensor Imaging (DTI)
HOW IT WORKS: The MRI scanner is used to detect water movement to create a map of the delicate fiber tracts that link brain regions.
WHAT IT'S USED FOR: Still mostly used in research, DTI scans may someday help show the cause of symptoms in people who have had a stroke that disrupted vital brain connections.
DRAWBACKS: Because it's used primarily for research, not many hospitals have access to technicians who have done this procedure.
COST: Medicare has not established a reimbursement schedule.
INSURANCE: Rarely covered.
Magnetic Resonance Angiography (MRA)
HOW IT WORKS: Makes detailed pictures of the brain blood vessels.
WHAT IT'S USED FOR: MRA reveals abnormalities in the arteries.
DRAWBACKS: Not as readily available as CTA.
COST: Medicare reimburses $594.
© 2009 American Heart Association, Inc.