Jun 2 2004
It's a no-brainer that the brain needs a constant supply of blood to keep it going. But some medical conditions can block or reduce that life-giving flow. Whether it's a stroke, a clogged artery or a brain tumor, any situation where blood can't get to the whole brain can lead to death or permanent disability. And it's often hard for doctors to tell just where blood is -- or isn't -- going.
But newly published research shows that an innovative type of brain scan can quickly tell doctors exactly where any of these problems lie, and help them decide how to restore blood flow. And it can be done on scanners found in many hospitals.
That's the conclusion of two new papers published in the June issue of the journal Radiology by a team from the University of Michigan Health System. The researchers detail the many potential uses of a technique called perfusion CT, including their own research results and data from a few other teams worldwide.
The U-M team, which has used perfusion CT clinically for several years, hopes their new findings and their comprehensive review of the literature will help many other hospitals decide to adopt the life-saving technique.
Perfusion CT scans can be made by any modern computed tomography machine, with help from special software. Several other brain-scanning techniques that can also reveal blood flow, such as PET, SPECT or xenon CT, require special equipment not found in many hospitals, cost a great deal or are more arduous for patients.
"It's still a relatively new technique, but it could be adapted to any newer-generation CT scanner and be used to image acute and chronic cerebrovascular conditions," says Ellen Hoeffner, M.D., the lead author of one of the two new papers and an assistant professor in the Department of Radiology in the U-M Medical School. "It still isn't completely validated for use in some conditions, but more research will help."
"Perfusion CT is going to be a terrific adjunct in the evaluation of certain patients with risk factors for stroke, especially those critical patients in whom an expeditious decision is needed," says co-author Gregory Thompson, M.D., an associate professor in the U-M Department of Neurosurgery. "I think it will become increasingly useful because it's easy to obtain, easily evaluated, quick and highly accurate."
The scan is made by passing X-rays through the brain, just like a regular CT scan. But in addition to revealing the structure of brain tissue, perfusion CT also shows how much blood is present in the brain and how quickly it is moving. This is done by scanning the patient several times every few seconds before, during and after the intravenous delivery of an iodine-containing contrast agent that absorbs the X-rays.
Hoeffner notes that commercially available computer software can calculate blood flow rates from this raw scan information. And doctors can zoom in on regions of interest: so, for example, the blood flow in the area fed by a clogged carotid artery on one side of the brain can be compared with the flow in an area fed by the other carotid artery, on the opposite side of the brain.
Clogs in the carotid arteries are caused by the same cholesterol-laden plaque that can cause chest pain and heart attacks when it occurs near the heart. In the brain, these blockages can cause problems with thinking or vision and are a major risk factor for the most common kind of stroke.
Often, surgeons will try to open or go around extremely clogged or narrowed carotid arteries, using procedures such as angioplasty, carotid endarterectomy or brain bypass. Or, if a nearby tumor must be removed, they may decide to close one carotid artery off for good, and let the other one feed the brain. But first they must test how well blood is getting through to the brain, and how the patient's brain will do if there's a temporary or permanent closure of the artery. Perfusion CT can help with both of these important tasks, and determine who might be helped -- or harmed -- by surgery.
To measure how strained the brain blood flow system is, a quality called cerebrovascular reserve, the perfusion CT scan is done after patients receive a dose of the drug acetazolamide to dilate their blood vessels and maximize flow. Those patients whose vessels don't dilate in response to acetazolamide, because they're already maximally dilated and letting as much blood through as they can, are most likely to benefit from a blood flow augmentation procedure.
Perfusion CT can also help during a balloon-test occlusion, in which doctors insert a tiny balloon into the narrowed blood vessel, inflate it to stop blood flow briefly, and measure any changes in brain activity. This shows how well a patient's brain could withstand surgery or one carotid closure.
In new results from eight patients, the U-M team shows that perfusion CT gave valuable information on patients who pass the balloon test but whose brain scan shows low blood flow that might not be sufficient if the closure is permanent. They say it may even be able to help predict which patients might go on to suffer a stroke after having one of their carotid arteries permanently closed.
Perfusion CT may also help when a patient has a stroke that's caused by blockage of blood supply to part of the brain, often because a clot or piece of plaque has lodged itself in a small blood vessel. This kind of stroke, called ischemic, is the most common form, accounting for 80 percent of the 730,000 strokes that occur nationally each year.
Regular CT scans are a standard exam for patients who arrive in the emergency room with symptoms of a stroke, because they can reveal whether a stroke is ischemic or hemorrhagic (caused by a burst blood vessel). CT scans guide doctors on whether they should deliver clotbuster drugs, which can save an ischemic stroke patient's life but can kill a hemorrhagic stroke patient.
Perfusion CT is proving useful as a way to get more information on some ischemic stroke patients' brains. It can tell, for instance, whether too much brain tissue has already died because of a lack of blood, or whether there are areas that are still getting some blood flow and could be saved if clotbusters were given. Recent research at other institutions has helped develop thresholds of brain blood flow, volume and flow rates to tell if tissue is dead (infarcted) or just blood-starved (ischemic).
The U-M team also notes that perfusion CT may be useful in patients who have survived a type of stroke known as a subarachnoid hemorrhage, caused by a burst aneurysm that bleeds onto the surface of the brain. Only a combination of quick action and neurosurgical skill can close the leak and save the patient's life. But even if they survive, patients have a high risk of ischemic stroke within hours or days of surgery, if their repaired blood vessels start to constrict uncontrollably. Many die or experience major disability. Perfusion CT can monitor for this condition, called cerebral vasospasm, and speed treatment.
Lastly, Hoeffner and her colleagues note that perfusion CT is in the early stages of development for patients with brain tumors. Because tumors need to form new blood vessels to feed themselves, and because new blood vessels have walls that are super-permeable, the technique lends itself to mapping tumors and assessing their rate of growth and stage.
Suresh Mukherji, M.D., director of the U-M Division of Neuroradiology, praises Hoeffner for spreading the word about the technique's many uses. "Dr. Hoeffner's work and her lectures at national meetings establishes her as one of the leading authorities on perfusion CT not only in the U.S., but worldwide," he says. "Her work will provide the framework for integrating perfusion CT into day-to-day practice."
In addition to Hoeffner, Thompson and Mukherji, the authors of the two papers include radiologists Rajan Jain, M.D., Sachin Gujar, M.D., Guarang Shah, M.D., John Deveikis, M.D., and Ruth Carlos, M.D., neurosurgeon Mark Harrigan, M.D., and radiology administrative associated Ian Case, RT (CT). The study was funded internally.