Imaging And Blood Flow Studies - Diagnosing A Stroke: Stroke

Content provided by the Faculty of the Harvard Medical School

Imaging and blood-flow studies

Advances in imaging technology have enabled doctors to diagnose a stroke with greater precision than had ever been possible. The more precise the diagnosis, the better the treatment. With improved imaging devices, doctors can also rule out brain tumors, head trauma, abscesses, and other conditions that can mimic a stroke.

Computed tomography, computed tomography angiography, magnetic resonance angiography, magnetic resonance imaging, and Doppler ultrasound can reveal within minutes whether a stroke is ischemic or hemorrhagic and what caused it. Imaging data can also help determine the evolution, extent, and location of any brain damage and show an obstruction in the artery supplying blood to the involved parts of the brain.

Imaging tests are often repeated several days after the onset of a stroke to survey the extent of damage. If your condition gets worse, imaging tests can show areas of the brain that have deteriorated since the previous images were taken.

Computed tomography (CT). Today most people who suffer a stroke will have a CT scan (see Figure 7). CT employs an x-ray source that surrounds the patient's head while recording many separate x-ray views; a computer then assembles these into a detailed, nearly three-dimensional picture of the brain. Most large hospitals have a CT scanner, which looks like a squared-off doughnut about 7 feet in diameter. You lie on a moving table that slides into an opening. Braces keep your head motionless during the procedure, which lasts about 20 minutes.

Figure 7: Computed tomography This CT scan shows a deep intracerebral hemorrhage in the brain of a 65-year-old who had longstanding hypertension.

CT scans can immediately determine whether hemorrhage (bleeding) is the cause of a stroke. This is very important because people with a hemorrhagic stroke can die if they are mistakenly given one of the drugs designed to break up small clots in the early stages of an ischemic stroke. CT scans are not perfect, however: They cannot reliably detect an ischemic stroke for the first 24–48 hours after it happens, and even then they may not be able to discern small ischemic strokes in certain areas of the brain.

CT angiography (CTA). High-speed CT scanning (called CT angiography or CTA) can safely and reliably study the major arteries supplying the brain and detect the type and location of the arterial pathologic process causing the stroke. To some extent, this technology can also distinguish which areas of the brain are ischemic, due to reduced blood flow, and at risk of infarction. It can also differentiate ischemic from hemorrhagic stroke.

This technique uses an intravenous injection of contrast dye to differentiate flowing arterial blood from the surrounding tissue (see Figure 8). It can also help diagnose an aneurysm in an artery at the base of the brain in people with subarachnoid hemorrhage. At many medical centers, CTA scanning is done immediately on anyone suspected of having had a stroke. This is the best and safest noninvasive method to diagnose a ruptured or unruptured berry aneurysm.

Figure 8: CT angiography This CT angiogram shows a narrowing of the left internal carotid artery caused by an atherosclerotic plaque (see arrow). Before the advent of CT angiography, this type of image could be produced only with a more invasive technique, such as cerebral angiography. Courtesy of the 3D Imaging Laboratory, Massachusetts General Hospital

Magnetic resonance imaging (MRI). MRI uses a powerful magnetic field to take highly detailed pictures of soft tissues such as the brain and heart (see Figure 9). Subtle variations show up clearly so that an ischemic stroke, tumor, or blood clot is visible against the surrounding healthy tissue. MRI can document the extent and location of an ischemic stroke deep inside the brain within minutes of its onset. By using a special pulse sequence, it can precisely distinguish infarcted brain from ischemic brain at risk of infarction. It can also detect intracerebral bleeding, although most physicians prefer to get this information from CT scans (or CTA scans, if they are available) because they're quicker and easier to do.

Figure 9: Magnetic resonance imaging This MRI scan shows an ischemic stroke in a 65-year-old who had atrial fibrillation. An embolus, which was only about 3 mm in diameter, lodged in the middle cerebral artery and blocked blood supply to the area of the brain shown in white.

Magnetic resonance angiography (MRA). This technique is similar to MRI, but uses an injected contrast dye to show blood flowing in vessels. In addition, it can assess whether brain blood flow is normal (see Figure 10).

Figure 10: MR angiogram MR angiograms help doctors determine whether stroke treatment has restored normal blood flow in the brain (as shown above).

Doppler ultrasound. Another noninvasive method, Doppler ultrasound enables doctors to view blood vessels and analyze blood flow in the neck and at the base of the brain. Like sonar in a ship, high-frequency ultrasound waves penetrate the artery in question — such as the carotid, vertebral, and intracranial arteries — and bounce back to produce an image of the blood vessel and a wave showing intensity of blood flow (see Figure 11). Using this image, the doctor can then calculate the velocity of blood flow.

Figure 11: Doppler ultrasound Sound waves produce images such as those shown above. The higher peaks in the image on top show normal blood flow. In contrast, the lower blips in the lower image indicate a narrowing of the carotid artery, which reduces blood flow to the brain.

Two Doppler ultrasound techniques are used for diagnosis of stroke. Carotid duplex Doppler scanning makes images of the carotid arteries. Transcranial Doppler scanning analyzes flow in the major intracranial arteries at the base of the brain. These techniques are essential to the stroke neurologist, as they can provide a minute-to-minute assessment of the adequacy of flow in the damaged artery and the availability of collateral flow from other arteries to help ischemic brain tissue. To put it more simply, Doppler ultrasound is the stroke doctor's stethoscope: It is used at the bedside to judge the effectiveness of targeted therapy to improve blood flow in the ischemic brain.

Researchers at Massachusetts General Hospital found that Doppler ultrasound, when combined with CTA or MRA, can accurately identify the presence and severity of carotid and intracranial artery narrowings and show whether blood flow has been rerouted around the narrowings through other vessels. This information is precise enough for most people considering carotid surgery to avoid the greater risks involved with conventional angiography, an invasive imaging procedure. Instead, a CTA is used to confirm diagnosis.

Cerebral angiography. This test is an invasive procedure that involves injecting a dye or contrast medium into an artery and taking x-rays to study blood flow (see Figure 12). Cerebral angiography is similar to heart angiography except that it studies blood flow to the brain instead of the heart. The doctor inserts a thin plastic tube, called a catheter, through a small surgical puncture into a blood vessel near the groin, and then guides it through the circulatory system to an artery feeding the brain. Next, the doctor squirts dye through the catheter into the blood vessels and takes detailed x-rays of blood flowing through the brain. This test can detect many of the abnormalities that cause a stroke, including the narrowing or blockage of a blood vessel, an embolus, atherosclerosis, arteriovenous malformations, and aneurysms.

Figure 12: Cerebral angiography The angiogram (left) shows a tight narrowing of the right internal carotid artery. The illustration (right) shows an artist's version of the same blockage. With the advent of noninvasive imaging techniques, angiography is used less often.

Nevertheless, the valuable information that angiography can provide must be weighed against its potentially harmful effects. If the catheter dislodges a blood clot or fragments of cholesterol, it can provoke an embolic stroke. The risk increases with a person's age. This test is usually done only when the doctors haven't been able to diagnose the cause of an ischemic stroke with noninvasive imaging and when treatment depends on knowing the cause. It is also used to identify the anatomy and location of an aneurysm (a bulge on the wall of a blood vessel) that has caused a subarachnoid hemorrhage, or an arteriovenous malformation that has caused an intracerebral hemorrhage.

Cardiac rhythm monitoring. In order to detect atrial fibrillation, a heart rhythm abnormality that comes and goes, continuous monitoring of the heart is required. The doctor may ask that you wear a Holter monitor, a portable device that records your heart rhythm over time, while you go about your daily activities and even while you sleep. Holter monitoring for 24 hours or longer may be needed.

Echocardiography. This type of ultrasound produces images of the heart and can help doctors pinpoint the origin of emboli (blood clots) that have made their way to the brain and caused a stroke. It is used when the evidence points to an embolic stroke that originated in the heart. Echocardiography can help to determine the type of cardiac abnormality responsible for the original embolic stroke, which is necessary before doctors can decide on a therapy to prevent another one. Types of cardiac abnormalities detected through echocardiography include an infected heart valve, a patent foramen ovale (a hole in the atrium), and a clot in the ventricle or the atrium. This test is also highly reliable in detecting left ventricular hypertrophy (an enlargement of the left side of the heart), which is caused by chronic hypertension, a strong risk factor for stroke.

Other blood-flow tests. Several other noninvasive imaging techniques, such as positron emission tomography, single photon emission computed tomography, and xenon blood-flow tomography, can be used to assess blood flow. These tests, however, are time-consuming, expensive, and not widely available.

Last updated:	September 05, 2008