Since contrast angiography was developed in the 1920s, great advances have been made in diagnostic tests to determine the type of stroke and its precise location.
The types of diagnostic tests include:
- Imaging tests
- Electrical activity tests
- Blood flow tests
- Other tests
Imaging tests can produce a detailed picture of the brain. These tests include computed tomography (CT or CAT) scans and magnetic resonance imaging (MRI).
A CT scanner sends a series of X-rays through the head that are analyzed by a computer to create a detailed picture of a “slice” of the area being studied. Each X-ray lasts a fraction of a second.
During a CT scan of the head, the head is positioned inside a CT scanner’s cylinder. The entire scanner can tilt, and the X-ray scanning cylinder within it can rotate to obtain the views needed. For a head scan, 10 to 30 slices are usually taken. The results are highly-detailed images of the head, including the brain, eyes, bones of the skull and sinuses within the bones around the nose. This is often one of the first tests given to patients who may have had a stroke. These scans provide important information about the cause of the stroke and the location and extent of brain injury. CT scans are clearer pictures of the brain than regular X-rays.
Sometimes a special dye (contrast material) that contains iodine is injected into the blood during a CT scan of the head. The dye makes blood vessels and certain structures inside the head more visible on the CT scan images. This is known as CT angiography.
An MRI produces a picture of the brain using a large magnetic field. It also can show the location and extent of brain injury, but the image is sharper and more detailed. An MRI can distinguish between the blockage of blood flow due to a clot, which causes transient ischemic attack and ischemic stroke, and bleeding, which causes hemorrhagic stroke. This type of diagnostic technique is often used to diagnose small, deep injuries. After the first 24 hours, MRI can identify the exact size and location of the area affected by a stroke. This information may help the doctor determine how well the person will recover from a stroke.
An MRI is more sensitive than a CT scan in picking up changes caused by lack of oxygen to brain cells during the first 72 hours after a stroke. An MRI is more accurate than a CT scan of the head in identifying multiple small strokes within the brain. An MRI is also better for detecting strokes in the lower, back part of the brain (cerebellum) and the part of the brain that connects with the spinal cord (brain stem). MRI can identify small areas of damage to the brain better than a CT scan. An MRI seems to be more accurate in detecting strokes caused by clots (ischemic strokes) during the first 3 days after a stroke, but the test is less accurate if it is done in the first 24 hours after symptoms first begin. A CT scan is sometimes preferable in the acute setting as detailed information regarding vessel anatomy and brain perfusion is rapidly available with this technology.
Electrical Activity Tests
Electrical activity tests record the electrical impulses of the brain. These tests include an electroencephalogram (EEG) and evoked response tests. In an EEG, electrodes are put on a person’s scalp to pick up electrical impulses, which are printed out as brain waves. An evoked response test measures how the brain handles different sensory information, using electrodes that record electrical impulses related to hearing, body sensation or vision.
Blood Flow Tests
Blood flow tests can reveal problems in the flow of blood to the brain, normally through the use of ultrasound technology. During these tests, a probe is placed over the artery in question – usually the arteries of the neck or at the base of the skull – and the amount of blood flow is measured. Such tests include B-mode imaging, Doppler testing and duplex scanning, which gives detailed information about the condition of arteries.
Angiography (also known as arteriography) is another type of blood flow test. In this, special dyes are injected into the blood vessels and an X-ray is taken. Angiography gives a picture of the blood flow through the vessels. This allows the size and location of blockages to be evaluated. This test can be especially valuable in diagnosing aneurysms and malformed blood vessels and providing information before surgery.
Carotid angiography is the best test available to identify and measure the blockage in the carotid arteries of the neck. It is usually done after a carotid ultrasound has shown that there probably is a blockage in the artery and if surgery (endarterectomy) is being considered to remove the blockage and reopen the artery. In this test, a tiny tube (catheter) is inserted into an artery (often in the arm) and threaded through other blood vessels to reach the carotid artery. A dye is then injected through the tube and into the artery. The dye outlines the blood vessel and X-rays are taken to evaluate the degree of narrowing and the condition of a plaque. If a plaque is rough, clots are more likely to form in the blood vessel. When the dye is injected, some people feel a burning sensation in the face and head, a brief headache, flushed on one side of the face or nauseous. The test usually takes from one to three hours, and for about six hours after the test, the place where the tube was inserted is carefully watched for bleeding. The patient may be given a drug to help relax during the test.
Cerebral angiography uses the same technique to study the arteries of the brain. It is usually done at the same time as carotid arteriography to evaluate blood flow through the brain. The results will help decide whether surgery to reopen a blocked artery (carotid endarterectomy) is appropriate. Angiography carries with it the risk that the procedure itself may cause a piece of plaque to break away and travel through the blood to the brain, causing a stroke during the procedure.
Transcranial Doppler (TCD) is an ultrasound study that allows physicians to measure the velocity at which blood is moving through the arteries of the brain. This important real-time information provides valuable insight into the adequacy of blood flow in the brain as well as the patterns of flow. Other important information that can be obtained with this procedure includes the arteries ability to adapt to higher demand for oxygen, whether tiny blood clots are entering the arteries in the brain, and whether a small hole in the heart is allowing a mixture of venous blood into the arterial circulation. TCD is commonly used to aid physicians in the diagnosis and management of diseases of the blood vessels in the brain including atherosclerotic disease, embolic events (an object moving through the blood vessels and gets lodged downstream blocking blood flow to that vessel), and vasospasm (a narrowing of the artery caused by squeezing of the vessel’s muscle). TCD is an outstanding method for evaluating the brain’s blood circulation. This diagnostic test offers the advantages of high sensitivity and specificity, non-invasive, does not use contrast agents (iodine dye), does not exposure the patient to ionizing radiation (X-rays), is available at the bedside in both inpatient and outpatient settings, is painless, and is relatively inexpensive as compared to other tests yielding similar information.
The evolution of TCD equipment has enabled the test to be highly portable and allows for the convenience of having the technologist perform the test at the patient’s bedside or in an outpatient setting. During a TCD test the patient will be asked to lie down on an exam table for approximately 30-45 minutes while the technologist uses an ultrasound transducer to acquire the necessary information. There is no requirement for fasting or other patient preparation for a TCD exam and all daily medicines can be taken in their normal routine.
We are pleased to offer this important and valuable diagnostic test in the Shands at UF Stroke Program. This test, as well as our other resources and expert personnel are here to assist you in primary and secondary prevention of stroke and cerebrovascular disease, as well as rehabilitation, education, and patient support.
For more information on TCD, please check out:
American Academy of Neurology TCD Assessment
“Neurosonology.” Garami and Alexandrov, Neurol. Clin. 27 (2008) 89-108
Duplex scans are a sensitive form of ultrasound done of the neck when narrowing of the carotid arteries due to plaque buildup is suspected. It is often the first test used when you are being evaluated for surgery to reopen a blocked artery (carotid endarterectomy). In carotid artery ultrasound scanning, high-pitched sound waves are bounced off the blood vessels and tissues of the neck to create an image of the arteries. Duplex scanning, which is a newer technique than traditional carotid artery ultrasound and now used more often, is able to measure blood flow at many points in the blood vessel at one time. It is used more often than older carotid ultrasound methods.
During duplex scanning, an instrument is moved over both sides of the neck. The resulting two-dimensional picture shows clearly the amount of blockage in the artery. This method also shows color pictures that indicate how fast blood is flowing in any point in the blood vessel. Although carotid ultrasonography is quicker, safer, less painful and less expensive than carotid arteriography, it may not always be as accurate in determining the amount of blockage of blood flow as some other tests. However, carotid ultrasonography is often the first test used and can be used to decide if further tests are needed. Using similar technology, transcranial Doppler imaging is an ultrasound of the head.
Magnetic Resonance Angiography
Magnetic resonance angiography (MRA) is a form of MRI that can measure blood flow through blood vessels. The test uses a strong magnetic field and radio signals to create pictures of the blood flow through blood vessels. With an MRA, both the blood flow inside of the vessel and the condition of the blood vessel walls can be seen. An MRA takes pictures quickly that can be seen individually or together as a three-dimensional picture.
- An MRA is often used to determine if narrowing of blood vessels (especially the carotid arteries), abnormally formed blood vessels or an aneurysm is present.
- MRAs are relatively safe and easy to perform, and they cost less than some other tests. People with pacemakers or certain metal implants cannot have an MRA done. Pregnant women should not have an MRA done.
- The pictures of the carotid arteries that are produced by an MRA are not as clear as those produced using carotid arteriography.
- MRA is no more sensitive than carotid ultrasonography/duplex scanning, but it is more expensive.
- MRAs do not produce clear pictures when the blood flow through the vessel is very rapid or when the vessel has severe narrowing.
- Holes (ulcerations) within plaque may not always be seen with an MRA.
Echocardiography (ECHO) is a sophisticated type of blood flow test that uses high-pitched sound waves to produce an image of the heart. The sound waves are sent through a device called a transducer and are reflected off the various structures of the heart. These echoes are converted into pictures of the heart that can be viewed on a monitor similar to a TV screen. An echocardiogram is used to evaluate how well the heart chambers fill with blood and pump blood to the rest of the body. ECHO can also be used to estimate the amount of blood pumped out of the left ventricle with each heartbeat (called the ejection fraction).
An ECHO can help evaluate heart size and heart valve function, identifying areas of poor blood flow in the heart, areas of heart muscle that are not contracting normally, previous injury to the heart muscle caused by impaired blood flow or evidence of congestive heart failure, especially in people with chest pain or a possible heart attack. In addition, ECHO can identify some heart defects that have been present since birth (congenital heart defects).
There are several different types of echocardiograms:
- Transthoracic echocardiogram (TTE). This is the standard, most commonly used method of echocardiography. Views of the heart are created by moving the transducer to different places on the chest or abdomen wall.
- Transesophageal echocardiogram (TEE). This is a special type of test in which the instrument that emits sound waves (transducer) is passed down the esophagus instead of being moved over the outside of the chest wall. A TEE may show clearer pictures of the heart because the transducer is located closer to the heart and the lungs and bones of the chest wall do not block the sound waves. A TEE requires a sedative and anesthetic applied to the throat to ease discomfort.
Another diagnostic test is the lumbar puncture (spinal tap), in which a needle is inserted into the spinal canal to collect samples of the clear, fluid that surrounds the brain and spinal cord. The pressure of this fluid is measured, and the samples are analyzed for color, blood cell counts, protein, glucose and other substances. Some of the fluid may be placed under conditions that promote the growth of infectious organisms (cultured), such as bacteria or fungi, to check for infection.