Transcranial Doppler Ultrasonography
Transcranial Doppler Ultrasonography
Transcranial Doppler ultrasonography (TCD) is a noninvasive method of evaluating cerebrovascular blood flow (CBF), the flow of blood in the vessels of the brain. The TCD technology allows changes in the rate of blood flow (velocity) over time to be easily followed, documented, and analyzed. Ultrasonography (ultrasound) is a diagnostic imaging technology that directs high-frequency sound waves into the body, where they either bounce off or pass through body tissues and fluids. Echoes from the tissues and fluids return to the ultrasound machine, where changes in pitch and direction are instantly measured and displayed on a monitor as a picture (image) of the tissue or body organ being scanned. Doppler ultrasonography measures what is called the Doppler effect, the frequency change that occurs when ultrasound is directed toward blood vessels and reflected back to the source. Unlike reflected ultrasound signals that are received as an image, reflected Doppler waves make an audible sound that corresponds to the heart beat.
The Doppler principle is a wave theory first described by an Austrian physicist, Christian Doppler, in 1842. It relates to the velocity of objects and wave frequencies either transmitted or received by these objects. In Doppler ultrasound, the rate and direction of blood flow in the vessels can be determined by the frequency of the reflected sound, which indicates the rate of blood flow in the reflecting vessel (blood vessel sending back the sound waves). While Doppler ultrasound has been in use since 1965 to monitor fetal heart rates and blood flow in the carotid artery in the neck, it has only been in use since 1981 to measure blood flow velocity in the arteries of the head.
TCD has proven to be a safe, fast, and reliable procedure for measuring the rate of CBF, especially as an assessment of risk for stroke. Individuals at risk for stroke usually have high blood velocities in the vessels of the brain. The rates of flow can be up to three or four times normal. Restrictions in blood flow may occur with the narrowing of blood vessels (stenosis), clot formation (thrombosis), blockage of blood vessels (embolism), or blood vessel rupture (hemorrhage). Lack of sufficient blood flow (ischemia) threatens brain tissue and may cause a stroke or other types of brain damage.
While ultrasonography typically receives inaudible echoes from tissues or organs and displays them as images, TCD measures changes in the frequency of transmitted waves, which are received as audible sounds. Just as a siren's pitch sounds higher when its source is moving toward the listener and lower as it moves away, so will ultrasound waves change pitch, or frequency, as they bounce off the blood flow in veins and arteries. Faster blood flow causes a greater change in frequency. These frequency shifts can be used to measure both the direction and the speed of blood flow in even the smallest of blood vessels.
Combined with other tests, this information can be used to locate restrictions in the blood vessels in the brain, and to track changes in blood flow over time. Ultrasound images can also be produced by the TCD equipment (as in ultrasound exams that view other body tissues or organs) from the reflected sound so that a vascular lesion (site of damage, blockage, or blood clot) can be found and examined. In this way, TCD can offer valuable information about the location of blockage or a clot that has caused a stroke and can help monitor the patient's response to therapy after a stroke. TCD is also used to evaluate the contraction of blood vessels that may occur if a blood vessel ruptures. Besides helping to diagnose stroke, TCD is used to evaluate brain death, head injury, abnormalities in veins and arteries, detection of blockage or rupture of vessels, and in surgical procedures such as heart bypass surgery or procedures requiring anesthesia.
Ultrasonography procedures, including TCD, are noninvasive and painless. They are considered to be safe procedures with no known side effects. There are no special precautions.
A TCD machine is an ultrasound scanner with Doppler capability. It is usually portable and is easy to set up in an examining room or at the patient's bedside. The first step in a TCD exam is to find an ideal location on the head (called an accoustic window) where the ultrasound beam can pass through the skull and allow the best transmission of sound waves. Because bone absorbs sound waves, areas where the bone is thinner are best for TCD exams. Children have thinner bones, and it is possible to obtain good signals from a large area of the head. The elderly have thicker bones, making it more difficult to obtain a good evaluation of blood flow velocity.
TCD is done with probes called transducers, which transmit and receive the ultrasound signals. These probes are placed against the skin of the head at the selected windows. The sonographer spreads a clear gel on the areas where a probe will be placed. Typical sites are the temple, the base of the skull at the back of the neck, and over the closed eyelid. These sites have the least amount of thick protective bone and will allow the best sound wave transmission. The sonographer adjusts the probe position and orientation to direct the sound waves toward the blood vessels of interest. Finding the best approach may take some time. A compression test may be performed during the exam. In this test, the main artery in the neck (carotid artery) is briefly compressed, and changes in blood flow patterns are observed. A full TCD exam may last 30 to 45 minutes, although a longer examination may be necessary in patients with known cerebrovascular disease.
No special preparation is needed. The patient should remove contact lenses, and may wish to avoid the use of eye makeup, since the gel is likely to smear it. For convenience and comfort during the procedure, the patient should wear loose, comfortable clothing and no earrings or hair ornaments.
The gel is washed off with soap and water. No other aftercare is needed.
TCD is noninvasive and has no notable complications. A compression test is occasionally, though very rarely, hazardous for a patient with narrowed arteries (atherosclerosis), since the increased pressure may dislodge a piece of the substance that causes the narrowing (plaque).
TCD ultrasonography calculates blood flow velocity, which, in turn, helps determine direction of flow and restrictions in flow. The sound being measured will vary depending on the direction and rate of flow through the vessel being examined. Each of the vessels in the brain has a characteristic direction of flow, which can be altered in various conditions. Flow rates are variable from person to person depending upon the condition of the vessels in the brain and the rate of blood flow from the heart. A normal result will correspond to typical flow rates and direction of flow for each of the brain's blood vessels. Blood flow velocity may be measured in several sites, after which a peak flow velocity and an average velocity will be calculated.
Diminished blood flow indicates that a vessel has been blocked to some extent. Lack of a signal may mean no blood flow due to complete blockage, although absence of a signal may also mean that sound waves have been absorbed by bone. If blood in a certain vessel flows in the wrong direction or alternates between normal and reverse flow, it may indicate a blockage elsewhere in the brain. This happens because blood is rerouted when a blockage causes differences in intracranial pressure.
An increased rate of flow may mean that blood is flowing through a restricted area just "upstream" from the probe. Although it seems that a restricted blood vessel would cause the speed of blood flow to slow down, the opposite is true. This is because the same amount of blood going through a narrower opening must go faster. Increased speed is also seen if a vessel is carrying rerouted blood.
Health care team roles
Ultrasound procedures, including TCD, are usually performed by a sonographer in an ultrasound or radiology department in a hospital or in a separate diagnostic imaging facility. When these procedures are performed during surgery, they may be performed by an anesthesiologist or other physician. The sonographer will explain the procedure to the patient, describing each step in a reassuring manner. A radiologist, who is a physician experienced in diagnostic imaging examinations, such as radiology (x ray) and ultrasound exams, will usually analyze the Doppler results and simultaneous images of the vessels examined. The testing physician will use the information to aid in diagnosis and treatment of the patient.
Sonographers are specifically trained to understand and use ultrasound equipment, including Doppler equipment, and to perform a broad range of ultrasound exams. They will have a good understanding of ultrasound electronics, of computer functions in the ultrasound scanning equipment, and they will be able to observe ultrasound images and interpret results, although they will neither diagnose nor advise patients.
Cerebrovascular— The blood vessels that make up the vascular system of the brain, including all veins and arteries that carry blood.
Doppler ultrasonography— Measures frequency changes that occurs when ultrasound signals are directed toward blood vessels and reflected back to their source. Transcranial Doppler ultrasonography (TCD) pertains to frequency changes measured in the blood vessles of the brain.
Frequency— The number of cycles of a wave over time, such the frequency of a sound wave.
Transcranial— Scanning through the skull.
Transducer— Also called a probe, a hand-held instrument that transmits and receives sound waves, which can then be measured by electronic equipment. In an ultrasound examination, a transducer is used to scan the body.
Ultrasonography— Also called ultrasound scanning or sonography; a safe, non-radiologic, noninvasive diagnostic imaging technology in which high frequency sound waves are bounced off or passed through body tissues to obtain a visual image of the tissue or body organs being evaluated.
Ultrasound image— Also called a scan or a sonogram; created on a computer monitor when high frequency sound waves are transmitted into the body and the resulting echoes are recaptured and displayed by the ultrasound system.
Samuels, M.A. and Feske, S. eds. Office Practice of Neurology. New York: Churchill Livingstone, 1996.
American Society of Neuroimaging. 5841 Cedar Lake Road, Ste. 204 Minneapolis, MN 55416. (952) 545-6291.
Society for Diagnostic Medical Sonographers (SDMS). 12770 Coit Road, Ste. 708, Dallas, TX 75251. (800) 229-9506.