Transcranial Doppler Ultrasonography Health Article

Definition

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.

Purpose

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.

Precautions

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.