Vascular Sonography Health Article

Definition

Vascular sonography, also called vascular ultra-sound, is a diagnostic procedure that uses sound waves to produce images of the blood vessels and blood flow.

Purpose

Vascular sonography is used to evaluate blood flow in the arteries and veins. This test has many applications, including diagnosis of deep vein thrombosis (DVT), claudication, atherosclerosis, and congenital vascular malformations. In addition to its diagnostic capabilities, vascular sonography can be used to determine whether a patient is a good candidate for a vascular procedure such as angioplasty. It can also be used to evaluate the success of a surgical procedure such as bypass surgery or graft transplantation (adequate blood flow to the graft would indicate a successful graft transplantation). Furthermore, vascular sonography may be used to determine the blood flow to tumors and chronic wounds in order to aid in treatment planning.

Finally, vascular sonography can be used to identify blood clots and other blockages to blood flow. The test can reveal blood clots requiring anticoagulant therapy, blood clots that may embolize (travel) to other organs (including the lungs), and blockages to blood flow in the brain that might result in a stroke.

Vascular sonography is usually performed in a hospital's radiology department, or its vascular laboratory, which focuses on vascular imaging and evaluation. However, because vascular ultrasound units are portable, vascular sonography can be performed at the bedside of patients in the emergency room, or anywhere else in the hospital, if necessary.

Vascular sonography may be recommended by a primary care physician after detecting sounds of abnormal blood flow (usually via stethoscope). It may be performed in patients with suspected narrowing of the carotid arteries in the neck (who are at increased risk of stroke), or in patients with suspected abnormalities in the superficial blood vessels in the arms and legs. It may also be used to detect narrowing of the deeper abdominal vessels (such as the renal arteries and superior mesenteric artery), or to rule out bleeding in the abdomen following trauma.

Precautions

Because smoking can cause constriction of blood vessels, patients should not smoke before vascular sonography.

Description

Medical ultrasound scanning works in a manner similar to sonar and radar. Vascular sonography relies on ultrasonic sound waves transmitted at high frequencies (approximately 2 to 10 megahertz) beyond the level of human hearing. The sound waves are aimed at the area of interest. Depending on the tissue or liquid the sound waves encounter, different echoes return to the scanner. The scanner then interprets these echoes to produce an image.

Vascular sonography may be performed using a handheld portable ultrasound scanner or a larger mobile scanner, both of which have an ultrasound probe with a transducer, and a computer that processes the echoed sound waves into an image. Larger scanners are usually equipped with a videotape recorder or digital image acquisition system to record the examination, as well as a medical image printer for hard copies.

For vascular sonography, the patient is positioned on a bed or table so that the area to be imaged can be easily accessed. A special gel, called acoustic coupling gel, is placed on the skin over the area to be imaged to enhance the transmission of sound waves. The probe is held over the area of interest and occasionally moved. The ultra-sound probe's transducer emits sound waves that are transmitted through the body and reflected back as echoes, which are then converted into an image. The probe can be positioned along the vessel for a longitudinal scan or across the vessel for a transverse scan. During an abdominal or neck examination, the patient may be asked to hold the breath or stop swallowing. Examinations of the arms and legs may require that the limb be elevated or compressed.

Vascular ultrasound images can be acquired and displayed as gray-scale or Doppler images. Gray-scale images use different shades of gray to indicate differences in the strength of echoes; echoes from blood are of lower strength and appear darker than surrounding tissue. Gray-scale images can depict the layers of the vessel wall and show real-time arterial motion.

Doppler imaging uses the frequency shift caused by the Doppler effect to produce images of blood flow. The Doppler effect is a principle of physics involving light and sound; relative to an observer, the frequency of any light or sound wave will vary as the source of the wave approaches or moves away. Ultrasound scanners with Doppler imaging capability detect and calculate the changes in frequency of the speed of blood flow relative to a computer marker placed by the sonographer. Color Doppler imaging superimposes color over moving structures on the gray-scale images. For example, red and yellow in a blood vessel image indicates flow away from the probe, while blue and green indicates flow toward the probe. Color Doppler imaging can be used to identify areas of arterial narrowing.

When areas of vessel narrowing or obstruction are detected, the sonographer can use the ultrasound scanner to take measurements and make calculations. The scanner's computer allows cursors to be placed on areas to be measured and automatic measurements to be recorded.