Doppler Ultrasonography Health Article

Definition

Doppler ultrasonography, also called Doppler ultrasound, is a noninvasive diagnostic procedure that uses an ultrasound scanner to convert sound waves into images of blood flow in body tissue and organs. Doppler ultrasonography does not use ionizing radiation and is used for a variety of clinical applications.

Purpose

Doppler ultrasonography is used during an ultra-sound examination to assess the direction, velocity, and turbulence of blood flow. It is frequently used in cardiac and vascular scanning to evaluate blood flow and diagnose abnormalities in flow. Cardiac applications include the detection of heart valve problems, the determination of arterial vessel narrowing (stenosis) or blockage, the diagnosis of congenital cardiac defects, and the evaluation of damage following myocardial infarction (heart attack). Vascular applications include the work-up of stroke patients, the assessment of blood flow in the major abdominal arteries, and the evaluation of vessels in the arms, legs, and neck. Vascular conditions that can be diagnosed using Doppler ultrasonography include deep vein thrombosis (DVT), a blocked carotid artery, blood clots, tumors with vascular involvement, and abdominal aortic aneurysm. Doppler ultasonography can also be used to determine whether a patient is a candidate for a surgical or other interventional procedure, such as vascular grafting, or it can be used during and after cardiac and

vascular surgical procedures to assess blood flow and the success of the procedure. In obstetric ultrasound, Doppler ultrasonography is used to check fetal cardiac activity.

Doppler ultrasonography can be performed in a hospital radiology or cardiology department, a hospital vascular laboratory, at the patient's bedside, in the emergency department, in an operating room, or in an outpatient imaging center, depending on the patient's medical condition. Doppler ultrasonography is a noninvasive, safer, and faster alternative to x-ray angiography, which involves radiation exposure, the injection of a contrast dye, and catheterization of blood vessels, although ultrasound may not yield images that are as detailed as those from x rays during catheterization.

A new device introduced in 2000 combines Doppler ultrasonography with endoscopy. The Doppler ultrasound scanner is interfaced with an endoscopy system, and the Doppler ultrasound images can be simultaneously displayed with the endoscopic images on one monitor, like a picture-in-picture display. Clinical applications include the evaluation of ulcers and hemorrhaging, vascular abnormalities of the intestinal tract, and enlarged vessels in the digestive tract, as well as assessment during endoscopic surgical procedures.

Precautions

The test is widely used because it is noninvasive, uses no x rays, and gives excellent images. It is harmless, painless, and widely available.

Because smoking can cause constriction of blood vessels, patients should not smoke before an ultrasound examination of the blood vessels.

Description

Doppler ultrasonography is performed using an ultrasound scanner with Doppler imaging capabilities; most scanners used for general-purpose abdominal, cardiac, and vascular scanning are equipped with Doppler. Ultrasonography involves the use of sound waves above the level of human hearing, and works similarly to sonar or radar. Sound waves are transmitted through the body and echoed back to produce an image of the area of interest. Ultrasound waves used for diagnostic imaging are typically in the range of 2 to 10 megahertz (MHz).

Doppler ultrasonography 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. With regard to medical ultrasound, the Doppler principle states that sound waves increase in frequency when they echo from objects (in this case, red blood cells) moving toward the transducer and decrease in frequency when they echo from objects moving away from it. This change in frequency, which is related to the velocity of the moving red blood cells, is then measured and used to determine blood flow velocity. Therefore, Doppler imaging allows the frequency of the speed of blood flow to be calculated relative to a computer marker placed by the sonographer.

There are several different modes of Doppler ultrasound. Most ultrasound scanners include both continuous-and pulsed-wave Doppler. Continuous-wave Doppler is the simplest mode, and is commonly used in cardiac studies for blood flow analysis. This mode receives flow information from all the moving reflectors in the path of the beam and can provide maximum velocity through the target area. Pulsed-wave Doppler allows the operator to select the area of interest for flow analysis using cursors superimposed on the 2-D image. Depth-selective information is obtained by acoustic pulses emitted from the transducer, allowing the precise location of the target area, as well as the flow, to be determined. Most ultrasound scanners also have color Doppler imaging capability, which superimposes color over moving structures on the gray-scale images. For example, red and yellow in a blood vessel image indicate flow away from the probe, while blue and green indicate flow toward the probe. Color Doppler imaging can be used to identify areas of arterial narrowing. Another Doppler feature is power Doppler, which is more sensitive than color Doppler imaging and can produce images of structures not normally able to be depicted with ultrasound, for example, inflammation or signs of congenital heart disease in a fetus. Power Doppler mode may only be included on advanced ultrasound scanners.

During an ultrasound examination, the patient is positioned on a bed or table so the area to be imaged can be easily accessed. An acoustic coupling gel, a special gel that enhances the transmission of ultrasound waves, is spread on the skin over the area of interest. A handheld ultrasound probe with a transducer (a crystal that transmits and receives the sound waves) is placed on the skin and positioned appropriately to acquire images of the blood vessels. Usually gray-scale images, which 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) are acquired first, and then the Doppler mode is selected to acquire Doppler images that are superimposed over the gray-scale images. The sonographer is able to use the scanner's computer to mark areas and calculate parameters of interest, such as blood flow velocity in vessels with narrowing or blockage. Ultrasound scanners are usually equipped with a videotape recorder or digital image acquisition system to record the Doppler examination, as well as a medical image printer for hard copies of still images.