Doppler ultrasonography is medical ultrasonography that employs the Doppler effect to generate imaging of the movement of tissues and body fluids, and their relative velocity to the probe. By calculating the frequency shift of a particular sample volume, for example, flow in an artery or a jet of blood flow over a heart valve, its speed and direction can be determined and visualized. Color Doppler or color flow Doppler is the presentation of the velocity by color scale. Color Doppler images are generally combined with grayscale images to display duplex ultrasonography images, allowing for simultaneous visualization of the anatomy of the area. This is particularly useful in cardiovascular studies and essential in many areas such as determining reverse blood flow in the liver vasculature in portal hypertension.
Operation
The Doppler data is displayed graphically using spectral Doppler, or as an image using color Doppler or power Doppler. This Doppler shift falls in the audible range and is often presented audibly using stereo speakers: this produces a very distinctive, although synthetic, pulsating sound. All modern ultrasound scanners use pulsed Doppler to measure velocity. Pulsed wave instruments transmit and receive series of pulses. The frequency shift of each pulse is ignored, however the relative phase changes of the pulses are used to obtain the frequency shift. The major advantage of pulsed wave Doppler over continuous wave is that distance information is obtained and gain correction is applied. The disadvantage of pulsed Doppler is that the measurements can suffer from aliasing. The terms Doppler ultrasound and Doppler sonography have been accepted to apply to both pulsed and continuous Doppler systems, despite the different mechanisms by which the velocity is measured. There are no standards for displaying color Doppler. Some laboratories show arteries as red and veins as blue, as medical illustrators usually show them, even though some vessels may have portions flowing toward and portions flowing away from the transducer. This results in the illogical appearance of a vessel being partly a vein and partly an artery. Other laboratories use red to indicate flow toward the transducer and blue away from the transducer. Still other laboratories display the Doppler color map in accordance with published data, with red shift representing longer wavelengths from blood flowing away from the transducer and blue representing the shorter wavelengths from blood flowing toward the transducer. Because of this confusion and lack of standards, the sonographer must understand the underlying physics of color Doppler and the physiology of normal and abnormal blood flow in the human body.
Vascular ultrasonography helps determine multiple factors within the circulatory system. It can evaluate central and peripheral arteries and veins, it helps determine the amount of vascular stenosis or occlusion within an artery, it assists in ruling out aneurysmal disease, and it is the main aid to rule out thrombotic events. Duplex is an inexpensive, non-invasive way to determine pathology. It is used in for example:
Duplex evaluation is usually done prior to any invasive testing or surgical procedure. Ultrasound duplex scanning can provide additional information that may guide therapeutic decisions. The location and severity of arterial narrowings and occlusions can be identified. The vascular sonographer can map disease in lower-extremity segments with great accuracy, though duplex scanning is more time-consuming than other lower-extremity arterial studies. An alternative to Doppler to visualize vessels is B-flow, which digitally highlights weak flow reflectors while suppressing the signals from the surrounding stationary tissue. It can visualize flowing blood and surrounding stationary tissues simultaneously. In addition to visualization, ultrasound can also be used in doppler auscultation, similar to using an ultrasound baby monitor, to locate clots or other vascular obstructions or collapse by tracing a blood vessel until sound is no longer heard. It is also used to conrfirm patency of Dorsalis Pedis arteries when edema or other conditions make manual palpation impractical.
Kidneys
Doppler ultrasonography is widely used in renal ultrasonography. Renal vessels are easily depicted by the color Doppler technique in order to evaluate perfusion. Applying spectral Doppler to the renal artery and selected interlobular arteries, peak systolic velocities, resistive index, and acceleration curves can be estimated .
Heart
is the use of Doppler ultrasonography to examine the heart. An echocardiogram can, within certain limits, produce an accurate assessment of the direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. One of the limitations is that the ultrasound beam should be as parallel to the blood flow as possible. Velocity measurements allow assessment of cardiac valve areas and function, any abnormal communications between the left and right side of the heart, any leaking of blood through the valves, calculation of the cardiac output and calculation of E/A ratio. Contrast-enhanced ultrasound using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements.
s, although usually not technically -graphy but rather sound-generating, use the Doppler effect to detect the fetal heartbeat for prenatal care. These are hand-held, and some models also display the heart rate in beats per minute. Use of this monitor is sometimes known as Doppler auscultation. The Doppler fetal monitor is commonly referred to simply as a Doppler or fetal Doppler. Doppler fetal monitors provide information about the fetus similar to that provided by a fetal stethoscope.
Superficial soft tissues
Doppler ultrasonography can help distinguishing benign from malignant soft tissue lumps.
