Doppler Studies Overview

• Doppler technology is a useful adjunct to USS

• Technological advances have increased its utility in the realtime investigation

• The Doppler effect is a physical phenomenon which demonstrates a change in the frequency of a signal due to the relative movement of the signal source compared to the observer

• The Doppler signal is usually in the audible range

• Application of an ultrasound beam will allow the measurement of flow direction and velocity of any structure which moves (e.g., flow of RBC in a blood vessel)

• Simple continuous-wave Doppler can detect velocity but not depth of the signal source

• Pulsed Doppler, on the other hand, uses intermittent bursts of ultrasound and is capable of assessing depth as well as velocity and can adequately display the flow characteristics at a desired depth

The flow, direction, pulsatile rhythm, and resistivity of flow can be displayed either—

• Graphically (spectral trace)

• By color (color Doppler imaging)

Color Doppler imaging (CDI) is currently the technique of choice for the evaluation of blood flow and directional information in vessels and works by superimposing color over the conventional gray-scale image. Red demonstrates blood flow toward the transducer, while blue demonstrates flow away. CDI, however, is angle dependent and lacks sensitivity when imaging small, deep blood vessels or areas of slow blood flow.

Power Doppler imaging attempts to overcome the limitations of CDI, by displaying the amplitude (or strength) of the Doppler signal rather than simply the change in frequency. Power Doppler imaging works by detecting the density and movement of red blood cells and as such is independent of blood flow velocity and direction. It is 3-5 times more sensitive in the study of small, deep, slow-flowing blood vessels.

Dupplex scans combine real-time anatomic scanning with pulsed Doppler hemodynamic information and permits blood velocity estimation at different sites within a two-dimensional field.

In addition, recent reports using intravascular contrast agents which enhance the Doppler signal by the production of micro-bubbles have shown promising results in increasing sensitivity and reducing motion artifact. Advances in ultrasound and transducer technology, though mostly experimental, are enabling higher-resolution imaging with 3D reconstruction and image manipulation of anatomical structures. Thus, in spite of significant advances in CT and MRI technology, Doppler-assisted ultrasound is likely to remain an invaluable tool in the armamentarium of urological investigative methods for some time yet.

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