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Physics, Techniques and Procedures

Colour Doppler sonography

ultrasound technique producing grey-scale B-mode images with superimposed colours indicating blood-flow velocity and direction (Fig. 1). Unlike pulsed Doppler ultrasound techniques, which acquire Doppler signals at restricted predetermined depths only, colour Doppler sonography acquires Doppler information at multiple locations along each scan line, i.e. at each position of the ultrasound beam during scanning. A commonly used method for measuring blood-flow velocity in colour Doppler sonography is autocorrelation, which involves repeating the pulse - echo-sequence several times (typically 4–8) along the same scan line, and comparing the phase of the echo signal at each depth from one pulse - echo sequence to the next. For stationary reflectors, the phase is the same from one echo to the next. For moving reflectors, like red blood cells, the phase of the signal will vary from echo to echo according to the flow velocity and direction. The autocorrelation technique estimates the mean velocity and variance at each depth location and places this information in a colour image memory, a process which provides data for a single scan line. The ultrasound beam is then moved to the next scan line position, and the procedure repeated. In the final image, each pixel containing flow information is colour-coded according to blood flow direction and mean velocity. To obtain more exact flow information such as Doppler wave form, maximum velocity, spectral broadening, resistance index etc., colour Doppler sonography must be combined with pulsed Doppler sonography. Since colour Doppler sonography requires multiple pulse - echo sequences at each scan line, the scanning frame rate is lower than in standard B-mode imaging. To improve time resolution, the colour-coded field may be restricted to only a part of the entire image (rectangle in Fig. 1).

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Fig.1

Transverse B-mode image of the neck showing cranial flow in the right common carotid artery (red colour) and caudal flow in the right internal jugular vein (blue colour).
Colour Doppler sonography, Fig.1