Color flow Doppler (ultrasound)

The use of colour flow Doppler (CFD) or colour Doppler imaging (CDI) (or simply colour Doppler) sonography allows the visualisation of flow direction and velocity within a user defined area. A region of interest is defined by the sonographer, and the Doppler shifts of returning ultrasound waves within are colour-coded based on average velocity and direction.

Physics

In a manner similar to pulsed wave Doppler (PWD), colour flow Doppler utilises intermittent sampling of ultrasound waves thereby avoiding the range ambiguity characteristic of continuous wave Doppler (CWD). Pulsed wave Doppler, however, is limited to the interrogation of flow velocity and direction along a single line at a certain depth (defined by the sample volume or gate); Colour flow Doppler simultaneously interrogates multiple sample volumes (with each pixel representing a sample volume) along an array of scan lines ¹.

Information regarding the flow velocity and direction is arbitrarily colour-coded and rendered onto a grey-scale (or M-mode) image. Flow that travels away from the transducer (negative Doppler shift) is depicted in blue, and flow that is travelling toward the transducer (positive Doppler shift) is depicted in red, with lighter shades of each colour denoting higher velocities. A third colour, usually green or yellow, is often used to denote areas of high flow turbulence. These colours are user-definable and may be reversed, however this is generally inadvisable as it may confuse later readers of the images.

When the Nyquist limit is reached, however, aliasing results in apparent flow direction occurring in the opposite direction to actual flow, a limitation shared with pulsed wave Doppler. The velocity at which flow aliases is further impacted by the transducer used and the depth at which one is insonating flow.

Clinical use

Colour flow Doppler is used frequently in sonography to semiquantitatesemiquantitative overall blood flow to a region of interest. Depiction of the general velocity and direction of blood flow within the heart and blood vessels is of primary importance in echocardiography and vascular ultrasound respectively. It also allows the generation of unique phenomena such as the fluid colour sign or the twinkling artifact and allows the targeting of spectral Doppler for a quantitative assessment of blood flow ².

Techniques

There are several general parameters that needsneed to be taken into account to obtain an optimum colour flow Doppler scan: types of transducers, location and number of focal zones, depth of field, 2D gain setting, scan orientation, and image zoom, Specific colour flow Doppler settings such as colour and spectral parameters should also be optimised.⁵

HighA high 2D gain setting would suppress the colour flow data while a low gain setting will highlight the colour Doppler information.⁵

Increasing the depth of field would slow the frame rate while reducing the depth of field would increase the frame rate.⁵

Using multiple focal zones would reduce the frame rate, thus single focal zone is preferred for the colour Doppler study to improve the frame rate.⁵

Setting a very high colour velocity scale would show an apparent absence of flow, while setting a very low colour velocity scale would show aliasing of the flow signals.⁵

History and etymology

Named after Austrian physicist, Christian Andreas Doppler (1803-1853) ⁴.