What is a colour Doppler test?

What is a colour Doppler test?

In Cardiology, colour Doppler would mean colour Doppler echocardiography. Colour Doppler is used in ultrasound imaging in general, to assess blood flow to the various organs by Doppler principles. Echocardiography is ultrasound imaging of the heart. The principle of Doppler is that the frequency of sound wave coming from an object which is moving towards the ultrasound probe increases while that from an object moving away from the probe decreases. It is similar to the difference in the sound of a train whistle, which is different when the train is moving towards you than when it is moving away from you. In Doppler echocardiography, the target is moving blood cells.

Colour Doppler echocardiography receives the ultrasound signals reflected from moving red blood cells in the heart. Ultrasound signals are emitted and received by the ultrasound probe used for echocardiography. By notation, flow of blood away from the probe is depicted as blue and flow towards the probe is depicted as red. When the blood flow velocity increases in abnormal situations, the flow becomes turbulent. A turbulent flow will be shown as a mosaic colour which is a mixture of the basic colour of the flow with shades of yellow added to it. All high velocity flows across the diseased valves will be shown as mosaic jets.

Colour Doppler is very useful in giving a quick visual assessment of leaks and obstructions across valves. It will also show abnormal flows as in a defect between the walls of the chambers. Mosaic coloured jet on colour Doppler is easily seen. For example, when the defect between the two lower chambers (VSD) is small, the pressure difference between the right and left ventricles is high. This produces a turbulent high velocity jet which can be easily seen on colour Doppler. But it is difficult to see a small defect by echocardiography without Doppler.

Narrowing of valves can be made out by the reduced opening at the time when it is supposed to open. There will be associated thickening of the valve as well. Doppler echocardiography will show a high velocity mosaic jet across the narrowed valve. But in case of a leaky valve, it is often difficult to make out a small opening between the valve leaflets when it is supposed to be fully closed. In that situation, Doppler echocardiography will show a high velocity mosaic jet across it, which is easily seen.

The extent to which the colour Doppler jet extends behind the leaky valve is an important indicator of the severity of the leak. The area of the mosaic jet into the chamber preceding the valve is another important indicator of the severity of the leak. The size of the leaky orifice in the valve (regurgitant orifice) can be assessed by the width of the mosaic jet on color Doppler at its origin between the valve leaflets. Sometimes a perforation in the valve leaflet may be causing the leak. This can also be seen as a mosaic jet originating away from the region where the valve leaflets meet, in the belly or base of the valve.