Colour Doppler

Understanding the physics

Colour in ultrasound picks up movement. So if your hand moves or the patient moves, and you have the colour box open, you will see a flash of artefactual colour across the screen. In ultrasound, colour is mainly used to detect blood flow: eg vascularity, valvular regurgitation. 

THE SETTINGS

The machine settings are important for colour. To understand colour doppler, you need to get your head around a lot of physics. We will go through these step by step below. But once you understand it, colour and what it represents will make a lot of sense. 

Colour spectrum

Firstly, the spectrum of colour you choose. You can set this on the machine to suit your preference. The traditional colours are blue transitioning to red. The colour at the top of the screen reflect the representative colour when the movement is towards the US transducer. The colour at the inferior most end of the spectrum is the representative colour when the movement is away from the transducer.

PLAx: Still image of aortic regurgitation. Colour box top left - red = towards the probe, blue = away. Hence the AR which is moving away from the probe is presented as a blue jet (thick arrow)

When the machine finds it difficult to locate a specific jet because the blood is swirling in all directions or moving too fast - eg turbulence, it will usually present this as a mix of all the colours. Ie you will start to see yellows and light blues mixed with the deep reds and blues. 

PLAx: turbulent AR

But you can also choose combinations which show a particular colour for turbulence. 

A5C: Still image of aortic regurgitation. Colour box top left - red- yellow= towards the probe, blue = away. Hence the AR which is moving towards the probe is presented as a red jet (thick arrow)

In the above setting, the green colour to the right of the colour box represents the colour depicted when there is turbulence: ie when the blood in moving in all directions at once. 

A5C: turbulent MR

So you see, you can tell a lot about blood flow by training your eye to pick upp the colour patterns. There are some patterns that are classic, such as turbulence. Another is the swirling yin and yang pattern seen in an aneurysm. 

Yin and Yang pattern: Aorta long, swirling blood moving first towards the anterior abdomen (towards the probe), across the wall and then down towards the spine due to ineffective forwards flow in the aneurysm.