Colour Doppler

Colour doppler is used to detect flow, to determine the direction of flow and to a lesser degree the speed of flow.  The machine does this by reading the doppler shift frequency. When an US wave hits a moving object, there is a small shift in the frequency of the returning echoes. The change in frequency is then translated by the machine to colour. 

This is detemined by the doppler equation: 

The Doppler Equation. 

∆ƒ = doppler shift frequency (MHz)

ƒ0 = Original transducer frequency (MHz)

v = velocity of the moving red cells (m/s)

cos(θ) = cosin of the insonating angle.

c = speed of sound (m/s)

You don't need to know this equation by heart. But the important elements are: 

1. the machine detects the change in frequency of the returning echoes. This is called the  doppler shift frequency.

2. One of the multipliers is cosin of the angle of insonation. Thus, the direction of flow compared to the direction of the beam will determine if flow is detected.

ANGLE OF INSONATION

For the machine to detect the direction of flow, the angle of insonation needs to be <60º.

Cosin of 90º is zero (see table below). So at 90º, the machine won't be able to detect movement. It won't place colour in that area, even though there is perfectly good flow within the vessel, or regurgitation from the valve. The smallest error in direction and velocity is made when the angle is 0-60. The more parallel the direction of flow to the beam of the US, the more accurate the doppler shift measured by the machine. 

This is the reason when looking for valvular regurgitation, you need to go hunting for the jet in lots of different windows. Eg MR jet from MV prolapse is often hidden in PLAx because the angle is close to 90º, it better seen and more parallel in A4C.

FRAME RATE

On POCUS machines, the frame rate is usually set by the machine. The frame rate is the number of times per second the machine samples the returning echoes. If the frame rate is low, the image is laggy and stilted. 

The size of the colour box affects frame rate. A wide colour box leads to a slower frame rate because the machine needs to calculate more information. Interestingly a long colour box doesn't do this. 

So make the colour box as narrow as possible to get the information you need.

NYQUIST LIMIT

The Nyquist limit is the positive and negative numbers on either end of the colour bar. It represents the threshold limits for the speed of flow towards (positive) or away (negative) from the transducer which can be accurately depicted before the machine gets confused. Usually, you don't need to change this setting. If you are in the correct modality setting, the Nyquist limit will be set automatically by the machine. For instance, in the echo setting, the Nyquist limit is typically 60-70cm/s or in the vascular/ MSK/ abdominal setting where you will be imaging vessels, the limit will be 30-40cm/s. 

In this way, the colour represented on screen also gives you an idea of the speed and direction of flow. If the image shows colour at the very ends of the spectrum, then the speed of flow is approaching the Nyquist limit which has been set. 

THE COLOUR SPECTRUM

You can set the colour spectrum on the machine to suit your preference. The traditional colours are red transitioning to blue. The colour at the top of the screen reflects 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. 

In this way, the jet of colour see gives you an idea of the direction of flow. 

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

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)

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. 

A4C: the Nyquist limit has been set quite low so there is a lot of yellow in the colour box

GAIN

Increasing the gain when in the colour doppler setting, increases the machine sensitivity to movement. This can make the colour artefactually bleed outside vessels and cardiac chambers. I usually increase the gain until this happens (speckle) and then decrease it until the colour is confined within walls of vessels. That is then the optimum gain. 

way too much gain!

slight amount of colour bleed: probably acceptable

TIPS AND TRICKS

1. Always get a good B mode (black and white) image first. A deteriorated B mode image will give you poor colour.

poor B mode image, poor colour

2. Ensure your angle of insonation is as parallel to the flow as possible

3. Ensure the Nyquist limit is correct

4. Increase the colour gain to speckle and then decrease it to the optimum level. 

5. Make the colour box as narrow as possible. 

One final thing, if the blood flow is a very low velocity, power doppler can detect flow. But this is very high intensity and can heat tissues quickly.