Ultrasound Doppler explained for vets

The Doppler Effect is the physical characteristic of the behaviour of ultrasound waves.

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Ultrasound Doppler explained for vets

Q. Peter, lots of small animal vets that may be new to ultrasound ask you if a scanner has Doppler. So what is Doppler?

A. Doppler is used to measure movement in the animal such as blood flow. The Doppler Effect is the physical characteristic of the behaviour of ultrasound waves. If an ultrasound wave strikes a moving object, the resultant echo is of a different frequency.  If a wave strikes a static object, the wave comes back at the same frequency. This is called Doppler shift and this shift can be expressed as colour by an ultrasound machine.

I should probably explain more about the two different types of Doppler – Colour and Spectral Doppler. Basically, with Colour Doppler you can find things, whereas with spectral you can actually measure the flow.

 

Colour Doppler

Colour Doppler expresses the Doppler Shift as colour on your ultrasound screen and gives you an idea of flow velocity.

Directional Colour Doppler expresses the direction of flow in different colours and intensity of colours. It is used widely in cardiology and for vascular studies.

Canine RPLA MV Colour Doppler with a Sonosite M-Turbo ultrasound scanner

 

Colour Power Doppler expresses the intensity of flow with different shades of one colour. You can often choose your own colour maps, so the colours used are a little bit up to you. Colour Power Doppler is a useful tool for highlighting low velocity blood flow and can be used to help identify adrenal glands or understand blood flow in a mass.

Canine kidney with Power Doppler using Sonosite M-Turbo

 

Spectral Doppler

This is where the speed of the flow is being measured and you can get a numerical value.
There are two types of Spectral Doppler:
Pulsed Wave (PW) and Continuous Wave (CW)

Pulsed Wave (PW)

A single pulse of ultrasound is emitted. The echoes received from a single point are received and analysed and plotted against time. The probe will not emit a second pulse until the echo from the first pulse has been received. This does limit the depth at which PW can be used and the velocities that can be measured.

You will get a trace on the screen. This is used to measure velocity at a specific point, such as over a heart valve or in systems with lower velocity flows. Usually, the size of the sample area can be adjusted, but the smaller the sample area the greater the accuracy of the measurement. 

Continuous Wave

An ultrasound beam is fired continuously from the probe and all the echoes received are plotted on an axis against time.

As Continuous Wave is not limited to the probe receiving one echo at a time, there is less of a limit on the distance you can use from the probe . You are also less limited on the velocities  you can measure, and you are able to measure higher velocities such as in the diagnosis of aortic stenosis.

Spectral Doppler CW and PW can also give us information about the nature of any flow of fluid – such as if it is turbulent of laminar.
 

Q. So what probes do I need for Doppler?

A. Colour Doppler can be used with linear, micro-convex, convex probes and phased array probes.
Pulsed Wave can often be used with a linear or micro-convex or convex probe along with a phased array probe.

    
Micro-convex probe

 

 
Linear probe

 

 
Phased array probe

 

However, Continuous Wave can usually only be used with phased array probes and requires an extremely high frame rate.

 

Q. What is frame rate?

A. This is the speed at which the screen refreshes. With cardiac work, as you are measuring movement, it is important that the screen view changes very quickly, which means having a high frame rate.

 

Q. Is there anything else a vet should consider with Doppler?

A. Doppler is angle dependant. Ideally, you should fire ultrasound so that it is in-line with the flow that you are measuring, so that you get the best reflection from the region of interest. So when making measurements with spectral Doppler it is impossible to overestimate velocity. However, it is possible to underestimate it if not in-line with your flow.
This means that you do need good technique in getting your standard views when scanning in B Mode.

 

Q. Are there any changes that you have seen recently with vets using Doppler?

A. Over the past couple of years I have seen more equine vets requesting Doppler for use in equine musculoskeletal and fertility scanning. For musculoskeletal, blood flow around injuries can be seen and the progression of healing tracked.
For equine fertility work, blood flow around ovarian structures can be measured. Before ovulation it may be possible to see an increase in blood flow. Doppler is also being used for looking at foetal blood flow and viability and of course in cardiac exams on horses. 

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