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Doppler ultrasound – how does it work?

Woman having ultrasound

The use of ultrasound to monitor the growth of a baby in the womb is well known but several other different types of ultrasound scan are now possible that can investigate problems with various parts of the body.

 

Like other ultrasound scans, Doppler ultrasound uses sound waves and their reflection to ‘see’ inside your body, in the same way that bats and submarines use sound to perceive their environment. These tests are particularly useful as they are non-invasive, which means they do not require any surgery or intrusion of the scanner into the body. Nor do they involve potentially harmful radiation, such as X-rays.

 

What is Doppler ultrasound?

Doppler ultrasound is a very specific branch of ultrasound scanning that enables technicians to see the flow of fluids – particularly blood – within the body. This technique enables your doctor to assess both the rate of blood flow, and which way blood is flowing, even in smaller, deeper veins and arteries.

 

How does Doppler ultrasound work?

Ultrasound works by bouncing sound waves off solid objects and using them to build a picture of that object. Most of us are familiar with ultrasound pictures of developing babies in the womb.

 

However, because Doppler ultrasound sound waves are bouncing off moving objects, their reflection is changed. If the object is moving away from the recorder, the sound wave will decrease in frequency, because the waves are being stretched away by the receding object. If the object is moving towards the recorder, then the sound waves will increase in frequency as they are squashed by the approaching object. The greater the increase or decrease in frequency, the faster the movement.

 

This is called the Doppler Effect and is most often observed when an ambulance or police car speeds past a stationary observer. The siren of the vehicle appears to drop in tone as it passes. This is because the sound waves are being stretched to a lower frequency as the vehicle moves away.

 

How is Doppler ultrasound used?

Doppler ultrasound uses the Doppler Effect to assess the speed and direction of blood flow around the body. This can be used to show where the flow is being blocked or restricted, and can indicate conditions such as narrowing or furring of the arteries, blood clots and deep vein thrombosis.

 

Being able to analyse blood flow through Doppler ultrasound can help in identifying and preventing a range of medical problems. For example, spotting blood clots early can allow time for treatment that can prevent major medical problems such as strokes and pulmonary embolisms (blood clots on the lungs), as well as heart attacks and other circulatory problems.

 

Doppler ultrasound can also prove useful during pregnancy to check the blood flow through the placenta, as it can show the flow of both the mother and the baby’s blood.

 

Are there different types of Doppler ultrasound?

Yes, there are, in fact, four types:

  • Hand held or ‘bedside’ Doppler ultrasound: as the name suggests, this is a simple and portable test which can be used to gather basic information about blood flow to identify possible damage or disease.
  • Duplex Doppler ultrasound: this is a more complex test that gives much more detail about the speed and direction of blood flow.
  • Colour Doppler ultrasound: this uses a computer to convert the readings from the scan into 3-d images of the vessels, colour coded to show the speed and direction of blood flow.
  • Power Doppler ultrasound: this is up to five times more sensitive than standard Doppler ultrasound and can be used to assess blood flow in vessels within solid organs, such as the liver.

 

What happens when you have Doppler ultrasound?

A Doppler ultrasound scan does not hurt; the only discomfort you might experience is the cold gel applied to your skin to aid the transmission of the sound waves. You will be asked to remove any jewellery or clothing that may interfere with the readings, and then lie still and relax on a treatment couch. A small amount of gel will then be applied to your skin at the area to be scanned.

 

The Doppler ultrasound equipment consists of a small emitter and receiver combined. This is about the size of a bar of soap and is connected by wire to the computer equipment. This emitter /receiver will be gently moved back and forth across your skin as different Doppler ultrasound readings take place.

 

The scan will normally take between half an hour and an hour and you may be required to change position, such as sitting up then laying back. This allows the ultrasound operator to assess the flow rates in different circumstances. Occasionally, your blood flow may be manipulated by the technician, either manually, by means of massage, or by using a blood pressure cuff to stop and start the flow.

 

At the end of the test, the results will be sent to your consultant as a series of images and graphs, which they will use to assess your condition and any treatment required. Thanks to Doppler ultrasound, they will now have an accurate idea of what is happening deep inside the blood vessels in your body, without any of the risks of invasive tests.

Kathryn Senior

Profile of the author

Dr Kathryn Senior is an acclaimed medical journalist who has written over 500 feature articles for leading international journals within The Lancet group. As Senior Writer at Freelance Copy she produces high quality scientific and medical content for websites and printed publications for companies and organisations in the health, medical and pharmaceutical sectors.