Duplex ultrasonography (also known as “colour flow duplex”, “Doppler ultrasound” or Triplex)
Duplex ultrasonography has completely revolutionised the study of veins and hence phlebology. Before Doppler, and then duplex ultrasound was available, we only had limited ways to observe venous function. Doctors were able to observe veins across the skin, at operation or at post-mortem. They could see the function of the whole venous system using non-invasive tests that measured the changes in volume of a limb (plethysmography) and they could make images of the vein using x-rays and the injection of contrast.
However all of these had massive limitations. Duplex ultrasonography has allowed doctors to observe blood flow in real time completely non-invasively and without any radiation. It is safe to say that without duplex ultrasonography, modern phlebology would not exist and the excellent results we are now seeing with endovenous techniques would not have been possible.
What is duplex ultrasonography?
Ultrasound is the name given to soundwaves that have a frequency higher than those which can be heard by the human ear. Medical ultrasound is in the range of 2 to 20 MHz – i.e. the frequency is between 2 and 20 million vibrations per second. Sound at this frequency cannot be heard by the human ear and also cannot travel through air. However it can travel through fluid and human tissue, bouncing back off certain structures and giving echoes. By using these echoes, a computer in an ultrasound machine can make a picture of structures deep inside the body during an ultrasound scan, without breaking the skin at all. Therefore ultrasound is completely non-invasive. As ultrasound is soundwaves only, no radiation is used at all and so it appears perfectly safe compared to x-rays.
Ultrasound gives a black-and-white picture in two dimensions.
In the mid-1950s, it was found that blood moving in the body changed the frequency of the sound that hit it, a process called the “Doppler effect”. Just as an ambulance siren going past you changes note and therefore changes the frequency of the soundwaves, blood cells flowing past an ultrasound beam changes the frequency of the ultrasound beam. The way the movement changes the frequency of the ultrasound depends upon several factors including the speed of the blood, the angle that the ultrasound beam hits it and the initial frequency of the ultrasound beam.
By the 1980s, ultrasound machines had become sophisticated enough to be able to show two different images at the same time. The first was the black-and-white ultrasound image showing the structures deep inside the body. The second was a colour picture that detected blood flow. The colour was superimposed over the black-and-white picture, allowing the ultrasound operator to see blood flowing in arteries and veins non-invasively.
Initially people called this combination of colour flow Doppler image and greyscale ultrasound image “colour flow duplex” or “Doppler ultrasound“. Of course, as with all new technologies, people often use combinations of names and so it is not uncommon to find references to “colour Doppler ultrasound“. However as it was the combination of two different technologies, it is now commonly called “duplex ultrasound“.
At this point it is probably sensible to note that in fact there were three technologies amalgamated into the machine. The first two have been discussed briefly, the “greyscale” two-dimensional ultrasound image and the colour flow image produced by Doppler mapping over the top. The third was called “gated Doppler”. Gated Doppler is the ability to choose one small area somewhere on the greyscale ultrasound image, and only listen to the Doppler waveform at that point. This is very useful when we want to know the speed of blood in a blood vessel, or the direction of blood flow.
Therefore duplex ultrasound machines actually have three main modalities:
• Greyscale ultrasound imaging of the structures present
• Colour Doppler imaging of any flow within blood vessels
• Gated Doppler waveforms of any flow
As such, although duplex ultrasound is the common name used, this can quite logically be called “triplex” scanning or “colour flow duplex ultrasonography“, both names showing that there are three different modalities possible.
Recently, ultrasound has been undergoing even more development and it is now possible to create three-dimensional images using ultrasound rather than the usual two-dimensional greyscale images. These images have been widely shown – particularly of babies in the womb. Although this produces three-dimensional pictures, the fact that the computers are so fast that movement can be seen means that some companies call this “4D scanning” – the fourth dimension being time and therefore being a moving 3-D image.