Duplex ultrasound – How it is performed

How is duplex ultrasonography performed?

Firstly, because the image is displayed on a screen, the room is usually dark with no bright lights which might cause reflections on the screen.

Secondly, ultrasound gel has to be used between the probe and the skin. This is because ultrasound cannot travel through air and therefore there has to be a gel between the probe and the skin to allow the ultrasound to travel into the body and then be reflected back to the probe.

The next most important thing in performing duplex ultrasound is the position of the patient.

If the scan is for varicose veins or venous reflux (“hidden varicose veins”), then it is very important that the blood is allowed to reflux due to gravity. As such, venous scans are usually performed with the patient standing up, with the weight on the leg that is not being scanned. This allows the very best chance to pick up venous reflux.

Sometimes, a patient may be put on a tilted couch or table, but to be able to see good reflux in the veins, the angle must be very steep. If there is only a little angle, mild or moderate venous reflux might be missed. For some of the small veins beneath the knee, the patient can be examined sitting on the edge of the couch with the leg dangling over the edge.

To be able to see the blood refluxing back down the veins, the blood has to have been pumped up the veins in the first place. Therefore the duplex ultrasonographer stimulates the muscle pump, usually by squeezing the calf or foot or other part of the leg below the area being examined. This is called “manual calf compression” although occasionally it can be done by machine.

The duplex ultrasound probe is then used to watch the blood flow upwards during the compression, followed by waiting to see if there is a reverse of flow during relaxation. If there is none, the valves are working and there is no venous reflux at that point in the vein. If there is flow back down the vein during relaxation, the opposite way from the flow when the compression was “pumping” the blood, then there is venous reflux at that point in the vein.

By examining all of the veins in the legs, a map of all the veins can be made.

Of course there are some more sophisticated tests for certain veins such as perforator veins and pelvic veins, but the above covers the general principles of duplex ultrasonography for varicose veins and venous reflux disorders.

Scanning for deep venous thrombosis (DVT)

When duplex ultrasound is used to scan for a deep vein thrombosis (DVT) or thrombosis of the superficial veins (superficial thrombophlebitis) then there is no need to look for reflux. Therefore patients can be examined lying down.

The ultrasound is used to check that the veins can be “squashed” by putting pressure on them with the probe. A normal vein can easily be squashed and this appears as a “kissing” of the vein as it is pressed. However a vein with thrombus within it, is solid and cannot be compressed.

Advantages and disadvantages of venous duplex ultrasonography in the examination of leg veins

In order to discuss the advantages and disadvantages venous duplex in relation to examination of leg veins, we need to compare to other methods that have been used in the past.

As varicose veins are so prevalent, and as they have been noted throughout the whole of human history, there have been a multitude of examination techniques and investigations that have been suggested and used. Therefore we are only going to discuss the most widely practised techniques to compare venous duplex ultrasonography. These are:

This website was last updated on 11/10/16.

Up until very recently, doctors thought that they would be able to identify the likely cause of varicose veins by merely examining the leg and seeing what “pattern” the varicose veins visible on the surface were arranged in. It was thought that varicose veins down the inner aspect of the calf related to Great Saphenous Vein (GSV) reflux, varicose veins on the back of the calf related to Small Saphenous Vein (SSV) reflux and varicose veins over the front of the thigh related to anterior accessory saphenous vein reflux.Although in a few patients this is possible, it must be remembered that half of the patients with venous reflux do not show any varicose veins on the surface at all. Furthermore, research over the last 10 years using duplex ultrasonography has shown that there is rarely only one cause of varicose veins, and the previously held view that all varicose veins came from the Great Saphenous Vein (GSV) or Small Saphenous Vein (SSV) has shown to be incorrect. We now know that incompetent perforating veins both below the knee and sometimes above the knee, double systems of Great Saphenous Veins (GSV) and Small Saphenous Veins (SSV) and pelvic vein reflux can cause varicose veins and other venous problems without conforming to the classic pattern.

Therefore when a doctor or nurse looks at varicose veins on the legs, they can merely say that there are varicose veins present and may be able to say if there is any skin damage, swelling or ulceration. However without a duplex ultrasound scan, they are unable to see what the cause of the varicose veins or venous problem is and more importantly, cannot say that there isn’t a problem in the leg that doesn’t exhibit any varicose veins.

Therefore clinical examination now is useful only for general examinations such as to note if there are any scars or venous abnormalities. It has very little use in helping to diagnose the cause of any venous problems.

The Trendelenberg tests are clinical tests that every medical student has to learn although they are virtually useless clinically. Basically the patient is laid down and a tourniquet is placed around the top of the leg. The patient stood up and the leg observed to see if any varicose veins fill up. The tourniquet is then released to observe if the veins fill on release of the tourniquet. The process is repeated with a tourniquet at the mid thigh and above the knee. The idea is meant to be to see where the lowest level of the venous reflux starts.However this supposes that all venous reflux is due to passive gravitational flow of blood, and we know that this is not the case as active reflux occurs in the Small Saphenous Vein (SSV) and incompetent perforators (see “Understanding Venous Reflux – the cause of varicose veins and venous leg ulcers“). It also assumes that seeing the varicosities dilate is a sensitive test as to whether venous reflux is occurring in the leg. Once again, as considerable damage is done in active reflux, this is erroneous.

Therefore the only two uses that the Trendelenberg tests are firstly, historical interest and secondly, to confound medical students. They should never be used in a real vein practice.

Venography is an x-ray technique where contrast is injected into veins and x-rays taken to see where it goes.The contrast can be injected at the foot in a lying down patient, and serial x-rays can be taken to watch the contrast be swept with the blood up the veins. In addition, contrast can be injected at the top of the leg or even via a catheter into pelvic or abdominal veins in a standing patient, and serial x-rays can be taken to see if the contrast reflux is down any veins.

There are several problems with venography, apart from the fact that it uses x-rays which are of course ionising radiation and therefore should only be used if there is no alternative.

The contrast used is heavier than blood and therefore, although it is swept along with blood to an extent, it does not behave as blood actually does. If the contrast goes into a vein, it is seen. However depending on where the contrast is injected, what volume and under what pressure, there are many cases where contrast may not flow into a vein that is actually there and so this will not be observed on x-ray.

Further, when looking for venous reflux, we already know that valves start failing lower in the leg first, and the failure only ascends to the valves at the top of the leg later in the reflux process (see “Understanding Venous Reflux – the cause of varicose veins and venous leg ulcers“). As such, if contrast is injected above a competent valve at the top of the leg, it will not reflux down to the level where the valves to become incompetent and so venous reflux lower in the leg will be completely missed.

Finally, as the contrast does not behave as blood donors in a physiological manner, functional abnormalities of the veins such as reflux incompetent perforating veins can be completely missed or over represented.

Therefore venography was useful when there was no alternative, but now has been totally superseded by venous duplex ultrasonography.

Plethysmography is the name given to a series of different techniques that measure the change of volume of the limb. The idea of this is that if the veins are working properly, the limb or decrease in size when blood is pumped out of it. When blood has been pumped out of the limb, or has flowed out due to gravity when the limb is elevated, any reflux will be noted by an increase in volume of the limb when the limb is once again still and hanging down.Plethysmography is a very good technique to measure the total amount of passive venous reflux in the limb. However it is useless for active venous reflux or for knowing which vein/combination of veins have lost their valves and are incompetent.

Plethysmography is very useful as an adjunct to venous duplex ultrasonography. Ultrasonography can give an overview as to how severe any passive venous reflux is, whilst duplex ultrasonography gives exact information as to which veins are involved and what size they are to enable planning of the optimal technique of treatment.

A hand held Doppler (previously sometimes called a “pencil Doppler”) is a very small machine that passes ultrasound at a set frequency from a piezoelectric crystal in a small probe through the skin and into the body, and then measures any frequency shift (or Doppler shift) in the returning ultrasound. Any shift in frequency shows that there has been blood flowing somewhere along the direction of the beam. This is usually represented by a sound that is emitted from the hand held Doppler.The problem with hand held Doppler is that it can detect flow, but does not say how deep the signal comes from, nor which vessel. In addition, if no signal is received, it does not mean that there wasn’t any flow as it might be that the Doppler beam was not aimed in exactly the right direction.

As such a hand held Doppler is better than nothing, but really has no place in a vein clinic that performs anything more than thread vein treatments.

As can be seen from the information so far on this website, venous duplex ultrasonography has advantages over any other venous investigation currently available. Virtually every vein that needs to be imaged can be imaged. The flow within the veins can be seen directly as it is the blood cells within the blood itself that reflect the ultrasound and give the Doppler shift. There is no ionising radiation and ultrasound is completely non-invasive meaning that it is perfectly safe.The only disadvantages are that venous duplex ultrasound machines are expensive (but not as expensive as getting the diagnosis and therefore treatment wrong!)and they only give accurate information when used by people who are trained and experienced in using them. As with everything in life, the more someone uses duplex ultrasound, the better their reports will be.

There are a great many different duplex ultrasound machines on the market ranging from very cheap with poor resolution to very expensive with excellent resolution.

Although cost isn’t necessarily an indication of quality, with duplex ultrasonography it is rare that a very cheap machine will give as good resolution and service as a more expensive machine. Having said that, as technology improves, computer processing speeds increase and costs of computing decrease, the price of duplex ultrasound machines is generally decreasing whilst the resolution of the pictures and images that can be obtained is increasing. It has often been said that every five years, ultrasound machines halve in price, halve in size and double in power. Although this is not strictly true, it is certainly true that the costs of duplex ultrasound machines are phenomenally low compared to the amazing images and insights that they now give the modern-day phlebologist.