How does duplex ultrasonography work?
However, before we can understand these different modalities, we need to understand the basics of medical ultrasound – the piezoelectric crystal.
The piezoelectric crystal
A piezoelectric crystal has the property that when an alternating electrical current is passed across it, it vibrates at a set frequency, producing ultrasound of that frequency. Therefore a piezoelectric crystal can be used to generate and transmit ultrasound at a set frequency.
However, what makes a piezoelectric crystal so useful is that it can also perform the same function in reverse. Hence ultrasound hitting the piezoelectric crystal at a certain frequency, makes the crystal vibrate and produce an electric current as a representative frequency. Therefore a piezoelectric crystal can also be used to receive ultrasound and convert it into alternating electrical current at a frequency represented to of the received ultrasound.
Thus a piezoelectric crystal can transmit ultrasound at a set frequency, and then receive echoes of that same ultrasound bouncing back, and can detect whether the frequency of the echoes have changed by Doppler shift.
How does two-dimensional greyscale ultrasound imaging work?
The way this is produced in principle is that an ultrasound probe is used to fire pulses of medical ultrasound into the area of the body to be investigated. The ultrasound probe has a line of piezoelectric crystals called an “array”. This array of piezoelectric crystals fire off a pulse of ultrasound in a coordinated fashion, starting with one crystal at the end and working along the array. As each crystal fires its pulse, any ultrasound echoes are picked up and fed back to the computer within the ultrasound scanner.
Echoes for greyscale imaging have two main properties that are used to make the image. The first is how long the echo has taken to come back to the crystal and the second is the strength of the echo.
As the speed of sound in the human body is known, then by measuring the length of time it is taken for the echo to come back, the depth of the structure that has caused the ultrasound to bounce back as an echo can be determined. The strength of the echo shows how reflective the structure is to ultrasound, and this is marked by an increasing brightness of the image at that point – no echo being black, weaker echoes being dark grey and the strongest echoes being white.
By building up all of the echoes received from all of the piezoelectric crystals in an array, a two-dimensional greyscale ultrasound image can be formed.
What is amazing is that with modern computing techniques, these images can be formed several times a second, allowing us to see movement of structures within the body in real time.
How does colour flow Doppler imaging work?
As all of the piezoelectric crystals within the array are firing off ultrasound pulses at the same frequency, then a third property of the echoes can also be measured.
The piezoelectric crystals are able to receive the ultrasound echoes and transmit a frequency representative of the frequency of the echoes. Thus the third property that the echoes can have is any change in the frequency of the pulse, compared to the frequency of the pulse when it was transmitted by the array. In other words, the piezoelectric crystals are monitoring any Doppler shift in frequency of the returning echoes.
As we have seen in the two-dimensional greyscale imaging, the time that the echos take to come back from each piezoelectric crystal in the array in turn, allows the computer in the ultrasound scanner to make a greyscale image. To make a colour flow Doppler image, the time that the echo takes to return is used along with any Doppler shift in the echo.
Any pulse that returns without a Doppler shift is ignored for the Doppler imaging, as this represents structures that are stationary. However any ultrasound echoes that return with a Doppler shift, are marked on the image with a colour. Traditionally, in vascular surgery, two colours are used. One colour is useful when the Doppler shift gets longer and another for when the Doppler shift gets shorter. In this way we can differentiate the direction of flow of the blood being imaged. In addition, the faster the blood flows, the greater the Doppler shift and the brighter the colour on the Doppler image.
Not surprisingly, as we are looking for flow in blood vessels, the two colours used are usually red and blue. When surgeons use Doppler ultrasound, they preferred to have their arteries in red and their veins in blue, as this is the colour scheme used in anatomy books for arteries and veins. However when a scientist or vascular technologist uses duplex ultrasound, the convention is to use red for blood flow away from the heart and blue for blood flow back to the heart. This usually correlates to the arteries and veins but not always.
However sometimes doctors wish to investigate the flow of blood in one vessel only or at one point in one vessel such as in a narrowed artery, or in a part of a vein where the valve might or might not be working.
The most accurate way of doing this is not to look for changes in colour as the blood flows, but to analyse the Doppler waveforms of the blood flow at just one point in the vessel. The way this is performed is using a process called gated Doppler.
The piezoelectric crystals in the probe can be programmed to send one pulse of ultrasound at a specific frequency at one angle through the greyscale image that is seen on the display. This is represented on the two-dimensional greyscale ultrasound image as a line at an angle.
As with all ultrasound, echoes return to the probe, with a delay depending on the depth of the echo. As we know the speed of ultrasound in tissue, we can know which echoes come from which point along this line by merely measuring the time the echo has taken to return. In gated Doppler, we choose to listen to only one point of the line by moving 2 markers on the greyscale screen. This tells the duplex ultrasound machine to ignore any echoes that come back from structures above the upper marker, and to ignore any echoes a comeback from below the lower marker. We only listen to the echoes coming back from between the two marks. In this way we are “gating” the echoes that we are listening to.
Thus although we are making our “gate” of what we want to listen to by using two markers on the screen, the software in the duplex ultrasound machine is actually allowing us to perform this “gating” by using time of the echo returning to the probe. Any change in frequency of the echoes within this “gate” can be analysed as a Doppler waveform.