Transcription
In the previous talk, we focused on the front end of our ultrasound transducer. We looked at the piezoelectric material that was actually responsible for generating our ultrasound pulse, and we looked at the matching layer, which allowed for transmission of those ultrasound pulses through to the patient's tissues.
In this talk, we're going to focus mainly on the damping block, which sits behind the piezoelectric material, and then we're going to briefly touch on the wiring that supplies the power to our PZT crystals.
Now, I said you can think of those PZT crystals as either a cymbal or a guitar string. And in the previous talk, it was useful to think of a guitar string because it showed us that the PZT crystal thickness was actually half of a wavelength.
Now, in this talk, we're going to think of the PZT crystal as a cymbal, like a cymbal that you have on a drum kit. And when you hit that cymbal, that cymbal will resonate at a certain frequency. Again, the wider that cymbal, the lower that frequency, and the narrower or the smaller the radius of that cymbal, the higher the frequency. The same is true with the thickness of our PZT material.
Now, if I was to strike this cymbal, it would resonate for a long time before coming to rest, and it would resonate at the same frequency. We would lose intensity of that wave slightly, but the frequency would remain the same.
Now, the function of the damping block is to shorten the period of time that that cymbal, or that that PZT crystal, is resonating. That's the first function of the damping block: to shorten our spatial pulse length. The second function is, when a PZT crystal expands and contracts, there are waves that can be formed towards the patient, but also waves that would be coming back towards our ultrasound transducer. And this damping block prevents those waves from coming into the transducer; it helps those waves to be transmitted forward into the patient's tissue.
Now, if you think of a cymbal like this and placed a rag on the top of that cymbal, imagine on the drum kit there was a wet rag sitting on that cymbal. If we were to hit that cymbal, it wouldn't resonate for a long period of time. It would make a note, and not a very pure note, and make a note and then come back to rest. That resonance frequency would be dampened down.
Now, that's exactly what the damping block is doing. It's like putting a wet rag on top of that cymbal. It shortens the period of time that the cymbal, or the PZT crystal, would resonate for.
Now, it also disturbs the characteristics of the cymbal, or the PZT crystal. It doesn't resonate so purely at a set frequency. We get a wider range of frequencies generated because that wet rag, or the damping block, is preventing the cymbal from moving in the way that it would naturally do without the rag on top of it.
So, if we were to look at these two cymbals, one without a damping block, one with this wet rag or a dampening block on it, the one without a damping block would create a long spatial pulse length that is relatively pure, has the same frequency, there's very little variation in that frequency; it's resonating at a set frequency.
Now, when we have a damping block, or we have a wet rag on top of a cymbal, and we hit that cymbal, it won't vibrate for very long. We get a very short spatial pulse length. But what that allows us to have is a longer receive time, a longer time to listen for echoes returning from those tissue boundaries within the patient.
Now, there's another advantage of having a short spatial pulse length, and that provides us with better axial resolution, better resolution as we're going into the depths of the patient. And we're going to look at axial resolution specifically in a later talk.
Now, when we have a long spatial pulse length, it becomes more and more difficult to separate individual units within the patient's tissue. So, having a short spatial pulse length has two advantages: we get better axial resolution, and we can listen for further depths within the tissue; we can image deeper into that tissue because we've got a longer receive time.
Here, now, when we look at pulse echo ultrasonography, what are the characteristics of the wave that are allowing us to make our image? The first thing we need to know is the speed of the wave traveling through the tissue. We use that speed to determine how far those tissue boundaries are. And the second thing we use is the intensity of that wave. The intensity of the echoes coming back correlate to a grayscale on our image. There's nothing about the frequency that helps us create that grayscale image, our B-mode image, that we're going to look at later.
Now, later on, when we look at Doppler imaging, the frequency becomes very important. Doppler imaging looks at a change in frequency. So, we need to know what frequency we are emitting from our ultrasound machine in order to know the change in the frequency as that wave comes into contact with structures within the body.
So, ultrasound machines with heavy damping blocks, heavy dampening of that ultrasound wave, is really good for pulse echo ultrasonography. And when we look at Doppler ultrasonography, we need to have light damping. We want a pure frequency coming through, and waves that have a frequency that remains the same. It's got a very narrow bandwidth. We call those waves high-quality waves, or waves that have a high quality factor. And as that frequency variation increases, as our bandwidth increases, our quality factor decreases.
So, we can actually go and calculate a quality factor by taking the resonance frequency of that PZT material. That resonance frequency is dependent on the speed of sound through the PZT material and the thickness of our PZT crystal. And then we can look at what range of frequencies are actually transmitted by that PZT material. Our frequency in a low-dampened system is very constant. There's very little range in frequencies. Most of the frequencies are at that resonance frequency. So, the difference in our lowest frequency and our highest frequency is very narrow. The higher our quality factor, the less variation in frequencies that we have in our wave.
Now, as we increase the dampening within our ultrasound transducer, we get a bigger variation in frequencies. Our frequency range is much higher. Our denominator in this equation becomes higher, and our quality factor decreases.
Now, importantly to note here, quality factor talks about the purity of the frequencies that are coming out of our ultrasound transducer. It does not determine the quality of the image we are able to produce. As we'll see later, short spatial pulse lengths actually give us better axial resolution, give us better quality images. And these long spatial pulse lengths, despite having a high quality factor, the beam being of good quality, actually we lose axial resolution. Our image quality can actually decrease as we get less dampening of this wave.
So, we use these waves in our Doppler imaging because we need pure frequencies. And we use highly dampened, low quality factor waves in our pulse echo ultrasonography, allowing us to get good axial resolution and to get good depth penetration, long receive times, listening for those echoes coming back.
Now, we can actually represent this graphically by looking at a frequency range on our x-axis here, and this is the amplitude of our waves here. We're still creating this same strength of wave, our electronic signal to that PZT crystal is the same, but our frequency range is very narrow in these high quality factor, low dampened systems. So, we get a very narrow graph here. The frequencies that we produce are all around that set resonance frequency.
As we increase the dampening in a system, the range of frequencies increase, and this is what's known as the bandwidth of our ultrasound machine. As we get more and more variation in that ultrasound frequency, our bandwidth increases. So, an increase in dampening results in an increase in bandwidth. We still have that center resonance frequency, but we have much more variation within the frequency. So, our quality factor determines how narrow our bandwidth is. It doesn't determine how good our image that we are producing is.
Now, we can move on to look at the wiring that supplies the PZT crystals. We can individually fire these PZT units depending on the type of beam we are trying to create. We can fire them all at the same time and create an ultrasound pulse that travels through the tissues, or we can fire individual groups of them depending on how we are trying to image the structures within the patient.
Now, you can see here, I've got these two lines here: a green line and a purple line. Now, this green line and purple line represent the timing of the firing of these PZT crystals. If we were to fire the PZT crystals at different times, the waves that are produced can interfere in such a way that the beam heading out of our ultrasound machine can be focused down onto a set point. And we can determine this focus distance depending on the timing that we use to fire these PZT crystals. We can bring that focus point closer to us, and we can push it further away. And we're going to look at this when we look at our beam characteristics in a future talk.
Another thing that we can do to the beam is fire them in a sequential manner. And as those waves are released from our ultrasound machine, they interfere with one another and steer the beam to a different direction. And we can sweep through a tissue. Say, we're going between a rib, but we want a wide view of the tissue, we can steer that beam through the tissue and scan our way, looking at a larger region while using a smaller area of our ultrasound machine. These are all things we're going to touch on more when we look at our beam characteristics, but I wanted to show you here why this wiring is important. We don't always fire the PZT crystals at the same time, all at once. We can manipulate those in order to manipulate our beam.
So, now we've looked at the various different components of our ultrasound transducer, and hopefully you have an appreciation for how they work together in order to create a specific ultrasound beam. And we want to use these in a way that will help us with the type of image we are trying to produce. If we need frequencies in our calculations, such as in Doppler, we want to decrease our dampening. If we want to be able to image at greater depths, we need more dampening, reducing our spatial pulse length, giving us a higher receive time, giving time for those ultrasound waves to go to a further depth and reflect back, and still have time for us to receive those echoes.
So, hopefully that has helped. I'll see you all in our next talk. Goodbye, everybody.