Transcription
Hello and welcome back. So, we've spent some time now looking at how ultrasound waves interact with tissue. We've looked at reflection, refraction, and scattering, as well as looking at the concept of attenuation – the loss of intensity of an ultrasound wave as it travels through tissue. We've seen that attenuation can happen via energy transfer into heat or via scattering of that ultrasound wave.
Now, we're going to look at the ultrasound transducer itself and how the various different components of that transducer go about creating a sound wave, as well as receiving echoes back and converting those received echoes, the energy from those echoes, into a digital signal that we can display on our monitors. So, if we look at the transducer itself, it has a power supply coming to the transducer and a plastic or metal housing that the operator can hold. It also protects the operator from the internal components of the machine.
If we were then to open up this transducer, we can see that there are many different components within the transducer itself. We've mentioned the power source and the plastic or metal housing. Then there's what's known as the acoustic absorber. This is either plastic or cork, and it prevents vibrations from the operator from affecting our ultrasound machine, as well as stray vibrations from the ultrasound machine where we are creating our ultrasounds from propagating within the machine. It attenuates all of those vibrations.
Now, I'm going to separate these two talks into a part one and a part two. In this talk, we're going to look at the piezoelectric material and the matching layer, the front layer of our ultrasound transducer, the part of the transducer that is responsible for creating ultrasound waves and propagating those waves into a tissue. In our second part, we're going to look at the damping block and the wiring that is supplying this piezoelectric material.
So, the piezoelectric material sits at the front of our transducer here, and it's responsible for creating the ultrasound waves. But it's also responsible for receiving those returning echoes, or the returning scattered waves, and converting that energy into an electrical signal that we can then convert into a digital image on our ultrasound machine. Now, the piezoelectric material is made up of what is known as a PZT crystal. Now, that PZT material has unique characteristics. Compression of that material will induce a current, and putting a current around that material will induce movement within that material.
Now, we are going to look specifically at this phenomenon known as the piezoelectric effect in a future talk. But for now, we can see that a sound wave heading towards our PZT material that is sandwiched between two electrodes can then compress that PZT material. So, as this sound wave, this region of compression, comes towards our PZT material and compresses it like this, what will happen then is a current will be induced within that material, and an electronic signal will head back up the wiring towards our ultrasound machine. So, this compression, you can see here, changes the orientation or the shape of that PZT material and causes an electronic signal to be induced. This is what's known as the piezoelectric effect.
Now, the reverse is also true, and it's called the reverse piezoelectric effect. If we were to alternate currents on these electrodes on either side of our PZT material, that alternating current will cause the PZT material to change shape like this. That change in shape will cause regions of compression and rarefaction within the units that abut that PZT material. Now, we can either run an alternating current and cause that shape change to happen continuously, propagating a wave at a set frequency through a tissue, or we can apply a large electric current over a short period of time over this PZT material, and that PZT material will then resonate at a set frequency. And it's that resonance of the PZT material that will cause the sound wave to propagate throughout tissue.
Now, when I think about the piezoelectric material, I think about it in two separate ways. The first way I think about it is one of these units, one of these PZT crystals, as being a cymbal on a drum set. You have the cymbal that can be hit here with a drumstick, and when you hit that cymbal, that cymbal will resonate at a set frequency. Now, the wider or the bigger the radius of that cymbal is, the lower the frequency of that sound when you hit it. The smaller that cymbal gets, the higher the pitch, the higher the frequency will be when you hit that cymbal.
The second way I like to think of a PZT material is as a guitar string, a string that is held between two fixed points here. Now, when you strum a guitar string, a certain note is played. No matter how hard you strum that guitar string, the same note will be played, the same frequency will come from strumming that guitar string. And both of these principles I'm going to use to describe various things that we see in the PZ layer.
In this talk, we're going to think about the piezoelectric material as being a guitar string. In our part two, where we look at the damping block, we're going to think of it as being a cymbal. Both have the same concepts: they resonate at a set frequency when a force is applied to them.
So, let's have a look at the individual units here, the PZT crystals. Now, we've got multiple transducer units making up the end of our transducer here. In pulse-echo ultrasonography, we want multiple different units in order to get lateral resolution within our image, a concept we're going to look at later.
Now, I've mentioned that frequency is something that is controlled by the ultrasound transducer, and a specific transducer has a specific frequency. We can't easily go and change the frequency of that transducer, and that's because frequency is determined by two things. It's determined by the speed of sound through our piezoelectric material, and it's determined by the thickness of that piezoelectric material. The thinner a piezoelectric material, the higher the frequency that will be emitted from the ultrasound machine. The thicker the material, the lower the frequency. And we've seen that high frequencies get attenuated quickly; they give us a shallower depth of view in the tissue. The lower frequencies travel a longer distance before being attenuated, but our higher frequency waves have shorter pulse lengths, allowing for better axial resolution. Again, we're going to look at resolution in our image.
Now, think of these as a guitar string, the length of this being the guitar string. If we've got a guitar string going the entire length of our guitar and we strum it, we get a low-frequency wave. If we were to place our finger on that guitar string, effectively shortening the length of that guitar string, and re-strum it, we get a much higher pitch, we get a higher note, a higher frequency coming from that guitar string. So, as we shorten that guitar string by compressing it closer to the fixed point at the end of our guitar string, we get a higher frequency. The same thing happens here. As we shorten our piezoelectric material, the frequency that it produces is higher, the pitch is higher. The longer our guitar string, or the thicker our piezoelectric material, the lower the frequency in our ultrasound wave.
Now, when you look at a guitar string like this, you can see that when you strum it, what you get is half a wavelength. If we were to add another guitar string on, we would get a full wavelength here. Now, when we go about calculating the frequency that a piezoelectric material will generate, it's this concept of the piezoelectric thickness being half a wavelength that helps us to calculate that frequency. So, we've seen these formulas before. I've just rearranged it to put frequency equal to the speed of sound over a wavelength. We're trying to calculate the frequency of sound that this specific piezoelectric material will be creating.
Now, I mentioned frequency is dependent on the thickness of the piezoelectric material and the speed at which sound travels through that material. Now, we've seen that the speed of sound within a material is constant for that material. It's dependent on the bulk modulus and the density of that material. So, we can see that the speed of sound between these two thicknesses will remain the same, and that is a value that is specific to the PZ material. What changes is the wavelength. As we saw, we change the length of our guitar string, we change the thickness of our piezoelectric material, our wavelength would change. And we saw that the thickness of our material, or the guitar string's length, made up half of a wavelength. The same is true here. We can use the piezoelectric material's thickness and multiply it by two to get the wavelength of the wave that is created here. The thickness of our material is half the wavelength. So, in order to get this wavelength here for our formula to calculate our frequency, we can take the thickness of our crystal, multiply it by two, and use that value to calculate our frequency.
Now, when we get an ultrasound transducer, it will have a resonance frequency, and that frequency is dependent on those piezoelectric crystals here, and these are values that you can calculate. And I've linked a question bank below where we go through calculating some of these frequencies.
The speed of sound traveling through these PZT crystals is very high, and it's going from our transducer into soft tissue where the speed of sound is much slower. Now, the difference in the acoustic impedance values of our piezoelectric material and our tissue boundary is quite high, and we need something to smooth that transition, coming from a high-speed piezoelectric material to a lower-speed soft tissue. And that's what's known as the matching layer.
Now, the matching layer is the front end of our transducer here. It protects these piezoelectric crystals from the tissue outside, as well as protecting the patient's tissue from the internal components here of the ultrasound transducer. Now, the matching layer has an acoustic impedance that lies between the acoustic impedance of our PZT crystals and the acoustic impedance of soft tissue. When we have a high differential in acoustic impedances, we saw that a large proportion of those sound waves will reflect off. This matching layer provides a smoothing of that transition. Fewer of those ultrasound waves that we are creating will be directly reflected back, and more will make it into our patient's tissue.
Now, we can calculate the ideal thickness of this matching layer by using this formula again. This is the exact formula that we just looked at, and we saw that the speed of sound is equal to the frequency times the wavelength of that sound. Now, we've seen that the frequency has been set by our piezoelectric material, dependent on the thickness of that material and the speed at which sound travels through that material. We will also know the speed of sound traveling through the matching layer. It depends on what material that matching layer is made of. Now, in order to not hinder that ultrasound wave that we are creating, we want our matching layer to be a quarter of the wavelength of the sound that is traveling through that matching layer. So, we will have the speed, and we will have the frequency, the frequency set by our PZT layer, the speed determined by the material that the matching layer is made of. Then we will be able to know what the wavelength of the sound traveling through our matching layer is. A quarter of the distance of that wavelength is how thick we want our matching layer to be. So, our matching layer thickness will be dependent on our PZT layer, because the PZ layer determines the frequency that the sound will be traveling through tissue.
Now, there's a problem here. Skin or tissue is not perfectly smooth and won't perfectly match with our matching layer. There will be air pockets between the matching layer and our soft tissue. And we've seen that the difference in acoustic impedance will determine how much of those sound waves are reflected back. And because air has such a low acoustic impedance, the differences will be very high, and we will struggle to get sound waves to be transmitted into our tissue. And what we need is a coupling gel here. And that gel will prevent air from coming between our matching layer and our soft tissue. The acoustic impedance of that gel is very similar to that of soft tissue, and it allows for that smooth transition from our piezoelectric material, through the matching layer, through that coupling gel, and into the tissue. We will have ultrasound waves now propagating into that tissue.
Now, our piezoelectric material has been hit by a current and caused it to resonate at a set frequency. Now, when you hit a cymbal or when you strum a guitar string, it will resonate at that frequency for a long period of time. It will ultimately come to rest. Now, we want to create short pulses of sound in our pulse-echo ultrasonography. So, what we want to do is hit that guitar string and then stop it, or hit the cymbal and then stop the cymbal. We want to create periods of time where we can listen or receive those returning echoes. Now, that is the function of the damping block, which we're going to look at in our next talk.
So, join me there where we will look at how exactly that damping layer shortens our spatial pulse length and allows us to have those long receive times, waiting for those echoes to return back in order to create our ultrasound image. So, I'll see you all in that talk. Goodbye, everybody.