Transcription
We've spent the previous two talks looking at the components of the ultrasound transducer, and we saw that the piezoelectric material was both responsible for generating ultrasound waves that propagate through tissues and receiving those returning echoes and converting that mechanical energy into an electrical signal that we can then use to create our ultrasound image. I said at the time it was the result of the piezoelectric effect and the reverse piezoelectric effect that allowed us to both receive and generate those ultrasound pulses.
Now, we saw that if we apply an alternating current to a piezoelectric material, that piezoelectric material will generate an ultrasound pulse that has a frequency that's equal to that alternating current. And if we applied a short, sharp burst of electric current to our piezoelectric material, the piezoelectric material would resonate at a frequency that's dependent on the thickness of our piezoelectric material and the speed at which sound travels through that material.
Now, we're going to look at the piezoelectric effect in a little bit more detail in order to understand how we go about both receiving and generating those pulses. This may be a little bit of an addendum to our ultrasound physics module. It's probably a little bit more depth than we need to know, but there are a couple of concepts that come up here that do come up in exams, and understanding it in this depth helps us to explain those concepts further.
So, if we have a look at our ultrasound transducer here, we can see that it has the ability to both receive these returning ultrasound pulses and generate ultrasound pulses. Now, what are ultrasound pulses exactly? They are regions of compression and rarefaction. They're regions of localized pressure changes. It's a mechanical energy that is moving through tissues, a mechanical force moving through tissues. And if we open up our ultrasound transducer, it's this layer here, the blue layer of piezoelectric material, that is responsible for both receiving those mechanical forces and generating those mechanical forces.
This transducer is a multi-element transducer. We've got multiple crystals, multiple transducer units that are separate from one another, and we've got individual control over each one of those units. We can get ultrasound machines where we have a single piezoelectric material transducer layer, which is known as a single-element transducer. We're going to look closely at the single-element transducer when we look at our beam characteristics, but for the most part, in our ultrasound course, we'll be looking at multi-element transducers.
So, how exactly does this piezoelectric material both receive those incoming pulses and generate outgoing pulses? Well, firstly, this piezoelectric material is flanked by two electrodes. There is an electrode on the patient's side and an electrode on the transducer side, and these electrodes are responsible for either generating current within the PZT material or receiving current that's coming from our PZT material.
So, let's start by looking at the piezoelectric effect. Now, the piezoelectric effect is the conversion of mechanical energy into electrical energy, and it's that electrical energy that we will convert into our ultrasound image. The piezoelectric effect is responsible for those returning echoes, that mechanical energy that we then want to convert into an electrical energy. So, let's look at our piezoelectric material that's flanked by those two electrodes, and we have this mechanical energy in the form of an ultrasound wave returning towards this electrode. This region of compression then compresses our PZT material, our piezoelectric material. That compression results in a current that can be measured by these electrodes, and that current can then be converted into a pixel value, a grayscale value that we will use on our ultrasound machine.
Now, the opposite is also true. If we run a current through these electrodes, that current will force the piezoelectric material to change shape. Now, that change in shape is what is responsible for the mechanical energy propagation into the tissue. As the piezoelectric material expands, we get regions of compression here, and as it then returns to its normal shape, it forms a region of rarefaction. And it's those alternating regions of compression and rarefaction that result in the propagation of our ultrasound wave into the patient's tissue. Now, the electric current conversion into a mechanical energy is what is the reverse piezoelectric effect.
So, let's have a closer look at the actual chemical structure of a piezoelectric material. Now, the most common material we use is what's known as a PZT crystal. P standing for lead, Z for zirconium, and T for titanium – a lead zirconium titanate crystal. Now, in its basic form, a PZT crystal has outer lead atoms that form these cuboid or rhombohedral shapes. Within that cuboid or rhombohedral shape is oxygen atoms that surround a central titanium or zirconium atom.
Now, when we think of oxygen atoms interacting with other atoms, let's take H2O, water, for example. That oxygen shares its valence electrons with those two hydrogens, and it's the sharing of those valence electrons that form the chemical bonds between the oxygen and the hydrogen. Now, the electrons within that H2O molecule spend most of their time by the oxygen atom relative to those hydrogen atoms. So, we can think of that oxygen atom as being relatively negatively charged compared to those hydrogen atoms.
The same is also true here. These oxygen atoms share valence electrons with that central titanium or zirconium, and the electrons spend greater time around those oxygen atoms. We can think of these oxygen atoms as being relatively negatively charged, and the central zirconium or titanium as being relatively positively charged. Now, if we were to take the charge of all of these oxygen atoms and average them out, we would see that that negative average would form in the center of this image here, and the relative positive charge of the zirconium or titanium atom would match the average of these negative charges in the center of this PZT crystal.
Now, if that zirconium was to shift, or if the titanium was to shift, our positive center here would not match our negative average center here. We would have formed what is known as a dipole. We will have a positive end to our PZT crystal and a negative end. We've formed a dipole here.
Now, how exactly do we go about forming these dipoles within a PZT crystal? Well, it's a process known as polling of the crystals, polarization of our PZT crystals. How we do that is we heat up our PZT crystals to a temperature known as the Curie temperature. Now, the Curie temperature for PZT is about 350 degrees Celsius or 660 degrees Fahrenheit. There is enough energy in that system there for our PZT crystal to have a non-dipole structure. Now, whenever we have a PZT crystal forming for the first time, these dipoles are in random orientations. When we heat it up to this Curie temperature, we get the reduction of those dipoles.
Then, what we can do is apply an electric current over our PZT crystal and cool it down slowly. As we cool it down while maintaining that electric current, we get all of our dipoles forming in the same direction. We've got all our PZT crystals now having dipoles in similar orientations. Then, the entire PZT crystal can take on that dipole structure, having a relatively positively charged and a relatively negatively charged end to that crystal. And this is the reason why we can't autoclave our ultrasound transducers to sterilize them. When we autoclave something, we would put it above the Curie temperature, and we would lose this dipole structure. We would lose this orientation. When the piezoelectric crystals then go back below that Curie temperature, these dipoles will again be in a random order, and that PZT crystal wouldn't function in the way we want it to function.
So, let's have a look at this dipole and flip it on its side. We've flipped this PZT crystal on its side, and we've placed electrodes on either side of that PZT crystal. Now, we can think of this as a single molecule, but in fact, in our PZT crystal, we've got thousands of molecules with all the dipoles lined up – a relative positive end and a relatively negative end.
The piezoelectric effect provides a mechanical compression here. As we compress this molecule, we move that titanium or zirconium into the center of those oxygen molecules. We are effectively reducing that dipole. That movement of charge results in a current being formed within our electrodes. That is the piezoelectric effect: mechanical force resulting in a current formation. So, as we compress this molecule, we get an electric current.
The reverse piezoelectric effect: we can then run a current through these electrodes. If we were to make this electrode negatively charged, it would attract that zirconium or the titanium and pull it towards that negatively charged electrode. We have then again compressed that PZT crystal. We've got the reverse piezoelectric effect causing compression here and ultimately rarefaction in our tissue. And if we were to then switch that current, we would create a compression within the tissue. Alternating between those two will give us those regions of compression and rarefaction – the reverse piezoelectric effect.
So, that, in a nutshell, is the piezoelectric effect and the reverse piezoelectric effect. Now, the diagrams I've used here are mainly for your understanding. There are some nuances that I've left out that I don't feel are necessary for our radiology physics exam and don't add any further understanding into how we go about creating and receiving ultrasound waves.
Now, in our next talk, we are going to be looking at the various different ultrasound modes and how we can use the different modes to create different ultrasound images depending on what information we want clinically. So, I hope this has helped. I hope your studying for your ultrasound physics exam is going well. And if you are studying for an exam, I've linked a curated past paper question bank below where I go through actual past paper questions in video format, showing you how I would go about answering those questions. So, if you're interested in that, go check it out. Otherwise, I'll see you all in our next talk. Goodbye, everybody.