Transcription
Hello and welcome back. In the next few talks, we're going to be looking specifically at the ultrasound beam. We'll describe the various different properties of the beam, as well as looking at how we can manipulate that beam in order to generate the type of image that we're trying to acquire.
Now, in order to understand the beam itself, we need to know what tools we have at our disposal. So today, I'm going to start off by looking at various ultrasound probes, the most commonly used ultrasound probes, and the different types of transducers.
Now, the ultrasound probe is what you hold in your hand when you are acquiring the ultrasound image. You would have seen these probes, I'm sure. We have a linear probe here, a curvilinear probe, and a phased array transducer.
Now, a linear probe has a flat face like this, and it sends out a rectangular field of view. These dark blue lines represent the field of view, what we can visualize within the patient's tissues. Now, the field of view represents how the ultrasound wave heads out into the tissue and how it's reflected back and received by the ultrasound probe. So, a linear probe sends out this rectangular field of view.
Now, we can take this same probe and curve the front of the probe. This is what's known as a curved linear probe, or a curvilinear probe. Now, you can see our field of view diverges out. As we get deeper into the tissue, we are sampling a greater area of that tissue.
Now, this shape here is what's known as a sector. A sector is a part of a circle, like a slice of pizza or a slice of cake. And here, we've got what's known as a blunted sector, because the front of our probe here blunts the apex of the sector.
And the last probe here is what's known as the phased array probe. It's got a much smaller face to the transducer, and it's got a pure sector here. It's not a blunted sector. We get this fanning out of the ultrasound wave.
Now, a phased array probe has much fewer transducer elements within its surface than our linear and our curvilinear probes. A linear and curvilinear probe generally have 200 to 300 discrete transducer crystals on the face of that probe. Our phased array probe generally has 64 to 128 transducer crystals on the face of it. And that's because we want a smaller real estate here. These probes allow us to use a smaller surface area but still get a large field of view. So, if we want to image the heart, say, we can get underneath the ribs or between the ribs and still get the entire heart within our field of view. And when we look at our transducer types, we're going to see exactly how we go about getting this large field of view with such small real estate at the front of our probe.
I'm going to mention two more probes for completeness' sake. We've got an endocavitary probe. It's got a very large field of view for a small transducer surface here. Now, this you would have seen used in Obstetrics and Gynecology. It can be a transvaginal or a transrectal probe. It allows you to scan from within a cavity but still get a large field of view to see the internal structures, to see the bladder, to see the uterus.
And the last probe here is an endovascular ultrasound probe. A very small transducer on the end of this wire that allows us to image from within vessels or within hollow viscera in the body.
Now, those are the various probes, the actual machine that we are holding. Now, we're going to look at the transducer types. Now, the transducer types refers to how we go about creating our wave. It generally refers to the crystals that we use on the face of our transducers. So, our linear and our curvilinear transducers can have the same transducer type. It talks to how we go about propagating that ultrasound wave.
Broadly, we can separate transducer types into single element transducers and array transducers. Now, single element transducers are exactly that. They have one element at the front, one PZT crystal that makes our ultrasound wave. Now, generally, you won't come across these in your practice because we've moved to array transducers, multi-element arrays.
What does the word array mean? Well, when you say there's a vast array of things on offer, we're talking about multiple different items here. We have an array of transducer elements. When we talk about an array in mathematics, we talk about objects or numbers that are organized in rows and columns. The same is happening here. We've got a long row of transducer elements.
Now, when we look at an array transducer, we can further subdivide that into linear arrays and phased arrays. Now, it's really important to understand the difference between these two types of transducers.
Now, a linear array sequentially fires either individual or small groups of transducer crystals at a time. In this example, we have fired these three crystals to generate an ultrasound wave. Once that wave has been generated and we've listened for the echoes coming back, we then shift along one transducer element, so it shifts along like that, and we make another ultrasound wave. We then do it again and make an ultrasound wave, one more time and make an ultrasound wave. Now, we are shifting one transducer crystal at a time, but in this example, we are firing three transducer elements at a time. Now, we want to fire multiple transducer elements at a time to allow our near field to be deep enough into the patient's tissue, a concept that we're going to look at in our next talk.
So, in linear arrays, we are sequentially firing either individual or small groups of transducer elements. In a phased array, we are firing the entire array of elements at the front of our transducer. And it's the order in which we fire those, the sequence in which we apply an electric current to these transducer arrays, will determine how our beam is steered through the patient. So, we can fire them in different sequences, and our beam will sweep through the field of view here.
Now, this was the phased linear array that we talked about when looking at the heart, going through the ribs. We've got a very small transducer surface, but we are able to sweep through the tissues. And in two talks time, we're going to be looking at beam steering and how we can steer this beam into a large field of view.
So, let's have a look at how a linear array works to start off with. Now, if we've opened up our transducer here, here we've got a flat linear array, a sequential linear array, and we fire three transducer crystals to create this small part of our ultrasound beam. Now, we'll be shifting across those transducer elements over time in order to get our B-mode image. It looks like a live image on our ultrasound screen. Now, each time we fire a group of elements, we get that A-mode data back, and we can stitch together all those A-mode data points in order to create our B-mode or our brightness image.
The same thing is happening in our curvilinear array here, but our transducer elements, our PZT crystals, have a curved shape. So, when we propagate the ultrasound beam, it's still coming off perpendicular to the face of the ultrasound probe, but the beam diverges as it heads out into our tissues. Now, strictly speaking, in a linear array, that ultrasound beam heads off perpendicular to the ultrasound face here. Now, as we'll look at later, because we are firing multiple elements at one time here, we can actually steer that beam a little bit if we need to.
The next type of array is a phased array, where we are using all of the PZT crystals to create each frame within our image. So, let's look at the phased array. As we saw earlier, the phased array can sweep through our diverging field of view here, and it gives us this sector-shaped field of view.
Now, look here where we are firing off our transducer elements. If we supply current to these PZT crystals with a small time difference between them, and we sequentially go along the face of our ultrasound transducer, what that will do is cause our ultrasound beam to steer off to the side here. If we supply current to all of the crystals at the same time, we get an ultrasound beam that heads directly into the tissue. And the opposite is also true. If we delay the firing of these transducer elements in the opposite direction to what we did initially, that beam will steer off to this direction. Now, you can draw a line here, and wherever that line is facing will show you the way in which that beam is steered. Again, this is a concept that we're going to look at later.
The key difference between a phased array and a linear array is that a phased array uses all of the PZT crystals on the transducer face to generate the data required for that single frame. A linear array sequentially fires off small groups of PZT crystals and stitches together that A-mode data to form a B-mode image. Here, in a phased array, it requires a little bit more complicated mathematics in order to generate the B-mode image.
So, we've looked now at the various different ultrasound probes. Now, they are a vast array of different probes that are available to us. I've just covered the main ones. And we've looked at transducer types. We can have single element transducers or array transducers, where we've got multiple PZT crystals on the face of our transducer. And the arrays can either be linear or phased arrays. Linear sequential arrays, where we fire small groups of transducers as we head across the transducer face, we're getting multiple A-mode lines of data that we can stitch together to create our B-mode image. Phased arrays use all of the transducer crystals, and depending on the timing of exciting those transducer crystals, we can then steer that beam through our large field of view.
Now, what's next? Well, you may have wondered that these beams are heading out towards our patient, and they've got this curved shape to them. We get a period where we focus down, and then the beam diverges. It converges to a focus point and diverges. And this happens no matter whether we're using a phased array or we're using a linear array. We still get that same shape of beam. And this is the beam geometry that we're going to cover in our next talk. And we'll see that depending on the transducer elements, depending on the frequency of the wave that we are generating, it will change the shape of this wave. And after that, we'll look at the various mechanisms we can use to focus the beam, to steer the beam, as well as to get better resolution within our image.
So, I'll see you all in that next talk. Goodbye, everybody.