Transcription
So we've seen now that the depth of the focal zone within the ultrasound beam is determined by both the frequency of the ultrasound pulse as well as the diameter of the transducer elements that are creating that ultrasound wave. In this talk, we are going to look at other mechanisms we can use to change the depth of our focal zone. Then we're also going to look at how we can manipulate the ultrasound beam in order to change the angle of intonation of that ultrasound beam as it travels into tissues.
So there are four mechanisms for changing the focal depth that I want to look at today. The first is what's known as an acoustic lens. This is a material that goes onto the front side of the transducer and it focuses the ultrasound beam to a set focal depth. This can be thought of much like a magnifying glass. Than if you were to shine light through a magnifying glass, it will focus that light at a set focal depth depending on the strength of that magnifying glass. The acoustic lens has a set focal depth. If you want to change that focal depth, you would need to change the acoustic lens.
The second mechanism we can use is to use a curved transducer array. The curved shape of these transducer elements will focus the ultrasound beam again to a set focal depth.
The third mechanism that we can use is by manipulating the diameter of the transducer elements that we use to create the ultrasound pulse. We've seen that changing the diameter of those transducer elements changes the depth of the focal zone. So in this example, one could fire off just the red transducer elements here, a very narrow diameter, and get a shallow focal zone. In the next frame, one could fire both the red and orange transducer elements here, increasing the diameter here by a factor of three. That would increase the depth of our focal zone. Lastly, if all the transducer elements were fired in the next frame, the focal zone would get even deeper into the tissue. We can then repeat this cycle, getting different focal zones and superimpose those images on top of one another. Those separate focal zones will then provide good resolution within each of those zones on our superimposed image.
Now, the last mechanism, and perhaps the most important mechanism that we'll cover today, is what's known as electronic focusing. When you take an ultrasound, you can see on the right-hand side normally of your image, you can adjust the focal depth. That adjustment is largely done by electronic focusing. Now, the way electronic focusing works is by delaying the central transducer element firing and firing the peripheral transducer elements first. The ultrasound wave is first created on the peripheries of the transducer element, and this delay in firing the central transducer elements will increase the angle of that ultrasound focusing and bring the focal depth closer to the transducer.
Now, in this third example, we saw how changing the diameter of the transducer elements changed the depth of our field of view. And when we looked at linear arrays, we saw that we sequentially fired groups of transducer elements and we repeated that process over and over again to create our B-mode image. Now, depending on the width of the transduced elements that we fire, the focal depth will change. If we were to use only one transducer element to create our ultrasound beam, the focal zone will be very shallow. The wider we make these transducer elements, in this example, firing four transducer elements per line of image that we are acquiring, we will increase the depth of our focal zone. Here, all of the transducer elements being fired at the same time will increase the depth of the focal zone to its maximum point. This focal zone now is dependent on the frequency of that ultrasound transducer as well as the diameter of these transducer elements. So you can see that choosing the width of the transducer elements that we use in order to create our linear array will have an effect on where the depth of our focal zone is, and this is using the change in diameter as the variable that changes focal depth.
Now, when we looked at phased arrays, we see that in each of these, we are firing the entire transducer element, and it's the timing of firing these transducer elements that will determine how deep our focal zone is. No delay in the firing of the transducer elements, all of them being fired at the same time, will give us the same ultrasound beam geometry as a single element transducer. As we increase the delay between the central transducer elements and our peripheral transducer elements, we bring the focal zone closer to the ultrasound transducer. This line here represents the timing at which we fire off these transducer elements, the peripheral ones being fired first, later followed by these central transducer elements. And as we increase that delay further and further, we bring that focal zone closer and closer to our transducer. And it's this delay, this phasing of the transducer elements, that is used when we are changing that focal zone on our ultrasound image, when sliding that scale on the ultrasound machine. This is what's known as electronic focusing.
So not only does the frequency of the ultrasound transducer and the diameter of the transducer elements change the depth of our focal zone, but these four mechanisms we can also use to further manipulate the depth of the focal zone.
Now, when we looked at phased arrays, we saw that a very small transducer element can get a large field of view, and we said that we can steer the ultrasound beam. Now, steering of the ultrasound beam is also to do with timing of the firing of these transducer elements. If we were to fire these transducer elements prior to firing these transducer elements, we would get steering of the beam here to the right. If all the transducer elements were fired at the same time, our beam would propagate forward into the tissue perpendicular to our ultrasound probe. Reversing the timing of the phased array here will then steer the beam in the opposite direction. And if we were to draw a line along this timing here, that line then faces the direction of the ultrasound beam steering.
Now, this helps us, for example, when we are trying to image within the chest, but say between two highly attenuating structures like two ribs. We have a very small area where the ultrasound probe can be placed, but we want to get a large field of view. If we want to look at the heart, we need a large field of view to see the heart. So a very small transducer array can create a very large field of view, and this is what's known as a phased array transducer. A small transducer with a large field of view accounted for by the fact that we can steer that ultrasound beam within the tissue.
Now, when we looked at linear arrays, we saw that this field of view was rectangular. There was no steering of the beam. And phased arrays, despite having a very small transducer surface, had a large field of view because we were steering that ultrasound beam. Now, we can actually combine these and get a linear array with some steering on the peripheries of the linear array, if we were to need a larger field of view than was available without simple linear arrays. So even though a linear array fires off just groups of transducer elements, we can also slightly steer those groups of transducer elements by phasing the timing of the firing in those separate transducer element arrays.
Beam steering and beam focusing can also be combined. Here we have steered this beam across to one side. Now, if you were to phase this steering in a way that also provides a further delay in the timing of these transducer elements being fired, we can bring that focal zone closer to our transducer element. Not only are we bringing that focal zone closer to the transducer element, but we are narrowing the width of this focal zone, getting us better lateral resolution. So you can see that focusing a beam also provides resolution benefit. The more we focus it down, the better resolution we get.
Now, here in beam steering, we are changing the angle of intonation of this ultrasound beam. And there's another mechanism known as spatial compounding, where we can slightly change the angle at which this ultrasound beam is passing in tissues, and we can get different views of the same structure within a tissue. So you can see here, if we make slight changes within the angles of the ultrasound beam that is traveling into the tissue, we can get slightly different data returning back to our ultrasound probe. I don't know if you remember those pictures that if you move them, the picture changes depending on the angle you are looking at that picture. They're called lenticular pictures. And because we are changing the angle, we can get different information of the structures within that picture. The same happens here in spatial compounding. We can then compute the different images that we are getting with those slight angle of intonation changes, and we can then process that information to reduce the background noise and get better resolution within our image.
So the ability to steer the beam ever so slightly to change that angle of intonation can give us multiple data points. And when we process those multiple data points, we can reduce that background noise, the random signal that is returning to that ultrasound transducer. And this spatial compounding gives us a much better spatial resolution within our image.
So as we've seen, there are multiple mechanisms for focusing the ultrasound beam. As we focus the ultrasound beam, we improve the resolution in the image. We can also steer the beam within tissues to increase our field of view, and we can also utilize steering in spatial compounding to increase the resolution within our image.
So that brings us to the end of the geometry of the ultrasound beam that we are creating. Next, we are going to look at the various factors that we can change in order to improve resolution in our image, both axial, lateral, elevational, and temporal resolution within the image, all of which are topics we are going to look at next. So until then, goodbye everybody.