Transcription
We've looked now at axial resolution, which is the ability to differentiate two discrete objects which are in the same longitudinal plane but different depths within our ultrasound beam. Now we're going to look at lateral resolution and then touch briefly on elevational resolution.
Lateral resolution is the ability to differentiate two discrete objects that are at the same depth within our ultrasound image but on different lateral planes. Now natural resolution relies heavily on beam geometry and beam focusing, both of which we've covered in previous talks. If we were to have a look at an ultrasound beam heading in towards tissue, this blue beam here, coming from a discrete group of transducer elements. Either these transducer elements cover the entire face of our ultrasound transducer or they groups of elements that are fired off separately when we're looking at our linear arrays. So the width of this is the width of the transducer elements that we are firing off when trying to create our ultrasound beam.
Now this ultrasound beam naturally converges to a focal point and then diverges in the far field of our ultrasound beam. We have a near field and a far field and a focal Zone here. Now the near field depth is dependent on the diameter of the transducer elements as well as the frequency of that ultrasound beam.
Now if we were to place discrete elements within the ultrasound beam at differing depths and then were to move that ultrasound beam across these discrete elements, we can see that our resolution in the lateral plane changes depending on where the discrete element is within the ultrasound beam. If you have a look at the closed near field here or the far far field here, we get a blurring of that lateral resolution. That's because as this beam moves from left to right here, this element stays within the beam. It will be plotted along that plane depending on how far we move that ultrasound beam across.
If we were to have a look at this element that lies within our focal Zone, as we move this ultrasound beam to the left here, the element no longer lies within the focal Zone. It will be plotted as gray, no longer registering as the element within the beam. And the same happens if the beam was to move to the right ear. We would get a grayscale plotier and get a good lateral resolution within our focal Zone. Now remember that the width of the focal Zone here is roughly half the width of the diameter of the elements that are creating that ultrasound beam.
Here's another example to illustrate that point. If we were to have six objects within the tissue of our patient, here is superficial, here is deep, and we put an ultrasound beam through those objects. The first ultrasound beam that passes these objects will register these three distinct objects. As we move that ultrasound beam across, we will see that these objects are still registered by the near field in our ultrasound beam. The two objects that lie in the middle here, at the focal zone of our ultrasound beam, are separated by the beam. The beam no longer registers these two objects. Now as the beam diverges again, these two objects are undifferentiated. The beam is still registering that there's an object within it. Then if we were to move across again, the same thing happens. We are still registering an object in the near field, still registering an object in the far field, and now we register another object in our focal zone. So this is the image that we will get. We will not have differentiated these two near field objects, and we won't have differentiated these two far field objects, but our lateral resolution within the focal zone is improved. We are able to resolve these two discrete objects.
So in order for the beam to resolve two objects that are at the same depth but in the same lateral plane, we need the distance between those two objects to be more than the ultrasound beam itself. So lateral resolution is equal to the beam width. Whatever the beam width is at a certain depth, the two objects need to be further apart than that beam width in order for them to be registered as two discrete objects in order to resolve those discrete objects. So we can see that lateral resolution is a function of beam width.
Now unlike axial resolution, which didn't change with depth, the frequency and the spatial pulse length of our wave doesn't change as the ultrasound wave heads into tissue. What does change with depth is our beam width. We start with a wide beam width that narrows down towards our focal Zone at the end around the Airfield, and then the beam again diverges and we get a greater beam width as we head into the far field of our beam. So lateral resolution starts improving as we head to our focal Zone and then gets worse as we head out into our far Zone. Lateral resolution changes with depth.
Now the shape of our ultrasound beam is dependent on the diameter of the transducer elements that are creating the wave as well as the frequency of that wave. Now depending on the type of anatomy that we are scanning, we generally keep the diameter of the transducer element as well as the frequency of that ultrasound probe the same. So how then do we go about improving our lateral resolution with depth within our tissue? If we want a good lateral resolution both 5 centimeters and 15 centimeters into our image, but we've got a set transducer probe, how do we go about improving that lateral resolution? Well, this all comes down to being focusing. When we can manipulate that ultrasound beam, and we've looked previously that there are four separate ways to focus a beam. Now I've gone over these topics within the beam focusing talk, but I want to focus today on phased arrays, which is the most common mechanism we use in order to get good lateral resolution throughout our ultrasound image.
Now phased arrays use the differential in timing of the firing of our transducer elements in order to shape or Focus that ultrasound beam. If we have a large delay between the firing of our Central elements and the firing of the lateral elements on our transducer array, we get a near field of focus. As we reduce that delay, our focal Zone heads further and further out into the tissue. And if we were to fire all the transducer elements at the same time, our focal depth will be the furthest that it will be for the specific ultrasound probe.
Now what we can do is sequentially take these three images, the purple image, the green image, and the pink image, and then superimpose those images onto one another. Now what that does is it creates a greater field of depth where our lateral resolution is improved. We've got good lateral resolution all the way from the top of this purple focal zone right down to the bottom of this pink focal zone. Now this comes at the cost of temporal resolution. We need to now take three separate frames for one specific scan line. So the number of pulses that we need to take per scan line has now tripled, but we've got better lateral resolution in the depth of our image. Now frame rates and temporal resolution is something that we're going to look at in our next talk. So keep this concept in your back of your mind when we look at temporal resolution.
Now when we look at lateral resolution, it's a function of our beam width. And we looked earlier at the concept of side lobes here. Now side lobes are propagated in the direction of our beam and can effectively widen our beam as it heads into tissues. You can see here that the beam is effectively wider. Echoes coming from tissue boundaries here are still going to make it back to our transducer. So in order to improve lateral resolution, we want to mitigate or reduce those side lobes. And we saw that we can reduce our side lobes in three different ways. We can dampen our ultrasound beam, reduce our spatial pulse glands, and we saw that reducing our spatial pulse length actually improved our axial resolution. So dampening our ultrasound pulse both improves axial resolution and it improves lateral resolution by reducing the side lobes. We can also make our transducer elements thinner. If they are less than half the wavelength of our ultrasound wave, we get a reduction in the side load production. And lastly, we can make the amplitude of the waves that we create on the peripheries of our transducer slightly less. That will reduce the amount of side lobe that we created image. So that's lateral resolution in a nutshell.
Let's briefly look at elevational resolution. Now elevational resolution talks to differentiating two discrete objects that are in the elevational plane or the height plane within our image. They are at the same depth, but they are in a different Z plane. Now elevational resolution is much like lateral resolution. Lateral resolution has to do with the beam width as it heads into tissue. Elevational resolution has to do with the beam height as it heads into tissue. Now much like our beam width narrows down, our elevational height also narrows down at a focal Zone, and that's dependent on the height of our transducer elements. Now depending on the height of those transducer elements, the beam heading into the tissue will come to a natural focal point in the elevational plane of our image.
Now we can place an acoustic lens over the front of our transducer element that can focus that beam height down to a set focal depth, much like the acoustic lens can focus our beam width at a set of focal point. Now we've seen that we can change our focal depth by using a phased array in the lateral plane. The same thing can happen with elevational resolution. If we were to add more transducer elements to our array here in the elevational plane, we get what is known as a 1.5 D transducer array. Now a 2D transducer Ray is when we have the same number of rows as we do columns within our transducer element. A 1.5 D array has fewer rows than it does columns within the transducer element. Now generally, we will have about five to seven rows within a 1D transducer array, and we can now phase those elements much like we face the elements in the lateral plane. We can phase them in the elevational plane in order to get our elevational focal spot at the distance that we require. This is called a 1.5 D transducer array.
Now when we're looking at ultrasound resolution, our axial resolution is the best resolutional plane that we have. It's then followed by our lateral resolution, which is worse than our axial resolution but better than our elevational resolution. The elevational resolution within our image is the worst resolutional plane that we have in ultrasound. Now when we are looking at elevational resolution, if we have an ultrasound beam that is this thick, it's got this height, and we are looking at a blood vessel here, then we will get this image generated in the elevational plane. Now if we're trying to put a needle into this blood vessel, and this is the image that we've got, and our needle is coming in in this direction, we won't know where that blood vessel is.
Now as we reduce our elevational plane, we will get a different image forming here. We get an idea of where the center of that blood vessel is, but if we were to place our needle into this image, it'll be difficult to differentiate our needle from the blood vessel itself. And as we improve our elevational plane, we get better and better elevational resolution. Now why am I mentioning a needle going into a blood vessel? Well, often we place our ultrasound transducer onto the patient's tissue, and we place our needle in the elevational plane of that ultrasound. Now we want to know that when we see our needle within this image, that that needle actually corresponds to what we are looking at. The better elevational resolution we have, the more confident we can be in the placement of our needle.
So now we've looked at both axial, lateral, and elevation resolution within the ultrasound image, and we've looked at the various factors that we can change in order to improve our resolution. Now often the improvement of resolution comes at a cost, and we need to trade off what type of image we are trying to create versus the resolution that is required in order to get a diagnostic image. In the next talk, we are going to be looking at temporal resolution, our ability to differentiate changes in the tissue over time as we are scanning an ultrasound image. That will then round off this section of ultrasound resolution. So I'll see you all in that talk. Goodbye everybody.