Transcription
At the end of our last talk, I mentioned that we need to compensate for that attenuation of our incident ultrasound beam as it travels into a tissue. And if we don't compensate for that attenuation, the A-mode signal that we get back, and subsequently the B-mode grayscale that we get back, won't actually truly reflect the differences in acoustic impedance values because we have also lost intensity just by the ultrasound beam being attenuated.
Now, the way we compensate for that is what's known as time gain compensation. Now, you may have heard of this in various different forms. Often, people call it depth gain compensation or time-varied gain. You may have also heard of swept gain. These are all synonyms; they all mean the same thing.
So, how do we go about ensuring that this A-mode signal that we get back here truly reflects the differences in acoustic impedances at tissue boundaries, and it doesn't integrate the loss of intensity as the incident ultrasound beam travels through a tissue? Now, we saw that when we looked at ultrasound beam attenuation, attenuation was dependent on a couple of factors. Firstly, it was dependent on the distance traveled into the tissue. Ultrasound beam gets attenuated the more it travels into tissue. Secondly, it was dependent on our attenuation coefficient. Now, our attenuation coefficient was dependent on the ultrasound frequency; the higher the frequency, the more the beam is attenuated. And it was dependent on the actual tissue itself; how much scattering does that tissue have? How much heat loss happens into that tissue? Now, based on the frequency, as we increase the frequency, we get a steeper loss of relative intensity of that ultrasound beam. And as we increase our depth into the tissue, we also get loss of intensity of that ultrasound beam.
Now, when we look at our ultrasound beam traveling through tissue, we've seen that it attenuates based on that graph that we've just looked at. And the intensity of the ultrasound beam also changes as it travels through tissue. That's based largely on the beam geometry, which we are going to look at in our next talk.
How do we compensate for this attenuation? So, if we were to have multiple layers of tissue here, the orange layer has one acoustic impedance, and our lighter blue layer has another acoustic impedance. At each one of these tissue boundaries, the reflection should be the same. The difference in acoustic impedance between our orange and blue doesn't change as we head down into the patient. But we can see, because of this attenuation, our A-mode signal, our amplitude mode signal that comes back, is less as we head down into our tissues. And the subsequent grayscale of these A-mode echoes is completely different. This first reflection and second reflection here should actually be the same as these last two reflections if the ultrasound beam wasn't being attenuated. If that ultrasound intensity stayed the same the entire way through this tissue, these grayscale values and these amplitude values should be exactly the same.
Now, what we can do is what's known as time gain compensation, which is represented by this line here. Now, time gain compensation is something that happens after our echoes have been received. When that echo comes back to our piezoelectric crystal and it's converted into an electronic signal, we can amplify that electronic signal. We can increase the gain of that signal depending on how long that echo took to come back. The longer the echo takes to come back, the more we amplify that signal. So, this is what this line represents here. As time increases, ergo, as depth increases, the amount that we amplify that returning echo increases. We require very little amplification of these initial echoes because there's been very little attenuation here. And as we get deeper and deeper into the tissue, we need to post-process that electronic signal and amplify it depending on the depth that has traveled through that tissue.
Once we have had time gain compensation, ideally, our amplitudes will all be the same because these differences in acoustic impedance values are the same throughout this depth of tissue. Our reflected portion of that wave is always the same if the acoustic impedance difference doesn't change as we're traveling through tissue. So, this is a good way to calculate the level of time gain compensation we require for a specific ultrasound probe. Remember, this will change if the type of tissue we are imaging changes. It will also change if the frequency of our ultrasound transducer has changed because both of those values determine how much this incident ultrasound beam is attenuated.
Now, you will see on your ultrasound machines, a lot of them will have inherently this time gain compensation built in. But generally, there's also an option for the operator to change the depth gain or the time gain compensation. There are generally some knobs on the ultrasound machine that correspond to different depths within our ultrasound image, and we can slide those knobs up and down depending on how much we want to amplify those echoes returning.
Now, if we have an ultrasound beam that travels through, say, a large cyst where very little of that ultrasound beam is attenuated, and we still increase this gain later on in the image, we could be increasing that brightness of those returning echoes deep in our image to higher than they actually are. So, we need to realize when we are operating the ultrasound scanner and changing our time gain compensation that changing the gain incorrectly could lead to some artifacts within our image. But time gain compensation is a really good way to get equal brightness distribution throughout the depth of our image. And this is something that comes up in almost every single ultrasound physics exam. That's why I've dedicated an entire video specifically looking at time gain compensation. So, I hope this helped. I hope these diagrams have helped you to conceptualize what time gain compensation is.
And an important note to remember: we are not changing the strength of our incident ultrasound beam, and we are not increasing the amplitude of those echoes that are returning. The echoes remain the same. The electronic signal that those echoes generate, we then amplify. We're not changing the properties of the ultrasound wave with time gain compensation; we are changing the electronic signal that we have received and amplifying the electronic signal. We are not amplifying the wave.
Now, as I've mentioned, we are going to be looking at beam geometry next, which is a crucial concept to understand the various different ultrasound modes, as well as understanding resolution within our image. So, I'll see you all in that talk. Goodbye, everybody.