Transcription
Hello everybody and welcome back. Now, in the previous talks, we've been looking at the actual sound wave itself, and we've looked at the various different parameters we can use to describe that sound wave. Now, I want to take a look at how we actually go about creating an ultrasound image that we can display on our machines.
Now, a lot of textbooks will leave this kind of topic right till the end, and only once we've learned everything, we'll bring it all together and show you this final outcome. I want to show you the broad, overarching view first, before we dive into these individual components. So, some things I mentioned today might seem a bit foreign. Don't worry, we're going to cover those topics in the future. It's like if I was to describe to you an ingredient list without telling you what we were baking. I want to show you the cake first, and then we're going to go into the ingredients, see how we can combine the various different parameters in order to get our end result.
So, what we're going to be looking at today is called pulse echo ultrasonography. That you can get continuous ultrasound and ultrasound wave that is continuously passed through a tissue, or pulse echo ultrasound. Now, in order to generate an image that we can display on a screen, we need to use pulse echo ultrasonography. Continuous ultrasound is continuously propagating or transmitting an ultrasound wave through a tissue and leaves no time for listening to the echoes, the waves that are bouncing off the various different tissues. So, pulse echo allows us to send a short pulse of ultrasound into a tissue and then wait in what is known as the receive time. That echo goes into the tissue and when it comes into contact with tissue boundaries, some of that pulse echo will be reflected back towards the ultrasound transducer, and we can then receive that echo. Some of the ultrasound pulse will pass through that barrier until it reaches the next tissue boundary, and another echo is transmitted back towards the patient. So, we have what is known as a transmit time, that's when we are generating an ultrasound pulse, and a receive time when the ultrasound machine isn't generating a pulse and it's listening for the echoes coming back off the tissue boundaries.
Now, pulse echo ultrasonography is the mainstay of our diagnostic imaging, and this is what we're going to spend most of our time looking at in this ultrasound physics module. Now, I want to look at five different parameters that we can use to describe this pulse echo sequence and how those parameters are changed depending on the depth that we are imaging in our tissues.
Now, I'm going to be representing these pulses with these sine waves here. Here I've got two wavelengths of a single pulse. Now, I just want you to remember that when we generate a pulse, we don't actually get this uniform, same amplitude happening throughout these pulses. There are varying pulses, the center of which will be at our transmit frequency. Now, we're going to look at these pulses later on when we look at the ultrasound transducer itself and the piezoelectric material within that ultrasound transducer, but just remember, these pulses aren't actually perfect, but I'm going to represent it like this for teaching purposes.
Now, when we place our ultrasound machine on the skin of a patient, those ultrasound waves are transmitted through the patient's tissue like this, and we've seen that there's a narrow field to this ultrasound wave and a far field with a focus point within the ultrasound, where all of which we're going to look at in future talks.
Now, various different tissues within the patient have different properties. They've got different bulk modulus values and different densities, and not only that, but we can define the tissues within a patient by a factor known as acoustic impedance, which we're going to look at in an upcoming talk. Now, the differences in the acoustic impedance determines how much of each pulse echo is either transmitted through a tissue boundary or echoed back towards our transducer. So, what happens is, we release a pulse from our ultrasound machine, and that travels through the patient tissue until it reaches a tissue boundary here. Now, what happens is, some of that pulse will be transmitted through that tissue boundary, and some of it will be echoed back towards our receiver. Now, during this time, the receiver is not releasing more pulses, it's listening for those echoes to come back, and that happens in each and every one of these tissue boundaries due to the difference in acoustic impedance of these different tissues.
Now, I've drawn a set distance here from which our ultrasound pulse will either be echoed back or transmitted through, and I've represented this graphically here. Our ultrasound pulse is coming into contact with this tissue boundary. Some of that pulse will be transmitted through, and some of it will be echoed back towards our transducer. Now, we can use what is known as the range equation to determine this depth, and it's this equation that the ultrasound machine uses itself to plot those echoes on our screen. The time taken for that echo to come back can then be used to calculate the distance that that tissue boundary was at. So, our time that the echo takes to go from the ultrasound machine to the object that we're imaging, to this tissue boundary, and return back, is represented by this equation here: two times the depth, this distance here, over the speed of sound.
Now, we use the speed of sound in soft tissue, an average speed of sound through soft tissue of 1540 meters per second. That's a value that you need to remember. Now, we can rearrange this equation in order to isolate distance here. So, when the machine, in its receive time, when it's listening for echoes to come back, it takes a time from when it released that pulse all the way to when it receives that echo, and it plugs that into this equation here. And that time times by the speed of sound will determine the distance that it traveled. But that's a round trip distance from the ultrasound probe to the tissue boundary and all the way back. So, we need to divide that by two in order to get the one-way distance of that ultrasound pulse traveling through the tissue.
Now, as I mentioned before, this is the critical value that you have to commit to memory. Many past papers ask you to calculate these equations here, and they don't give you this value. This is a value that is assumed knowledge, and you need to remember it. Now, in the ultrasound question bank that I've linked below, we'll go through multiple different equations, and I'll get you to calculate various different values. At no stage will I give you this value. This is something that you need to remember. So, 1540 meters per second.
Now, you can see that the speed of sound actually does vary quite a lot in the various different materials, but we use this soft tissue average. So, this is a little bit of a false assumption, and we can actually get some artifacts based on the fact that the sound doesn't actually travel at this speed all the way through the various different tissues.
Now, when we look at an ultrasound machine trying to image a particular piece of anatomy, what the ultrasound machine does is it fires off various different lines of ultrasound pulses. So, what it will do is, we'll fire off a single ultrasound pulse here and wait for those echoes to come back. It plots there in the tissue boundaries based on the time that those echoes took to come back to the ultrasound probe. And each line, we will do multiple of those pulses in order to get good data acquisition back. The ultrasound machine will then sequentially work its way all the way across the ultrasound transducer like this. At each point, the tissue boundary distance will be calculated by the machine, and a grayscale value will be assigned to these tissue boundaries based on the intensity of the echo that is returning to our ultrasound machine. The ultrasound machine then continues to scan all the way through this piece of anatomy until it has scanned our entire field of view. Once it's scanned that field of view, it can then compute all of those echo values and plot them on a grayscale value on a machine here. Tissue boundaries that have high differences in acoustic impedances will appear bright on our image, and we can get an idea of the underlying anatomy here.
Now, the echoes coming back represent the tissue boundaries, and the grayscale within the tissues is what's known as the echogenicity of that tissue. All of these, we're going to look at in more depth later on.
Now, when we look at our ultrasound pulse here, we've seen that we've defined various values in previous talks. Now, let's look at the five different values that we can look at when looking at pulse echo ultrasonography. Now, again, everything above this line represents a time value, and everything below represents a distance value.
Now, the first value that we can look at is what's known as the pulse duration. The time taken for an entire pulse to be emitted from our ultrasound machine. And we can calculate this by multiplying the number of cycles in our pulse by the period of a single pulse. Remember, the period is the amount of time it takes for a single wavelength to pass a particular point. If we times that amount of time by the number of cycles, we will get what's known as our pulse duration. And because period is inversely proportional to frequency, we can also calculate our pulse duration as the number of cycles divided by the frequency of our ultrasound pulse here. Now, this is representing the transmit time, the time that the ultrasound is actually generating an ultrasound wave. Remember, we then pause and wait for echoes to come back, our receive time here. This is the time that the ultrasound machine is actually creating ultrasound waves.
The next value that we can look at is what is known as the spatial pulse length. Now, the spatial pulse length is a distance measurement. It's the length of our pulse as it moves through space. Our spatial pulse length is the number of cycles within our pulse times by the wavelength of a single wave. This is the distance from the start of our pulse to the end of our pulse. Now, these two factors, the pulse duration and the spatial pulse length, are dependent on the frequency of the wave and the wavelength of the wave. These are things that we cannot change when we put our ultrasound transducer to the skin. We are generating a set frequency, so we can't change the length of this pulse, we can't change the distance or the time that the pulse is emitted during our transmit part of our cycle.
Now, when we were calculating the depth of the various different tissue boundaries, we used these equations up here. Now, our pulse has both a transmit time and a receive time, I've just mentioned. Now, we cannot change this pulse duration here, the pulse duration is fixed. So, if we want to image at a further depth within the patient, the only thing that we can change is the receive time, the amount of time that we wait to listen for those echoes to come back. We can't repeat another pulse until all of our echoes have come back. If we were to prematurely repeat a pulse, we would get interference with those returning echoes coming towards our ultrasound machine. So, if we want to image superficial structures, we can reduce our receive time, the amount of time it takes for those echoes to return, it's very short, and we don't need a long listening time. The deeper we want to image within our tissue, the longer our receive time needs to be. So, when we set the depth on our ultrasound machine, we are changing the receive time in our pulse echo ultrasonography, and that change in receive time is going to influence three separate factors, the remaining three factors that we need to look at in our pulse echo sequence.
Now, the first two factors I'm going to share together because they are related to one another. The first is known as the pulse repetition period here. Now, the pulse repetition period describes the amount of time between the start of one pulse and the start of the next pulse. It is the time taken for our pulse, our transmit time, added to the time taken for our receive time. How long does it take from when we generate a pulse until we generate our next pulse? That is our pulse repetition period. You can see that as we image shallower depths, we can reduce our receive time and reduce our pulse repetition period.
Now, our pulse repetition period is inversely proportional to what is known as our pulse repetition frequency. Now, the pulse repetition frequency determines the number of pulses, these pulses here, the number of pulses that we can fit in in one second. It's a time value as well, much like frequency was the number of waves within a cycle passing a particular point in a set period of time. The pulse repetition frequency is no longer the number of wavelengths passing a particular point, but the number of pulses passing a particular point in a period of time. So, don't get confused between our pulse repetition frequency and the frequency of our wave. In the same breath, don't get confused with our pulse repetition period and the period of our wave. They're not describing the same thing here. So, we can calculate our pulse repetition period if we're given our pulse repetition frequency.
Later on, when we look at these concepts in more depth, I will show you a separate way that we can calculate these when we don't need either the pulse repetition frequency or the repetition period to calculate the other value. But for now, I want you to understand the relationship between these two values. The way I like to think of this is as a seesaw between our pulse repetition period and our pulse repetition frequency. As our pulse repetition period increases, as this time gets longer, we can image a further and further depth. The depth in our image increases because we've got more received time. Remember, we're not changing this pulse duration, we're only changing the receive time here. Now, because we're changing our receive time, we're increasing our receive time, we're increasing the depth that we can measure, and we are increasing our listening time. Our listening time is synonymous with our sleep time. Increasing this reduces the number of pulses that we can fit in an individual second. Our pulse repetition frequency will be decreasing, and the opposite is true. If we increase the frequency of our pulses, we decrease our listening time, we decrease the amount of depth that we can image within a tissue. So, these values are something that we can actually change when we change the depth on our ultrasound transducer. When we change the amount of depth that we want to image within a patient, we will inadvertently be changing the pulse repetition period and the pulse repetition frequency.
Now, the last parameter I want to look at is what is known as the duty factor of our pulse echo ultrasonography. Now, the duty factor determines the percentage of time that the ultrasound machine is transmitting a pulse compared to the entire time that we are taking an image. It determines the intensity that our patient will be receiving in a given examination. If we were to put an ultrasound machine on a patient and scan them for one minute, our duty factor will help us determine of that minute, how long are we actually creating ultrasound waves compared to how long we're actually listening for ultrasound waves to come back. In that listening period, when we're waiting for echoes to come back, we are not importing any intensity onto our patient. There's no power being transmitted into the tissue, we're just passively waiting for echoes to come back. And our duty factor will allow us to determine what percentage of time are we actually generating an intensity or power, are we actually making that wave work through tissue in our patients. And to calculate our duty factor, we take our pulse duration, this is a time value, and divide it by our pulse repetition period. So, we are saying, what is the proportion of our transmit time over the entire time of our pulse here? So, we take our transmit time and we divide it by our transmit time and our receive time, otherwise known as our pulse repetition period. Now, if we want to get this as a percentage value, we can times this ratio by 100, and that will give us a true percentage known as our duty factor. And when we look at biologic effects of ultrasonography, we are going to revisit this concept of a duty factor.
So, by now, we've covered an entire spectrum of parameters: those that relate specifically to the ultrasound wave itself, and those that relate to the pulse when we're looking at pulse echo ultrasonography. Now, I want you to become familiar with these terms, and there are terms that we are going to revisit when we look at various different components of our ultrasound physics module. But I thought that I would introduce these concepts to you now, so that when we revisit them later, they're not foreign concepts to you. You understand what I'm talking about. And I will show you that when we look at our spatial pulse length, that's going to become critically important when we look at the axial resolution within our image. We've seen how these parameters change with the depth that we're imaging within our tissues. All of these are crucial when we're trying to understand how we generate an ultrasound image.
So, in our next talk, we're going to look at how these pulses actually interact with the tissues within our patient. There are multiple different interaction types that we need to be aware of, and we need to know how they affect our image quality in the end. So, I hope this has helped you to get a broad, overarching overview of how pulse echo ultrasonography works. Now, we can look at the individual ingredients within our recipe, see how they combine, see how they work with one another in order to create that ultrasound image that we can analyze with our patients. So, I'll see you all in the next talk. Goodbye everybody.