Transcription
Hello and welcome back. So, we've looked now at the generation of ultrasound pulses, as well as the reception of those echoes returning back to the ultrasound transducer. And we saw it was the piezoelectric material that both created and received those ultrasound pulses.
So now that we're receiving those echoes and we've converted that mechanical force into an electronic signal, how can we go about interpreting that electronic signal? Well, the ultrasound machine has multiple different ways of displaying that information, and we call this ultrasound modes. Now, in this talk, we're going to be looking at the A, B, and M modes in ultrasound scanning, and we reserve Doppler scanning for a whole section coming up in the future.
So let's start by having a look at the most simplest form, the A mode. Now, A stands for amplitude, and you'll see why that's important now. So we place our ultrasound machine on the patient's tissue that has various different tissues of differing acoustic impedances, and it's those differences in acoustic impedances act tissue boundaries which will determine how much of that incident wave will reflect back, echo back towards our ultrasound machine, and how much will be transmitted through.
Now, in A mode, we take a single line in this field here. We are not scanning an entire broad field. We are taking one line into these tissues here. So we've got one transducer element firing and receiving those signals. Now, it can either be one element in itself, or it can be a group of elements fired at the same time, but we are getting one piece of information coming back. We are receiving those echoes as one signal.
This part of our graph here, this represents the incident ultrasound beam as it's traveling into our tissue. And we know that incident ultrasound beam attenuates as it goes into tissue. The frequency doesn't change of that ultrasound wave. The speed changes depending on the type of tissue it's traveling through, and our intensity or our amplitude of that wave decreases as that ultrasound wave attenuates in tissue. And it attenuates by the giving of heat into the tissue or by scattering in the tissues.
Now, when that incident ultrasound wave comes into contact with a tissue boundary, we will get a reflected echo heading back towards our transducer. Now, those echoes are going to get smaller and smaller as they head away from the body because the incident ultrasound wave itself has gotten smaller. It has been attenuated. Its intensity or its amplitude is less.
Now, these small waves coming back, these are these scattered ultrasound waves coming back. These small waves give us our echo texture of the specific tissue. They are not reflected waves from tissue boundaries. They are scattered waves, waves that have come into contact with units that are smaller than the wavelength of our incident ultrasound beam, and those scattered waves have headed back towards our transducer here.
A mode, amplitude mode, helps us to determine where the tissue boundaries are, and the amplitude of our wave is what is registered at the transducer. And we get this series of high amplitude waves coming back, which represent our tissue boundaries. Now, it's important to remember, if there's no difference in the acoustic impedance of those two tissues, we won't get any echo coming back. So this A mode can only detect tissue boundaries with differing acoustic impedance values.
Here, I've represented these amplitudes, these peaks, these spikes of the echoes coming back with a grayscale value. Now, don't get this confused. This grayscale value is going to be looked at when we look at how beam mode imaging. Amplitude, A mode, only takes into account the amplitude of the returning echoes here.
Now, just a reminder, when we are sending a pulse into the tissues here, we are sending or transmitting a pulse and then waiting or receiving the echoes coming back. There's a transmit time and a receive time, and the combination of that transmit and receive time is our pulse repetition period. Now, this entire pulse repetition period happens in the depth of the tissue that we are imaging. The echoes return, and then only does our next pulse happen. We are gaining all of this information in a single pulse. And if we want to combine this information, we can release multiple pulses and stack those amplitudes on top of one another to get a better or a clearer image of our A mode ultrasound image.
So, the number of pulses that we release in a given period of time is our pulse repetition frequency. If we increase our frequency, we decrease our listening time, and we can only image shallower depths.
Now, if we were to take these amplitudes and plot them onto a graph, we can move that graph around, and we can see that from our ultrasound source, we can measure the time taken for these echoes to occur. Now, we know that time is equal to two times the depth over the speed of sound through that tissue, and we use the speed of sound in soft tissue, 1514 meters per second, as our average speed of sound when calculating ultrasound speed.
Now, we can see if we know the time it takes for that ultrasound wave to reach our tissue boundary and to come back, we can then go and calculate the distance of that tissue boundary. So we can plot these amplitudes now as a distance. Distance is the time taken for that ultrasound wave to be released into the tissue, interact with the tissue boundary, and return back. We take that time, times it by the speed of sound, so we get the total distance traveled, and we divide it by two because it's an out and back distance. That will give us the depth of the tissue boundary. So these spikes, these amplitudes, correspond to the depths of the tissue boundaries.
For the most part, we no longer use amplitude mode. There are very specific circumstances where it's used. The most common place it's used is in ophthalmology, where an ultrasound probe can be placed onto the eye wall, and we can see where the lens starts, where the vitreous humor is, and where the retina is. And if there's a mass within the retina, that retina may be moved closer towards the front of our eye. So we can use these distances to see where the various components of the eye are. And in the past, it was also used to see if there was midline shift in a neonate. If there was midline shift in the brain, and that falx cerebri was moved to the side, we could see that the distance between our skull, the falx cerebri, and the other skull was no longer symmetric, as there was midline shift. That falx cerebri has moved closer towards our ultrasound scanner or away from our ultrasound scanner.
These grayscale values correspond to the amplitude. They correspond to the strength of that echo returning back, and we can use these grayscale values when creating our B mode ultrasound image.
B mode stands for brightness mode, and this is probably the ultrasound that you are most familiar with. Now, here in our A mode, we were scanning a single line. Now, if we were to move to our next transducer element and scan another line, and continue moving across the face of our transducer here, scanning line by line by line, and getting the amplitudes back of the tissue boundaries here, and changing those amplitudes to a grayscale value, the higher the amplitude, the lighter that grayscale value. We can create a series of grayscale values that will represent now this entire tissue boundary here. Now, we have scanned along the front of our transducer, one line at a time, a single line of A mode information at a time.
In order for our eyes to perceive this image that we have created here as a continuous image, this entire process here needs to happen at least 24 times within one second. We need to scan through that tissue 24 times or more in order for that image to look like a live image. Our eyes won't then see the flickering between the rows here. We will perceive that as a continuous image, much like when we are watching a video. We're actually seeing a series of still images at a speed that looks normal to us. When you're watching this, this is going at 24 frames a second. It looks like my movement is continuous because our eye can't tell the difference between those separate frames. The same happens here in brightness mode.
Now, each A line again is created from a pulse and a receive time, a pulse repetition period. And we can determine how much time we have for each scan line if we know the number of scan lines in our image and we know that we need to get through those scan lines at least 24 times within a second. And given that information, we can then calculate the pulse repetition frequency that we need in order to create our image. So we're going to calculate these later on. I just want to make you aware of the fact that our pulse repetition period and frequency affect the depth and the resolution that we can get in our B mode or our brightness mode images.
Previously, we looked at this diagram that I made showing you how we go about scanning in B mode, and we take a single scan line at a time, watching that scan line go through the tissue like that until we have scanned the entire length of our transducer. We can then create our B mode or our brightness mode image here.
Now, you may be wondering, how are we getting different levels of gray within the tissues themselves, not the tissue boundaries? We've seen the tissue boundaries are our A mode echoes coming back. How are we getting this grayscale value, and how is this grayscale value different? Well, that was that scatter coming back. B mode not only takes those A mode tissue boundaries, but it also takes the scatter coming back towards the transducer and it plots that low-level scatter as a signal in our image. That's the echo texture of the organ that we are imaging, and that echo texture is largely due to the scattering of that specific tissue.
If we have a closer look at our B mode scanning here, we can see that we take a single line and plot our A mode image here. These amplitudes are going to be converted into a grayscale or a brightness scale. We then look at our next line, and our next line, and our next line, and we can see how, as we scan each line, we are plotting the tissue boundaries here. We know these depths here because we can calculate those depths based on the time those echoes took to return. Again, I've simplified this ever so slightly by not showing these scatters coming back. That scatter coming back, that background noise, as well as the scatter, is contributing to the grayscale or the echo texture of the tissue that we are imaging. So this is how we go about creating our B mode images. So we can see we have an ultrasound scan with our field of view here. We're scanning through that tissue, getting our echoes coming back in separate A mode lines, and we can convert that electronic signal into an image that we display on our ultrasound machine. This is our B or our brightness mode.
The final mode we'll look at is what's known as our M mode, or our motion mode, and that actually combines our A mode and our B modes. So you can see here, we've got a B mode image being created by our ultrasound machine. We've got our chest wall, and this is actually our heart here, right ventricle, left ventricle. This is the anterior wall, our anterior right ventricular wall, our right ventricle, our interventricular septum, our left ventricle, and our posterior left ventricular wall.
What we can do is place a single scan line. We've seen a single scan line before, an A mode scan line, coming through the heart here. So we can superimpose that A mode scan line over the B mode image that we have created. Now, you'll know that the heart itself moves in ventricular contraction. The distance between our interventricular septum and the walls of our ventricles will get smaller, and in diastole, when we are filling those ventricles, the distance will get bigger.
Now, we can take this A mode scan line and plot it on a graph with depth here. And that A mode scan line, we can then represent the various different echoes coming back as a grayscale value, just as we did in our brightness mode. Now, what we can then do is plot those A mode signals that we've converted into a brightness mode over a period of time, and we can see how those A mode signals change on that specific line that we have put on our B mode image here. So this now will then scan over a period of time. Importantly, this tissue that we are now representing on this graph here is the same tissue. It's along the same line here. As our ventricles contract, we can see that the walls become closer together, and the space within the ventricle gets smaller. We can plot this over time. Our motion mode allows us to assess the motion of the walls here. We can also place this A mode line over a valve leaflet within the heart itself, or we can look at the movement of lung over a period of time. This motion mode allows us to see that movement within the axial plane here.
Now, motion mode has become less relevant over time now that we have developed Doppler mode, where we can actually look at the flow of blood over time. We can measure the velocity changes within our ventricular walls and within the valve leaflets. But motion mode still holds a lot of value that we can use on a daily basis.
Now, there's one issue that I've left out. I haven't covered while we were looking at our A, B, and M modes, and that's that our intensity of our incident ultrasound actually gets exponentially smaller as it travels through depth. We saw that attenuation of the incident ultrasound beam was dependent on the type of tissue it's traveling through, the depth of that tissue it's traveling through, and the frequency of that ultrasound being. The higher the frequency, the faster it is attenuated. Also, when we looked at intensity, we saw that the intensity changes as the ultrasound travels into our tissues. As our ultrasound beam gets narrower, as we focus down that ultrasound beam, the intensity gets higher because we are reducing the area that the power is distributed over. And as our far field develops, as that ultrasound wave diverges, we get a loss of intensity here.
Now, when we were calculating our amplitude mode here, we didn't take into account that loss of intensity. This reflection that is happening deep in our tissue here, we don't know if that reflection is small based on the fact that there's very little difference in acoustic impedance here, or if it's small because our incident ultrasound wave has been highly attenuated.
Now, what we need to do is compensate for this loss in attenuation. We need to ensure that ultrasound echoes that are returning from a far distance away from our transducer are amplified in order to correct for that attenuation. And this is what's known as time gain compensation, which we're going to look at briefly in our next talk. So I'll see you all in that talk. Goodbye, everybody.