Transcription
So let's move on to our final type of resolution, which is temporal resolution.
Now, temporal resolution is the ultrasound machine's ability to detect and display real-time movement within our ultrasound image. Now, either that movement comes from the organ we are scanning, the heart beating or the lungs moving as a patient breathes in, or the movement comes from the operator moving the ultrasound probe. And how accurately that movement is represented within our image is what's known as temporal resolution.
So, when we go about actually creating an ultrasound image, we've seen that the ultrasound probe projects a field of view into the tissue that we're scanning. Now, this field of view represents what's known as a frame. And in order to create one B-mode frame, we've seen that multiple A-lines of data have been acquired and stitched together and created that single frame. Now, multiple frames played one after the other in quick succession will give us what looks like a moving image. Now, there'll be a certain point that the frames played quick enough, we won't be able to tell that we are seeing actually individual frames. It will look like smooth motion to us.
Now, in most video that we watch, anything above 24 frames per second is perceived to us as smooth movement. Now, this concept applies in ultrasound imaging. Our frame rate is synonymous with our temporal resolution. The speed at which we can display frames on our ultrasound image will determine how much motion we can detect within our ultrasound image.
Now, the frame rate is determined by the number of frames per second of image acquisition. We can also think of a frame rate as taking one second and dividing it by the time it takes for our ultrasound machine to acquire one single frame. The time it takes to acquire that frame, how many times can we fit that into one second? That gives us our frame rate.
So, what goes into acquiring a single frame? We've seen that acquiring a single frame takes multiple A-lines of data and then stitches those A-lines of data together and creates a single frame. So, the time it takes to create a single frame is the number of scan lines within our field of view times by the time it takes to acquire a single scan line.
Now, we've seen that to acquire a single scan line, the ultrasound machine needs to emit a pulse into the tissue. That pulse travels into the tissue to a set depth that we've set on our ultrasound machine and then reflects off a surface, returning back to the ultrasound machine. That time it takes for that round trip is the time it takes to acquire a single line of ultrasound data. And we've looked at this concept before. We can calculate that round trip time.
To calculate that round trip time, we take the depth in our image, the depth that we want to image, and we times it by two. We've traveled five centimeters into tissue, we need to travel five centimeters back in order to complete that round trip. That's why we multiply that depth by two. Then we can divide that total distance that has traveled by the speed of sound within that tissue. That will give us the total round trip time that that wave took to travel in tissues. And we know what the speed of sound in soft tissue is. It's 1540 meters per second.
Now, we're not measuring meters into our patient, we're measuring centimeters into the patient. So we can convert those units into centimeters per microsecond and use this depth value as centimeters. Now, our speed of sound is constant. So the only thing that's going to change the amount of time that it takes is the depth that we want to image within our tissues.
Now, we can divide this coefficient here, the number two, by the speed of sound to give us this formula here. The time it takes for one line of data to be acquired is 13 microseconds times the depth that we want to image within our field of view. Two divided by 0.154 is 13 microseconds. Now, this is specific to soft tissue. If we were imaging another tissue type, we wouldn't be able to use this formula here. This only works for speed of sound in soft tissue.
And what we can see with this equation now is that increasing our depth is going to increase the amount of time it takes to acquire a single A-line piece of data for our entire field of view. And we can see that changing the depth is going to ultimately affect our frame rate.
So, what have we actually calculated here? Well, we've calculated the time it takes for one pulse to go into our patient's tissues to a depth that we have specified and returned back before we can release our next pulse. And we've seen that before, that's our pulse repetition period. The amount of time we can have between our first pulse and the next pulse in our sequence. So, our pulse repetition period is both our transmit time and our receive time. This combined time is the pulse repetition period, the amount of time it takes before we can release our next pulse.
So, if we look at our green pulse repetition period here, we can see that imaging shallower depths here, the pulse is released, reaches the depth that we want to image and returns back. It's a much shorter time. Our time taken for an individual A-line of data is much shorter. If we want to image deep into the patient's tissue, we need a longer receive time. Our pulse repetition period gets longer. The time it takes to acquire a single A-line piece of data is ultimately longer.
So, what does this mean for us? What does it mean to the person actually holding the ultrasound probe wanting to improve their temporal resolution? Well, what can we change within these formulas? We can't change the speed of sound, that's constant in soft tissue. We can change the depth in our tissues, and we can change the number of scan lines within our field of view. So, this section of formulas becomes important when trying to determine what factors we change in order to improve our temporal resolution.
So, let's have a closer look at these formulas. The time it takes to acquire a single frame within our field of view is the number of scan lines within the field of view multiplied by the time it takes for a single A-line of data acquisition. So, when we're trying to improve our temporal resolution, we can alter the number of scan lines.
The first way we can alter the number of scan lines is by taking our field of view with a set number of scan lines and simply reducing the line density, reducing the number of A-lines that we take within our field of view. We can see that the A-lines per centimeter of our field of view here has decreased. Our line density has decreased. Now, the time it takes to acquire a single frame has decreased. We've got better temporal resolution. We can acquire a frame quicker, so we can have a higher frame rate in our image.
Now, you can see here, we're obviously going to lose lateral resolution when we decrease our line density. Our A-lines are further apart. We are going to miss some anatomy within this field of view.
Another term we have for line density is lines per degree, the number of lines that are represented within each degree of our field of view in the ultrasound transducer. Now, what if we don't want to sacrifice our lateral resolution? What if we want to reduce the number of scan lines without compromising our lateral resolution? What we can do then is narrow down our field of view. Our line density remains the same as our initial large field of view, but we've reduced the number of scan lines by narrowing down that field of view. If we were scanning a kidney here and there was a cyst in one small region of that kidney, and we were only interested in that cyst, we could narrow down our field of view. We effectively reduce the number of scan lines. We've got a narrower field of view. We've got a quicker frame rate, a better temporal resolution.
So, we can either decrease the line density or we can decrease the size of our field of view. And we've seen from this equation that it's changing the depth that is going to change the amount of time it takes to acquire that single A-line of data. So, as we reduce the depth in our image, the time it takes to acquire a single line of data also reduces. If we halve the depth, we double the frame rate that we can get within our image here. So, we always want to image at the most shallow depth that includes the anatomy that we're interested in. The deeper we make our field of view, the longer the round trip time for that pulse, the longer our pulse repetition period, the longer it takes for a single line of data to be acquired, and ultimately that will result in a longer time for our entire frame to be acquired.
Now, there's one more way that we can change the time it takes per A-line of data, and we looked at this when we looked at lateral resolution. In order to improve our lateral resolution in the longitudinal plane, we fired multiple different focal points along a single A-mode line. Now, changing those focal points means that for each line within our field of view, we need to fire a single pulse repetition period for each focal point. Now, on your ultrasound machine, you will be able to set focal points within the depth of your field of view. Every time you add a focal point here, it will extend the amount of time it takes to get that single line of ultrasound data. Now, for this single scan line here, we need three pulse repetition periods. In this example, our frame rate will decrease by a factor of three. So, although we get better lateral resolution within the longitudinal plane of our ultrasound image, that comes at the expense of temporal resolution.
Now, knowing these parameters is all you really need to know for temporal resolution. And knowing how these parameters will affect our frame rate, the number of frames per second displayed on our ultrasound machine, will help us understand how these factors affect our temporal resolution.
So, in order to get the best temporal resolution, we want to image at the shallower depth possible, with the smallest field of view possible, with the fewest focal points possible, and the least number of scan lines possible. Now, obviously, changing these parameters comes at a cost, and we need to trade off what is more important in the image we are trying to acquire. Is lateral resolution more important than our temporal resolution? Are we trying to track fast movements over time? There, maybe temporal resolution is better.
So, hopefully, by now, you've got a good conceptual understanding of axial, lateral, elevational, and temporal resolution within our ultrasound image, and which factors we can change in order to manipulate the resolution in our image.
Now, when we've been looking at temporal resolution within this talk, we are looking at pulse-echo ultrasonography, short pulses with long receive times, waiting for that echo to return. Those short pulses have a very low quality factor. There's a wide variation in frequency in those pulses. Now, in our next talk, we're going to be looking at Doppler ultrasound, and we're not interested in the echoes returning. We're interested in the frequency change of our ultrasound beam that's heading into the tissue and returning back to our ultrasound transducer. And because Doppler is dependent on frequency, we need an ultrasound beam that has a longer pulse length, has very little variation in the frequencies, so a better quality factor, in order to use that frequency change to detect movement within our tissues.
Now, because of that longer pulse length, we are going to reduce our temporal resolution in Doppler ultrasound imaging. So, another factor that prevents us from improving our temporal resolution is by turning on Doppler mode on our ultrasound machine. So, I'll see you all in that talk where we're going to take a deep dive into Doppler ultrasonography. Until then, goodbye everybody.