Transcription
Hello everybody and welcome back. Today we're going to be reviewing some of the common ultrasound artifacts.
Now, what exactly is an artifact? Well, an artifact is a misrepresentation of the anatomy on our ultrasound image. If you cast your mind back to Doppler ultrasound imaging, we looked at the concept of aliasing, where our calculated velocity of the blood was actually incorrect. We misplotted those spectral waveforms; we weren't accurately representing what was happening in the tissues, and that's exactly what an artifact is.
Now, today we're going to be looking at B-mode ultrasound artifacts, and they all occur because of assumptions that the ultrasound transducer makes. Now, these assumptions are crucial in order to actually create an ultrasound image, but they are actually false assumptions, and it's these false assumptions that lead to the development of artifacts within the image.
Now, the first assumption that the machine makes is that sound travels at a constant speed. And we've seen that sound travels actually at different speeds through different tissues, depending on the bulk modulus and the density of those tissues. We just assume that sound travels at 1,514 meters per second in soft tissue. We know that's actually not the case.
We also assume that sound travels in straight lines. And we've seen sound reflecting off surfaces at angles. We've also seen sound refracting, and in those cases, sound isn't actually traveling in a straight line.
It also assumes that echoes returning to the ultrasound machine have returned from a perpendicular surface. They are congruent with the initial pulse that we've sent out, and that's not always the case. We can get scattered echoes returning; we can get echoes bouncing off a surface, bouncing off an object, and returning. The machine assumes that those returning echoes have come from the A-line in which it sends out the pulse.
It also assumes that sound only reflects off a tissue boundary once. Now, sound, when it comes into contact with a tissue boundary, some of it will echo back towards the ultrasound machine, and some of it will be transmitted through the tissue. Now, that transmitted ultrasound wave will also send back echoes towards the ultrasound machine, and that echo wave will again come into contact with that tissue boundary, and some of that echoed wave will then echo back into the tissues. Now, the ultrasound machine assumes that that doesn't happen, that there isn't multiple echoes between tissue boundaries.
And the last assumption that is made by the transducer is that the attenuation of sound throughout our B-mode image is linear, depending on the depth that the ultrasound machine is traveling. And we will see it actually depends on the types of tissue through which the ultrasound machine is traveling. Ultrasound is highly attenuated by bone, for instance, but travels through water with very little attenuation. And when it creates the ultrasound image, it assumes that all of those A-lines of data have been attenuated in the same way as it travels into the tissue depths.
So, the first artifacts that we're going to look at occur because of refraction. Now, refraction occurs when the incident ultrasound pulse doesn't come into contact with the tissue boundary in a 90-degree angle. It comes in at an angle, an angle of incidence here. Now, if this ultrasound pulse was to speed up as it heads into the second tissue, the speed of sound increases because of the differing bulk modulus and density in the second tissue, we will get an angle of transmittance that is less than our angle of incidence. If that sound was to slow down, our transmittance angle will be bigger than the incidence angle, and it's this changing of the angle of the ultrasound pulse that can lead to two different artifacts.
Now, the first artifact we're going to look at is known as edge shadowing. When we have an object within our ultrasound image, we can send out a pulse towards that object. Some of it will be reflected back to the ultrasound machine; some of it will be reflected at an angle that will make the edge of this object look a bit brighter than it actually is, and this will also occur on the other edge of this object. Now, some of the ultrasound pulses that we send out into the tissues will make it into this object, and because the angle of incidence is not 90 degrees, that sound will be refracted if there's a speed change within this object. Now, that refraction of sound means that the transmittance angle will be different to the incidence angle, and there will be a region of tissue here that receives no ultrasound pulses. Either the sound is refracted or it is reflected away. We are not getting any echoes returning from this edge here. We've formed an edge shadow as a result of refraction of sound waves on the edge of this object here, and this obviously occurs on the other side of the object. Now, this isn't happening in the center of the object here because we are getting perpendicular interactions with the object surface here; we aren't getting refraction.
The second type of artifact due to refraction is when an ultrasound pulse heads into tissues and it comes into contact with the tissue boundary. Again, that pulse is refracted. We've changed our angle; a transmittance angle has been created, and an echo can come off an adjacent object heading back to the ultrasound transducer. Now, this transducer, when it receives that echo, how it plots where this object is, is by taking the round trip time for the pulse to head out into the tissue and to return. And it assumes that that pulse has traveled straight down the A-line. It's gone in a straight line into our image and echoed back off a perpendicular surface straight back to our ultrasound transducer. So, it takes an increased amount of time here, but the ultrasound transducer will now plot that object as being directly in line with this initial A-line, and we get this object being duplicated here. Now, there will be ultrasound pulses that come into an angle here that has a 90-degree angle with this tissue boundary and correctly plot the object within the tissues, but we are incorrectly plotting that object a second time. This doesn't represent the anatomy within the tissues. So, we can see that this here is the artifact; it is actually incorrect. And the way we can reduce this artifact is by changing our angle of incidence, maybe digging in the heel of the ultrasound in order to get these ultrasound waves perpendicular with this tissue boundary. If that occurs, then this duplication should disappear. Now, notice where the duplication happens; it is both in the second tissue here. They are at similar depths within the ultrasound image.
Our next ultrasound artifact we're going to look at is called mirroring, and it can look quite similar to that refraction artifact. Now, mirroring occurs when an ultrasound pulse heads out towards an object. Now, this object can be correctly imaged by a pulse heading out towards the object and returning an echo back, plotting that object directly in the B-mode image. Another pulse may head all the way out to the tissue boundary and head back to the ultrasound transducer, plotting that tissue boundary correctly. Now, if this is a highly reflective tissue boundary, a specular reflector here, what can happen is this pulse that reaches the specular reflector can return towards our ultrasound machine but echo off the undersurface of this object here. That echo will then head back towards the specular reflector before heading back to the ultrasound transducer. That pulse has now taken a longer time to return to the transducer, and again, the transducer only takes the time it takes for the pulse to return in order to plot where the anatomy is on that B-mode image, and we're going to create a mirror image of this object on the other side of this specular reflector. Now, this often happens at the diaphragm when we've got lungs here, we've got liver here, we have a lesion in the liver, and we get a mirroring of that lesion within the liver. This is incorrect here; this object, it doesn't in fact exist; it is an artifact, a mirror artifact. Now, this mirroring is on either side of the specular reflector, not like our refraction, which was on the same side of the tissue, and these should be equal distances away from the specular reflector, mirror images of one another.
Let's move on to the next artifact, which is called reverberation, quite similar to the mirroring artifact. When we have a strong reflector perpendicular to our ultrasound transducer, what can happen is we can send out an ultrasound pulse that returns an echo back, correctly plotting this first specular reflector. But what can happen is that returning echo can actually echo itself off of the face of the ultrasound transducer and cause a second echo pulse to be created. Now, the ultrasound transducer is now going to plot this returning echo as a separate specular reflector that in fact doesn't exist within our image. And these reverberations can happen multiple times if this is a strong specular reflector, and what we will get is repetition of that specular reflector at equal spacings throughout our image, losing some brightness because of the time it takes for those echoes to return. Now, this can happen between the transducer and a specular reflector, or it can happen between two specular reflectors that start echoing pulses between one another. This is what's known as the reverberation artifact. These aren't reflecting true tissue boundaries.
Now, the reverberation artifact has a close cousin known as the ring down or comet tail artifact. Now, the ring down artifact occurs when there's fluid between a conglomerate of microbubbles. And when ultrasound interacts with the air around that fluid, the fluid will resonate at a set frequency, sending off echoes towards the ultrasound transducer. And those echoes, those resonant echoes, will give this ring down effect here, where we get an artifact occurring. There's actually no anatomy that is causing these separate little reflections; it's a result of fluid being wedged between some microbubbles.
Now, the comet tail artifact looks the same or very similar to the ring down artifact, and it is a type of reverberation between two small, highly reflective surfaces that could be small pieces of calcium, like small renal calculi, causing sound to reverberate within the calcium, giving this comet tail effect. You also see it between metallic objects. If you have a surgical clip within your field of view, you can get this comet tail artifact here.
The next artifact we're going to look at is called side lobe or grating artifact. You'll recall when we looked at the ultrasound beam itself, we saw this phenomenon of side lobes and grating artifacts occurring. This grating artifact happened in our multi-array transducers, and this side lobe artifact is as a result of the radial expansion of the transducer elements themselves. Now, if the side lobe sound energy heading out into our tissues reflects off a highly reflective surface that is outside of our ultrasound beam, it can send back falsely ultrasound echoes towards the ultrasound transducer. So, if we take this example here, we can see a side lobe of ultrasound energy heading towards a highly reflective surface. Say, this is some bile gas here. This is our field of view; this is what we're actually interested in. But that bile gas then sends back a reflection, an echo that heads off towards our ultrasound transducer. When the transducer detects this echo, it will plot this reflection within our field of view, and this is what's known as side lobe artifact. This anatomy doesn't actually represent anatomy that's within our field of view; it represents anatomy outside of the field of view that has interacted with these side lobes of our ultrasound machine.
We can then move on to what is known as shadowing. Now, this is a very common ultrasound artifact. When an ultrasound beam comes into contact with a highly attenuating structure or a very highly reflective structure, we will get no ultrasound heading past that object. This can be a rib; bone is highly attenuating to ultrasound, and no ultrasound pulses will make it through the rib. We will get no echoes returning from behind this rib. If this was an air bubble, we've seen that the acoustic impedance difference between soft tissue and air is very high. Most of that incident pulse will be reflected back towards the ultrasound transducer. Here, we will get no pulses heading out into this region of our field of view. We get what's known as shadowing in our image.
Now, the opposite can also happen: acoustic enhancement behind an object within our image. If we have an object here that doesn't attenuate ultrasound very strongly, if this is a fluid-filled structure, we will get ultrasound pulses passing through here without attenuation happening. So, the pulse is now reaching this part of our image have a higher intensity than the machine would expect. They have a higher intensity here than the ultrasound that is passing lateral to that object. These ultrasound pulses here have been attenuated more than the ultrasound pulse that has passed through a tissue that is not highly attenuating, and we get what is known as enhancement here. And enhancement can help us to figure out what this is. Is this air that is reflecting most of the sound back? Well, it can't be, because we would get shadowing behind it. This must be fluid-filled because we're getting this enhancement behind that structure. So, you can see how we can use artifacts to try and determine what type of tissue we are dealing with here.
So, the enhancement artifact is taking the false assumption that ultrasound pulses are attenuated equally throughout our field of view, and it's this asymmetric attenuation that causes the enhancement behind this object. We can then look at the false assumption of ultrasound pulses traveling at the same speed through tissue, this soft tissue average that the transducer uses in order to plot the various different A-mode signals heading back and make our B-mode image. We can see that sound travels slower through fat than it does through soft tissue, and faster through muscle than it does through our soft tissue average. So, when we look at an image like this, we can have a specular reflector here or a tissue boundary that is actually continuous. If this was our diaphragm, we wouldn't have a step-off on our diaphragm here. But if the incident pulse travels through an object in which sound slows down, say this was a large lipoma here, consisting of fat, the sound would travel slower through the lipoma than it would through the surrounding tissue. Now, when that echo returns off that tissue boundary, it would have taken a longer time to reach our transducer than echoes heading to the tissue boundary that didn't travel through the lipoma. And when those echoes come back to the transducer, the transducer will use that round trip time to plot that tissue boundary incorrectly, plot that tissue boundary. So, we get a step-off away from the transducer when ultrasound pulses travel through an object that have a slower than the average speed of sound through soft tissue. If it travels through an object that is faster than the average speed of sound in soft tissue, we will get a step-off closer to the transducer here. The return time will be quicker traveling through the object because the speed of sound is quicker in this object than it would be not traveling through the object, and a step-up, our speed error happens towards the transducer.
The last artifact that we're going to look at today is called ambiguity. You will note that depending on the depth that we're trying to image within our tissue, we can change our pulse repetition period. The transmit time remains the same; we change the receive time, the amount of time we're taking to listen for those returning echoes. Now, this is what's known as our pulse repetition period, the amount of time from one pulse to the next pulse. And depending on how long or how short the pulse repetition period is, will depend on how deep we can image within the structure. The deeper we want to image within the tissue, the longer our receive time needs to be, the longer the pulse repetition period needs to be. Now, when we take a small pulse repetition period, we are imaging superficial structures. We only accept pulses that come backwards in this time period, but it doesn't mean that those initial pulses don't head out towards the deeper structures here and send off returning echoes. Now, when we look at a field of view like this with two separate objects, we've got a deep field of view, a long pulse repetition period here, we can image both of these structures. Now, when we reduce our pulse repetition period, what we're doing is reducing this depth of field here. We are reducing the receive time, and we are only receiving echoes in this small depth here. Now, when an A-line is sent out in this region of the tissue, the pulse will still head out into the tissues. This object is still here; we are just imaging more superficially. And if the echo returning off of this object heads back towards the ultrasound transducer and hits the transducer in one of the receive times of the A-lines within this small pulse repetition period, we will get a false plotting of this object within our field of view. It will have a darker B-mode value because the returning echoes here have been attenuated; it's traveled a much larger distance than if the object was actually here. And what we can get is this ghosting here, this object being displayed within this shallower field of view, and that's what's known as ambiguity, where pulses returning from deeper structures are then plotted within this smaller field of view when we've reduced our depth of field here.
So, now we've looked at the major types of artifacts that you'll come across when using ultrasound imaging. Now, artifacts come up over and over again in exams because it tests your knowledge of the underlying physics, and it's a common question that comes up. Now, below this, I've linked a curated question bank where I've taken ultrasound questions from multiple different radiology physics exams and I've curated them into a question bank where I answer those questions in video format. So, if you are studying for an exam, go and check out that curated question bank. Otherwise, I'll see you in the last video of the course, where we're going to be looking at ultrasound safety. So, until then, goodbye everybody.