Transcription
Hello and welcome back. Today we're going to be looking at ultrasound safety. Now, as medical procedures go, ultrasound scanning is a relatively low-risk test, but like any medical test, it comes with inherent risks. And we need to understand some of the bioeffects, or at least potential bioeffects, that can occur in ultrasound scanning, as well as how we can reduce those risks to our patients.
Now, for the most part, we are thinking about tissue heating and cavity formation, or cavitation, within the ultrasound beam. And these are the two properties that we're going to end this talk by looking at. But how does ultrasound go about heating up tissues or creating the risk of cavitation? Well, it brings us right back to the start of this ultrasound course, where we looked at exactly what an ultrasound wave was. An ultrasound wave is a longitudinal wave in which pressure or energy moves through a medium through transfer of energy between units in that medium. And the ultrasound wave itself creates regions of compression and rarefaction. And we can represent those regions of compression and rarefaction by this sine wave here.
Now, the amplitude of the sound wave represents the local pressure changes within the tissue or within the medium. So, this amplitude here can be measured as a pressure, which we measure in Pascals. Now, when we talk about the power of an ultrasound wave, we are still talking about the pressure changes here. But the power, measured in watts, is proportional to the pressure change, or the amplitude squared. So, as we change the amplitude of our wave, we exponentially increase the power within the wave. Now, when we're talking about power, we're talking about the energy that is moving through tissues. And if we calculate that energy over a certain area, that's what's known as intensity. So, the intensity is the amount of energy, the movement of energy over time, per unit area. So, the more we increase our power, the more we increase the amplitude of the wave, the more the intensity of that wave is. The wider we spread that energy over a greater surface area, the more we reduce that intensity.
Now, how do we go about actually calculating what intensities they are within an ultrasound beam? Well, we can actually test and calibrate machines by using some devices that I want to cover here. Now, the most common device that's used is what's known as a hydrophone. Now, a hydrophone either places a microprobe within the beam or replaces a membrane within the beam. The microprobe has a PZT crystal on the end, and it can measure the amplitude changes of the wave traveling through this beam. And we can place this microprobe anywhere within this beam, so we can get very specific measurements of intensity throughout the ultrasound beam. The membrane hydrophone can be placed across the beam and also measure wave changes and ultimately intensity within the beam. So, both the microprobe and the membrane are both hydrophones. And these boxes here represent oscilloscopes, where the data is sent back, and we can represent this ultrasound beam in a waveform. We can see the amplitude of those waves, the frequency, the pulse repetition of the ultrasound wave.
Now, the hydrophone is measuring intensity at a very specific region within the ultrasound beam. We're either measuring a single point using the microprobe, or we're measuring the intensity at one point at a certain depth within the beam, across the entire beam. Now, if we want to calculate the energy or the intensity of the entire ultrasound beam, we can use what is known as a calorimeter. Now, we can place the ultrasound beam into the calorimeter, and this ultrasound beam will be attenuated. Now, ultrasound attenuation predominantly occurs through the production of heat. And the calorimeter uses the time taken to increase the temperature of the substance within the calorimeter by one degree. The more intense the ultrasound beam is, the faster it will heat up the substance within the calorimeter. So, this is a temperature probe here.
Now, the last device I want to mention here is what's known as the thermocouple. I like to think of it as a combination between the hydrophone and the calorimeter. We have a probe here that we can place within the beam, much like our hydrophone, but this probe now has a temperature sensor on it. And again, we measure the temperature change at a specific point within the ultrasound beam, and we can convert that temperature change into an intensity for that specific region in the ultrasound beam. Now, these measuring devices are used to measure or test an ultrasound machine, as well as calibrate ultrasound machines. We can't obviously do this in a patient. We can't put a thermocouple within the patient or a hydrophone within the patient. What we need to do is calculate certain indices based on the ultrasound's parameters, as well as the type of tissue that the ultrasound is traveling through, to serve as a proxy for the ultrasound machine intensity.
Now, when we look at the ultrasound beam itself, we can see that intensities change over depth. That beam becomes more and more intense per centimeter squared as the beam narrows down to this focal point, then it becomes less intense as we head out into the far field. We can also see that the center of the beam, because of the constructive interference, results in a more intense beam than the outer portions or lateral portions of the beam. So, we can see that beam intensity not only varies through depth but also varies depending on where we are within the ultrasound beam.
What we can do is actually plot a graph of beam intensities as we slice across the ultrasound beam. Now, this graph of beam intensity shows us that in the center of the beam, we have the highest intensities, and that intensity peters out to the lateral portion of the ultrasound beam. Now, this is calculating intensity in an actual area. It's a spatial intensity. We're dealing with an actual physical intensity across the ultrasound beam. So, this x-axis here represents points across the ultrasound beam. It represents the width of the ultrasound beam here. We are dealing with intensities in a spatial plane.
In the center of the ultrasound beam, we get what's known as the spatial peak intensity. That's the region of the beam where we have the most intense ultrasound waves. As we head out laterally, we get the least intense ultrasound waves out towards the periphery of the beam. The spatial average intensity takes the average intensity throughout the entire beam width. So, if we were to add up all these intensities throughout the width of the beam and then average it out over that distance, we would get what's known as the spatial average intensity. Now, the ratio between these two intensities is what's known as the beam uniformity ratio. The less the difference between the lateral portions of the beam and the central portion of the beam, the higher the beam uniformity ratio, the more uniform that beam is. So, you can see now by the focal point here, we've got a much more uniform beam than we do in the near field and far field here. Now, this beam uniformity ratio comes in handy later on when we're trying to calculate temperature change and mechanical bioeffects in the tissues.
Now, these parameters here are spatial parameters. But we know that when we send an ultrasound pulse through tissues, we are sending a pulse and then we're waiting for echoes to return. There's a receive time where no ultrasound pulse, no energy, is heading through the tissue. So, we can also get intensities over time. There will be periods of time where we have high intensities and periods of time when we have no intensity.
Now, we've seen these various different parameters that describe this pulse-echo ultrasonography, and I want to review some of them now because they become important when looking at bioeffects in tissues. We've seen that we have a transmit time when we are transmitting the pulse into the tissues, and we have a receive time when we are waiting for the returning echoes and using that data to create our B-mode image. Now, the time it takes for one pulse to head into the tissue is what's known as the pulse duration. During the pulse duration, that period of time, we are emitting energy, intensity into the tissues. The spatial pulse length is a distance measurement. It's the length of that pulse as it heads into the tissue, and that becomes important, remember, when we're looking at axial resolution.
The pulse repetition period is the amount of time from one pulse until the next pulse. And the ratio between the pulse duration and the pulse repetition period is what's known as the duty factor, or the duty factor percentage. The percentage of time that we are actually transmitting pulse versus the entire pulse repetition period. Now, the number of pulses that we send out per second is what's known as our pulse repetition frequency. Now, why are these parameters actually important? Well, if we increase our pulse duration, we're increasing the amount of time that we are exposing tissues to a certain intensity. If we reduce our pulse repetition period, reduce that receive time, the total time that we are transmitting ultrasound pulses relative to the pulse repetition period will then increase. We've got more on time than we have off time. Increasing ultrasound intensity, the same is true for pulse repetition frequency. The more frequent those pulses, the more time in total the tissue is exposed to these pulses, to actual intensities traveling through tissues. The longer our receive times, the less intensity will be received by those tissues.
Now, remember, these pulses aren't actually uniform. We can represent them like this. When the PZT crystal is fired, it has a bandwidth. It's got a variation in the amount of frequencies, and the intensity of that wave will change throughout that pulse duration. Now, we can measure intensities during the pulse itself and during the pulse repetition period, and we're measuring these intensities over time, either over the pulse duration, which is a time, or the pulse repetition period, which also represents a time. So, if you were looking at spatial intensities, where we're actually looking at intensities over space, now we're looking at temporal intensities, intensities over time.
Now, the highest intensity of a single pulse is what's known as the temporal peak. And we don't actually use this much when calculating bioeffects. What we do use is what's known as the pulse average and the temporal average. The pulse average is the average intensity of the pulse during the pulse duration. It doesn't take into account this downtime here. It's the average intensity of a single pulse. The temporal average takes into account this off time here. It's the pulse average times by our duty factor. It's spreading out that intensity over the entire pulse repetition period. Now, the temporal average is the average intensity over a pulse repetition period that the tissues will experience.
Now, any amplitude underneath our x-axis here is rarefaction, and any amplitude above represents compression. And it's actually rarefaction, or the peak rarefaction amplitude, that will influence our mechanical bioeffects in tissues. I'm going to look at that later. So, now we've calculated two separate types of intensities: intensity changes over time, the temporal intensities, and intensity distribution over space, the spatial intensities. And we can combine these two types of intensities to calculate various different parameters that will indicate to us what thermal changes and which mechanical changes will happen within tissues.
Most of our calculations will be derived from this initial calculation, which is known as our SATA, our spatial average temporal average intensity. This is the average power over the area of the beam for at least one pulse repetition period. Now, what are we doing? We're taking the spatial average, so we're taking the average power or the average intensity over the entire width of the beam, and we're timing it by our temporal average, the average intensity experienced over one full pulse repetition period. Now, this is an estimate of the average power that the tissue will receive over one full pulse repetition period. It's not taking into account these maximum intensities or even the average intensity that is experienced during one specific pulse. This is the average intensity over time.
Now, the spatial average and the temporal average can both be used to calculate the pulse average and the spatial peak. So, our spatial average is actually the spatial peak divided by that beam uniformity ratio. We saw that the ratio between these two will give us our beam uniformity ratio, and we can use the beam uniformity ratio, which will be specific to our ultrasound probe, to then calculate the spatial peak. Now, when we look at the temporal average, we can see that the pulse average, the average intensity experienced over the pulse duration, times by the duty factor, the percentage of which the pulse duration makes up the pulse repetition period, will give us that temporal average. And we will know our duty factor based on the pulse repetition frequency that we've set on the ultrasound probe, and we can use that duty factor then to calculate this pulse average.
Now, why does this become important? Well, we can use a combination of both the spatial peak and spatial average, as well as the pulse average and temporal average, to calculate various different parameters. We've looked now at the SATA, the spatial average temporal average. Now, we can also calculate the spatial average pulse average, which takes the average intensity across the ultrasound beam during the ultrasound pulse passing through that beam. We can then also calculate the spatial peak temporal average. Now, this becomes really important when looking at thermal changes in tissue. What are we calculating here? We are looking at the most intense part of the beam, the spatial peak, and then we're saying, at that most intense part of the beam, what is the average energy that will be experienced during one pulse repetition period?
How I like to think about this is placing a pot of water on the stovetop. Now, the stovetop has a really small hob, but the pot is really large. The most intense part of that pot will be in the center of the pot over the hob, and most of the heating will happen at this spatial peak. So, we need to calculate what intensities are happening at the spatial peak, where the heating is going to happen in tissues. It's at the center of the ultrasound beam, at the spatial peak, and we want to calculate what's the average energy that the hob is applying into that body of water at this area here. So, we take our spatial peak and we time it by the temporal average, the average energy over a period of time.
The next thing that we can look at is the spatial peak and pulse average. We take the most intense part of the beam, but now we're taking, what's the average energy during the ultrasound pulse passing through this part of the beam? Now, mechanical bioeffects, as we will see, is when we're looking at the ability for the ultrasound beam to cause cavitation within tissue. So, we're looking for those periods of peak rarefaction. So, we want to look at the most intense part of the beam, that's where most likely the cavitation will be to occur, and we want to look at what's the average energy while that pulse is going through. As that pulse goes through the tissue, that's when cavitation can occur.
We can then also look at the maximum intensity that will be experienced by the tissue at any given point, and that is our spatial peak temporal peak. We take the most intense part of the beam and the most intense part of the pulse. What is the maximum amplitude at that very specific point? Now, actually calculating these is not important. What is important is understanding what they represent.
Now, what we worry about when looking at bioeffects and tissue is both heating of the tissue and cavitation within the tissue. So, let's start by looking at heating of tissues. Now, when an ultrasound beam is traveling through tissues, there are local regions of pressure changes. We either get less pressure in regions of rarefaction or more pressure in regions of compression. And that ultrasound beam, the amplitude of that wave diminishes as it heads out through tissue, and it diminishes because of attenuation. Now, when we looked at scatter, we said that scatter contributes to attenuation, but by far and away the most, or the biggest contributor to attenuation, is heat production within the tissues. So, as the ultrasound beam travels through a depth of tissue, it gets attenuated, and that attenuation produces heat.
Increasing heat within a tissue can lead to negative biological effects, and we want to reduce the amount of heating that happens within tissues. Now, when we are using an ultrasound probe, we can't actually measure the increase in temperature. And what we need to use is what's known as the thermal index. It's a calculation, it's a mathematical calculation of what the temperature rise would be given certain ultrasound parameters. Now, what we are doing is taking the parameters of the ultrasound, taking the geometry of the ultrasound beam based on those parameters, and combining them to give us what's known as the thermal index.
The thermal index represents the beam's ability to increase temperature within tissues. And a thermal index of one will be a uniform beam increasing tissue temperature by one degree. A thermal index of two will be increasing temperature by two degrees Celsius. Now, as we've looked at, intensity varies depending on where we are within the beam and depending on where we are within our pulse repetition period. And there are multiple variables that will influence beam intensity and ultimately the thermal index. It goes without saying that because of these variations, the thermal index is dependent on multiple variables, and the machine has to make some assumptions when calculating thermal index.
By far and away, thermal index is most dependent on the duration of the scan. These changes are cumulative. If you think back to our pot analogy sitting on a hob, if we have a pot of water sitting on a heating plate, as we increase the amount of time that that pot of water is on the plate, the more and more that temperature increases. It's cumulative, depending on the period of time. It's not like me standing in some wind, and as that wind blows at a constant speed over a long period of time, that wind doesn't accumulate until I fall over. It doesn't add up to one another. Here, thermal index is cumulative. The changes, or the thermal changes within tissue, build up over time, and the longer we're scanning for, the more the thermal index will be.
Now, also, as we've looked at, thermal index depends on location within the beam. We said the thermal index is largely dependent on spatial peak and temporal average. Now, the spatial peak determines the most intense part of the beam, and we can see that the beam is most intense at the focal point. So, our thermal index will be high at this focal point. We also looked at temporal average, which looked at the amplitude of a specific pulse, the intensity of that pulse spread over a full pulse repetition period. And when we look at beam attenuation, we see that the highest intensities are near field. As that beam travels out into tissue, it gets more and more attenuated, and the amplitude of that wave decreases. So, again, our thermal index will be really high superficially. Both superficially and at the focal zone is where our thermal index is highest. The thermal index is dependent on location within the beam.
Now, because thermal index is dependent on attenuation, we can see that the tissue type is also important. Certain tissues attenuate the beam more than others. We've seen that bone is highly attenuating to ultrasound waves. That's why we get that posterior acoustic shadowing. And we can use different modes to compensate or adjust the thermal index. We can use a thermal index in soft tissue. We can look at a thermal index in bone, which will be higher because bone is highly attenuating, there's more heat production at the bone, and the soft tissue surrounding the bone are now more vulnerable to that extra heat. And we can look at what's known as the thermal index of cranial ultrasound. Now, because the bone is so close to the ultrasound probe when we are doing cranial ultrasound, we're getting a combination of factors. We've got a highly attenuating substance, the bone, and we're at the most intense part of our beam. The bone is lying superficially here, close to the probe, where the beam has the highest intensities, and the TIC will represent the highest thermal index.
There are various different transducer parameters that we can change which will influence our thermal index. If we increase the power or the intensity of the beam heading out of the ultrasound probe, we're going to increase the thermal index. If we increase our pulse repetition period, increase the amount of time between different pulses, we reduce our temporal average, we will reduce our thermal index. As we change the ultrasound mode or the ultrasound type that we're using, we will also change the thermal index. Ultrasound Doppler uses much more energy than B-mode scanning. Pulse wave Doppler is looking at a very small point within the B-mode image, a lot of intensity going into one point, that gate that we've set, and we have a high pulse repetition frequency in order to accurately represent those spectral changes. And red blood cells are not very good reflectors, so we need to use a really intense beam with a longer spatial pulse length. That increased spatial pulse length gives us a more pure frequency, and we're using frequency shifts in Doppler in order to calculate velocities of blood. So, different modes have different thermal indexes, and our pulse duration obviously will affect the thermal index. The longer the pulse duration, the more time the tissue is being exposed to those local pressure changes, the more the thermal index will be.
All of these are used when the machine is calculating a thermal index value, and ultrasound machines are required to display this thermal index value. Now, the last parameter that affects the thermal index is the perfusion to a tissue. The more blood flow that we have through the ultrasound beam, the more we will remove that heat away. Going back to our pot analogy, where we've got a pot of water on a hob, if I was to remove some of that hot water and replace it with cold water at a constant rate, the thermal increase of the water within that pot would be slower. I'm removing some of the hot water and adding cold water. That's the same with perfusion to the tissue. As we perfuse a tissue, we are absorbing some of that heat into the blood. That blood is moving away and being replaced by new blood that has not been exposed to the ultrasound beam.
So, you can see that using these various parameters, we can now make changes to how we scan a patient in order to reduce the thermal risk to that patient. We can scan for shorter periods of time. We can lower our power rating, increase our pulse repetition period, or decrease our pulse repetition frequency. We can use a specific mode that is specific for the image we are trying to create. We don't want to perform an obstetric scan with an abdominal mode. We want to use the obstetric mode because amniotic fluid attenuates ultrasound beams less, so we can use lower powers on that obstetric mode. And if we move our ultrasound beam around, we are changing the location of where that intensity is depositing some of its heat. We can reduce those thermal effects.
The second type of effect is what's known as a mechanical effect. Now, mechanical effects are largely looking at cavitation within tissues. Now, as an ultrasound machine is passing over a bubble of gas, we can see that that bubble will expand and compress depending on the compression and rarefaction. As a region of rarefaction passes, that bubble, the bubble will expand. A region of compression passes, a localized pressure increase, it will cause that bubble to contract. Compression and contraction of a bubble can lead to cavitation.
Now, we can have both stable and transient cavitation. Stable cavitation refers to when a bubble gets bigger and smaller but doesn't burst. The bubble is creating its own waves into the tissue, and the pressure changes that the bubble releases into the local tissues around the bubble can cause mechanical effects on the surrounding tissues, what's known as shearing or microstreaming. What's more concerning to us is what's known as transient cavitation. Transient cavitation is when there's enough pressure changes that causes the bubble to burst. The bubble reacts non-linearly to the ultrasound probe. Now, the bursting of that bubble causes the bubble to implode, as well as forming free radicals. That water vapor can then form hydrogen ions or hydroxy ions that can then damage important structures like DNA.
Cavitation most commonly occurs in regions of the body where there is air already. So, the lungs and the intestines, which already contain air, are vulnerable to this cavitation. And at the general settings that we are using ultrasound, this transient cavitation doesn't occur. But we need to be cognizant of the fact that we want to avoid these mechanical complications. We want to reduce our factors enough to prevent these happening.
Now, the mechanical index can be calculated using this formula. The peak rarefactional pressure, as I mentioned, is the most important factor when it comes to assessing the risk of mechanical bioeffects within the tissue. As the peak rarefactional pressure increases, our mechanical index increases. We can also see that it's inversely proportional to the frequency of the wave. The higher the frequency of the ultrasound pulse, the less the mechanical index will be.
Now, when we are thinking of bioeffects within tissue, we want to stick to the ALARA principle, which is as low as reasonably achievable. We want to reduce scanning times. We want to use parameters that will allow us to get a good enough image to make a diagnosis, but we don't want to increase those parameters unnecessarily. We also want to ensure that the scan we are doing is actually going to be of benefit to the patient. We don't want to be doing unnecessary ultrasound scanning. So, that is patient safety within ultrasound in a nutshell. And there are lots of different concepts to go through. And what you want to do is just understand the broad concepts and how they apply to both thermal increases within tissue, as well as mechanical bioeffects within tissue.
So, I hope you've enjoyed this ultrasound course. Linked below is a curated question bank. You can use that to test your knowledge, find gaps in your knowledge that you can come back to these lectures, shore up some of those gaps, and then ace your Radiology Physics Exam. So, until next time, I'll see you all. Goodbye, everybody.