Welcome to Module 4 of the Cardiac Pet Curriculum. The title of this section is Instrumentation, Hybrid Imaging, Pet CT. My name is James Case. I'm from CVIT in Kansas City, Missouri. Here are my disclosure slides. Please take a moment to review those. The learning objectives of this module are, first, to understand hybrid imaging's uses for attenuation correction, secondly, examine CT-related artifacts and review mitigation strategies, third, understand strategies for acquiring CT data for assessing coronary calcium, and then finally, we will examine strategies for maximizing image quality while minimizing radiation dose. So why should we do hybrid imaging? There are several very good reasons why a hybrid system might be well-suited for cardiac pet assessment. For one, we get very high-count transmission studies for use in attenuation correction. Secondly, the addition of CT information provides us an assessment of anatomical features such as coronary calcium and other features such as examining aorta and other anomalies. We also have the capacity on many of these scanners for acquiring CT angiography to leverage the investment on this type of instrumentation beyond just perfusion imaging. It also allows for equipment sharing between other departments, such as neurology and oncology. And the two primary sorts of systems that are used in hybrid imaging are a PET-CT, which is a combination system, which includes a complete PET system attached to a CT system, and the other sort of hybrid system is a PET system attached to an MR scanner. Since the second of these, the PET-MR, is uncommon in cardiac PET applications, we won't be discussing this in detail. At the heart of the hybrid imaging system is a CT scanner, and the heart of the CT scanner is its X-ray tube. The X-ray tube, the way that this system works is it uses accelerated electrons that are then crashed into a block of tungsten, and as these electrons slow down within the tungsten, they emit a cascade of X-rays, of bremsstrahlung radiation. Bremsstrahlung literally means breaking radiation, and so as those electrons slow down, they give up that energy, their kinetic energy, in the form of X-rays, and those X-rays are then emitted out through an aperture within the X-ray tube onto our target. These X-ray tubes have two primary settings. The first is the KVP. That is the high voltage between the cathode and the anode, which allows for the acceleration of the electrons. As electrons get emitted, they will travel across the gap between the cathode and the anode, accelerating in proportion to the amount of voltage that's applied between the two surfaces. The second is the MAS, the milliamps, and this determines the flux of electrons or the number of electrons being emitted through the gap and onto the tungsten surface. In general, the higher the KVP, the harder, and what that means by harder, the more higher energy photons are emitted by the beam, and these higher energy photons have a greater penetrating power through the patient. They also carry with it a higher radiation dose. The higher MAS, or the more electrons that are being emitted, the higher the signal-to-noise. The way in which we take an X-ray tube and create a tomogram from it is the same way we do it in SPECT and PET. We're going to acquire a series of images around the patient and then apply a tomographic reconstruction algorithm to then determine the anatomical volume that created that series of transmission projections. The types of CT scanners out there are single and multi-detector CT. The earliest CT systems that were available used only a single slice, so they would take an image and it would go around the patient. They'd move the scanner, take another, or it would go in a spiral with one row of detectors capturing images across the patient. It was quickly realized that a more efficient way to cover the entire volume is to use a multi-detector or an array of detectors for receiving the X-rays coming from the X-ray tube, 4, 16, 32, and all the way up to modern day 128-slice systems. Those numbers account for the number of rows of detectors that are receiving our X-ray dose. The more rows, the more volume can be covered in each one of the passes over the patient, and the faster we can scan our patient. So why do we use that multi-slice CT? The use of a multi-slice system does not improve our temporal resolution because that's determined by how fast our system can rotate. What it does do is it increases the volume that can be scanned, and that shortens our total scan time. It allows us to hold our breath throughout the coverage of the heart, creates more redundancy, so overlap between one pass of the scanner and the next, and that overlap improves the image quality. And then it also allows us to do finer in-plane resolution. Now there are a couple of different ways in which we can do our scanning. One is a step-and-shoot, and the way that that works in step-and-shoot sequential scanning is we would take a picture and then move the patient and then take another picture, and that's the image on the left. So we acquire the desired volume one segment at a time by scanning, stopping the tube, moving the patient, then scanning again. And the other approach is a spiral scanning, where the tube is constantly spinning around the patient, and the patient is fed through the scanner, and that circle of volume slices is then reconstructed as a single acquisition. The sequential scanning has some benefits that we do end up having very consistent images from one pass to the next. The spiral scanning has the benefit of running much faster for the patient. Generally speaking, for most acquisitions, the spiral scanning is utilized, though sequential scanning is still used for some CT calcium scoring applications. So what is the practical resolution of the different systems out there today? The earliest systems, four-slice systems, their limitation was approximately one millimeter. In-plane resolution and plane-to-plane resolution, so that's the slice thickness, would be 3.5. And you can see we end up having a mismatch between the in-plane resolution, that would be the resolution of the image within an individual slice, and then the individual slices were then of lower resolution, which when we try to reorient the heart into cardiac oblique angles or trying to track a coronary tree, the mismatch in resolution would create artifacts. As we increase the number of slices, we can cover the heart more quickly and use finer in-plane resolution. The later 16-slice systems reached near isotropic, so the same plane-to-plane resolution as in-plane resolution, and then more modern 64-slice machines do acquire a true isotropic. And our goal of trying to achieve isotropic resolution allows us to rotate and change the view of the heart in three dimensions without changing the appearance of the image. Now the temporal resolution is the other feature that we have with the scanner. The scanner is, we want to be able to capture the heart using ECG gating so we can stop the coronary tree and be able to resolve in coronary calcium. What we're limited by is the amount of g-forces that are produced due to the rotation. At a rotation speed of one rotation per second, there is a total g-force, so gravitational acceleration of four g's around the, as that system, and that puts tremendous strain on the equipment. Now as we try to accelerate the rate in which we are scanning the patient down to 0.6, we go from four g's to 11 g's, so a big increase in the g-forces on all of the instrumentation, and that puts a bigger engineering challenge. So as we try to get down to where we would like to be, sub-second, half-second, and lower resolution to be able to stop the coronary tree in space and then be able to resolve the coronaries, once we get to half-second and lower, we're looking at 16 g's all the way up to 0.33, and even lower, up to 40 g's of force are on the instrumentation as it's rotating around the patient trying to make the whole scanner fall apart, those g-forces, so all the engineering has to hold it together. So we are limited in temporal resolution by the mechanics of trying to rotate that big a piece of equipment around the patient several times a second. The main use of hybrid imaging for cardiac is to use the map that's created, the CT map for attenuation correction. Now, there are two approaches in cardiac PET for doing this. One is using a PET-CT system, and the other is a line source or dedicated system. Both of these systems are perfectly capable of acquiring an attenuation map for attenuation correcting the cardiac PET image. And if we remember, attenuation has its origins with scatter. The photons from our radionuclide pass through the material and are scattered by electrons within the medium, the most common of which at the energy range that we look at in cardiac PET is Compton scattering, and that is the scattering of the photons off of electrons within the atom, producing a recoil electron and reducing the energy of the photon. Now, one of the nice things that we have with attenuation correction for PET that we don't have with SPECT is attenuation correction is very straightforward. And the reason why attenuation correction is so simple with IMPACT compared to SPECT is if we look at the image over on the right, what we have is that if we imagine and follow the red dot, an emission within the SPECT scan at this point traversing to the detector face, the amount of attenuation is going to be different depending on where that photon is emitted along this line of sight. So if it were emitted from here, very near the surface, it'll have a short distance to travel through the medium, so less attenuation. But if that photon were emitted from back here, it would have to go through much more material. So just by knowing where a photon hits the scanner, we don't necessarily know how much attenuation compensation to apply. With PET, our situation is quite different. If we have an emission from here, the total attenuation is the attenuation of one photon traveling in the forward direction plus the attenuation in the posterior direction. So the total attenuation correction applied is the total distance. So the forward-facing photon plus the backwards-facing photon. So it doesn't matter whether it's emitted from here, it should be that attenuation plus that attenuation, or here, which is that attenuation plus that attenuation. The only attenuation correction value that's important is the total attenuation along the line of sight. And this makes attenuation correction very simple to apply and very accurate to apply. That is why, when we talk about PET, we always do attenuation correction, and why the PET-CT is so important part of this process. So here's an image of a PET-CT scanner. Inside of the PET-CT scanner, with the crate taken off, we can see how these systems are put together. Over here is the array of detectors, and this is the PET scanner, piece of our scanner. You can see the blocks surrounding the tube. Now, from this view, on the other side, we can see the PET-CT scanner, which is the Now, from this view, on the other side, we can see the CT portion, and you can see the entire apparatus. And so, literally, these two machines are glued one next to another. So the CT portion would rotate rapidly around while the PET system is sitting behind it, and it all comes under the same housing. Now, transmission CT, the way that this works is our x-ray source is sitting outside the patient. It emits the photons through the patient, and then it's received at the detector. And what's displayed here is the detector phase. It doesn't matter whether it's a spec detector, an x-ray tube, a PET detector. All of these have the same features, which is an exponential loss signal as a function of depth. The way the dedicated systems function is they use a line source for doing the attenuation correction, such as germanium. They have a lower count than the PET-CT systems. The benefits of them that we'll have to take care of in the hybrid systems is that breathing, metal, and cardiac motion have less impact on these attenuation corrections. Less impact on these types of systems. So we look at line source attenuation correction. The way that we produce this transmission map is we have a line source. Then when we slide that line source into the field of view, it projects counts through the patient to create that total line source, a total loss of counts along a line of sight. And then what we can do, the unique property of PET, is multiply this fractional loss of counts times the emission data to come up with the attenuation corrected emission data. And we can, with line source systems, end up with very accurate transmission maps with very short scan times, as short as 90 seconds. So the challenges that we're going to have for CT-based attenuation correction is, unlike cardiac PET, dedicated cardiac PET, where all of our emissions happen right at the 5.11, the X-ray tube emits a broad spectrum of photons over a wide range of energy, far from the 5.11 that we're trying to acquire. The second challenge that we have is the CT images are often acquired over a single line respiratory cycle. So we need to find the best average position for the diaphragm. And then finally, even at a low dose configuration for the CT, we're going to have a high radiation dose when compared to line source attenuation. And of course, X-rays are readily absorbed by implanted metal devices and metallic objects in the body. So here's some spectra to look at. If we look at the most common spectra that we're familiar with, the technetium spectrum, we have our photon photo peak centered at 140 keV, and then a broad Compton scatter window peak spreading downward from that. So these are the photons that are the primary photons, and these are the scattered photons produced by the X-ray tube. Within the material, the 5.11 broad peak up at the top. Now, physically, it's all truly at 5.11, but given the energy, the finite energy resolution of the PET scanner, that peak is spread out. And the same sort of appearances we would have with spect technetium is we have our photo peak window, and then a broad Compton plateau. And by setting our energy window, we can focus just on the primary photons. Now, by comparison, X-rays don't have that same sharp peak. It's a broad peak that starts at as low as 10 keV and extending up to 100. Now, this can change depending on the KVP. The tube current can increase the maximum energy, and this can be stretched out. So 100 KVP tube voltage will produce the maximum energy at 100. And as we increase that tube voltage, this peak will spread out, and the maximum energy will be increased. The primary unit that's measured within CT is the Hounsfield unit. And what the Hounsfield unit is, is it is a measure of the density of the material or the opacity of the material to X-ray radiation. At the lowest level, it's a negative value, and that's of vacuum or air. And then it increases. The values increase from the negative value up until it reaches zero for an attenuation coefficient, for matching that attenuation coefficient at zero. Zero is the turning point where it's equivalent to water. And as we go to materials more dense than water, that value increases. So, again, the Hounsfield unit starts at a negative value, and that's for air, and then it goes up to zero at the water value. And then for materials such as bone, calcium, metal, higher than that, the Hounsfield units continue upward. So the attenuation in low-density materials, we all have Compton scattering. We all have Compton scattering in higher-density materials. One of the differences that we have that can create artifacts is PET and CT photons behave very similarly for low-density materials, such as water. For high-density materials, such as bone, metal, and calcium, the CT is dominated by a combination of scatter and photoelectric effect, and PET is dominated by Compton scatter. And what this translates into is CT photons are more opaque in higher-density materials than they are with PET. So the way that CT-based attenuation deals with this is our CT map. We first acquire a CT map using our hybrid system and measuring the line of response from the X-ray tube through the patient and to the detector. But this map has all of the familiar differences that we see with a CT, bone being more dense than water, metal artifacts, etc. And what we have to do is we have to have a translation algorithm for converting this image, which is a CT image measured in Hounsfield units, into this image, which is an attenuation map, where the numbers inside of it are the attenuation coefficients for 511 KeV photons. And this is usually done with a pixel-by-pixel translation of the values using a mapping algorithm. Some things that also might come into this translation are corrections for metallic artifacts, reducing the resolution to match the resolution of the transmission map to the resolution of the PET scan, etc. Essentially massaging the very high-quality transmission state down to something that looks an awful lot like a dedicated PET study. Once we have the CTAC map, we can do the same thing we would do with the dedicated system. We translate it over into a set of raw projection set, transmission sets, multiply it times our raw data to create our attenuation corrected data set for reconstruction. So that's how we create the transmission data set from the CT. Now, the CT scan, because it's acquired differently than a dedicated system, it has its own family of artifacts. The type of artifacts that we see within cardiac PET, some are similar to what we have in dedicated PET. Some are different. The ones that are similar is we're also going to have issues with misregistration between the transmission and emission data sets. So as you can see here, the misregistration between the transmission and emission data sets has the heart laying out in the lung field. We also have motion that we would have within the emission data set, and that would be similar between the two. But we have a new family of artifacts, as we can see in the middle, of motion created that's exclusive to hybrid imaging. And in this case, we have a breathing artifact, which is caused by the diaphragm being imaged at two different locations. So you can see at this point, the diaphragm is elevated. And down here, it's reduced. So at this point, the diaphragm was at end expiration. And down here, the diaphragm is at a different place at end inspiration. So the patient took a breath, and the diaphragm dropped during the study, creating an artifact. So let's look a little deeper into these artifacts. First thing I would like for you to do is try to interpret this study. I think we would all agree that this particular study, it would be very difficult to call this as anything other than an abnormal study. We clearly have a reversible defect laterally, high laterally, and to some degree, also septally. So this would be a very clear cut in the absence of any quality control example of an abnormal cardiac PET study. And this is an example of misregistration. You can see here in this illustration that the heart is out in the lung field, and it's creating an artifact in the high lateral range. Then when we do the correction, we get a normal study. Now, this happens in both dedicated and PET-CT. In dedicated studies, as many as 21% of all resting studies have misregistration. The stress is higher, and as little as 1 centimeter. But this is one point where it diverges in PET-CT, as opposed to dedicated PET. Dedicated PET tends to have a very high misregistration because of the fact we're trying to capture the heart in two different phases. During the PET study, the patient is free breathing. The patient's free breathing. And so we're capturing the heart through its entire cycle, where in CT, most of the time, we're trying to capture it at one particular place during the breath hold. So depending on how close that breath hold matches the average location during the PET, we'll introduce more or less misregistration artifacts. We have two different sources of misregistration in PET-CT. We have the breath hold, which contributes, and then we also have the effect of any motion of the patient during the study. So the recommendation in cardiac PET is that we inspect all studies for misregistration correct. And this is more acute in PET-CT studies because of the need to do this breath hold or breathing compensation with the CT portion of the study. So let's dive a little deeper into the breathing artifacts. Breathing is probably the second biggest source of artifact in all of PET-CT and SPECT-CT, so all branches of hybrid imaging. There are a couple of strategies that can be employed. We can either slow down the scan to make it mimic what we see during the PET study and allow it to average over the entire scan. Now this, we have to be very careful in applying this and make sure we're using a scanner with the capability of doing this because this will require the X-ray tube passing several times over the same place, we can end up having a very high dose being transmitted onto that spot. Some scanners have the ability to reduce the tube current, the MAS, to reduce the dose of a single pass of the detector to allow us to do this at a reasonable dose. So make sure if you're going to approach things using the slowed CT scanning and the breathing average, that you're using a scanner with software and tube capabilities that will allow you to do this without an unreasonable amount of radiation. The second common approach is to use a breath hold. Now this is a, the way that this is done is the patient is asked to hold their breath at end expiration. So what you would do is instead of breathe in, breathe out, breathe in, and then hold, the way most people hold their breath is after inspiration, we're going to ask the patient to lightly exhale and hold their breath. So breathe in, breathe out, breathe in, breathe out, and hold. And that's what we're trying to do is capture the heart or the diaphragm at an elevated state. Most of the time during the respiratory cycle, the diaphragm is in its relaxed state, a little bit below expiration. So be it hold, hold, hold, inspiration, hold, hold, hold, inspiration. And that's usually the most light end expiration is kind of where that average position would be. Now, this is another approach that's used is a light free breathe. So if you have a larger number, a smaller number of slices, so we're capturing less volume and covering the heart in a longer period of time, like a four slice, a breath hold may not be possible for these patients. And what's recommended is a light breathing. So kind of a puff, puff, puff type of breathing. It's very important. These are not natural ways a patient would breathe in particular cardiac patients who may also have pulmonary problems. So you may have to practice with your patient to see how they can get through the study in the time. So be sure you practice breathing with the patient prior to the scan to make sure they're capable of doing the type of breath hold you want within the time that the scanner will be over the heart. So here's another example of that breathing artifact. So we have the patient where the diaphragm is captured at two different spots. You can see in the resting study, it's a little high in one place and low in the other, and it creates a streak through the anterior wall. And that can corrupt our images. So the top row is one with the breathing artifact and then using the resting scan for correction down below. The other problem, here's an example of a study where it looks like we don't have misregistration and the heart is clearly within the mediastinal region, but there's an artifact in here, a breathing artifact. So we've instructed the patient to do a breath hold, and this time we haven't gone through and practiced the breath hold. And what you can notice about this image is how low the diaphragm is. This is an example of a patient with an end expiration breath hold. So the patient breathes in, it inflates the lungs, the diaphragm drops. So the diaphragm is no longer sitting in front of the inferior wall. So when we do our attenuation correction without the diaphragm, well, that's the same thing as a diaphragm artifact. So the diaphragm is attenuating the inferior wall, but we're not applying the attenuation correction. So this is an example of that study where we've looked at the patient, they have an end expiration breath hold, and it creates effectively a huge diaphragm artifact because we have attenuation present in the diaphragm and we haven't compensated for it. Concern over breathing is also present within the admission data. So take a look at this particular study. We can see there's a difference in image fidelity between the two images, but there's no misregistration between the two studies. Now, looking at the study, we see it's a relatively normal looking study, but with a chop out of the apex, you can see it right here, here at rest, a little less so at stress. This is another common breathing artifact that we see when you have a big loss of counts. It tends to be either anywhere from the six o'clock to the four o'clock range at the apex. That's also breathing artifact. It can be caused by two things, either apex bobbing up and down on top of the diaphragm, or it can be caused by doing the misregistration correction on one slice, but not looking all the way through to the apex and the heart coming out of the mediastinum and out into the lung field at the apex while being correctly preserved in the mid ventricular slices. So it's very important to look for this type of breathing artifact and this registration artifact throughout the entire ventricle. So here we can see this is a heart's poking out. The diaphragm is low through here in this apical slice, and that's also being caused by the diaphragm being a little bit lower out of place for the transmission scan. The last type of artifact that we're going to discuss here is an artifact caused by implanted metal devices. Patients these days have everything from ICDs to surgical clips inside of their bodies that are metallic. And because our x-rays are very efficiently absorbed by metal, they can create very sharp artifacts. So as you can see here, the ICD across here causes a sharp line of attenuation along the right ventricle. Then when we translate it and try an attenuation correct with this map, we end up with a hotspot that follows that wire all the way down to the apex. So most PET-CT systems have the ability to apply a metal artifact correction. It's important for those patients that have implanted metal devices or surgical clips and that sort of thing, that metal artifact correction is applied to remove the star artifact that can be in your transmission images. The last thing we're going to discuss briefly is the adjunct information that we get from PET-CT that we don't get from traditional perfusion imaging. This is a technical talk that we're going to be going over and there'll be other talks in the series to go into the clinical aspects of this. But as you can see in this particular image on the left is a normal perfusion scan. And within the absence of any kind of additional information it would be very easy to recognize this as a normal study. On the right is the same patient's CT scan. And as you can see along the LAD, there's a huge amount of calcium in the patient. It's not the sort of, it's not blockage, but it does significantly change the risk profile and the treatment strategy of this patient. So the added value of being able to visually detect calcium is an essential part of the patient workup. In hybrid imaging, we have the capacity to do this in virtually every patient and it can have a significant impact on what we do. And one of the key value adds that we have in hybrid imaging. So briefly we'll go over the CT quality control program. The CT quality control program is a little similar to, is similar to what we do with dedicated. We need to be able to daily assess certain quality features to confirm that the scanner is functioning properly. The things that we have to assess for image quality is high and low contrast resolution, image uniformity. So that'd be similar to our blank scans, image noise, slice thickness, alignment, light accuracy. That's only if we're using tilt pantries. Typically, if you're not doing neurological applications, you won't do this and the CT number. So making sure we're, the water value is centered directly on that zero ounce field unit. Now there's a specific phantom that we use for SPECT-CT. It contains several different chambers, all of which can be, all of which test these things from the previous slide. What our water value is, high and low contrast resolution, et cetera. So this odd looking phantom is the phantom that we use for running these tests. It's placed in the scanner, and we image it and assess it using, usually with automatic tools provided by the manufacturer. Now, when we acquire these PET-CT images, these are going to be lower quality studies, but look how much more information that we're going to have. And this is about as low quality study that would be recommended. This is a data set, a non-gated study off of a four-slice system. And look at all the additional information. We can see pleural effusions, pericardial thickening, calcium. When you acquire PET-CT, you do open up a whole new world of trying to being able to see things that wouldn't be able to be seen with a dedicated PET system. And you're going to need to make yourself familiar with how to look for these things. And if necessary, involve radiology and radiologists to your practice to help you understand these. Each state has different rules in terms of assessing these. So you'll want to check with your own jurisdiction to make sure that the assessment of the CT scan, your assessment of that is consistent with what your local rules are. Now, one of the things you have to do with computed tomography is computed tomography has the capacity of emitting a lot more radiation dose than you would with the emission study. The things that can affect the radiation dose in CT is the type of scanner we use, the X-ray tube output. So things like the KBP and the tube current, beam collimation and shielding. And what beam collimation is, is that's how big an aperture that's opened. Are we focused CT on the heart? Are we focused on the whole field of view? Pitch, which is the rate in which the patient is fed through the scanner and how much overlap there is between each one of the slices. X-axis scan range. So do we cover shoulder to pelvis? Are we going to just focus on the cardiac region? Tube modulation, which is trying to capture only the peak of the ECG, so diastole, and then the patient body habitus. Now, the way these things work together to either increase or decrease dose, if we add tube current, we can improve the counts and it comes at the expense of patient dose. If we add KBP, it increases counts, decreases contrast because we increase the penetrating capacity of our beam through the patient. It will add dose, but because these are harder x-rays, they have better penetrating abilities. Increasing pitch, which means we're increasing the speed of the feed. It reduces the overlap, which if there's too much speed and not enough overlap, we can introduce some artifacts, but it does speed the scans up, but increasing pitch will bring down the radiation dose. Then finally, ECG pulsing or ECG modulation that allows the tube current to be up at diastole and come back down at systole. It should have no impact on the image quality of diastole because we only are capturing it at that peak when we're acquiring, and it will bring down the dosing. By far, the bottom one of these is the most effective way of reducing dose without compromising image quality. Now, a few differences that we have if you're making that transition from dedicated PET over to PET-CT is the way in which staff is exposed to radiation is quite different than in the nuclear lab. CT is a very fast, high-intensity beam of low-energy photons. Because it's fast, the most important thing we need to do is stay out of the room while we're scanning, but the nice thing about a CT is when we turn that scanner off, there's no radiation in the room. So unlike the patient who may still be hot, we don't have to worry about any radiation from the scanner when it's turned off. It's a high-intensity burst, and that'll allow it to penetrate a lot of unshielded areas with a fairly bright glow. So you need to make sure in the shielding of the room and personnel that nobody's present in that room during the scan. The patient's only gonna receive one scan maybe per year of CT, whereas a technologist, if they're in the room or a nurse or anyone like that, they'll receive several in a day, and that's an important difference that we're gonna have in our workflow that your staff need to stay out of the room during the scan. And then the last thing that's very different between PET-CT and dedicated is that PET, the 511 KEBs, takes a lot of lead to stop those photons, whereas because of the low-energy nature, lead is very effective for shielding. So using shielding is a very important part of your radiation protection plan. Reviewing these key differences between 511 KEB and CT photons, the energy of these photons is much lower than that of PET photons. So in most applications, our CT photons are gonna be less than 120 KEB or less with peaks around 40 and 50 KEB versus the 511 that we see with PET. So we can use lead to protect during the CT scan very effectively. And then finally, the other key difference is that the scanner delivers a high blast of photons and then shuts off completely. So shielding, as we can see, makes a very big impact. The green triangles are the effect of lead shielding on 511 positron. So you can see a half a centimeter of lead will only knock down 60% of your 511 KEB photons, where as little as one millimeter of lead will reduce the number of X-rays by almost 99%. So lead needs to be a very important part of switching from dedicated to PET-CT. Again, going over the key teaching points of this module are the most important function of hybrid imaging in nuclear cardiology is to correct for attenuation. CT scanners feature such as number of slices and tube modulation can improve our CT image quality and also simultaneously reducing patient dose. Even ungated CTs for attenuation correction may have important secondary findings such as coronary calcium and other anatomical features. And you'll need to be able to inspect those ungated CTs for any secondary findings. Then lastly, the high intensity, low energy of X-rays that we have in hybrid imaging require a different strategy for radiation protection. Thank you for attention to this module.

Hybrid Imaging

PET-CT

Attenuation Correction

CT Scanner

Multi-slice Systems

CT Artifacts

Metal Artifact Correction

Radiation Dose Considerations