Understanding CBCT Imaging Artifacts and Findings

hello and welcome to this presentation on comb beam ct imaging artifacts and incidental findings my name is nyle and i'm a consultant in dental and maxillofacial radiology based at the leeds dental institute the aims and objectives of this presentation are to be able to identify common comb beam ct artifacts to be aware of how these artifacts occur and how their effects can be avoided or minimized and also to be aware of the common incidental findings that is seen on cone beam ct of the dental and maxillofacial region and if we start with artifacts these are structures that are present in the final image which are not actually part of the object that has been imaged and these act to degrade image quality and reduce the diagnostic yield by either obscuring or mimicking pathology and it's important to recognize these artifacts and to be able to differentiate them from the true nature of the imaged object and one of the most common artifacts that we see in combium ct is noise and noise relates to unwanted or randomly distributed disturbance of the signal in the final image and this arises due to two processes one being quantum model whereby where there is less information reaching the image receptor, there is a greater degree of noise. And the other cause being scattered radiation, which is reaching the image receptor and detracting from the final image. If we compare cone beam CT with conventional CT, there's actually very little noise inherent within conventional medical or multi-slice CT. And that's because it uses a much higher MA, which results in a greater number of... x-ray photons reaching the detector so we get an increased signal to noise ratio conventional ct also employs pre and post collimation of the beam and that results in a very thin and fan shaped beam which results overall in reduced scatter and we'll see how that differs from comb beam ct in a moment so if we look at our conventional ct scanner we have an example here of a patient being viewed from the top of the table and they're lying flat on the CT table. Surrounding the patient there is the CT gantry which houses the x-ray tube and an array of detectors, multiple detectors, and when the x-ray is switched on it produces this thin fan-shaped x-ray beam and during the scan the CT gantry will rotate around the patient acquiring slices as the patient has moved through the gantry as the table moves through the gantry if we look at this now from a side view we can see again the patient lying flat on the ct table this time viewing from the side the ct gantry we have the x-ray tube at one end and our array of detectors at the other and on this view we can see that there is post patient collimation and this is part of the reason why there's less noise conventional ct because this will remove some of that scattered radiation so again when the x-rays are switched on it produces this very thin but as we've seen in the other view a fan shaped beam and then the gantry as we've seen rotates around the patient as the ct table moves through the gantry and that's how the slices are required in our conventional ct scan so by comparison combium ct is inherently noisier than conventional medical ct and that's because it uses a lower ma so we have fewer number of x-ray photons and so a greater signal to noise ratio and this post-patient collimation is not present and that's because of the very nature of the the comb beam shaped x-ray beam that's used in combium ct so if we look at a cone beam ct scanner we can see here on one side the x-ray tube and on the opposite side the image detector and when the x-ray tube is switched on it produces this cone-shaped x-ray beam if we look at that on a diagrammatic view of the patient from above again x-ray tube on one side image single image receptor this time of Combeam CT scanners. the x-ray source is switched on producing our cone-shaped x-ray beam and as it rotates around the patient instead of acquiring slices this time acquires 2d what we call basis projections from different angles and it is these that is reconstructed at the end of the scan to form our 3d volume another influencing factor within comb beam ct is the form of image receptors and these typically arise as flat panel detectors which can have both direct and indirect sensors we'll come on to those in a moment but also more historically but still on the market image intensifiers have been used as image receptors within combium ct machines so flat panel detector technology is much more common within combium ct and this is based on very similar detector technology to what we see in digital panoramic and capillometric units and this creates the typical cylindrical volume that we're used to seeing within cone beam ct imaging and as mentioned earlier this arises in two main forms of detected technology in indirect detectors as the name suggests there are additional steps whereby the x-rays are initially converted into light by a scintillant layer at the image receptor this light is then converted into electrical signal by photodiodes and it is this electrical data that is then converted and used to reconstruct the final image with direct detector technology x-rays are converted directly into electrical signal leaving out that additional step of conversion to light and it is this that is fed then to the computer to reconstruct the final image where there are fewer steps involved in the reconstruction or the detection of data then there is less chance of noise being introduced into that process image intensifiers were used more commonly in the past but are still used on some comb beam ct units on the market as we can see from the image here that is much bulkier because there are additional components within the image receptor and another difference that would be visible is that instead of producing the cylindrical volume as we see in flat panel detectors these create a spherical volume but because of those additional steps that are involved in conversion much as and even more so than would be with the indirect flat panel detectors there's a greater chance of noise being introduced into the final image if we look at the settings in terms of the combium ct machine as well we can also see that there are other methods of reducing noise and if we look namely at the kv and the ma by increasing the kv we can increase the penetrative power of the x-rays so we have the same number of x-rays but they are more energetic and therefore more likely to pass through the patient and reach the image receptor and we would take this into consideration based upon the size or perhaps bone density of the patient where a larger patient or greater boner density would require an increase in kv to be able to penetrate the patient and reach the image receptor if we have more x-rays reaching the image receptor that adds to the detected signal and therefore we have an increased signal to noise ratio so the noise will be less apparent however by increasing the kv this will also affect the contrast resolution of the image if we increase the kv too high all the x-rays will pass through the patient and reach the image receptor and we need a certain number of the x-rays absorbed within the patient to be able to create the image so if we look here we have our x-rays passing through the patient and onwards to the image receptor with some of them being absorbed if we increase the kv we essentially beef up make x-ray is more energetic and therefore more likely to be transmitted through the patient and so we have a greater number reaching the image receptor if we increase the ma or the time of the exposure what we do is we increase the number of x-rays produced we don't change the energy of those we increase the overall number as a result with a greater number of x-rays reaching the image receptor once again we have a increase in signal to noise ratio so the noise will be less apparent however we also have a greater number of x-rays absorbed within the patient and therefore we add to the patient dose so there's always a balance to be struck what we really need to consider is the level of detail that we require to answer the clinical question yes we could get a very high resolution image but if the clinical question is for something that doesn't require that level of detail then we should look at trying to reduce the dose to the patient while still answering the clinical question so diagrammatically if we increase the ma so we have our again our x-rays passing through the patient and being absorbed but by increasing the me or the time of exposure we increase the number of x-rays which are the same average energy but therefore there will be more going through the patient but they'll also be more absorbed using an appropriate field of view can also influence the amount of noise in our final image by reducing the field of view while still covering our region of interest we produce less scatter and therefore less scattered radiation reaching the image receptor results in less noise a smaller field of view is typically associated with smaller voxels essentially the 3d equivalent of pixels therefore we can create a greater level of detail in the final image and also by exposing a smaller area of the patient there are less tissues or less less tissue types exposed and therefore we've reduced the overall dose and there are a number of fields of view depending on the different combium ct units that will be available and as mentioned earlier it's the most important part is to select the field of view that is appropriate to cover your region of interest so therefore we consider using the smallest possible field of view if we were needed to image an individual tooth or potentially a larger field of view if we had to image the dentition of the myxilla and mandible we would reduce the field of view to our area of interest if it was for example trauma in relation to the orbits or we may have to justify using a very large field of view depending on the clinical indication such as planning for orthognathic surgery in some units it may be possible to independently vary the voxel size these are essentially as i mentioned the 3d equivalent of pixels on our 2d images and where we have smaller voxels therefore a greater degree of detail in the final image we require more information reaching the image receptor to be able to produce an image of adequate detail without having significant levels of noise therefore by increasing the size of the voxels we can reduce the amount of information that's required at the image receptor and therefore reduce the overall dose to the patient but again we have to correlate this with our clinical indication do we require that high level of detail or can we use a slightly lower dose to the patient and accept a slightly lower level of detail but still answer our clinical question in many combium ct units the voxel size may be fixed according to the field of view selected so again that's important factor in selecting the smallest possible field of view the second most common artifact within comb beam ct is metal or beam hardening artifact and this results in dark and light bands that extend in all directions due to the inherent nature of that divergent cone-shaped x-ray beam arising from radio-dense objects such as restorations like crowns or amalgam or any other metal objects in the head and neck region so as we'll see in a moment that's why we have to appropriately prepare the patient by removing any jewelry hair clips or cobalt chromium dentures the issue with metal beam hardening artifact is that it can both obscure or mimic pathology such as caries so we see here an example of metal or beam hardening artifact of those characteristics light and dark bands emanating from the metal restoration and here's an example where it can cause issues we can see this left mixillary molar and we can see this area that on conventional radiographs would be very much in keeping with carries however we can see that on these other views that this appearance within the premolar is actually due to beam hardening artifact from adjacent Nettle restorations mimicking carries within that tooth Again, another example of something that looks very suspicious for Kerries, but on another view we can see that this again is in relation to a metal restoration casting this dark shadow. And where there is ambiguity over whether a shadow on comb beam CT represents either Kerries or beam hardening artefact, it may be recommended that the imaging is supplemented by conventional radiographs such as bite wings to determine whether it is truly Kerries or is it beam hardening artifact mimicking the presence of caries or obscuring it. Again, we see another example here within this incisor tooth, the appearance of either root filling material or perhaps fractured endodontic instrument. And on our other views, we can actually see that it is the contralateral central incisor and the endodontic restoration within that that is producing beam hardening or metal artifact. that's mimicking the presence of root filling material or a fractured instrument within the adjacent incisor some comb beam ct machine manufacturers will provide software that attempts to reduce the conspicuity of metal artifact as we can see in these images however the problem with this metal artifact producing software is that it not only smooths out some of that beam hardening artifact but also can smooth out some of the detail than the rest of the image as well and therefore although these do reduce the appearance of these metal artifacts they also act to overall smooth the image and therefore lose some of that required detail not dissimilar to something that's been photoshopped it becomes less true of the original object movement artifact can also occur in combim ct scans due to the relatively long acquisition times the computer does not compensate for patient movement as it assumes a stationary object and reconstruction and therefore if this is particularly subtle it can result in loss of definition or blurring of the margins or if it is severe it can cause a double outline of all the structures or even mimic certain pathologies such as periosteal bone formation so whereas movement artifact can be very obvious in conventional radiographs it can be much more subtle in cone beam ct imaging as we can see here where there's some generalized loss of definition and some slight blurring or loss of sharpness and where this is particularly severe it could render the combium ct image non-diagnostic depending again on the clinical indication and just to demonstrate the length of time that's involved in acquiring a combium ct see in a moment as the scan begins it starts to acquire these basis projections and therefore any movement of the patient during the acquisition of these basis projections will result in that movement artifact being reconstructed into the final combium CT volume and some of the pathology that movement artifact can mimic is periosteal bone formation so we can see here where there's that slight double outline of the margins particularly if we were looking for features that might include periosteal bone formation subtle movement artifact can mimic that pathology and we can see another example here of movement artifact mimicking periosteal bone formation but how we would differentiate that is that that would be present throughout uh the image volume it wouldn't be localized to one region because the movement artifact will affect the entire image we also advise the patient during the scan to not to swallow or to move their tongue as again we can see here it can also develop movement artifact of the soft tissues on comb beam ct movement artifact can be influenced by the scanner configuration and there are three main forms that are available standing seated and supine units In standing Conebeam CT units there is the greatest risk of patient movement, however we can mitigate that somewhat by introducing a chair and lowering the machine to the patient and therefore reducing the risk of them moving during the scan. With seated Conebeam CT units there is increased stability for the patient and with supine Conebeam CT scanners there's probably the least risk of movement. during the scan we can also reduce movement artifact by appropriately selecting patients by instructing them of what be involved in the scan and reassuring them and a confident operator is important in avoiding an inefficient technique and therefore resulting in loss of compliance from the patient occasionally because of the nature of some of the units the gantry can touch the patient during the scan this may be related to the design for a particular population and in those scenarios where it's not clear if the patient will be able to either tolerate remaining stationary for the scan or if there's a risk of the gantry touching the patient during the scan you can perform a dry run where the gantry will rotate around the patient without exposing them to any radiographs prior to actually undertaking the actual cone beam ct scan so as part of appropriate preparation of the patient for the comb beam ct scan will ensure all removable metal objects have been removed from the field of view will explain to the patient what will happen during the scan warning them that the gantry will move around potentially very close to them will stress the importance of remaining stationary throughout reassure them of what's involved advising them they can close their eyes if they feel more comfortable to do so and then as we can see here there'll be appropriate restraints to reduce the risk of movement so we're stabilizing the patient with things like a chin rest or head support We can also reduce the risk of movement by employing shorter scan times and there are a number of methods by which we can do this. In some units we've seen initially that the cone beam CT scan is reconstructed from a 360 degree rotation of the gantry around the patient but in some cone beam CT units a 180 degree rotation can be used and therefore a shorter exposure time. There are some units that have high speed scan settings. so we can acquire the scan in much less time but both of these will result in fewer basis projections so less information reaching the image detector and therefore the trade-off for the reduction in scan time is that it will result in an overall noisier image but again this may be perfectly acceptable depending on the clinical age clinical indication both in terms of what the patient is able to tolerate and the level of detail that we require so if we look at a standard setting on this particular scanner we can see the scan time is 17 and a half seconds if we go to very high resolution scans the scan time will increase so there'll be an increased risk of patient movement but we can get a greater degree of detail if that is required to answer the clinical question high resolution very similar but we have these high speed settings as well and again reducing the scan time from 17.5 to 10.5 seconds in this case the other setting that we mentioned was the 180 degree setting instead of 360 and again we can see the scan time is reduced down to nine seconds so as you've seen in the initial animation in a standard comb beam ct scan the gantry will do 360 degree rotation around the patient whereas with just half a rotation half of a full rotation in some units we can reconstruct the final volume But we don't get something for nothing. As I say, there's always a trade off. So we have a shorter scan time. However, we have less basis projections or information reaching the image receptor. And therefore, overall, we can see that results in a noisier image when we employ that 180 degree setting. another benefit of the units that can reconstruct from 180 degree is if we have undertaken a 360 degree scan however the patient has moved during the portion of that scan then afterwards we can use the software on the computer to select an appropriate 180 degree portion of the 360 degree rotation and reconstruct the image from that 180 degree portion removing the part where the patient is moved so we can see here an example of movement artifact that's occurred during a 360 degree scan and by excluding that portion of the scan in which movement occurred if it is isolated to one specific part of the scan we can reconstruct um the 180 degrees of that 360 degree information and get an image that may go from something that was non-diagnostic to something that is potentially diagnostic and save having to repeat the scan. Combeam CT manufacturers have also been developing post-processing software much in the same way that metal artifact reduction software is used to remove movement artifact in reconstructed volumes. The next Combeam CT image artifact we'll discuss is Ring artifact. and this arises due to a defective site on the image receptor. As a result of the image receptor rotating around the patient, this defective site results in a dark ring on the final image. And we can see an example of ring artefact here. And if we look at how this develops in an illustration, we can see that the green area on the image receptor is not detecting any of the x-rays because it's defective. So when the X-ray beam is switched on and the scan begins, that portion of the image receptor is not detecting any information. And therefore, that's left as this dark ring on the final image where there's been no data collected from that region of the image receptor. And we can see another example here of ring artifact on this small volume cone beam CT, which is even apparent. the volume rendered image. So how we can avoid or fix ring artefact is by performing regular quality assurance, ensuring that the cone beam unit is regularly maintained and contacting the manufacturer for repair and potentially putting the unit out of service until it is repaired as soon as it is first identified. A liaising artefact occurs due to under sampling and this is typically seen at the very periphery of scanned cone beam CT volumes. This is related to the divergent nature of the x-ray beam used within cone beam CT and results in something that's termed a moiré pattern. If we look at this first as an illustration we can see as the x-ray beam rotates around the patient we get a lot of information from the most central region in green but we get less information arising from the peripheries of the scan volume if we look at this on a real life example we can see how the x-ray beam is mainly focused on the central region of the scan volume and we have less information from the periphery so again the green region tends to have the greatest level of detail and the yellow peripheral region has less levels of detail and this results in that artifact we mentioned earlier these linear light and dark bands that's uh termed a moiré pattern how we can minimize this having an impact on the clinical yield of our combium ct volume is by centering our region of interest within the field of view as as we've seen from the examples this artifact tends to be confined to the peripheries of the scanned volume also by using the smallest possible field of view we will have less divergence of the x-ray beam so moving on now from image artifacts to incidental findings these are chance radiographic findings that we note within a scanned image that was taken for an unrelated reason. and given the increased utilization of cross-sectional imaging including comb beam ct this has led to a marked increase in a number of these incidental findings one of the most common incidental findings we see in comb beam ct would be caries so we can see an example here with indenting and confined to enamel so although caries is demonstrated on comb beam ct it is not indicated as a primary use In fact, as we've seen from the early examples, the opposite can often be true where caries can be mimic or obscured as a result of metal artefact within cone beam CT. And as we mentioned earlier, if there is any suspicion or ambiguity over the presence of caries, then we may recommend supplementing the cone beam CT image with diphthene radiographs. Periodontal disease is another common incidental finding on cone beam CT imaging. We can see examples here of deposits of calculus, periodontal bone loss associated with this impacted wisdom tooth, and frication involvement. Tonsilloliths, as we commonly see on conventional dental panoramic radiographs, are often also visible on cone beam CT, particularly when imaging the mandibular wisdom teeth. other radiopacities such as dense bone islands or bony anastosis we can see on this periapical are picked up as a common incidental finding on comb beam ct imaging typically continuous with the adjacent buccal or lingual cortex and typically these are benign lesions that don't require any intervention but where we do have to raise awareness of them is where there is planned orthodontic tooth movements through such regions of dense bone as typically that would result in an increased risk of root resorption as we can see here. Bony exostosis again as demonstrated on this dental panoramic radiograph are often detected in the form of mandibular tori. or certainly maxillary torus on cone beam CT as an incidental finding. However, where we see numerous or more pedunculated bony projections, we would have to raise the clinical possibility of Gardner syndrome for investigation. Mucosal thickening within the maxillary antrum is another common incidental finding. cone beam CT imaging and there is a normal thickness to the lining of the maxillary antrum the Snyderian membrane and that can vary approximately one millimeter but potentially up to three millimeters in some patients where there is localized mucosal thickening this is likely to represent pathologies such as mucous retention cysts Other more radiodense radiopacities which we may see projected over the maxillary antrum on these periapical radiographs on and on this dental panoramic radiograph would include things such as antraliths or potentially where there's an inappropriate history these may represent roots that have been displaced into the maxillary antrum. Partial bony septations within the maxillary antrum can also mimic the appearance of a root displaced, as you can see on this periapical and dental panoramic radiograph. And then on the cone beam CT image, we can see the site of that partial bony septation. Retained roots are often identified as an incidental finding on cone beam CT imaging. and in scenarios where none of the permanent teeth are absent but we still see these radiopacities these are more likely to represent retained roots of the primary dentition certain anatomical features can also be more apparent on conveying ct and this would include things like the greater palatine groove along which the greater palatine nerve runs within the heart palate and common incidental findings on conventional radiographs such as embedded restorative material will also be detected commonly on combium ct imaging another normal anatomical variant that may also be seen on combium ct imaging is the presence of accessory maxillary ostea in terms of drainage pathways for the maxillary antra or again these anthraceptations we've seen the example earlier of it mimicking a root displaced into the antrum again they can mimic cystic lesions which on combium ct we can then identify as partial septations of the antrum the nasal septum is often deviated to some degree and there can be the presence of these septal bony spurs another anatomical variation we can see within the sinuses is whereby the middle turbinate curves in the opposite direction to which it typically does this is termed paradoxical curvature or there can be pneumatization of the turbinates in a process called Concha Bellosa. Osteomas can be detected within the paranasal sinuses and occasionally can obstruct a drainage pathway and septal perforation can be seen as an incidental finding. This could be part of a disease process such as Wegener's granulomatosis. or can be related to chronic cocaine abuse and it's important for us to be able to differentiate some of those anatomical variations with the post-surgical appearance whereby the patient may have undergone endoscopic sinus surgery to improve the drainage of the maxillary antra either by widening the drainage pathways or by removing some of those obstructive components calcified or ossified stylohyoid ligaments are commonly seen on dental panoramic radiographs and again as a result are a common incidental finding on comb beam ct they can produce a pseudo-articulated appearance but the presence of them in themselves doesn't necessarily mean a diagnosis of eagle syndrome as this is more related to irritation of the adjacent musculature which occurs much less commonly degenerative change within the temporomandibular joints is also a common incidental finding within cone beam ct imaging and we can see here examples of flattening of the anterior superior surface of the condyle with early anterior osteophyte formation and developmental abnormalities of the tmj may also be detected as an incidental finding such as the presence of a bifid condyle as well as degenerative change within the temporomandibular joints degenerative change can also be detected as an incidental finding on the partially imaged region of the cervical spine on combium ct and as well as developmental abnormalities within the tmj congenital abnormalities of the cervical spine can also be detected. An incidental finding that can be associated with ectopic and uninterrupted teeth is the presence of the gubernacular canal and what this represents is the remnants of the eruption pathway or the potential eruption pathway of the tooth. This is often seen with impacted maxillary teeth. We can see here the point at which it exits onto the alveolar crest. Coronal defects can also be incidentally detected on comb beam CT volumes such as the buccal defects we can see in this mandibular wisdom tooth as well as the presence of enamel pearls. which may predispose to periodontal disease intracranial calcifications may be detected on particularly large volume comsium cts such as this calcification within the pineal gland and calcification of the walls of the internal carotid artery Other incidental findings arising within the adjacent soft tissues include salivary stones, as well as calcification occurring within long-standing chronic skin infections such as acne, which is called calcinosis cutis. and although not primarily indicated for the imaging of soft tissues incidental findings at the soft tissue air interfaces can be detected on combium ct and therefore swelling such as this one we see on the heart palette should be highlighted in the final report back to the referring clinician for clinical correlation should you wish to read more about the topics we've discussed today these are some recommended papers and thank you for your time.

Surrounding the patient there is the CT gantry which houses the x-ray tube and an array of detectors, multiple detectors, and when the x-ray is switched on it produces this thin fan-shaped x-ray beam and during the scan the CT gantry will rotate around the patient acquiring slices as the patient has moved through the gantry as the table moves through the gantry if we look at this now from a side view we can see again the patient lying flat on the ct table this time viewing from the side the ct gantry we have the x-ray tube at one end and our array of detectors at the other and on this view we can see that there is post patient collimation and this is part of the reason why there's less noise conventional ct because this will remove some of that scattered radiation so again when the x-rays are switched on it produces this very thin but as we've seen in the other view a fan shaped beam and then the gantry as we've seen rotates around the patient as the ct table moves through the gantry and that's how the slices are required in our conventional ct scan so by comparison combium ct is inherently noisier than conventional medical ct and that's because it uses a lower ma so we have fewer number of x-ray photons and so a greater signal to noise ratio and this post-patient collimation is not present and that's because of the very nature of the the comb beam shaped x-ray beam that's used in combium ct so if we look at a cone beam ct scanner we can see here on one side the x-ray tube and on the opposite side the image detector and when the x-ray tube is switched on it produces this cone-shaped x-ray beam if we look at that on a diagrammatic view of the patient from above again x-ray tube on one side image single image receptor this time of Combeam CT scanners. the x-ray source is switched on producing our cone-shaped x-ray beam and as it rotates around the patient instead of acquiring slices this time acquires 2d what we call basis projections from different angles and it is these that is reconstructed at the end of the scan to form our 3d volume another influencing factor within comb beam ct is the form of image receptors and these typically arise as flat panel detectors which can have both direct and indirect sensors we'll come on to those in a moment but also more historically but still on the market image intensifiers have been used as image receptors within combium ct machines so flat panel detector technology is much more common within combium ct and this is based on very similar detector technology to what we see in digital panoramic and capillometric units and this creates the typical cylindrical volume that we're used to seeing within cone beam ct imaging and as mentioned earlier this arises in two main forms of detected technology in indirect detectors as the name suggests there are additional steps whereby the x-rays are initially converted into light by a scintillant layer at the image receptor this light is then converted into electrical signal by photodiodes and it is this electrical data that is then converted and used to reconstruct the final image with direct detector technology x-rays are converted directly into electrical signal leaving out that additional step of conversion to light and it is this that is fed then to the computer to reconstruct the final image where there are fewer steps involved in the reconstruction or the detection of data then there is less chance of noise being introduced into that process image intensifiers were used more commonly in the past but are still used on some comb beam ct units on the market as we can see from the image here that is much bulkier because there are additional components within the image receptor and another difference that would be visible is that instead of producing the cylindrical volume as we see in flat panel detectors these create a spherical volume but because of those additional steps that are involved in conversion much as and even more so than would be with the indirect flat panel detectors there's a greater chance of noise being introduced into the final image if we look at the settings in terms of the combium ct machine as well we can also see that there are other methods of reducing noise and if we look namely at the kv and the ma by increasing the kv we can increase the penetrative power of the x-rays so we have the same number of x-rays but they are more energetic and therefore more likely to pass through the patient and reach the image receptor and we would take this into consideration based upon the size or perhaps bone density of the patient where a larger patient or greater boner density would require an increase in kv to be able to penetrate the patient and reach the image receptor if we have more x-rays reaching the image receptor that adds to the detected signal and therefore we have an increased signal to noise ratio so the noise will be less apparent however by increasing the kv this will also affect the contrast resolution of the image if we increase the kv too high all the x-rays will pass through the patient and reach the image receptor and we need a certain number of the x-rays absorbed within the patient to be able to create the image so if we look here we have our x-rays passing through the patient and onwards to the image receptor with some of them being absorbed if we increase the kv we essentially beef up make x-ray is more energetic and therefore more likely to be transmitted through the patient and so we have a greater number reaching the image receptor if we increase the ma or the time of the exposure what we do is we increase the number of x-rays produced we don't change the energy of those we increase the overall number as a result with a greater number of x-rays reaching the image receptor once again we have a increase in signal to noise ratio so the noise will be less apparent however we also have a greater number of x-rays absorbed within the patient and therefore we add to the patient dose so there's always a balance to be struck what we really need to consider is the level of detail that we require to answer the clinical question yes we could get a very high resolution image but if the clinical question is for something that doesn't require that level of detail then we should look at trying to reduce the dose to the patient while still answering the clinical question so diagrammatically if we increase the ma so we have our again our x-rays passing through the patient and being absorbed but by increasing the me or the time of exposure we increase the number of x-rays which are the same average energy but therefore there will be more going through the patient but they'll also be more absorbed using an appropriate field of view can also influence the amount of noise in our final image by reducing the field of view while still covering our region of interest we produce less scatter and therefore less scattered radiation reaching the image receptor results in less noise a smaller field of view is typically associated with smaller voxels essentially the 3d equivalent of pixels therefore we can create a greater level of detail in the final image and also by exposing a smaller area of the patient there are less tissues or less less tissue types exposed and therefore we've reduced the overall dose and there are a number of fields of view depending on the different combium ct units that will be available and as mentioned earlier it's the most important part is to select the field of view that is appropriate to cover your region of interest so therefore we consider using the smallest possible field of view if we were needed to image an individual tooth or potentially a larger field of view if we had to image the dentition of the myxilla and mandible we would reduce the field of view to our area of interest if it was for example trauma in relation to the orbits or we may have to justify using a very large field of view depending on the clinical indication such as planning for orthognathic surgery in some units it may be possible to independently vary the voxel size these are essentially as i mentioned the 3d equivalent of pixels on our 2d images and where we have smaller voxels therefore a greater degree of detail in the final image we require more information reaching the image receptor to be able to produce an image of adequate detail without having significant levels of noise therefore by increasing the size of the voxels we can reduce the amount of information that's required at the image receptor and therefore reduce the overall dose to the patient but again we have to correlate this with our clinical indication do we require that high level of detail or can we use a slightly lower dose to the patient and accept a slightly lower level of detail but still answer our clinical question in many combium ct units the voxel size may be fixed according to the field of view selected so again that's important factor in selecting the smallest possible field of view the second most common artifact within comb beam ct is metal or beam hardening artifact and this results in dark and light bands that extend in all directions due to the inherent nature of that divergent cone-shaped x-ray beam arising from radio-dense objects such as restorations like crowns or amalgam or any other metal objects in the head and neck region so as we'll see in a moment that's why we have to appropriately prepare the patient by removing any jewelry hair clips or cobalt chromium dentures the issue with metal beam hardening artifact is that it can both obscure or mimic pathology such as caries so we see here an example of metal or beam hardening artifact of those characteristics light and dark bands emanating from the metal restoration and here's an example where it can cause issues we can see this left mixillary molar and we can see this area that on conventional radiographs would be very much in keeping with carries however we can see that on these other views that this appearance within the premolar is actually due to beam hardening artifact from adjacent Nettle restorations mimicking carries within that tooth Again, another example of something that looks very suspicious for Kerries, but on another view we can see that this again is in relation to a metal restoration casting this dark shadow. And where there is ambiguity over whether a shadow on comb beam CT represents either Kerries or beam hardening artefact, it may be recommended that the imaging is supplemented by conventional radiographs such as bite wings to determine whether it is truly Kerries or is it beam hardening artifact mimicking the presence of caries or obscuring it.

Again, we see another example here within this incisor tooth, the appearance of either root filling material or perhaps fractured endodontic instrument. And on our other views, we can actually see that it is the contralateral central incisor and the endodontic restoration within that that is producing beam hardening or metal artifact. that's mimicking the presence of root filling material or a fractured instrument within the adjacent incisor some comb beam ct machine manufacturers will provide software that attempts to reduce the conspicuity of metal artifact as we can see in these images however the problem with this metal artifact producing software is that it not only smooths out some of that beam hardening artifact but also can smooth out some of the detail than the rest of the image as well and therefore although these do reduce the appearance of these metal artifacts they also act to overall smooth the image and therefore lose some of that required detail not dissimilar to something that's been photoshopped it becomes less true of the original object movement artifact can also occur in combim ct scans due to the relatively long acquisition times the computer does not compensate for patient movement as it assumes a stationary object and reconstruction and therefore if this is particularly subtle it can result in loss of definition or blurring of the margins or if it is severe it can cause a double outline of all the structures or even mimic certain pathologies such as periosteal bone formation so whereas movement artifact can be very obvious in conventional radiographs it can be much more subtle in cone beam ct imaging as we can see here where there's some generalized loss of definition and some slight blurring or loss of sharpness and where this is particularly severe it could render the combium ct image non-diagnostic depending again on the clinical indication and just to demonstrate the length of time that's involved in acquiring a combium ct see in a moment as the scan begins it starts to acquire these basis projections and therefore any movement of the patient during the acquisition of these basis projections will result in that movement artifact being reconstructed into the final combium CT volume and some of the pathology that movement artifact can mimic is periosteal bone formation so we can see here where there's that slight double outline of the margins particularly if we were looking for features that might include periosteal bone formation subtle movement artifact can mimic that pathology and we can see another example here of movement artifact mimicking periosteal bone formation but how we would differentiate that is that that would be present throughout uh the image volume it wouldn't be localized to one region because the movement artifact will affect the entire image we also advise the patient during the scan to not to swallow or to move their tongue as again we can see here it can also develop movement artifact of the soft tissues on comb beam ct movement artifact can be influenced by the scanner configuration and there are three main forms that are available standing seated and supine units In standing Conebeam CT units there is the greatest risk of patient movement, however we can mitigate that somewhat by introducing a chair and lowering the machine to the patient and therefore reducing the risk of them moving during the scan.

With seated Conebeam CT units there is increased stability for the patient and with supine Conebeam CT scanners there's probably the least risk of movement. during the scan we can also reduce movement artifact by appropriately selecting patients by instructing them of what be involved in the scan and reassuring them and a confident operator is important in avoiding an inefficient technique and therefore resulting in loss of compliance from the patient occasionally because of the nature of some of the units the gantry can touch the patient during the scan this may be related to the design for a particular population and in those scenarios where it's not clear if the patient will be able to either tolerate remaining stationary for the scan or if there's a risk of the gantry touching the patient during the scan you can perform a dry run where the gantry will rotate around the patient without exposing them to any radiographs prior to actually undertaking the actual cone beam ct scan so as part of appropriate preparation of the patient for the comb beam ct scan will ensure all removable metal objects have been removed from the field of view will explain to the patient what will happen during the scan warning them that the gantry will move around potentially very close to them will stress the importance of remaining stationary throughout reassure them of what's involved advising them they can close their eyes if they feel more comfortable to do so and then as we can see here there'll be appropriate restraints to reduce the risk of movement so we're stabilizing the patient with things like a chin rest or head support We can also reduce the risk of movement by employing shorter scan times and there are a number of methods by which we can do this. In some units we've seen initially that the cone beam CT scan is reconstructed from a 360 degree rotation of the gantry around the patient but in some cone beam CT units a 180 degree rotation can be used and therefore a shorter exposure time. There are some units that have high speed scan settings. so we can acquire the scan in much less time but both of these will result in fewer basis projections so less information reaching the image detector and therefore the trade-off for the reduction in scan time is that it will result in an overall noisier image but again this may be perfectly acceptable depending on the clinical age clinical indication both in terms of what the patient is able to tolerate and the level of detail that we require so if we look at a standard setting on this particular scanner we can see the scan time is 17 and a half seconds if we go to very high resolution scans the scan time will increase so there'll be an increased risk of patient movement but we can get a greater degree of detail if that is required to answer the clinical question high resolution very similar but we have these high speed settings as well and again reducing the scan time from 17.5 to 10.5 seconds in this case the other setting that we mentioned was the 180 degree setting instead of 360 and again we can see the scan time is reduced down to nine seconds so as you've seen in the initial animation in a standard comb beam ct scan the gantry will do 360 degree rotation around the patient whereas with just half a rotation half of a full rotation in some units we can reconstruct the final volume But we don't get something for nothing.

As I say, there's always a trade off. So we have a shorter scan time. However, we have less basis projections or information reaching the image receptor. And therefore, overall, we can see that results in a noisier image when we employ that 180 degree setting.

another benefit of the units that can reconstruct from 180 degree is if we have undertaken a 360 degree scan however the patient has moved during the portion of that scan then afterwards we can use the software on the computer to select an appropriate 180 degree portion of the 360 degree rotation and reconstruct the image from that 180 degree portion removing the part where the patient is moved so we can see here an example of movement artifact that's occurred during a 360 degree scan and by excluding that portion of the scan in which movement occurred if it is isolated to one specific part of the scan we can reconstruct um the 180 degrees of that 360 degree information and get an image that may go from something that was non-diagnostic to something that is potentially diagnostic and save having to repeat the scan. Combeam CT manufacturers have also been developing post-processing software much in the same way that metal artifact reduction software is used to remove movement artifact in reconstructed volumes. The next Combeam CT image artifact we'll discuss is Ring artifact.

and this arises due to a defective site on the image receptor. As a result of the image receptor rotating around the patient, this defective site results in a dark ring on the final image. And we can see an example of ring artefact here.

And if we look at how this develops in an illustration, we can see that the green area on the image receptor is not detecting any of the x-rays because it's defective. So when the X-ray beam is switched on and the scan begins, that portion of the image receptor is not detecting any information. And therefore, that's left as this dark ring on the final image where there's been no data collected from that region of the image receptor. And we can see another example here of ring artifact on this small volume cone beam CT, which is even apparent. the volume rendered image.

So how we can avoid or fix ring artefact is by performing regular quality assurance, ensuring that the cone beam unit is regularly maintained and contacting the manufacturer for repair and potentially putting the unit out of service until it is repaired as soon as it is first identified. A liaising artefact occurs due to under sampling and this is typically seen at the very periphery of scanned cone beam CT volumes. This is related to the divergent nature of the x-ray beam used within cone beam CT and results in something that's termed a moiré pattern.

If we look at this first as an illustration we can see as the x-ray beam rotates around the patient we get a lot of information from the most central region in green but we get less information arising from the peripheries of the scan volume if we look at this on a real life example we can see how the x-ray beam is mainly focused on the central region of the scan volume and we have less information from the periphery so again the green region tends to have the greatest level of detail and the yellow peripheral region has less levels of detail and this results in that artifact we mentioned earlier these linear light and dark bands that's uh termed a moiré pattern how we can minimize this having an impact on the clinical yield of our combium ct volume is by centering our region of interest within the field of view as as we've seen from the examples this artifact tends to be confined to the peripheries of the scanned volume also by using the smallest possible field of view we will have less divergence of the x-ray beam so moving on now from image artifacts to incidental findings these are chance radiographic findings that we note within a scanned image that was taken for an unrelated reason. and given the increased utilization of cross-sectional imaging including comb beam ct this has led to a marked increase in a number of these incidental findings one of the most common incidental findings we see in comb beam ct would be caries so we can see an example here with indenting and confined to enamel so although caries is demonstrated on comb beam ct it is not indicated as a primary use In fact, as we've seen from the early examples, the opposite can often be true where caries can be mimic or obscured as a result of metal artefact within cone beam CT. And as we mentioned earlier, if there is any suspicion or ambiguity over the presence of caries, then we may recommend supplementing the cone beam CT image with diphthene radiographs. Periodontal disease is another common incidental finding on cone beam CT imaging.

We can see examples here of deposits of calculus, periodontal bone loss associated with this impacted wisdom tooth, and frication involvement. Tonsilloliths, as we commonly see on conventional dental panoramic radiographs, are often also visible on cone beam CT, particularly when imaging the mandibular wisdom teeth. other radiopacities such as dense bone islands or bony anastosis we can see on this periapical are picked up as a common incidental finding on comb beam ct imaging typically continuous with the adjacent buccal or lingual cortex and typically these are benign lesions that don't require any intervention but where we do have to raise awareness of them is where there is planned orthodontic tooth movements through such regions of dense bone as typically that would result in an increased risk of root resorption as we can see here. Bony exostosis again as demonstrated on this dental panoramic radiograph are often detected in the form of mandibular tori. or certainly maxillary torus on cone beam CT as an incidental finding.

However, where we see numerous or more pedunculated bony projections, we would have to raise the clinical possibility of Gardner syndrome for investigation. Mucosal thickening within the maxillary antrum is another common incidental finding. cone beam CT imaging and there is a normal thickness to the lining of the maxillary antrum the Snyderian membrane and that can vary approximately one millimeter but potentially up to three millimeters in some patients where there is localized mucosal thickening this is likely to represent pathologies such as mucous retention cysts Other more radiodense radiopacities which we may see projected over the maxillary antrum on these periapical radiographs on and on this dental panoramic radiograph would include things such as antraliths or potentially where there's an inappropriate history these may represent roots that have been displaced into the maxillary antrum.

Partial bony septations within the maxillary antrum can also mimic the appearance of a root displaced, as you can see on this periapical and dental panoramic radiograph. And then on the cone beam CT image, we can see the site of that partial bony septation. Retained roots are often identified as an incidental finding on cone beam CT imaging. and in scenarios where none of the permanent teeth are absent but we still see these radiopacities these are more likely to represent retained roots of the primary dentition certain anatomical features can also be more apparent on conveying ct and this would include things like the greater palatine groove along which the greater palatine nerve runs within the heart palate and common incidental findings on conventional radiographs such as embedded restorative material will also be detected commonly on combium ct imaging another normal anatomical variant that may also be seen on combium ct imaging is the presence of accessory maxillary ostea in terms of drainage pathways for the maxillary antra or again these anthraceptations we've seen the example earlier of it mimicking a root displaced into the antrum again they can mimic cystic lesions which on combium ct we can then identify as partial septations of the antrum the nasal septum is often deviated to some degree and there can be the presence of these septal bony spurs another anatomical variation we can see within the sinuses is whereby the middle turbinate curves in the opposite direction to which it typically does this is termed paradoxical curvature or there can be pneumatization of the turbinates in a process called Concha Bellosa. Osteomas can be detected within the paranasal sinuses and occasionally can obstruct a drainage pathway and septal perforation can be seen as an incidental finding.

This could be part of a disease process such as Wegener's granulomatosis. or can be related to chronic cocaine abuse and it's important for us to be able to differentiate some of those anatomical variations with the post-surgical appearance whereby the patient may have undergone endoscopic sinus surgery to improve the drainage of the maxillary antra either by widening the drainage pathways or by removing some of those obstructive components calcified or ossified stylohyoid ligaments are commonly seen on dental panoramic radiographs and again as a result are a common incidental finding on comb beam ct they can produce a pseudo-articulated appearance but the presence of them in themselves doesn't necessarily mean a diagnosis of eagle syndrome as this is more related to irritation of the adjacent musculature which occurs much less commonly degenerative change within the temporomandibular joints is also a common incidental finding within cone beam ct imaging and we can see here examples of flattening of the anterior superior surface of the condyle with early anterior osteophyte formation and developmental abnormalities of the tmj may also be detected as an incidental finding such as the presence of a bifid condyle as well as degenerative change within the temporomandibular joints degenerative change can also be detected as an incidental finding on the partially imaged region of the cervical spine on combium ct and as well as developmental abnormalities within the tmj congenital abnormalities of the cervical spine can also be detected. An incidental finding that can be associated with ectopic and uninterrupted teeth is the presence of the gubernacular canal and what this represents is the remnants of the eruption pathway or the potential eruption pathway of the tooth. This is often seen with impacted maxillary teeth. We can see here the point at which it exits onto the alveolar crest.

Coronal defects can also be incidentally detected on comb beam CT volumes such as the buccal defects we can see in this mandibular wisdom tooth as well as the presence of enamel pearls. which may predispose to periodontal disease intracranial calcifications may be detected on particularly large volume comsium cts such as this calcification within the pineal gland and calcification of the walls of the internal carotid artery Other incidental findings arising within the adjacent soft tissues include salivary stones, as well as calcification occurring within long-standing chronic skin infections such as acne, which is called calcinosis cutis. and although not primarily indicated for the imaging of soft tissues incidental findings at the soft tissue air interfaces can be detected on combium ct and therefore swelling such as this one we see on the heart palette should be highlighted in the final report back to the referring clinician for clinical correlation should you wish to read more about the topics we've discussed today these are some recommended papers and thank you for your time.

Transcript for:Understanding CBCT Imaging Artifacts and Findings

Transcript for:
Understanding CBCT Imaging Artifacts and Findings