Radiography plays an important role in implant dentistry. These investigations are used to assist in the diagnosis of pathology, aid in implant treatment planning, and monitor peri-implant bone levels on maintenance assessments, such as this image taken at a 1-year follow-up visit. To achieve these goals, radiographic images need to meet basic standards of quality. They need to have sufficient density and contrast to clearly show changes and to demonstrate sharp detail. Distortion, blurring from movement, and other artifacts limit a radiograph's diagnostic value.

Failure to meet these quality standards can impact on subsequent treatment. Missed diagnoses and poor planning can increase the risk of undesirable outcomes in treatment. In this example, overexposure and a bent film have resulted in a dark and distorted image that is virtually useless for either diagnosis or implant treatment planning. In such situations, repeat exposures are often needed, increasing the patient's radiation exposure. This ITI Academy Learning Module will discuss the sources of errors and artifacts in radiographic imaging. Knowing how these occur will help you to avoid these problems and assist in controlling radiation exposure and risk to your patients.

After completing this ITI Academy Module, you should be able to describe errors and artifacts that can affect 2D imaging, and how to deal with them; describe the errors and artifacts that can affect 3D imaging, and how to deal with them; and interpret linear measurements in 2D and 3D imaging, and translate them into the specific clinical situation.

Intraoral images can be captured using a number of techniques: using conventional silver-halide film technology; or digital capture using solid-state detectors (commonly called sensors) or photostimulable phosphor plates or PSPs, which capture a latent image that is later scanned. While each method has the potential for system-specific technical errors, some potential errors are common to all image-capture systems. Despite the potential for problems, many suboptimal radiographs can still be used. It is only when the quality of the film is so poor as to be totally undiagnostic that repeat exposures should be made.

Common errors with film-based intraoral radiographs include: light images from underdeveloping or underexposure; dark images from overdeveloping or overexposure - either too long an exposure or too high a kilovoltage setting; "tire track" marks on the film - usually a herringbone-type pattern superimposed over the image - that result from the film being placed with the wrong side facing towards the x-ray beam; crescent-shaped black lines on the image from excessive bending or fingernail pressure on the film surface; distortion related to bending of the film; double exposures; and finally dark radiolucent spots on the image that are caused by drops of developer being splashed onto the film prior to processing. Here are two examples that demonstrate some of these errors. In this radiograph the dark image is related to overexposure or overdeveloping. The film has also been bent excessively, resulting in distortion and elongation of parts of the image. In this example the image is too light due to underexposure or underdevelopment. Finally, it should be noted that digital image capture can be affected by similar errors, especially those that relate to film placement and x-ray exposure.

Common errors with digital imaging include image noise, which can be a problem with both types of digital capture. This is normally associated with low-dose images and presents as a grainy, salt-and-pepper type pattern over the image. This can be rectified by slightly increasing the exposure in a subsequent image; alternatively, noise can be reduced to a level where the image can still be used by manipulating the image using software filters. Digital intraoral imaging also can be affected by double exposures, resulting in two visible images that are superimposed. This is mostly an issue with phosphor plates, as digital sensor software usually prevents double exposures. Phosphor plates can also be easily scratched, leading to white lines on the image where the phosphor coating on the plate has been lost. This problem can only be fixed by replacing the plate and retaking the image. Digital sensors have some potential difficulties due to their limited range of sizes, rigidity, and bulk. Unlike films and phosphor plates, the digital sensors can be bulky and uncomfortable for patients. Additionally, they tend not to comply with standard film sizes, which may limit their usefulness.

Errors in positioning the capture device and the x-ray beam can adversely affect the resulting image. All image-capture systems can be affected by these positioning errors, although the appearance of the affected image may differ. Errors in angulation of the x-ray beam relative to the film or sensor will result in elongation with any system, as will cone cutting where the image is partly cut off by the positioning of the x-ray generating apparatus and collimating cone. If films are placed with their back surface facing the x-ray beam, superimposition of the embossed pattern from the lead foil in the film packet leads to a pattern overlaid on the image. Similar patterns are visible on phosphor plate systems when the plate has been incorrectly placed. With digital sensors, the wire and sensor electronics can be seen superimposed on the captured image if these sensors are incorrectly placed.

Panoramic images remain one of the most frequent radiographs taken for implant treatment planning, as they detail the facial structures of the maxilla and mandible including the dental arches and the supporting structures in one single image. A significant disadvantage of panoramic images is that they are of a lower resolution than intraoral radiographs and do not display fine anatomic detail. Additionally, panoramic radiographs are not dimensionally accurate; they distort the size and shape of depicted objects. Thus linear and angular measurements are not truly reliable. Horizontal magnification varies between anterior and posterior regions of the jaws, whereas vertical magnification is relatively constant thorough the different areas of the image. An example of a good-quality panoramic radiograph is shown. It clearly shows the dental and paradental structures, including the dental implants and sinus grafts that have been used to enable their placement.

Many problems with panoramic images relate to positioning the patient in the x-ray machine. Because these machines rely on a geometric projection to arrive at the final image, the position of the patient in the machine and the alignment of their head will have a significant impact on image quality and potential distortion. The teeth and jaws must also be positioned in the focal trough of the machine to obtain good quality imaging of these structures. Failure to achieve this will result in poorly focused images of the teeth and/or jaws. In this example, the patient was placed in the machine with their head too high. This has resulted in an image in which the maxillary sinuses and temporomandibular joints are not well demonstrated. An incorrect head alignment will also distort the image, resulting in apparent asymmetry of the jaws when the patient's head is turned to one side, or enlargement or reduction in the size of the jaws if their chin is too high or too low. In this example, the patient's chin is tilted down, resulting in a so-called "smiley" image. Here the temporomandibular joints are also not well displayed. These errors are easily detected when compared against a good quality image.

Ghost images are another common artifact affecting panoramic radiographs. These are normally related to extraneous items that are situated outside of the focal trough. Ghost images can obscure regions of interest. Again, because of the geometry of these projections, ghost images tend to be enlarged, poorly focused, and situated on the opposite side and higher when compared to the true image of the object. In this example, the patient's left side earrings result in ghost images superimposed on the maxillary sinus on the right side while the nose ring in the left anterior nares can be seen superimposed over the right TMJ.

When taking 2D images for diagnosis and planning, anticipating problems or difficulties is the key to obtaining radiographs of diagnostic quality. It is important to understand the limitations of the imaging technique that you are using, as well as the common problems that can arise. Choosing the correct view for the task at hand will simplify processes and reduce the risks of problems. As noted earlier, it is important to ensure that patients are not exposed to radiation unnecessarily, and therefore adherence to the principles of ALARA and ALADA is essential. This means that you should only retake images if they are totally unusable. Suboptimal images may be sufficiently detailed to use for implant planning purposes. Preventing problems is likely the only reliable way of avoiding poor imaging outcomes. Therefore, ensure that: films and patients are correctly positioned to get good outcomes, the correct exposure settings are used, and appropriate processing protocols are followed for film-based images. Finally, using positioning aids like bite-blocks and head guides to correctly place patients in a panoramic machine or film holders to assist in correct aiming of the x-ray beam can limit the potential for incorrect positioning that leads to distorted or unusable images.

2D Imaging, Key Learning Points: Certain errors such as underexposure, overexposure, and image elongation are common to all intraoral imaging techniques. When the film, sensor, or phosphor plate is reversed in the mouth, a pattern will be superimposed on the image. Digital images with too much graininess or noise can sometimes be corrected using software. A disadvantage of panoramic radiographs is the horizontal and vertical distortion of the image. Patient positioning is important in obtaining panoramic images of diagnostic quality. Only retake images that are not sufficiently detailed for implant planning purposes.

For cone beam computed tomography (or CBCT), there are a number of problems that can occur during imaging because of either patient motion or the image capture and reconstruction process. Beam hardening is one of the most prominent sources of artifacts. As the beam passes through an object, it becomes "harder," that is, its mean energy increases because the lower-energy photons are absorbed more rapidly than the higher-energy photons. The beam therefore becomes proportionally richer in these high-energy photons. This results in dark streaks and bands in the image. In the example shown, beam hardening has resulted in the darker streaks as well as the areas of radiolucency that can be seen between the radiodense implants in this image. Unless aware of beam hardening as a source of artifacts, one might misinterpret these areas as representing regions of peri-implant bone loss.

Highly radiodense metallic objects in CBCT images can result in distortion referred to as "cupping." This is, in effect, a shadow area where all of the x-ray photons in the beam have been blocked by the metal, resulting in distortion of the shape of the object. This type of artifact can be seen in this example image, where the shape of the metallic object has been distorted by this photon starvation. This artifact is sufficiently severe as to make accurate identification of this object in the image highly uncertain.

Because the resolution of CBCT devices currently available on the market is very high - with voxel sizes ranging from 0.08 to 0.4 mm - even small movements can have a detrimental effect on image quality. This can result in double images similar to the example shown here.

Understanding the potential for imaging artifacts in CBCT images, and the causes of these problems, will help you avoid misinterpretation of what you see. This, in turn, requires good training in CBCT radiology and in interpretation of the images. Again, prevention is the key to minimizing image artifacts. Using limited fields of view will help reduce the incidence of beam hardening and cupping artifacts by avoiding the involvement (where possible) of radiodense metallic objects in the field. Small field-of-view investigations will also reduce exposure times, limiting the risk of motion artifacts. Motion-related distortions can also be reduced by using positioning aids and instructing patients to hold their breath for the duration of the exposure. When patients cannot remain still, as may happen with patients afflicted by dyskinetic movements related to Parkinson's Disease, other imaging methods that employ shorter exposures or are less affected by motion should be used. In such cases intraoral films, whilst not ideal, might be a better choice of technique.

3D CBCT Imaging, Key Learning Points: The presence of radiodense metallic objects such as implants can cause beam hardening, seen as dark streaks and bands in the image. Photon starvation causes a distortion of metallic objects known as cupping. Patient movement reduces image quality. When taking CBCT images, training and preventive measures are the key to avoiding artifacts.

For two-dimensional intraoral images, distortion is an important limiting factor with regard to the precision of linear measurements. To account for potential magnification of the measurements, it is necessary to calibrate the image using a known dimension within the radiograph. If no reference is available, metallic objects of known dimensions such as wires or balls can be introduced for calibration. Otherwise, if a dental implant is already present in the film, the implant thread pitch (that is, the distance between two radiographically visible threads) can be used for calibration by identifying the magnification factor. In this example the thread pitch of the implants can be used to calibrate measurements. Here the implants have a pitch of 1.25 mm.

In panoramic images, the enlargement factor is not constant across all areas of the image. This enlargement can be in the order of 120 to 130 percent. Posterior sites tend to be less variable. If the image does not have features near the area of interest that can be used to calibrate measurements in that site, radiopaque markers can be added in a radiographic template. In this example, 5-mm steel ball bearings were positioned near the areas of interest while the patient had the image taken. These balls were cemented to tongue depressor sticks and placed into the buccal fornix adjacent to the areas of interest prior to image capture. This technique is possibly less accurate than using a template, but it is faster, simpler, and less expensive, making it an option in straightforward cases.

We can calculate an unknown dimension from a panoramic radiograph if we know the size of the radiographic marker that has been used. In this example, a 5-mm stainless steel ball bearing has been used as the radiographic marker in the region of the lower right first molar that is planned for replacement with an implant. In the panoramic image, this ball appears to be 6.3 mm in the dimension parallel to the planned long axis of the implant. Using these numbers we can calculate that the enlargement factor in this part of the film is 1.26 or 126%. In this region, we need to ensure that we do not injure the inferior alveolar nerve, so our unknown dimension is the distance from the alveolar crest to the superior aspect of the mandibular canal. Measured on the image, this distance is 28.4 mm. To get the true dimension of the ridge height in this site, the measured dimension must be divided by the enlargement factor, giving a real dimension of 22.5 mm. This can also be expressed as an equation, where the real unknown dimension is equal to the real size of the marker multiplied by the distance in question measured on the image, then divided by the apparent size of the marker that has been measured from the film. These proportionality calculations can be done on any film as long as you know the real dimension of one measurement.

Now it's your turn to practice calculating a linear measurement. Let's say that you measure the stainless steel marker next to the mandibular molar, and the dimension parallel to the planned long axis of the implant is 6 mm. You then measure the distance from the crest to the mandibular canal on the image and find it to be 30 mm. You know that you can't rely on 30 mm as an accurate measurement of the real distance. You remember that this formula can be used to calculate the actual distance. Select the correct actual distance.

For CBCT imaging, a recent systematic review indicated that CBCT provides cross-sectional images that demonstrate high accuracy and reliability for bony linear measurements related to implant treatment. A voxel size of 0.3 to 0.4 mm is adequate to provide CBCT images of acceptable diagnostic quality for implant treatment planning, as shown in this image from an implant planning software application.

However, a wide range of error has been reported when performing linear measurements on CBCT images, with both over- and underestimation of dimensions in comparison to a gold standard. The range of error can exceed 1 mm in selected cases. As such, a 2-mm safety margin is recommended. In this closeup view of the image on the previous slide, a radiographic template has been used in conjunction with planning software to determine the appropriate implant length for this site, ensuring that the inferior alveolar canal is safely avoided. Note the 2-mm safety margin between the chosen implant and the inferior alveolar canal.

Interpreting Linear Measurements, Key Learning Points: When measuring available bone on a 2D image, objects of known dimension should be used for calibration. Metallic markers and implant threads can be used as reference objects of known dimension. In CBCT images a safety zone of 2 mm is recommended with respect to crucial structures due to the potential for error in linear measurements.

Recognizing and Troubleshooting Radiographic Imaging Errors, Module Summary: Adequate training in radiologic technique is necessary to recognize and avoid errors and artifacts. Some potential errors and artifacts are specific to certain intraoral imaging methods, while other errors are common to all techniques. Accurate linear measurements on 2D radiographs require the use of objects of known dimension for calibration. Because image enlargement is not constant across all areas of a panoramic radiograph, objects used for calibration should be near the site of interest. A 2-mm safety margin to crucial structures is recommended when using CBCT images for implant planning.