From Panoramic, to Periapicals, to Cone Beam CT

How to Ensure Excllence in Radiographic Interpretation

Primum non nocere, “First do no harm” is a guiding principle in all of the health professions. Patient care is predicated upon accurate assessment of clinical data, formulation of an appropriate treatment plan and implementation of that plan. Inaccurate interpretation of diagnostic data can result in failure to diagnose and treat dental disease or it can generate unnecessary treatment. Neither of these scenarios embraces the concept of “do no harm.”

Radiographic imaging provides a wealth of data that allows the clinician to generate the most accurate and complete treatment plan for each patient. Digital radiographic imaging has made the acquisition of radiographic information much easier for the clinician; however, the parameters that affect film based imaging still have an impact on digital imaging. Additionally, digital imaging systems provide numerous software tools that impact the ability to accurately interpret the radiographic image.

Appropriate Survey

Selecting the appropriate imaging for each patient is essential for providing adequate diagnostic information. Caries experience, periodontal status, systemic health and current medications all factor into the selection of appropriate radiographic images. Utilization of the above-mentioned factors when determining the appropriate imaging is termed selection criteria. Selection criteria guidelines were updated in 2004.

The new guidelines place more emphasis on panoramic imaging for new patients (Figure 1). For example, you don’t need a full-mouth series on an 18 year-old who only has a few occlusal restorations. Bitewings and a panoramic image would be sufficient. Conversely, an 18 year-old with rampant caries might require a full mouth survey and a panoramic image.

Use of vertical bitewings for patients with alveolar bone loss is another good example of the use of selection criteria. The advent of cone beam computed tomography (CBCT) has also affected the way that clinicians evaluate their patients (Figure 2). CBCT can provide three-dimensional information that assists the clinician in evaluating potential implant sites, planning orthodontic treatment, localizing unerupted teeth or assessing the expansion of a pathologic entity. One of the advantages of CBCT over conventional computed tomography is dose (Figure 3). The dose from CBCT is slightly more than a conventional panoramic image as opposed to more than ten times the dose from conventional CT. While CBCT technology provides a wealth of information it is important to remember that the clinician is responsible for all of the information contained in the data set, not just in a particular region of interest.

Image Quality

The five factors of image quality apply to digital imaging as well as conventional imaging. Photons don’t care about the type of receptor used. These factors are separated into visual and geometric factors. Visual factors are density and contrast.

Visual Factors: Density is the overall darkness of a radiographic image and is controlled by exposure time. It is essential to assess the size of a patient before you expose the image. Large patients require more exposure time. If the resultant image is too light, the clinician can darken the image electronically. Unfortunately, you can’t enhance information that wasn’t captured in the first place. Conversely, a dark image can be electronically lightened (as long as the receptor isn’t saturated). The linear relationship between exposure and density allows the clinician to alter density but it also can create an image that is too dark in the anterior region of the image or too light in the posterior portion of the image (Figure 4). To allow optimum use of the image always use the exposure setting for the most posterior teeth in the image (e.g. use the molar exposure setting for the premolar bitewing so that the molars will be of appropriate density and the density of the premolars can be diminished).

Contrast: is defined as the difference in density visualized on an image. Contrast can only be manipulated by altering kVp. Digital systems have the ability to manipulate the contrast of the output image by altering the slope of the exposure/density curve. Altering the slope of this curve allows the clinician to accentuate small density differences but again, if the subject contrast isn’t captured, it can’t be enhanced.

Geometric Factors: The three geometric factors of image quality are image unsharpness, magnification and image distortion. Image unsharpness and magnification both create a fuzzy image or penumbra because the object of interest is too far away from the image receptor. The impact of the object-receptor difference can be minimized by using a long target receptor difference or long cone technique. Image distortion is created when the alignment of the source, object of interest and image receptor is incorrect. The goal is to align the long axis of the object of interest parallel to the image receptor and direct the center of the x-ray beam perpendicular to the long axes of the object and the receptor.

Sound familiar? This is the description of the paralleling technique. Achieving these relationships can be difficult with film based or phosphor plate imaging. Creating these relationships with rigid sensors is even more challenging. It isn’t a good idea to bend the corner of a sensor so the clinician must utilize the space in the patient’s mouth to their advantage. The palate is the deepest in the midline towards the posterior. Place the receptor towards the posterior when taking anterior projections and there won’t be a shadow of the soft tissue of the nose superimposed over the image. The floor of the mouth is deepest in the midline and towards the posterior. Don’t try to place the sensor right next to the posterior teeth, the patient won’t bite down and will turn their tongue into an immoveable mass of tense muscle. Projection geometry has a huge impact on image quality and can affect multiple image enhancements if the principles of parallelism are not applied.

Digital radiographic imaging provides both clinician and patient with the opportunity to view larger than life images of the dentition. Monitor placement can affect how well the image is displayed. Make sure that the monitor isn’t in a brightly lit area of the operatory and that it’s angulation can be changed to give the patient and the operator the best possible view.

Systematic Approach

The best way to interpret radiographs is to use a systematic approach every time images are viewed. Regardless of the order, the following components should be included in the interpretation: maxillofacial bone, alveolar bone, teeth, paranasal sinuses, structures outside the jaws and the temporomandibular joint complex. The temptation is for the eye to be drawn to the largest radiographic finding if a systemic approach isn’t followed.

Maxillofacial Bone: Maxillofacial bone supports the alveolar bone. The overall density and trabecular pattern of the bone should be evaluated. Any biological process that affects calcium homeostasis (e.g. osteoporosis, Paget’s Disease of Bone) can affect the trabecular patterns in the bone. The apical regions of the teeth should also be evaluated. Subtle changes in the density of the lamina dura or the periodontal ligament space provide an early warning system that the pulpal tissues of the tooth are injured. Evaluation of the maxillofacial bone is necessary for the edentulous patient. As the alveolar bone resorbs, normal anatomic structures appear to change. The genial tubercles appear to become more prominent when in fact they just become more visible. The mental foramina appear to migrate to the superior aspect of the residual ridge. The foramina don’t move, the bone just goes away.

Alveolar Bone: Periodontal disease affects the alveolar bone of almost every dental patient. Loss of alveolar bone height is a radiographic sign of periodontal disease. However, density changes in both cortical and trabecular bone occur prior to loss of bone height. Radiographic images are two dimensional representations of three-dimensional structures. Consequently, architectural subtleties may be obscured due to errors in projection geometry. Finally, radiographic images demonstrate loss of alveolar bone but provide no information about disease activity. It is essential to correlate radiographic findings with the clinical examination when evaluating the periodontal status of the patient.¹

Teeth: Radiographic imaging has long been the method by which dental caries are detected on proximal surfaces. Sufficient destruction of the dental hard tissues must occur before the lesion can be radiographically detected. Carious lesions on the occlusal surfaces are less likely to be discovered radiographically due to the amount of hard tissue destruction that must occur. Proximal caries present just gingival to the proximal contact regardless of the alignment of the teeth. Occlusal caries are found in dentin, apical to the deepest pits found on clinical examination (e.g. central pits of molars). Others conditions that increase the difficulty of caries detection are resin restorations, liners, cervical burnout and lesions of toothbrush abrasion. It is common to find a radiolucent margin between the restoration and the tooth (Figure 5). Block out the restoration and determine if the radiolucency still exists. The mach effect is an optical illusion that creates a distinct radiolucent edge at the junction of two dissimilar structures. Sometimes, the anatomy of the tooth causes an altered radiographic appearance. The fluting of the roots of the maxillary first premolars commonly generates a radiolucency that could be confused with dental caries. Finally, the ability to alter density and contrast can increase the likelihood of creating cervical burnout (Figure 6). Always correlate radiographic findings with the clinical examination.

Paranasal Sinuses: The paranasal sinuses are airspaces that can be captured in dental imaging. The maxillary sinus can be seen in maxillary posterior periapicals and in panoramic imaging. The normal content of these sinuses is air and they appear radiolucent. Alterations in the contents of the sinuses can be seen in many dental images. Cloudiness in the maxillary sinus suggests the presence of fluid in the sinus. Long standing inflammation in the maxillary posterior teeth can cause thickening of the mucosal lining of the sinus. The antral pseudocyst can be easily identified in radiographic images.

Structures outside the Jaws: There are numerous radiographic entities that exist outside the jaws but are visible on panoramic imaging. Panoramic images can be used to evaluate air spaces, soft tissue calcifications, the stylohyoid and stylomandibular ligaments, and mastoid air-cells. Perhaps the most significant soft tissue one can find is the carotid artery calcification. Carotid artery calcifications can be found adjacent to the cervical spine. The presence of these calcifications is indicative of heart disease. It is the responsibility of the clinician to inform the patient and the patient’s physician if these soft tissue calcifications are identified. There is no way to tell the amount of carotid artery occlusion from radiographic imaging so the patient should be referred to their physician for further evaluation.

Temporomandibular Joint Complex: The most common view of the temporomandibular complex is the panoramic view. The condylar head can have numerous appearances that are normal due to patient positioning and the problems associated with three dimensional objects captured in a two dimensional image. The shape of the Glenoid Fossae and articular eminentiae should also be evaluated. The asymmetric slope of the Glenoid fossae can present as a deviation on opening during the clinical examination. Radiographic imaging can tell the clinician a great deal about the bony components of the temporomandibular joint complex but photons are no match for the articular disc. The best method to evaluate the articular disc is with Magnetic Resonance Imaging (MRI). MRI evaluates the hydrogen content of tissues. Consequently, it is a very good method for evaluating soft tissues due to their water content (Figure 7).

Using Enhancement Tools

A systematic approach to image interpretation is essential for the development of a complete treatment plan and is applicable for either film-based or digital imaging. Let’s look at some of the image enhancements available with digital imaging systems.

Numerous investigators have evaluated the effects of various enhancements on the diagnostic efficacy of digital images. The results are varied. Some authors report no benefit from enhancements^2,3,4,5,6 some find increased diagnostic efficacy⁷ and some state that the enhanced image is less diagnostic than the unenhanced image⁸. A global assessment of the literature suggests that the effects of image enhancement have more to do with the visual system of the operator than with the digital information itself. Consequently, the benefit of the enhancements will vary among individuals. The clinician must experiment with enhancements to determine what works and what doesn’t work. The benefits will be dependent upon the operator and the task.

Density and Contrast Enhancement: These two enhancements have already been discussed. There are obvious advantages to changing the density or contrast on the output image (Figures 8 and 9). The ability to lighten, darken and accentuate contrast are valuable diagnostic tools.^3,9It is however important to remember that you can’t necessarily salvage a light image and that too much enhancement can produce distortions that can be misinterpreted as dental disease.

Measurement Tool: The ability to measure length in a radiographic image is a great advantage of digital imaging (Figure 10).^10,11Most systems allow the clinician to calibrate this tool to maintain the accuracy of measurements. The measurements are made of the image and are only as good as the projection geometry. A foreshortened image will not be measured with a foreshortened ruler. Adherence to the principles of the paralleling technique is essential for accurate measurement.

Flashlight: The flashlight tool is simply a histogram equalization performed on a specific region of interest (Figure 11). Histogram equalization is a mathematical technique that evens out the distribution of gray levels in a particular area. This enhancement accentuates the density differences within a region of interest and affords the clinician a valuable diagnostic tool.^2,3,12,13

Image Inversion: Image inversion generates an image with the gray scale flipped (Figure 12). What used to be dark on the image is now light. This enhancement may have some utility in certain diagnostic tasks but is entirely dependent upon the visual system of the clinician.¹⁴This is one of the enhancements that must be evaluated by the individual clinician to determine the level of benefit derived from the manipulation.

Pseudocolor Enhancement: This enhancement has been studied by several investigators with varied results. Pseudocolor enhancement has been reported to improve diagnosis of periapical defects5 but has impeded the diagnosis in a variety of other diagnostic tasks.^9,15This is another enhancement that’s utility is determined by the visual system of the clinician (Figure 13). There is no question that pseudocolor enhancement serves as a valuable patient education tool.

Now let’s apply a systematic approach to evaluating the non-tooth bearing areas of the panoramic radiograph (Figure 14). There are five notable findings in this image. Figure 15 displays the positive findings.

Excellence in radiographic interpretation is essential for quality patient care. Selecting the appropriate imaging study with good image quality provides the clinician with the materials to begin the interpretive process. A systematic approach coupled with judicious use of electronic image enhancements will allow the clinician to derive the optimum amount of information from the radiographic data. The correlation of radiographic findings with the clinical examination affords the clinician to develop a complete and accurate treatment plan.

References

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