Digital Dental Radiography

Digital radiography (DR) is ubiquitous in medicine, with more than 75% of medical clinics in the United States having converted to digital use since 2000. In fact, within medicine, the conversion to digital has been mandated by the US government. ^{[1, 2]}

In contrast, based on several recent dental surveys, ^[3] a minority of dental practices in the United States and elsewhere have converted to digital radiology or other digital systems. These surveys suggest that the use of this technology in dentistry appears to depend on specialty (more often used by general dentists), location (large population centers vs small cities), ^[4] and cost. ^{[5, 6]}

In one 2007 dental survey conducted by the American Dental Association, only 36.5% of dentists in the United States used digital imaging, and this was primarily for bitewing and periapical radiography. Approximately 20% used this technology for panoramic studies. ^[7] Nonetheless, awareness of the potential benefits of digital imaging generally and digital radiography specifically is increasing with each new technical innovation being introduced. It has been estimated that, by 2016, the proportion of digital dental imaging systems will double from the number estimated in 2009. ^[8]

Digital imaging is a term that describes the presentation of a radiologic image on a computer monitor as a digital picture. This is made possible via the development of high-luminance and high-resolution display monitors combined with high-performance computers coupled with the development of image plates that are coated with photostimulable phosphors that capture “reinterpreted” x-rays.

When these image detectors are stimulated by light following radiation exposure, they release the energy, and this light is then converted to rows and columns of pixels that represent the various x-ray intensities. These pixels are then processed via mathematical algorithms to be displayed on the viewing screen as an image that can be manipulated in various ways. In dentistry, numerous acquisition sensors or detectors are available for use.

Capture sensor devices used in dentistry fall into two categories: indirect flat-panel detectors and direct flat-panel detectors.

The indirect sensors use a technology as described in simple terms in Digital Technology.

Direct flat-panel detectors (FPDs) convert x-ray photons directly into an electrical charge, which is read out by a thin film transistor array, active matrix array, electrometer probe, or microplasma line. The material used in the FPDs includes amorphous selenium (a-Se). Another direct sensor is the high-density line-scan solid-state detector, which is lined by photostimulable barium fluorobromide “doped” with europium or caesium bromide phosphor. The energy collected by x-ray exposure is then scanned by a laser diode, and the resulting excitable energy that is released is read by a digital image capture array charge-coupled device (CCD) and converted to display.

Photostimuable phosphor plates (PSPs) are digitally scanned prior to display, whereas the direct imaging sensors need no secondary step in processing prior to display on a computer monitor. There are benefits and drawbacks associated with both of these sensor systems.

Indirect intraoral sensors

Advantages of indirect intraoral sensors are as follows:

• PSP plates are flexible

• PSP plates can be easily placed in the mouth without discomfort

• PSP plates are more economical than direct sensors

• PSP plates offer more office flexibility in the multitreatment room setting

• PSP plates match the sizes of traditional film

• PSP plates offer similar workflow dynamics as used with analog systems

• Warranty typically covers 2 years

Disadvantages of indirect intraoral sensors are as follows:

• PSP plates require the intermediary step of development of the sensor

• PSP plates, while reusable, do wear out over time and must be replaced

• PSP developing technique must be learned and correctly performed

• PSP plates can be damaged by excessive bending during oral placement

• PSP packaging requires precise staff handling

• PSP packaging must be performed and is time consuming

• Incorrect PSP plate handling may result in bacterial contamination

• PSP plates can be easily damaged by scratching, fingerprints, etc

• A developer must be purchased

• PSP systems may require IT support and software upgrades

Direct intraoral sensors

Advantages of direct intraoral sensors are as follows:

• Complementary metal-oxide semiconductor (CMOS) sensor technology is reported to enhance image quality

• The image is transmitted directly to the monitor without need for an intermediary step

• CMOS sensors provide portability

• There is no need for USB controllers, adapters, or docking stations

• Some CMOS sensors come with rounded corners to make placement easier

• The Size 1 Pedo sensors are fairly close in size to their legacy film counterparts

• The process of image taking, from insertion of the sensor to screen display, is fast

• Some wired systems now have detachable connecting wires to make replacement less costly

• There is industry competition for sensors

• There is seamless integration with associated software and some picture archiving and communications systems (PACS)

Disadvantages of direct intraoral sensors are as follows:

• High cost

• CMOS sensors are much less flexible than PSP plates

• CMOS sensors are larger than PSP plates or legacy film

• CMOS sensors with squared edges are not well tolerated by patients

• The line from the sensor to the computer can be damaged (wireless is now available)

• Wireless CMOS sensors may be subject to radiofrequency noise interference

• Replacement parts are expensive with this system

• There may be limited integrated with all PACS

• CMOS systems may require IT software updating and support

• The positioning of the sensor wire may interfere with ease of placement

• Sensor thickness (4.2 mm)

This x-ray provides a 2-dimensional image of the maxillary and mandibular arch, including the teeth, the maxillary sinus, and the anatomic structures immediately inferior to the mandibular. This type of x-ray can be used to identify impacted wisdom teeth, relative tooth position, and jaw bone pathology (including that of the TMJ) and to plan for implants. It is also used forensically to identify persons involved in fires, crashes, or other fatalities who may be otherwise unrecognizable.

Other types of digital imaging used in assessing pathology include multislice computed tomography (MCT), cephalometric assessment, sialography, digital photography, and digitally driven CAD/CAM scanning systems.

Digital imaging systems have been compared with conventional film processes in detecting proximal caries, with the results suggesting comparative diagnostic accuracy. In a study involving 150 proximal-surface lesions of varying depth radiographed using either Ektaspeed Plus film or Schick CMOS-APS sensors, ^[9] with histological evaluation used as verification of decay extension, no significant differences were found between the systems in defining lesion depth. The problem was that neither system performed adequately in detecting enamel lesions.

In another study assessing agreement among visual inspection, conventional radiography, and digital radiography for diagnosing interproximal and occlusal dental caries in the posterior teeth of 30 patients considered at low risk of decay, ^[10] subsequent kappa analysis suggested that imaging, regardless of the type used (analog or digital x-ray imaging), improved caries detection over visual examination. However, in a comparison between digital and conventional radiology, digital assessment resulted in 3.23 times more caries lesions identified versus 2.88 times more for conventional radiology. The authors conclude that the two radiographic techniques show high agreement for lesion detection.

Other comparative studies have demonstrated similar results. ^{[11, 12, 13]} However, studies assessing different digital machines for diagnostic accuracy in caries detection are somewhat limited. A glimpse of potential differences is found in the experimental research study of Ferreira et al. ^[14] The authors induced experimental caries and evaluated the results using 3 digital systems (CygnusRay MPS, DenOptix, DIGORA) and contrasted these with InSight film. They found that the DenOptix (phosphor plate) system and conventional radiographs were more accurate than the other radiographic systems tested. They also found that contrast-enhanced subtraction images were significantly more accurate than conventional, digital, or digitized radiographs for defining enamel demineralization.

Digital imaging has also been compared to conventional radiographs in defining experimentally induced root fractures, ^[15] with digital imaging found to be superior in terms of detection accuracy in molars. However, applying colorizing techniques or reverse-contrast to digital images in at least one experimental study did not appear to improve diagnostic accuracy in defining vertical root fractures. ^[16]

Digital panoramic images have been compared with legacy panoramic images for diagnostic accuracy in defining anatomic features such as the root morphology of impacted mandibular third molars. Both systems appear to be equally effective in defining relative tooth position, the number of roots, and the proximity of the roots to the mandibular canal. However, PSP-based radiography was found to be significantly more accurate in defining root morphology. ^[17]

Cone beam computed tomography (CBCT) allows 2- and 3-dimensional image processing of maxillary and mandibular bone. It can be used to identify tumors and fractures; to guide implant diagnosis, planning, and placement; ^[18] to evaluate periodontal structures and third molar position presurgery; to identify temporomandibular joint (TMJ) and fossa anatomy and disease; and to estimate bone density. ^[19]

Digital display via proprietary software associated with an acquisition machine or via an independent PACS typically allows manipulation and measurement of identified structures. The disadvantage of digital CBCT imaging is relative x-ray exposure, which is much greater than that from panoramic radiographic assessment. ^[20] However, it is reported to be much less than that which occurs with standard CT imaging. ^[21]

CBCT assessment is used in many dental disciplines, including oral surgery, periodontal therapy, orthodontics, radiology, and oral medicine and pathology.

Diagnostic validity

Diagnostic image quality can vary considerably between CBCT machines and depending on parameter settings. This represents a serious issue in need of standardization. Segmental and linear accuracies vary considerably between machines depending on how they are calibrated. ^[22] Based on limited studies, CBCT has been shown to be reasonably accurate for assessing volume related to intraosseous defect size, ^[23] tooth volumes, ^[24] and 3-dimensional reconstructions of the teeth. ^[25]

There also appears to be significant differences between CBCT machines in terms of 2-dimensional reconstruction (as a panoramic image) from 3-dimensional sectional imaging. In a comparison study of 2-dimensional digital panoramic images and panoramic images reconstructed from 3-dimensional imaging via CBCT, it was found that the type of machine used for imaging affected visualization of mandibular structure. Digital panoramic radiographs appeared to provide superior visualization of mandibular anatomy with the exception of the condyle region. The NewTom VGi and the 3-dimensional Accuitomo 170 provided images that were rated as fairly close to digital panoramic projections (odds ratio estimates of 1.2 and 1.6 at 95% Wald confidence limits). ^[26]

CBCT has been compared to multislice spiral CT (MSCT) for assessing jaw width, the lamina dura, and the periodontal ligament space in 25 dry human mandibles. The findings in one study suggest that MSCT offers a significant advantage over CBCT in terms of image quality of gingiva and cortical bone, while CBCT may offer better visualization of details associated with small bony structures. ^[27]

In terms of dental tooth impactions and anatomic aspects associated with impactions, a recent literature search suggests that, currently, there is only limited support for the diagnostic accuracy of CBCT for assessing impactions. ^[28]

Using Medline, Embase, and CENTRAL and a reference list of identified studies, the author’s search resulted in the selection of 96 titles, of which only 7 were included based on their inclusion criteria, which specified the expression of sensitivity, specificity, and predictive values in the report. The authors determined that only 2 studies comparing CBCT and panoramic radiographs used a valid reference method and presented the results appropriately. This is not necessarily an indictment of CBCT but suggests a need for additional studies validating its diagnostic accuracy in this regard.

Numerous studies have assessed potential landmark identification errors that could occur with CBCT-derived cephalograms used in orthodontic evaluation. Most have found that errors on CBCT-derived cephalograms are comparable to those seen on conventional digital cephalograms. ^[29] One study found that a program of professional calibration in the variability of landmark identification made a greater difference than the type of cephalogram used (conventional radiograph vs CBCT-derived cephalogram) in measurement validity. ^[30]

A CBCT acquisition modality, which is relatively inexpensive, allows purchase by outpatient clinics other than dental schools or medical centers. The radiation exposure using CBCT is much less than that associated with hospital CT scans. As a result, outpatient dental clinic use of 3-dimensional CBCT digital imaging is becoming much more common as a result of this technological breakthrough.

The digital image resulting from a CBCT study is able to be displayed with adequate resolution on most computer monitors in various angles that can be reconstructed in interrelational image slices based on the type of angulation selected (axial, sagittal, coronal). Reconstruction and monitor presentation is relatively fast depending on the speed of the server, and the scrolling function allows for movement through the selected planes.

Scrolling through multiple slices defined by restructuring, a clinician can observe anatomic irregularities that reflect possible pathology that, in the past, had not been as visible using any other 2-dimensional imaging solutions. ^[31]

Since the development CBCT, there has been considerable research evaluating applications in the realm of diagnostics, implant planning, surgical guidance, ^[32] and postimplant evaluation. The research suggests that CBCT, which allows for 3-dimensional presentation of anatomic structures, as opposed to the 2-dimensional imaging from standard digital radiography or legacy imaging, may improve the diagnoses of periodontal disease. ^[33]

The International Congress of Oral Implantologists in a Consensus Report on the use of Cone Beam Computed Tomography in Implant Dentistry has concluded that the literature supports CBCT for use in evaluating alveolar ridge topography and bone density, as well as for fabricating surgical guides for surgical navigation during implant preparation. ^[34] The authors of this report also suggest that the technology might be used when bone augmentation sites are questionable for standard implant placement and 2-dimensional imaging is not sufficient to define the anatomy of the considered site.

X-ray imaging via CBCT allows visualization of numerous conditions, including cysts, benign tumors, inflammatory lesions of the teeth and jaws, and bone diseases associated with the jaw bones. ^[35] Other findings associated with CBCT imaging include TMJ disease/abnormalities (eg, condylar flattening, condylar cysts, subcortical sclerosis, osteophytes, cortical irregularities), sinus disease (eg, retention cysts, sinus mucosal thickening, sinus polyps, sinus fluid, sinus opacification), ^[36] cleft palate assessment, ^[37] and airway evaluation.

Incidental findings identified on a CBCT that require additional consultation have been reported to range from 24.6% ^[38] to 65% ^[39] . In the orthodontic patient population, which is typically much younger than the general population, the percentage of individuals with incidental findings on CBCT examination has been reported to be 37%. ^[40]

While few studies have assessed the sensitivity and specificity of CBCT with respect to diagnostic capability in terms of oral pathology, in at least one published study, CBCT was found to be quite accurate based on radiology interpretation of gray value measurement of periapical lesions. In 13 of 17 periapical lesion cases in which the diagnosis could have been granuloma or cyst, the pathologist interpretation of the subsequent biopsy report coincided with the radiological interpretation of the lesion. In 4 of 17 cases, the pathology assessment was granuloma, while the CBCT was read as suggesting a cyst. ^[41]

The authors suggest that CBCT, given its accuracy, may be useful in providing a diagnosis when invasive surgery is contraindicated or when there is presurgical consideration of nonsurgical therapy prior to invasive intervention.

A study by Venskutonis et al compared the accuracy of intraoral digital periapical radiography and cone beam computed tomography in the detection of periapical radiolucencies in endodontically treated teeth. Although the study had limitations (only 20 patients met the inclusion criteria), the authors found that cone beam computed tomography scans were more accurate compared to digital periapical radiographs for detecting periapical radiolucencies in endodontically treated teeth. ^[42]

CBCT has also been found, in the detection of bone invasion of the jaw bones from oral malignancies, to be comparative in diagnostic performance with MSCT and single photon emission CT (SPECT). ^[43] As reported, the sensitivity, specificity, and positive predictive value for CBCT were 0.92, 0.965, 0.98, and 0.875, respectively, while those of MSCT were 0.8, 1.0, 1.0, and 0.75, respectively, and those for SPECT were 0.91, 0.4, 0.7, and 0.75, respectively. Receiver operating characteristic (ROC) analysis was also comparable.

In orthodontics, CBCT is increasingly being used in preorthodontic evaluation of maxillary and mandibular anatomy, ^[44] presurgical assessment, ^[45] and posttreatment evaluation ^[46] . It is also being used in various research protocols. ^[47]

While clinical use appears to be increasing, the scientific evidence supporting the use of CBCT in orthodontics is generally limited. In a 2012 literature review, van Vlijmen et al ^[48] assessed, via systematic review (with the lowest level of evidence accepted for inclusion being a case series with ≥5 participants), 550 articles with only 50 meeting their very lenient inclusion criteria. They report that the study topics incorporated into their review included such items as temporary anchorage devices, cephalometry, combined orthodontic and surgical treatment, airway measurements, root resorption and tooth impactions, and cleft lip and palate.

The score for methodological quality was quite poor, with an average of 53%. In essence, the authors did not find that there were high-quality studies validating the use of CBCT in orthodontics but suggest that it may still have utility in orthodontic treatment planning and outcome evaluation if the risk of radiation exposure does not outweigh the potential benefit of CBCT assessment. They conclude that only the airway studies with CBCT appear to provide added value to diagnostics.

Relevant studies suggest that CBCT imaging is useful in providing guidance for oral surgical procedures, ^[49] presurgical treatment planning for third molar extraction (eg, tooth anatomy, tooth position) ^[50] and surgical removal of mesiodentes or supernumerary teeth ^[51] , or correction of cystic lesions. It also appears to be useful in the presurgical assessment of osteomyelitis. ^[52]

One of the more important aspects of digitization of x-ray systems is that they can be integrated with PACS. PACS facilitate connectivity and the sharing of information between dental and medical providers and meet federal guidelines mandated to take effect in 2012 regarding the electronic communication of patient information. The format for PACS storage and transfer is Digital Imaging and Communications in Medicine (DICOM). A PACS allows for the integration of multiple digital x-ray imaging machines, as well as scanned documents and the interfacing with electronic dental records (EDR), providing workflow efficiency with the acquired digital information.

There are 4 components of a PACS, as follows:

• The imaging modality, such as a panoramic machine, CBCT, or CMOS or PSP digital x-ray machine for bitewing or periapical imaging

• A network allowing transmission of patient information

• Workstations (computer monitors) for viewing images/information

• An archive (a server or multiple servers) for storage and retrieval of images and reports

PACS can be combined with web technology, which allows for transfer of images for viewing over secure networks or via the Internet between diverse settings such as hospitals and clinics, dental and medical clinics, and different departments within the medical setting or dental school(s). More recently, PACS have been integrated with cloud technology to provide another means of digital storage.

Several areas of potential concern with respect to digital systems are continuing to be addressed and improved on by the industry. The first is resolution power. This is a function of pixel size. Theoretically, the smaller the pixel size on a CCD, the greater the image quality, since a greater number of pixels can be placed on the sensor. Increased pixel size means that a lesser number of pixels can be placed on the sensor, translating into greater signal noise and reduced image clarity with image display (via pixilation, as the image is enlarged). ^[53]

With improvement in technology, smaller and smaller pixels should become the norm for imaging CCDs, thus improving screen display and diagnostic accuracy over current standards. This also applies to display capabilities of computer monitors and other display devices included in phones and various flat-screen pads or monitors.

The digital specifications of the computer monitor associated with a digital system can also significantly affect the diagnostic quality of the displayed digital image. ^[54] Resolution is critical to diagnostic sensitivity and specificity in identifying possible disease. Other important monitor features include luminescence or brightness, spatial resolution, and contrast (tone scale) or dynamic range. Other factors, such as ambient light and eye position, also appear to be important in reading displayed images as suggested by evidence from medical studies assessing this potential confounder. ^[55]

The brightness of a display monitor is likely to be significantly less than that associated with a typical display box for reading standard film. In fact, it has been reported that the best monitors that are currently available may be only a fifth as bright as the typical view box. ^[56] Furthermore, monitors that display color tend to degrade the dynamic range of blacks and whites in contrast to black-and-white monitors. Most monitors used by dentists are in color, and, unfortunately, these lack the contrast and spacial resolution that is considered important in defining abnormality.

The typical desktop or laptop computer monitor used by dental clinicians has 1024 x 1280 pixels or 1200 x 1600 pixels. Whether this resolution is sufficient to identify pathology within the dental setting appears to be substantiated by the indirect studies assessing diagnostic comparisons with plain film imaging. However, there is little to no research looking specifically at these technical issues as they relate to, for example, CBCT imaging, in which soft tissue assessment may occur. Of course, the question is whether, in general dentistry, it is necessary to have the highest technological standards in computer monitor capacity to be able to identify the type of pathology that is typically encountered (eg, hard structure [bone and teeth] disease).

In one published study, ^[57] two factors, field of view and voxel size, affected diagnostic efficacy in terms of the detection of erosions of the TMJ.

In medicine, it is recommended that monitors with at least 2048 x 2560 pixels be used for diagnostic interpretation. However, it is not unusual to find medical facilities that are using commercial-grade monitors that are not calibrated or do not have the required contrast range to adequately diagnose disease. ^[58]

In medicine, there are two different guidelines related to monitor specifications, one published by the American College of Radiology (ACR) and a second from the American Association of Physicists in Medicine (AAPM). No specific guidelines have been published in dentistry by established organizations (eg, American Dental Association) or by any of the relevant disciplines within dentistry, and the general reviews that have been published do not consider monitor specification recommendations for digital display. ^{[59, 60]}

Furthermore, no published guidance related to flat-panel devices are currently available and that may be increasingly used in the future for dental image viewing. An overview of the problem related to display performance has been published by Butt et al. ^[61]

An additional concern regarding dental digital radiology is radiation exposure. For single bitewing, periapical, and panoramic imaging series, radiation exposure using digital systems is significantly less than corresponding analogue systems. For necessary CBCT studies, x-ray exposure is significantly less than corresponding CT imaging. ^[62] However, the general public has been made aware of the possible overuse of CBCT by dentists, which could constitute a problem in terms of overall head and face x-ray exposure. ^[63] The basis for the article is research that suggests considerable variation in x-ray exposure from the different systems on the market.

In a study by Pauwels et al, ^[64] 14 CBCT devices were assessed for amount of absorbed organ dose and effective dose using different exposure protocols and geometries. They found that the effective dose ranged between 19 and 368 μSv, representing a 20-fold range. Dose received was strongly related to the field size, which ranged from small, medium, and large field protocols. The measured values were found to depend on differences in exposure factors, diameter and height of the primary beam, and the positioning of the beam relative to the radiosensitive organs. The two organs of concern with respect to relative dose were the salivary glands and thyroid gland. The authors suggest that “the optimization of dose should be performed by an appropriate selection of exposure parameters and field size, depending on the diagnostic requirements.”

There has been movement in defining standards and regulations for CBCT use in dentistry, but the efforts are limited. The Ontario Health Technology Advisory Committee, based on reviewed evidence, issued recommendations regarding safety of CT, including CBCT use, related to x-ray exposure in 2006. ^[65] At the time of this report, there were no specific qualifications or even continuing education requirements for CBCT scanner operation in Canada. Dental assistants were performing the scans with only limited in-office training from the manufacturers and without formal certification. Unfortunately, the HARP Act, a prior Canadian regulation that covers the installation, use, and testing of standard dental x-ray machines, does not address the use of dental CBCT scanners.

The European Academy of Dental and Maxillofacial Radiology, constituting 339 members, laid out a set of 20 basic principles on the use of CBCT as comprehensive use standards. ^[66] The regulations suggest “adequate theoretical and practical training” for dentists and other allied staff using CBCT that has been validated by an academic institution or equivalent. The document also suggests that all new installations need to undergo examination and acceptance tests prior to use to ensure appropriate radiation protection for staff and patients.

The Health Protection Agency (HPA) of the United Kingdom has also published a set of standards for CBCT use, ^[67] and, in a press release dated November 23, 2010, the American Dental Association suggested that diagnostic radiation procedures must be used sparingly to reduce dental radiation risk. ^[68] . A joint task force of the American Academy of Orthodontics and the American Academy of Oral and Maxillofacial Radiologists is said to have a submission that is soon to be submitted to their organizations. ^[69] . Endodontists have also published standards for their members in the United States for appropriate CBCT use.