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Despite significant advances in new diagnostic techniques that use no ionizing radiation, intraoral radiographic technique in dentistry is still the most commonly used method for detecting pathology associated with dental disease. The use of conventional dental x-ray film is continuing to decline worldwide, and the number of dental offices using digital radiography is increasing. Digital intraoral techniques in dentistry have now been available for more than 25 years, and the choice of two different types of system configurations still exists today.
Digital Intraoral Radiographic System Configurations
The solid-state CCD (charge-couple device) or CMOS (complementary metal oxide semiconductor) sensor and the photo-stimulable storage phosphor (PSP) plate are still the two basic configurations that continue to be widely used for dental x-ray examinations. Dental teams must decide upon the system configuration that best fits their practice or clinic setting; the decision may be based upon sensor parameters, image capture and display times, spatial resolution, and practice workflow, to name just a few of the considerations.
The advantages of digital radiographic over conventional film-based techniques have been well documented.1,2 Patient radiation dose reduction; immediate or faster image production; interactive image display; image security; and image storage, retrieval, and archiving are all noted advantages for digital over film radiography. It has been suggested that integration of digital radiography into the dental practice can take four different routes: completely film-based, hybrid of film and digital, completely digital without integration into an electronic health record (EHR), or completely digital with integration into the EHR.1 Once the level of integration is determined, the dental team must next decide on the system configuration or combination of systems that best meets the needs of the office or clinic.
CCD and CMOS systems provide a direct connection to the computer via a cord that allows for image viewing within seconds. However, the presence of the cord and the thickness of sensor/cord interface may complicate positioning of the sensor and could prohibit optimal sensor placement.3 Digital sensors are rigid, their corners cannot be bent, and placement may create some patient discomfort. In one study, all digital sensors were uncomfortable for patients during imaging as compared to dental x-ray film.4 In a similar study of bitewing examinations, only 76% of the patients reported that the CCD sensor felt more uncomfortable than film.5 Therefore, it may be possible that patient comfort during digital radiographic examinations is likely more dependent on operator technique than the physical contours of the sensor itself.
A second choice for sensor and system configuration is the photostimulable phosphor plate (PSP). PSP system sensors are thinner and more flexible than the hardwired CCD or CMOS sensors, and in some cases, may be more comfortable for the patient when positioned in the mouth. However, plates require an extra processing step to acquire the image, which takes a few extra seconds; but these systems have the advantage of potentially needing only one scanner to service multiple operatories. PSP systems also have a wider exposure latitude or “dynamic range” than do the CCD or CMOS sensors, which means that over- or under-exposing images is less likely with PSP systems, thus often requiring fewer retakes. PSP plates are significantly less expensive than the solid-state sensors (CCD or CMOS), but plates can be damaged with repeated use and must be discarded when scratching or surface degradation leads to image disfiguration. At least one study determined that in a moderately sized dental school clinic, PSP plates needed to be replaced after approximately 50 uses due to scratches and surface irregularities.6
Choice of sensor and/or system configuration for a dental practice or clinic depends on many factors, but some key elements include: a determination of workflow needs, budget, and preferences. Every practice or clinic has a different radiographic workflow, a different mixture of experienced dentists and staff, and varying budgetary needs, as well as a preference for a particular system or systems. Each practice or clinic must weigh all of the aspects associated with radiographic services, decide which ones are critical for success, and make a selection of a system or systems that will satisfy the need.
Caries Detection with Digital Systems
Spatial resolution—ie, the ability of the eye to distinguish fine detail in an image—is a factor often discussed when considering the image quality of dental x-ray film versus any digital system. Resolution is recorded and presented in line pairs per millimeter (lp/mm). Film provides more than 20 lp/mm, but digital systems average anywhere from 7 lp/mm up to a theoretical average of 25 lp/mm, depending on the size of pixels in the image. However, the ultimate limiting factor for any imaging modality—film or digital—is the human eye. The eye without the aid of magnification is restricted to a resolution that is equivalent to approximately 6 lp/mm. Numerous studies have been done to validate the presence or absence of a carious lesion using many different digital systems, and the majority of digital receptors demonstrated diagnostic accuracy equivalent to film, with only a few exceptions.7
The most challenging carious lesion to accurately diagnose is the carious lesion of the outer approximal enamel or the incipient proximal carious lesion. Numerous studies have examined the efficacy of digital radiography for the detection of proximal caries in enamel. Among recent articles comparing x-ray film and digital techniques for the detection of proximal carious lesions of enamel, none found any significant differences in the ability of the observer to detect approximal carious lesions.8-11 These findings agree with many previous studies of artificial and natural carious lesions of enamel that found no significant differences between film and digital systems.
System and software enhancements have been promoted by digital systems manufacturers as being necessary features of digital systems to provide the dental office with the tools to adequately diagnose pathoses, including caries detection. Wenzel7 reported that image enhancement functions do not seem to influence diagnostic accuracy, and features such as inversion, filters, and pseudocolors may be more or less superfluous for the detection of carious lesions. One study compared both standard and high-resolution CCD and PSP images that were both magnified and unmagnified and found that there was no significant difference in their detection of proximal caries.12 Based upon many studies comparing film to various digital systems, as well as studies comparing different digital sensors and digital system features, it can be concluded that, regardless of the radiographic imaging modality used, the dental practitioner should find no difference in his or her ability to detect the proximal carious lesion.
Finally, the use of digital technique does hold a potential promise of providing a “leg up” over x-ray film for the detection of caries. While x-ray film provides only an unchangeable static image, a digital image provides the potential for further analysis. Commercially available computer-assisted diagnostic software programs are available to aid the dental practitioner in the diagnosis of carious lesions. One study evaluated the efficacy of a computer-assisted program to aid in the diagnosis of caries using as observers seven inexperienced dentists, who evaluated 50 extracted teeth.13 Considering all of the teeth observed—each with a varying lesion size—there was no significant difference when using the computer-assisted software compared to observations without the software; however, when looking strictly at lesions confined to the inner half of enamel or into dentin, detection ability was significantly increased.13 Rather than further refinements to existing CCD-, CMOS-, or PSP-based systems, the potential for the development of computer-assisted diagnostic aides for the dental practitioner is likely.
Factors Associated with Image Display
In addition to questions regarding the efficacy of digital systems to acquire the image in a manner suitable for adequate diagnosis of pathology, the manner in which the image is displayed and the conditions under which images are viewed have also been questioned. Medical imaging has long been a proponent of high-end high-resolution monitors, especially for viewing images where soft-tissue display is critical, such as mammography. Medical imaging has established standards for display function to ensure the consistent presentation of the image with the optimum contrast and to match this display as closely as possible to the contrast sensitivity of the human eye.14 The author suspected that monitor quality might be an issue in dentistry as well, so a study was designed to examine observer performance in the recognition of incipient artificial lesions of enamel when viewed on standard computer monitors versus high-resolution medical-grade monitors.15 Theoretically, monitors with smaller pixel sizes, greater spatial resolution, and better bit depth, dot pitch, and luminance should have a positive influence on observers’ ability to detect subtle differences in lesion outline. However, this study demonstrated that trained and calibrated observers could recognize lesions of enamel regardless of the quality of the monitor used to display the image.15 Ludlow et al16 concluded that, even with the inclusion of a laptop display, there were no significant differences in monitor performance. A recent review article on computer display performance in dentistry suggests a need for further development of guidelines and standards for display of dental images, but it also indicated that there has been no conclusive proof that there are any advantages of medical-grade displays over standard displays in dentistry.17
Conditions in and around the dental operatory or clinic setting have also been considered important factors in viewing radiographic images, and radiology textbooks continue to recommend that ambient light in the viewing room be reduced.18 While some studies have reported that lighting conditions impact adequate detection of proximal incipient carious lesions,19 others have not determined that lighting conditions have a significant impact on lesion recognition.20 A 2006 study using both dental students and faculty in a dental school setting found that lighting conditions did not make a difference for student observers, but that ambient lighting did improve lesion recognition for faculty.21 While these studies suggest that it is optimal to use a room with reduced ambient lighting when performing a radiographic interpretation, in reality, dental practice workflow often precludes this. The inconclusive results of these studies would seem to indicate that when the dental practitioner is unable to use ambient lighting to view digital images, it will likely not grossly hamper his or her diagnostic abilities when performing radiographic exams in fully lighted clinic settings such as those found in most dental operatories.
Image Storage, Retrieval, and Archiving
Management of radiographs once they are captured is critical to the long-term usefulness of the images for the dental practitioner. In terms of storage, each intraoral digital image consumes between 100 and 250 kilobytes of space on a computer hard drive. For a dental practice, this amounts to several gigabytes per year of needed space.22 Although the cost per megabyte of storage space has fallen, there is still a great demand for space due to the need to store complex radiographic images, high-resolution photographs, and other large documents.22 One solution is compression of the image. File sizes using compression algorithms with ratios as high as 1:16 can still maintain diagnostic accuracy for detection of proximal caries lesions, but reduces file sizes by 94%, as reported in a study by Pabla.22 A subsequent study using the same JPEG compression format found no significant differences in proximal caries recognition at a 1:12 ratio, which still provided a significant savings in storage space.23
A major advantage of digital radiographic technique in dentistry is the ability to review images that are many years old as a means of comparison with radiographs taken for recall appointments. Also important is the ability to review images that may have been taken in a different proprietary format for other practitioners. DICOM (digital imaging and communication in medicine) provides a specific standard for format, exchanging images, and associated information.24 DICOM is universally accepted in medicine, and widespread acceptance in dentistry will permit the reading of images from different digital imaging systems using the same computer.24 DICOM standards for intraoral digital image capture has become widely accepted, and models have been proposed for archiving analog film radiographs into a searchable digital database so that patient radiographic information can be easily shared with all treatment providers.25 The universal adoption of DICOM formatting in dentistry will be beneficial to patient treatment in that once images are captured, it will be possible to view them using any digital imaging system, thus streamlining the sharing of patient health information among practitioners.
Future of Intraoral Digital Imaging in Dentistry
There is currently little debate that intraoral digital imaging in dentistry is no longer an experimental technique. Direct digital imaging is becoming the universal and accepted standard for capturing dentistry’s mainstream diagnostic images—the periapical and the bitewing. The resolution of current sensors is diagnostically accurate, and the choice of system configurations rests on a determination of workflow needs and preferences. Detractors have suggested that conventional x-ray film outperforms digital radiographic technique in the detection of caries, but numerous studies have demonstrated that the capabilities of digital are equivalent to radiographic film. Additionally, many have questioned the need for high-end high-resolution monitors such as those used in medicine to accurately diagnose dental pathoses, when standard computer monitors have performed as well as high-end monitors for the detection of pathology. Also, standardization of digital file formats, storage, archiving, and sharing of images has allowed patient radiographs to be easily managed. Hardware, software, and many other aspects associated with digital imaging have been improved, and the costs have become reasonable.
Intraoral digital radiography will undoubtedly continue to be a viable technique for the practicing dentist. Of course, dental practitioner will continue to rely on periapical and bitewing images for routine diagnosis of dental disease for the foreseeable future. However, technologies will likely continue to evolve, whether they use non-ionizing radiation or perhaps complex imaging systems such as cone beam computed tomography (CBCT), in addition to producing panoramic, cephalometric, and tomographic views of the jaws. They may be able to produce periapical and bitewing images that are equivalent to current direct intraoral image capture.
Conclusion
Intraoral digital imaging has made a significant impact on the practice of dentistry. These techniques have lowered patient radiation dose, greatly increased the speed of image capture, eliminated the need for darkroom and processing chemicals, enhanced the practitioner’s ability to demonstrate pathology to the patient, and provided a means to make patient records accessible, portable, and secure. Dental digital intraoral radiography is a reliable, versatile, and cost-effective diagnostic aide for the entire dental team.
ABOUT THE AUTHOR
Robert A. Cederberg, MA, DDS
Associate Dean for Patient Care and Professor, Department of General Practice and Dental Public Health, University of Texas Health Science Center at Houston School of Dentistry, Houston, Texas
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