Digital radiography has

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1 Better imaging The advantages of digital radiography Paul F. van der Stelt, DDS, PhD Digital radiography has been available in dentistry for more than 25 years, but digital imaging has not replaced conventional film-based radiography completely. Studies investigating the number of dentists using digital radiographic systems report percentages ranging from 11 percent to 30 percent; for instance, the percentage of dentists using digital radiographic systems was 11 to 14 percent in a 2001 study conducted in Norway, 1 12 percent in a 2002 study in the Netherlands 2 and 30 percent in a 2007 study in Indiana. 3 There may be several reasons for this relatively low rate of use of digital radiography. The main reason given by general practitioners is the financial investment required to replace conventional radiography with digital imaging. 2,4 However, when a dentist is starting a new practice, there is not much difference between conventional and digital radiography in implementation costs. Maintenance costs of chargecoupled device (CCD) or complementary metal-oxide semiconductor (CMOS) based systems could be even lower than those related to film-based imaging. Practitioners should remember that conventional radiography also involves costs for items such as film mounts, pro- ABSTRACT Background. Digital radiography has been available in dentistry for more than 25 years, but it has not replaced conventional film-based radiography completely. This could be because of the costs involved in replacing conventional radiographic equipment with a digital imaging system, or because implementing new technology in the dental practice requires a bit of courage. When the practitioner is fully aware of the new possibilities offered by digital radiography, he or she can make a more informed decision about adopting it. This article offers information about digital radiography, not just as a replacement of conventional radiography, but also as a concept offering benefits beyond those of conventional radiography. Overview. Digital radiographs are composed of a set of numbers arranged as a grid of rows and columns. The dentist can perform mathematical operations on these numbers to create a new image in which certain characteristics are enhanced, thus making interpretation of the image easier. The dentist also can correct, to some extent, overexposed or underexposed images and can optimize contrast and brightness for specific diagnostic procedures, such as caries detection and bone level assessment. More advanced procedures are available as well, such as digital subtraction radiography and computer-aided recognition of image features. Conclusions and Clinical Implications. The author presents a selection of the advantages of digital radiography that are not achievable with conventional film-based radiography. Implementing digital radiography in the dental office requires additional training. However, once members of the dental team have gone through this initial phase, they have the benefits of several new diagnostic possibilities. With a digital system, information from radiographic images is collected more easily and in a more objective way, which will improve the performance of the diagnostic process. Key Words. Dental radiography; digital imaging; image processing; image analysis; diagnosis. JADA 2008;139(6 supplement):7s-13s. Dr. van der Stelt is a professor of oral and maxillofacial radiology, Department of Oral and Maxillofacial Radiology, Academic Center for Dentistry (ACTA), Louwesweg 1, 1066EA Amsterdam, Netherlands, p.vdstelt@acta.nl. Address reprint requests to Dr. van der Stelt. JADA, Vol June S

2 offered by digital imaging. Admittedly, clinicians will not use several of these advanced features frequently in daily practice. However, it is helpful to know that the features are available in cases in which conventional diagnostic procedures are not sufficient. Knowledge of digital imaging technology will enhance the diagnostic tools available to general practitioners, as well as dental specialists, thus improving the contribution of radiography to patients dental welfare. THE BASICS OF DIGITAL IMAGING Figure 1. A digital image. A. X-ray shadow the x-ray beam after it has passed through the patient. B. Image superimposed on the grid of pixels. C. Numerical representation of the pixel values corresponding with x-ray intensities. D. Digital image on the computer screen. Each pixel of the sensor corresponds with a pixel on the computer screen. cessing solutions and time needed for cleaning the film processor, as well as the costs of the films themselves. Another explanation could be that introducing new technology in the dental practice requires a bit of courage. After all, if your practice runs smoothly, why change things? Some dentists have mentioned the complexity of the software and the hardware as the reason for not converting to a digital system. 2 The dentist and his or her team will require additional training to understand the new equipment and new routines. 5 It is not always clear beforehand how the new approach will affect existing logistics in the practice. These reasons could make the practitioner hesitate to change current procedures. It also is possible, however, that the practitioner is not fully aware of the new possibilities offered by digital radiography. This supplement to The Journal of the American Dental Association provides information about digital radiography not just as a replacement of conventional radiography, but also as a concept offering benefits beyond those of film-based imaging. This article provides some background information about the characteristics of digital images. Understanding these characteristics can help practitioners appreciate the advanced features Although a digital image seen on the screen as a collection of brighter and darker areas very much resembles the traditional film-based image, the nature of a digital image is completely different. A traditional radiographic image is composed of radiolucent (dark) areas in which the silver grains in the emulsion are densely packed and radiopaque areas in which the grains are more dispersed, having been washed out during the film processing. A digital image, on the other hand, is composed of a set of cells that are ordered in rows and columns. The rows and columns form a table. Each cell is characterized by three numbers: the x-coordinate, the y- coordinate and the gray value. The gray value is a number that corresponds with the x-ray intensity at that location during the exposure of the sensor. Individual cells are called picture elements, which has been shortened to pixels. The numbers describing each pixel are stored in an image file in the computer (Figure 1). This is an essential difference between analog and digital radiographs. Traditional images cannot be changed once they are processed. The exposure conditions and the developing procedure determine the final result; brightness and contrast are fixed. Digital images, however, can be altered after they have been produced. The user can apply mathematical operations to alter the pixel values, which can change certain characteristics of the image. This is called image processing. Dedicated software is available to enable the practitioner to perform such processes. I described image processing in more detail in a previous JADA article. 6 An example of useful ABBREVIATION KEY. AI: Artificial intelligence. CBCT: Cone beam computed tomography. CCD: Charge-coupled device. CMOS: Complementary metal-oxide semiconductor. DSR: Digital subtraction radiography. 8S JADA, Vol June 2008

3 image processing is the optimization of contrast and brightness of an image. This can be used to correct overexposure or underexposure of an image, although of course it is no excuse to pay less attention to the correct exposure settings. Nevertheless, it can help to rescue an image in which exposure conditions were not optimal and thus prevent the need for a remake, saving the patient from an extra dose of radiation. It also is possible to adjust the contrast and density of an otherwise correctly exposed image to optimize the recognition of caries. If the clinician increases the contrast within certain limits, of course he or she will find it easier to recognize initial caries lesions. Similarly, decreasing the contrast to a certain extent will improve the assessment of periodontal bone lesions. Changing contrast and density can be a somewhat arbitrary process, but a clinician with some experience can extract much more information from a digital radiograph than he or she could from an analog radiograph. To avoid the subjectivity of selecting image density or contrast by means of slider bars, some clinical imaging software offers the use of standard gamma optimization procedures. This is done by distributing the gray values of the pixels more evenly over the full scale of gray values (Figure 2). It is reasonable to assume that in the near future, software for digital radiography will include tools to optimize contrast and brightness automatically for specific diagnostic tasks. Thus, the practitioner could use a single image, and thus a single exposure, to assess more than one diagnostic issue. Other image-processing tools that are available in most clinical software are inversion of the gray scale of the image (the result of which is called negative image ) and edge enhancement. Edge enhancement converts contrast gradients into a texture that is visible as a shape. The human eye recognizes shapes better than it does small contrast gradients; this ability allows the practitioner to detect, for instance, the point of an endodontic file in an image more easily. 7 Another simple but effective tool is the ability to zoom in on an image. By using a twofold or threefold magnification, the user can recognize details more easily. To perform this action, the computer duplicates or interpolates rows and columns of the digital image, thus increasing the size of the image on the screen. The resolution of the current sensor systems is A Figure 2. Automated optimization of the pixel gray values is achieved by distributing them over the full scale of gray levels. A. Underexposed image: the histogram of gray values is moved to the right (light) side of the histogram. B. Image after correction of the gray value distribution, in which the full scale of gray levels is used. high, although the added diagnostic value of these high-resolution sensors still is questionable. 8 In other words, the size of each pixel is rather small, and the sensor system uses a large number of pixels horizontally and vertically to represent the image on the screen. The computer screen itself has a fixed number of pixels (for example, a standard monitor setting is pixels). Most intraoral sensors, and certainly the extraoral sensors manufactured to make panoramic images and skull radiographs, produce images that contain more pixels. Part of the image, therefore, would fall outside the border of the computer screen, but most software reduces the resolution of the image so that the image fits within the dimensions of the screen. The zoom option is helpful in this situation because of its ability to show part of the image in its original resolution. Because of the high resolution of the original image, which makes no interpolation of image pixels necessary, the zoom action prevents the appearance of artifacts in the image, called pixelation (Figure 3). This is an advantage of the high-resolution sensor systems that are available, although the size of these image files consequently is much larger than it was in the past. It is obvious that larger files require more storage space and increase transmission time of the image over a network or the Internet. In one of the articles in this supplement, Farman and colleagues 9 provide more information about image transmission. B JADA, Vol June S

4 A IMAGE ANALYSIS A logical continuation of image processing is computer-aided image analysis. It is difficult to define the difference between image processing and B Figure 3. The effect of zooming in on a digital image. A. The original image extends partly outside the screen because of the large number of pixels vertically and horizontally. B. Resolution is reduced to fit the image within the size of the screen. C. Magnification is increased beyond the original pixel resolution, resulting in pixelation. Figure 4. Calibrated length measurement in a digital image. The endodontic file length is shown in the dialog window; the correct length of the root is shown at the bottom of the dialog window. C image analysis. When the user adjusts the whole image to make it more suitable for diagnostic purposes, the term image processing usually is applied. When the user performs certain calculations extracting specific information from the image, it is considered image analysis. An important aspect of computer-aided image analysis is the fact that the extraction of information is done not only quickly but also more objectively, not influenced by any potential bias on the user s part. An example of image analysis is the measurement of a distance in a digital image. To measure distances on conventional radiographs, a simple ruler suffices. This will not work for a digital image on the computer screen, because the user does not know the physical dimensions of the image on the screen beforehand. However, when the user draws a line with the cursor in the digital image, it is easy for the software to determine the number of pixels that form the line. This can even be a curved line, something that is not easily achievable on a traditional radiograph. When the software used to measure the length recognizes which sensor was used to create the image, the software uses the correct pixel size from an internal table of sensor characteristics. This allows for measurements expressed directly as a distance in millimeters. This measurement outcome may deviate from the real distance because of the projection error of the radiographic image. If the user did not direct the x-ray beam perpendicular to the object plane, the sensor plane or both, structures in the image may appear elongated or shortened. More exact length measurements, therefore, require calibration of the measuring tool. This is a useful approach when determining the length of a root canal during endodontic treatment. 10 The clinician inserts an endodontic file of known length in 10S JADA, Vol June 2008

5 A B C Figure 5. Example of ideal digital subtraction radiography. A. Initial radiograph. B. Second radiograph obtained a few weeks later. The arrow is pointing at a lesion. C. The lesion itself, made more noticeable by the removal of anatomical noise from the image. the root canal and produces a radiograph. When the user clicks the cursor on the rubber stopper, the end of the file and the end of the root, the software calculates the length of the root in millimeters automatically, so that the user can decide the appropriate working length for the continuation of the root canal procedure (Figure 4). Advanced image analysis. More advanced image analysis tools are available with different types of imaging software. Several studies have been published that describe the diagnostic importance of digital subtraction radiography (DSR) This procedure allows practitioners to distinguish small differences between subsequent radiographs that otherwise would have remained unnoticed because of overprojection of anatomical structures or differences in density that are too small to be recognized by the human eye. Mathematically, subtraction radiography is quite simple. DSR software subtracts corresponding pixels of two images obtained within an interval of a few weeks or a few months, and it uses the outcome to calculate a new image. When the gray levels of the corresponding pixels are the same, the output pixel is zero. As an example, when the second image shows periodontal bone loss, the subtraction outcome in this area is different from zero, because the gray values of the corresponding pixels are different in that area. This is visible in the subtraction image as a darker area when there is bone loss (or, similarly, as a brighter area when there is bone repair). The radiographic pattern of the trabecular bone makes the recognition of these small changes difficult; for that reason, it often is called anatomical noise. The DSR procedure removes the anatomical noise so that the small differences stand out against the background (Figure 5). To perform DSR reliably, the two images being compared have to be identical with respect to gray values and projection geometry. In film radiography, this was achieved by means of a film holder with an individual bite block. The patient had to bite on the film holder to be connected reproducibly to the x-ray device and the film or sensor. Today, software tools are available to do the image matching, making DSR a procedure that easily can be carried out in general practice. The image matching makes the gray value distribution of the second image identical to that of the first image, and it re-projects the second image according to the projection direction of the first image. It even is possible to combine images made with sensors manufactured by different companies. 11 Applications of DSR in general practice include the diagnosis and follow-up of periodontal bone resorption, 12 assessment of bone levels around implants 13 and the progression of healing of periapical lesions. 14 FUTURE DEVELOPMENTS New software tools are under development by researchers and manufacturers to improve the diagnostic outcome of digital radiography. Threedimensional visualization of dental structures, as done with cone beam computed tomography (CBCT), is expanding rapidly, and it would not have been possible without the availability of digital images. CBCT, however, is somewhat outside the scope of this article and is mentioned here briefly only as another form of digital radiography. An area that is continuously under development is the application of artificial intelligence (AI) to the interpretation of digital radiographs. Ten or 15 years ago, AI was expected to solve all JADA, Vol June S

6 intraoral dental radiographs could be used to identify patients with an increased risk of experiencing osteoporosis. 15,16 More research is needed to prove the feasibility of this concept under clinical conditions. If the outcome confirms the preliminary results, then this approach will be an inexpensive diagnostic tool that requires no additional radiation dose to the patient beyond that of the initial radiographs made for dental purposes and that can identify patients at risk who need further medical examination. Figure 6. Quantitative assessment of bone density around implants (used with permission of Oral Diagnostic Systems, Amsterdam, Netherlands). The bone density immediately adjacent to the implant is compared with bone at a short distance from the implant, which is outside the region affected by the implant procedure. The red areas represent less dense bone. diagnostic problems. The conventional wisdom was that computers would be able to collect information independently and solve diagnostic problems without intervention by the clinician. Today, this optimism is reduced, and the goal is more modest. Today s systems leave the initiative with the clinician and support him or her only in performing the diagnostic task. An example of this approach is the quantitative analysis of osseointegration for the assessment of the success of implant procedures (Figure 6). Other studies suggest that specific pattern recognition techniques applied to the radiographic trabecular pattern on DOSE REDUCTION Practitioners and manufacturers frequently use the reduction of the radiation dose that the patient receives as a reason to implement digital radiography. There are several reasons why the dose reduction is not as large as often suggested. 17 The most important reasons are dose per exposure, an increase in the number of radiographs made and an increase in the number of remakes made. ddose per exposure: Sometimes manufacturers claim a dose reduction of 90 percent compared with film for digital sensors. The reality, however, is that the reduction compared with the current standard of F-speed film is somewhere between 0 percent and 50 percent. Storage phosphor plate systems, with their wide exposure latitude, carry the risk of using even more radiation, as the image quality will not alert the practitioner to possible overexposure in the same way in which a film image would. dincrease in the number of radiographs made: Dentists indicate that the decision to make a radiograph is reached more easily with a digital system. In a Dutch study, 55 percent of clinicians using storage phosphor plates and 65 percent of clinicians using solid-state systems reported making more radiographs than they had with film. 18 dincrease in the number and ease of remakes: It takes some time for the practitioner to get used to the positioning of the sensor inside the patient s mouth. It is obvious that this will result in a more frequent need to remake an image. After some time, however, when the dentist and other members of the team are trained in making digital radiographs, they still may have a tendency to decide sooner to repeat an exposure than they would have with analog radiographs. This was reported in one study by 60 percent of clinicians using phosphor plates and even by 80 percent of 12S JADA, Vol June 2008

7 those using solid-state systems. 18 This probably is because it does not require much time to make another radiograph and, subsequently, the threshold for remakes is lower. In my 2005 JADA article, 6 I published more information about digital radiology and radiation dose to the patient. The International Commission on Radiological Protection also has expressed concern about the claims of dose reduction of digital radiography that are made by manufacturers of digital systems. 19 Users of digital systems should be aware of this concern and try not to increase the number of radiographs when they switch from analog to digital radiography. CONCLUSION Some of the advantages of digital radiography are not achievable with conventional film-based radiography. Implementing digital radiography into the dental office requires additional training. Once the dental team has gone through the initial training phase, however, they and their patients can benefit from several new diagnostic possibilities. Disclosure. Dr. van der Stelt is involved in the development of Emago software (Oral Diagnostic Systems, Amsterdam, Netherlands) for clinical digital radiography. 1. Wenzel A, Møystad A. Experience of Norwegian general dental practitioners with solid state and storage phosphor detectors. Dentomaxillofac Radiol 2001;30(4): Berkhout WE, Sanderink GC, van der Stelt PF. A comparison of digital and film radiography in Dutch dental practices assessed by questionnaire. Dentomaxillofac Radiol 2002;31(2): Brian JN, Williamson GF. Digital radiography in dentistry: a survey of Indiana dentists. Dentomaxillofac Radiol 2007;36(1): Flores-Mir C, Palmer NG, Northcott HC, Khurshed F, Major PW. Perceptions and attitudes of Canadian dentists toward digital and electronic technologies. J Can Dent Assoc 2006;72(3): Hellén-Halme K, Nilsson M, Petersson A. Digital radiography in general dental practice: a field study. Dentomaxillofac Radiol 2007;36(5): van der Stelt PF. Filmless imaging: the uses of digital radiography in dental practice. JADA 2005;136(10): Kal BI, Baksi BG, Dündar N, Sen BH. Effect of various digital processing algorithms on the measurement accuracy of endodontic file length.oral Surg Oral Med Oral Pathol Oral Radiol Endod 2007;103(2): Berkhout WE, Verheij JG, Syriopoulos K, Li G, Sanderink GC, van der Stelt PF. Detection of proximal caries with high-resolution and standard resolution digital radiographic systems [published correction appears in Dentomaxillofac Radiol 2007;36(6): ]. Dentomaxillofac Radiol 2007;36(4): Farman AG, Levato CM, Gane D, Scarfe WC. In practice: how going digital will affect the dental office. JADA 2008;139(6 suppl): 14S-19S. 10. Mentes A, Gencoglu N. Canal length evaluation of curved canals by direct digital or conventional radiography. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2002;93(1): van der Stelt PF, Geraets WG. Matching pairs of radiographs from different sensor systems for subtraction radiography. Paper presented at: 81st General Session of the International Association for Dental Research; June 28, 2003; Göteborg, Sweden. 12. Gröndahl K, Gröndahl H-G, Webber RL. Digital subtraction radiography for diagnosis of periodontal bone lesions with simulated highspeed systems. Oral Surg Oral Med Oral Pathol 1983;55(3): Bittar-Cortez JA, Passeri LA, Bóscolo FN, Haiter-Neto F. Comparison of hard tissue density changes around implants assessed in digitized conventional radiographs and subtraction images. Clin Oral Implants Res 2006;17(5): Nicopoulou-Karayianni K, Bragger U, Patrikiou A, Stassinakis A, Lang NP. Image processing for enhanced observer agreement in the evaluation of periapical bone changes. Int Endod J 2002;35(7): Geraets WG, Verheij JG, van der Stelt PF, et al. Prediction of bone mineral density with dental radiographs. Bone 2007;40(5): Geraets WG, Verheij JG, van der Stelt PF, et al. Osteoporosis and the general dental practitioner: reliability of some digital dental radiological measures. Community Dent Oral Epidemiol 2007;35: Berkhout WE, Beuger DA, Sanderink GC, van der Stelt PF. The dynamic range of digital radiography systems: dose reduction or risk of overexposure? Dentomaxillofac Radiol 2004;33(1): Berkhout WE, Sanderink GC, van der Stelt PF. Does digital radiography increase the number of intraoral radiographs? A questionnaire study of Dutch dental practices. Dentomaxillofac Radiol 2003;32(2): Valentin J. Managing patient dose in digital radiology. Oxford, England: Published for the International Commission on Radiological Protection by Elsevier; JADA, Vol June S