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FOUNDATIONS Digital Dental Radiography Brook A. Niemiec, DVM Digital dental radiography has recently become more economically feasible for the general practitioner. As with all progressive technology, costs for these systems have decreased. Positioning techniques for intraoral digital dental radiography are very similar to those used for standard dental radiographs (see previous Foundations features in issues 21-3 and 21-4) with a few minor variations. This article will review the physics of digital radiography, advantages and disadvantages of digital compared with analog film, and the minor technique adjustments required for its use. The Physics and Technology of Digital Radiographic Systems Indirect digital radiographs are standard film “analog” radiographs that have been digitized by either scanning with a flatbed or slide scanner, or by taking a picture with a high quality digital camera (Fig. 1).1 This process requires that an analog film be exposed first, however it is an inexpensive way to experience several of the advantages of digital radiography (such as archiving and telemedicine) that are listed below. These images will not match the quality obtained with either straight digital or analog films,2 and should not be relied on for critical analysis of fine defects (e.g., endodontic fills). However, indirect digital images are likely sufficient for the diagnosis of many dental abnormalities.

Figure 1 Digital photograph of an analog radiograph. The image is not the quality of a true digital image, but is permanent and can be used for telemedicine.

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The two major types of true digital systems include semidirect and direct digital systems. These systems do not require analog (film) radiographs to create the image. Semi-direct systems employ a photo-stimulative phosphor (PSP) plate covered with phosphor crystals which temporarily stores the x-ray photon energy for future scanning (Fig. 2). The storage time can range from minutes to hours depending on the method of storage. Exposure of these plates to direct bright light prior to scanning will destroy the image. Following exposure, the film is removed from the patient’s mouth and scanned with a near-red wavelength laser beam. The scanner then transfers the information to the computer for processing. It is important to note that when an image is scanned, some (but not all) of the energy in the plate is lost. This knowledge is valuable because an image that is overexposed can be made diagnostic by repeated scannings, thereby limiting retakes. Once the image is scanned and stored, it is “erased” from the PSP plate by exposure to a bright visible light. Direct digital systems employ solid state sensors (Fig. 3). The two major types of solid state sensors are CCD (charge-coupled device) and CMOS (complementary metal oxide semiconductor). CCD systems take the energy from x-ray photons hitting the sensor and convert it into electronic signals. To improve efficiency, a scintillation layer is placed on top of the CCD sensor in order to turn x-ray photons into light photons which are subsequently absorbed by the microprocessing chip. This is similar to an intensifying screen used with analog films. The

Figure 2 Photograph showing typical photo-stimulative phosphor (PSP) plates. Following exposure, the plate will be scanned and converted to a digital image.

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sensor then transfers the information to a computer via a USB cable or Bluetooth technology. CMOS systems are essentially the same, but have more of the electronic components integrated into the chip, resulting in a thicker sensor plate. CMOS systems were initially less expensive, but advances in CCD technology have resulted in decreased cost with the vast majority of direct digital sensors currently utilizing CCD technology. Regardless of the digital system (PSP or solid state sensor), the images are created in a similar fashion. Both plates measure the intensity of the photons following their passage through the oral tissues. The intensity is electronically measured on a 256 unit gray scale. Using this scale, zero corresponds to the maximum measurable radiation seen as “black”, and 255 corresponds to no exposure at all or seen as “white”. This measurement is performed over an array of very small regions of the sensor called pixels. Each pixel is 20 to 30-mm2 in size, corresponding to thousands of pixels on a size 2 sensor. The intensity measurement for each pixel is then transferred to the computer. The computer then assigns a gray-scale value to each pixel and places each pixel in the correct position. The computer creates the image a pixel at a time (Fig. 4). An interesting point is that some systems utilize a more detailed gray scale (up to a 64,000 unit gray scale). However, standard monitors are only capable of reading the 256 scale. Computers have the ability to manipulate this numerical information in order to change the appearance of the image. This manipulation is performed by subjecting the information to a mathematical procedure called an algorithm. Algorithms can

Figure 3 Photograph showing a typical solid state sensor. The sensor is connected to a computer using a USB cable for almost instant image availability.

Figure 4 Schematic showing how a digital image is created by a computer using radiology software beginning with the x-ray shadow (A); image detected by the digital sensor where each square equals a pixel (B); numerical representation of the pixels sent to the computer (C); and the digital image transferred to the computer screen (D).1

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Figure 5

Figure 6

Photograph showing a postoperative radiograph of root canal therapy in a dog. Since the tube head and sensor are still in place, minor positioning adjustments can be made to create a diagnostic (or superior) image without beginning the process again.

Dental radiographs showing the value of optimizing a digital image showing the raw image after initial filtering (A) and the same image following radiographic optimization (B). Note the increased clarity of the retained tooth root (arrow) of the maxillary first molar tooth.

range from simple to complicated. An example of a simple algorithm would be reversing the gray scale (white becomes black) to create a “negative”. However, more complicated algorithms are available which can improve contours or contrast, and can even adjust an image that is too light or dark. These changes can be done manually, however some systems have the ability to “optimize” the image automatically. These algorithms are responsible for many of the differences in the appearance of images between manufacturers. In many cases, the raw image is very similar but the algorithm gives it a slightly different appearance. Systems are often purchased due to the perceived “prettiness” of the image, regardless of the lack of any diagnostic difference. Although there are no published studies that demonstrate a significant difference between the digital images produced by the various sensors available there is variation between systems.3 The prospective purchaser should evaluate all available systems before making their decision. Numerous studies have compared solid state sensor to PSP produced images. The majority of these studies report that while solid state sensor systems have a higher resolution, the overall image quality is equal. These same studies report, however, that PSP systems will produce quality images over a wider exposure range.4,5 In contrast, several other studies found that solid sensor plates had superior resolution to PSP plates when properly exposed.4,6 Finally, one study reported that PSP plates had superior image quality compared with solid state sensor images.4 Advantages of Digital Dental Radiography The major advantage of digital dental radiography is speed. With solid state sensor technology, the image is available in seconds. Additionally, since the image is produced while the sensor and tube-head are still in position, adjustments to the angle can be made without starting over (Fig. 5). Digital images are of great value to the clinician. The large size of the image projected on the computer screen coupled with 194

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the ability to manipulate the image allows for easier reading of the films (Fig. 6). Additionally, a large computer screen image allows much easier discussions with clients, and the ability to “mark” the images further facilitates client communication (Fig 7). Digital images are permanent and do not degrade over time. Furthermore, with no development process, the use of toxic chemicals is avoided, as well as the logistical problems associated with chemical disposal. Digital images require less storage space compared with standard film and are generally easier for staff to locate. Digital images can be quickly uploaded to telemedicine sites or e-mailed for consultation with specialists. Solid state sensors have an additional advantage of creating far less x-ray exposure compared with standard film, which, is positive for both patients and staff. The scanning of the PSP plates takes approximately the same

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Figure 7 Dental radiograph showing utilization of the marking tool in the digital system. Pointing to the size of the periapical lucencies of the left maxillary fourth premolar tooth abscess allows the clinician to explain the pathology to the client more easily.

amount of time as development of standard film (minus the additional archival time), and the film must be removed from the patient’s mouth. This process tends to negate much of the speed advantage of digital dental radiography. However, when compared with solid state sensor and standard film, the PSP plates have an increased latitude of exposure limiting the need for retakes. Disadvantages of Digital Dental Radiography The main criticism of digital dental radiographs has always been that they have less detail than standard radiographic film. This is a controversial issue and depends on several factors. When image quality in general is studied, digital radiographs are often rated inferior to images produced by standard film,7,8 yet some studies report that digital images are superior to standard film.9 When particular pathology or procedural aspects are studied, digital images can be superior to plain film. For the general practitioner, accurate periodontal assessment is likely the most important parameter when assessing dental radiographs. Studies have shown variable results in regard to periodontal image detail

with digital dental radiographs considered superior,10 inferior,11 or comparable to analog dental films.4,12 For endodontic disease, which may be the most important parameter for dental specialists when assessing dental radiographs, there are also conflicting reports depending on what aspect of the procedure is being studied. For assessment of working length accuracy, some studies show that digital images are inferior when compared with standard film, especially when fine files are used in the canal.13,14 Other studies report that digital images are comparable to standard films for working length determination.15,16 Finally, some report digital images are superior to conventional films.17 The one aspect that appears consistent in different studies is that enhanced digital images are superior to plain digital films for endodontic assessment.15,18 When endodontic obturation is studied, enhanced digital images are at least as good, and in some studies are superior to standard film.19,20,21 This correlates well with the author’s experience. When rechecking previous root canals (not obturated with resorbable material), minor voids which were not visible on standard film are easily visible on the enhanced digital image. Finally, digital J VET DENT Vol. 24 No. 3 September 2007

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images appear to be equal to standard film when evaluating periapical lesions.22 Caries evaluation is also controversial, with one study reporting that the digital system was inferior to film23, two which reported that enhanced digital systems are superior,24,25 and others that were equivicable.24 Again, it appears that the superiority of the digital image lies in the enhancement of the image rather than in the raw pixel information. In summary, improvements in computer monitors as well as image manipulation (enhancement) have improved the quality of digital dental images to the point where they are at least as good as standard film, and in some instances better.1 The one point of agreement in almost every study was that enhanced digital images were superior to the raw images.15,18, 24 The biggest drawback for veterinarians using solid state sensor systems is the lack of a size-4 sensor plate. This is mostly a problem in large breed dogs since some teeth (canine and carnassial) cannot be completely imaged on one film. In addition, exposing full mouth radiographs in dogs (especially larger breeds) will require substantially more exposures. Another concern when using solid state sensor systems is the cost of sensor replacement if it gets damaged. Warrantees will cover many types of damage, but drops and bites are excluded. Therefore, these sensors must be handled carefully. Finally, solid state sensors have a very narrow exposure range to create an acceptable image, consequently more retakes may be required. The PSP plate system negates these problems. It has a size4 plate and the individual plates are less that $100 (smaller sizes are significantly less). Therefore, replacement of damaged plates is not a major expense. However, a concern with PSP plate systems is that their optimum radiographic exposure is higher than ektaspeed film.1 In addition, the wide latitude of dose levels allow a diagnostic radiograph to be created with exceedingly high exposure. Practitioners must be mindful to use the lowest possible setting. An overall disadvantage when converting to digital dental radiography is the initial set-up cost. However, there are cost savings in x-ray film and development. The speed of diagnostic image acquisition and greater compliance in regard to equipment use by the professional staff compensates for these costs. Differences Between Standard and Digital Radiographic Techniques The most obvious difference in technique between direct digital and standard dental radiography is the decrease in radiation exposure necessary to create the image. This requires that the x-ray generator be “current”, as older models may not have a digital setting. Furthermore, digital dental radiography utilizes only 4 to 5 different settings between cats and the largest dog. PSP plates have a similar exposure range as analog film. The lack of a true embossed dot makes determining right and left more difficult. However, there is a “box” on one corner of the digital image that serves the same function. Additionally, the majority of digital systems default to labial mounting. Finally, most systems have a template where the film can be placed, thus permanently identifying the film. In order to image the maxillary fourth premolar in large 196

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breed dogs using a distal tube shift technique, the solid state sensor must be moved mesially on the tooth. All three roots will be imaged in their entirety only when the film appears too mesial to capture the distal root. Finally, in many cases, the entire tooth cannot be imaged on one film image, necessitating two film images if an image including the crown is required.

Conclusion Digital dental radiography is the future of veterinary dentistry. It is more efficient, requires less radiation, utilizes no toxic chemicals, allows for easy telemedicine consults, costs less than standard radiographs over time, and is permanent. In addition, numerous studies looking at many parameters have shown that digital images are at least equal to, if not superior to standard film. Finally, the positioning technique differences between analog and digital systems are minimal, allowing for an easy transition.

Author Information From Southern California Veterinary Dental Specialties, 5610 Kearny Mesa Road, Suite B1, San Diego, CA 92111. Email: [email protected]

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10. Li G, Engstrom PG, Nasstrom K, et al. Marginal bone levels measured in film and digital radiographs corrected for attenuation and visual response: an in vivo study. Dentomaxillofac Radiol 2007;36:7-11. 11. Khocht A, Janal M, Harasty L, Chang KM. Comparison of direct digital and conventional intraoral radiographs in detecting alveolar bone loss. J Am Dent Assoc 2003; 134: 1468-1475. 12. Kappler G, Vogel A, Axmann-Krcmar D. Intra-oral storage phosphor and conventional radiography in the assessment of alveolar bone structures. Dentomaxillofac Radio 2000; 29:362-367. 13. Freidlander LT, Love RM, Chandler MP. A comparison of phosphor-plate digital images with conventional radiographs for the perceived clarity of fine endodontic files and periapical lesions. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2002; 93:321-327. 14. Hedrick RT, Dove SB, Peters DD, McDavid WD. Radiographic determination of canal length direct digital radiography versus conventional radiography. J Endod 1994; 20:320-326

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15. Woolheiser GA, Brand GW, Hoen MM. Accuracy of film-based, digital, and enhanced digital images for endodontic length determination. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005; 99:499-504. 16. 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; 93:88-91. 17. Cederberg RA, Tidwell E, Frederiksen NL, Bensen BW. Endodontic working length assessment. Comparison of storage phosphor digital imaging and radiographic film. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1998;85:325-328. 18. Kal BI, Baksi BG, Dundar 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;103:280-4. 19. Kositbowornchai S, Hanwachirapong D, Somsopon R, Pirmsinthavee S, Sooksuntisakoonchai N. Ex vivo comparison of digital images with conventional radiographs for detection of simulated voids in root canal filling material. Int Endod J 2006; 39:287-292. 20. Sogur E, Baksi BG, Grondahl HG. Imaging of root canal fillings: a comparison of subjective image quality between limited cone-beam CT, storage phosphor and film radiography. Int Endod J 2007; 40:179-185. 21. Akdeniz BG, Sogur E. An ex vivo comparison of conventional and digital radiography for perceived image quality of root fillings. Int Endod J 2005; 38:397-401. 22. Gundappa M, Ng SY, Whaites EJ. Comparison of ultrasound, digital and conventional radiography in differentiating periapical lesions. Dentomaxillofac Radio 2006; 35:326-333. 23. Versteeg KH, Sanderlink GC, Velders XL, van Ginkel FC, vander Stelt PF. In vivo study of approximal caries depth on storage phosphor plate images compared with dental x-ray film. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997;84:210-213. 24. Svanaes DB, Moystad A, Larheim TA. Approximal caries depth assessment with storage phosphor versus film radiography. Evaluation of the caries-specific Oslo enhancement procedure. Caries Res 2000; 34:448-53. 25. Moystad A, Svanaes DB, Risnes S, Larheim TA, Grondahl HG. Detection of approximal caries with a storage phosphor system. A comparison of enhanced digital images with dental X-ray film. Dentomaxillofac Radiol 1996; 25:202-206.

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