Worldwide medical and scientific journal for animal health professionals

Issue number 22.3 Dental

Veterinary dental radiology – an overview

Published 12/04/2021

Written by Michael Bailey

Also available in Français , Deutsch , Italiano and Español

Dental care is necessary to promote optimal health and quality of life, but the most visible part of the tooth, the crown, is only a small portion of the dental anatomy, with the majority of dental morphology and potential disease situated - and therefore hidden - subgingivally.

Veterinary dental radiology

Key points

Dental radiography is an essential tool for the diagnosis and treatment of both dental disease and whole body health.


Radiation doses are low in dental radiography but no exposure can be considered risk free; by following basic guidelines the risk can be minimized.


Digital dental radiography is now widely available and offers many advantages to the clinician.


Technical errors can occur at any stage in dental radiology and can be due to various factors; good technique should minimize errors.


Introduction

Dental care is necessary to promote optimal health and quality of life 1 2, but the most visible part of the tooth, the crown, is only a small portion of the dental anatomy, with the majority of dental morphology and potential disease situated - and therefore hidden - subgingivally.

Early detection of disease has been shown to simplify treatment, improve overall patient outcomes for common diseases 3 4 and prevent the need for more expensive, invasive care resulting from missed diagnoses or late-stage oral health issues and associated systemic disease 5 6. Dental radiography is therefore an essential tool for both the diagnosis and treatment of dental disease and to maintain whole body health. Radiology can also demonstrate disease to the client, encouraging an understanding for the need of an appropriate treatment plan. 

To be a valuable tool dental radiology depends on optimal image quality obtained by good technique, i.e. proper exposure and positioning. Understanding the geometric influences of the X-ray beam will ensure the best possible results, and following basic radiographic principles will reduce health risks as far as possible.

Radiation safety

Although radiation doses to the patient and workers are low in dental radiography 7 8, no exposure can be considered risk-free; the principle of As Low As Reasonably Achievable (ALARA) should always be followed to minimize unnecessary radiation exposure to personnel, patient, and the general public 9. The three guiding principles of ALARA, Distance, Shielding and Time, are easy to remember. Utilize distance wherever practical by maximizing the distance from any X-ray source; an operator must stand at least 6 feet (2 meters) from a useful beam that is angled away from personnel. The inverse square law applies; a person 6 feet from a primary beam will receive ~75% less radiation than someone standing 3 feet from the beam 10. The direct primary beam should never be directed towards an entrance or other non-protected areas and no-one should ever stand in the path of the beam. If the benefit of distance is not achievable then shielding, such as approved barriers or personal protective devices (e.g. aprons) should be employed. Time should always be a consideration; personnel should strive to minimize time spent near an X-ray source by using the shortest exposure possible, obtaining the fewest images needed for diagnosis, processing film using optimized time-temperature methods, using high speed X-ray film or digital radiology, and optimizing X-ray technique 9 10. It is generally agreed that settings greater than 60 kVp are the optimal operating potential for intra-oral imaging, maintaining image contrast while reducing radiation absorption by soft tissue and bone 9 10.
 
The type of image receptor used has a direct effect on the radiation exposure required. Film-based imaging still predominates in veterinary medicine and there are currently three speeds of intraoral films available for dental radiology, D, E and F speed. Many clinicians use the slower D speed films because of its perceived greater contrast resolution. The original E-speed films reduced the amount of radiation required by ~50%; however these produced a lower contrast image, were sensitive to age and depleted processing solutions, and lost their high speed advantage at higher densities 11. Subsequent E-speed emulsions have improved 11 12 and the newer F group films offer dose reductions of 20-25% compared with even E-speed film 12 13. Recent studies have demonstrated that there is no loss of diagnostic image quality with faster speed films, which can allow up to 80% decrease in exposure factors 12 13
 
The recent move to digital dental radiology has had a significant benefit of reducing radiation exposure by 50-80% while achieving an image comparable to dental film systems 14.

General X-ray generators

General radiographic systems can be used for dental radiology but are not very convenient (Figure 1). Using D-speed intraoral films with a standard X-ray generator, the operator should reduce the film-collimator distance to 12-16 inches (30-40 cm), collimate to the film size, employ the smallest focal spot (if available), and select 60-85 kVp at 100 mA and an exposure time of 1/10th sec (=10 mAs) dependent on patient size; the film should be exposed and processed using an approved method. As with standard radiographs, a technique chart should be developed to allow repeatability of first-time images. If the dental radiograph is underexposed but shows adequate penetration, double the mAs by doubling time. If the image is over-exposed, halve the mAs by halving the time. If penetration is inadequate, increase the kVp by 15% which will double the radiographic density; conversely, reducing the kVp by 15% reduces density. Remember contrast is inversely proportional to kVp, so a decreased kVp will give more contrast whilst reduced contrast is achieved with an increased kVp. Because of a resulting change in radiographic density a concurrent inverse doubling or halving of the mAs setting is required to maintain density.

 
Figure 1. A general X-ray generator can be used for dental work but manipulating the machine to achieve satisfactory image angles can be difficult. © Michael Bailey

Dental X-ray generators

Dedicated dental radiography units are relatively inexpensive, low maintenance and allow for accurate image positioning with minimum patient manipulation. They are compact, maneuverable, have user-friendly controls and limit the amount of radiation scatter. The kVp and mA are often preset, or the settings are limited to those appropriate for dental anatomy.

Until relatively recently, most dental X-ray generators were half-wave self rectified units and applied alternating current (AC) to the tube when generating X-rays. With an AC generator, voltage across the tube produces a sinusoidal power output, generating X-ray photons with a wide range of energies. Low energy (non-useful) photons are removed by filtration; the average, useful, photon energy released by an AC tube for a given kVp is only 33% of the peak photon energy selected. A consequence or benefit of this is that high contrast images are obtained.

New dental X-ray generators apply a near-constant electrical potential to the tube and are often referred to as direct current (DC), constant potential or digital generators. These produce a relatively constant stream of useful high energy photons; this higher energy output means that a DC generated image has inherently lower contrast compared to an AC generator but the actual exposure (photons arriving at the image receptor) will be higher and tissue absorption lower 15 16.

Although both AC and DC generators provide satisfactory exposures the latter are more consistent. All dental X-ray units, regardless of the generator type, use a Position Indicating Device (PID) (or cone) (Figure 2) attached to the front of the collimator. Typically the PID length will be 4, 6, 8, 12 or 16 inches. The short 4 inch cones require the least amount of radiation to be produced by the generator, and are therefore often found on low power units, but they result in more scatter radiation and hence less image contrast and more patient exposure, as well as loss of image detail. A longer cone gives improved image quality with better detail, superior contrast (due to reduced scatter) and lower patient exposure. A trade-off exists between the choice of PID and the required exposure factors; the inverse square law means that if the PID length is doubled (e.g. from 4 inches to 8 inches) only 25% of the generated photons arrive at the image receptor. To ensure the image density remains the same for both PIDs it is necessary to increase the radiation generated by a factor of 4 when doubling the PID distance and if the distance is tripled (from 4 to 12 inches PID) the radiation generated must be increased by a factor of 9 to maintain the same density. There is a significant diagnostic benefit to an increased PID length which results in enhanced image quality by decreasing edge distortion known as penumbra 15 16.

 
Figure 2. Dental generators with different PID lengths; the dental unit with a longer PID will produce a higher quality image but will require more power to generate X-ray photons. © Michael Bailey

Dental films come in five sizes (0, 1, 2, 3, and 4) with the most common sizes being 2 & 4. Size 4 is an occlusal film and as the largest size available can only be used in large breed dogs or for whole mouth or nasal radiographs in cats or small dogs (Figure 3a and b). For smaller dogs and cats a single root radiograph is most commonly obtained with a size 2 film. Dental film has a bubble on the upper left hand corner; the convex surface of the bubble should always be placed towards the X-ray beam source. Note that a dental film pack has multiple layers that include a white plastic outer layer, front and back paper layers, the film, and a silver lead foil layer; the foil can be an environmental contaminant, and for health reasons caution should be taken when handling it during radiograph processing 17.

Figure 3a. Size 4 dental films may be used for high detail nasal radiographs. © Michael Bailey

Figure 3b. Size 4 dental films may be used for high detail nasal radiographs. © Michael Bailey

Processing 

Film processing procedures can affect the quality of the radiographic image. Poor processing can severely compromise the diagnostic quality and may result in increased radiation exposure for both patient and personnel. Chair-side processing is an easy and inexpensive dip-tank method that provides excellent, rapid results as long as fresh chemicals and a time/temperature chart (instead of the unreliable “by-sight method”) are used. The time/temperature compensation chart is a quick and easy guide for users to adjust temperature-dependent processing times to ensure proper and consistent development and fixing. 

All solutions, including wash water, should be at the same temperature (within 5°C/10°F) to ensure proper processing. Films must be secured by holder clips to avoid fingerprints and to reduce chemical skin contact. 

The use of automatic processors allows greater film consistency and is time-efficient. Dental film is too small to pass through a standard large-format processor unless a dental film carrier/transport system is employed, the transporter doubling as a permanent film mount. Small-format, dental X-ray specific automatic processors are available but they can be expensive and require a large throughput of films to be cost effective. 

Note that if converting from D-speed to F-speed film the appropriate safelight filter is also required; F-speed films allow reduction of mAs (60% if using an automatic process or 50% if using manual tanks). 

Technical errors can occur at any stage in dental radiology. This can be due to film placement, patient positioning, angle of the X-ray beam, exposure, processing, storage or any combination of the above. Table 1 addresses the most common problems encountered. 

 
Table 1. Common errors in dental radiography.

Digital dental radiology

Digital dental radiography is now widely available and comes in two forms: direct and indirect. 

  • Direct radiology (DR) systems employ solid-state sensors 14 that detect radiation and deliver an almost immediate radiographic image to the attached computer. However DR sensors are currently limited in size, equivalent to film sizes 1 and 2.
  • Indirect systems, or computed radiology (CR), use photo-stimulable phosphor (PSP) plates that are exposed then digitally scanned by a laser processor and converted to an image on a computer; the image is then erased from the plate immediately after processing, leaving it ready for reuse. The advantage of this technology is that the size and thickness of the phosphor plates are almost identical to those of traditional film. However the intra-oral sensors may degrade if scratched, and the time needed to scan (and then erase) an exposed plate is longer than with a DR system.

Both forms give diagnostic results 14 but the DR system offers a limited size selection while CR systems, with their varying plate sizes, offer flexibility. Digital machines greatly reduce (by 50-80%) the exposure necessary compared to film systems, and the images can be electronically stored and manipulated as necessary for radiographic evaluation of dental disease (Figure 4).

 
Figure 4. Computer software offers the ability to manipulate digital radiographs, as can be seen here with differing contrasts of the same radiograph. © Michael Bailey

Conventional film displays 16 shades of gray, which is a narrow range for diagnostic imaging. Digital dental radiographs, by comparison, offer up to 65,536 shades of gray and a digital image may be enhanced, correcting various parameters to produce a more diagnostic image and better visualization of disease. Studies have shown that altering contrast and brightness have the greatest effect on diagnostic accuracy 18 and a single image can be enhanced to reveal features or details of diagnostic importance without additional exposures. The pros and cons are summarized in Table 2. 

 

Table 2. Digital dental radiography.

Advantages

  • Immediate image production with solid-state devices.
  • Improved contrast resolution.
  • Ability to computer-enhance features.
  • Image can be duplicated and distributed as necessary (e.g. the patient file, referring veterinarian or telemedicine consultant).
  • Security mechanisms allow identification of original images and differentiation from altered images.
  • Easy storage and retrieval of the image, including integration with practice management software systems.
  • 50-80% reduction in radiation needed to expose an image.
  • Elimination of hazardous processing chemicals.
  • Reduced anesthetic time.
  • Thin, flexible plates which provide easy placement in confined spaces (CR systems).
  • DICOM compatibility allows practitioners with different equipment and software to share, view and enhance the same images.
Disadvantages
  • Sensors are initially expensive (although over time they are less expensive than film-based radiology).
  • DR sensors are currently limited in size.
  • System requires a computer in the dental area.
  • Extra time may be needed for input of computer data.
  • Lack of DICOM compatibility can be a problem.


DICOM and telemedicine

Film images can be read anywhere - assuming an adequate light source – and therefore have a universal utility. Digital radiology has come of age but hardware and software compatibility issues exist between different manufacturers; inter-operability of images across all manufactures is essential, and Digital Image Communication in Medicine (DICOM) is an international open standard for medical images created to promote this concept 19; whilst this standard has been adopted for medical radiography, not all dental systems are as yet compatible. 

Telemedicine - delivering healthcare services via electronic means 20 - facilitates earlier and more accurate care not previously deliverable by accessing highly trained consultants at a distance, thus affording better diagnostic abilities. Digital imaging – assuming compatibility issues do not exist – makes the many benefits of telemedicine a reality for veterinary medicine, and also offers improved professional education and reduced costs with a more efficient and timely delivery of care 21.

Positioning of the dental radiograph image

There are two intra-oral radiograph techniques commonly utilized in veterinary dentistry. The simpler is the parallel technique; the oral anatomy means that its use is limited to the caudal mandible, but will visualize the molars and caudal premolars. The X-ray beam is set at an angle of 90º to the film, which is placed on the lingual surface of the teeth 22.

The alternative technique is the bisecting angle, which minimizes distortions of the teeth and is used for the rostral teeth, maxilla and mandible, and the caudal maxillary teeth. With this technique the beam is aimed at an imaginary line bisecting the plane of the tooth and the plane of the film 22.

A full radiographic survey will include 8 radiographs: 

  • occlusal view of the maxillary incisors.
  • lateral view of the maxillary canine teeth.
  • rostral maxilla-P1-P3-M2.
  • caudal maxilla-P4-M2.
  • occlusal view of the mandibular incisors and canine teeth.
  • lateral view of the mandibular canine teeth.
  • rostral mandible-P1-P4.
  • caudal mandibular-P4-M3.

All but the last employ a bisecting angle technique, which requires a parallel technique. The upper fourth premolar requires additional radiographs to permit adequate visualization of all three roots using the SLOB (Same Lingual Opposite Buccal) rule. The methodology of performing the above studies is covered in various publications (e.g. 22 23 24) to which the clinician is referred as necessary.

Dental radiography critique 

Various organizations, including the American Veterinary Dental College and the Academy of Veterinary Dentistry, have established guidelines which, if followed, will produce meaningful diagnostic films. These are as follows:
 
  • Exposure and developing technique are adequate.
  • Contrast and density of the radiograph are correct.
  • No artifacts appear on the film.
  • Radiographs are well positioned.
  • Proper angulation has been used: foreshortening or elongation should be avoided.
  • All teeth to be evaluated are clearly visible and complete; there should be adequate visualization of all roots and apices with at least 3 mm of periapical bone visible.
  • Maxillary cheek teeth and incisors should have the roots facing upward and the crowns downward.
  • Mandibular cheek teeth and incisors should have the crowns facing upward and the roots downward.
  • When viewing the right side of the mouth, the rostral teeth are on the right side.
  • When viewing the left side of the mouth, the rostral teeth are on the left side.

Conclusion

There is no doubt that dental radiology can be frustrating and is underutilized in veterinary medicine, yet good imaging is essential when investigating dental disease. Recent advances in dental films, better X-ray generator technology, and new digital dental radiology systems are all significant developments; with the correct equipment, and the ability to detect and eliminate common radiographic faults, the clinician should be able to obtain excellent images which will permit better diagnosis and treatment of patients.

 

 

 

References

  1. Harvey CE. Periodontal disease in dogs. Etiopathogenesis, prevalence, and significance. Vet Clin North Am Small Animal Pract 1998;28(5):1111-1128. 
  2. Lund EM, Armstrong PJ, Kirk CA, et al. Health status and population characteristics of dogs and cats examined at private veterinary practices in the United States. J Am Vet Med Assoc 1999;214(9):1336-1341.
  3. Lommer MJ, Vertraete FJ. Prevalence of odontoclastic resorption lesions and periapical radiographic lucencies in cats: 265 cases (1995-1998). J Am Vet Med Assoc 2000;217(12):1866-1869. 
  4. DuPont GA. Radiographic evaluation and treatment of feline dental resorptive lesions. Vet Clin North Am Small Anim Pract 2005;943-962. 
  5. DeBowes LJ, Mosier D, Logan E, et al. Association of periodental disease and histologic lesions in multiple organs from 45 dogs. J Vet Dent 1996;13(2):57- 60. 
  6. Glickman LT, Glickman NW, Moore GE, et al. Evaluation of the risk of endocarditis and other cardiovascular events on the basis of the severity of periodontal disease in dogs. J Am Vet Med Assoc 2009;234(4):486-494. 
  7. Freeman JP, Brand JW. Radiation doses of commonly used dental radiographic surveys. Oral Surg Oral Med Oral Pathol Oral Radio Endo 1994;77(3):285-9. 
  8. Gibbs SJ, Pujol A Jr, Chen TS, et al. Patient risk from intra-oral dental radiography. Dentomaxillofac Radiol 1988;17(1):15-23. 
  9. National Council for Radiation Protection & Measurements. Radiation protection in dentistry. Bethesda, Md.: National Council for Radiation Protection & Measurements; 2003. 
  10. European Commission, Radiation Protection 136, European guidelines on radiation protection in dental radiology. The safe use of radiographs in dental practice. Directorate General for Energy and Transport, Directorate H- Unit H.4-radiation Protection 2004. 
  11. Horton PS, Francis H, Sippy FJ, et al. A clinical comparison of speed group D and E dental X-ray films. Original Research Article. Oral Surg Oral Med Oral Pathol Oral Radio Endo 1984;58(1):104-108. 
  12. White SC, Yoon DC. Comparison of sensitometric and diagnostic performance of two films. Comp Cont Ed Dentistry 2000;21:530-2,534,536 passim. 
  13. FDA, The Nationwide Evaluation of X-ray Trends (NEXT), Dental radiography: doses and film speed, 2009. 
  14. Mupparapu M. Digital dental radiography - a review of the solid-state and semi-direct digital detector. Orofac J Sci 2011;3(1):40. 
  15. USAF. Dental evaluation & consultation services. Synopsis of Intra-Oral X-ray Units (Project 05-02) (4/05). 
  16. Bellows J. Dental radiography. In: Bellows J, ed. Small animal dental equipment, materials and techniques, a primer. Oxford: Blackwell Publishing, 2004;63-103. 
  17. Suji LJS, Wainman BC, Ruwan K, et al. Foil backing used in intraoral radiographic dental film: a source of environmental lead. J Can Dent Assoc 2005;71(1):35-8. 
  18. Gormez O, Yilmaz HH. Image post-processing in dental practice. Eur J Dent 2009;3(4):343-347. 
  19. Dean Bidgood W, Horii SC, Prior FW, et al. Understanding and using DICOM, the data interchange standard for biomedical imaging. J Am Med Inform Assoc 1997;4(3):199-212. 
  20. American Dental Association Standards Committee on Dental Informatics. Technical report no. 1023-2005: Implementation requirements for DICOM in dentistry. Chicago: American Dental Association; 2005. 
  21. Hjelm NM. Benefits and drawbacks of telemedicine. J Telemed Telecare 2005;11(2):60-70. 
  22. Mulligan TW, Williams CA, Aller MS. Atlas of Canine & Feline Dental Radiography. Trenton: Veterinary Learning Systems, 1998;27-44. 
  23. Brook NA. How to obtain the best dental radiographs. Vet Med Supp Oct 1, 2007. 
  24. Holmstrom SE, Frost-Fitch P, Eisner ER. Veterinary Dental Techniques for the Small Animal Practitioner. 3rd ed. Philadelphia: WB Saunders, 2004;131-174.

Michael Bailey

Michael Bailey

Michael Bailey, Banfield Pet Hospital, Portland, Oregon, USA Read more

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