Veterinary dental radiology – an overview
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.
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.
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.
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.
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.
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.
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.
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).
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.
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
- 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.
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.
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