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

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 1/10^th 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.

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. 

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.