Dental X-ray Film and Image Characteristics

• Dental x-ray film and image characteristics are examined including film composition, film speed, latent image formation, visual, and geometric characteristics.

Film Composition

• Dental radiography film is photographic film adapted for dental use.
• When exposed to x-rays passing through teeth and adjacent structures, a photographic image is produced.
• X-ray film in dentistry has four basic components: film base, adhesive layer, film emulsion, and protective layer.

Film Base

• A 0.2 mm-thick piece of polyester plastic provides strength and stable support for the emulsion.
• It is constructed to withstand heat, moisture, and chemical exposure.
• The film base forms the actual radiograph.

Adhesive Layer

• A thin adhesive layer covers both sides of the film base.
• This layer attaches the emulsion to the base.

Film Emulsion

• The film emulsion consists of a homogenous coating with gelatin and silver halide crystals attached to both sides of the film base by the adhesive layer.
• Gelatin is used to suspend and disperse millions of microscopic silver halide crystals over the film base.
• During film processing, the gelatin absorbs processing solutions and allows chemicals to react with the silver halide crystals.
• Halide crystals are chemical compounds sensitive to radiation or light.
• The silver halide crystals absorb radiation during exposure and store energy from the radiation.

Protective Layer

• A thin coating over the emulsion to protect it from damage during handling and processing.

Latent Image Formation

• Silver halide crystals absorb x-rays during exposure and store energy.
• Silver halide crystals will contain various levels of stored energy, depending on the density of the objects radiated.
• Crystals behind dense amalgam are not energized because the amalgam absorbs x-rays.
• Silver halide crystals corresponding to air receive more radiation and are highly energized.
• Stored energy within the silver halide crystals forms a pattern known as the latent image.

Latent Image Visibility

• The latent image is invisible until chemical processing produces a visible image.
• Processing transforms the latent image with dark areas visible on the radiograph.
• Dark areas on the radiograph are areas exposed to radiation.
• White areas are unexposed, such as areas blocked by amalgam; silver crystals forming these areas are not ionized.

Film Speed

• Film speed refers to the amount of radiation required to produce a radiograph of standard density.
• Film speed is determined by silver halide crystal size, emulsion thickness, and the presence of special radiosensitive dyes.
• Fast film requires less radiation exposure.
• Film responds more quickly and the silver halide crystals in the emulsion are larger.
• Current speeds available are D and F; F speed film requires about half the exposure time as D speed film (60%).

Dental X-Ray Image Characteristics

• Dental x-ray image characteristics include both visual and geometric attributes.
• Quality is determined by proper density and contrast, with sharpness, minimal magnification, and distortion.

Radiolucent

• A dental radiograph appears as a black and white image or picture with varying shades of gray.
• A radiolucent structure lacks density and permits x-ray beam passage with little resistance, appearing dark or black on the processed film.

Radiopaque

• A radiopaque structure absorbs or resists passage of the x-ray beam, appearing white on the processed film.

Diagnostic Radiograph Image

• In a diagnostic radiograph, images that have proper density and contrast, sharp outlines, and are the same shape and size as the object radiographed provide a great deal of information.

Visual Characteristics of Radiographs

• Visual characteristics influence diagnostic quality: density and contrast.

Density

• Overall darkness or blackness of a dental radiograph.
• The film appears too light, and images cannot be visually separated when density is too weak.
• Relative transparency depends on the distribution of black silver particles in the emulsion.
• Correct density is achieved when black, white, and gray are present.
• Factors that influence density include milliamperage, operating kilovoltage, exposure time and subject/client thickness.

Contrast

• Difference in degrees or shades of blackness between adjacent areas.
• A film that represents a compromise between low and high contrast is preferred.
• High contrast has very dark and very white areas with striking differences between light and dark.
• Low contrast does not have very dark and very light areas and has a lot of gray.
• Increasing kilovoltage impacts contrast by increasing the energy of the x-rays, making them more penetrating and resulting in more variations in tissue density recorded as shades of gray.
• Short-scale contrast (high contrast) with only black and white densities occurs with machines functioning at a low kVp.
• Long-scale contrast (low contrast) with many shades of gray occurs with machines functioning at a high kVp.
• The overall contrast is determined by film contrast, inherent qualities of film and film processing and subject contrast, the characteristics of the subject that influence radiographic contrast.
• An aluminum stepwedge can demonstrate short-scale and long-scale contrast.
• The stepwedge consists of uniform-layered thicknesses of x-ray absorbing material placed on top of a film and exposed to x-rays leading to differences in film density.
• An aluminum stepwedge is used for quality control tests.

Geometric Characteristics of Radiographs

• Sharpness, magnification, and distortion influence the diagnostic quality of a radiograph.

Sharpness

• The capability of the x-ray film to reproduce the distinct outlines of an object.
• The smallest details of an object are well-reproduced on the radiograph.
• A certain lack of image sharpness is present in every radiograph called the penumbra
• Penambra is the "blurring" of edges; a fuzzy or unclear area surrounding an image.
• A lack of sharpness can change a diagnosis dramatically and cause a problem to be undetected at an early stage.
• Three factors: focal spot size, film composition and movement, influence sharpness.

Focal Spot Size

• At the tungsten target (anode).
• The focal spot is determined by the x-ray equipment manufacturer.
• The smaller the focal spot, the sharper the image appears.
• Penumbra is the shadow/area on x-ray
• Always present, the smaller focal point makes for a smaller penumbra and sharper the image.

Film compostion

• Faster film contains larger crystals that produce less image sharpness.
• Slower film contains smaller crystals that produce more image sharpness.

Movement

• Movement causes a loss of image sharpness.
• Caused by movement of film or client, even slight amount.

Magnification

• Refers to a radiographic image that appears larger than the actual size of the object.
• Magnification results from the beam radiating from the focal spot along divergent paths
• Because of divergence, magnification is always present, but can be limited.
• Influencing factors: target-film distance, also called source-film distance and object-film distance.
• A longer PID and target-film distance results in more parallel rays from the middle of the x-ray beam striking the object rather than diverging rays, results in less images.
• The closer the film is to the tooth(object), the less magnification
• An increase in object-film distance results in an increase in image magnification
• Film as close as possible to tooth.

Distortion

• A variation in the true size and shape of object being radiographed.
• Results from unequal magnification of different parts of the same object becase of improper film alignment or angulation of the x-ray beam.
• Influencing factors: the object-film alignment- The object and film must be parallel to each other or there will be distortion.
• Angular relationships produce variations of distances between the tooth and the distorted film image.
• Influencing factors: distortion occurs as a result of the x-ray beam angulation
• The x-ray beam must be directed perpendicular to the tooth and film to record the adjacent structures in their true shape.