Image-characteristics Self Reading

Image Characteristics Image characteristics include density, contrast, speed, and latitude.

Film Density Film density represents the degree of darkening of an exposed x-ray film. White areas (e.g., metallic restorations) have no density and black areas (air spaces) have maximum density. The areas in between these two extremes (tooth structure, bone) are represented by various shades of gray.

Film Density Radiolucent: refers to high film density, which appears in a range from dark gray to black. Soft tissue, air spaces, and pulp tissue, all of which have low object density, appear as radiolucent areas on a film (see next slide). Radiopaque: refers to area with low film density, which appear in a range from light gray to white on the film. (The “white” areas of the film are actually clear, but appear white when the light from a viewbox passes through the film). Structures with high object density, such as enamel, bone and metallic restorations will appear radiopaque (see next slide).

Radiolucent

Radiopaque

Soft tissue

Cement base

Air space

Enamel

Pulp tissue

Amalgam

Mental foramen

Bone

The overall density of the film affects the diagnostic value of the film. Only the center film below has the proper density. The one on the left is too light (low density) and the film on the right is too dark (high density); both of these films are non-diagnostic.

Film Density influenced by: Patient size: the larger the patient’s head, the more x-rays that are needed to produce an ideal film density Exposure factors (mA, kVp, exposure time). Some patients require a change in exposure factors (increase for large adult, decrease for child) to maintain proper film density. An unnecessary increase in any of these factors results in an increase in film density.

Film Density influenced by: Object density: determined by type of material (metal, tooth structure, composite, etc.) and by amount of material. Metallic restorations have higher object density than tooth structure. Film density decreases (film gets lighter) when object density increases, assuming no changes are made in the exposure factors. In the film at right, the post and core in each tooth has a high object density, resulting in low film density.

Film Density influenced by: Film fog: This is an increased film density resulting from causes other than exposure to the primary x-ray beam. This includes scatter radiation, improper safelighting, improper film storage, and using expired film. All of these things will cause extra silver halide crystals on the film to be converted to black metallic silver, resulting in an overall increase in the film density and making the film less diagnostic.

fog

Contrast Contrast refers to the difference in film densities between various regions on a radiograph. Structures with different object densities produce images with different film densities.

High Contrast High contrast implies that there is a pronounced change from the light to the dark areas of the film. There are fewer shades of gray, the predominant densities being either very light or very dark. High contrast is also known as short scale contrast. Theoretically, high contrast is best for caries detection, the radiolucent carious lesion showing up distinctly against the surrounding radiopaque enamel.

Low Contrast With low contrast, there are many shades of gray seen on the film, with less pronounced changes from light to dark. This is also known as long scale contrast. Low contrast is best for periapical or periodontal evaluation. Slight changes caused by bone loss will be more evident, showing up as a darker gray than the surrounding area.

Contrast influenced by: Subject Contrast: In order to see an image on the film, the objects being radiographed must have different object densities. If everything had the same object density, the film would be blank. In the film at right, the teeth, restorations, bone, air spaces, etc., all have different object densities, allowing us to see them on the film.

Contrast influenced by: kVp: kVp controls the energy (penetrating ability) of the xrays. The higher the kVp, the more easily the x-rays pass through objects in their path, resulting in many shades of gray (low contrast). At lower kVp settings, it is harder for xrays to pass through objects with higher object densities, resulting in a higher contrast (short scale).

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kVp settings

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Contrast influenced by:

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Density

Film contrast: this is incorporated into the film by the manufacturer. In general, high film contrast (green curve below) requires very precise exposure of the film; if it is too high or too low, the film will be too dark or too light, resulting in a non-diagnostic film. With low film contrast (purple curve) the film will be diagnostic over a broader range of film exposure.

Exposure of film

Contrast influenced by: Film fog: as discussed under density, film fog makes the whole film darker. This makes it harder to see the density differences (contrast), making the film less diagnostic.

fog Fogged film

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Latitude The latitude of a film represents the range of exposures that will produce diagnostically acceptable densities on a film. A wide latitude film will more readily image both hard and soft tissues on a film. As the latitude of a film increases, the contrast of the film decreases.

Density

High Contrast

Wide Latitude Log Relative Exposure

Speed The speed of a film represents the amount of radiation required to produce a radiograph of acceptable density. The higher the speed, the less radiation needed to properly expose the film. Higher speed films have larger silver halide crystals; the larger crystals cover more area and are more likely to interact with the x-rays. F-speed film (Insight) has the highest speed of intraoral films. An F-speed film requires 60% less radiation than a D-speed film.