Table of Contents
Note: This page, in addition to the following three, relate nearly exclusively to the handling, processing and technology of radiographic film. As the popularity of digital sensors grows, more and more dentists are abandoning film for computerized images. Many offices, however, are still wedded to film and its processing. Thus, it is still essential that anyone involved in radiographic technique understand this material.Although the terms “density”, “contrast” and “subject contrast” have the same meaning when viewing a digital image as they do when viewing a film, digital radiographs are “processed” electronically, and not chemically. Thus the terms “Film contrast” and “processing” are relevant to the type of software and sensors owned by the practice.
To properly evaluate dental x-ray quality, and to optimize your practice’s imaging activities, it is helpful to understand some key imaging terms. Two measures of dental x-ray quality are density and contrast.
The optical density is the degree of blackening of film after exposure and processing. The blacker the area in question, the higher the density. Density is measured by the ability of the developed silver in the piece of film to stop light from passing through.
X-ray films that have too little density appear “too light.” Films that have too much density appear “too dark.” In either case, detail can be lost. If a film is too light, detail is washed out in the lighter areas of the film. If a film is too dark, detail is lost in the dark areas.
Two terms used to express two common values of density are: D-min and D-max. D-min corresponds to the lowest possible density that a particular type of film can achieve. D-max corresponds to the highest possible density a particular type of film can achieve. For intraoral film, the D-min value is generally in the range of 0.2 to 0.3. The D-max value is 5.0 or greater.
In the three images below, notice the difference in appearance of the caries on the mesial of #2 The upper right image shows high contrast, the upper left shows low contrast and the bottom is medium contrast.
Contrast is the difference in optical density (film blackening) between areas of interest in a radiograph. For this reason, contrast is critical for distinguishing objects in a radiograph. Contrast in a dental x-ray image is influenced by three factors: subject contrast, film contrast and processing.
Subject contrast is the result of the differential attenuation of the x-ray beam by the subject being imaged. As x-ray radiation passes through patient’s soft tissue, bone and teeth, it is absorbed to varying degrees depending on the type of tissue it encounters. Teeth, for example, absorb more x-ray radiation than soft tissue. Therefore, fewer x-ray beams pass through teeth to strike the x-ray film. The area of the film that corresponds to teeth will, as a result, appear lighter than the area that corresponds to soft tissue.
Film contrast is a characteristic of the film itself. These films, if exposed and processed correctly, are optimized to deliver very dark blacks and very light whites. Films with excellent film contrast help dental professionals adjust for other variables in the imaging process.
For example, if processing temperatures are too high, the image density will be too high and detail will be washed out. If processing temperatures are too low, the image will be too light. Chemistry solutions must also be fresh to work properly. Old chemicals will not be as chemically active, which will result in images that are “too light”.
The use of cold chemicals during processing produces also produces coarse grained radiographs in which the details of the subject may not properly resolve, even if the time is increased to compensate for density.