Radiographic Contrast

nder contrast, there are subject factors and film factors.As mentioned on the previous page, radiographic contrast describes the differences in photographic density in a radiograph. The contrast between different parts of the image is what forms the image and the greater the contrast, the more visible features become. Radiographic contrast has two main contributors: subject contrast and detector (film) contrast.

Subject Contrast

Subject contrast is the ratio of radiation intensities transmitted through different areas of the component being evaluated. It is dependent on the absorption differences in the component, the wavelength of the primary radiation, and intensity and distribution of secondary radiation due to scattering.

Generating x-rays using a low kilovoltage will generally result in a radiograph with high contrast.It should be no surprise that absorption differences within the subject will affect the level of contrast in a radiograph. The larger the difference in thickness or density between two areas of the subject, the larger the difference in radiographic density or contrast. However, it is also possible to radiograph a particular subject and produce two radiographs having entirely different contrast levels. Generating x-rays using a low kilovoltage will generally result in a radiograph with high contrast. This occurs because low energy radiation is more easily attenuated. Therefore, the ratio of photons that are transmitted through a thick and thin area will be greater with low energy radiation. This in turn will result in the film being exposed to a greater and lesser degree in the two areas.

There is a tradeoff, however. Generally, as contrast sensitivity increases, the latitude of the radiograph decreases.  Radiographic latitude refers to the range of material thickness that can be imaged This means that more areas of different thicknesses will be visible in the image. Therefore, the goal is to balance radiographic contrast and latitude so that there is enough contrast to identify the features of interest but also to make sure the latitude is great enough so that all areas of interest can be inspected with one radiograph. In thick parts with a large range of thicknesses, multiple radiographs will likely be necessary to get the necessary density levels in all areas.

Film Contrast

Film contrast refers to density differences that result due to the type of film used, how it was exposed, and how it was processed. Since there are other detectors besides film, this could be called detector contrast, but the focus here will be on film. Exposing a film to produce higher film densities will generally increase the contrast in the radiograph.

The typical film characteristic curve plots the log of relative exposure versus film density.A typical film characteristic curve, which shows how a film responds to different amounts of radiation exposure, is shown to the right.  (More information on film characteristic curves is presented later in this section.) From the shape of the curves, it can be seen that when the film has not seen many photon interactions (which will result in a low film density) the slope of the curve is low. In this region of the curve, it takes a large change in exposure to produce a small change in film density. Therefore, the sensitivity of the film is relatively low. It can be seen that changing the log of the relative exposure from 0.75 to 1.4 only changes the film density from 0.20 to about 0.30. However, at film densities above 2.0, the slope of the characteristic curve for most films is at its maximum. In this region of the curve, a relatively small change in exposure will result in a relatively large change in film density. For example, changing the log of relative exposure from 2.4 to 2.6 would change the film density from 1.75 to 2.75. Therefore, the sensitivity of the film is high in this region of the curve. In general, the highest overall film density that can be conveniently viewed or digitized will have the highest level of contrast and contain the most useful information.

Lead screens in the thickness range of 0.004 to 0.015 inch typically reduce scatter radiation at energy levels below 150,000 volts. Above this point they will emit electrons to provide more exposure of the film to ionizing radiation, thus increasing the density and contrast of the radiograph.  Fluorescent screens produce visible light when exposed to radiation and this light further exposes the film and increases contrast.