Scattered radiation and characteristic film curve.

Abstract

It is well known that scattered radiation has both favorable and adverse effects on the diagnostic image. The favorable effect consists of the increased intensity of x-rays after passing through strongly absorbent objects. In turn, the image is adversely affected by the decrease of contrast in the “straight-line” portion of the film curve. Depending on the radiologist's approach, emphasis is mostly either on visualizing as large a subject range as possible or on rendering maximum contrast. Basically, the interrelations between scattered radiation and characteristic film curve have been known for some time, but nevertheless incorrect descriptions continue to appear. The following comments are calculated to clarify matters and to contribute to the quantitative understanding of the problem. Subject Contrast and Subject Range The effects brought about when a homogeneous phantom is traversed by x-rays have been dealt with frequently in recent times (1, 2). Radiologists therefore have a sufficient knowledge of the absorption coefficients for the primary radiation and the amount of scattered radiation produced by a specific layer of water with a given field size and voltage. The degree of transmission of the normal x-ray grids for primary and for scattered radiation and the correlation between them, the so-called selectivity as a function of phantom dimension and voltage (3), are also known. But to date only a few tests (4, 5) have been undertaken to render contrasts in the phantom by introducing, for example, an air bubble or a stepped wedge. It is understood that here only relatively small contrasts, i.e., subject contrasts, are involved as described by Seemann and Splettstosser (5). The subject range has been studied only on the side when such measurements were made. This led to the implication that radiologists should be able to expose sections of the human body so as to bring them within the “straight-line” portion of the characteristic film curve. With regard to relative attenuation, and also for anatomical reasons, this seems not always possible (e.g., the mediastinum when postero-anterior chest radiography is performed). Consequently, phantom measurements must be extended to the visualization of a large subject range. Nowadays plastic phantoms simulating the radiological properties of the human body are available. Automation in exposure and development is widely used, and therefore a more detailed study of the problem is possible. If we expose a film with intensifying screens to a logarithmic scale of x-ray intensities, e.g., 0.1;0.3; 1;3;10; etc. mr, develop it by a standardized method, and measure the blackening S = − log10T (T = transparency) we get the well known characteristic film curve (Fig. 1: “0 per cent scatter”). Next we add increasing amounts of scattered radiation. In a real x-ray picture the amount of scattered radiation varies not very much over the film area. For a first approach, we can consider it constant.