Why do the backs of cassettes contain lead?

The presence of lead on the backs of cassettes serves a crucial purpose in improving image quality by preventing backscatter radiation from fogging the image receptor (IR). Backscatter refers to radiation that comes from the patient and bounces back towards the IR after being absorbed or scattered within the patient's body or surrounding environment. When backscatter occurs, it can create a haze or unwanted fog on the image, resulting in decreased contrast and clarity. By incorporating lead into the design of the cassette, the backscatter is effectively absorbed before it has a chance to reach the IR, thus maintaining the integrity of the radiographic image and enhancing its overall quality.

Furthermore, while the other options address various aspects of radiation interaction, they do not specifically focus on the prevention of backscatter in relation to imaging. The lead's primary function is to act as a barrier against this backscatter, directly contributing to better imaging outcomes.