What effect does increasing OID have on image contrast?

Increasing the Object-to-Image Distance (OID) generally leads to an increase in image contrast. This phenomenon occurs because a greater OID effectively reduces the amount of scatter radiation reaching the imaging detector. Scatter radiation can degrade image quality and reduce contrast by adding unwanted exposure to all areas of the image, thereby reducing the differences between high-density and low-density tissues.

When OID is increased, the radiation that is not directly transmitted through the object undergoes more scatter before reaching the detector, which can obscure details. Therefore, with a higher OID, the contrast improves as the image retains more of the primary radiation's defining qualities, allowing for better differentiation between different tissue densities or structures. This principle is essential in optimizing image quality in radiography and other imaging modalities.