If the source-to-image distance (SID) is increased or decreased, how is the density on the image affected?

When discussing how changing the source-to-image distance (SID) affects image density, it is important to understand the principles of radiographic image formation and the inverse square law. The density of an image is influenced by the quantity of radiation that reaches the film or detector, which is determined by factors such as SID.

Increasing the SID reduces the intensity of the radiation reaching the image receptor due to the inverse square law, which states that radiation intensity is inversely proportional to the square of the distance from the source. Conversely, decreasing the SID increases the radiation intensity reaching the receptor. However, when these changes are made while keeping other exposure factors (such as milliampere-seconds and kilovolt peak) constant, the overall image density remains largely unchanged.

In practice, adjustments may be needed to exposure settings to maintain optimal density, but the inherent relationship dictated by SID itself suggests that under controlled conditions, the density will not be significantly affected by merely changing the distance. Therefore, option A is correct in stating that density remains unchanged in this context, provided that all other exposure parameters are kept constant.