RTBC Advanced Exposure Factors Practice Test

When the source-to-image distance (SID) increases, beam intensity decreases due to the inverse square law, which states that the intensity of a radiation beam is inversely proportional to the square of the distance from the source. This means that as the distance from the source increases, the energy spread over a larger area diminishes the intensity experienced at a particular point.

Specifically, when you double the distance from the source, the intensity becomes one-fourth of what it was at the original distance. This principle is crucial in radiography and radiation therapy because accurate dosing and image clarity are highly dependent on the SID.

An increase in SID can lead to the need for adjustments in exposure time or the overall energy delivered to ensure that image quality is maintained while considering the reduced intensity at greater distances.