If the source-to-image distance (SID) increases, what happens to the beam intensity?

When the source-to-image distance (SID) increases, the beam intensity decreases. This phenomenon can be explained using the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source.

As the SID increases, the X-ray beam spreads out over a larger area, resulting in a lower intensity of radiation reaching the image receptor. Specifically, if the distance is doubled, the intensity becomes one-fourth of its original value. This principle is essential for radiography, as it underscores the importance of maintaining appropriate distances to ensure optimal image quality.

In practical terms, understanding this relationship helps radiologic technologists adjust their technique properly to account for changes in SID and ensure that the diagnostic images produced are of suitable quality while also minimizing patient exposure to radiation.