If the source-to-image distance (SID) is doubled, how is x-ray photon intensity affected?

When the source-to-image distance (SID) is doubled, the intensity of x-ray photon exposure at the image receptor decreases. This phenomenon is explained by the Inverse Square Law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source of radiation.

In simpler terms, as you move further away from an x-ray source, the radiation spreads out over a larger area, resulting in a decrease in intensity. Specifically, if the distance is doubled, the intensity is reduced to one-fourth (or decreased by a factor of four) of its original value at the initial distance.

Therefore, when the SID is increased twofold, the correct interpretation is that the x-ray photon intensity is decreased by four times. This relationship is crucial in understanding how changes in distance can significantly affect radiation exposure in imaging and therapeutic applications.