What happens to patient dose when the source-to-image distance (SID) increases?

When the source-to-image distance (SID) increases, the patient dose decreases. This relationship is grounded in the principles of radiation physics and geometry. As the SID is increased, the x-ray beam diverges more before reaching the patient and the image receptor. Because of this divergence, the intensity (or exposure) of the x-ray beam is spread over a larger area, which leads to a reduction in the amount of radiation absorbed by the patient.

The reduction in patient dose can be quantitatively understood through the inverse square law, which states that the intensity of radiation from a point source is inversely proportional to the square of the distance from that source. Thus, when distance is doubled, the intensity—and therefore the dose to the patient—becomes one-fourth of what it was at the original distance.

This decrease in dose is an important consideration in radiographic technique, as it helps to minimize the exposure to patients while still achieving the necessary image quality. Reducing patient dose through increased SID is a common practice, especially in applications requiring meticulous imaging and safety.