What happens to x-ray beam intensity when changing the SID from 72 inches to 40 inches?

When the source-to-image distance (SID) changes from 72 inches to 40 inches, the intensity of the x-ray beam increases. This is grounded in 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 distance decreases, more of the x-ray photons converge on a smaller area, leading to an increase in the beam intensity. When moving from a longer distance (72 inches) to a shorter distance (40 inches), the x-ray beam becomes more concentrated, resulting in a higher intensity at the receptor. Therefore, reducing the SID from 72 inches to 40 inches increases the intensity of the x-ray beam received at the image receptor, which is why this choice accurately reflects the outcome of the change in distance.