If the distance between the X-ray source and the object increases, what happens to the radiation intensity?

• A. Decreases
• B. Increases
• C. Varies with kilovoltage and miliamperage
• D. Remains the same

Answer: (A)

The key idea here is the inverse square law: radiation intensity falls off with the square of the distance from the source. As the X-ray beam spreads out, the same number of photons covers a larger area, so the photons per unit area—and thus the intensity at the object—decreases proportional to 1/d^2. So when you increase the distance, the intensity drops. For example, doubling the distance makes the intensity about one-quarter, and tripling makes it about one-ninth. Changes in kilovoltage or current affect the beam’s energy or quantity, but distance alone follows this 1/d^2 relationship.