What is the inverse square law and how does it apply to x-ray imaging?

• A. Intensity falls with the square of distance from the source; doubling the source-to-image distance reduces exposure to one-quarter, reducing patient and operator dose when geometry allows.
• B. Exposure doubles when distance doubles.
• C. Distance does not affect exposure due to shielding.
• D. Intensity increases with distance from the source.

Answer: (A)

The main idea is the inverse square law: radiation intensity falls off with the square of the distance from the source. In x-ray imaging, the tube acts like a nearly point source, and the beam spreads as it travels. That means the exposure rate at a given point—whether at the image receptor or at the patient’s skin—decreases in proportion to 1 over the distance squared. So when you double the source-to-image distance, the primary beam intensity at the receptor drops to about one-quarter. This reduction helps lower both patient entrance skin dose and operator scatter dose, as long as you can maintain diagnostic image quality by adjusting exposure settings accordingly. In practice, increasing distance reduces magnification and improves sharpness, but you may need to raise mA or exposure time to compensate for the lower intensity to keep the image properly exposed. The inverse square relation applies to the primary beam and to scatter reaching the operator, making distance a crucial factor in dose management during radiography.