During a fluoroscopy procedure, the radiographer notes that doubling the distance from the X-ray tube to the patient reduces the radiation dose rate to the patient by a factor of four. This relationship is an example of which physical law?

• A Beer-Lambert law
• B Compton scatter law
• C Bremsstrahlung law
• D Inverse square law ✓

Explanation

The inverse square law states that radiation intensity is inversely proportional to the square of the distance from the source: I ∝ 1/d². Doubling the distance therefore reduces intensity by a factor of 4 (2²). This principle is fundamental to radiation protection in fluoroscopy and interventional radiology — increasing the distance between the X-ray source and operator is one of the three cardinal radiation protection principles (time, distance, shielding). The Beer-Lambert law describes exponential attenuation through material. Compton scatter describes the interaction of photons with outer shell electrons. Bremsstrahlung describes X-ray production by electron deceleration.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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