Reducing the effective focal spot size decreases geometric unsharpness but increases anode heat loading.

• A. True
• B. False
• C. Not enough information
• D. Only for rotating anodes

Answer: (A)

Smaller effective focal spot size reduces geometric unsharpness because the blur introduced by the focal spot is tied to its diameter; a smaller spot means crisper edges and sharper detail since the penumbra is decreased. At the same time, the X-ray energy used for a given exposure must be dissipated as heat in the anode. Concentrating that energy onto a smaller area increases heat density, raising the anode heat loading. Rotating anodes help spread heat over a larger surface, but the fundamental trade-off remains: decreasing focal spot size improves sharpness yet increases heat loading.