In general radiography, quantum noise is a function of which factors?

• A. mAs and kVp
• B. SID and OID
• C. Focal spot size and grid ratio
• D. Patient age and anatomy

Answer: (A)

Quantum noise comes from the random, particle-like nature of photons arriving at the detector. The image quality improves as more photons are detected because the statistical fluctuations (the noise) become a smaller fraction of the signal. The two factors that most directly control how many photons reach the detector are mAs and kVp: increasing mAs raises the number of photons produced, so noise decreases; increasing kVp raises the beam’s penetrating power, so more photons reach the detector after passing through the patient, which also reduces noise for a given exposure. The other factors change exposure or image properties in other ways—such as geometric distance affecting intensity, focal spot size and grid influencing sharpness and scatter, and patient age or anatomy altering attenuation—but they don’t set the photon count at the detector as directly as mAs and kVp.