Which parameter does NOT affect the patient dose?

• A. SID (FFD)
• B. Focal Spot Size
• C. kVp
• D. mAs

Answer: (B)

Focal spot size mainly influences image sharpness and geometric blurring, not how many photons are produced or how penetrating those photons are. The patient dose is governed by how many photons reach the patient and how energetic they are, which come from mAs (photon quantity) and kVp (photon energy), as well as distance from the source to the patient (SID) via the inverse square law. A longer SID reduces entrance skin dose; increasing mAs raises dose; adjusting kVp changes energy and, in practice, often allows lowering mAs to maintain image receptor exposure, which can reduce dose overall but affects image quality. Because the focal spot size does not change photon output or energy, it does not affect patient dose, making it the parameter that does not influence dose.