Which one does NOT affect the patient dose?

Which one does NOT affect the patient dose?

• A. kVp
• B. mAs
• C. Focal spot size
• D. SID (FFD)

Answer: (C)

The amount of radiation a patient receives is driven by how many photons reach the tissues and their energy. mAs directly controls photon quantity, so increasing mAs raises dose. SID changes with distance: the farther away the source, the lower the dose to the patient via the inverse-square law. kVp affects the beam’s energy and penetrability, which influences dose and image quality, and may require adjustments in technique to keep receptor exposure appropriate. The focal spot size, however, mainly affects image sharpness and geometric blur; it does not change the total energy or number of photons reaching the patient in a standard exposure, so it does not affect patient dose.

The amount of radiation a patient receives is driven by how many photons reach the tissues and their energy. mAs directly controls photon quantity, so increasing mAs raises dose. SID changes with distance: the farther away the source, the lower the dose to the patient via the inverse-square law. kVp affects the beam’s energy and penetrability, which influences dose and image quality, and may require adjustments in technique to keep receptor exposure appropriate. The focal spot size, however, mainly affects image sharpness and geometric blur; it does not change the total energy or number of photons reaching the patient in a standard exposure, so it does not affect patient dose.