Final answer:
In a variable kVp x-ray system, the voltage of the x-ray tube is adjusted to produce x-ray photons with different energies, ensuring effective penetration of body parts of varying densities for optimal imaging.
Step-by-step explanation:
In a variable kilovolt peak (kVp) x-ray system, the factor that is varied depending on the projection for a particular body part is the voltage of the x-ray tube, or more specifically, the energy of the x-ray photons it produces. This adjustment in voltage is necessary to ensure that the x-rays can penetrate the body part being imaged effectively.
For example, a lower voltage of 50.0 kV is used for a chest x-ray, while a higher voltage of 100 kV may be needed to examine a broken leg within a cast. By adjusting the voltage, and therefore the energy of the x-ray photons, technicians can tailor the beam to produce optimal images for the examination of different body parts with varying densities.