Which operator console knob controls x-ray beam quality?

• A. mA
• B. kVp
• C. exposure time
• D. source-to-image distance

Answer: (B)

Beam quality refers to the energy or penetrating power of the x-ray photons, which is set by the kilovoltage peak. Increasing kVp raises the average photon energy, making the beam more penetrating and altering image contrast. The control on the operator console that adjusts this energy is the kVp knob, so it directly sets beam quality.

The other controls affect beam quantity rather than energy: milliampere (mA) changes the tube current and thus the number of photons produced per second; exposure time changes how long the tube is on, altering total photon output; and source-to-image distance changes intensity at the image receptor via the inverse-square law without changing photon energy.