Which parameter controls the penetrating power of the X-ray beam?

• A. kVp
• B. mA
• C. s
• D. Focal length

Answer: (A)

Penetrating power comes from the energy of the X-ray photons. The kilovoltage peak sets that energy: increasing kVp raises the average photon energy, producing a beam that can pass more readily through tissue. Lower kVp yields more low-energy photons, which are absorbed more readily and thus reduce penetration but increase image contrast. The other parameters don’t change photon energy: mA changes how many photons are produced (affecting brightness and dose but not penetration), s controls exposure duration, and focal length (distance from source to patient) alters beam intensity and magnification via the inverse square law but not the photons’ energy. So the parameter that controls penetrating power is kilovoltage peak.