Above what kVp percentage are 85% of photons created by bremsstrahlung?

The correct answer is based on the physics of bremsstrahlung radiation, which occurs when high-speed electrons are suddenly decelerated or deflected by the electric fields of atomic nuclei in the target material of the x-ray tube. As the kinetic energy of the electrons (indicated by kilovolt peak, or kVp) increases, the efficiency and quantity of bremsstrahlung radiation produced also increase.

At 70 kVp, it is observed that approximately 85% of the x-ray photons generated are due to bremsstrahlung. This is a critical point in diagnostic radiology because operating at this voltage maximizes the production of useful x-ray photons for imaging while minimizing unwanted scatter and higher energy radiation.

Values below 70 kVp do not produce adequate bremsstrahlung photons to reach the 85% threshold. Higher values, while still producing bremsstrahlung, exceed the threshold necessary for the intended imaging applications, making 70 kVp the key point of reference in many clinical settings.