# X-ray quantity and quality

Last revised by Dr Hamish Smith on 29 Apr 2019

X-ray photon quantity refers to the number of photons produced during an exposure.

Factors influencing x-ray quantity includes:

• peak voltage (kVp): beam quantity is approximately proportional to the square of the tube potential
• generator type/voltage waveform: reducing ripple increases beam quantity
• beam filtration: increasing filtration reduces beam quantity
• distance from the beam: inverse square law
• current (mA): beam quantity is directly proportional to current
• exposure time (seconds): beam quantity is directly proportional to exposure time
• anode material: beam quantity is directly proportional to the atomic number (Z) of the anode material

X-ray photons quality relates to the x-ray spectrum changes and the effective photon energy. The effective photon energy is approximately equal to between one third to one half of the maximum photon energy.

Factors influencing x-ray quality include:

• peak voltage (kVp)
• voltage waveform: reducing ripple increases quality
• beam filtration: increasing filtration increases quality through beam hardening
• anode material: photon energy depends on the binding energies of shells in the anode material