F-factor (conversion factor)

The F-factor, in diagnostic radiology, is the conversion factor between exposure and absorbed dose. In other words, it converts between the amount of ionization in air (roentgens or coulombs/kg) and the absorbed dose in air (rads or grays). The two determinants of the F-factor are the effective Z of the material and the type of ionizing radiation being considered. Since the effective Z of air and soft tissue is approximately the same, the F-factor is approximately 1 for many x-ray imaging applications. However, bone has an F-factor of up to 4, due to its higher effective Z.
Radiation-related quantities.
The following table shows radiation quantities in SI and non-SI units.
References.
Bushberg et al., 2002. "The Essential Physics of Medical Imaging." Philadelphia: Lippincott Williams & Wilkins. (p. 55)