F-factor (conversion factor)

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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 tissue (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[edit]

The following table shows radiation quantities in SI and non-SI units.

Ionising radiation related quantities view  talk  edit
Quantity Unit Symbol Derivation Year SI equivalence
Activity (A) becquerel Bq s−1 1974 SI unit
curie Ci 3.7 × 1010 s−1 1953 3.7×1010 Bq
rutherford Rd 106 s−1 1946 1,000,000 Bq
Exposure (X) coulomb per kilogram C/kg C⋅kg−1 of air 1974 SI unit
röntgen R esu / 0.001293 g of air 1928 2.58 × 10−4 C/kg
Absorbed dose (D) gray Gy J⋅kg−1 1974 SI unit
erg per gram erg/g erg⋅g−1 1950 1.0 × 10−4 Gy
rad rad 100 erg⋅g−1 1953 0.010 Gy
Dose equivalent (H) sievert Sv J⋅kg−1 × WR 1977 SI unit
röntgen equivalent man rem 100 erg⋅g−1 1971 0.010 Sv

See also[edit]


Bushberg et al., 2002. The Essential Physics of Medical Imaging. Philadelphia: Lippincott Williams & Wilkins. (p. 55)