In diagnostic radiology, the F-factor is the conversion factor between exposure to ionizing radiation and the absorbed dose from that radiation. In other words, it converts between the amount of ionization in air ( roentgens or, in SI units, coulombs per kilogram of absorber material) and the absorbed dose in air ( rads or grays). The two determinants of the F-factor are the effective atomic number (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.
The following table shows radiation quantities in SI and non-SI units.
| Quantity | Unit | Symbol | Derivation | Year | SI equivalent |
|---|---|---|---|---|---|
| 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 | |
| Equivalent dose (H) | sievert | Sv | J⋅kg−1 × WR | 1977 | SI unit |
| röntgen equivalent man | rem | 100 erg⋅g−1 × WR | 1971 | 0.010 Sv | |
| Effective dose (E) | sievert | Sv | J⋅kg−1 × WR × WT | 1977 | SI unit |
| röntgen equivalent man | rem | 100 erg⋅g−1 × WR × WT | 1971 | 0.010 Sv |
See also
References
Bushberg et al., 2002. The Essential Physics of Medical Imaging. Philadelphia: Lippincott Williams & Wilkins. (p. 55)
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