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.

Ionizing radiation related quantities

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

• Gray (unit)
• Sievert
• Equivalent dose
• Relative biological effectiveness

References

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