Units of measurement

For units of measurement the use of SI units (both base and derived units) in articles and cases on Radiopaedia.org is preferred. This is in line with best scientific practice and helps maintain consistency across the site.

Terminology

By scientific convention:

• for eponymous units, the full name is not capitalized, but the symbol is, e.g. tesla is the correct form - not Tesla - but the symbol is T

  • degree Celsius is the only exception to this rule, even kelvin is only ever written with a lowercase initial when written out in full

• for non-eponymous units, neither the full name nor the symbol is capitalized, e.g. ‘second’ is the correct form and the symbol is ‘s’

  • only exception to this rule for non-eponymous units is the liter

    • strictly-speaking the symbol should be 'l', i.e. the non-capital form of the letter 'L'. However, in view of the risk of mixing up 'l' with '1' in some fonts, it was decided in some countries, e.g. USA, Australia, to legally mandate the upper case for safety reasons

    • the full name remains liter, not Liter

    • other countries, e.g. UK, have kept it as 'l'

    • on Radiopaedia.org we have decided to use L, and therefore mL, etc.

• a single-space should always be inserted between the numeric quantity and the symbol of the unit, e.g. 3 T magnet, and not 3T magnet. This becomes easier to remember and appreciate if one considers that the symbol is just a shortening of a word, i.e. using the same example, 3 tesla magnet, no-one would write 3tesla magnet! 

  • only exception to this is the symbol for angle measurements, e.g. ninety degrees is 90° and not 90 °

Length

• SI unit of distance is the meter (m)

• for most of our purposes the meter is too long, and we prefer the centimeter (cm) or millimeter (mm)

• a single-space should always be inserted between the numeric quantity and the symbol of the unit, e.g. 3 cm cyst, not 3cm cyst. The latter incorrect usage is seen all too commonly in radiology reports. This becomes easier to remember and appreciate if one considers that the symbol is just a shortening of a word, i.e. using the same example, "3 centimeter cyst", no-one would write "3centimetre cyst"!

• for cases (like imaging reports) try to stick to one unit rather than switching between them e.g. "1 cm calculus in the gallbladder with a 0.4 cm wall", rather than a "1 cm calculus in the gallbladder with a 4 mm wall"

Volume

• liquid/gas volumes or for organ size (e.g. ovary), we prefer milliliter (mL) rather than ml, cc or cubic centimeter

  • remember that 1 mL is numerically equivalent to 1 cm³

• solid lesions e.g. lung nodules, mm³ or cm³, are best

Mass

• SI unit of mass is the kilogram (kg)

  • the spellings kilogramme/gramme are archaic ³

Time

• SI unit of time is the second (s)

Electricity

• SI unit of electrical current is the ampere (A), often shortened to amp

  • unit for tube current is milliampere-seconds (mAs)

Temperature

• SI unit of temperature is the kelvin (K), although it is acceptable to use degree Celsius (°C) for everyday temperatures as a change of one degree Celsius is equal to a change of one kelvin (i.e. Δ(Τ) / °C ≡ Δ(Τ) / K).

  • unlike other temperature units, kelvin is never expressed as degrees, therefore "degree kelvin"/°K is scientifically incorrect and should never be used

  • Celsius is the only official eponymous unit that retains a capital first letter, the rationale being that the unit's full name is "degrees Celsius"

  • the term centigrade as a synonym for Celsius should not be used

Amount of substance

• SI unit of amount of substance is the mole (mol)

Ionizing radiation

For ionizing radiation, SI units should be used even though older units (e.g. rad) remain in common use in a few countries (e.g. USA). It should be noted, that even though this is the case the USA's National Institute of Standards and Technology (NIST) "strongly discourages" the use of non-SI units ². 

Absorbed dose

• derived SI unit of absorbed dose is the gray (Gy)

  • the old equivalent cgs unit is the rad

Equivalent dose

• derived SI unit of equivalent dose is the sievert (Sv)

  • the old equivalent cgs unit is the rem

Radioactivity

• derived SI unit of radioactivity is the becquerel (Bq)

  • commonly used for doses in nuclear medicine

  • one becquerel is a tiny unit, nuclear medicine doses are most commonly expressed in megabecquerels (MBq) or gigabecquerels (GBq)

    • the old equivalent cgs unit is the curie (Ci)

    • however as the curie remains in common use in some parts of the world we express all doses in Bq, with the curie equivalent following in brackets, e.g. 37 MBq (1 mCi)

Magnetism

• derived SI unit of magnetic field strength is the tesla (T)

• when stating the field strength of an MRI scanner, the normal scientific convention is followed, e.g. 1.5 T and 3 T, not 1.5T and 3T

  • the old equivalent cgs unit is the gauss (G)

Frequency

• derived SI unit of frequency of a wave is the hertz (Hz)

• one hertz is one cycle per second