(eV) Electron volt is an energy unit defined as 1.60919 x10^-19 joules (in older unit 1.60919 x10^-12 erg). One electron volt is equal to the kinetic energy required to raise an electron through a potential difference of one volt (in a vacuum). The electron volt is not an SI unit but its use is valid within the International System for atomic (eV), electronic (keV), nuclear (MeV), and subnuclear processes (GeV or TeV).

In medical imaging used units:
MeV: One million electron volts
keV: One thousand electron volts.

This unit of temperature is still used customarily in the United States.
Definition: 0° is the coldest temperature achieved by using an ice and salt mixture, and 100° is set at the temperature of the human body. On this scale, the freezing point of water turned out to be about 32°F and the boiling point about 212°F.
1°F equals 5/9°C. To convert a temperature in °F to the Celsius scale, first subtract 32° and then multiply by 5/9. In the other direction, to convert a temperature in °C to the Fahrenheit scale, multiply by 9/5 and then add 32°. The unit was defined by the German physicist Daniel Gabriel Fahrenheit.

See also Kelvin, Celsius.

(Gy) Gray is the SI unit of absorbed radiation dose. 1 Gray is equal to the energy absorption of 1 Joule per kilogram. The absorbed dose due to any type of radiation is measured in Gray. 1 Gray is equal to 100 rads in the older terminology.

(Hz) The standard SI unit of frequency.
Definition: The number of repetitions of a periodic process per unit time. It is equal to the old unit cycles or oscillations each second of a simple harmonic motion. The unit is named for the German physicist Heinrich Rudolf Hertz.
Larger units are:
kilohertz (KHz) = 1 000 Hz = 10³ Hertz
megahertz (MHz) = 1 000 KHz = 10⁶ Hertz
gigahertz (GHz) = 1 000 MHz = 10⁹ Hertz
terahertz (THz) = 1 000 GHz = 10¹² Hertz
petahertz (PHz) = 1 000 THz = 10¹⁵ Hertz
exahertz (EHz) = 1 000 PHz = 10¹⁸ Hertz

See also Oscillation, Coherence, Duty Cycle, Cine Mode, and System International.

(H) The Hounsfield scale displays radiodensity in a linear scale of gray shades expressed in Hounsfield units (HU). The Hounsfield scale is a quantitative transformation of the attenuation coefficient.
The Hounsfield value -1000 is defined as the radiodensity of air, 0 H that of distilled water at standard pressure and temperature, and denser tissues like for example cranial bone can reach 2000 H. The radiation attenuation of dental fillings or artificial implants depends on atomic number of the elements used. Titanium usually has an amount of +1000 HU. Iron steel can have a density greater than the highest range (traditional 3095 H) covered by the standard Hounsfield scale of a CT scanner. Areas with attenuation coefficients that exceed the scale's maximum are white areas in which no detail is visible.
Some CT machines are relatively tolerant, precise representing regions with very high densities. Sometimes, an option is available to select an extended CT number scale.

(HU) [CT scanner display unit] The arbitrary scale is defined by air, which has a CT number of -1000 HU, and water, with a CT number of 0 HU. It is named after Sir Godfrey Hounsfield, who developed the first clinical CT scanner.

See also Hounsfield Scale.