The energy of a photon is direct proportional to its frequency.
E = h * v
E = Energy (J joule)
h = Planck's constant, 6.626 x 10^-34 J-s (Joule-seconds)
v = Frequency (Hz)
This relation between energy and frequency of a photon is known as Planck's relation.

See also Photon.

The absorbed dose is the average energy absorbed per unit mass.

The tissue absorbed energy in a small mass volume:
D = (dW/dm) [ Gy ]
D = absorbed dose in Gray (Gy); dW = in the tissue energy absorbed; DM = small volume of the mass.

The SI unit of absorbed dose is the joule per kilogram and its special name is the gray (Gy). In units often used by federal and state agencies, absorbed dose is given in rad; 1 rad = 0.01 Gy.

Absorbed dose is a feature that should increase dose awareness and help users in dose optimization. Absorbed dose in CT is quoted using the CTDI (computed tomography dose index)

CTDIvol (volume-averaged CT dose index) and the dose-length product (DLP) give an indication of the average absorbed dose and relative radiation risk to a standard patient. The user is being warned to scan parameter settings that may lead to high doses, and can adjust the protocol if appropriate. It should be noted that CTDIvol and DLP do not take patient size into account, and will give overestimates and underestimates for large and small patients, respectively.

The directional dose equivalent is the equivalent dose at a point in a radiation field that would be produced by the corresponding expanded field at depth on a radius in a specified direction. The unit of directional dose equivalent in the SI system of units is joule per kilogram (J/kg) and sievert (Sv).

(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.

In physics, energy is described as the ability to do work by a force, measured in joules (J).
Several different forms of energy exist to describe various natural phenomena.

See also Photon Energy, Rest Energy, Binding Energy, Kinetic Energy and Effective Energy.