(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.

The filament is the source of electrons (cathode) in x-ray tubes. A thin wire (0.1- 0.5 mm, usually tungsten) emits electrons due to thermionic emission, operating in a vacuum and energized with electric current.
A CT tube utilizes a larger filament with larger size of the effective focal spot than a conventional x-ray tube.

A photon is a discrete packet of electromagnetic energy. The amount of energy depends on the frequency (wavelength) of the photon. Highest frequency, most energetic photon radiations are gamma rays, up to 300 EHz - 1.24 MeV. In addition to energy, photons are also carrying momentum.
Photons have no electrical charge or rest mass and exhibit both particle and wave behavior.
Photons are traveling in vacuum (without interactions with matter) with the constant velocity of 2.9979 x 10⁸ m/s (c, speed of light).
Photons get absorbed or scattered away from their original direction of travel when interacting with matter.
High energy photons as for example x-rays cause damages to exposed tissue and cells. Radiation exposure is measured in roentgen, radiation absorption in Roentgen//min.
Photon radiation in the frequency ranges of x-rays and gamma rays are used for medical diagnostic and treatment.

See also Photon Energy and Gamma Ray.

[Rest Mass Energy] The term refers to the concept of mass-energy equivalence. Einstein proposed that the equivalence of mass and energy is a general principle.
E=mc²
E = energy,
m = mass,
c = the speed of light in a vacuum.
c² is the conversion factor required to convert from units of mass to units of energy.
The rest energy of an electron or a positron for example amounts to 0.511 MeV.

See also Photon, Photon Energy.

X-ray tubes are devices for the production of x-rays. X-ray tubes consist of an evacuated glass vessel and two electrodes. An electrical current with very high voltage passes across the tube and accelerates electrons emitted by thermionic emission from a tungsten filament (cathode also called electron gun) towards the anode target. The electrons collide with the anode and this deceleration generates x-rays (bremsstrahlung).
The high vacuum allows the electron beam an unimpeded passage. The electron beam heats the anode (usually copper), which is cooled by water to prevent melting. A copper target emits x-rays with a characteristic wavelength. Other used metals soften or harden the x-ray beam.
The x-rays pass through a very thin beryllium (Be) foil. This beryllium window absorbs a high amount of the elastically scattered electrons (produced by the target) and allows the radiation to get out of the tube without substantial absorption.
In conventional x-ray tubes, the anode is also the target. In nanofocus and microfocus x-ray tubes, the electron beam is transmitted through a hole in the anode where it is then focused onto a small spot on the target.

See also X-Ray Tube Housing, Fine Focus X-Ray Tube, Transformer, Diode, Digital to Analog Converter and Angular Response.