(A or amp) The SI base unit of electric current.
Definition: Two parallel conductors, infinitely long and having negligible cross section should be placed 1 meter apart in a perfect vacuum. One ampere is the current that creates between them a force of 0.2 µN (micronewton) per meter of length.
One ampere represents a current flow of 1 coulomb of charge per second.
One ampere of current results from a potential distribution of 1 volt per ohm of resistance, or from a power production rate of 1 watt per volt of potential.
The unit is known informally as the amp, but A is its official symbol and is named for the French physicist André-Marie Ampère.
See also System International.

The anode is the positive terminal of an x-ray tube, usually consisting of a tungsten block embedded in a copper stem. Electrons flow from the cathode toward the anode and the anode emits x-rays from the focal spot.

A tomographic imaging plane, parallel to the ground, perpendicular (rotated 90°) to the long axis of the human body; the axial plane separates the superior from the inferior part (the head from the feet). Also called transaxial, transversal, transverse plane.

Natural background radiation originates from radioactive elements in the environment, including food, water, soil and rock (also building materials), the atmosphere and cosmic rays. The level of natural exposure to radiation can vary greatly between different locations. In the US, the average annual exposure from natural sources to humans is about 3 mSv (millisievert) corresponding to 0.3 rem. Radon gas accounts for two-thirds of this exposure.
Background radiation may also interfere with measurements. Background radiation includes radioactive contamination of samples or incomplete absorption of radiation in a detector.

A stream of electromagnetic or particulate radiation that could be collimated and is generally unidirectional or divergent usually from a small source and restricted to a small-solid angle.

See also Ion Beam, Broad Beam, Useful Beam and Beam On Time.