The X-ray tube

• X-rays are generated in an x-ray tube.  The tube consists of a cathode side (negative electrical charge) and an anode  side (positive electrical charge).

• An x-ray beam is generated by passing an electron beam through a vacuum between a cathode (-) and an anode (+). The positively charged anode attracts the rapidly moving, negatively charged electrons.

• In the x-ray tube, a stream of fast moving electrons are attracted and directed from the cathode to the anode.  As the electrons collide and interact with the atoms on the anode target, a great amount of energy is produced.  1% of this energy is in the form of x-radiation. 99% appears as heat and must be removed from the anode.

• The tube enclosure is shielded and a series of lead shutters allow the diagnostic beam to exit.

• The cathode consists of a wire filament that emits electrons when heated. The temperature of the filament is controlled by the milliamperage (mA) setting on the control panel of the machine.

• As the mA is increased, the temperature of the filament is increased and the filament produces more electrons.  The period of time during which the x-rays are permitted to leave the x-ray tube is measured in fractions of a second. The number of electrons available and the time period set for their release from the filament determines how many x-rays are produced from the anode. The mAs thus controls the total number of x-rays produced.

• The anode, which attracts the negatively charged electrons, is constructed at an angle so that the x-rays produced are directed downward (toward the film) through a window in the metal housing of this x-ray tube.  In some machines the anode spins on a rotor.

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