An x-ray tube produces x-rays used in radiography and computed tomography. It receives electrical energy and converts it into two other forms of energy: x-radiation (1%) and heat (99%). Heat is considered the undesirable product of this conversion process.
This article contains an overview of the construction and basic working principles of modern vacuum x-ray tubes. For details about specific tube elements and their function, see the linked dedicated articles.
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Construction
Principal elements
filament (part of cathode): boils off electrons by thermionic emission
target (part of anode): electrons strike it to produce x-rays
Additional elements
anode rotor and induction stator
evacuated tube envelope with window
coolant with expansion bellows
tube housing
Other parts of an x-ray system
Function
Filament (cathode) and target (anode)
Current applied to the filament heats it, so that it emits electrons in the process of thermionic emission. High potential difference (in the range of 30-150 kV) between the cathode (where the filament releases electrons) and the anode accelerates the electrons.
Electrons strike the anode target, releasing most of their energy as heat, and a small part (1%) as radiation. Most of the radiation appears in the form of continuous spectrum of Bremsstrahlung (German for "braking radiation") and some in the form of discrete characteristic x-ray peaks.
The process of x-ray production is described with more detail in the dedicated article.
Anode rotor and induction stator
Most modern x-ray tubes facilitate a rotating anode ref, in order to distribute the heat of x-ray production over a larger target area.
For some demanding applications, such as dual-energy CT,?rotating envelope tubes (RET) are used. Unlike conventional x-ray tubes, in rotating envelope tubes, not only the anode, but the entire vacuum tube rotates. Furthermore, the anode is in direct contact with the liquid coolant, resulting in improved heat conduction and increased performance 4.?
Tube envelope
The filament/cathode and target/anode are contained in the envelope, which provides vacuum, support and electrical insulation. The envelope is made of glass (Pyrex or borosilicate glass) 8, ceramic, or metal 4. A glass envelope is more prone to failure because tungsten filament vapourises and coats the surface of the glass, altering the electric potential of the tube, and resulting in the formation of an electric arc, causing tube failure. A metal envelope maintains the electric potential across the tube, making it less likely to fail. Thus, most high-output x-ray tubes use metal envelopes 6.
Tube window
Tube window is a small part of the tube envelope made of low-attenuation material (usually beryllium), which lets the desired imaging x-ray beam out of the tube. Radiation emitted in all other directions is absorbed by the tube envelope or housing.
Coolant
A coolant, e.g. a dielectric (non-conducting) oil, surrounds the tube envelope. Expansion bellows allow the coolant to expand when heated.
Tube housing
Provides structure and protection for the rest of the elements. Captures undesired x-rays in directions other than the imaging beam going through the tube window.
Generator
The generator converts the supplied alternating current (AC) into:
high voltage direct tube current (DC)
low voltage and high amperage alternate filament heating current (AC)
The quality and the quantity of the x-radiation are controlled by adjusting the electrical parameters:
tube voltage, kV: potential difference applied across the tube between the cathode and anode and thus energy of the accelerated electrons
tube current, mA: electron flux between the cathode and anode
exposure time, usually a fraction of a second
The two latter parameters, together regulating the amount of x-rays generated, are often referred to by their product: milliampere-seconds (mAs).
History and etymology
Wilhelm Roentgen discovered x-rays using a Crookes tube in 1895. Until the invention of the Coolidge tube in 1913, all x-ray tubes were based on the Crookes or cold cathode gas tube technology. From the late 1910s onwards there was a rapid replacement of gas tubes by the far more effective Coolidge tubes.
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