X-ray tube

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An X-ray tube is a vacuum tube that produces X-rays on demand. X-ray tubes are part of X-ray machines. X-rays are part of the electromagnetic spectrum, which extends from radio waves to microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. The development of X-ray tubes was an important historical event which enabled the field of radiology, for both imaging and treatment applications. Over decades, the development and usefulness of imaging using X-rays encouraged the growth of radiology to encompass radioactive isotope decay (nuclear medicine), nuclear magnetic resonance (MRI) and others. X-rays are also widely used in industrial inspection.

[edit] X-ray tube function

As with any vacuum tube, there is an emitter, either a filament or cathode, which emits electrons into the vacuum and an anode to collect the electrons, thus establishing a flow of electrical current, known as the beam, through the tube. A high voltage power source is connected across cathode and anode, for example 30 to 150 kilovolts (kV). In many applications, the current flow (typically in the range 1mA to 1A) is able to be pulsed on for between about 1ms to 1s. This enables consistent doses of x-rays, and taking snapshots of motion. Many types of vacuum tube can produce X-rays. (historically color TV high voltage rectifier tubes were a significant domestic source). Common CRTs do not produce significant amounts of x-rays. Until the late 1980s, X-ray generators were merely high-voltage, AC to DC variable power supplies. In the late 1980s a different method of control was emerging, called high speed switching. This followed the electronics technology of switching power supplies (aka switch mode power supply), and allowed for more accurate control of the X-ray unit, higher quality results, and reduced X-ray exposures.

Electrons from the cathode collide with the tungsten (and sometimes molybdenum) target deposited on the anode and accelerate other electrons, ions and nuclei within the deposited material. About 1% of the energy generated is emitted/radiated, perpendicular to the path of the electron beam, as X-rays. Over time, tungsten will be deposited from the target onto the interior surface of the tube, including the glass surface. This will slowly darken the tube and was thought to degrade the quality of the X-ray beam, but research (cf., Half-Value-Layer Increase Owing to Tungsten Buildup in the X-ray Tube: Fact or Fiction, John G. Stears, Joel P. Felmlee, and Joel E. Gray; Radiology, Vol 160, Number 3, pp 837 - 838, Sept 86). Eventually, the tungsten deposit may become sufficiently conductive that at high enough voltages, arcing occurs. The arc will jump from the cathode to the tungsten deposit, and then to the anode. This arcing causes an effect called "crazing" on the interior glass of the X-ray window. As time goes on, the tube becomes unstable even at lower voltages, and must be replaced. At this point, the tube assembly (also called the "tube head") is removed from the X-ray system, and replaced with a new tube assembly. The old tube assembly is shipped to a company that reloads it with a new X-ray tube.

The X-Ray photon-generating effect is generally called the Bremsstrahlung effect, a contraction of the German brems for braking, and strahlung for radiation.

The range of photonic energies emitted by the system can be adjusted by changing the applied voltage, and installing aluminum filters of varying thicknesses. Aluminum filters are installed in the path of the X-ray beam to remove "soft" (non-penetrating) radiation. The number of emitted X-ray photons, or dose, are adjusted by controlling the current flow and exposure time.

Simply put, the high voltage controls X-ray penetration, and thus the contrast of the image. The tube current and exposure time affect the dose and therefore the darkness of the image.

[edit] Crookes tube

Historically, the first X-ray tube was invented by sir William Crookes. It was used to make a visible fluorescence on minerals. The Crookes tube is also called discharge tube or cold cathode tube.

It is a glass bulb with around a thousandth of sea-level atmospheric pressure of air (approximately 100 pascals or 1 torr). It contains an aluminum cathode with a curved shape to concentrate the electron flow on the anode, or "target".

A high tension (known in the US as voltage) is made between the electrodes; this induces an ionization of the residual air, and thus an electron flow or "discharge" from the cathode to the anode. When these electrons hit the target, they are slowed down, producing the X-rays (Bremsstrahlung and X-ray fluorescence of the target).

This tube can not produce X-rays continuously. It is no longer used on modern devices.

See also Crookes tube and glow discharge tube.

[edit] Coolidge tube

The Crookes tube was improved by William Coolidge in 1913. The Coolidge tube, also called hot cathode tube, is the most widely used. It works with a very good quality vacuum (about 10^-4 Pa, or 10^-6 Torr).

In the Coolidge tube, the electrons are produced by thermionic effect from a tungsten filament heated by an electric current. The filament is the cathode of the tube. The high voltage potential is between the cathode and the anode, the electrons are thus accelerated, and then hit the anode.

There are two designs: end-window tubes and side-window tubes.

In the end-window tubes, the filament is around the anode, the electrons have a curved path.

What is special about side-window tubes is:

• An Electrostatic Lens to focus the beam onto a very small spot on the anode
• The anode is specially designed to dissipate the heat and wear resulting from this intense focused barrage of electrons:
  • Mechanically spun to increase the area heated by the beam.
  • Cooled by circulating coolant.
• The anode is precisely angled at 1-20 degrees off perpendicular to the electron current so as to allow escape of some of the X-ray photons which are emitted essentially perpendicular to the direction of the electron current.
• The anode is usually made out of tungsten or molybdenum.
• The tube has a window designed for escape of the generated X-ray photons.

The power of a Coolidge tube usually ranges from 1 to 4 kW.

[edit] Rotating anode tube

The rotating anode tube is an improvement of the Coolidge tube. Because X-ray production is very inefficient (99% of incident energy is converted to heat) the dissipation of heat at the focal spot is one of the main limitations on the power which can be applied. By sweeping the anode past the focal spot the heat load can be spread over a larger area, greatly increasing the power rating. With the exception of dental X-ray tubes, almost all medical X-ray tubes are of this type.

The anode consists of a disc with an annular target close to the edge. The anode disc is supported on a long stem which is supported by bearings within the tube. The anode can then be rotated by electromagnetic induction from a series of stator windings outside the evacuated tube.

Because the entire anode assembly has to be contained within the evacuated tube, heat removal is a serious problem, further exacerbated by the higher power rating available. Direct cooling by conduction or convection, as in the Coolidge tube, is difficult. In most tubes, the anode is suspended on ball bearings with silver powder lubrication which provide almost negligible cooling by conduction.

A recent development has been liquid gallium lubricated fluid dynamic bearings which can withstand very high temperatures without contaminating the tube vacuum. The large bearing contact surface and metal lubricant provide an effective method for conduction of heat from the anode.

The anode must be constructed of high temperature materials. The focal spot temperature can reach 2500°C during an exposure, and the anode assembly can reach 1000°C following a series of large exposures. Typical materials are a tungsten-rhenium target on a molybdenum core, backed with graphite. The rhenium makes the tungsten more ductile and resistant to wear from impact of the electron beams. The molybdenum conducts heat from the target. The graphite provides thermal storage for the anode, and minimizes the rotating mass of the anode.

Increasing demand for high-performance CT scanning and angiography systems has driven development of very high performance medical X-ray tubes. Contemporary CT tubes have power ratings of up to 100 kW and anode heat capacity of 6 MJ (8 million heat units), (the heat unit is a simple product of voltage times current - which given the inefficiency of the X-ray tube approximates to the heat input) yet retain an effective focal spot area of less than 1 mm².