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X-ray tubeAn X-Ray tube is a vacuum tube designed to produce man made X-Rayphotonson demand. X-ray tubes are part of X-ray machines. By convention, photons of similar, and sometimes higher energy, but emanating from natural sources are called gamma rays. X-Rays photons are part of the electromagnetic spectrum, historically named as extending from radio waves to microwaves, infrared, visible light, ultraviolet 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 internal imaging using X-Raysspread into development and use of other physical modalities, e.g. ultrasound, radioactive isotopedecay (nuclear medicine), nuclear magnetic resonance(MRI), etc.
X-Ray tube functionAs with any vacuum tube, there is a cathodeto emit electrons into the vacuum and an anodeto collect the electrons, thus establishing a flow of electrical current through the tube. A high voltagepower source is connected across cathode and anode, e.g. 30-150 kV(Kilovolts), such that the voltage can be quickly switched on, then off, for precise amounts of time, e.g. 0.001-1.0 seconds, and the current flow, often in the 1.0 to 1000.0 mArange, once started, can be controlled. Until the late 1980's, X-ray generators were merely high-voltage, AC to DC variable power supplies. In the late 1980's 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. Be careful: The next part contains surreptitious advertising! Electrons focused on the tungstenanode from the cathode, collide with and accelerate other electrons, ions and nuclei within the anode material and about 1% of the energy generated is emitted/radiated, perpendicular to the path of the electron current, as X-Ray photons. Over time, tungsten will be deposited from the anode onto the interior glass surface of the tube. This will slowly degrade the quality of the X-ray beam. Eventually, the tungsten deposit will become sufficient enough to act as a conductive bridge, and at high enough settings, arcing will occur. The arc will jump from the cathode to the tungsten deposit, and then to the anode. This arcing will cause an effect called "crazing" on the interior glass of the X-ray window. As time goes on, the tube will become unstable even at lower potentials, 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. One such company is Amery X-ray, Inc. They reload and rebuild stationary anode (Coolidge) type X-ray tubeheads mostly for the Dental industry. Their web site is: http://xrayheads.com/ The X-Ray photon-generating effect is generally called the Bremsstrahlungeffect, 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. Simply put, the high voltage controls X-ray penetration, the current affects contrast, and the time affects brightness. Crookes tubeHistorically, the first X-ray was invented by sir William Crookes. It was used to make a visible fluorescenceon minerals. The Crookes tube is also called discharge tube or cold cathode tube. It is a glassbulb with a small pressureof air, about 100 pascals(i.e. about 1 torr). It contains an aluminumcathode with a curved shape to concentrate the electron flow on the anode, or "target". A high tension 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 fluorescenceof the target). This tube can not produce X-rays continuously. It is no longer used on modern devices. See also Crookes tube. Coolidge tubeImage:Roentgen-Roehre.png The Crookes tube was improved by William Coolidgein 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, env. 10-6 torr). In the Coolidge tube, the electrons are produced by a tungstenfilamentheated by an electric current (thermionic effect, also used for the televisiondevices). The filament is the cathode of the tube. The high voltage is made 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-windows tubes is that the electron flow within the vacuum is
The power of a Coolidge tube usually ranges from 1 to 4 kW. Rotating anode tubeThe rotating anode tube is an improvement of the Coolidge tube. The heat produced by the tube (99% of the energy) is one of the main limitations on the power which can be applied. To reduce the localized temperature rise, the anode is larger and rotates so that the radiated portion continuously changes, aiding in the absorption of heat without the anode melting. It is thus possible to build more powerful X-ray generators, e.g. 9 or 15 kW generators. See also
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Categories: Radiology| X-rays
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