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An X-ray generator is a device that produces X-rays. Together with an X-ray detector, it is commonly used in a variety of applications including medicine, fluorescence, electronic assembly inspection, and measurement of material thickness in manufacturing operations. In medical applications, X-ray generators are used by radiographers to acquire x-ray images of the internal structures (e.g., bones) of living organisms, and also in sterilization.
An X-ray tube is a simple vacuum tube that contains a cathode, which directs a stream of electrons into a vacuum, and an anode, which collects the electrons and is made of tungsten to evacuate the heat generated by the collision. When the electrons collide with the target, about 1% of the resulting energy is emitted as X-rays, with the remaining 99% released as heat. Due to the high energy of the electrons that reach relativistic speeds the target is usually made of tungsten even if other material can be used particularly in XRF applications.
An X-ray generator also needs to contain a cooling system to cool the anode; many X-ray generators use water or oil recirculating systems.
In medical imaging applications, an x-ray machine has a control console that is used by a radiologic technologist to select x-ray attributes suitable for the specific exam, a power supply that creates and produces the desired kVp (peak kilovoltage), mA (milliamperes, sometimes referred to as mAs which is actually mA multiplied by the desired exposure length) for the x-ray tube, and the x-ray tube itself.
The discovery of X-rays came from experimenting with Crookes tubes, an early experimental electrical discharge tube invented by English physicist William Crookes around 1869-1875. In 1895, Wilhelm Röntgen discovered X-rays emanating from Crookes tubes and the many uses for X-rays were immediately apparent. One of the first X-ray photographs was made of the hand of Röntgen's wife. The image displayed both her wedding ring and bones. On January 18, 1896 an X-ray machine was formally displayed by Henry Louis Smith. A fully functioning unit was introduced to the public at the 1904 World's Fair by Clarence Dally.
In the 1940s and 1950s, X-ray machines were used in stores to help sell footwear. These were known as Shoe-fitting fluoroscopes. However, as the harmful effects of X-ray radiation were properly considered, they finally fell out of use. Shoe-fitting use of the device was first banned by the state of Pennsylvania in 1957. (They were more a clever marketing tool to attract customers, rather than a fitting aid.) Together with Robert J. Van de Graaff, John G. Trump developed one of the first million-volt X-ray generators.
An X-ray imaging system consists of a generator control console where the operator selects desired techniques to obtain a quality readable image(kVp, mA and exposure time), an x-ray generator which controls the x-ray tube current, x-ray tube kilovoltage and x-ray emitting exposure time, an X-ray tube that converts the kilovoltage and mA into actual x-rays and an image detection system which can be either a film (analog technology) or a digital capture system and a PACS.
X-ray machines are used in health care for visualising bone structures, during surgeries (especially orthopedic) to assist surgeons in reattaching broken bones with screws or structural plates, assisting cardiologists in locating blocked arteries and guiding stent placements or performing angioplasties and for other dense tissues such as tumours. Non-medicinal applications include security and material analysis.
The main fields in which x-ray machines are used in medicine are radiography, radiotherapy, and fluoroscopic type procedures.
Radiography is generally used for fast, highly penetrating images, and is usually used in areas with a high bone content but can also be used to look for tumors such as with mammography imaging. Some forms of radiography include:
- orthopantomogram — a panoramic x-ray of the jaw showing all the teeth at once
- mammography — x-rays of breast tissue
- tomography — x-ray imaging in sections
- angiography — used to examine blood vessels in real time along with the placement of stents and other procedures to repair blocked arteries.
- barium enema — a procedure used to examine problems of the colon and lower gastrointestinal tract
- barium swallow — similar to a barium enema, but used to examine the upper gastrointestinal tract
- biopsy — the removal of tissue for examination
X-rays are highly penetrating, ionizing radiation, therefore X-ray machines are used to take pictures of dense tissues such as bones and teeth. This is because bones absorb the radiation more than the less dense soft tissue. X-rays from a source pass through the body and onto a photographic cassette. Areas where radiation is absorbed show up as lighter shades of grey (closer to white). This can be used to diagnose broken or fractured bones.
X-ray machines are used to screen objects non-invasively. Luggage at airports and student baggage at some schools are examined for possible weapons, including bombs. Prices of these Luggage X-rays vary from $50,000 to $300,000. The main parts of an X-ray Baggage Inspection System are the generator used to generate x-rays, the detector to detect radiation after passing through the baggage, signal processor unit (usually a PC) to process the incoming signal from the detector, and a conveyor system for moving baggage into the system. Portable pulsed X-ray Battery Powered X-ray Generator used in Security as shown in the figure provides EOD responders safer analysis of any possible target hazard.
When baggage is placed on the conveyor, it is moved into the machine by the operator. There is an infrared transmitter and receiver assembly to detect the baggage when it enters the tunnel. This assembly gives the signal to switch on the generator and signal processing system. The signal processing system processes incoming signals from the detector and reproduce an image based upon the type of material and material density inside the baggage. This image is then sent to the display unit.
The colour of the image displayed depends upon the material and material density : organic material such as paper, clothes and most explosives are displayed in orange. Mixed materials such as aluminum are displayed in green. Inorganic materials such as copper are displayed in blue and non-penetrable items are displayed in black (some machines display this as a yellowish green or red). The darkness of the color depends upon the density or thickness of the material.
The material density determination is achieved by two-layer detector. The layers of the detector pixels are separated with a strip of metal. The metal absorbs soft rays, letting the shorter, more penetrating wavelengths through to the bottom layer of detectors, turning the detector to a crude two-band spectrometer.
Advances in X-ray technology
A film of carbon nanotubes (as a cathode) that emits electrons at room temperature when exposed to an electrical field has been fashioned into an X-ray device. An array of these emitters can be placed around a target item to be scanned and the images from each emitter can be assembled by computer software to provide a 3-dimensional image of the target in a fraction of the time it takes using a conventional X-ray device. The system also allows rapid, precise control, enabling prospective physiological gated imaging.
Engineers at the University of Missouri (MU), Columbia, have invented a compact source of x-rays and other forms of radiation. The radiation source is the size of a stick of gum and could be used to create portable x-ray scanners. A prototype handheld x-ray scanner using the source could be manufactured in as soon as three years.
- Backscatter X-ray e.g., for security scanning passengers (rather than baggage)
- X-ray crystallography
- X-ray fluorescence
- X-ray astronomy Just detectors.
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