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Monday, November 19, 2007

X-ray Machine

Researched by Alex
An X-ray machine utilizes electromagnetic radiation to produce an image of an object, usually with the purpose of visualizing something located below the object's surface. The machine is made up of an X-ray source or X-ray tube, an x-ray detection system, and positioning hardware to align these two components with the object to be imaged.

X-rays are produced by bombarding a surface with high speed electrons (in a vacuum). 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.

How they work

An X-ray imaging system consists of a X-ray source or generator, and an image detection system which can either be comprised of film (analog technology) or a digital capture system.

X-ray Sources

In the typical X-ray source of less than 450 kV, X-ray photons are produced by an electron beam striking a target. The electrons that make up the beam are emitted from a heated cathode filament. The electrons are then focused and accelerated towards an angled anode target. The point where the electron beam strikes the target is called the focal spot. Most of the kinetic energy contained in the electron beam is converted to heat, but around 10% of the energy is converted into X-ray photons, the excess heat is dissipated via a heat sink. At the focal spot, X-ray photons are emitted at 180° from the target surface, the highest intensity being around 60° to 90°, there is a small round window in the X-ray tube directly above the angled target. This window allows the X-ray to exit the tube with little attenuation while maintaining a vacuum seal required for the X-ray tube operation.

X-ray machines work by applying controlled voltage, current, and time to the X-ray tube, which results in a beam of X-rays. The beam is projected on matter. Some of the X-ray beam will pass through the object, while some are reflected. The resulting pattern of the radiation is then ultimately detected by a detection medium including rare earth screens (which surround photographic film), semiconductor detectors, or X-ray image intensifiers.

X-Ray Detection

In health care applications in particular, the x-ray detection system rarely consists of the detection medium. For example, a typical stationary radiographic x-ray machine also includes an ion chamber and grid. The ion chamber is basically a hollow plate located between the detection medium and the object being imaged. It determines the level of exposure by measuring the amount of x-rays that have passed through the electrically charged, gas-filled gap inside the plate. This allows for minimization of patient radiation exposure by both ensuring that an image is not underdeveloped to the point the exam needs to be repeated and ensuring that more radiation than needed is not applied. The grid is usually located between the ion chamber and object and consists of several lead slats stacked next to each other (resembling open window blinds). In this manner, the grid allows straight x-rays to pass through to the detection medium but absorbs reflected x-rays. This improves image quality by preventing reflected (non-diagnostic) x-rays from reaching the detection medium allowing for lower exam doses overall.

Images taken with such devices are known as X-ray photographs or radiographs. The older term Röentgenogram continues to be used by radiologists.

Medical Uses

There are two basic areas in which Health Care uses X-radiation; Radiography, and Dental Hygine.

Radiography is used for fast, highly penetrating images. Usually it's used on areas with a high bone content. Some forms of radiography uses are Panoramic X-rays, Radiography, Mammography, Tomography, and Radiotherapy.

Fluoroscopy is used in cases where real-time visualization is necessary. You may have seen a type of fluorography at the airport. Some of the uses of Fluorography are Angiography, barium enemas, barium swallows, biopsies, and hip replacement.

X-rays are highly penetrating, ionizing radiation, and X-ray machines are used in radiology to take pictures of 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 gray (closer to white). This can be used to diagnose broken or fractured bones. Imaging of the digestive tract is done with the help of a radiocontrast agent such as barium sulfate, which is opaque to X-rays.

Source: Wikipedia

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