X-ray

An X-ray (or Röntgen ray) is a form of electromagnetic radiation with a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30 PHz to 30 EHz. They are longer than Gamma rays but shorter than UV rays. X-rays are primarily used for diagnostic radiography and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous. In many languages it is called Röntgen radiation after one of the first investigators of the X-rays, Wilhelm Conrad Röntgen.

Structures that are dense (such as bone) will block most of the x-ray particles, and will appear white. Metal and contrast media (special dye used to highlight areas of the body) will also appear white. Structures containing air will be black and muscle, fat, and fluid will appear as shades of gray.

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How the Test is Performed
Much like conventional photography, motion causes blurry images on radiographs, and thus, patients may be asked to hold their breath or not move during the brief exposure (about 1 second).
The test is performed in a hospital radiology department or in the health care provider's office by an x-ray technologist. The positioning of the patient, x-ray machine, and film depends on the type of study and area of interest. Multiple individual views may be requested.

How the Test Will Feel
There is no discomfort from x-ray exposure. Patients may be asked to stay still in awkward positions for a short period of time.

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Risks

For the exposures encountered in conventional radiography, the risk of cancer or heritable defects (due to damaged ovarian cells or sperm cells) is very low. Most experts feel that this low risk is largely outweighed by the benefits of information gained from appropriate imaging. X-rays are monitored and regulated to provide the minimum amount of radiation exposure needed to produce the image.

During a single radiograph, a small fraction of the x-rays pass right through the body. The remaining photons are absorbed by tissues in the body. The energy of the absorbed photons can break apart (ionize) compounds, and this may cause cell damage. Most cell damage is soon repaired. However, some is permanent.
Young children and developing fetuses carried by pregnant women are more sensitive to the risks of x-rays. Women should tell health care providers about suspected pregnancy.

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X-ray Sources
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.

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 180deg from the target surface,the highest intensity being around 60deg to 90deg 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.

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