Glossary of terms used on this site
All
- Bremsstrahlung (Braking Radiation)
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The term "bremsstrahlung" is retained from the original German term to describe the radiation which is emitted when electrons are decelerated or "braked" when they are fired at a metal target. Accelerated charges give off electromagnetic radiation, and when the energy of the bombarding electrons is high enough, that radiation is in the x-ray region of the electromagnetic spectrum. Bremsstrahlung is the continuum component of the X-ray spectrum* and is characterized by a continuous distribution of radiation which becomes more intense and shifts toward higher frequencies when the energy of the bombarding electrons is increased. The highest energy in the spectrum, that means the shortest wave length, corresponds to the maximum acceleration potential of the electrons. The higher the tube voltage, the higher the energy of the photons, the higher their capability of passing through material.
- Characteristic radiation
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The discrete part of the X-ray spectrum*. The electrons suddenly decelerate upon colliding with the material target and knock out electrons from the inner shell of the target atoms. As a result, electrons from higher energy levels fill up the vacancies and X-ray photons are emitted to compensate for the difference in energy between the two shells. The wavelength distribution of the characteristic radiation varies from material to material- different anode materials create more or less high-energy X-rays, thus directly influencing their capability to pass trough material.
- Contrast
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Radiographic contrast describes the differences in photographic density in a radiograph. The larger the difference in thickness or density between to areas of the subject, the larger the difference in radiographic density or contrast, the more visible features become. Contrast is caused by the fact that different parts of the object absorb X-rays differently. How great a difference in radiation absorption is necessary depends on the detector. With two parts of an object with radiation intensities I[A] and I[B], the difference in contrast is defined as 2*| I[A] � I [B] | / ( I[A] + I [B] ). As a rule of thumb, a difference in radiation absorption of 2% (0.5% for a digital detector) is needed in order for an image intensifier to produce a visible image. Moreover, contrast depends on the wavelength of the primary radiation.
- Coolidge, W.D.
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Inventor of the Coolidge tube, also called hot cathode tube (1913). It is still used today, since most X-ray tubes used today are based on the design of the Coolidge tube.
- Cosslett, V.E.
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This British physicist was the first to build a microfocus X-ray tube, based on a suggestion of M. v. Ardenne, and to use it in his �Shadow X-ray microscope� in 1951 (Nature 10, 1951, pp 24).
- Detail detectability
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The highest amount of detail that can be shown in an image. Defined by the size of the smallest object that can be conveniently viewed, which, for nanofocus and microfocus X-ray tubes, is about half the size of the focal spot*.
- Electromagnetic lens
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Electron-optical device used to focus the electron beam by means of a magnetic field. The magnetic lens is a rotationally symmetric electromagnet consisting of a wire coil, a magnetic iron yoke and iron pole pieces. Like optical lenses, magnetic lenses are characterized by focal length and principal planes.
- Filament
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Source of electrons in the X-ray tube. A thin tungsten wire (0.1- 0.5 mm) emitting electrons due to thermionic emission when in a vacuum and energized with electric current.
- Filter
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Thin plates made of materials such as iron, copper and aluminium that filter out lower energy (soft) X-rays, for instance to prevent overexposure.
- Image intensifier
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Electron-optical device transforming X-rays into optically visible rays, thus producing an amplified radioscopic image that can be captured by a CCD camera.
- Image resolution
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Image resolution has two main contributors: the size of the focal spot and the detector or image chain resolution. For nanofocus� and microfocus tubes, resolution lies in the range of several hundred of nanometers or in the very low micrometer range. X-ray image resolution is defined as the period length of the finest grid that can be comfortably viewed in the image. This grid may consist of gold structures on a silicon surface, but there are other alternatives as well. With a grid period length of 2 micron, for example, the spaces in between are 1 micron wide each. At the lowest magnification possible, image resolution equals detector resolution and can be as high as several hundred of micrometers. At very high magnifications, the resolution that can be obtained is limited by the size of the focal spot.