"In a field emission cathode, electrons are emitted as a result of a sufficiently high electric field being applied. Field emission is achieved e.g. by means of a simple diode mode in which a bias voltage is applied between anode and cathode. Electrons are emitted from the cathode when the electric field exceeds the threshold for the emission. A triode construction can also be provided in which a gate electrode is arranged close to the cathode. Electrons are emitted in this case as a result of a bias voltage being applied between gate and cathode. The emitted electrons are then accelerated by means of a high voltage between gate and anode. Field emission cathodes permit a very high, readily controllable and easily focusable electron beam current. Furthermore, the electron beam can also be deflected magnetically or electrostatically.
"Overall, by virtue of the field emission guns the invention has the advantages of low heat generation by the X-ray source and a low weight, attributable not only to the field emission guns themselves but also to the omission or reduction in size of a cooling system. Compared with conventional X-ray emitters, field emission guns are also extremely compact, which is a prerequisite for making a high-quality, planar X-ray source having a surface of many focal points arranged adjacent to one another possible in the first place. This is ensured in particular by an array having a plurality of field emission guns. The lifespan of field emission guns is also significantly longer than that of known X-ray emitters with thermal cathodes. Compared with a thermal cathode, a field emission cathode can also be started quickly without heating. By virtue of the easily focusable electron current a higher spatial resolution can additionally be achieved for X-ray images. All in all, an X-ray source having field emission guns is also particularly suitable for new applications in which rapid movements of the X-ray source or of the entire imaging systems consisting of X-ray source and X-ray detector are necessary.
"According to an embodiment of the invention the field emission cathode in each case has a nanostructured material with carbon nanotubes (a so-called CNT cathode). Materials of this kind exhibit particularly good emission characteristics, are stable even at high currents, and furthermore can be manufactured in a particularly small format. Alternatively the field emission cathodes can also have nanocrystalline graphite.
"The surface of the X-ray source is advantageously at least twice, in particular between twice and eight times, as large in size as the sensor surface of the X-ray detector. This enables the advantages of the inverse geometry to be exploited to optimal effect. The X-ray detector is beneficially formed by a digital flat-panel detector.
"According to another embodiment of the invention the field emission guns are arranged on a surface in a rectangular, elliptical or circular array.
"The X-ray source and X-ray detector are advantageously carried jointly by a C-arm or U-bracket. Such a C-arm or U-bracket can also be arranged on a multi-axis robot arm, in particular on an articulated arm robot having six axes of rotation, so that it can be moved along any trajectories.
"According to further embodiments of the invention the X-ray imaging system is formed by a fluoroscopy system or an angiography system or a projection X-ray system."
URL and more information on this patent, see: Boese, Jan;
Keywords for this news article include: Robotics, Machine Learning, Emerging Technologies,
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