Conventional diagnostic use of x-radiation includes the form of radiography, in which a still shadow image of the patient is produced on x-ray film, fluoroscopy, in which a visible real time shadow light image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient, and computed tomography (CT) in which complete patient images are electrically reconstructed from x-rays produced by a high powered x-ray tube rotated about a patient's body.
Typically, a high power x-ray tube includes an evacuated envelope made of metal or glass which holds a cathode filament through which a heating current is passed. This current heats the filament sufficiently that a cloud of electrons is emitted, i.e. thermionic emission occurs. A high potential, on the order of 100-200 kV, is applied between the cathode and an anode which is also located in the evacuated envelope. This potential causes the electrons to flow from the cathode to the anode through the evacuated region in the interior of the evacuated envelope. A cathode focusing cup housing the cathode filament focuses the electrons onto a small area or focal spot on the anode. The electron beam impinges the anode with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through an x-ray transmissive window of the envelope to a beam limiting device, or collimator, attached to an x-ray tube housing. The beam limiting device regulates the size and shape of the x-ray beam directed toward a patient or subject under examination thereby allowing images of the patient or subject to be reconstructed.
During the production of x-rays, many electrons from the electron beam striking the anode are reflected from the anode and fall upon other regions of the x-ray tube. The reflected electrons are often referred to as secondary electrons, and the act of such reflected electrons falling on other regions of the x-ray tube is often referred to as secondary electron bombardment. Secondary electron bombardment causes substantial heating to the regions in which the secondary electrons fall.
In x-ray tubes having a metal envelope, secondary electrons are often attracted to the metal envelope which is at ground potential. Thus, portions of the metal envelope closest to where the x-rays are being produced are often substantially heated during operation of the x-ray tube due to secondary electron bombardment. The region along the metal envelope closest to where the x-rays are produced also is the region in which the window is coupled to the metal envelope. An air tight junction between the window and the metal envelope is therefore made such that it can withstand high temperatures without failure. With an ongoing desire to provide x-ray tube producing higher power exposures and shorter imaging times, the intensity of the electron beam striking the anode continues to increase. Unfortunately, this in turn has caused the amount of secondary electron bombardment to proportionally increase thereby making it increasingly difficult to provide a reliable air tight junction between the window and the metal envelope.
One known method of reducing the amount of secondary electron bombardment occurring at a junction between the window and the metal frame is described in U.S. Pat. No. 5,511,104 assigned to Siemens Aktiengesellschaft. The '104 Patent provides a first electrode at anode potential and a second electrode at cathode potential positioned such that secondary electrons emanating from the anode must pass through a space between the first and second electrodes in order to reach the window. Since secondary electrons passing through the space are attracted to the electrode at anode potential, fewer secondary electrons reach the window thus avoiding excessive heating at the junction between the window and the envelope. One main drawback to the '104 patent is that x-ray tubes configured with this design are typically limited to single ended designs where the anode is at ground potential and the cathode is at -150,000 volts, for example. If a bi-polar arrangement was used in conjunction with the design described in the '104 patent where the anode was at a positive voltage potential (i.e. +75,000 volts) and the cathode was at a negative voltage potential (i.e. -75,000 volts), for example, positioning the electrodes such that arcing does not occur between the electrodes and the anode and/or the cathode becomes extremely difficult since placement of the electrodes between the anode and the cathode would likely alter the electric field concentration between these elements in a manner that would cause arcing to occur. Unfortunately this makes it difficult for such x-ray tubes to be used in a retrofit manner since most x-ray tubes have generators which are configured to handle only a bi-polar topology.
Therefore, what is needed is an apparatus for reducing the amount of secondary electron bombardment at a junction between the window and a metal envelope which overcomes the shortfall described above.