X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
2000-06-13
2002-09-17
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S127000, C378S141000
Reexamination Certificate
active
06453010
ABSTRACT:
TECHNICAL FIELD
The present invention relates to x-ray tube technology. More specifically, the present invention relates to a method and apparatus for directing cooling fluid supplied from a reservoir to (i) a bearing cooling apparatus and (ii) an x-ray tube housing chamber to reduce the heating effects on x-ray tube bearings caused by heat dissipated from the anode during operation.
BACKGROUND OF THE INVENTION
Conventional diagnostic use of x-radiation includes the forms of (i) radiography, in which a still shadow image of the patient is produced on x-ray film, (ii) 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 (iii) computed tomography (CT) in which complete patient images are digitally constructed from x-rays produced by a high powered x-ray tube rotated about a patient's body.
Typically, an x-ray tube includes an evacuated envelope made of metal or glass which is supported within an x-ray tube housing. The x-ray tube housing provides electrical connections to the envelope and is filled with a fluid such as oil to aid in cooling components housed within the envelope. The fluid is circulated through the housing and a heat exchanger external to the housing for removing heat from the cooling fluid. The envelope and the x-ray tube housing each include an x-ray transmissive window aligned with one another such that x-rays produced within the envelope may be directed to a patient or subject under examination.
In order to produce x-rays, the envelope houses a cathode assembly and an anode assembly. The cathode assembly includes 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 assembly and the anode assembly. This potential causes the electrons to flow from the cathode assembly to the anode assembly through the evacuated region in the interior of the envelope. A cathode focusing cup containing the cathode filament focuses the electrons onto a small area or focal spot on a target of the anode assembly. The electron beam impinges the target with sufficient energy that x-rays are generated. A portion of the x-rays generated pass through the x-ray transmissive windows of the envelope and x-ray tube housing to a beam limiting device, or collimator, attached to the 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 to be constructed.
In order to distribute the thermal loading created during the production of x-rays a rotating anode assembly configuration has been adopted for many applications. In this configuration, the anode assembly is rotated about an axis such that the electron beam focused on a focal spot of the target impinges on a continuously rotating circular path about a peripheral edge of the target. Each portion along the circular path becomes heated to a very high temperature during the generation of x-rays and is cooled as it is rotated before returning to be struck again by the electron beam. In many high powered x-ray tube applications such as CT, the generation of x-rays often causes the anode assembly to be heated to a temperature range of 1200-1400° C., for example.
In order to provide for rotation, the anode assembly is typically mounted to a rotor which is rotated by an induction motor. The rotor in turn is rotatably supported by a bearing assembly. The bearing assembly provides for a smooth rotation of the rotor and anode assembly about its axis. The bearing assembly typically includes at least two sets of ball bearings disposed in a bearing housing. The ball bearings often consist of a ring of metal balls which are lubricated by application of lead or silver to an outer surface of each ball thereby providing support to the rotor with minimal frictional resistance.
During operation of the x-ray tube, the anode assembly is passively cooled by use of oil or other cooling fluid flowing within the housing which serves to absorb heat radiated by the anode assembly through the envelope. However, a portion of the heat radiating from the anode assembly is also absorbed by the rotor and bearing assembly. For example, heat radiated from the anode assembly has been found to subject the bearing assembly to temperatures of approximately 400° C. in many high powered applications. Unfortunately, such heat transfer to the bearings may deleteriously effect the bearing performance. For instance, prolonged or excessive heating to the lubricant applied to each ball of a bearing can reduce the effectiveness of such lubricant. Further, prolonged and/or excessive heating may also deleteriously effect the life of the bearings and thus the life of the x-ray tube.
One known method to reduce the amount of heat passed from the anode assembly to the bearing assembly is to mechanically secure a heat shield to the rotor. The heat shield serves to protect the bearing assembly from a portion of the heat radiated from the anode assembly in the direction of the bearing assembly. Unfortunately, heat shields are not able to completely protect the bearing assembly from heat transfer from the anode assembly and a portion of the heat radiated will be absorbed by the bearing assembly. Additionally, although the heat shield is useful in preventing some heat transfer to the bearing assembly, the heat shield does not play a role in cooling the bearing assembly by removing heat already absorbed therein. Further, given that the bearing assembly is enclosed by the rotor, the bearing assembly is not able to easily radiate heat to the cooling fluid contained in the housing as done by the anode assembly. In fact, some rotor and bearing assembly configurations operate as a heat sink. For these reasons, a substantial amount of heat is typically transferred into the bearing assembly and the heat is not readily dissipated.
Another method to reduce heating of bearings is to pass cooling fluid through an internal conduit in the bearing assembly. For example, as described in U.S. Pat. No. 6,011,829, cooling fluid is supplied through two separate input tubes from a heat exchanger into the x-ray tube housing. A first supply tube provides cooling fluid through a first opening in the housing to be directed to a cooling fluid shaft along an inner surface of the bearing housing. A separate second supply tube provides cooling fluid through a second opening in the housing directly into the chamber surrounding the x-ray tube. A fluid flow regulator consisting of conventional valve controls is located outside the tube housing in the heat exchanger. The regulator valves control the flow rate of cooling fluid through each of the respective inlet tubes and openings in the housing wall. A third cooling fluid return port circulates the cooling fluid back to the heat exchanger. However, it is desirable to reduce the number of supply tubes, openings and fluid connections in the housing. In addition it is desirable to simplify the fluid flow regulator.
Therefore, what is needed is an apparatus for effectively and simply directing the appropriate volume of cooling fluid into each of (i) the chamber within the housing that surrounds the x-ray tube and (ii) the cooling fluid shaft along the inner surface of the bearing housing for the x-ray tube located within the housing.
SUMMARY OF THE INVENTION
In accordance with the present invention, an x-ray apparatus is provided. The x-ray apparatus includes a housing defining a chamber. The x-ray tube housing has a fluid input port. The x-ray tube includes a cathode assembly having a filament which emits electrons when heated, an anode assembly defining a target for intercepting the electrons such that collision between the electrons and the anode assembly generate x-rays from an anode focal spot and a bearing assembly rotatably supporting the anode asse
Lu Qing
Miller Thomas R.
Panasik Cheryl L.
Clair Eugene E.
Dunn Drew A.
Koninklijke Philips Electronics , N.V.
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