X-ray tube heat barrier

Details X-ray tube heat barrier X-ray tube heat barrier

2001-11-14

2004-03-16

Arana, Louis (Department: 2859)

X-ray or gamma ray systems or devices

Source

Electron tube

C378S141000

Reexamination Certificate

active

06707882

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention pertains to the vacuum tube arts, and in particular to a heat barrier for an x-ray tube. It finds particular application in conjunction with rotating anode x-ray tubes for CT scanners and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in the generation of radiation and in vacuum tubes for other applications.
Conventional diagnostic uses of x-radiation include shadowgraphic projection images of the patient on x-ray film or electronic pick-up, fluoroscopy, in which a visible real time shadowgraphic image is produced by low intensity x-rays impinging on a fluorescent screen after passing through the patient, and computed tomography (CT) in which projection images from many directions are electrically reconstructed into a volume reconstruction. A high powered x-ray tube is rotated about a patient's body at a high rate of speed to generate the projection images.
A high power x-ray tube typically includes a thermionic cathode and an anode, which are encased in an evacuated envelope. A heating current, commonly of the order of 2-5 amps, is applied through a filament or thin layer to create a surrounding electron cloud. A high potential, of the order of 100-200 kilovolts, is applied between the cathode and the anode to accelerate the electrons from the cloud towards the anode. The electrons are focused into an electron beam which impinges on a small area of the anode, or target area, with sufficient energy to generate x-rays. X-radiation is emitted from the anode and focused into a beam, typically through a beryllium window.
The acceleration of electrons causes a tube or anode current of the order of 5-200 milliamps. Only a small fraction of the energy of the electron beam is converted into x-rays, the majority of the energy being converted to heat which heats the anode white hot.
In high energy tubes, the anode rotates relative to the cathode at high speeds during x-ray generation to spread the heat energy over a large area and inhibit the target area from overheating. Due to the rotation of the anode, the electron beam does not dwell on the small impingement spot of the anode long enough to cause thermal deformation. The diameter of the anode is sufficiently large that in one rotation of the anode, each spot on the anode that was heated by the electron beam has substantially cooled before returning to be reheated by the electron beam.
The anode is typically rotated by an induction motor. The induction motor includes driving coils, which are placed outside the evacuated envelope, and a rotor supported by a bearing assembly, within the envelope, which is connected to the anode. When the motor is energized, the driving coils induce electric currents and magnetic fields in the rotor which cause the rotor to rotate.
The temperature of the anode can be as high as 1,400° C. Part of the heat is transformed through the vacuum by radiation. Part of the heat is transferred by conduction to the rotor, and to the bearings assembly. Heat travels through the bearing shaft to the bearing races and is transferred to the lubricated bearing balls in the races. The lubricants, typically lead or silver, on the bearing balls become hot and tend to evaporate.
One way to reduce bearing temperatures is to provide a thermal block to isolate the bearing lubricant from the heat of the target. A variety of thermal blocks have been developed for reducing the flow of heat from the anode to the bearing shaft. In one low power design, the rotor stem is brazed to a steel rotor body liner that is then screwed to the bearing shaft. This provides a slightly more thermally resistive path.
Another thermal block that has been used in the industry is known as a top-hat design. A top hat-shaped piece of low thermal conductivity material, such as Hastelloy™ or Inconel™, is screwed onto the hub of the x-ray bearing shaft. The rotor body is then attached to the brim of the top hat with screws, welds, or other fastening means. The thermal conduction path from the rotor body to the bearing is then extended by the length of the top hat. Analysis shows that a 20-50° C. temperature decrease may be achieved at the front bearing race when the top hat design is employed. Another thermal block uses a thin molybdenum cone with a highly reflective surface which is pinned to the stem connecting the target with the bearing assembly. The cone follows the contours the target, blocking the view of the target from the bearing assembly. The cone reflects heat radiating from the target, reducing the radiative mode of heat transfer to the bearing assembly.
Another method of reducing heat flow is to use a spiral groove bearing shaft. The spiral groove bearing is a relatively complex, large bearing that employs a gallium alloy to transfer heat. The bearing shaft is limited to a rotational speed of about 60 Hz. This limits operating power of the x-ray tube.
A trend toward shorter x-ray exposure times in radiography has placed an emphasis on having a greater intensity of radiation and hence higher electron currents. Increasing the intensity can cause overheating of the x-ray tube anode. As such higher power x-ray tubes are developed, the diameter and the mass of the rotating anode continues to grow. Further, when x-ray tubes are combined with conventional CT scanners, a gantry holding the x-ray tube is rotated around a patient's body in order to obtain complete images of the patient. Today, typical CT scanners revolve the x-ray tube around the patient's body at a rate of between 60-120 rotations-per-minute (RPM). This increased rotation speed has resulted in increased stresses on the rotor stem and bearing shaft. For the x-ray tube to operate properly, the anode needs to be supported and stabilized from the effects of its own rotation and, in some instances, from centrifugal forces created by rotation of the x-ray tube about a patient's body.
One way to reduce these stresses to a non-critical level is to reduce the length of the rotor stem while increasing the cross sectional area. This, however, shortens and widens the heat conduction path from the target to the bearing shaft, resulting in higher thermal transfer. Recently, x-ray tubes have been developed in which the anode surrounds the bearing shaft, as shown, for example, in U.S. Pat. No. 5,978,447. However, many of the conventional types of thermal radiation blocks, such as the cone design, are unsuited to use in such a configuration, since there is no stem to which a cone may be attached.
The present invention provides a new and improved x-ray tube and method which overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, an x-ray tube is provided. The x-ray tube includes an envelope which encloses an evacuated chamber. A cathode disposed within the chamber provides a source of electrons. An anode disposed within the chamber is positioned to be struck by the electrons and generate x-rays. A bearing assembly is surrounded by the anode, the bearing assembly including a stationary portion and a rotatable portion. The rotatable portion is connected with the anode and rotates with the anode relative to the stationary portion during operation of the x-ray tube. A heat shield between the bearing assembly and the anode reduces the radiative transfer of heat from the anode to the bearing assembly.
In accordance with another aspect of the present invention, an x-ray tube is provided. The x-ray tube includes an envelope which defines an evacuated chamber. A cathode is disposed within the chamber for providing a source of electrons. An anode is disposed within the chamber and positioned to be struck by the electrons and generate x-rays. A bearing assembly is concentrically aligned with the anode. The bearing assembly includes a rotating portion connected with the anode by a shaft and a stationary portion thermally connected with a heat sink outside the envelope. A first g

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