X-ray or gamma ray systems or devices – Source – Electron tube
Reexamination Certificate
1999-10-28
2001-09-25
Dunn, Drew (Department: 2882)
X-ray or gamma ray systems or devices
Source
Electron tube
C378S125000, C378S127000, C384S615000
Reexamination Certificate
active
06295338
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to rotating anode x-ray tube technology and is particularly related to apparatus that improves cooling and reduces heating of x-ray tube bearings. The invention also improves the ability of the x-ray tube bearing assembly to handle mechanical loads associated with larger rotating anodes and Computed Tomography (CT) systems.
Typically, an x-ray tube housing assembly includes an x-ray tube having an 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 x-ray tube supported within. The housing is filled with a fluid that surrounds the envelope, such as oil, to aid in cooling the x-ray tube by absorbing heat radiated from internal components of the x-ray tube.
In
FIG. 1
, a prior art x-ray tube
120
is schematically shown illustrating a common bearing assembly construction that limits bearing cooling effectiveness and bearing size, thereby limiting the thermal and mechanical loading of the bearings. The x-ray tube
120
includes a cathode assembly
122
, an anode assembly
124
, and an envelope
126
. A housing
128
encloses the x-ray tube
120
and is filled with a cooling oil, or other suitable medium, which surrounds the tube
120
.
The cathode assembly
122
includes a cathode focusing cup and at least one cathode filament. A support bracket mounts the cathode cup within the envelope. Electrical conductors are attached to the focusing cup and cathode filament. The conductors provide an appropriate source of electrical energy to each of the cup and filament respectively.
The anode assembly
124
includes a circular anode disk
130
that is mounted on a stem
132
in a conventional manner. A typical annular target area is located about the peripheral edge of the anode disk. The stem
132
is attached to a bearing shaft
133
which defines inner bearing races
134
,
136
. An outer bearing member
146
is frictionally received in a high purity copper bearing housing
148
. Outer bearing races
142
,
144
are formed in the outer bearing member
146
. A plurality of ball or other bearing members
140
are received between the inner bearing races
134
,
136
and the outer bearing races
142
,
144
. The bearing housing
148
is attached to a non-electrically conducting portion of the envelope
128
with a bolt
125
.
An induction motor
150
rotates the anode assembly
124
. The induction motor includes a stator having driving coils
152
which are positioned outside the vacuum envelope
126
. A rotor assembly
154
inside the envelope encloses the bearing assembly and is operatively attached to the anode stem
132
. The rotor assembly
154
includes a cylindrical sleeve
156
attached in a known manner to a generally cylindrical support member
155
connected to the stem
132
. Typically the sleeve
156
, is formed of a thermally and electrically conductive material such as copper. When the motor is energized, the rotor assembly
154
rotates within the envelope
126
.
In order to produce x-rays, the cathode filament is heated with an electric current such that thermonic emission occurs thereby producing a cloud of electrons. A high electrical potential, on the order of 100-200 kV, is applied across the cathode assembly and anode assembly. This potential causes the emitted electrons to flow from the cathode through the evacuated region in the envelope to the target on the rotating anode. The cathode cup focuses the electrons into a beam that is directed onto the annular target track. The electron bean impinges the target with sufficient energy that x-rays are generated.
The electron beam produces substantial heat when striking the anode during x-ray generation. Rotating anode configurations have been adopted to distribute the thermal loading created during the production of x-rays. Each portion along the path of the annular target portion 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 Computed Tomography (CT), the generation of x-rays often causes the anode assembly to be heated to a temperature range of 1200-1400° C., for example.
During operation of the x-ray tube, the x-ray tube is cooled by use of oil or other cooling fluid that surrounds the evacuated envelope and flows within the housing. The oil serves to absorb heat radiated by the anode assembly through the envelope. However, a portion of the heat radiating from the anode
130
is also absorbed by the rotor and bearing assembly. In addition, some heat is conducted from the anode
130
along the stem
132
into the bearing assembly. Some of the heat in the bearing assembly is radiated through the envelope
126
and a portion of the heat is conducted to the end of the bearing housing
148
near the mounting bolt
125
. These mechanisms for removing heat from the bearing assembly are inefficient and result in bearing assembly component temperatures higher than desired.
Present x-ray tubes, as shown in
FIG. 1
, have a number of components surrounding the bearings such as the bearing housing and the rotor for the induction motor. These components (i) limit the efficiency of heat removal from the bearings, and (ii) limit the size of the bearing assembly components for a given tube and thus their ability to handle larger mechanical loads. As a result of the limits on the cooling of the bearing assembly, bearing temperatures of approximately 400° C. are common in many high powered x-ray tube applications. Unfortunately, such high temperatures may deleteriously effect bearing performance. For instance, prolonged and/or excessive heating of the lubricant applied to each ball of a bearing can reduce the effectiveness of the lubricant. In addition, the lubricant can be boiled off causing contamination of the vacuum in the x-ray tube. Further, prolonged and/or excessive heating may also shorten the life of the bearings, and thus the life of the x-ray tube. For these reasons it is desirable to (i) reduce the amount of heat that reaches the bearings and (ii) effectively remove heat in the bearings, regardless of its source.
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 between the anode and the bearing assembly. The heat shield serves to protect the bearing assembly by intercepting a portion of the heat radiated from the anode
130
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
130
and a portion of the radiated heat will be absorbed by the bearing assembly. Additionally, although the heat shield is useful in reducing heat transfer to the bearing assembly, the heat shield does not play a role in cooling or removing heat already absorbed in the bearing assembly. Furthermore, given that the bearing assembly is enclosed by the rotor, the bearing assembly is not able to efficiently radiate heat to the cooling fluid contained in the housing. Thus, once heat has been transferred to the bearing assembly it is not readily dissipated.
Another disadvantage caused by the limit on bearing temperature is that various processes during manufacture of the tube, such as exhausting and seasoning the tube, are deleteriously affected. Exhausting the tube is the process in which vacuum is drawn in the tube. The tube is operated with internal components at high temperatures while a vacuum pump is operatively attached to the tube. The rate at which gas is removed from the tube and the resulting final pressure of the tube are related to the temperature of the components, such as the anode, during exhaust. The higher the temperature of the component the more effectively the gas is removed from the tube and the lower the pressure of the tube after exhaust. The bearing temperature limit results in reducing the temperature at which the compon
Carlson Gerald J.
Harris Jason P.
Kuzniar Daniel E.
Clair Eugene E.
Dunn Drew
Fry John J.
Marconi Medical Systems, Inc.
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