Information
-
Patent Grant
-
6256375
-
Patent Number
6,256,375
-
Date Filed
Monday, March 29, 199925 years ago
-
Date Issued
Tuesday, July 3, 200123 years ago
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Inventors
-
Original Assignees
-
Examiners
- Kim; Robert H.
- Ho; Allen C.
Agents
- Haushalter; Barbara Joan
- Cabou; Christian G.
- Price; Phyllis Y.
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CPC
-
US Classifications
Field of Search
US
- 378 119
- 378 121
- 378 122
- 378 125
- 378 136
- 378 144
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International Classifications
-
Abstract
A rotating x-ray tube comprises an anode assembly having an anode target for distributing heat generated at a focal spot. The x-ray tube further comprises a cathode assembly for producing X-rays upon impact with the anode. A target angle matching cathode cup structure is associated with the cathode assembly. The angled cathode cup structure creates a parallel surface between the cathode and the target to provide a focused electron beam.
Description
TECHNICAL FIELD
The present invention relates to rotating x-ray tubes and, more particularly, to rotating x-ray tubes which employ an angled cathode structure to maintain parallelism of the surface between the cathode and the x-ray target.
BACKGROUND ART
X-ray tubes used in medical diagnostic imaging are built with a rotating anode structure for the purpose of distributing the heat generated at the focal spot. The anode is rotated by an induction motor consisting of a cylindrical rotor built into a cantilevered axle that supports the disc shaped anode target, and an iron stator structure with copper windings that surrounds the elongated neck of the x-ray tube that contains the rotor. The rotor of the rotating anode assembly being driven by the stator which surrounds the rotor of the anode assembly is at anodic potential while the stator is referenced electrically to ground. The X-ray tube cathode provides a focused electron beam which is accelerated across the anode-to-cathode vacuum gap and produces X-rays upon impact with the anode.
In designing the cathode for an x-ray tube, the surface of the cathode is typically desired to be parallel to the target. However, there are instances that the surface between the cathode and the target are not paralleled by design. Unfortunately, if the surface between the cathode and the target is not paralleled, the focal spot will be skewed. The skew of the focal spot could cause the tube to be out of specification, resulting in failure. The focal skew is caused by the electron beams accelerating from the cathode and landing at different coordinates on the target because of the non-parallel surface.
In order to minimize focal skew, it is desirable for the surface between the cathode and the target to be parallel.
BRIEF SUMMARY OF THE INVENTION
The present invention provides for a target angle matching cathode design for an x-ray tube, for generating the desired parallel surface between the cathode and the target. Using the target angle matching technique to maintain parallelism of the surface between the cathode and the target minimizes the opportunity of the focal spot from being skewed.
A rotating x-ray tube comprises an anode assembly and a target. The target, associated with the anode assembly, distributes heat generated at a focal spot. A cathode produces X-rays upon impact with the anode. A target angle matching cathode cup creates a parallel surface between the cathode and the target. The target angle matching cathode cup maximizes the gap between a frame containing the cathode and anode, and the cathode. The angled cathode cup configuration also minimizes focal spot skew.
Accordingly, the present invention provides a parallel surface between the cathode and the target. This provides the advantage of minimizing focal skew.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
is a prior art cross-sectional illustration of a typical x-ray tube;
FIGS. 2-4
illustrate the relationship between the cathode and the target of an x-ray tube, such as is illustrated in
FIG. 1
, and certain of the problems with such a structure; and
FIG. 5
is a cross-sectional illustration of a cathode constructed applying the teachings of the present invention, to overcome certain of the problems illustrated by FIGS.
2
-
4
.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings,
FIG. 1
illustrates a typical prior art X-ray tube
10
. The x-ray tube
10
has a rotating anode assembly
12
, including an anode target
22
, for the purpose of distributing the heat generated at a focal spot, and an x-ray tube cathode assembly
14
for providing a focused electron beam which is accelerated across a large anode-to-cathode vacuum gap
16
and produces X-rays upon impact with the anode.
Continuing with
FIG. 1
, the anode assembly
12
is rotated by an induction motor comprising a cylindrical rotor
18
built around a cantilevered axle
20
. The cantilevered axle
20
supports the disc shaped anode target
22
connected via the hub and stud
24
to rotor
18
and cantilevered axle
20
which contains bearings facilitating rotation. The stator
26
of the induction motor includes an iron stator core
28
with copper windings
30
that surround the rotor
18
. The rotor
18
of the rotating anode assembly
12
, driven by the stator
26
, is at anodic potential while the stator is referenced electrically to ground. Since the stator
26
operates at or near ground potential and the anode may be raised to 75,000 VDC positive with respect to ground, a large air gap
32
, typically on the order of 0.400 inches or greater, is needed to maintain stable, discharge free operation. In a typical assembly, the anode assembly
12
and the cathode assembly
14
are sealed in a glass frame
40
and mounted in a conductive metal housing
42
, and an insulation material
44
is provided between the stator
26
, and the glass frame
40
and rotor
18
.
Referring now to
FIGS. 2-4
, in designing a cathode
34
, associated with cathode structure
14
of
FIG. 1
, it is generally desirable to design the cathode so that its surface
36
facing the target
22
is parallel to the target, as shown in FIG.
2
. Electrons
38
transfer between the surface
36
and target
22
. However, in some x-ray tube designs, the surface
36
between the cathode
34
and the target
22
are not parallel, such as is illustrated in
FIGS. 3 and 4
. If the surface between the cathode and the target is not parallel, the focal spot
46
will be skewed, as illustrated in FIG.
4
. As can be seen, the focal skew is caused by electron beams accelerating from the cathode and landing at different coordinates on the target.
There are certain design options that can generate the paralleled surface between the cathode and the target. In the prior art, it is common practice to have the cathode arm at the same angle as the target. However, such a design reduces the spacing between the cathode and the frame which could cause high voltage arcs between the cathode and the frame. The present invention therefore proposes a target angle matching cathode structure
48
, as is illustrated in FIG.
5
. Rather than angle the cathode support arm
54
, the cathode cup
56
is angled to match the angle of the target surface
58
. The target angle matching technique for matching the angle of the target and the angle of the cathode generates a parallel surface
50
between the cathode and the target
22
. This, in turn, minimizes the opportunity of the focal spot
46
in
FIG. 4
to be skewed. That is, since KV
1
is significantly less in the target angle matching structure of
FIG. 5
, as compared to the angled cathode arm structure of the prior art, the focal skew is minimized.
It is a further advantage of the angle matching cathode structure of the present invention that increasing the gap or distance D between the cathode and the frame
60
. Referring to
FIG. 5
, the electric field is equal to the value of KV
1
minus KV
2
, divided by the distance D. Thus, the increased distance D minimizes the probability of occurrence of discharges between the angled cathode
56
and the frame
60
. Additionally, the increased distance D decreases the potential for high voltage arcing between the angled cathode
56
and the frame
60
. Furthermore, when space limitations exist, the structure of the present invention is advantageous over the prior art.
It will be obvious to those skilled in the art that various modifications and variations of the present invention are possible without departing from the scope of the invention, which provides for a parallel surface between the cathode and the target in an x-ray tube assembly, by providing a target angle matching cathode structure.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
Claims
- 1. A rotating x-ray tube comprising:an anode assembly; a target associated with the anode assembly for distributing heat generated at a focal spot; a cathode for emitting a high-energy electron beam for focus on the anode; and a target angle matching cathode cup for creating a parallel surface between the cathode and the target.
- 2. A rotating x-ray tube as claimed in claim 1 further comprising a frame for containing and sealing the cathode and anode assembly.
- 3. A rotating x-ray tube as claimed in claim 2 further comprising a gap between the cathode and the frame.
- 4. A rotating x-ray tube as claimed in claim 3 wherein the target angle matching cathode cup maximizes the gap.
- 5. A rotating x-ray tube as claimed in claim 1 wherein focal spot skew is minimized.
- 6. A cathode structure for an x-ray tube, the x-ray tube having an x-ray tube target for distributing heat generated at a focal spot, the cathode structure comprising:a cathode for emitting a high-energy electron beam for focus on an x-ray emissive region of an anode; a linear cathode support arm for supporting the cathode and the cathode structure; and a cathode cup angled to match in parallel relation a facing surface of the target.
- 7. A cathode structure as claimed in claim 6 wherein the cathode cup angle generates a parallel surface between the cathode and the target.
- 8. A cathode structure as claimed in claim 6 further comprising a frame for containing the cathode.
- 9. A cathode structure as claimed in claim 8 further comprising a gap between the cathode and the frame.
- 10. A cathode structure as claimed in claim 9 wherein the angled cathode cup maximizes the gap.
US Referenced Citations (5)