X-ray tube

Details X-ray tube X-ray tube

1999-12-30

2001-12-25

Dunn, Drew (Department: 2882)

X-ray or gamma ray systems or devices

Source

Electron tube

C378S134000, C378S136000, C313S421000

Reexamination Certificate

active

06333969

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to an X-ray tube with reduced distortion of a configuration of X-ray focal spot.
DISCUSSION OF THE BACKGROUND
An X-ray tube is an electron tube in which X-rays are generated out of a target surface when thermal electrons generated out of a cathode filament impinge on the target surface of an anode, and is used, for example, for radiography of an object.
To take radiographs of an object, fluoroscopy in which the object is observed while being exposed by X-rays, and ordinary radiography in which X-ray images of the object are printed on such as photographic films, are in use. The fluoroscopy is done under a small dose of X-rays, while the ordinary radiography is done under a large dose of X-rays.
Usually an X-ray tube having plural focal spots is used when switching of the dose of X-rays to take radiographs of the object is required. For example, there is a method where plural kinds of focal spots whose sizes are different from each other, such as large focal spot and small focal spot, can be so formed that the small focal spot is used for fluoroscopy and large focal spot is used for ordinary radiography.
In a conventional X-ray tube, the case in which large and small X-ray focal spots are prepared, will be explained referring to FIG.
15
. In the tube, there are cathode body
50
emitting thermal electrons, and disc shaped rotating anode
52
placed facing the cathode body. Upper and lower surfaces of rotating anode
52
in the figure are flat, and the target surface is inclined to the surface planes thereof.
Inside rectangular shaped focusing slots
54
a,
54
b
in focusing electrode
51
forming cathode body
50
, there are two cathodes for emitting thermal electrons, for instance, coil shaped direct heated filaments
53
a,
53
b.
One of the two filaments
53
a,
for example is for large focal spot, and the other filament
53
b
is for small focal spot. Focusing recesses
54
a,
54
b
shape an electrostatic field to converge electrons emitted out of filaments
53
a,
53
b
onto the surface of the anode target and to confine within the focal spot whose size and configuration are predetermined. The electron beam impinging area is, of course, the X-ray focal spot.
For an X-ray tube with two focal spots, it is necessary that a discrepancy between position of taking radiographs of an object using large focal spot and the object using small focal spot does not occur. For this purpose, two focal spots of electrons emitted out of filaments
53
a,
53
b
are placed on substantially the same position of the target surface of anode
52
.
For the reason, bottom portion M is formed, making the central portion of focusing electrode
51
recessed. At slopes
51
a,
51
b
located on the opposite side of bottom portion M to each other to shape a V-character, focusing recesses
54
a,
54
b
are formed respectively, the openings of focusing recesses
54
a,
54
b
facing toward inside. In this case, the opening end of one of focusing recesses
54
a
is inclined at predetermined angle &agr; to the straight line connecting the bottom portion with the focal spot, i.e., to the surface H normal to a center axis C, and the opening end of the other of focusing recesses
54
b
is inclined at predetermined angle &bgr; in the same manner. Here, Lw denotes the width of the opening end along the slope of focusing recess
54
a
and Sw denotes the width of the opening end along the slope of focusing recess
54
b.
Next,
FIG. 15
c
shows the structure of filaments
53
a,
53
b
and focusing recesses
54
a,
54
b
seen from anode
52
. This figure is the plan view of each slope seen in directions
15
c,
15
d
perpendicular thereto.
The overall contour of the focusing electrode is making substantially cylindrical configuration. Two filaments
54
a,
54
b
are located parallel to each other in the same direction along linear bottom portion M. Thus focusing recesses
54
a,
54
b
are substantially rectangular in the direction of the extension of bottom portion M, in compliance with filaments
53
a,
53
b
accommodated therein. To manufacture X-ray tubes, two focusing recesses
54
a,
54
b
are prepared by recess machining in the same process. Four corners L
1
to L
4
of the walls forming focusing recess
54
a
and four corners S
1
to S
4
of the walls forming focusing recess
54
b
are round surfaces having the same curvature radius. In conventional X-ray tubes, curvature radii of corners L
1
to L
4
, and S
1
to S
4
are generally not greater than 0.3 time of the recess widths Lw, Sw along slopes
51
a,
51
b
of the openings end of focusing recesses
54
a,
54
b.
In above mentioned structure, thermal electrons emitted from filaments
53
a,
53
b
are focused by the electrostatic field inside focusing recesses
54
a,
54
b
and form focal spots
57
a,
57
b
on the target surface of anode
52
, after following trajectories
56
a,
56
b
as shown in
FIG. 15
a.
In this case, the configurations of the focal spots of thermal electrons are shown in
FIG. 15
b.
Marks
57
a
and
57
b
denote large focal spot and small focal spot respectively, and bow shaped distortions take place along the direction in which filaments
53
a,
53
b
are laid. Distortions of the focal spot results in expansion of effective size thereof, by the area swelling outside as noted by A
1
, A
2
.
In conventional X-ray tubes, distortion of the configuration of the focal spot formed on the target surface of the anode results in expansion of the size of the focal spot, and as a result, in deterioration of quality of radiographs. The distortion of the configuration of focal spot is due to losing the uniformity of running trajectories of thermal electrons, caused by the distortion of focusing field for electron beams. The reason of aforementioned distortion of focusing field is that the surface of the focusing electrode facing the disc shaped rotating anode inclines in V shape, and the focusing recesses formed in the focusing electrodes are rectangular and have some discrepancy of the position to each other, even though the focusing electrode is cylindrical.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome the above mentioned disadvantages and to provide an X-ray tube having X-ray focal spots with less distortion.


REFERENCES:
patent: 4868842 (1989-09-01), Dowd
patent: 4894853 (1990-01-01), Dowd
patent: 5884963 (1998-12-01), Koller et al.
patent: 2-144835 (1990-06-01), None
patent: 5-121020 (1993-05-01), None

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