JPH0589810A - Rotary cathode x-ray tube - Google Patents

Abstract

PURPOSE:To prevent a rotary cathode from deforming in a place, where a convergence electrode is installed, and eliminate displacement of the X-ray focal point resulting from thermal expansion of the rotary cathode. CONSTITUTION:A convergence electrode 3 is installed with possibility of displacement in the radial direction of a rotary cathode 1, and a supporting plate 4 having a high coefficient of thermal expansion is interposed between the convergence electrode 3 and rotary cathode 1. In association with thermal expansion of the rotary cathode 1, the supporting plate 4 makes thermal expansion to push the convergence electrode 3 toward the center of rotation, which cancels the displacement of the convergence electrode 3 resulting from the thermal expansion of the rotary cathode 1. The product of the center-of-gravity distance and the weight of a cut-off piece of a notch 2 where the convergence electrode 3 is to be installed shall be on the design basis approx. identical with the product of the center of-gravity distance and the weight of the supporting plate 4 and convergence electrode 3, and thereby the centrifugal forces acting on them are made equal to eliminate deformation of the rotary cathode 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in an X-ray CT apparatus for making a tomographic image of an object, and in particular, by rotating a cathode equipped with an electron beam emitting section for X-ray generation. The present invention relates to a rotating cathode X-ray tube that irradiates X-rays from the entire circumference of a specimen.

[0002]

2. Description of the Related Art A partial cross section of a conventional rotary cathode X-ray tube is shown in FIG. 4 and will be described below. A ring-shaped target 41 and a rotating cathode 42 are arranged opposite to each other inside a vacuum container 40 formed in a ring shape. Target 41 is a vacuum vessel
Installed and fixed on the inner wall surface of 40, the rotating cathode 42 is
It is supported in a magnetically levitated state by an electromagnetic magnet 43 attached to the outer peripheral wall surface of the. Levitated rotating cathode 42
There is a gap sensor 46 that senses a gap with the inner wall surface of the vacuum container 40. Based on a detection signal of the gap sensor 46, the power supplied to the electromagnetic magnet 43 is controlled so that the gap becomes constant. It is controlled by a vessel.

On the surface of the rotating cathode 42 facing the target 41, a filament 44 for emitting thermoelectrons for generating X-rays is provided toward the target 41, and the rotating cathode 42 is fixed to the stator.
The thermoelectrons are radiated over the entire circumference of the target 41 by being rotationally driven by 45, and X-rays are radiated from the entire circumference direction to the subject carried into the cavity of the vacuum container 40.

The finer the collision point of the thermoelectrons on the target 41 (the X-ray focal point at the X-ray generation point), the better the image quality of the tomographic image. Therefore, the filament 44 focuses the thermoelectrons toward the target 41. It is attached to the focusing electrode 47. The focusing electrode 47 has, for example, a cylindrical shape having a flange portion, penetrates a hole formed in a required portion of the rotating cathode 42, and projects onto a surface facing the target 41, and is screwed to the rotating cathode 42 at the flange portion. It is fixed.

[0005]

However, in order to reduce the weight of the rotating cathode 42, the portion other than the portion where the electromagnetic magnet 43 acts is formed of aluminum, whereas the focusing electrode 47 is formed of nickel or stainless steel. It is made of a metal having a larger specific gravity than aluminum, and the weight of the rotating cathode 42 is not uniform at the location where the focusing electrode 47 is attached. Therefore, the centrifugal force acting on the rotating cathode 42 at the time of high-speed rotation during tomography may be different between the place where the focusing electrode 47 is attached and the place where the focusing electrode 47 is not attached, and the rotating cathode 42 may be deformed.

Generally, the centrifugal force F at the mass point A is
Is m, the radius of gyration to the mass A is Ra, and the mass A is
When the angular velocity at is ω, it is expressed by the following equation (1). F = m × Ra × ω ² (1) Therefore, the centrifugal force acts largely at a place with a large mass (where the focusing electrode 47 is attached), and compared with this. The centrifugal force that acts on a place with a small mass (a place where the focusing electrode 47 is not attached) is small. From this, the rotating cathode 42 is deformed into the shape shown by the phantom line in FIG. At the position where the focusing electrode 47 is attached, it is suitable to expand along the direction of the rotation diameter.

When the rotating cathode 42 is deformed so as to expand in places, a gap sensor 46 for detecting the gap between the rotating cathode 42 and the inner wall surface of the vacuum container 40 detects this and outputs it to a controller (not shown). .. Since the controller controls the power supplied to the electromagnetic magnet 43 so that the gap becomes constant,
Each time the deformed portion passes through the gap sensor 46, the power supplied to the electromagnetic magnet 43 is changed, and the magnetically levitated state of the rotating cathode 42 becomes unstable.

When the rotating cathode 42 is deformed during rotation,
The position of the filament 44 attached to the focusing electrode 47 is displaced, and the position of the X-ray focus formed on the target 41 is displaced. Along with the displacement of the X-ray focal point, there is a problem that the irradiation angle of the X-ray toward the subject and the distance from the X-ray focal point to the subject change, which significantly impairs the image quality of the tomographic image.

The above are the problems associated with the deformation of the rotary cathode 42 at the location where the focusing electrode 47 is attached.
The same problem occurs due to the thermal expansion of. Rotating cathode 42
The thermal expansion of the target is (a) the heat conduction from the heated filament 44, and (b) the target whose temperature is raised by the collision of thermoelectrons.
Radiant heat from 41, (c) Frictional heat with the power supply brush in contact with the rotating cathode 42 to supply power to the filament, (d)
This is caused by the induction heat of the stator 45, etc., and the position of the filament 44 (the position of the X-ray focus) is displaced by thermal expansion, causing the same problem as described above.

The present invention has been made in view of the above circumstances, and eliminates the deformation of the rotating cathode at the location where the focusing electrode is attached and the displacement of the X-ray focal point due to the thermal expansion of the rotating cathode. It is an object of the present invention to provide a rotating cathode X-ray tube which can be manufactured.

[0011]

In order to achieve the above object, the present invention has the following constitution. That is, the invention according to claim 1 is provided with a ring-shaped target installed and fixed on the anode side in a ring-shaped vacuum container, and an electron emission unit for emitting an electron beam for X-ray generation toward the target. A rotating cathode X-ray tube having a ring-shaped rotating cathode and a focusing electrode attached to the rotating cathode to focus an electron beam from the electron emitting portion toward a target. Cut to attach the focusing electrode, the product of the weight of this cutting piece and the distance from the center of gravity of the cutting piece to the center of rotation of the rotating cathode, and the weight of the focusing electrode and the center of rotation of the rotating cathode from the center of gravity of the focusing electrode. It is characterized in that it is configured so that the product of the distance to and the distance to becomes almost the same.

According to a second aspect of the present invention, a ring-shaped target fixed and set on the anode side in a ring-shaped vacuum container, and an electron emission for emitting an electron beam for X-ray generation toward the target. A ring-shaped rotating cathode with a part attached,
In a rotating cathode X-ray tube equipped with a focusing electrode that is attached to this rotating cathode and focuses an electron beam from the electron emitting portion toward a target, the focusing electrode is attached so as to be displaceable in the radial direction of the rotating cathode, A member that is displaced toward the center of the rotating cathode due to thermal expansion is interposed between the focusing electrode and the rotating cathode.

[0013]

The operation of the invention described in claim 1 is as follows. The weight of the cut piece is m, the distance from the center of gravity of the cut piece to the center of rotation of the rotating cathode is L, the weight of the focusing electrode is m ′, and the distance from the center of gravity of the focusing electrode to the center of rotation of the rotating cathode is L ′. If so, m · L≈m ′ · L ′. Letting F be the centrifugal force acting on the position of the center of gravity of the cut piece, F = m · L · ω ² (ω is the angular velocity at the position of the center of gravity of the cut piece), and F is the centrifugal force acting on the position of the center of gravity of the focusing electrode. Then, F '= m'
L ′ · ω ′ ² (ω ′ is the angular velocity at the center of gravity of the focusing electrode). Here, since both the center of gravity of the cut piece and the center of gravity of the focusing electrode are points on the rotating cathode, ω ² = ω ^'2 . Further, as described above, since the configuration is such that m · L≈m ′ · L ′, the centrifugal force F that acts on the position of the center of gravity of the cut piece and the centrifugal force F that acts on the position of the center of gravity of the focusing electrode. 'Is almost equal to. That is, in the rotating cathode, the centrifugal force at the place where the focusing electrode is attached and the centrifugal force at the place where the focusing electrode is not attached (the place where the above-mentioned cut-out piece is attached to the part cut off to attach the focusing electrode) Since the forces substantially match, the deformation of the rotating cathode due to the difference in centrifugal force can be suppressed.

The operation of the invention described in claim 2 is as follows. When the rotary cathode is heated and thermally expanded due to the reasons described above, the temperature of the member interposed between the rotary cathode and the focusing electrode also rises and thermally expands. Due to the thermal expansion of the member, the focusing electrode is displaced toward the center of rotation of the rotating cathode. That is, since the rotating cathode is displaced in the direction opposite to the thermal expansion direction, the displacement of the focusing electrode due to the thermal expansion of the rotating cathode is offset, and the position of the focusing electrode remains substantially unchanged.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a partial front view of a rotating cathode to which a focusing electrode is attached. A focusing electrode 3 having a flange-shaped bottom surface in a notch 2 having an opening on the inner peripheral side of the rotating cathode 1.
Is attached. A support plate 4 made of a material having a higher thermal expansion coefficient than that of the rotating cathode 1 such as titanium is interposed between the focusing electrode 3 and the outer peripheral wall surface of the cutout 2.

Elongated holes 6 and 7 are formed along the radial direction of the rotary cathode 1 at both ends of the flange-like portion on the bottom surface of the focusing electrode 3. The focusing electrode 3 is movably attached toward the center of rotation of the rotating cathode 1 by pins 8 and 9 penetrating these elongated holes 6 and 7. Reference numeral 10 is a spring plate, which is a member that prevents the focusing electrode 3 from moving down when the rotation of the rotating cathode 1 is stopped.

The focusing electrode 3 is positioned by the centrifugal force acting on the focusing electrode 3 during the high speed rotation of the rotating cathode 1. That is, the focusing electrode 3 is pressed against the outer peripheral wall surface of the cutout portion 2 via the support plate 4 by the centrifugal force, and the position is fixed there. The support plate 4 as a high thermal expansion metal is interposed between the rotating cathode 1 and the focusing electrode 3 because the focusing electrode 3 is displaced by the thermal expansion of the rotating cathode 1 (the filament 5 attached to the focusing electrode 3). This is to eliminate the displacement).

When the temperature of the rotary cathode 1 rises due to the reasons described above, the rotary cathode 1 thermally expands outward in the radial direction. The diameter of the rotating cathode 1 expands by about 1 mm due to a temperature rise of about 50 to 70 degrees Celsius. The heat of the rotating cathode 1 at this time is transferred to the support plate 4, and the temperature of the support plate 4 also rises similarly. However, since the support plate 4 is in direct contact with the focusing electrode 3 having the filament 5 serving as a heat source attached thereto. ,
It rises to a temperature higher than that of the rotating cathode 1 (about 200 degrees Celsius).

The outer peripheral surface (upper side of the drawing) of the support plate 4 is in contact with the rotary cathode 1, and similarly, the side surface side (left and right side of the drawing) of the support plate 4 is also in contact with the rotary cathode 1. .. For this reason,
The support plate 4 whose temperature has risen thermally expands toward the inner peripheral side of the rotating cathode 1 (the focusing electrode 3 side in the lower side of the drawing). Focusing electrode 3
Is attached so as to be displaceable along the radial direction of the rotating cathode 1, so that it is pushed toward the inner peripheral side of the rotating cathode 1 by thermal expansion of the support plate 4. That is, the rotating cathode 1 is displaced in the direction opposite to the thermal expansion direction. If the thermal expansion coefficient of the support plate 4 is selected so that the displacement amount of the focusing electrode 3 and the displacement amount of the rotating cathode 1 due to the thermal expansion are equal, even if the rotating cathode 1 thermally expands, the focusing electrode 3 , That is, the position of the filament 5 is almost unchanged.

Since the supporting plate 4 has a smaller radial length than the rotating cathode 1, when it is made of a material having the same coefficient of thermal expansion as the rotating cathode 1, the displacement amount due to thermal expansion is smaller than that of the rotating cathode 1. turn into. Therefore, it is preferable that the support plate 4 is formed of a material having a higher thermal expansion coefficient than that of the rotating cathode 1. As in the above example, the rotating cathode 1 is 50 degrees Celsius or
If the thermal expansion is about 1 mm when the temperature is raised to 70 degrees, the coefficient of thermal expansion may be selected so that the support plate 4 is displaced by about 1 mm when the temperature is raised to 200 degrees Celsius.

Next, the structure for eliminating the deformation of the rotating cathode 1 due to the difference in centrifugal force between the place where the focusing electrode 3 is attached and the place where it is not attached will be described. In order to eliminate the displacement of the filament 5 due to the thermal expansion of the rotating cathode 1 at the same time, it is preferable to have the same external appearance as that shown in FIG. 1, but the present invention is not limited to this configuration, and various forms are possible. That is, in order to solve the above problem, the centrifugal force acting on the portion where the focusing electrode 3 is attached is equal to the centrifugal force acting on the portion where the focusing electrode 3 is not attached.
This is because there is no choice but to design the focusing electrode 3. This will be described with reference to FIG.

An area indicated by reference numeral D'is set for the location where the focusing electrode 3 is attached. The same region is set for a portion where the focusing electrode 3 is not attached. The area is indicated by reference numeral D. First, the centrifugal force F acting on the region D where the focusing electrode 3 is not attached is calculated by using the above equation (1). That is, assuming that the weight of the rotating cathode 1 in the region D is m, the distance from the center of gravity G in the region D to the center of rotation O is L, and the angular velocity at the center of gravity G is ω, the centrifugal force F is calculated by the following equation (2). To be done. ^{F = m · L · ω 2} ·············· (2)

On the other hand, the weight of the area D'where the focusing electrode 3 is attached is m ', the distance from the center of gravity G'of the area D'to the center of rotation O is L', and the angular velocity at the center of gravity G'is ω '. Then, the centrifugal force F ′ acting on the area D ′ is similarly calculated by the following equation (3). F '= m' ・ L '・ ω ^{' 2}・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (3)

Here, the center of gravity G of the area D where the focusing electrode 3 is not attached and the center of gravity G ′ of the area D ′ where the focusing electrode 3 is attached are on the same one rotating cathode 1, so the center of gravity is The angular velocities at G and the center of gravity G'are equal (ω ² = ω ^'2 ). Therefore, the condition that the centrifugal force F acting on the area D and the centrifugal force F ′ acting on the area D ′ are equal is expressed by the following equation (4). m ・ L = m '・ L' ... (4)

In order to substantially satisfy the equation (4), for example, the weights of the focusing electrode 3 and the support plate 4 shown in FIG.
The center of gravity position (shape) may be appropriately designed and configured.
That is, as shown in FIG. 3, the weight and the center-of-gravity distance of the piece 11 cut to form the cutout portion 2 of the rotating cathode 1 to which the focusing electrode 3 and the support plate 4 are attached are obtained (the center-of-gravity distance is Distance from the rotation center O of the cathode 1 to the center of gravity of the piece 11).

Next, the weight of the arbitrarily designed supporting plate 4 and the weight of the focusing electrode 3 are added, and this is calculated and the center of gravity distance between the supporting plate 4 and the focusing electrode 3 is obtained and multiplied. Using this value as a starting point, the product of the weight of the piece 11 and the center-of-gravity distance obtained above is compared. And, so that both values are almost the same,
The weight of the focusing electrode 3 and the support plate 4 and the distance of the center of gravity are changed. If there is no possibility of coincidence, the size of the notch 2, that is, the piece 11, is changed, and the above-mentioned comparative verification is repeated to obtain a solution.

[0027]

As is apparent from the above description, according to the rotating cathode X-ray tube according to the invention described in claim 1, the weight of the cut piece and the center of gravity of the cut piece cut for attaching the focusing electrode. To the center of rotation of the rotating cathode, and the product of the weight of the focusing electrode and the distance from the center of gravity of the focusing electrode to the center of rotation of the rotating cathode are configured to substantially match. Therefore, the centrifugal force acting on the portion where the focusing electrode is attached and the centrifugal force acting on the other portion are substantially the same, and the deformation of the rotating cathode due to the difference in centrifugal force can be suppressed. Therefore, the magnetic levitation control of the rotating cathode is stabilized, and the displacement of the filament due to the deformation of the rotating cathode is suppressed, and the image quality of the tomographic image is also stabilized. According to the rotating cathode X-ray tube of the invention described in claim 2, the focusing electrode is attached so as to be displaceable in the radial direction of the rotating cathode, and the rotating cathode is thermally expanded between the focusing electrode and the rotating cathode. Since a member displacing toward the center of the rotating cathode is heated and thermally expanded, the temperature of the member also rises and thermally expands, pushing the focusing electrode toward the rotating center of the rotating cathode, The displacement of the focusing electrode due to the thermal expansion of the rotating cathode is offset. Therefore, the position of the focusing electrode is almost unchanged, and the position of the filament is not changed, so that the image quality of the tomographic image can be stabilized.

[Brief description of drawings]

FIG. 1 is a front view showing a part of a rotating cathode to which a focusing electrode is attached in an embodiment of the present invention.

FIG. 2 is a partial front view of a rotating cathode illustrating a centrifugal force at a place where a focusing electrode is attached and a centrifugal force at a place where a focusing electrode is not attached.

FIG. 3 is a partial front view of a rotating cathode showing a cut piece to which a focusing electrode is attached.

FIG. 4 is a cross-sectional view illustrating a rotary cathode X-ray tube in a conventional example.

FIG. 5 is a front view of a rotating cathode showing a deformation of the rotating cathode due to a difference between a centrifugal force at a place where the focusing electrode is attached and a centrifugal force at a place where the focusing electrode is not attached.

[Explanation of Codes] 1 ... Rotating cathode 2 ... Notch portion 3 ... Focusing electrode 4 ... Support plate (member) 5 ... Filament 6, 7 ... Slot 8, 9 ... ..Pin 11 ... Pieces (cut pieces)

Claims (2)

[Claims] 1. A ring-shaped rotary cathode having a ring-shaped target fixed and installed on the anode side in a ring-shaped vacuum container, and an electron emission section for emitting an electron beam for X-ray generation toward the target. And a focusing electrode which is attached to the rotating cathode and focuses an electron beam from the electron emitting portion toward a target. In the rotating cathode X-ray tube, a part of the rotating cathode is attached to the focusing electrode. Cut, the product of the weight of the cut piece and the distance from the center of gravity of the cut piece to the center of rotation of the rotating cathode, and the weight of the focusing electrode and the distance from the center of gravity of the focusing electrode to the center of rotation of the rotating cathode. And X and X are configured so as to substantially coincide with each other. 2. A ring-shaped rotating cathode provided with a ring-shaped target fixedly installed on the anode side in a ring-shaped vacuum container, and an electron emission unit for emitting an electron beam for X-ray generation toward the target. And a focusing electrode attached to the rotating cathode for focusing an electron beam from the electron emitting portion toward a target, in a rotating cathode X-ray tube, the focusing electrode being displaceable in a radial direction of the rotating cathode. A rotary cathode X-ray tube, characterized in that a member which is mounted and displaced toward the center of the rotary cathode by thermal expansion is interposed between the focusing electrode and the rotary cathode.