CN113327830A

CN113327830A - High-power X-ray tube

Info

Publication number: CN113327830A
Application number: CN202110595054.7A
Authority: CN
Inventors: 邹昀
Original assignee: Individual
Current assignee: Individual
Priority date: 2021-05-28
Filing date: 2021-05-28
Publication date: 2021-08-31
Anticipated expiration: 2041-05-28
Also published as: CN113327830B

Abstract

The present invention relates to an X-ray tube and a CT system using the X-ray tube. X-ray tube has the vacuum cavity that is enclosed by the vacuum cavity wall, and the one end of vacuum cavity sets up the negative pole, and the other end of vacuum cavity sets up the positive pole target disc, and the electron beam sends from the negative pole, strikes positive pole target disc at the high speed of focus position to arouse and produce X ray, X-ray tube have control assembly, its characterized in that: the anode target disc is a fixed anode target disc, and when the X-ray tube works, the control assembly controls the focal position of the electron beam on the fixed anode target disc to change rapidly. The invention adopts the fixed anode target disc, can achieve the peak power similar to a rotating target by quickly changing the focal position of the electron beam on the fixed anode target disc, can reduce vacuum rotating assemblies by adopting the scheme of the invention, has the characteristic of very simple structure, can directly cool the target disc by oil by adopting the scheme of the invention, thereby achieving the effect of continuous work and improving the stability of the system.

Description

High-power X-ray tube

Technical Field

The present invention relates to high power X-ray tubes for medical or industrial CT or two-dimensional imaging or quasi three-dimensional imaging (such as limited angle three-dimensional imaging).

Background

The X-ray tube is mainly used for medical or industrial equipment such as an X-ray machine, a CT machine and the like, and generates X-rays under the action of applied external high voltage for doctors to diagnose or treat patients or perform nondestructive detection on articles.

The X-ray tube may have a power of from several kilowatts to 150 kilowatts, and has a fixed target to which a high energy electron beam (60kV to 500kV) bombards to generate X-rays. In an X-ray tube, a high-energy electron beam is directed from a cathode to an anode target to generate X-rays, wherein less than 1% of the energy is converted into X-ray radiation, and another 99% of the energy is converted into heat energy of the anode, resulting in a rapid temperature rise at the impacted site.

In a fixed anode X-ray tube, the electrons continue to impinge on the same location, causing the local temperature of the fixed anode X-ray tube to rise very quickly, albeit continuously, but at a very low power. In view of this, whether the heat energy generated by the X-ray tube during operation can be conducted away in time becomes a decisive factor for limiting the continuous power of the X-ray tube (especially, the fixed anode X-ray tube). In practical application, phenomena that the target surface is cracked or melted due to overhigh anode temperature of the X-ray tube, the working stability and the service life of a product are seriously affected due to ignition in the tube, cracking of insulating oil and the like caused by overhigh anode temperature often occur.

The current high power target disks for CT are all rotating anode targets. The rotating anode target adopts a method that an anode bearing drives an anode target disc to rotate at a high speed in a vacuum tube shell, so that heat is dispersed to the whole target disc. The advantage of a rotating target is that the peak power can be very high, but the continuous power is not high, and the structure is very complex, useful for vacuum bearings, and very fragile. The bearings have poor heat transfer capabilities and have to be stopped for cooling after CT scans have been performed on the patient, possibly because the target disk is too hot. Under high vacuum conditions, heat on the rotating anode X-ray tube target disk is transferred to the vacuum envelope primarily through thermal radiation, and then is carried away by the cooling fluid flowing through the envelope. At the same time, a part of heat is inevitably transferred to the anode bearing by heat conduction and heat radiation, so that the temperature of the metal ball is raised. If the heat transmitted by heat conduction or heat radiation is excessive, the metal ball can exceed the limit working temperature, and further the bearing is blocked, and the whole X-ray tube fails.

Disclosure of Invention

Based on the problems in the prior art, the technical problem to be solved by the invention is to provide a high-power non-rotating target disk X-ray tube which is simple in structure and has good heat dissipation efficiency, the heat dissipation efficiency of the X-ray tube can be improved, the heat on an anode target can be dissipated quickly, and the service life of the X-ray tube is prolonged.

The invention provides an X-ray tube, which is provided with a vacuum cavity enclosed by vacuum cavity walls, wherein one end of the vacuum cavity is provided with a cathode, the other end of the vacuum cavity is provided with an anode target disk, electron beams are emitted from the cathode and impact the anode target disk at a focus position at a high speed so as to excite and generate X-rays, the X-ray tube is provided with a control component, and the X-ray tube is characterized in that: the anode target disc is a fixed anode target disc, and when the X-ray tube works, the control assembly controls the focal position of the electron beam on the fixed anode target disc to change rapidly.

Further, in the X-ray tube of the present invention, the control unit includes a deflection and focusing magnetic pole unit, and the focus position of the electron beam on the fixed anode target disk is changed by the deflection and focusing magnetic pole unit.

Further, downstream of the cathode are two pairs of two-pole electromagnets that generate a magnetic field in a range that enables the deflection range of the electron beam to cover the entire area of the stationary anode target disk.

Furthermore, one or more quadrupole electromagnets are superposed at the same position or different positions in front of and behind the dipolar electromagnet, so that the quadrupole electromagnet has better focusing effect on electron beams and can keep the size of a target spot to be optimized.

Further, the X-ray tube has a plurality of cathodes, and the control unit includes cathode switches corresponding to the plurality of cathodes one-to-one, and controls on/off of the respective cathode switches to realize rapid change of the focal position of the electron beam on the fixed anode target disk.

Further, the cathode is a cold cathode or a hot cathode, the cold cathode is formed by applying a strong electric field to the surface of the cathode, and electrons are emitted from the cathode by the effect of field emission. The hot cathode heats the filament by current, thereby increasing the energy of electrons in the filament, and the electrons are emitted across the potential barrier of the filament material under the external electric field.

Furthermore, each cathode is provided with a pair of electrodes or two pairs of electrodes or a pair of dipolar electromagnets or two pairs of dipolar electromagnets or one or two quadrupole electromagnets downstream, which can generate strong electrostatic fields or magnetic fields, thereby realizing that the electron beams are deviated by half a pixel distance in one or two directions of the fixed anode target plate.

Further, the electron beam jump position between different points on the stationary anode target disk according to the invention may be every time a fixed distance apart or generated according to a specific algorithm or by letting the next jump target point be the farthest from a weighted average of the last several target points.

Further, the surface of the fixed anode target disk bombarded by the electron beam is a layer of tungsten material with a thickness of several micrometers to several hundred micrometers, and a layer of transition material is arranged in the middle, and the requirement of the transition material is that the melting point is relatively high, the thermal conductivity coefficient is high, the thermal expansion coefficient is between that of tungsten and the material with high thermal conductivity at the outside, the material can be, but is not limited to, aluminum nitride (AlN), silicon carbide (SiC) or other high temperature material with a thickness of several micrometers to several tens of micrometers, the other function of the material is to act as a transition layer to reduce the stress caused by the thermal expansion between the tungsten material and the outermost heat conducting material, and the outermost is the material with high thermal conductivity, and the materials can include, but are not limited to, diamond, copper, silver, gold and the like.

Under certain conditions, the tungsten coating may be directly plated on the outermost thermal conductivity material without using an intermediate material.

The invention also provides a CT system, which is provided with the X-ray tube, an X-ray detector, a CT frame and a computer system; the computer system can control a cathode and a deflection and focusing magnetic pole assembly in the X-ray tube in real time, and realizes that electron beams form target points at different positions of a fixed anode target surface; the computer system also has an image processing module capable of reconstructing the plurality of two-dimensional images acquired by the detector into a three-dimensional image or other suitable representation.

Furthermore, in the CT scanning process, the electron beam is enabled to move in the direction opposite to the CT rotation direction during imaging of each two-dimensional image, the change of the position of the electron beam target point in space in each two-dimensional imaging process is reduced or the position of the electron beam target point in space is kept unchanged, the position of the focus of each X-ray is recorded by the controller, and the position information is used for establishing a three-dimensional image in the reconstruction process.

The invention has the beneficial effects that:

1. the invention adopts the fixed anode target disc, and can achieve the peak power similar to a rotating target by quickly changing the focal position of the electron beam on the fixed anode target disc;

2. the scheme of the invention can further optimize the algorithm of focus jump, so that the heat dissipation efficiency of the fixed anode target plate is maximized;

3. according to the invention, the number of focuses on the fixed anode target plate can be increased through the distance of half pixel point of electron beam deflection, and the spatial resolution can be obviously improved;

4. the invention enables the electron beam to move in the direction opposite to the CT rotation direction when each two-dimensional image is imaged, reduces the change of the position of the electron beam target point in the space in each two-dimensional imaging process or keeps the position of the electron beam target point in the space unchanged, thereby further improving the spatial resolution.

The present invention will be described in further detail below with reference to the accompanying drawings.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and together with the description serve to explain the invention and not to limit the invention. In the drawings:

FIG. 1 is a schematic view of an X-ray tube according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of an electromagnet according to one embodiment of the present invention;

FIG. 3 is a schematic view of a stationary anode target disk according to one embodiment of the present invention;

FIG. 4 is a schematic view of a CT system incorporating an X-ray tube of the present invention;

FIG. 5 is a schematic view of a focal point of a fixed anode target disk according to an embodiment of the present invention.

The reference numerals are explained below:

1-a cathode; 2-fixing an anode target disc; 3-vacuum cavity wall; 4-electron beam; 5-deflecting and focusing pole pieces.

Detailed Description

In order to make the technical solutions of the present invention better understood, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example one

The invention provides an X-ray tube, which is provided with a vacuum cavity enclosed by vacuum cavity walls, wherein one end of the vacuum cavity is provided with a cathode, the other end of the vacuum cavity is provided with an anode target disk, electron beams are emitted from the cathode and impact the anode target disk at a focus position at a high speed so as to excite and generate X-rays, the X-ray tube is provided with a control component, and the X-ray tube is characterized in that: the anode target disc is a fixed anode target disc, and when the X-ray tube works, the control assembly controls the focus position of the electron beam on the fixed anode target disc to change rapidly, so that the effect of rapid scanning of the electron beam on the fixed anode target disc is achieved.

The invention generates the effect of fast scanning of the electron beam on the fixed anode target disc by changing the focal position of the electron beam on the fixed anode target disc, thereby forming the function of a virtual rotating target; the focus position of the electron beam is changed rapidly, so that the retention time of the electron beam at a single position on the anode target disc is short, the focus position of the electron beam is changed rapidly, heat can be dispersed to the whole area of the anode target disc, and the anode target disc is guaranteed to have a good cooling effect.

Further, referring to fig. 1, the control assembly of the X-ray tube of the present invention comprises a deflection and focusing magnetic pole assembly 5, which can change the focal position of the electron beam 4 on the fixed anode target disk 2 by the deflection and focusing magnetic pole assembly 5 to realize the fast scanning of the electron beam;

more specifically:

the cathode is a hot cathode, which is composed of tungsten or molybdenum or tantalum or other materials with low work function, the cathode material is heated to the temperature of thousands of degrees, electron beams escape from the cathode material and strike a fixed anode target disc under the driving of a strong electric field. The hot cathodes may be one or more, say two, or three or five. Two pairs of two-pole electromagnets are arranged at the downstream of the hot cathode, and can deflect the electron beams in a larger range, and the magnetic force range of the two pairs of two-pole electromagnets can enable the deflection range of the electron beams to cover the whole area of the fixed anode target disc, so that the electron beam focus positions which are arranged very closely are formed on the fixed anode target disc.

Further, one or more quadrupole electromagnets may be disposed at the position where the two pairs of dipole electromagnets coincide, for example, as shown in fig. 2, that is, a quadrupole electromagnet may be disposed at the position where the two pairs of dipole electromagnets coincide, and the quadrupole electromagnet may control the size of the focus in the width and length directions. The two-pole and four-pole electromagnets may be wound around a core and may share windings to form a combined magnet. Two such combined magnets can be used to precisely control the length and width dimensions of the X target.

In this way, the cathode can also be a cold cathode, which is formed by applying a strong electric field to the surface of the cathode, and electrons are emitted from the cathode by the effect of field emission.

Furthermore, the fixed anode target plate can bear high power and has a very quick heat dissipation function. Which consists of one or several different materials. One composition is such that: the surface of the fixed anode target disk, which is bombarded by the electron beam, is a layer of tungsten material with a thickness of several micrometers to several hundred micrometers, and a layer of transition material is arranged in the middle, wherein the transition material has a requirement of relatively high melting point and high thermal conductivity, and the thermal expansion coefficient is between that of tungsten and the material with high thermal conductivity at the outside, the material can be, but is not limited to, aluminum nitride (AlN), silicon carbide (SiC) or other high temperature material with a thickness of several micrometers to several tens of micrometers, and the material has another function of reducing the stress caused by the thermal expansion between the tungsten material and the outermost heat conductive material as a transition layer, and the outermost material is a material with high thermal conductivity, and the materials can include, but are not limited to, diamond, copper, silver, gold and the like.

The size of the fixed anode target disk can be from a few centimeters to a dozen centimeters square, and can be rectangular or circular.

Fig. 3 shows one possible focus distribution of a stationary anode target disk whose X-ray focus is very densely and uniformly or non-uniformly distributed over the target, as shown in fig. 3 (a). The focal spot positions of the X-rays are very close, and adjacent focal spots may be completely adjacent or may even partially overlap. The number of focal spots ranges from a few to several hundred. The partially overlapping focal points can be controlled to be very precise, only by a distance of half a pixel, as shown in fig. 3(b), which can improve the imaging resolution of CT. The distribution range of the focal points can reach dozens of centimeters (X direction) on the left and the right, and a plurality of centimeters (Z direction) on the upper and the lower. The X-ray tube is very small in size, and a CT, equipped with 1 or more X-ray tubes, can provide multiple high voltage power supplies, detectors for cathode emission.

During CT operation, the CT support, X-ray tube and detector may rotate at 0.5 to 4 revolutions per second, as shown in fig. 4, during which the focal point of the electron beam endlessly jumps at these possible positions. At each focus position the time is very short, e.g. tens of microseconds, and then the next position is jumped so that the temperature of the focus on the stationary anode target disk does not rise very fast. Hundreds to thousands of projection images are obtained in the CT scanning process, and a three-dimensional image is automatically established by a reconstruction algorithm according to the projection images and the angle position information.

The schematic diagram of the X-ray runout between different points on the fixed anode target disk is shown in fig. 3(c), and the X-ray runout can be sequential runout, or random runout by control, for example, the runout can be performed according to the numerical sequence indicated in fig. 3(c), or can be optimized according to a certain algorithm.

Furthermore, the jump position of the X-rays between different points on the stationary anode target disk is optimized using a certain algorithm, so that the maximum power that can be sustained on the target or the temperature of the target surface is kept to a minimum. For example, one algorithm is to have the next target point jump farthest from a weighted average of the last several target points. For example, the position of the weighted average of the N target points is calculated first by selecting the last N target points. The position of the (N + 1) th target point is P_N+1Pi, i is chosen to maximize the distance in the following equation:

furthermore, the jitter variation of the focus is divided into two levels,

the 1 st layer takes 4 closer points as a group of focuses, two focuses are respectively arranged in the Z direction and the X direction, the distance between the 4 focuses is half of the pixel of the CT detector or (the specific value is adjusted along with the position of the imaging center) in the two-dimensional imaging process, the focus position jumps at the 4 focuses, and the spatial resolution can be increased. This arrangement, which shifts the focus by half a pixel distance, can increase the spatial resolution of the CT. This arrangement of the focus offset by half the focus distance is equivalent to doubling the number of X-ray focus positions, with the increased X-ray focus position in the middle of the original X-ray focus position, so that the number of X-ray focus positions is doubled, and the pitch of each focus is halved compared to the original, allowing for increased spatial resolution, without changing other conditions (same X-ray focus size, same detector pixel size, etc.).

The 2 nd aspect is run-out by a wide margin, namely in the process of different two-dimensional map imaging, the focus spatial position changes greatly, the focus run-out distance is far away, and the interval is far away and is more favorable for heat dissipation.

As a specific example, the two-level jumping mode of the present invention can be illustrated with reference to fig. 5, for example, the jumping mode can be a group of 4 jumps performed by shifting to other distant areas after the end of the jumping in 4 focuses of each small area, corresponding to the jumping mode performed according to 11-12-13-14-21-22-23-24 in fig. 5.

As an alternative embodiment, the variation jitter of the electron beam focus of the present invention may be any combination of the jitter of the large range level 2 and the jitter of the small range level 1, for example, the jitter may be performed in the order of 11-21-31-41-12-22-32-42-13-23-33-43-14-24-34-44 shown in fig. 5.

Example two

According to another mode of the invention, a plurality of cathodes are arranged, the control assembly comprises cathode switches which correspond to the cathodes one by one, and the fast change of the focal position of the electron beam on the fixed anode target disc is realized by controlling the on or off of each cathode switch, so that the fast scanning of the electron beam is realized; more specifically:

the cathode is a cold cathode, and electrons are emitted from the cathode through the field emission effect by applying a strong electric field on the surface of the cathode. A plurality of cathodes are arranged, the number of the cathodes corresponds to the number of the focuses, and each focus position corresponds to one cathode, so that a deflection system is not needed, and the emission control of the cathodes is carried out only through the switch of one cathode, and the generation of target points of the electron beams at different places is realized.

Further, for the plurality of cathodes in this manner, each cathode is further provided with a pair of electrodes or two pairs of electrodes or a pair of dipolar electromagnets or two pairs of dipolar electromagnets or one or two quadrupole electromagnets downstream, which can generate a strong electrostatic field or magnetic field, thereby achieving a half pixel distance deviation of the electron beam in one or two directions of the stationary anode target disk.

This arrangement, which shifts the focus by half a pixel distance, can increase the spatial resolution of the CT. This arrangement of the focus offset by half the focus distance is equivalent to doubling the number of X-ray focus positions, with the increased X-ray focus position in the middle of the original X-ray focus position, so that the number of X-ray focus positions is doubled, and the pitch of each focus is halved compared to the original, allowing for increased spatial resolution, without changing other conditions (same X-ray focus size, same detector pixel size, etc.).

EXAMPLE III

The invention also provides a CT system, which is provided with the X-ray tube, an X-ray detector, a CT frame and a computer system; the computer system can control a control assembly in the X-ray tube in real time to realize that the electron beams form target points at different positions of the fixed anode target surface; the computer system also has an image processing module capable of reconstructing the plurality of two-dimensional images acquired by the detector into a three-dimensional image or other suitable representation.

Furthermore, in the CT scanning process, the electron beam moves in the direction opposite to the CT rotation direction when each two-dimensional image is imaged, and the position change of the target point of the electron beam in the space in each two-dimensional imaging process is reduced.

During the CT rotation, the focus on the stationary anode target disk is moved during each two-dimensional imaging and rotates along with the rotation of the CT support, for example, about 2 mm during a two-dimensional imaging, and the following movement of the focus will cause the resolution of the CT image to decrease. The X-ray tube of the invention has the function of deflecting the electron beam, and can enable the electron beam to move in the direction opposite to the CT rotation direction when each two-dimensional image is imaged, so that the position of a target point in space can be kept unchanged or reduced and changed in each two-dimensional imaging process. This may be advantageous for improving the spatial resolution of the CT image.

The speed of the focal spot can be adjusted with the following formula:

V_F＝-α×ω_CT×R_CT

ω_CTangular frequency speed of CT gantry rotation

R_CTRadius of focus to center of CT

V_FSpeed of movement of the focal point

An optimization factor in the range of 0-2

It should be noted that the terms "comprises" and "comprising," and any variations thereof, in the description and claims of the present invention and the above-described drawings, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.

While the invention has been described with respect to a preferred embodiment, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the scope of the invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. An X-ray tube having a vacuum chamber defined by a vacuum chamber wall, a cathode provided at one end of the vacuum chamber, an anode target disk provided at the other end of the vacuum chamber, an electron beam emitted from the cathode striking the anode target disk at a high speed at a focal position to excite generation of X-rays, the X-ray tube having a control assembly, characterized in that: the anode target disc is a fixed anode target disc, and when the X-ray tube works, the control assembly controls the focal position of the electron beam on the fixed anode target disc to change rapidly.

2. The X-ray tube of claim 1, wherein: the control assembly includes a deflection and focusing magnetic pole assembly by which the focal position of the electron beam on the stationary anode target disk is changed.

3. The X-ray tube of claim 2, wherein: downstream of the cathode are two pairs of two-pole electromagnets that generate a magnetic field range that enables the deflection range of the electron beam to cover the full area of the stationary anode target disk.

4. The X-ray tube of claim 3, wherein: one or more four-pole electromagnets are superposed at the same position or different positions in front of and behind the two-pole electromagnet.

5. The X-ray tube of claim 1, wherein: the control assembly comprises cathode switches corresponding to the cathodes one by one, and the rapid change of the focal position of the electron beam on the fixed anode target disc is realized by controlling the on or off of each cathode switch.

6. The X-ray tube according to any one of claims 1 to 5, wherein: each cathode is also provided downstream with a pair of electrodes or two pairs of electrodes or a pair of two-pole electromagnets or two pairs of two-pole electromagnets or one or two four-pole electromagnets, which can generate strong electrostatic or magnetic fields, thus achieving a deflection of the electron beam by half a pixel distance in one or both directions of the stationary anode target disk.

7. The X-ray tube according to any one of claims 1 to 5, wherein: the algorithm for the variation of the position of the electron beam between different points on the stationary anode target disk is either every time a fixed distance is spaced or according to a specific algorithm or the next jumping target point is located farthest away from a weighted average of the last several target points.

8. The X-ray tube according to any one of claims 1 to 5, wherein: the surface of the fixed anode target disk bombarded by the electron beam is a layer of tungsten material with the thickness of several micrometers to several hundred micrometers, the middle is a layer of transition material, the thermal expansion coefficient of the transition material is between that of the tungsten and the material with high thermal conductivity outside, the thickness of the transition material is several micrometers to several tens micrometers, and the outermost layer is the material with high thermal conductivity.

9. A CT system having an X-ray tube according to claims 1-8, a detector, a CT gantry, a computer system; the computer system can control a control assembly in the X-ray tube in real time to realize that the electron beams form target points at different positions of the fixed anode target surface; the computer system also has an image processing module capable of reconstructing the plurality of two-dimensional images acquired by the detector into a three-dimensional image or other suitable representation.

10. The CT system of claim 9, wherein: in the CT scanning process, the electron beam is moved in the direction opposite to the CT rotation direction during imaging of each two-dimensional image, so that the change of the spatial position of the electron beam target point in each two-dimensional imaging process is reduced or the spatial position of the electron beam target point is kept unchanged.