X-ray tube, and method for operating an x-ray tube

Abstract

The invention relates to an X-ray tube (1), in particular of a medical imaging device, comprising a vacuum container (2) in which an electron source (4) is arranged, the electrons of which are directed towards an anode (5) that is designed to emit X-ray radiation and is likewise arranged in the vacuum container (2), wherein a cable (19) supplying the anode (5) with high voltage is guided through a wall (3) of the vacuum container (2). A linearising resistor (21) is inserted into the high-voltage cable (19) between the wall (3) of the vacuum container (2) and an anode voltage supply unit (18) located outside the vacuum container (2).

Description

[0001] X-ray tube and method for operating an X-ray tube

[0002] The invention relates to an X-ray tube which can be used in particular in a medical imaging device according to the preamble of claim 1. Furthermore, the invention relates to a method for operating an X-ray tube.

[0003] An X-ray tube of this type is known, for example, from WO 2019 / 042587 A2. The known X-ray tube has a plurality of cathodes configured to emit electron beams directed toward an anode also located within the X-ray tube. An anode current control unit is connected to a cathode power supply unit. In the case of WO 2019 / 042587 A2, a programmable module is provided, among other things, to control the cathodes.

[0004] DE 31 36 881 A1 discloses an arrangement for generating a variable bias voltage for an X-ray tube with adjustable focus. In this arrangement, a cathode of the X-ray tube is below ground potential, while a corresponding anode is above ground potential. Furthermore, a focusing electrode is set to a defined potential using a voltage divider.

[0005] The invention is based on the object of further developing X-ray tubes compared to the state of the art with regard to a particularly favorable ratio between operational reliability, equipment expenditure and diverse application possibilities.

[0006] This object is achieved according to the invention by an X-ray tube having the features of claim 1. The object is likewise achieved by a method for operating an X-ray tube according to claim 14. Configurations and advantages of the invention explained below in connection with the operating method also apply mutatis mutandis to the device, i.e., the X-ray tube, and vice versa. The X-ray tube, in a basic concept known per se, has a vacuum container in which an electron source is arranged, the electrons of which are directed at an anode provided for emitting X-rays and also arranged in the vacuum container. A cable supplying the anode with high voltage is guided through a wall of the vacuum container.According to the application, there is a linearization resistor inserted into the said cable, i.e. high-voltage cable, between the wall of the vacuum vessel and an anode voltage supply unit located outside the vacuum vessel.

[0007] The linearization resistor is, in particular, an electrical resistance in the range from 100 ohms to 100 kOhms, for example, an electrical resistance in the range from 2 kOhms to 50 kOhms. The distance between an anode connector of the X-ray tube and the linearization resistor is, for example, at least 10 cm and at most 40 cm. Alternatively, the linearization resistor can be embedded in the anode connector. Likewise, designs are possible in which the linearization resistor is designed as a resistance cable connected to the anode connector. Regardless of the exact design and positioning of the linearization resistor, it is preferably designed as a high-voltage-insulating, pulse-resistant resistor.

[0008] Cooling of the linearization resistor can be passive or active, particularly liquid cooling. In the latter case, the linearization resistor can be integrated into a common cooling circuit together with the anode.

[0009] In various designs, a focusing electrode arrangement is located between the electron sources and the anode. This can—viewed in projection from the anode onto the electron source—cover a large portion of the surface of a substrate on which the electron source is located, for example, more than half the surface of this substrate, particularly a ceramic substrate. The focusing electrodes interact with all emitters. This means that the focusing electrodes—measured in the longitudinal direction of the X-ray tube—are at least as long as all the emitters located on the ceramic substrate. The focusing electrodes can be screwed to all the ceramic substrates, thus forming a single structural unit. This contributes to reducing the risk of a breakdown down to the electron source.

[0010] As a further measure to minimize the risk of breakdown to the electron source, the individual focusing electrodes of the focusing electrode arrangement can be electrically connected, in particular welded, to foil strips. The foil strips are aligned in the longitudinal direction of the X-ray tube and electrically connected to the grounded tube housing. In particular, the focusing electrodes can be welded to the foil strips along their entire length. Likewise, the focusing electrodes can be connected to the tube housing without foil strips or be part of the housing. In either case, a low-impedance connection exists between the focusing electrodes and the tube housing.

[0011] The operating method according to the application generally assumes an X-ray tube in which an electron source comprising field emission cathodes, in particular NT (nanotube) cathodes, is arranged. The electron source directs an electron beam onto an anode, also arranged in the X-ray tube, which is supplied with high voltage in a generally known manner by means of an anode voltage supply unit connected to the anode via a linearization resistor. Various measures and combinations of measures have been shown to be suitable for drastically reducing the likelihood of damage.These include, particularly with the help of the linearization resistor, supplemented by the optional foil strips on the focusing electrodes, limiting arcing energy, avoiding the occurrence of induction voltage, improving emitter protection in the event of arcing, and dissipating arcing energy via the shortest path via system ground. These improvements, in particular, enable an optimized HV conditioning process and an extension of the service life.

[0012] In conventional, unclaimed arrangements and methods, the high-voltage generator is connected to the tube via a 5-meter-long cable, for example. Although the generator would limit the current to less than 100 mA in the event of a short circuit, a significant amount of energy is still present in the cable and the generator's capacitors, which, without current limitation, could be released in a potential discharge channel. The problem is not only the amount of energy, which is likely to be in the single-digit joules range, but also the time in which the energy can be released.

[0013] In contrast to such conventional solutions, in the context of the solution according to the application, in addition to damping the arcing energy, conductive connections from the focus electrode to the housing can be provided at regular intervals instead of continuously welded foils.

[0014] Regarding the estimation of the energy in the cable, the following figures can be cited as examples: Assuming a cable capacitance of 130pF / m, a cable capacitance of 32.5pF is given in a 25cm section of cable. At 160kV, there is still 0.42J of energy in the 25cm section of cable. With the help of the linearization resistor, the energy to be dissipated can be reduced to approximately one-twentieth. By positioning the linearization resistor close to the anode, the energy dissipation that must be managed in the event of arcing is particularly effectively limited. The energy limitation contributes in particular to the protection of the emitters and the control module (ECS = electronic control system) used to control the electron source.

[0015] In principle, it is conceivable that a flashover could either strike the extraction grid or extend to the emitters. In both cases, the linearization resistor exerts its protective effect on the components of the X-ray tube. This effect can be supplemented by a surge arrester located outside the X-ray tube, particularly in the form of a bidirectional gas arrester.

[0016] The addition of such a surge arrester to the protection provided by the linearization resistor is based on the consideration that potential flashovers reaching the mesh (extraction grid) or the emitters can damage not only the emitters but also the controlling electronics (ECS electronics). Therefore, precautions must be taken to ensure that high currents do not flow from the emitters during flashovers, nor do high voltages enter the electronics.

[0017] In the typical X-ray tube design, the most vulnerable electrode connected to the ECS system is the mesh. To divert any discharge currents to the tube housing via the shortest possible path, the surge arrester used should be located directly on or in the ECS connector. If the mesh voltage needs to be adjustable in a range of 1kV to +1kV, a bidirectional gas-filled surge arrester is recommended. If a positive mesh voltage is not expected, a simple diode is sufficient, since only positive voltage peaks are to be expected in the event of a flashover.

[0018] As already mentioned, gas discharge tubes (GDTs) are generally considered as devices for protecting electrical components. Regarding the protection of the emitters, in this case, in order to operate the emitters without having to accept measurement distortions caused by a GDT, the voltage of the GDT would have to be many volts higher than the voltage of the emitters. This would mean that the GDT would no longer be able to provide effective protection in the event of a breakdown.

[0019] Instead of a gas discharge tube, a resistor can be placed between the emitter and the cable, creating a high-impedance connection between the emitter and the cable in the event of a breakdown. This allows the emitter to respond more quickly to potential changes than variants that don't have the resistor, thus limiting any damage.

[0020] The described safety mechanisms, which drastically minimize damage caused by flashovers compared to older solutions, are ideally used in conjunction with a well-thought-out grounding concept.

[0021] This assumes that the grounding of the X-ray device's system components must ensure that, in the event of an arcing event, the energy is dissipated without causing damage. For this purpose, a central grounding point is sensibly defined, for example, an anode generator or the X-ray tube housing, where all grounding points converge at the shortest possible distance. This central grounding point is then connected to the system ground.

[0022] Several embodiments of the invention are explained in more detail below with reference to a drawing. In the drawings:

[0023] Fig. 1 Components of an X-ray device in an overview,

[0024] Fig. 2 shows an X-ray tube of the device according to Fig. 1 in perspective view, Fig. 3 shows another X-ray device in a representation analogous to Fig. 1, with three tubes,

[0025] Fig. 4 to 6 Details of X-ray tubes, each in perspective view,

[0026] Fig. 7 shows an X-ray tube in section and components connected to the X-ray tube,

[0027] Fig. 8 shows a detail of an emitter control in an X-ray tube.

[0028] The following explanations apply, unless otherwise stated, to all exemplary embodiments. An X-ray device, designated overall by 10, which in the examples outlined represents a medical imaging device, namely a tomography device, comprises a plurality, for example three, X-ray tubes 1 of essentially or completely identical construction.

[0029] Each X-ray tube 1 comprises a vacuum vessel 2, whose elongated tube housing provides a pressure-tight wall 3. Located within the vacuum vessel 2 are an electron source, designated overall by 4, and an associated anode 5, which emits X-ray radiation, in particular bremsstrahlung, in a manner known per se when electrons strike it.

[0030] The electron source 4 includes field emission cathodes 6 in the form of NT cathodes (nanotubes) located on a ceramic substrate 7. The ceramic substrate 7 is partially covered by a molybdenum intermediate layer 8. On this intermediate layer 8 is an extraction grid 9, also referred to as a mesh. The extraction grid 9, in turn, is partially covered by a ceramic intermediate layer 11, on which a molybdenum cover layer 12 is located. The beam path of the electrons emitted by the electron source 4 is influenced by a focusing electrode arrangement, designated overall by 13, which is located between the electron source 4 and the anode 5. Individual focusing electrodes are designated by 14. As can be seen from Figs. 4 to 7, the focusing electrodes 14 are electrically connected to the tube housing 3 via foil strips 20. In the arrangement according to Fig.4, a second foil strip 20 is located on the right side of the field emission cathodes 6, in a manner not shown.

[0031] To operate the electron source 4, a control module 15 is provided, which is connected to the X-ray tube 1 via a control cable 16, with a vacuum feedthrough in the tube housing 3 designated 17. Regarding the supply of high voltage to the anode 5, there is an anode voltage supply unit 18 and a high-voltage cable 19 connected to it.

[0032] A linearization resistor 21 is located in the high-voltage cable 19, a maximum of a few tens of centimeters from an anode plug 25 protruding from the vacuum vessel 2. A zero distance can also be achieved by integrating the linearization resistor 21 into the anode plug 25. In this way, stored energy in a cable remnant, which might be located at a distance between the anode plug 25 and the linearization resistor 21 and would discharge within the X-ray tube 1 in the event of arcing, can be eliminated.

[0033] The linearization resistor 21 has an electrical resistance of at least 2 kOhm and at most 50 kOhm. During normal operation of the X-ray tube 1, the linearization resistor 21 has hardly any effect on the control of the anode 5. The main purpose of the linearization resistor 21 is to minimize the adverse effects of breakdowns, so-called arcing events, which are conceivable, for example, during conditioning of the X-ray tube 1, by drastically reducing the energy that must be absorbed by components of the X-ray tube 1 during such events compared to older device concepts. The linearization resistor 21 can be integrated into a common cooling circuit 31 together with the anode 5. This means that the same fluid, for example, oil, which flows through the anode 5 for cooling purposes also flows through the linearization resistor 21.A heat exchanger within the cooling circuit 31 is designated by 22. The various components 5, 21, 22 of the cooling circuit 31 are connected to one another via coolant lines 23.

[0034] In the embodiment according to Fig. 1, only a single X-ray tube 1 is controlled. Details of this X-ray tube 1 including the connected linearization resistor 21 can be seen in Fig. 2. In contrast to the design according to Fig. 2, the X-ray tube 1 could also have a curved shape. In both cases, it is possible to arrange a plurality of X-ray tubes 1 in a polygonal or ring shape. This also applies to the embodiment according to Fig. 3, in which three identical X-ray tubes 1 are present, wherein, among other things, a splitter box 24 is provided to control these X-ray tubes, which is connected to the anode voltage supply unit 18. The splitter box 24 prevents arcing energy from being exchanged between the X-ray tubes 1 and the creation of reflection waves. For this purpose, it is not necessary for the splitter box 24 to be designed as a separate device.It is also possible, for example, to integrate the Splitterbox 24 into the generator.

[0035] The electron sources 4 shown in Figures 4 to 6 are suitable for use both within the arrangement shown in Figure 1 and within the arrangement shown in Figure 3. Electrical conductors 30, among other things, are provided for connecting the field emission cathodes. Grounding lines are designated 32. The one-piece design of the focusing electrodes 14 is clearly visible.

[0036] Additional protective measures in addition to the linearization resistor 21

[0037] The devices for absorbing the effects of breakdowns are outlined in Fig. 8. This figure should be understood to mean that the connection to the control module 15 (ECS) is located at the lower edge of the depicted field, while in this illustration, vertically aligned conductors 30 are connected to the field emission cathodes 6 at the upper edge of the depicted field. A resistor 27 is inserted into each of these conductors 30. Furthermore, there is a conductor 30 connected to the extraction grid 9, which is connected to the wall 3 of the vacuum vessel 2 via a surge arrester in the form of a gas discharge tube 29. Furthermore, an optional diode 28 can be seen, which—in the same circuit arrangement as the gas discharge tube 29—also represents a surge arrester. The surge arrester 29 is located outside the vacuum vessel 2 to prevent damage to the ultra-high vacuum in the X-ray tube 1.

[0038] Concerning possible arcing from the anode 5 towards the electron source 4, please refer to Fig. 7. The following arcing cases can be distinguished from one another:

[0039] Arcing directly on focusing electrode 14 Arcing directly on molybdenum cover layer 12 Arcing directly on mesh 9 Arcing directly on emitter 6

[0040] In Fig. 7, the third case, i.e. "arcing directly on mesh 9", is visualized as an example, whereby the arrangement of the individual components of the X-ray tube 1 corresponds in all cases to the arrangement shown in Fig. 7.

[0041] In the first case, i.e., in the case of "arcing directly on the focusing electrode 14," the arcing energy is limited in the HV feedthrough in conjunction with the metal foil 20 welded to the wall 3 of the vacuum vessel 2, i.e., to the tube housing. Important here is the fact that a weld seam is formed over the entire length of the focusing electrode 14, through which the arcing energy is dissipated via the system ground. This closes the circuit to the HV generator. This discharge path represents the lowest resistance for the arcing energy. This largely eliminates the risk of damage to the emitters 6. The expected discharge current and the temporal progression will not change. However, the extensive grounding will prevent dangerous voltages from building up along the focusing electrode 14 in the event of arcing and from breaking through to the mesh 9.

[0042] The second case, i.e., the case of "arcing directly on the molybdenum cover layer 12," is similar to the first case, since the focusing electrode 14 is electrically connected to the molybdenum cover layer 12 via the metal foil 20, i.e., the foil strip. The underlying ceramic intermediate layer 11 serves as further insulation to the extraction grid 9. Therefore, in this case, the arcing energy is also dissipated via the main ground. This closes the circuit to the HV generator. This discharge path represents the least resistance for the arcing energy. In contrast to case 1, the proximity to the emitters 6 could allow the material released by arcing to reach the emitters 6 and impair their function.

[0043] In the third case, i.e., the case of "arcing directly on mesh 9," it is essential that the circuit to the HV generator is closed as short as possible. This is possible by establishing the electrical connection from the ECS GND to the main ground at the vacuum feedthrough. This is achieved with the help of the module, which additionally contains a cathode-side linearization resistor and a gas discharge tube to limit the arcing energy and voltage. In the arcing case, the largest portion of the discharge is diverted via the top spacer above it, i.e., the molybdenum cover layer 12. Partial discharges via mesh 9 must be diverted to the tube housing 3 via the shortest possible path, i.e., directly at the tube housing 3 using a gas discharge tube or a diode.

[0044] Any remaining overvoltages must be absorbed by the ECS, i.e., the control module 15. In case four, i.e., the "arcing directly on emitter 6" case, limiting the emitter current is essential. The emitters 6 are operated in pulsed mode with a negative voltage during normal operation. Since the emitters 6 are well protected from the anode 5 by the mesh 9 and the top spacer, a direct strike to the emitters 6 is unlikely. If at all, only small partial discharges are likely to penetrate to the emitters 6. The series resistors in the connectors allow the emitters 6 to yield voltage-wise, thus limiting the emitter current to safe values. Any remaining overvoltages must also be absorbed by the ECS in this case.

[0045] The probability that one of cases two to four occurs is minimized, on the one hand, by limiting and quickly dissipating the arcing energy, and, on the other hand, is already significantly reduced by the fact that the focusing electrode arrangement 13 covers a large part of the electron source 4. Looking from the anode 5 to the electron source 4, at least half of the surface of the electron source 4 is shielded by the focusing electrode arrangement 13 in the exemplary embodiments.

[0046] List of reference symbols

[0047] X-ray tube

[0048] Vacuum container

[0049] wall

[0050] Electron source

[0051] anode

[0052] Field emission cathode ceramic substrate

[0053] Molybdenum interlayer

[0054] Extraction grid

[0055] X-ray machine ceramic interlayer

[0056] Molybdenum top layer

[0057] Focusing electrode arrangement

[0058] Focusing electrode

[0059] Control module

[0060] Control cable

[0061] Vacuum feedthrough

[0062] Anode voltage supply unit

[0063] High-voltage cables

[0064] foil strips

[0065] Linearization resistor

[0066] heat exchanger

[0067] coolant line

[0068] Splitter box

[0069] Anode connector

[0070] resistor diode gas discharge tube electrical conductor cooling circuit ground wire

Claims

Patent claims 1. X-ray tube (1), with a vacuum container (2) in which an electron source (4) is arranged, the electrons of which are directed onto an anode (5) provided for emitting X-radiation and also arranged in the vacuum container (2), wherein a cable (19) supplying the anode (5) with high voltage is guided through a wall (3) of the vacuum container (2), characterized by a linearization resistor (21) inserted into the high-voltage cable (19) between the wall (3) of the vacuum container (2) and an anode voltage supply unit (18) located outside the vacuum container (2).

2. X-ray tube (1) according to claim 1, characterized in that an electrical resistance of at least 100 ohms and at most 100 kOhm is provided as the linearization resistor (21).

3. X-ray tube (1) according to claim 1 or 2, characterized in that the distance between an anode plug (25) and the linearization resistor (21) is at least 10 cm and at most 40 cm.

4. X-ray tube (1) according to claim 1 or 2, characterized in that the linearization resistor (21) is embedded in the anode plug (25).

5. X-ray tube (1) according to claim 1 or 2, characterized in that the linearization resistor (21) is designed as a resistance cable connected to the anode plug (25).

6. X-ray tube (1) according to one of claims 1 to 5, characterized in that the linearization resistor (21) is designed as a high-voltage-resistant, pulse-resistant resistor.

7. X-ray tube (1) according to one of claims 1 to 6, characterized in that the linearization resistor (21) is passively cooled.

8. X-ray tube (1) according to one of claims 1 to 6, characterized in that the linearization resistor (21) is fluid-cooled.

9. X-ray tube (1) according to claim 8, characterized in that the linearization resistor (21) is integrated together with the anode (5) into a common cooling circuit (31).

10. X-ray tube (1) according to one of claims 1 to 9, characterized in that between the electron source (4) and the anode (5) there is a focusing electrode arrangement (13) which - viewed in projection from the anode (5) onto the electron source (4) - covers more than half the area of ​​a substrate (7) on which the electron source (4) is located.

11. X-ray tube (1) according to claim 10, characterized in that the focusing electrode arrangement (13) comprises a number of focusing electrodes (14) which are electrically conductively connected, in particular welded, to film strips (20), wherein the film strips (20) are aligned in the longitudinal direction of the X-ray tube (1) and are electrically conductively connected to the grounded tube housing (3).

12. X-ray tube (1) according to claim 11, characterized in that the foil strips (20) are welded to the focusing electrodes (14) over the entire length thereof, measured in the longitudinal direction of the X-ray tube (1).

13. X-ray tube (1) according to one of claims 1 to 12, characterized by a surge arrester (29), in particular in the form of a bidirectional gas arrester, provided for protecting the electron source (4) and arranged outside the wall (3) of the vacuum container (2).

14. A method for operating an X-ray tube (1), wherein an electron source (4) arranged in the X-ray tube (1) and having field emission cathodes (6) directs an electron beam onto an anode (5) also arranged in the X-ray tube (1), which anode is supplied with high voltage by means of an anode voltage supply unit (18) which is connected to the anode (5) with the interposition of a linearization resistor (21).

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