Patent Grant
11558950
The present application claims priority to German Patent Application No. 102017008264.2, filed on Sep. 2, 2017, and International Application No. PCT/EP2018/025225 filed on Aug. 31, 2018, titled “Control Device for an X-Ray Tube and Method for Operating an X-ray Tube,” and assigned to the assignee of the present invention. German Patent Application No. 102017008264.2 and International Application No. PCT/EP2018/025225 are incorporated by reference herein.
The invention relates to a device for controlling an X-ray tube and a method for operating an X-ray tube.
A method for controlling an X-ray tube is known, for example, from U.S. Pat. No. 7,751,582 B2. In this case the X-ray system is designed as a tomosynthesis system having a plurality of stationary X-ray sources arranged in a row.
In general, X-ray tubes have electron emitters, the function of which can depend on various physical principles. In DE 10 2011 076 912 B4, among other things, dispenser cathodes are named as thermal emitters. Information on the use of dispenser cathodes, for example, can be found in DE 10 2010 043 561 A1.
Electronic control devices for multifocus X-ray tubes, the cathodes of which are intended for the thermal emission of electrons, are known, for example, from documents EP 1 617 764 B1 and EP 1 618 368 B1.
Particularly suitable emitters for field emission of electrons are emitters containing nanorods, especially carbon nanorods. In this connection reference is made to documents WO 2018/086737 A1 and WO 2018/086744 A2.
A method for regulation of the emission current for X-ray tubes is disclosed in DE 10 2009 017 649 B4. Here, a current regulation can be superimposed on a voltage regulation.
The investigation is based on the problem of further developing the control of X-ray tubes, especially X-ray tubes with field emission cathodes, compared to the prior art, wherein a particularly high operational reliability is to be achieved.
This problem is solved according to the invention by a device for controlling an X-ray tube according to claim 1. The problem is also solved by an operating method according to claim 13. In the following, embodiments and advantages of the invention explained in connection with the method will also apply to the controlling device and vice-versa.
The control device is intended for operating an X-ray tube comprising an anode designed as an X-ray emitter and a plurality of cathodes intended for generating electron beams directed toward the anode.
The control device includes a housing, designed as a shield, in which an anode current regulating device is arranged. The anode current regulating device is connected to a cathode power supply unit, with a plurality of cathode voltage switches each to be connected to a cathode, as well as with a programmable assembly in which the control of the cathodes is determined. In this case also the cathode power supply unit, the cathode voltage switch, as well as the programmable assembly are arranged in the housing noted.
As a result of the shielded placement of the power and control electronics for the electron tube in the common housing, together with a suitable circuit board layout, the electromagnetic radiation emittance is distinctly reduced compared with conventional solutions. Thus effects on and interference with other electronic equipment as well as between different circuit sections of the electronic system can be prevented.
For example, the programmable assembly of the control device comprises an FPGA (Field Programmable Gate Arrangement) and at least one digital-analog converter. The anode current control unit, which is a central voltage-controlled current source, is controlled by the FPGA or another programmable assembly or an arrangement of such assemblies over the at least one digital-analog converter. The FPGA or an element with comparable function controls a number of subsystems. Possible subsystems in the present case may include the voltage supply unit—i.e., the power supply unit—of the cathodes, an anode power supply unit, various supply units for focusing devices and grids, as well as a power source to be assigned to the anode current control unit and the cathode voltage switches.
Even before the performance of a pulse sequence of the cathodes, the FPGA is already programmed such that the pulse sequence is triggered in real time. The timing of the pulse sequence takes place purely through the FPGA or an element of similar function. To allow for rapid switching between various current values, for example two analog-digital converters are each programmed with the voltage value corresponding to the equivalent current. Through a multiplexer it is possible to switch between the desired voltage level for a boost or for the actual pulse. The boost is defined here as a peak generated at the beginning of the pulse, with which a rectangular form of the pulse with improved approximation to the theoretical ideal form can be achieved compared with pulses generated without a short-term voltage overshoot.
The cathode voltage switch over which the cathodes, i.e., the electron emitters of the X-ray tube, are supplied with electric power, are structured for example as a high-voltage switch bank with a number of MOSFETs. Here, several MOSFETs are connected in series, optionally within a single cathode voltage switch.
The anode current control unit makes it possible to control the electron current emitted by the cathodes, i.e., electron emitters, from cathode to cathode in real time. In each case, an actual current flowing through the cathode and an assigned nominal-value current enter into the control. In addition, currents flowing through extraction grids and through focusing devices can enter into the control.
Since the order in which the high-voltage switches can be controlled is freely programmable, the sequence and number of emitters used can be freely programmed. Thus not all emitters must be operated, and the X-ray tube can also be operated as a single-beam tube. When a corresponding multiplexer is used, several or all channels may be activated simultaneously and thus electron emitters activated in parallel.
Typically, focusing electrodes are assigned to the individual cathodes. In a preferred embodiment, an extraction grid located between the cathodes and the focusing electrode is grounded independently of the focusing electrodes.
Through the energy supply of the focusing electrodes and grid, the thermal focal spot size on the anode can be adjusted individually from emitter to emitter. The thermal focal spot size is to be considered without projection in this situation. The X-ray focal spot size to be viewed under a projection is to be distinguished from this. It is also true for the X-ray focal spot size that this can be adjustable from pulse to pulse for each emitter. As long as the focusing electrodes can be operated at constant voltage, the focal spot size can be adjusted by varying the grid voltage, also in the form of fine tuning. This is true both in the continuous mode and for a pulsed mode, wherein in each case adjustments different from emitter to emitter are possible in all cases.
Fundamental considerations on controlling the cathodes and advantages achieved are summarized as follows:
Through the bank of high-voltage switches, each of which is assigned to a respective cathode, it is possible to switch rapidly between the individual channels. Each switching channel of the bank in this case preferably comprises several serial SiC MOSFETs to achieve the necessary cutoff voltage. In the case of a flashover that is detected by the FPGA via the gate emitter voltage after the power source or by the anode control via the altered anode current, for protecting the emitter, the total gate drive circuit is separated from the MOSFETs, which form the bank from high-voltage switches. This is realized via the multiplexers with which the output of the joint gate driver is distributed to the individual channels of the bank of high-voltage switches in normal operation. To protect the bank of high-voltage switches from destruction in case of a flashover, the voltage is preferably monitored by a circuit via the MOSFET cascade.
According to an advantageous further development, the programmable assembly of the control device is designed for storing the operating parameters, especially including current and voltage values, measured during operation of the X-ray tube.
Monitoring the control device is especially significant with regard to flashovers, which may conceivably occur during operation of the X-ray tube because of the high voltages at the anode. A flashover is a short circuit between electron emitters and anodes. In this process, the anode current may reach a current peak, which lasts only nanoseconds. Because of the rapidity of the anode current control, in the microsecond range, this current pulse will very likely not be detected by the control. However, the current pulse can be demonstrated in the measured anode current.
Therefore, in advantageous process control to protect against flashover, the measured anode current is compared with an adjustable maximum value of the anode current in a comparator. In case of a flashover and thus exceedance of the maximum current value, a positive voltage is achieved at the outlet of the comparator, representing a digital value of one. When the value is below the maximum, the comparator puts out the base value, in other words, digital zero. The duration of this detection mechanism depends almost exclusively on the duration of detection of the comparator. Depending on the comparator, this is in the pico- or nanosecond range. As soon as the maximum value is exceeded, the digital value of the comparator is transmitted with the aid of an optocoupler over an additional connecting cable between the anode power supply unit and the voltage supply unit of the cathodes, and the electron emission of the cathode is stopped immediately by a MOSFET switch so that no damage to the electron emitter will occur. Furthermore, in a certain form of flashover, conclusions can be drawn regarding a flashover that will occur in the future, based on the change in the anode current trend and the cathode current trend. For this purpose the anode current is measured, as described, and when the anode current and the cathode current decline without a reason being apparent from the control (the anode current target value not having been changed), the predicted occurrence of a flashover will be transmitted based on the same transmission mechanism as described above to the voltage supply unit of the cathodes. Then the electron emission of the cathode will be turned off even before flashover takes place. In this form of flashover avoidance, the time to shut-off of the electron emission is less critical, since as measurements have shown, the decrease in the anode current can already be detected microseconds before the flashover occurs.
If a flashover should nevertheless occur, the influence thereof is minimized in an advantageous configuration in that the electrical voltages of the power supply unit of the cathode are based on the grid. Thus the voltage difference between the grid and the emitter will not be changed in the case of a flashover onto the grid and thus also the number of electrons released in the emitter will not be changed. This ensures a long useful life of the emitter. The voltage between anode and grid altered by flashover to the grid does not present a threat to the lifetime of the emitter.
In the X-ray tubes operated using the control device, for example, dispenser cathodes are used as electron emitters.
In particularly preferred embodiments, the cathodes of the X-ray tube are field emission cathodes, especially cathodes with nanorods, also called nanosticks.
The nanosticks are preferably made of a material that has the lowest possible electron work function relative to the quantum mechanical field emission effect. Here, the nanosticks have a composition that is inherently uniform or nonuniform and are formed either as hollow bodies, i.e., tubes, or in solid form. The cathodes may be nanosticks of the same kind or a mixture of different kinds of nanosticks, wherein the kind of the nanosticks relates to their material composition and material modification.
Suitable materials in pure or doped form for the field emission of electrons are, for example, single- or multiple-walled carbon nanotubes, single- or multiple-walled hetero-nitrogen-carbon nanotubes, rare earth borides, especially lanthanum hexaboride and cerium hexaboride, metal oxides, especially TiO2, MnO, ZnO and Al2O3, metal sulfides, especially molybdenum sulfide, nitrides, especially boron nitride, aluminum nitride, carbon nitride, gallium nitride, carbides, especially silicon carbide, silicon. Starting products for producing the nanosticks, which emit electrons during operation of the cathodes, also include rod-shaped, optionally hollow, elements made of polymeric materials. The nanosticks of the cathodes are optionally made from starting products that are only partially made of polymer materials, especially in the form of a coating.
In a particularly preferred embodiment, on their surfaces the cathodes have nanosticks in a preferably vertical direction, in other words, in the direction of the anode. Upon operation of the X-ray emitter and at adequate distances from one another, very strong electric fields can be generated at the tips of the nanosticks, substantially simplifying the emission of electrons.
In pulsed operation of the cathodes, capacitances of the cathodes and the elements electrically connected to the cathodes, especially supply lines, play a role. To minimize undesirable effects of such capacitances, optionally a discharge circuit is connected to the cathode voltage switch. The discharge circuit represents a complementary solution component to the previously described voltage overshoots, at the beginning of a rectangular peak to be generated.
In addition to the pulsed operation of the cathodes, in a preferred embodiment, pulse operation of the anode of the X-ray tube is also possible. Here, an anode voltage supply unit supplies a direct current in the form of a pulsed unipolar voltage. In this embodiment the anode voltage supply unit to be assigned to the control device is preferably a Marx generator. The level of the voltage pulse applied to the anode can differ from pulse to pulse.
The method of the invention for operating the X-ray tube is characterized by the following features:
The anode current regulation can take place in various ways, explained in the following. First, the commonalities of all control possibilities will be discussed, and finally the differences in them will be pointed out.
Upon emission of electrons in the X-ray tube, an anode current flows through both a cascade assigned to one of the control devices and connected to the X-ray tube and also through a control unit forming a component of the control device. This is converted into a voltage and measured either in the control unit or in the cascade by a measuring resistance or an operating amplifier circuit.
This voltage, proportional to the anode current, serves as the input variable for anode current regulation. Here, the voltage values can be present, either in digital form through an analog-digital converter or as an analog value. An additional input value is the information about the current setpoint. Here also the information may consist of a digital value or an analog voltage value obtained from a voltage value proportional to the current setpoint, wherein an analog value is obtained with the aid of a digital-analog conversion.
In any case, the current setpoint of the cathode is obtained as the initial value. This means that an internal control loop for regulating the cathode current is present, so that this will follow the cathode current setpoint as rapidly as possible. Furthermore there is an external control loop that regulates the anode current by changing the cathode current setpoint. For anode current regulation through specification of the cathode current setpoint, the anode current information must be transmitted, by either digital or analog means, from the circuit board that accomplishes power supply for the anode to the circuit board over which the cathode is supplied with electric power. In the case of analog transmission, the circuit boards are connected with a cable that is as interference-free as possible. For this purpose the reference potential of the anode voltage, proportional to the anode current or the digital value must be altered because of the different voltage ranges on the individual circuit boards. This is done by using analog or digital optocouplers.
Basically two possibilities exist for accomplishing the control. The control can either be established digitally in the form of an algorithm or in an analog manner as an operational amplifier. There is an advantage to digital control in that it is easily adaptable. However, the control is not as rapid as with the analog variants. On the other hand, it was found by measurements that the anode current is constant over a long time period and differs from the cathode current only by a constant transmission factor. Therefore, even without active control, the anode current can be adapted to the anode current by determining the transmission factor in an initial calibration run and storing the transmission factor in a lookup table of the anode current. These two control methods may also be combined, so that first the transmission factor is determined and the anode current is set using this, and then the anode current is kept constant even if the transmission rate changes using analog or digital control.
During operation of the X-ray tube, as a result of already mentioned focusing mechanisms, each of which is assigned to a cathode, focal spots can be produced on the anode, differing from one cathode to another. Variation of the focal spot size is possible both at constant anode voltage and in the case of pulsed anode voltage with voltage different from pulse to pulse. The possibility also exists of influencing the geometry of a focal spot with an extraction grid located before the electron-emitting material, i.e., using the extraction grid as a means for focusing the electron beam.
According to an advantageous process variant, changes in the current flowing through the anode are detected so that a trend of the changes can be determined if necessary. By automatically determining and evaluating a trend of this type, under certain circumstances an increasing risk of flashover between anode and electron emitter can be inferred. In such a case the power supply of the cathode will be cut off automatically to prevent damage to the X-ray tube and minimize downtime.
If the anodes are operated in pulsed fashion, capacitances of the anode and connected components are also significant. As a rule, a rectangular pulse form is desired during pulsed operation of the anode. To achieve the rectangular form as well as possible, a voltage overshoot can be generated at the beginning of a pulse to compensate for the effect of unwanted capacitances. A particular advantage of pulsed operation of the anode is the fact that successive pulses may be at different voltage levels. As a result of the different voltage levels, X-ray pulses with different wavelengths of X-ray radiation are generated. The wavelengths in these cases can be adapted to the X-ray properties of different materials found in the object to be examined. This allows various materials in the object to be examined to be distinguished very well. This is preferably done with a stationary, especially non-rotating, arrangement of X-ray sources.
Unless stated otherwise, the explanations that follow pertain to all exemplary embodiments. Corresponding parts or parameters are labeled with the same reference symbols in all Figures.
An X-ray apparatus 1 comprises an X-ray tube 2 and a control device 3. Components of the X-ray tube 2 are a cathode 4 as electron source and an anode 5, which is struck by an electron beam EB generated by the cathode 4, generating X-rays XR. Between the electron source 4 and the anode 5, a focusing device 6 for the electron beam EB is located.
In the exemplary embodiment according to
The focusing device 6 comprises various focusing electrodes 11, 12 connected sequentially. Design variants of the focusing electrodes 11, 12 are sketched in
The control device 3 used for operating the x-ray tube 2 comprises an anode power supply unit 14, which supplies the anode 5 with high voltage. The electric current actually flowing through the anode 5 is designated as IA-actual. In contrast, IA-S designates the anode setpoint.
The value of the anode setpoint, IA-S is entered into an anode current control unit 19. The anode current control unit 19, as the power source, constitutes a central unit of a current control loop, which can be of various types, as will be further explained in the following.
Independently of the detailed design of the anode current control, the control device 3 includes a voltage supply unit 15 of the focusing electrode 12 and a voltage supply unit 16 of the focusing electrode 11. In addition, a voltage supply unit 17 of the extraction grid 10 is present. The voltage supply unit 17 comprises an insulating transformer. With this, galvanic separation between the reference potential designated as BP in
The anode current control unit 19 is connected with a voltage supply unit 18 of the cathodes 4 and a cathode switch arrangement 20. In addition, the anode current control unit 19 is connected with a programmable assembly 25, which comprises a microcontroller 26 and a FPGA (Field Programmable Gate Arrangement) 27. The components 18, 19, 20, 25 mentioned are assembled into a cathode control device 28, which is located in a housing 29 designed as a shield. An external housing 30 shown in broken lines in
These additional components include, among other things, the anode power supply unit 14. As is apparent from
The control of the emitter current source, i.e., the anode current control unit 19, is visualized in
The signal Sig delivered by the anode current control unit 19 is conducted to the cathode switch arrangement 20, as is sketched in
With the aid of the circuit according to
As is apparent from
The control device 3 offers the possibility of operating not only the cathodes 4 but also the anode 5 in pulsed mode. As is apparent from
The anode power supply unit 14 according to
Prepulse compensation PPC of the control device 3 is provided for avoiding a short-term voltage decrease, a so-called drop, at the beginning of a voltage pulse, and as is indicated in
In contrast to
In addition, a heating element is present, which is to be connected via a heating connection HA.
The anode power supply unit 14 according to
The embodiment according to
In contrast to the arrangements according to
In the last-named case, the circuit shown for use in
The current control can be designed in the form of various control loops CR1, CR2, CR3, CR4. In all cases a certain anode current setpoint IA-S is preset. This current setpoint IA-S is compared with measured values. In the simplest case this is merely a matter of the actual anode current IA-actual. The corresponding control loop is designated by CR2. If the grid current designated by IG is also included in the control, i.e., the current flowing out through the extraction grid 10, the control loop CR4 is present. The focusing electrodes 11, 12 also play a role in the control loops CR3 and CR1. In the case of control loop CR3, the focusing electrodes 11, 12 are operated passively, i.e., at the same potential as the housing of the X-ray tube 2. On the other hand, in the case of control loop CR1, active focusing is used. In this case the focusing electrodes 11, 12 can be operated with constant or pulsed voltages on the order of −10 KV to +10 KV. The current flowing through the focusing electrodes 11, 12 is designated by IF1 and IF2 respectively. The control loop CR1 is the most complex form of current regulation overall.
With the diagram according to
The trigger signals T2 and T1 mark the end and the beginning of idling states IP. After the end of the idling phase IP shown first chronologically in
The plurality of individual cathodes 4, which are located within the X-ray tube 2 and are controlled by the central anode current control unit 19, are shown schematically in
Although the structures, devices, methods, and systems have been described in accordance with particular embodiments, one of ordinary skill in the art will readily recognize that many variations to the particular embodiments are possible, and any variations should therefore be considered to be within the spirit and scope disclosed herein. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.
The claims following this written disclosure are hereby expressly incorporated into the present written disclosure, with each claim standing on its own as a separate embodiment. This disclosure includes all permutations of the independent claims with their dependent claims. Moreover, additional embodiments capable of derivation from the independent and dependent claims that follow are also expressly incorporated into the present written description. These additional embodiments are determined by replacing the dependency of a given dependent claim with the phrase “any of the claims beginning with claim [x] and ending with the claim that immediately precedes this one,” where the bracketed term “[x]” is replaced with the number of the most recently recited independent claim. For example, for the first claim set that begins with independent claim 1, claim 3 can depend from either of claims 1 and 2, with these separate dependencies yielding two distinct embodiments; claim 4 can depend from any one of claim 1, 2, or 3, with these separate dependencies yielding three distinct embodiments; claim 6 can depend from any one of claims 1, 2, 3, or 4, with these separate dependencies yielding four distinct embodiments; and so on.
Recitation in the claims of the term “first” with respect to a feature or element does not necessarily imply the existence of a second or additional such feature or element. Elements specifically recited in means-plus-function format, if any, are intended to be construed to cover the corresponding structure, material, or acts described herein and equivalents thereof in accordance with 35 U.S.C. § 112(f). Embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.
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| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/025225 | 8/31/2018 | WO |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2019/042587 | 3/7/2019 | WO | A |
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| Number | Date | Country | |
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| 20200367350 A1 | Nov 2020 | US |
