X-ray source and method for compensating focal spot movement

Abstract

X-ray source (1) with an anode (3) arranged within a vacuum housing (4), wherein at least the anode (3) is rotatably mounted, can be set into a rotational movement by an electric drive (8) and can be acted upon in the region of a focal spot (B) by an electron beam (E) emitted by a cathode (2), characterized in that a control unit (7), depending on at least one operating parameter of the electric drive, controls an electromagnetic deflection unit (5) deflecting the electron beam (E) in such a way that a movement of the focal spot (B) caused by electromagnetic fields of the electric drive (8) can be at least partially compensated.

Description

[0001] The invention relates to an X-ray tube with an anode arranged within a vacuum housing, wherein at least the anode is rotatably mounted and can be set into rotation by an electric drive, and wherein the anode can be acted upon by an electron beam emitted from a cathode in the region of a focal spot. The invention further relates to a method for compensating for focal spot movement during the operation of such an X-ray tube.

[0002] X-ray equipment, particularly medical imaging X-ray equipment such as computed tomography (CT) scanners, comprises one or more X-ray sources whose rotatably mounted anodes are illuminated by electron beams, which may be focused, to generate X-rays. The area where the electron beam strikes the rotating anode material is usually referred to as the focal spot. In X-ray sources whose anodes are designed as rotating anodes, these are typically set in motion by an electric drive to distribute the heat generated in the focal spot over a larger area of ​​the rotating anode.

[0003] It has been shown that the position and extent of the focal spot can vary during operation of the X-ray tube. This causes fluctuations in the generated X-ray radiation, which can negatively affect the quality of acquired X-ray images. As a countermeasure, DE 103 01 071 A1 proposed the use of position detection of the focal spot and classical control of its position as a controlled variable. This means that a value of the control parameter used to adjust the controlled variable is generated based on a control deviation of a measured actual value of the controlled variable from a predetermined setpoint. A disadvantage of this approach is that measurable deviations of the controlled variable—in this case, the focal spot position—must first exist before compensation can be performed. Due to the control dynamics, some movement of the focal spot remains, which depends on the amplitude of the original, uncompensated focal spot movement.Furthermore, direct, active control requires the detection of the spatial position and extent of the focal spot, which is sensor-intensive. However, such methods, implemented as control loops, are designed to detect disturbances of both known and unknown origin by detecting the focal spot position and, if necessary, to compensate for them.

[0004] DE 10 2012 204 841 A1 proposes a rotating anode X-ray source for particularly interference-free X-ray generation, comprising an X-ray tube for generating X-rays, a rotatable rotating anode arranged in a vacuum vessel with a rotating axis, a rotor arranged on the rotating axis and a stator which is at least partially located outside the vacuum vessel, wherein the stator generates an alternating electromagnetic field for driving the rotor, and wherein the stator has at least one stator coil for generating the alternating electromagnetic field, as well as a cathode for generating an electron beam that can be accelerated onto the rotating anode, wherein at least one counter coil is provided to compensate for the alternating electromagnetic field of the at least one stator coil in the area of ​​the cathode and / or the electron beam.

[0005] DE 100 38 176 C1 relates to a medical examination system with an MR system and an X-ray system, which includes an X-ray source with an X-ray tube and a solid-state X-ray image detector for producing X-ray images, wherein the X-ray system includes sensors for detecting the spatial dependence of the stray field of the MR system in the three spatial axes, coils for compensating the stray field and a computer that calculates a current for the coils from the output signal of the sensors, as a result of which the stray field in the area of ​​the electron beams of the X-ray tube is reduced.

[0006] The object of the invention is to provide a device and a method which enable efficient compensation of the focal spot movement.

[0007] With regard to the device, the aforementioned problem is solved by an X-ray emitter of the type mentioned at the outset with the further features of claim 1.

[0008] Advantageous further developments of the invention are the subject of the dependent claims.

[0009] An X-ray tube has an anode arranged within a vacuum housing, wherein at least the anode is rotatably mounted and can be set into rotation by an electric drive. The anode can be acted upon by an electron beam emitted from a cathode in the region of a focal spot. According to the invention, a control unit is provided which, depending on at least one operating parameter of the electric drive, controls an electromagnetic deflection unit for deflecting the electron beam in such a way that a movement of the focal spot caused by electromagnetic fields of the electric drive can be at least partially compensated.

[0010] The invention is based on the understanding that the movement of the focal spot is at least partially caused directly by electromagnetic fields generated during the operation of the electric drive. This measurable influence on the position and potentially also on the extent of the focal spot can be compensated for by controlling the electromagnetic deflection unit, which, for example, comprises one or more coils deflecting the electron beam, according to measured values ​​for at least one operating parameter of the electric drive. The relationship between the at least one operating parameter of the electric drive and the focal spot movement can be measured and recorded before the X-ray tube is put into operation and thus used as the basis for the control.For example, the periodic influence of the alternating fields caused by the electric drive on the electron beam can be largely compensated before a change in the electron beam's position becomes apparent. The compensation of the focal spot movement thus occurs immediately and, in particular, faster than with conventional control systems, where a significant deviation of the actual position of the focal spot from a predetermined target position must first be present and detected.

[0011] A control system should be characterized here by an open or a closed control path, whereby the output variables influenced by the input variables do not act on themselves again via the same input variables.

[0012] However, the control unit of the X-ray tube can be integrated into a higher-level control loop, particularly within the framework of a disturbance variable feedforward. Such a control loop requires active position detection of the focal spot during operation of the X-ray tube using appropriately designed measuring instruments. Implementing the control unit within a disturbance variable feedforward in the higher-level control loop has the advantage that it allows for direct compensation of the portion of the focal spot movement caused by the electromagnetic fields of the electric drive, and additionally, the elimination of focal spot movements of other, especially unknown, origins by the control system.

[0013] In other cases, a simple compensation control without a higher-level control loop is provided. In this case, active compensation of the focal spot movement is still possible based on at least one operating parameter of the electric drive, whereby the position of the focal spot does not necessarily have to be detected during the operation of the X-ray tube.

[0014] The electromagnetic deflection unit includes, for example, one or more electromagnetic deflection coils with or without ferromagnetic cores or electrostatically chargeable deflection plates.

[0015] In possible embodiments of the invention, the anode is designed as a rotating anode rotatably mounted within the vacuum housing, which is particularly stationary. In these embodiments, the rotating anode is set into rotational motion relative to the stationary vacuum housing and the cathode during operation, in order to distribute the heat input acting on the anode over a larger surface area.

[0016] In other possible embodiments, the vacuum housing is rotatably mounted and set into rotation by an electric drive. The cathode and anode are fixedly connected to the vacuum housing. In other words, the design of the X-ray tube corresponds to a rotary piston tube, in which the vacuum housing, which supports both the anode and the cathode, is set into rotation during operation.

[0017] Preferably, the operating parameter of the electric drive, on which the control of the electromagnetic deflection unit is based, is a stator current amplitude and / or a stator current phase angle. Particularly preferably, the control is based on several of the aforementioned parameters.

[0018] The movement of the focal spot can be decomposed into two geometric components: a radial component and a tangential component. The dependence of these components on one or more operating parameters, particularly the applied stator current amplitude and / or the stator current phase, can be determined, especially in a single calibration step. The recorded dependencies can be stored in a storage medium that is operatively connected to the control unit, so that they can be used as the basis for controlling the operation of the X-ray tube. The storage medium is preferably a non-volatile data storage device, such as a ROM (read-only memory), EPROM (erasable programmable read-only memory), or flash memory.

[0019] The X-ray emitter preferably comprises a measuring unit that detects at least one operating parameter of the electric drive and sends a measurement signal to the control unit.

[0020] In a further development of the invention, it is provided that the control unit controls the electromagnetic deflection unit depending on at least one additional operating parameter of the X-ray tube. It has been shown that the focal spot movement correlated with the operating parameter(s) of the electric drive depends on further quantities, in particular operating parameters assigned to the X-ray tube. In this way, these influences, which are measurable in themselves, can be taken into account within the framework of a control system and / or a disturbance variable feedforward.

[0021] The operating parameter of the X-ray tube is, for example, a tube voltage. Alternatively or additionally, temperature-dependent electromagnetic effects can be taken into account by measuring the temperature, in particular the operating temperature, of the X-ray tube.

[0022] In this context, it is advantageous to provide a further measuring unit that detects at least one operating parameter of the X-ray tube. For example, a suitably designed measuring unit is provided on the high-voltage generator to detect the tube voltage. Alternatively or additionally, a temperature sensor is integrated into the X-ray tube.

[0023] With regard to the method, the aforementioned problem is solved by a method for compensating for focal spot movement with the further features of claim 9. The associated advantages will follow directly from the preceding description with reference to the X-ray tube according to the present invention.

[0024] In a method for compensating focal spot movement during the operation of the aforementioned X-ray tube, an anode is arranged within a vacuum housing and is stimulated by an electron beam to generate X-rays. At least the anode is rotatably mounted and is set into rotation by an electric drive. According to the invention, a control unit controls an electromagnetic deflection unit that deflects the electron beam, depending on at least one operating parameter of the electric drive, such that a movement of the focal spot caused by electromagnetic fields of the electric drive is at least partially compensated.

[0025] The dependence of the focal spot movement on the operating parameters of the electric drive is a measurable effect that can be detected and recorded, particularly in a single calibration measurement. This forms the basis for compensating the portion of the focal spot movement caused by the electromagnetic fields generated during the operation of the electric drive. In this respect, no control deviation detection is necessary; that is, position detection of the focal spot during operation is not essential. The method can therefore be implemented in a simple compensation controller.

[0026] The method proposed by the invention can also be advantageously implemented within a higher-level control system with active detection of the focal spot movement, using feedforward control. The control of the electromagnetic deflection unit for compensating the movement of the focal spot caused by electromagnetic fields from the electric drive is then performed as a subsystem in a control loop where the actual position of the focal spot is actively detected as the controlled variable during operation. The influence of the electric drive on the focal spot movement can thus be at least partially eliminated in advance, without requiring a response from the higher-level control system. Ideally, the influence of the electric drive on the focal spot movement is completely eliminated, so that any control deviation where the actual position of the focal spot deviates significantly from the target position has a different origin.The feedforward control system forms a control mechanism superimposed on the control loop. In such training examples, the influence of the electric drive is at least partially eliminated by the additional control signals of the feedforward control system, while the behavior of the remaining control loop, in particular its stability and tracking behavior, ideally remains unchanged.

[0027] The control unit preferably controls the electromagnetic deflection unit based on at least one additional operating parameter of the X-ray tube. In these embodiments, complex control is achieved based on several variables, which can be determined in a comprehensive calibration measurement prior to commissioning the X-ray tube. This allows for the consideration of further measurable disturbances, which directly or indirectly influence, in particular, the electromagnetic fields occurring during operation, within the framework of a control system or disturbance feedforward. Examples of such operating parameters of the X-ray tube are the applied tube voltage or a temperature, especially an operating temperature, of the X-ray tube.

[0028] The dependence of the focal spot movement on at least one operating parameter of the electric drive and / or on at least one operating parameter of the X-ray tube is recorded in a calibration step and preferably stored as a discrete data structure in a storage medium, particularly in a non-volatile data storage device such as an EEPROM or flash memory. A discrete data structure is defined as one in which discrete values ​​of the correlated quantities are assigned to each other. For example, the discrete data structure takes the form of a multidimensional lookup table.

[0029] Preferably, the discrete data structure is interpolated to generate the control signals for the electromagnetic deflection unit. The intermediate values ​​required for controlling the electromagnetic deflection unit are thus generated from the stored discrete values ​​using a suitable interpolation. For this purpose, the control unit is equipped with appropriate computing resources, such as microprocessors, microcontrollers, integrated circuits, or similar devices. Preferably, the discrete data structure is interpolated linearly; in other applications, a higher-order interpolation, i.e., quadratic or higher-order interpolation, is used. Reducing the results to analytical equations, and thus reducing the number of parameters, is also proposed as a possible implementation.

[0030] Preferably, the X-ray source and / or the method for compensating focal spot movement described above is used in an X-ray imaging device. The X-ray imaging device is intended, for example, for medical imaging, materials testing, or baggage screening. The X-ray imaging device is particularly preferably designed as a computed tomography scanner or a C-arm X-ray unit.

[0031] The properties, features and advantages of the invention described above, as well as the manner in which these are achieved, will become clearer and more easily understood in connection with the following description of the exemplary embodiments, which will be explained in more detail with reference to the drawings.

[0032] For a further description of the invention, reference is made to the exemplary embodiments shown in the drawings. These are shown in a schematic representation: Fig. 1: an X-ray source with a rotary piston X-ray tube according to a first embodiment in a schematic sectional view; Fig. 2: an X-ray source with a rotating anode according to a second embodiment in a schematic sectional view; Fig. 3: the construction of a control system for an electromagnetic or electrostatic deflection unit according to a first embodiment; Fig. 4: the construction of a control system for an electromagnetic or electrostatic deflection unit according to a second embodiment; Fig. 5: a control of the focal spot movement with disturbance variable feedforward.

[0033] Corresponding parts are marked with the same reference symbols in all figures.

[0034] Fig. Figure 1 shows an X-ray source 1 designed as a rotary piston source according to a first embodiment. The X-ray source 1 comprises a cathode 2 and an anode 3, which are arranged non-rotatably within a rotatably mounted vacuum housing 4. During operation of the X-ray source 1, the evacuated vacuum housing 4 is evacuated by means of a Fig. 1 electric drive not shown in detail (cf. the electric drive designated 8 of the Fig. 2) is set into a rotational motion. A high voltage is applied between the cathode 2 and the anode 3 when the X-ray source 1 is in operation, causing an electron beam E to emanate from the cathode 1 and strike the anode 3. To ensure that the electron beam E strikes the anode 3 in the designated lateral edge region, the electron beam E is appropriately focused and deflected. For this purpose, a deflection unit 5 is provided, which in the exemplary embodiment shown is designed as an electromagnetic deflection coil. The electron beam E strikes the material of the anode 3 in the region of the so-called focal spot B. The resulting X-ray radiation R exits the X-ray source 1 laterally through an exit window 6.

[0035] The position of the focal spot B is generally influenced by various disturbances during operation. To compensate for focal spot movement caused by these disturbances, the electromagnetic deflection unit 5 generates a correspondingly opposing, time-varying deflection field. For this purpose, the electromagnetic or electrostatic deflection unit 5 is connected to a control unit 7, which provides control signals that are executed according to pre-determined correlations that control the focal spot movement as a function of operating parameters of the [system / device]. Fig. 1. The electric drive (not shown in detail) is characterized by these correlations. These correlations take into account, at least partially, the influence of the electric drive on the time-varying position P of the focal spot B and are stored in a discrete data structure, for example, in the form of a lookup table on a storage medium 71 of the control unit 7. The control unit 71 also includes digital computing devices 9, such as microprocessors or integrated circuits, which are configured to perform any calculations necessary for control. The computing devices 72 are specifically configured to calculate further intermediate values ​​necessary for control from the values ​​stored in the discrete data structure by means of first- or higher-order interpolation.

[0036] The values ​​to be stored in the data structure, which characterize the dependence of the time-varying position P of the focal spot B on operating parameters of the electric drive 8, are recorded in advance, i.e. during the calibration of the X-ray tube 1 in calibration measurements, and stored in the storage medium 71.

[0037] Fig. Figure 2 shows a further embodiment of the X-ray source 1 with cathode 2 and an anode 3 designed as a rotating anode. In this embodiment, the electric drive 8 driving the rotating anode is explicitly shown. The anode 3, designed as a rotating anode, has a hollow shaft 9 which is rotatably mounted relative to a stationary shaft 11 via bearings 10, in particular ball bearings.

[0038] In the illustrated embodiment, the electric drive 8 is a squirrel-cage motor and comprises, in a manner known per se, a stator 12 and a rotor 13 which is non-rotatably connected to the rotating anode 3.

[0039] The X-ray tube 1 of the in Fig. The second embodiment shown in Figure 2 further comprises a protective housing 14 surrounding the evacuated vacuum housing 4, which has a further outlet window 15. The protective housing 14 is filled with a coolant, for example, an insulating oil.

[0040] The deflection unit 5 of the second embodiment is operated by the in Fig. The control unit 7 (not shown in detail) is controlled as a function of the operating parameters of the electric drive 8. The operating parameter of the electric drive 8 under consideration is preferably a stator current amplitude A or a stator current phase Ph, whereby the load-dependent rotor slip can also be taken into account when determining the focal spot movements generated by the electric drive 8.

[0041] Fig. Figure 3 schematically illustrates a method for compensating the focal spot movement within the framework of a simple compensation control system. Position detection of the focal spot B during the operation of the X-ray tube 1 is not necessary in this case, since the control system is based entirely on the correlations between values ​​for the operating parameters of the electric drive 8 and the position P of the focal spot B, which are stored in the form of a discrete data structure.

[0042] During operation of the X-ray tube 1, the stator current amplitude A and stator current phase Ph are measured by measuring units 16. The currently available values ​​for these operating parameters of the electric drive 8 are supplied to the control unit 7. Based on the correlations stored in the storage medium 71 between the stator current amplitude A and stator current phase Ph on the one hand, and the position P of the focal spot B on the other, the control unit 7 generates control signals St for the electromagnetic deflection unit 5 such that the variation in the focal spot position induced by the fields of the electric drive 8 is at least partially compensated. For this purpose, the discrete values ​​stored in the storage medium 71 are interpolated linearly or with a higher order, if necessary, using the computing means 72.

[0043] Fig. Figure 4 shows a version where the control of the Fig. 3 is extended by the fact that additional operating parameters assigned to the X-ray tube 1 are recorded and thus taken into account during operation by further measuring units 17. Specifically, these additional parameters are a tube voltage S applied between cathode 2 and anode 3 and a temperature T. The data stored in the storage medium 71 are supplemented by the corresponding dependencies with regard to the position P of the focal spot B. The data structure stored in the storage medium 71 has the form of a multidimensional lookup table. In this way, the influence of the tube voltage S and temperature-dependent effects on the position P of the focal spot B is taken into account within the framework of the illustrated compensation control.

[0044] Fig. Figure 5 illustrates a control loop for active control of the position P of the focal spot B, where the in Fig. 3 or Fig.The control system illustrated in section 4 is implemented as a disturbance feedforward. The position P of the focal spot B is therefore the controlled variable, which is used as the actual position P. Ist actively detected and fed to an input of a control device 18. From a predefined target position P soll is based on the current position P Ist A control deviation ΔP is calculated in a manner known per se. Depending on this control deviation ΔP, the control unit 18 controls the electromagnetic deflection unit 5, taking into account the control signal St provided by the control unit 7 within the framework of a disturbance feedforward. In this way, the focal spot movements caused by the electromagnetic fields of the electric drive 8 are already compensated, so that any remaining control deviations ΔP ideally have a different origin.

[0045] Although the invention has been illustrated and described in detail with reference to preferred embodiments, the invention is not limited by this. Other variations and combinations can be derived by those skilled in the art without deviating from the essential concept of the invention.

Claims

[1] X-ray source (1) with an anode (3) arranged within a vacuum housing (4), wherein at least the anode (3) is rotatably mounted, can be set into a rotational motion by an electric drive (8) and can be acted upon in the area of ​​a focal spot (B) by an electron beam (E) emitted from a cathode (2), characterized by , that a control unit (7) depending on at least one operating parameter of the electric drive controls an electromagnetic deflection unit (5) that deflects the electron beam (E) in such a way that a movement of the focal spot (B) caused by electromagnetic fields of the electric drive (8) can be at least partially compensated. [2] X-ray tube according to claim 1, characterized by , that the anode (3) is designed as a rotating anode rotatably mounted within the stationary vacuum housing (4). [3] X-ray tube according to claim 1, characterized by, that the vacuum housing (4) is rotatably mounted and can be set into a rotary motion by the electric drive (8), wherein the cathode (2) and the anode (2) are connected to the vacuum housing (4) in a rotationally fixed manner. [4] X-ray tube (1) according to any one of the preceding claims, characterized by , that the operating parameter of the electric drive (8) is a stator current amplitude (A) and / or a stator current phase position (Ph). [5] X-ray tube (1) according to any one of the preceding claims, characterized by a measuring unit (16) that captures at least one operating parameter of the electric drive (8). [6] X-ray tube (1) according to any one of the preceding claims, characterized by , that the control unit (7) controls the electromagnetic deflection unit (5) depending on at least one additional operating parameter of the X-ray tube (1). [7] X-ray tube (1) according to claim 6, characterized by, that the operating parameter of the X-ray tube (1) is a tube voltage and / or a temperature. [8] X-ray emitter (1) according to claim 6 or 7, characterized by a further measuring unit (17) that captures at least one operating parameter of the X-ray emitter (1). [9] Method for compensating for focal spot movement during the operation of an X-ray source (1) comprising an anode (3) arranged within a vacuum housing (4) which is subjected to an electron beam (E) to generate X-ray radiation (R), wherein at least the anode (3) is set into a rotational motion by an electric drive (8), characterized by, that an electromagnetic deflection unit (5) deflecting the electron beam (E) is controlled as a function of at least one operating parameter of the electric drive (8) in such a way that a movement of the focal spot (B) caused by electromagnetic fields of the electric drive (8) is at least partially compensated. [10] Method according to claim 9, characterized by , that the control of the deflection unit (5) depends on at least one operating parameter of the electric drive (8) within the framework of a disturbance feedforward in a control loop in which an actual position (P Ist ) of the focal spot (B) is recorded as a controlled variable, is implemented. [11] Method according to claim 9 or 10, characterized by , that the control unit (7) controls the deflection unit (5) depending on at least one additional operating parameter of the X-ray tube (1). [12] Method according to any one of claims 9 to 11, characterized by, that the dependence of the focal spot movement on at least one operating parameter of the electric drive (8) and / or on at least one operating parameter of the X-ray tube (1) is stored in a storage medium (71) assigned to the control unit (7) as a discrete data structure, in particular in the form of a multidimensional look-up table. [13] Method according to claim 12, characterized by , that the discrete data structure for generating control signals (St) for the deflection unit (5) is interpolated.

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