SILICON FIELD EFFECT EMITE

Description

[0001] The invention relates to an X-ray tube, an X-ray device, a method for generating X-ray radiation in a predetermined spatial distribution on an anode and an associated computer program product.

[0002] Typically, X-rays are generated in an X-ray tube by bombarding an anode with electrons. The resolving power of the X-ray tube is determined by the spatial distribution of the electrons upon impact with the anode. The electrons are conventionally emitted by a thermionic tungsten emitter, a carbon field-effect emitter, and / or a field-effect emitter with a dispenser cathode. The emitters described above have in common that an electron emission density is usually limited to approximately 3 A / cm^2. However, for imaging examinations, an electron emission density of approximately 10 A / cm^2 is usually required, which is why the electrons from conventional emitters are usually focused onto the anode, particularly by means of a deflection unit.The focusing of the electrons typically depends on an accelerating voltage between a cathode with an emitter and an anode, as well as the space charge density of the electrons. Thus, the spatial distribution of the electrons usually changes depending on the accelerating voltage and / or the intensity of the electron beam.

[0003] From EP 3 531 437 A1 an electron emission device with at least one electron emitter and with at least one barrier grid is known.

[0004] EP 3 518 266 A1 discloses a thermionic emission device with a flat emitter and with a switchable field-effect electron emitter.

[0005] Guerrera et al. describe a silicon field-effect emitter with an electron emission density exceeding 100 A / cm^2 in "Silicon Field Emitter Arrays With Current Densities Exceeding 100 A / cm^2 at Gate Voltages Below 75 V" (IEEE ELECTRON DEVICE LETTERS, VOL. 37, NO. 1, JANUARY 2016).

[0006] US 2009 / 0 185 660 A1 and US 2010 / 0 260 317 A1 disclose a multifocal X-ray source based on a plurality of field-effect emitter units. US 2010 / 0 239 064 A1 discloses a computed tomography system with a plurality of X-ray sources arranged at different angles. Brodie et al. investigate the use of microelectronic units in vacuum devices (ADVANCES IN ELECTRONICS AND ELECTRON PHYSICS, VOL. 83, ISBN 0-12-014725-4).

[0007] The invention is based on the object of specifying an X-ray tube, an X-ray device, a method for generating X-ray radiation in a predetermined spatial distribution on an anode and an associated computer program product in which the emission of electrons is controlled more flexibly.

[0008] The problem is solved by the features of the independent claims. Advantageous embodiments are described in the subclaims.

[0009] The method according to claim 1 for generating X-ray radiation in a predetermined spatial distribution on an anode of an X-ray tube comprises the following steps: Specifying the spatial distribution of electrons impinging on the anode of an X-ray tube, selecting a group of several field-effect emitter needles of an emitter of the X-ray tube depending on an accelerating voltage in the control unit, switching on the selected group of several field-effect emitter needles, whereby X-rays are generated in the specified spatial distribution on the anode.

[0010] The spatial distribution corresponds, in particular, to a modulation transfer function. The spatial distribution can be specified, for example, by the user and / or physician using input devices, in particular a keyboard, a mouse, and / or a screen. Alternatively, a control unit can automatically determine the spatial distribution, in particular depending on the measurement protocol specified by the user and / or physician.

[0011] The group is preferably selected automatically in the control unit, in particular depending on the measurement protocol specified by the user and / or doctor.

[0012] Switching on the selected group of multiple field-effect emitter needles may involve applying a gate-emitter voltage. By switching on the selected group of multiple field-effect emitter needles, electrons are emitted and aligned toward the anode in such a way that X-rays are generated when the electrons strike the anode at the specified spatial distribution.

[0013] The X-ray tube has an anode, a first switching device, a second switching device, a control unit and an emitter with a plurality of field-effect emitter needles, wherein at least one field effect emitter needle of the plurality Field effect emitter needles have a diameter of less than 1 µm and silicon,wherein a first group of the plurality of field-effect emitter needles can be switched on or off by means of the first switching device, wherein a second group of the plurality of field-effect emitter needles can be switched on or off by means of the second switching device, wherein the first group differs from the second group in an arrangement and in the number of the plurality of field-effect emitter needles and in the applied acceleration voltage, wherein the first group and the second group have at least partially the same field-effect emitter needles, and wherein the control unit is designed to control the first switching device and the second switching device. The emitter with the plurality of field-effect emitter needles is preferably designed according to the silicon field-effect emitter of Guerrera et al.The at least one field-effect emitter needle can in particular have a diameter between 10 nm and 800 nm, preferably between 100 nm and 500 nm, particularly advantageously between 150 nm and 250 nm, for example 200 nm. Typically, the at least one field-effect emitter needle has no carbon and only silicon. In principle, it is conceivable for each field-effect emitter needle to have silicon and / or be made of silicon. The emitter has in particular between 2 and 2,500,000, preferably between 100,000 and 1,000,000,000 field-effect emitter needles per square millimeter. The plurality of field-effect emitter needles are typically spaced apart from one another by between 10 nm and 500 µm, preferably between 200 nm and 1 µm. The plurality of field-effect emitter needles are usually aligned parallel.

[0014] Typically, the at least one field-effect emitter needle, preferably each field-effect emitter needle, can be switched on with a gate-emitter voltage, in particular greater than 0 V, less than 1 kV, preferably less than 200 V, and particularly advantageously less than 100 V. In particular, the gate-emitter voltage forces such an electric field strength at a tip of the switched-on field-effect emitter needles that electrons are emitted from the switched-on field-effect emitter needles. Switching off the first group and / or the second group can be achieved, in particular, by applying a gate-emitter voltage equal to or less than 0 V.

[0015] The anode can, in particular, be a stationary anode or a rotating anode. The anode comprises, for example, tungsten for generating X-rays from the electrons. The rotating anode can be part of a rotary piston X-ray tube or a rotating anode X-ray tube. The X-ray tube is typically evacuated. To accelerate the electrons emitted by the switched-on field-effect emitter needles, an accelerating voltage is typically applied between the emitter and the anode. The accelerating voltage is provided, for example, by a voltage supply. The X-ray tube comprises, for example, the voltage supply for applying a first acceleration voltage or a second acceleration voltage between the emitter and the anode. The first acceleration voltage can be different from or equal to the second acceleration voltage.The control unit controls the first switching device or the second switching device depending on the applied acceleration voltage.

[0016] The plurality of field-effect emitter needles are connected in groups. The first group and the second group differ, typically in at least one field-effect emitter needle. The first group and the second group have at least some of the same field-effect emitter needles.

[0017] The first group and the second group are not the same.

[0018] The multiple field-effect emitter needles are typically distributed in two spatial directions and / or arranged in a grid pattern. The arrangement of the multiple field-effect emitter needles is usually such that the emitter is a two-dimensional emitter. Advantageously, the multiple field-effect emitter needles are arranged in a plane and / or aligned parallel. In principle, it is conceivable for the multiple field-effect emitter needles to be arranged linearly or multidimensionally.

[0019] It is conceivable for each field-effect emitter needle to have a switch connected to the first switching device and / or the second switching device. The switch can be a field-effect emitter needle voltage supply for providing the gate-emitter voltage. The first switching device and the second switching device can, in particular, be logically mapped in program code means, while in particular the switches of the plurality of field-effect emitter needles can electronically switch the emission of electrons at the respective field-effect emitter needles. In particular, if the first switching device and the second switching device are logically mapped, the first group and the second group can preferably be repeatedly occupied by the plurality of field-effect emitter needles.In other words, the control unit can advantageously select the field-effect emitter needles for the first group and / or the field-effect emitter needles for the second group, for example, depending on a measurement protocol. In this case, the X-ray tube can preferably be used for the various measurement protocols without the field-effect emitter needles being electronically connected again. The groups of the multiple field-effect emitter needles can be controlled, in particular, by means of the control unit such that the control unit programs the first switching device and / or the second switching device with the respective switches of the first group and / or the second group.

[0020] Switching the emission of electrons may involve completely blocking, particularly during turn-off, and / or continuously regulating the emission of electrons, particularly during turn-on. Continuously regulating the emission of electrons may correspond to adjusting the emission current of the respective switched-on field-effect emission needle. The continuous regulation may depend on the gate-emitter voltage.

[0021] The switching on and off, preferably the temporally staggered group-related switching, of the field-effect emitter needles can be repeated, in particular, at a frequency of greater than 0.1 Hz, advantageously greater than 10 kHz, and particularly advantageously greater than 1 MHz. Compared to a thermionic emitter, the high-frequency switching on and off is particularly advantageous because the heat distribution within the emitter can be efficiently controlled and / or the total number of emitted electrons, in particular a total emission current as the sum of the emission currents, can be flexibly adjusted.

[0022] The first switching device is linked to the first group, for example via the switches of the respective field-effect emitter needles of the first group, and the second switching device is linked to the second group, for example via the switches of the respective field-effect emitter needles of the second group. The link corresponds in particular to an electronic circuit. The control unit is in particular designed to control the first switching device and the second switching device in such a way that, by emitting a first switch-on signal, for example, the first switching device switches on the first group of the plurality of field-effect emitter needles, in particular their switches, and / or by emitting a second switch-on signal, for example, the second switching device switches on the second group of the plurality of field-effect emitter needles, in particular their switches.The first switching device can switch on the field-effect emitter needles of the first group according to the switch-on signal, for example, by switching on the switches of the field-effect emitter needles of the first group. As an alternative to switching on, the control additionally includes switching off, for example, by transmitting a switch-off signal.

[0023] Advantageously, the X-ray tube configured in this way can provide the electron emission density required for an imaging examination, in particular approximately 10 A / cm^2, preferably on the anode. Only a portion, for example, the first group or the second group, of the plurality of field-effect emitter needles is switched on. If not all field-effect emitter needles are switched on, this is advantageous, for example, for the service life of the X-ray tube, in particular for the emitter with the plurality of field-effect emitter needles, especially when heat development is homogeneously distributed within the emitter. The longer service life is particularly advantageous with regard to cost reduction.

[0024] Advantageously, the X-ray tube does not have an electron beam deflection unit between the emitter and the anode. A conventional electron beam deflection unit usually has a magnetic deflection unit. The number and / or arrangement of the first group and / or the second group are advantageously selected such that the X-ray tube does not require an electron beam deflection unit between the emitter and the anode. As a result, the weight of the X-ray tube is, in particular, lower than that of a conventional X-ray tube with a deflection unit. In this case, the X-ray tube is usually less expensive than the conventional X-ray tube. Furthermore, control of the electron beam is preferably simplified if the X-ray tube is less complex due to the lack of a deflection unit.

[0025] The fact that the multiple field-effect emitter needles can be divided into groups and / or switched enables, in particular, the switching on of the field-effect emitter needles depending on different measurement protocols for the imaging examination, wherein the spatial distribution of the electrons on the anode preferably remains unchanged. The different measurement protocols can also differ, in particular, in an X-ray dose and / or in a resolution. The multiple field-effect emitter needles of the first group and / or the multiple field-effect emitter needles of the second group can be specified, for example, by a manufacturer of the X-ray tube and / or a user and / or a physician of the X-ray tube. The specification of the field-effect emitter needles for the respective group can correspond to programming the emitter.Advantageously, in addition to the first group and the second group, further groups of field-effect emitter needles can be defined. Preferably, the control unit can define a number of circuit devices and the field-effect emitter needles assigned to the respective circuit device in groups, depending on the measurement protocol.

[0026] Furthermore, switching on the field-effect emitter needles in groups enables flexible shaping of the electron beam. Particularly advantageously, the field-effect emitter needles of the first group and the field-effect emitter needles of the second group are arranged such that a predetermined spatial distribution of the electrons impinging on the anode is essentially independent of the switched-on group.

[0027] The first group differs from the second group in an arrangement of the multiple field-effect emitter needles. The arrangement is not congruent, in particular with respect to a reference point. If the field-effect emitter needles of the two groups differ in their arrangement, the shape of the electron beam is usually influenced, in particular, by physical interactions between the emitted electrons and thus the spatial distribution of the electrons. Taking the physical interactions into account preferably allows the omission of the deflection unit. The physical interactions include, in particular, electrostatic effects due to the space charge density of the electrons. The arrangement of the switched-on field-effect emitter needles can be, for example, star-shaped, circular, or arbitrary.

[0028] The first group differs from the second group in the number of field-effect emitter needles. A variation in the number usually automatically results in a difference in the arrangement. Varying the number of field-effect emitter needles in each group preferably enables scaling of the emission current.

[0029] The first group differs from the second group in the applied acceleration voltage. This is particularly advantageous because, depending on the acceleration voltage, the heat distribution can be distributed across the field-effect emitter needles.

[0030] The arrangement and / or the number of field-effect emitter needles and / or the applied acceleration voltage can be determined, for example, in a simulation of the X-ray tube or during a test measurement with the X-ray tube and / or stored group-specifically in a memory unit of the control unit.

[0031] According to the invention, the first group differs from the second group in the number and arrangement of the multiple field-effect emitter needles, as well as in the applied acceleration voltage. This advantageously allows the physical interactions between the emitted electrons, particularly those due to the space charge density, to be taken into account such that the spatial distribution of the electrons on the anode is the same regardless of the connected circuit device. In other words, in this exemplary embodiment, the electrons advantageously impinge on the anode with the same spatial distribution, regardless of which field-effect emitter needles are connected.A further advantage may be that the X-ray dose of the X-rays generated by the electrons is independent of the field-effect emitter needles in use, with the X-ray dose typically depending on the applied accelerating voltage. The X-ray dose corresponds, in particular, to the intensity of the X-rays.

[0032] It is conceivable that the first switching device and / or the second switching device are configured to switch on the respective plurality of field-effect emitter needles in such a way that each switched-on field-effect emitter needle supplies a saturation current. This is particularly advantageous since the first group differs from the second group in the number of the plurality of field-effect emitter needles. In this case, the total emission current of the emitter typically correlates with the number of switched-on field-effect emitter needles. The total emission current can be controlled in a particularly flexible manner. According to Guerrera et al., the saturation current can be determined by a specific structure of the plurality of field-effect emitter needles, wherein the specific structure includes, in particular, an effective cross-sectional area and / or a doping density of the respective field-effect emitter needle and / or a saturation velocity of the electrons.Operating the field-effect emitter needles in saturation is particularly advantageous because it keeps the emission current of the respective switched-on field-effect emitter needle constant. In other words, controlling the first switching device and the second switching device comprises binary switching of the first group or the second group, in particular without continuous current regulation. In saturation, the respective switched-on field-effect emitter needle supplies a constant current, regardless of the applied electric field. The respective switched-on field-effect emitter needle, in particular, has a current limit. Compared to a silicon field-effect emitter, a conventional carbon field-effect emitter can be destroyed by an excessively large electric field because the conventional carbon field-effect emitter typically does not have a current limit.In this embodiment, the emission current is typically not continuously adjustable. The gate-emitter voltage is preferably greater than or equal to a saturation voltage.

[0033] The X-ray device according to the invention comprises the X-ray tube and the X-ray detector. The X-ray detector can advantageously detect the X-ray radiation passing through a patient during an imaging examination. For example, a medical image can be reconstructed using the detected X-ray radiation. The medical image can, for example, be provided to the user or the physician on a display unit and / or stored in a radiology information system and / or in a PACS image archiving system. The X-ray device can be designed, in particular, as a conventional X-ray system, as a single-source computed tomography system, as a mammography system, or as a C-arm angiography system.

[0034] One embodiment provides that the X-ray device is designed for an imaging examination with alternating acceleration voltage. The alternating acceleration voltage is specified, in particular, according to the dual-energy measurement protocol. The acceleration voltage preferably changes at a frequency greater than 1 Hz, preferably greater than 100 Hz, and particularly advantageously greater than 1 kHz. A further advantage of this embodiment is that the alternating acceleration voltage can be switched quickly using the respective group of field-effect emitter needles. In this embodiment, for example, materials within the patient can be differentiated, wherein the materials typically have an attenuation coefficient as a function of the acceleration voltage. The materials can, in particular, be tissue and / or bone and / or contrast agents.In particular, the X-ray system has only a single X-ray tube, preferably without an additional X-ray tube as in a conventional dual-energy X-ray system. In other words, no additional X-ray tube is required for the imaging examination according to the dual-energy measurement protocol.

[0035] The computer program product can be a computer program or comprise a computer program. The computer program product has the program code means that map the method steps according to the invention. This allows the method according to the invention to be carried out in a defined and repeatable manner, and control over the forwarding of the method according to the invention can be exercised. The computer program product is configured such that the computing unit executes the method steps according to the invention using the computer program product. The program code means can be loaded into a memory of the computing unit and typically executed by a processor of the computing unit with access to the memory. If the computer program product, in particular the program code means, is executed in the computing unit, typically all inventive embodiments of the described method can be carried out.The computer program product is, for example, stored on a physical, computer-readable medium and / or digitally stored as a data packet in a computer network. The computer program product can represent the physical, computer-readable medium and / or the data packet in the computer network. The invention can therefore also be based on the physical, computer-readable medium and / or the data packet in the computer network. The physical, computer-readable medium can usually be connected directly to the computing unit, for example by inserting the physical, computer-readable medium into a DVD drive or plugging it into a USB port, as a result of which the computing unit can access the physical, computer-readable medium, in particular for reading purposes. The data packet can preferably be retrieved from the computer network. The computer network can have the computing unit or can be connected to the computing unit by means of a wide area network (WAN) ora (wireless) local area network connection (WLAN or LAN) to the computing unit. For example, the computer program product can be stored digitally on a cloud server at a storage location of the computer network, transferred to the computing unit via the WAN over the Internet and / or via the WLAN or LAN, in particular by accessing a download link that points to the storage location of the computer program product.

[0036] Features, advantages, or alternative embodiments mentioned in the description of the device are also applicable to the method, and vice versa. In other words, claims to the method can be further developed with features of the device, and vice versa.

[0037] The invention is described and explained in more detail below with reference to the exemplary embodiments illustrated in the figures. In the following description of the figures, essentially identical structures and units are designated by the same reference numerals as when the respective structure or unit first appeared.

[0038] They show: Fig. 1 an emitter having a first group of said plurality of field effect emitter needles, Fig. 2 an emitter with a second group of the plurality of field effect emitter needles, Fig. 3 Trajectories of the emitted electrons as a function of the applied accelerating voltage, Fig. 4 an X-ray device in a case not according to the invention and Fig. 5 a method for generating X-rays in a given spatial distribution on an anode.

[0039] Fig. 1 shows a plan view of an emitter E with a first group G1 of the plurality of field-effect emitter needles F1, F2, FN. The plurality of field-effect emitter needles F1, F2, FN are arranged distributed in two spatial directions and, in this embodiment, comprise 8x10 field-effect emitter needles.

[0040] The first group G1 of the plurality of field-effect emitter needles F1, F2, FN is marked with crosses and, in this exemplary embodiment, comprises 12 field-effect emitter needles. The first number is therefore 12. In principle, it is conceivable that the first group comprises more or fewer than 12 field-effect emitter needles. If the control unit S has a Fig. 1 When the switching device shown is activated and switched on, the first group G1 of the plurality of field-effect emitter needles F1, F2, FN emits electrons. These electrons are accelerated by a first accelerating voltage, which is, for example, between 30 kV and 150 kV, preferably 120 kV.

[0041] The first group G1 has a first arrangement and the first number of the plurality of field effect emitter needles F1, F2, FN, which differ from the Fig. 2 different from what is shown.

[0042] Fig. 2 shows a top view of an emitter E with a second group G2 of the plurality of field-effect emitter needles F1, F2, FN. The second group G2 differs from the first group G1. The second group G2 has a second arrangement and a second number of the plurality of field-effect emitter needles F1, F2, FN. The second number is 20. The second group G1 of the plurality of field-effect emitter needles F1, F2, FN is marked with dashed crosses.

[0043] The electrons emitted from the second group G2 are accelerated by a second acceleration voltage (which, according to the invention, differs from the first acceleration voltage), which is, for example, between 30 kV and 150 kV, preferably 80 kV.

[0044] The Fig. 2 switched field effect emitter needles of the second group G2 are arranged more compactly than those in Fig. 1 switched on field-effect emitter needles of the first group G1. The Fig. 1 and Fig. 2 The groups G1, G2 of the plurality of field-effect emitter needles shown can be switched on at different times, in particular one after the other, for example according to the dual-energy measurement protocol.

[0045] Fig. 3 shows schematic trajectories of the emitted electrons as a function of the time-delayed acceleration voltage. The electrons are emitted in particular when the gate-emitter voltage is applied by means of a Fig. 3 The field-effect emitter needle voltage supply shown is provided accordingly. The electrons emitted by the emitter E are accelerated by a voltage supply V toward an anode A, preferably in a vacuum.

[0046] The solid trajectories differ from the dashed trajectories in the number of emitting field-effect emitter needles and thus in the effect of the physical interactions. The solid trajectories show, for example, electrons of the Fig. 1 shown configuration and the dashed trajectories of the Fig. 2 shown configuration, if the first number of Fig. 1 is less than the second number of Fig. 2 Due to the higher emission current and the associated higher space charge density of the Fig. 2 In the embodiment shown, which results in comparatively stronger physical interactions between the electrons, the dashed trajectories diverge more than the solid trajectories.

[0047] Fig. 4 shows an X-ray tube R in an arrangement not according to the invention. The X-ray tube R has an anode A, a first switching device, a second switching device, a control unit S, and an emitter E with a plurality of field-effect emitter needles F1, F2, FN. At least one field-effect emitter needle of the plurality of field-effect emitter needles F1, F2, FN has a diameter of less than 1 µm and is made of silicon. A first group G1 of the plurality of field-effect emitter needles F1, F2, FN can be switched on or off by means of the first switching device. A second group G2 of the plurality of field-effect emitter needles F1, F2, FN can be switched on or off by means of the second switching device. The first group G1 differs from the second group G2. The control unit S is designed to control the first switching device and the second switching device. According to the invention, the control unit S is part of the X-ray tube R.The control unit S can, in particular, comprise an FPGA or a processor. In this case, which is not according to the invention and is illustrated here, the control unit S is arranged outside the X-ray tube R and is connected to the X-ray tube R for controlling the first switching device and the second switching device.

[0048] In principle, it is conceivable that the first switching device and / or the second switching device are designed to switch on the respective plurality of field-effect emitter needles F1, F2, FN in such a way that each switched-on field-effect emitter needle supplies a saturation current.

[0049] The X-ray system comprises the X-ray tube R and an X-ray detector D. The X-ray system is configured for imaging examinations with alternating acceleration voltages. The acceleration voltage changes during the imaging examination, preferably according to the dual-energy measurement protocol. The X-ray system is shown as part of a single-source computed tomography (CT) scanner. A patient P is positioned on a patient couch L.

[0050] Fig. 5 shows a flowchart of a method for generating X-rays in a given spatial distribution on an anode.

[0051] Method step S100 characterizes a specification of the spatial distribution of electrons impinging on the anode of an X-ray tube.

[0052] Method step S101 denotes a selection of a group of several field-effect emitter needles of an emitter of the X-ray tube as a function of an acceleration voltage.

[0053] Method step S102 identifies a switching on of the selected group of several field effect emitter needles, whereby X-ray radiation is generated in the specified spatial distribution on the anode.

[0054] Although the invention has been illustrated and described in detail by the preferred embodiments, the invention is nevertheless not limited by the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention, which is defined exclusively by the claims.

Claims

1. Method for generating X-ray radiation in a predefined spatial distribution on an anode (A) of an X-ray tube (R), wherein the X-ray tube (R) furthermore has a first switching device, a second switching device, a control unit (S) and an emitter (E) with multiple field effect emitter needles (F1, F2, FN), wherein at least one field effect emitter needle of the multiple field effect emitter needles (F1, F2, FN) has a diameter of less than 1 µm and silicon, wherein a first group (G1) of the multiple field effect emitter needles (F1, F2, FN) can be activated or deactivated by means of the first switching device, wherein a second group (G2) of the multiple field effect emitter needles (F1, F2, FN) can be activated or deactivated by means of the second switching device, wherein the first group (G1) and the second group (G2) have at least partially the same field effect emitter needles, wherein the first group (G1) differs from the second group (G2) in an arrangement and in the number of the multiple field effect emitter needles (F1, F2, FN) as well as in the acceleration voltage applied, wherein the control unit (S) is designed to actuate the first switching device and the second switching device, comprising the steps: - predefining the spatial distribution of electrons striking the anode (A), - selecting a group (G1, G2) as a function of an acceleration voltage in the control unit (S), - activating the selected group (G1, G2) of multiple field effect emitter needles (F1, F2, FN), as a result of which X-ray radiation is generated in the predefined spatial distribution on the anode (A).

2. X-ray tube (R), configured to perform the method according to claim 1.

3. X-ray device, having - the X-ray tube (R) according to claim 2 and - an X-ray detector (D).

4. X-ray device as claimed in claim 3, wherein the X-ray device is designed for an imaging examination with an alternating acceleration voltage.

5. Computer program product which can be loaded directly into a memory of a computer unit of the control unit of the X-ray tube according to claim 2, having program code means in order to carry out a method according to claim 1 when the computer program product is executed in the computer unit.