CONSTANT DISCHARGE CURRENT BLEEDER

Abstract

The present invention relates to a rotary anode X-ray source. In addition to a primary cathode of a rotary anode X-ray tube, an auxiliary cathode is provided in the rotary anode X-ray tube. Electrons from the auxiliary cathode are focused into an area on the anode, from which X-rays cannot enter the used X-ray beam generated by the primary cathode. An emission current controlling device is used to control the electron emission of the auxiliary cathode. Thus, the voltage down-ramp for dual energy scanning is kept constant even though the primary X-ray output changes for the sake of dose modulation or during a transient of the primary electron current.

Claims

1. An X-ray tube for generating an X-ray beam, comprising: a primary cathode; an auxiliary cathode; a rotatable anode; and an electron current controller; wherein the primary cathode is configured to emit first electrons establishing a flow of primary electron current, the first electrons being focused on a first area on the rotatable anode for generating the X-ray beam; wherein the auxiliary cathode is configured to emit second electrons establishing a flow of auxiliary electron current, the second electrons being directed to a second area, which is different from the first area, on the rotatable anode for generating X-rays, wherein the generated X-rays are configured to be directed to a direction different from that of the X-ray beam, such that the X-rays do not enter the X-ray beam; and wherein the electron current controller is configured to adjust the auxiliary electron current in response to a change of the primary electron current, such that a sum of the primary electron current and the auxiliary electron current remains constant.

2. The X-ray tube according to claim 1, wherein the electron current controller comprises an emission control grid arranged between the auxiliary cathode and the anode; and wherein the emission control grid is configured to control the flow of the auxiliary electron current between the auxiliary cathode and the anode.

3. The X-ray tube according to claim 2, wherein the emission control grid has a grid control voltage that is configured to sufficiently reduce the auxiliary electron current, such that the X-ray beam is generated with a maximum X-ray intensity.

4. The X-ray tube according to claim 1, wherein the electron current controller comprises at least one heating supply configured to supply the primary and the auxiliary cathodes with different heating powers, such that the sum of the primary electron current and the auxiliary electron current remains constant.

5. The X-ray tube according to claim 4, wherein the at least one heating supply comprises an alternating current AC heating circuit with a variable frequency; wherein the AC heating circuit is configured to supply the primary and the auxiliary cathodes with different heating powers using at least one of a inductor and a capacitor.

6. The X-ray tube according to claim 4, wherein the at least one heating supply comprises: a primary heating supply associated with the primary cathode; and an auxiliary heating supply associated with the auxiliary cathode, wherein the auxiliary heating supply is configured to change a heating current of the auxiliary cathode to adjust the auxiliary electron current in response to a change of the primary electron current, such that a sum of the primary electron current and the auxiliary electron current remains constant.

7. The X-ray tube according to claim 1, further comprising an emission control grid arranged between the primary cathode and the anode, and wherein the emission control grid is configured to control a shape of the first electrons to adjust a focal spot on the first area on the rotatable anode.

8. The X-ray tube according to claim 7, wherein the emission control grid is configured as a focusing electrode or a set of focusing electrodes to keep a size of the focal spot constant when the tube voltage changes.

9. The X-ray tube according to claim 1, wherein the primary cathode and the auxiliary cathode are connected in series or in parallel.

10. The X-ray tube according to claim 9, wherein, when connected in series with the primary cathode, the auxiliary cathode is configured to produce a sufficiently high auxiliary electron current at a lower heating power compared with a heating power required for the primary cathode to keep the sum of the primary electron current and the auxiliary electron current constant for the case that the primary cathode carries only a minimal primary electron current close to zero.

11. The X-ray tube according to claim 10, wherein the auxiliary cathode is configured to have a slew rate for rise and/or fall of the auxiliary electron current upon a change of the heating current, which is configured to be equal or higher than that of the primary cathode.

12. The X-ray tube according to claim 11, wherein the auxiliary cathode is configured to have: a higher heat conduction from a wire of the auxiliary cathode to a surrounding than that of the primary cathode; and/or a higher heat radiation from a wire of the auxiliary cathode to a surrounding than that of the primary cathode.

13. The X-ray tube according to claim 1, wherein the auxiliary cathode comprises a field emission cathode.

14. An X-ray imaging system, comprising: an X-ray tube comprising: a primary cathode; an auxiliary cathode; a rotatable anode; and an electron current controller; wherein the primary cathode is configured to emit first electrons establishing a flow of primary electron current, the first electrons being focused on a first area on the rotatable anode for generating the X-ray beam; wherein the auxiliary cathode is configured to emit second electrons establishing a flow of auxiliary electron current, the second electrons being directed to a second area, which is different from the first area, on the rotatable anode for generating X-rays, wherein the generated X-rays are configured to be directed to a direction different from that of the X-ray beam, such that the X-rays do not enter the X-ray beam; and wherein the electron current controller is configured to adjust the auxiliary electron current in response to a change of the primary electron current, such that a sum of the primary electron current and the auxiliary electron current remains constant; and an X-ray detector arranged to be opposite to the X-ray tube, wherein the X-ray tube is configured to generate an X-ray beam towards an object of interest; and wherein the X-ray detector is configured to detect attenuated X-rays passing through the object of interest.

15. A method of controlling an X-ray tube control, comprising: emitting, by a primary cathode of an X-ray tube claim 1, first electrons establishing a flow of primary electron current, the first electrons being focused on a first area on a rotatable anode of the X-ray tube for generating an X-ray beam; emitting, by an auxiliary cathode of the X-ray tube, second electrons establishing a flow of auxiliary electron current, the second electrons being directed to a second area, which is different from the first area, on the rotatable anode for generating X-rays, wherein the generated X-rays are configured to be directed to a direction different from that of the X-ray beam such that the X-rays do not enter the X-ray beam; and adjusting, by an electron current controller, the auxiliary electron current in response to a change of the primary electron current such that a sum of the primary electron current and the auxiliary electron current remains constant.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] These and other aspects of the invention will be apparent from and elucidated further with reference to the embodiments described by way of examples in the following description and with reference to the accompanying drawings, in which

[0066] FIG. 1 illustrates a schematic central cut-through view of a conventional rotary anode X-ray tube assembly.

[0067] FIG. 2 illustrates the tube ramp down between the high and low tube voltages during discharging from 140 kV down to 80 kV of a conventional X-ray tube.

[0068] FIG. 3 shows a schematic central cut-through view of a rotary anode X-ray tube 12 according to some embodiments of the present disclosure.

[0069] FIG. 4 shows a schematic central cut-through view of a rotary anode X-ray tube 12 according to some further embodiments of the present disclosure.

[0070] FIG. 5 shows an X-ray system according to some embodiments of the present disclosure.

[0071] FIG. 6 shows a flow diagram of a method of X-ray tube control according to some embodiments of the present disclosure.

[0072] It should be noted that the figures are purely diagrammatic and not drawn to scale. In the figures, elements which correspond to elements already described may have the same reference numerals. Examples, embodiments or optional features, whether indicated as non-limiting or not, are not to be understood as limiting the invention as claimed.

DETAILED DESCRIPTION OF EMBODIMENTS

[0073] In a CT or C-arm X-ray system, a rotatory anode X-ray tube rotates about a region of interest configured to accommodate an object of interest. The rotatory anode X-ray tube generates a beam of X-rays. Opposite to the rotary anode X-ray tube, held on a gantry rotor assembly of a CT scanner or a C-arm assembly, is a detector subsystem which converts attenuated X-rays into electrical signals.

[0074] FIG. 1 illustrates a schematic central cut-through view of a conventional rotary anode X-ray tube assembly. Housing 10 provides a mounting point mounting point for the X-ray source assembly, and typically holds an insulating oil 14 used to provide more effective thermal management by conducting heat away from a rotary anode X-ray tube in operation. A rotary anode X-ray tube 12 is arranged inside the housing 10. The rotary anode X-ray tube 12 is typically formed from glass, and encloses a vacuum 16.

[0075] A stator 18a would be mounted to the housing and typically entirely encompasses X-ray tube 12. The stator is denoted in FIG. 1 as portions 18a and 18b, but these are section views of the same, unitary circular stator. In FIG. 1, a single circular stator 18a is shown in cross-section. An anode support shaft 20 supports a rotor body 22, a bearing system 24, and a rotatable anode disk 26. Rotor body 22, bearing system 24 and anode disk 26 are all arranged to be rotatable around the anode support shaft 20 (aligned with the centre axis 28) inside the rotary X-ray tube 12. Rotor body 22 is, typically, made from copper. The stator 18a and rotor body 22 are arranged in a facing relationship such that when a driving current is applied to stator 18a a magnetic field induces a current in rotor body 22. The current circulating in the rotor body 22 itself opposes the stator magnetic field causing the rotor to exert a rotational force on the bearing system, thus rotating the anode disk 26. Typically, anode disk 26 rotates between fifty and two hundred revolutions per second.

[0076] The bearing system 24 typically comprises a spiral groove bearing (hydrodynamic bearing) having a thrust bearing portion and a radial bearing portion. This ensures a relatively low maintenance and temperature resistant support of the rotational components of the X-ray tube. The bearing system is typically lubricated with a liquid metal lubricant to enable an electrical connection between the anode disk and the outside of the X-ray tube envelope.

[0077] A cathode 30 is provided at the opposite end of the tube to the rotor, and comprises an electrode 32 configured, when energized with a high negative voltage relative to the voltage of the rotary anode, to emit electrons across the gap between the cathode and the anode disk 26. The cathode 30 typically comprises a wire filament or a flat emitter that emits electrons when heated. The temperature of the emitter is controlled by the tube current control of the machine. As the tube current is increased, the temperature of the filament is increased and the filament produces more electrons. The number of electrons available and the time period set for their release from the filament determines the amount of x-rays produced from the anode. Given a tube voltage, the tube-current-time-product thus controls the total number of x-ray photons produced. Electrons are accelerated between electron emitter inside the cathode 30 and the focal spot 34 on the anode disk 26. Upon colliding with the anode disk, the energy of the emitted electrons is substantially converted to heat, which must be dissipated from the anode disk 26, partly through the bearing system and partly by heat radiation into the insulating oil 14. Less than one percent of the electron energy is converted into X-rays emitted from the focal spot 34 on the anode disk 26 outside of the X-ray tube. The X-rays emitted from the focal spot 34 may then be collimated and applied to an object of interest.

[0078] The rotary anode X-ray tube 12 described in FIG. 1 may send X-rays of different spectra allowing imaging the object of interest with spectral material decomposition. This technique has proven to yield better diagnostics, and save toxic contrast agent. The tube voltage may be rapidly switched between a low (for example 70 kV) and high (for example 140 kV) value during a CT scan. Thus, smoothing capacitance in the generator and cables have to be charged and discharged at a fast pace. However, because of the significant ramp up and down between the high and low tube voltages, tube current modulation may become difficult, as the slope of the down ramp may depend on the (modulated) tube current. This uncertainty of the applied X-ray spectra may impair the material decomposition during image reconstruction. As an example, FIG. 2 illustrates the tube ramp down between the high and low tube voltages during discharging from 140 kV down to 80 kV of a conventional X-ray tube. The discharge pattern will depend on the tube current and, thus, on the emitter temperature.

[0079] FIG. 3 shows a schematic central cut-through view of a rotary anode X-ray tube 12 according to some embodiments of the present disclosure. The rotary anode X-ray tube 12 comprises a primary cathode 30a, an auxiliary cathode 30b, a rotatable anode 26, and an electron current controlling device 40. In other words, in addition to the primary cathode 30a, an auxiliary cathode 30b is provided.

[0080] The primary cathode 30a is arranged and configured to emit first electrons establishing a flow of primary electron current 42a. Examples of the primary cathode 30a may include, but not limited to, a field emission cathode, a photo cathode, and an indirectly heated cathode. The first electrons are focused on a first area 34a on the rotatable anode for generating an X-ray beam 44. Typically, less than one percent of the electron energy is converted into X-rays emitted from the focal spot 34a on the rotatable anode 26 outside of the X-ray tube. The X-ray beam 44 emitted from the focal spot 34a may then be collimated and applied to an object of interest. The X-ray beam 44 may also be referred to as used X-rays.

[0081] The auxiliary cathode 30b is arranged and configured to emit second electrons establishing a flow of auxiliary electron current 42b. Examples of the auxiliary cathode 30b may include, but not limited to, a field emission cathode, a photo cathode, and an indirectly heated cathode. Preferably, the auxiliary cathode 30b may be a field emission cathode. The second electrons are directed to a second area 34b, which is different from the first area 34a, on the rotatable anode 26 for generating X-rays 46. The generated X-rays may also be referred to as unused X-rays. The focal spot on the rotatable anode 26, which is created by the auxiliary cathode 30b may not need to be well defined, as the generated X-rays are not used. Hence, the second area 34b may be configured to be large enough to carry high current. The generated X-rays 46 are configured to be directed to a direction different from that of the X-ray beam 44 such that the X-rays 46 do not enter the X-ray beam 44. For example, as illustrated in FIG. 3, the first area 34a and the second area 34b may be both tilted with respect to the centre axis 28, but have faces or fronts pointing in two different directions, e.g., in two opposite directions.

[0082] The electron current controlling device 40 is configured to adjust the auxiliary electron current 42b in response to a change of the primary electron current 42a such that a sum of the primary electron current and the auxiliary electron current remains constant during a multi-energy CT scan or other multi-energy X-ray exposure of an object.

[0083] In an example, as illustrated in FIG. 3, the electron current controlling device 40 may comprise an emission control grid 40a arranged between the auxiliary cathode 30b and the anode 26. The emission control grid 40a is configured to control the flow of the auxiliary electron current 42b between the auxiliary cathode 30b and the anode 26. For example, the intensity of the electron emission the primary cathode 30a emits may be changed by varying the temperature of the primary thermionic electron emitter. The emission control grid 40a may be charged such that the sum of the electron emission from both cathodes 30a, 30b is kept constant no matter what electron emission the primary cathode 30a generates. Typically, the emission control grid may be charged and discharged on a time scale of around 100 ms, when the primary cathode 30a comprises a thermionic emitter. This may enable a suitably fast response to a change of the primary electron current by changing the temperature of the primary emitter. In the exemplary rotatory anode X-ray tube illustrated in FIG. 1, the auxiliary cathode 30b is placed and charged with about the negative potential of the primary cathode 30a. In other words, the primary emitter 30a and the auxiliary emitter 30b may be connected in series. Thus, filaments or flat tungsten emitters are sharing the same heating current.

[0084] Optionally, when connected in series with the primary cathode 30b, the auxiliary cathode 30b may be configured to produce a sufficiently high auxiliary electron current at a lower heating power compared with a heating power required for the primary cathode to keep the sum of the primary electron current and the auxiliary electron current constant for the case that the primary cathode carries only a minimal primary electron current close to zero. In other words, when connected in series with the primary cathode 30a, the auxiliary cathode 30b may be required to be powerful enough. The auxiliary cathode 30b may need to produce high electron current even at low heating current, e.g., when dose modulation with the primary cathode is on, to deliver the full tube current, e.g., at minimal absolute grid voltage, for the case that the primary cathode 30a carries only a minimal current close to zero.

[0085] Optionally, the auxiliary cathode 30b may have a slew rate for rise and/or fall of the auxiliary electron current upon a change of the heating current, which is configured to be equal or higher than that of the primary cathode. The slew rate refers to the change of emission current per unit of time. The slew rate of emission current of the auxiliary cathode upon sudden change of the heating current, notably when heating is stopped, is higher at least for falling current than that of the primary cathode. The slew rate may be higher even at low emission current. The higher slew rate allows to synchronize the adjustment of the auxiliary electron current 42b with the change of the primary electron current 42a such that the sum of the primary electron current and the auxiliary electron current remains constant during the change of the heating current. To achieve a higher slew rate, the auxiliary cathode may be configured to have a higher heat conduction from a wire of the auxiliary cathode to a surrounding than that of the primary cathode. For example, the auxiliary cathode may be arranged on a thicker emitter holder such that the auxiliary cathode cools much faster than the primary cathode. Alternatively or additionally, the auxiliary cathode may be configured to have a higher heat radiation from a wire of the auxiliary cathode to a surrounding than that of the primary cathode. For example, the auxiliary cathode may have a thinner emitter wire such that it cools much faster than the primary cathode.

[0086] Alternatively to connection in series, the primary cathode 30a and the auxiliary cathode 30b may be connected in parallel (not shown), thus sharing the same heating voltage.

[0087] Both, connection in series and in parallel, may reduce the number of feed-throughs.

[0088] Optionally, the emission control grid 40a has a grid control voltage that is configured to sufficiently reduce the auxiliary electron current 42b such that the X-ray beam 44 is generated with a maximum X-ray intensity. In other words, the grid control voltage may be configured to allow to substantially, or totally, blank the auxiliary electron current 42b for the case that the rotary anode X-ray tube 12 has to produce the maximum of used X-ray intensity. As the auxiliary cathode 30b may not need to produce fine focal spots—this may be even unwanted to prevent anode melting, the filament of the auxiliary cathode 30b may be long and narrow. In this way, the cut-off grid voltage may be minimized.

[0089] Optionally, the X-ray tube 12 may comprise a further emission control grid (not shown) arranged between the primary cathode 30a and the anode 26. The further emission control grid is configured to control a shape of the first electrons to adjust a focal spot on the first area 34a on the rotatable anode 26. With the further emission control grid, the primary cathode 30a may be switched with a second analogue output or second digital output. A second analogue output may also be foreseen which may control the electron emission from the primary cathode 30a. It is noted, however, that the focal spot on the anode 26 is not overheated during such analogue current control. The emission from the auxiliary cathode has then to be controlled in such a way that the sum of both currents is kept constant.

[0090] Optionally, the further emission control grid may be configured as a focusing electrode or a set of focusing electrodes, e.g. a pair, in addition to its function as a current control grid, to keep the size of the focal spot constant when the tube voltage changes. The “grid” is typically a pair of electrodes at the same of slightly different negative bias voltages (different for focal spot deflection). This pair of electrodes may be used to keep the focal spot, the cross-sectional area of the electron beam at the target surface, constant even when the tube voltage changes, e.g. in dual energy mode of CT. Typically, the focal spot size changes when the bias grid voltage is constant and the tube voltage changes. This is not desired, as the X-ray projections at high tube voltage and low tube voltage should be exactly the same, except being acquired with different spectra. By keeping the focal spot size constant when the tube voltage changes, image artifacts or poor spectral performance of the X-ray system may be avoided. The grid control voltage, which controls the size of the focal spot, would be adapted accordingly. In addition, the auxiliary cathode may be equipped with a similar emission control grid. The grid control voltage of the auxiliary control grid would be adapted in sync with the changing primary control voltage to keep the total tube current constant even though the primary electron current would change with the changing tube voltage, the changing grid control voltage for focusing as well as with the changing heating current of the primary cathode. The total tube current, the sum of primary and auxiliary current, may also be controlled in a different way, other than keeping the total current constant.

[0091] In another example, instead of or in addition to the emission control grid 40a optionally with a further emission control grid, the electron current controlling device 40 may comprise at least one heating supply 48 configured to supply the primary and the auxiliary cathodes 30a, 30b with different heating powers such that the sum of the primary electron current and the auxiliary electron current remains constant.

[0092] FIG. 4 shows a schematic central cut-through view of a rotary anode X-ray tube 12 according to some further embodiments of the present disclosure. In the exemplary rotatory anode X-ray tube illustrated in FIG. 4, at least one heating supply 48 is provided. The at least one heating supply 48 comprises a primary heating supply 48a associated with the primary cathode 30a. The primary cathode 30a comprises a wire filament or a flat sheet that emits electrons when heated. The temperature of the wire filament or flat emitter of the primary cathode 30a may be controlled by the primary heating supply 48a. As the heating current of the primary heating supply 48a is increased, the temperature of the wire filament of the primary cathode 30a is increased and the wire filament produces more electrons. The primary heating supply 48a thus controls the total number of X-rays produced by the primary cathode 30a. The at least one heating supply 48 further comprises an auxiliary heating supply 48b associated with the auxiliary cathode 30b. The auxiliary heating supply 48b is configured to change a heating current of the auxiliary cathode 30b to adjust the auxiliary electron current 42b in response to a change of the primary electron current 42a such that a sum of the primary electron current and the auxiliary electron current remains constant. In other words, the temperature of the auxiliary cathode is steered by the auxiliary heating supply 48b. The primary heating supply 48a and the auxiliary heating supply 48b may be controlled by a processing unit, which changes the heating current of the auxiliary heating supply 48b based on the primary electron current 42a generated by the primary cathode 30a. Thus, the down-ramp of the tube voltage for dual energy scanning is kept constant even though the X-ray output of the tube changes e.g., for dose modulation.

[0093] In the exemplary rotatory anode X-ray tube illustrated in FIG. 4, the auxiliary cathode 30b is placed and charged with about the negative potential of the primary cathode 30a. In other words, the primary cathode 30a and the auxiliary cathode 30b may be connected in series, thus sharing the same heating current. When two cathodes 30a, 30b are connected in series, the auxiliary cathode may be powerful enough to produce high electron current even at low heating current, e.g., when dose modulation with the primary cathode is on, to deliver the full tube current, e.g., at minimal absolute grid voltage, for the case that the primary cathode carries only a minimal current close to zero. Optionally, the auxiliary cathode 30b may be configured to produce a sufficiently high auxiliary electron current at a lower heating power compared with a heating power required for the primary cathode to keep the sum of the primary electron current and the auxiliary electron current constant for the case that the primary cathode carries only a minimal primary electron current close to zero. Optionally, the auxiliary cathode 30b may have a slew rate for rise and/or fall of the auxiliary electron current upon a change of the heating current, which is configured to be equal or higher than that of the primary cathode. To achieve a higher slew rate, the auxiliary cathode may be configured to have a higher heat conduction from a wire of the auxiliary cathode to a surrounding than that of the primary cathode. Alternatively or additionally, the auxiliary cathode may be configured to have a higher heat radiation from a wire of the auxiliary cathode to a surrounding than that of the primary cathode.

[0094] Alternatively, the primary cathode 30a and the auxiliary cathode 30b may be connected in parallel (not shown), thus sharing the same heating voltage.

[0095] In a further example (not shown), instead of using two heating supplies, i.e. the primary heating supply and the auxiliary heating supply, an AC heating circuit with a variable frequency may be provided. The AC heating circuit is configured to supply the primary and the auxiliary cathodes with different heating powers using at least one of an inductor and a capacitor. The distribution of the current then takes place in the tube with inductors and/or capacitors. Since it is possible to set the frequency almost arbitrarily high, a few strategically distributed additional turns in the coils of the primary cathode may be sufficient. Such a variable frequency heating circuit may not be much more expensive than a conventional one.

[0096] FIG. 5 shows an X-ray imaging system 100 according to some embodiments of the present disclosure in a C-arm X-ray imaging suite. Other examples of the X-ray imaging system may include, but not limited to, a CT imaging system or a fluoroscopy system.

[0097] The C-arm imaging system 100 has a support arrangement 102 which may translate through azimuth and elevation axes around the object of interest 104. For example, the C-arm X-ray imaging system 100 may be supported from the ceiling of an X-ray facility. The support arrangement holds a rotary anode X-ray source 12 as described above and below, and an X-ray detector 106.

[0098] The C-arm imaging system (or CT imaging system) is optionally provided with motion sensors (for example, rotary encoders in the C-arm or CT gantry axes). This enables the feedback of motion information to the X-ray imaging system state detector.

[0099] Alternatively, or in combination, the X-ray imaging system state detector is configured to receive a list of motion commands representing a pre-planned imaging protocol.

[0100] The C-arm X-ray imaging system is controlled, for example, from a control console 108, comprising, for example, display screens 110, computer apparatus 112 optionally functioning as a stator control system, controllable via a keyboard 114 and a mouse 116.

[0101] The C-arm 118 is configured to translate around the object of interest 104, not simply in a flat rotational sense (in the sense of a CT scanner), but also by tilting.

[0102] In operation, an object of interest 104 is placed in between the detector 106 and the X-ray source 12 of a C-arm imaging system 100. The C-arm may rotate about the patient for acquisition of an image data set which is then used for 3D image reconstruction. An X-ray imaging system scanning protocol is initiated using the control console 114.

[0103] FIG. 6 shows a flow diagram of a method 200 of X-ray tube control according to some embodiments of the present disclosure. In step 210, i.e. step a), first electrons are emitted by a primary cathode establishing a flow of primary electron current. The first electrons are focused on a first area on a rotatable anode of the X-ray tube for generating an X-ray beam. The primary cathode may comprise at least one of a field emission cathode, a photo cathode, and an indirectly heated cathode.

[0104] In step 220, i.e. step b), second electrons are emitted by an auxiliary cathode establishing a flow of auxiliary electron current. The second electrons are directed to a second area, which is different from the first area, on the rotatable anode for generating X-rays. The generated X-rays are configured to be directed to a direction different from that of the X-ray beam such that the X-rays do not enter the X-ray beam. In an example, the primary cathode and the auxiliary cathode are connected in series or in parallel. When connected in series with the primary cathode, the auxiliary cathode may be configured to produce a sufficiently high auxiliary electron current at a lower heating power compared with a heating power required for the primary cathode to keep the sum of the primary electron current and the auxiliary electron current constant for the case that the primary cathode carries only a minimal primary electron current close to zero. The auxiliary cathode is configured to have a slew rate for rise and/or fall of the auxiliary electron current upon a change of the heating current, which is configured to be equal or higher than that of the primary cathode. To achieve a higher slew rate, the auxiliary cathode may be configured to have a higher heat conduction from a wire of the auxiliary cathode to a surrounding than that of the primary cathode. Alternatively or additionally, the auxiliary cathode may be configured to have a higher heat radiation from a wire of the auxiliary cathode to a surrounding than that of the primary cathode. The auxiliary cathode may comprise a field emission cathode.

[0105] In step 230, i.e. step c), the auxiliary electron current is adjusted by an electron current controlling device in response to a change of the primary electron current such that a sum of the primary electron current and the auxiliary electron current remains constant.

[0106] In an example, the electron current controlling device may comprise an emission control grid arranged between the auxiliary cathode and the anode. The emission control grid may be configured to control the flow of the auxiliary electron current between the auxiliary cathode and the anode. Optionally, the emission control grid may have a grid control voltage that is configured to sufficiently reduce the auxiliary electron current such that the X-ray beam is generated with a maximum X-ray intensity. Optionally, the X-ray tube may comprise a further emission control grid arranged between the primary cathode and the anode. The further emission control grid is configured to control a shape of the first electrons to adjust a focal spot on the first area on the rotatable anode.

[0107] In another example, the electron current controlling device may comprise at least one heating supply configured to supply the primary and the auxiliary cathodes with different heating powers such that the sum of the primary electron current and the auxiliary electron current remains constant. Optionally, the at least one heating supply may comprise an alternating current (AC) heating circuit with a variable frequency. The AC heating circuit is configured to supply the primary and the auxiliary cathodes with different heating powers using at least one of an inductor and a capacitor. Alternatively, the at least one heating supply may comprise a primary heating supply associated with the primary cathode and an auxiliary heating supply associated with the auxiliary cathode. The auxiliary heating supply may be configured to change a heating current of the auxiliary cathode to adjust the auxiliary electron current in response to a change of the primary electron current such that a sum of the primary electron current and the auxiliary electron current remains constant.

[0108] In another exemplary embodiment of the present invention, a computer program or a computer program element is provided that is characterized by being adapted to execute the method steps of the method according to one of the preceding embodiments, on an appropriate system.

[0109] The computer program element might therefore be stored on a computer unit, which might also be part of an embodiment of the present invention. This computing unit may be adapted to perform or induce a performing of the steps of the method described above. Moreover, it may be adapted to operate the components of the above described apparatus. The computing unit can be adapted to operate automatically and/or to execute the orders of a user. A computer program may be loaded into a working memory of a data processor. The data processor may thus be equipped to carry out the method of the invention.

[0110] This exemplary embodiment of the invention covers both, a computer program that right from the beginning uses the invention and a computer program that by means of an up-date turns an existing program into a program that uses the invention.

[0111] Further on, the computer program element might be able to provide all necessary steps to fulfil the procedure of an exemplary embodiment of the method as described above.

[0112] According to a further exemplary embodiment of the present invention, a computer readable medium, such as a CD-ROM, is presented wherein the computer readable medium has a computer program element stored on it which computer program element is described by the preceding section.

[0113] A computer program may be stored and/or distributed on a suitable medium, such as an optical storage medium or a solid state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

[0114] However, the computer program may also be presented over a network like the World Wide Web and can be downloaded into the working memory of a data processor from such a network. According to a further exemplary embodiment of the present invention, a medium for making a computer program element available for downloading is provided, which computer program element is arranged to perform a method according to one of the previously described embodiments of the invention.

[0115] It has to be noted that embodiments of the invention are described with reference to different subject matters. In particular, some embodiments are described with reference to method type claims whereas other embodiments are described with reference to the device type claims. However, a person skilled in the art will gather from the above and the following description that, unless otherwise notified, in addition to any combination of features belonging to one type of subject matter also any combination between features relating to different subject matters is considered to be disclosed with this application. However, all features can be combined providing synergetic effects that are more than the simple summation of the features.

[0116] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing a claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

[0117] In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfil the functions of several items re-cited in the claims. The mere fact that certain measures are re-cited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.