Balancing X-ray power for dual energy X-ray imaging systems

Abstract

An X-ray source 100 is proposed for generating X-ray radiation of first and second energy spectra. The imbalance in X-ray intensity between the first and second energy spectra is reduced compared to conventional X-ray sources. The reduction in X-ray intensity imbalance is achieved by setting a smaller electron impact angle 141 on the anode 102 when a higher tube voltage is applied compared to when a lower tube voltage is applied.

Description

[Technical field]

[0001] The present invention relates to an X-ray source for generating X-ray radiation of a first and a second energy spectrum, an X-ray imaging system, a method for generating X-ray radiation, a computer program, and a computer readable medium. [Background technology]

[0002] X-ray imaging systems are used in a variety of applications, including medical diagnostics, airport security, and material analysis. For example, in a computed tomography (CT) system, an X-ray source and an X-ray detector are positioned on either side of a patient. The X-ray source can generate a fan beam of X-rays. The photons of the X-ray beam are partially absorbed by the patient's body. This causes bone to absorb more photons compared to lean tissue. The photons that pass through the patient's body are then received by the X-ray detector, which generates a shadow image of the patient's anatomy. The resulting image is a two-dimensional projection of the three-dimensional structure of the patient's body. In a CT system, the X-ray source and the X-ray detector rotate around the patient to acquire images from various viewing directions. These images can be processed by a computer system to generate a three-dimensional image of the patient's anatomy.

[0003] X-ray sources typically include a cathode and an anode located inside a vacuum tube. The cathode emits electrons. The electrons are accelerated toward the anode by a tube voltage provided by a power supply. For example, if the tube voltage is 80 kV, the electrons are accelerated from the cathode to the anode and reach a kinetic energy of 80 keV when they impinge on the anode. This energy is fully or partially converted into emitted X-rays, which are emitted through an aperture in the housing of the X-ray tube.

[0004] The image quality provided by an X-ray imaging system can be improved by scanning a patient with radiation X-rays having multiple energy spectra. This allows imaging of the patient with spectral material decomposition. This technique has been demonstrated to improve images while reducing the amount of toxic contrast agent (usually iodine). By applying different tube voltages between the cathode and anode of the X-ray tube, radiation X-rays of various energy spectra can be produced. For example, tube voltages of 80 kV and 140 kV can be applied to produce soft radiation X-rays and hard radiation X-rays, respectively. Summary of the Invention [Problem to be solved by the invention]

[0005] It has been determined that the intensity of emitted x-rays is higher at high tube voltages compared to low tube voltages. This imbalance in x-ray intensity is at least partially compensated for by using a longer integration period at low tube voltages compared to high tube voltages in the x-ray imaging system's x-ray detector. However, in CT systems, the x-ray source and x-ray detector typically rotate around the patient at a constant angular velocity, so the longer integration period at low tube voltages reduces image resolution.

[0006] It is therefore desirable to provide an improved x-ray source for producing x-ray radiation of first and second energy spectra in which the imbalance in x-ray intensity at different tube voltages is reduced. [Means for solving the problem]

[0007] This is achieved by the subject matter of the independent claims, further embodiments are incorporated in the dependent claims and the following description, it being understood that all features, elements and / or functions of the X-ray source described below equally apply to the X-ray imaging system, the method for generating X-ray radiation, the computer program for controlling the X-ray source and the computer readable medium described below and vice versa.

[0008] In the present disclosure, an X-ray source for generating radiation X-rays of a first and a second energy spectrum is presented. The X-ray source includes a cathode for emitting an electron beam, an anode for at least partially converting the electron beam into radiation X-rays, an electron optics for controlling an impact angle at which electrons of the electron beam impinge on the anode, a power supply for applying first and second tube voltages between the cathode and the anode (the second tube voltage is higher than the first tube voltage), and a control circuit coupled to the electron optics, the control circuit controlling the electron optics such that when the first tube voltage is applied, the electrons of the electron beam impinge on the anode at a first average impact angle, and when the second tube voltage is applied, the electrons of the electron beam impinge on the anode at a second average impact angle, the second average impact angle being smaller than the first average impact angle.

[0009] The impact angle is defined as the angle between the electron's path of propagation and a plane tangent to the anode surface where the electron's path of propagation impacts the anode surface.

[0010] Radiation X-rays having a first energy spectrum can be generated by applying a first tube voltage between the cathode and the anode. Radiation X-rays having a second energy spectrum can be generated by applying a second tube voltage between the cathode and the anode. The second tube voltage is higher than the first tube voltage. The power supply generates, for example, a first tube voltage of 80 kV and a second tube voltage of 140 kV. The tube voltage accelerates electrons emitted from the cathode towards the anode. For example, when the tube voltage is 80 kV, the electrons reach a kinetic energy of 80 keV before impinging on the anode. At the anode, this energy can be partially or completely converted into radiation X-rays such that the X-ray photons have an energy of up to 80 keV. In contrast, when the tube voltage is 140 keV, the X-ray photons reach an energy of up to 140 keV. Thus, the energy spectrum of radiation X-rays can include higher energy at the second tube voltage compared to the first tube voltage.

[0011] The x-ray source of the present disclosure can use several types of cathodes, including conventional thermal cathodes, field emitter cathodes, pierce type cathodes, dispenser cathodes, photocathodes, or plasma cathodes. The anode of the x-ray source can include a material that produces x-ray radiation, such as tungsten or molybdenum. The anode can rotate so that the heat generation at the focal point, i.e., the surface section where the electrons impinge on the anode, is spread over a larger area compared to a stationary anode. However, the present invention is not limited to the case of a rotating anode.

[0012] Electrons of the electron beam from the cathode to the anode can impinge on the anode from different impingement angles. The impingement angle value is from 0° to 90°. An impingement angle of 90° corresponds to the electron propagation path perpendicular to the surface of the anode. The smaller the impingement angle, the more the electron propagation path grazes the surface of the anode.

[0013] The electron optics of the x-ray source control the impact angle of the electrons of the electron beam on the anode. The electron optics may include electrostatic and / or magnetic devices. For example, the electron optics may include one or more dipoles, one or more quadrupoles, etc. The electron optics may shape the electron beam. In one scenario, the electron optics shapes a narrow electron beam such that the impact angle of the electrons on the anode is substantially the same for all electrons. In another scenario, the electron optics shapes a wide electron beam such that the electrons impacting on the anode form a wide range of impact angles. In another scenario, the electron optics deflects the electron beam to change the average impact angle of the electrons on the anode without changing the width of the electron beam at the surface area of ​​impact. In another scenario, the electron optics deflects and also shapes the electron beam.

[0014] The control circuit may include analog and / or digital components. For example, the control circuit may include passive electronics such as resistors, capacitors, or electronic filters, active electronics such as amplifiers or analog-to-digital converters, microcontrollers, microprocessors, memory units, etc. The control circuit may be operatively coupled to the electro-optical system and control the electro-optical system. Additionally, the control circuit may be operatively coupled to a power source. Also, the control circuit controls the power source, including the voltage generated by the power source. Alternatively or additionally, the control circuit may receive a signal indicative of the generated tube voltage from the power source. Also, the control circuit may control the electro-optical system based on the signal received from the power source.

[0015] The power supply generates first and second tube voltages and applies these voltages between the cathode and the anode. The power supply may frequently switch between these tube voltages. For example, in a CT system, the tube voltage may be switched frequently to reduce imaging artifacts due to movement of the patient (or the patient's internal organs) in the period between acquisition of low-energy x-ray images from a similar direction and acquisition of high-energy x-ray images. The power supply may switch between the first and second tube voltages at a rate of 1 kHz, 2 kHz, 4 kHz, or 10 kHz. The voltages generated by the power supply may also be switched slower or faster. The cathode may not allow switching of the tube current at such a rate, or the ability to switch at such a high rate may be intentionally reduced. In the x-ray tube of the present disclosure, the tube current may be kept constant either throughout the entire scan time, or at least during the period when the tube voltage is changed (especially during the drop in the tube voltage). The tube current may be substantially the same during the period when the first and second tube voltages are applied. Since the second tube voltage is higher than the first tube voltage, the power of the electron beam is higher at the second tube voltage compared to the first tube voltage.

[0016] At tube voltages used for medical imaging, most of the power of the electron beam is converted into heat, and a higher power of the electron beam at the second tube voltage may exceed the heat capacity of the anode. However, according to the present disclosure, the control circuit controls the electron optics such that, when the second tube voltage is applied, the electrons of the electron beam impinge on the anode at a second average impingement angle, which is smaller than the first average impingement angle of the electrons set when the first tube voltage is applied. By setting a smaller impingement angle for the second tube voltage, the electron backscatter rate is increased, resulting in a smaller proportion of the power of the electron beam being converted into heat. Thus, the smaller impingement angle of the electrons for the second tube voltage allows a higher power of the electron beam. This can be utilized to apply a higher tube current when the second tube voltage is applied without exceeding the power limit of the anode.

[0017] The conversion efficiency, i.e., the percentage of the power of the electron beam that is converted into emitted X-rays, is higher for the second tube voltage compared to the first tube voltage when similar impact angles are set for the first and second tube voltages. Furthermore, the percentage of emitted X-rays that are absorbed by an object, such as a patient's body, in a medical X-ray imaging system is higher for the first tube voltage compared to the second tube voltage. Thus, assuming similar impact angles and similar tube currents are set for the first and second tube voltages, the X-ray intensity at the X-ray detector of the X-ray imaging system is significantly higher for the second tube voltage compared to the first tube voltage.

[0018] As explained above, the imbalance in x-ray intensity can be at least partially corrected by using a longer integration period at the first x-ray voltage compared to the second tube voltage at the x-ray detector of the x-ray imaging system. However, in a CT system, the x-ray source and x-ray detector typically rotate around the patient at a constant angular velocity, so a longer integration period at the first tube voltage reduces image resolution.

[0019] Alternatively, the imbalance in x-ray intensity can be mitigated by controlling the tube current emitted from the cathode such that the tube current is higher when a first tube voltage is applied compared to when a second tube voltage is applied. However, as explained above, it may be desirable to switch the tube voltage frequently, and tube current control may be too slow to correct the imbalance in x-ray intensity, especially when the tube voltage switches between each integration period. For example, in a conventional thermal cathode, the tube current may be switched at a rate of 10 Hz, whereas the tube voltage may be switched at a rate of 10 kHz.

[0020] Alternatively, the tube current emitted from the cathode may be increased to increase the X-ray intensity at the first tube voltage. However, when the same high tube current is applied at the first and second tube voltages, the power limit of the anode may be exceeded during the application of the second tube voltage, resulting in a shortened life of the X-ray source.

[0021] In the present disclosure, the imbalance in X-ray intensity can be reduced by setting a smaller impact angle for the second tube voltage compared to the first tube voltage. The conversion efficiency is smaller for small electron impact angles compared to large electron impact angles. Therefore, at small electron impact angles, a smaller percentage of the power of the electron beam is converted into emitted X-rays. Thus, for the second tube voltage, a smaller electron impact angle on the anode results in a reduction in X-ray intensity at the X-ray detector, which can reduce the imbalance in X-ray intensity between the first and second tube voltages.

[0022] In one example, the second average impact angle at which the electrons of the electron beam impact on the anode is less than 70°.

[0023] By setting a smaller impact angle of electrons on the anode for the second tube voltage compared to the first tube voltage, the conversion efficiency for the second tube voltage is reduced, and the imbalance of X-ray intensity between the first and second tube voltages can be mitigated. The first average impact angle for the first tube voltage may be close to 90°. That means that when the first tube voltage is applied, the electron propagation path is substantially perpendicular to the surface of the anode. To achieve a significant reduction in the conversion efficiency for the second tube voltage, the second average impact angle of electrons can be reduced to less than 70°. Thus, when the second tube voltage is applied, a second average impact angle of less than 70° may significantly reduce the conversion efficiency compared to a second average impact angle of 90°. This immediately reduces the X-ray intensity at the X-ray detector of the X-ray imaging system. In other examples, the second average impact angle at which electrons impinge on the anode is less than 60°, 50°, 40°, 30°, or 20°.

[0024] In another example, electrons of the electron beam impinge on a first surface section of the anode when a first tube voltage is applied and impinge on a second surface section of the anode when a second tube voltage is applied, the first surface section overlapping the second surface section.

[0025] The X-ray source can be applied to an X-ray imaging system. Differences between the first and second surface sections of the anode can result in imaging artifacts, making it more difficult to compare or combine the X-ray images acquired for the first and second X-ray energy spectra. Therefore, it is essentially desirable to make the first and second surface sections of the anode identical. That is, it is desirable for the first and second surface sections to be approximately the same size and approximately in the same location relative to the interface of the X-ray source in the X-ray system. Usually, this means that the area on the anode where the electrons impinge, the first and second surface sections of the anode, are essentially coincident. The first and second surface sections of the anode are considered to be approximately the same size when they do not differ in size by more than 20%. Thus, the control circuit can control the electron optics such that the size of the first surface section differs from the size of the second surface section by, for example, less than 20% or less than 10%. Alternatively or additionally, the control circuitry can control the electron optics such that a parameter indicative of the size of the first surface section (e.g. the diameter of the first surface section in a particular direction) differs from a corresponding parameter indicative of the size of the second surface section, e.g. by less than 20% or less than 10%. In other examples, it is desirable for the second surface section to be larger than the first surface section and / or for the second surface section to overlap the first surface section. As a result, the first and second surface sections are spread over a larger section of the anode's surface compared to the same first and second surface sections. Thus, the section of the anode's surface where heat is generated by electron impingement is larger compared to examples with the same first and second surface sections. This spreading of the heat generation over a larger surface section of the anode allows the power of the electron beam to be increased without exceeding the power limitations of the anode.

[0026] In another example, the center of the first surface section coincides with the center of the second surface section of the anode.

[0027] The center of the first surface section can be defined, for example, as the average position on the surface of the anode where the electrons impinge when the first tube voltage is applied. Alternatively, the center of the first surface section can be defined, for example, as the position on the surface of the anode where the electrons are most likely to impinge when the first tube voltage is applied. The center of the second surface section is preferably defined similarly to the center of the first surface section. Thus, the center of the second surface section can be defined as the average position on the surface of the anode where the electrons impinge when the second tube voltage is applied. Alternatively, the center of the second surface section can be defined as the position on the surface of the anode where the electrons are most likely to impinge when the second tube voltage is applied.

[0028] Here and in the following, the centre of a first surface section is said to coincide with the centre of a second surface section when the two centres differ by a negligible amount compared to the size of the smaller of the first and second surface sections. In particular, the control circuitry is capable of controlling the electron optics such that the centre of the first surface section differs from the centre of the second surface section by less than 20%, or preferably by less than 10%, relative to the smaller diameter of the first and second surface sections in the direction of the difference between the centres of the first and second surface sections.

[0029] In another example, the electron optical system includes a first deflection device and a second deflection device, the first deflection device being switchable between a first state and a second state. The control circuit switches the first deflection device from the first state to the second state according to switching of a power supply from a first tube voltage to a second tube voltage. When the first deflection device is in the second state, the first and second deflection devices provide a deflection of the electron beam.

[0030] The first and second deflection devices may each include one or more magnetic devices and / or one or more electrostatic devices (dipole, quadrupole, etc.).

[0031] The control circuit can control the first deflection device so that the first deflection device deflects the electron beam when the second tube voltage is applied, and so that the first deflection device does not deflect the electron beam when the first tube voltage is applied. Alternatively, the control circuit can control the first deflection device so that the first deflection device deflects the electron beam more strongly when the second tube voltage is applied compared to when the first tube voltage is applied. The control circuit can set the first state when the first tube voltage is applied by the power supply. On the other hand, the second state can be set when the second tube voltage is applied.

[0032] The second deflection device can steer the electron beam, which may be deflected by the first deflection device, onto a first surface section of the anode when a first tube voltage is applied, and onto a second surface section of the anode when a second tube voltage is applied. The first and second surface sections of the anode may overlap. Or the centers of the first and second surface sections of the anode may coincide. Or the first and second surface sections of the anode may be identical. The second deflection device is also switchable, and the second deflection device can control the control circuit.

[0033] The deflection of the electron beam in the second state is provided such that the second average impact angle is smaller than the first average impact angle. The electrons of the electron beam may be deflected by the first deflection device in a radial direction relative to the axis of rotation of the anode. Additionally or alternatively, the electrons of the electron beam may be deflected by the first deflection device in both a radial direction and the axis of rotation of the rotating anode, or in a direction orthogonal to a normal to the anode target of a stationary anode.

[0034] In another example, the electron optical system includes a focus device and a defocus device. The electron optical system is switchable between a third state and a fourth state. The control circuit switches the electron optical system from the third state to the fourth state according to switching of the power supply from the first tube voltage to the second tube voltage. In the fourth state, the defocus device first provides a spread of the electron beam, and then the focus device provides a focus of the spread electron beam.

[0035] The focus and defocus devices may each include one or more magnetic devices and / or one or more electrostatic devices (dipole, quadrupole, etc.).

[0036] The control circuit can control the defocusing device such that the defocusing device increases the width of the electron beam when the second tube voltage is applied and such that the defocusing device does not widen the electron beam when the first tube voltage is applied. Alternatively, the control circuit can control the defocusing device such that the defocusing device provides a stronger widening of the electron beam when the second tube voltage is applied compared to when the first tube voltage is applied.

[0037] Furthermore, the control circuitry can control the focusing system to focus the electron beam, which may be widened by the defocusing device, on the first surface section of the anode when the first tube voltage is applied and on the second surface section of the anode when the second tube voltage is applied. The sizes of the first and second surface sections of the anode may be comparable or identical as described above. And / or the first and second surface sections may overlap. And / or the center of the first surface section of the anode may coincide with the center of the second surface section of the anode.

[0038] The control circuit can set a third state when the first tube voltage is provided by the power supply. Meanwhile, a fourth state can be set when the second tube voltage is provided. The broadening and subsequent focusing of the electron beam in the fourth state is provided to reduce the second average impact angle. For example, the central direction of the electron beam is perpendicular to the surface of the anode. However, in a wide electron beam, the electrons may impact on the anode surface at an impact angle significantly smaller than 90°. Thus, in a wide electron beam, the average impact angle of the electrons is significantly smaller than 90°, even when the central direction of the electron beam is perpendicular to the surface of the anode.

[0039] The electron beam may be spread in a radial direction relative to the axis of rotation of a rotating anode or the normal to the anode target of a stationary anode. Additionally or alternatively, the electron beam may be spread in a direction perpendicular to both the axis of rotation or the normal to the anode target of a stationary anode and in a corresponding radial direction.

[0040] In another example, the electron optical system includes a first deflection device, a second deflection device, a focus device, and a defocus device. The electron optical system is switchable between a fifth state and a sixth state. The control circuit switches the electron optical system from the fifth state to the sixth state according to switching of a power supply from a first tube voltage to a second tube voltage. In the fifth state, the first and second deflection devices provide deflection of the electron beam, and the focus and defocus devices provide spreading and subsequent focusing of the electron beam.

[0041] In another example, the defocusing device provides a stronger spread of the electron beam when the electron-optics is in a fourth state compared to a third state.

[0042] The central direction of the electron beam may be perpendicular or nearly perpendicular to the surface of the anode, such that the broadening of the electron beam by the defocusing device and subsequent focusing of the broadened electron beam onto a surface section of the anode reduces the average impact angle compared to a configuration in which the electron beam is not broadened.

[0043] In another example, the anode includes a substrate. The anode is at least partially coated with a first and a second coating layer. The first coating layer is disposed between the second coating layer and the substrate. The substrate includes a first material that generates X-ray radiation, and the first coating layer includes a second material that is transparent to the X-ray radiation and electrons of the electron beam. The second coating layer includes a third material that generates X-ray radiation. An X-ray filter is attached to the second coating layer to filter the X-ray radiation generated in the second coating layer.

[0044] The electron beam may first impinge on the second coating layer. When the electron beam impinges on the second coating layer, the power of the electron beam may be partially converted into emitted X-rays. The average impingement angle of the electrons on the second surface section of the anode is smaller when the power supply generates the second tube voltage compared to when the first tube voltage is supplied.

[0045] At the second tube voltage, the average impact angle of the electrons is smaller, so the propagation path through the second coating layer is longer on average at the second tube voltage than at the first tube voltage. Therefore, the proportion of electrons propagating through the second coating layer is smaller at the second tube voltage than at the first tube voltage. Therefore, the proportion of electrons impinging on the substrate is smaller at the second tube voltage than at the first tube voltage. Therefore, the proportion of the radiation X-rays generated in the second coating layer to the total of the radiation X-rays generated in the second coating layer and the substrate is higher at the second tube voltage than at the first tube voltage. That is, when the second tube voltage is applied, most of the radiation X-rays are generated in the second coating layer, and only a small portion of the radiation X-rays are generated in the substrate. On the other hand, when the first tube voltage is applied, most of the radiation X-rays are generated in the substrate.

[0046] The x-ray filter can be positioned to filter x-ray radiation generated in the second coating layer and not filter x-ray radiation generated in the substrate, and to accomplish this, the first coating layer can provide a sufficient gap between the substrate and the second coating layer such that the x-ray filter is positioned sufficiently far from the substrate.

[0047] The x-ray filter includes a material such as molybdenum. Filtering the x-ray radiation generated in the second coating layer increases a measure of difference between a first energy spectrum of the x-ray radiation generated when the first tube voltage is applied and a second energy spectrum of the x-ray radiation generated when the second tube voltage is applied. For example, the x-ray filter increases the difference between the average photon energies of the first and second energy spectra. Providing x-ray radiation having more distinct first and second energy spectra allows the dual energy x-ray imaging system to capture more meaningful x-ray images.

[0048] Additionally, the x-ray filter filters a greater percentage of x-ray radiation when the second tube voltage is applied compared to when the first tube voltage is applied. Filtering the emitted x-rays reduces the x-ray intensity. Thus, the x-ray filter provides a greater reduction in x-ray intensity at the second tube voltage compared to when the first tube voltage is applied. This can reduce imbalances in x-ray intensity at the x-ray detector of the x-ray imaging system.

[0049] The first material of the substrate may be rhenium. The second material of the first coating layer may be diamond or a carbon-based fiber-reinforced composite. The third material of the second coating layer may be tungsten. The x-ray filter may include molybdenum or another filtering material that does not have a k-edge in the portion of the x-ray spectrum relevant to x-ray imaging. These materials may include aluminum, titanium, tin, or iron, or combinations thereof.

[0050] In another example, the second coating layer has a thickness of less than 1 micrometer at a surface section where the electrons of the electron beam impinge on the second coating layer.

[0051] The thickness of the second coating layer can generally be determined so that the difference between the first rate of radiation X-rays and the second rate of radiation X-rays is large or maximum. The first rate of radiation X-rays is the rate of radiation X-rays generated in the second coating layer to the total radiation X-rays generated in both the second coating layer and the substrate when the first tube voltage is applied. Similarly, the second rate of radiation X-rays is the rate of radiation X-rays generated in the second coating layer to the total radiation X-rays generated in both the second coating layer and the substrate when the second tube voltage is applied. The thickness of the second coating layer is 1 μm. However, it is also possible to make the second coating layer thinner or thicker depending on the composition of the second coating layer, the composition of the substrate, the first and second tube voltages, and the electron collision angle.

[0052] According to the present disclosure, an X-ray imaging system is also presented, comprising an X-ray source as defined in any one of the preceding claims and an X-ray detector, the X-ray detector receiving X-ray radiation emitted from the X-ray source.

[0053] The X-ray imaging system can be applied to medical imaging systems, airport security systems, and the like. When an X-ray source generates X-ray radiation of a first energy spectrum, an X-ray detector can acquire first image data of an object. Similarly, when an X-ray source generates X-ray radiation of a second energy spectrum, an X-ray detector can acquire second image data. Since the first and second energy spectra may be different, the first image data may provide different object information than the second image data. The first and second image data may be displayed separately. Or, the first and second image data may be processed separately to detect, for example, structures or materials. Or, the first and second image data may be combined to render an image based on both the first and second image data.

[0054] In one example, the X-ray imaging system further includes a detector control circuit that controls the X-ray detector to generate first image data when a first tube voltage is applied between a cathode and an anode of the X-ray source, and to generate second image data when a second tube voltage is applied, such that an integration period for generating the first image data can have the same time interval as an integration period for generating the second image data.

[0055] The X-ray source of the present disclosure can provide emitted X-rays with reduced intensity imbalance for the first and second tube voltages compared to conventional X-ray sources. This can be achieved by setting a smaller average impingement angle for the second tube voltage compared to the first tube voltage. With the reduced X-ray intensity imbalance for the first and second tube voltages, more similar X-ray detector integration periods can be used when the first and second tube voltages are applied. Specifically, a first X-ray detector integration period is used when the first tube voltage is applied, and a second X-ray detector integration period is used when the second tube voltage is applied, and the time intervals of the first and second integration periods can be the same or similar.

[0056] According to the present disclosure, a method for generating X-ray radiation of a first and a second energy spectrum using an X-ray source is also provided. The X-ray source includes a cathode for emitting an electron beam, an anode for at least partially converting the electron beam into X-ray radiation, a power supply for applying a tube voltage between the cathode and the anode, an electron-optical system for controlling an impact angle at which electrons of the electron beam impinge on the anode, and a control circuit operably coupled to the electron-optical system. The steps for generating X-ray radiation of a first energy spectrum include, but are not necessarily in this order: (i) applying a first tube voltage by the power supply; and (ii) controlling the electron-optical system by the control circuit such that electrons of the electron beam impinge on the anode at a first average impact angle.

[0057] Further, the steps of generating radiation of a second energy spectrum include, although not necessarily in this order, (i) applying a second tube voltage by a power supply, the second tube voltage being higher than the first tube voltage, and (ii) controlling, by a control circuit, the electron optics system such that electrons of the electron beam impinge on the anode at a second average impact angle, the second average impact angle being smaller than the first average impact angle.

[0058] According to the present disclosure, a computer program is also presented, the computer program comprising instructions for causing an X-ray source to perform the steps of a method for generating X-ray radiation of a first and a second energy spectrum as defined in the independent claims.

[0059] According to the present disclosure, a computer readable medium is also provided having stored thereon a computer program of the present disclosure.

[0060] It is to be understood that the X-ray source, the X-ray imaging system, the method for generating X-ray radiation, the computer program for controlling an X-ray source and the computer movable medium on which such a computer program is stored according to the independent claims have similar and / or identical preferred embodiments as in particular defined in the dependent claims. It is also to be understood that a preferred embodiment of the invention can be any combination of the dependent claims with the respective independent claim.

[0061] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. [Brief description of the drawings]

[0062] Exemplary embodiments of the invention are described below with reference to the accompanying drawings.

[0063] [Figure 1] FIG. 1 shows a schematic and exemplary first embodiment of an X-ray source for generating X-ray radiation of a first and a second energy spectrum. [Diagram 2] FIG. 2 shows a second exemplary embodiment of an X-ray source for generating X-ray radiation of a first and a second energy spectrum. [Diagram 3] FIG. 3 shows a schematic and exemplary portion of an anode of an X-ray source with an electron beam impinging on the anode from a large impact angle. [Figure 4] FIG. 4 shows a portion of the anode of FIG. 3 with the electron beam impinging on the anode from a small impact angle. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] 1 shows a schematic and exemplary first embodiment of an X-ray source 100 for generating X-ray radiation of a first and second energy spectrum. The X-ray source comprises a cathode 101 and an anode 102. The cathode and the anode are arranged in a tube 120. The tube is preferably a vacuum tube. The anode can rotate about an axis 103, but this is not required for the present disclosure.

[0065] A power supply (not shown) can be used to apply a tube voltage between the cathode and the anode, so that the cathode insulator 122 and the anode insulator 121 can be in electrical contact with the cathode and the anode, respectively. The power supply can provide a first and a second tube voltage, the second tube voltage being higher than the first tube voltage. For example, the power supply can provide a first tube voltage of 80 kV and a second tube voltage of 140 kV. The tube voltage accelerates electrons emitted from the cathode toward the anode. For example, when a first tube voltage of 80 kV is applied between the cathode and the anode, the electrons can reach a kinetic energy of 80 keV before impinging on the anode. This energy is partially or completely converted into emitted X-rays when the electrons impinge on the anode. Thus, the X-ray photons can have an energy of up to 80 keV when the first tube voltage is applied and up to 140 keV when the second tube voltage is applied. Therefore, by applying different tube voltages, radiation X-rays having different energy spectra can be generated. Also, the power supply can generate three or more voltages and can be used to generate radiation X-rays of three or more energy spectra. X-rays 151 and 152 indicate the emission of radiation X-rays through the aperture of the X-ray tube 120.

[0066] FIG. 1 also shows an electron optics 110. The electron optics is operatively connected to a control circuit 104 that controls the electron optics. In FIG. 1, the control circuit is located outside the tube 120, but it can also be located inside the tube. In FIG. 1, the electron optics 110 includes a first deflection device 111 and a second deflection device 112. The control circuit 104 controls the first deflection device such that, when the first tube voltage is applied, electrons propagate along a path 130 from the cathode to the anode, and when the second tube voltage is applied, electrons propagate along a path 140. In addition to this, the control circuit controls the first deflection device such that, when the second tube voltage is applied, the first deflection device deflects the electron beam. On the other hand, the first deflection device weakly or does not deflect the electron beam when the first tube voltage is applied. The second deflection device 112 has a negligible effect on the path of the electrons as they propagate from the cathode to the anode along path 130. On the other hand, the second deflection device significantly changes the direction of the electrons' path as they propagate from the cathode to the anode along path 140. In Fig. 1, a narrow electron beam is assumed; that is, the electrons propagate along approximately the same path 130 when the first tube voltage is applied, and along approximately the same path 140 when the second tube voltage is applied.

[0067] 1 also shows that when the second tube voltage is applied, the electrons impinge on the anode at a small impact angle 141, and when the first tube voltage is applied, the electrons impinge on the anode at a large impact angle 131. Thus, the control circuit 104 controls the electron optical system 110 such that when the first tube voltage is applied, the electrons impinge on the anode at the first impact angle 131, and when the second tube voltage is applied, the electrons impinge on the anode at a smaller second impact angle 141.

[0068] Most of the power of the electron beam is converted to heat. Since the tube current may be the same at the first and second voltages, the electrical energy of the electron beam is higher at the second tube voltage compared to the first tube voltage. The higher power of the electron beam with the second tube voltage may exceed the power limit of the anode when the electrons impinge on the anode at an impact angle close to 90°. However, because the impact angle 141 is smaller when the second tube voltage is applied, the backscatter rate of the electrons increases and the percentage of the power of the electron beam that is converted to heat is smaller. Thus, for the second tube voltage, the smaller impact angle of the electrons results in a higher power of the electron beam. This can be utilized to apply a higher tube current when the second tube voltage is applied without exceeding the power limit of the anode.

[0069] Assuming the electron impact angle is the same for the first and second tube voltages, the conversion efficiency, i.e., the percentage of the power of the electron beam that is converted into emitted X-rays, is higher for the second tube voltage compared to the first tube voltage. Furthermore, the percentage of the emitted X-rays that are absorbed by an object, such as a patient's body, in a medical X-ray imaging system is lower for the second tube voltage compared to the first tube voltage. This results in a significantly higher X-ray intensity at an X-ray detector of the X-ray imaging system for a similar impact angle for the second tube voltage compared to the first tube voltage.

[0070] However, in the present disclosure, a smaller impact angle is set for the second tube voltage compared to the first tube voltage. At a smaller electron impact angle, the conversion efficiency is smaller compared to a larger electron impact angle, so that at a smaller electron impact angle, a smaller percentage of the power of the electron beam is converted into emitted X-rays. Thus, for the second tube voltage, the smaller electron impact angle reduces the X-ray intensity at the X-ray detector, which can reduce the imbalance in X-ray intensity between the first and second tube voltages.

[0071] 2 shows a schematic and exemplary second embodiment of an X-ray source 200 for generating X-ray radiation of a first and a second energy spectrum. The X-ray source comprises a cathode 201 and an anode 202. The cathode and the anode are arranged in a tube 220. The tube is preferably a vacuum tube. The anode preferably rotates about an axis 203, although the invention also applies in the case of a stationary anode.

[0072] A power supply (not shown) can be used to apply a tube voltage between the cathode and the anode, allowing the cathode insulator 222 and the anode insulator 221 to be in electrical contact with the cathode and the anode, respectively. The power supply can provide a first and a second tube voltage, the second tube voltage being higher than the first tube voltage. The tube voltage accelerates electrons emitted from the cathode towards the anode. When impinging on the anode, the kinetic energy of the electrons is partially or completely converted into emitted X-rays. By applying different tube voltages, emitted X-rays of different energy spectra can be generated. Also, the power supply can provide three or more tube voltages and can be utilized to generate emitted X-rays of three or more energy spectra. X-rays 251 and 252 indicate the emission of emitted X-rays through an aperture of the X-ray tube 220.

[0073] FIG. 2 also shows the electron optics 210. The electron optics is operatively connected to a control circuit 204 that controls the electron optics. In FIG. 2, the control circuit is located outside the tube 220, but it can also be located inside the tube. The electron optics 210 includes a defocusing device 213 and a focus device 214. The control circuit 204 controls the electron optics 210 such that the defocusing device 213 provides weak or no defocusing of the electron beam from the cathode to the anode when a first tube voltage is applied, and provides strong deflection of the electron beam when a second tube voltage is applied. Furthermore, the control circuit controls the focus device 214 such that the electrons are focused on a first surface section of the anode when a first tube voltage is applied, and on a second surface section of the anode when a second tube voltage is applied. The first and second surface sections of the anode are identical. Alternatively (not shown), the control circuitry controls the focusing device 214 so that the first and second surface sections overlap, or the first surface section is within the second surface section, or the centers of the first and second surface sections coincide as described above. Thus, when a first tube voltage is applied between the cathode and the anode, the electron beam is confined by the propagation paths 232 and 234, while when a second tube voltage is applied, the electron beam is confined by the propagation paths 242 and 244. That is, the electron optics forms a narrow electron beam when the first tube voltage is applied, and a wide electron beam when the second tube voltage is applied. FIG. 2 also shows the minimum impact angles 233 and 235 when the first tube voltage is applied. The minimum impact angles 233 and 235 are equal to each other when the central direction of the electron beam is perpendicular to the surface of the anode and the electron beam is symmetrical with respect to this central direction. Similarly, the minimum impact angles are 243 and 245 when the second tube voltage is applied. Apparently, the minimum impact angles 243 and 245 are smaller than the minimum impact angles 233 and 235. That is, the impact angles of electrons on the anode are spread over a wider range of angles when the second tube voltage is applied compared to when the first tube voltage is applied.Therefore, the average impingement angle is smaller when the second tube voltage is applied compared to when the first tube voltage is applied.

[0074] The smaller average impact angle when the second tube voltage is applied increases the electron backscatter rate and reduces the conversion efficiency, which reduces the imbalance in x-ray intensity for the low and high tube voltages as explained above.

[0075] FIG. 3 shows a schematic and exemplary portion of an anode of an X-ray source. The anode comprises a substrate 301 at least partially coated with a first coating layer 302 and a second coating layer 303. An electron beam 330 impinges on the second coating layer of the anode from an impingement angle 331 of 90°. The second coating layer comprises a material that generates X-ray radiation, such as tungsten. Thus, the energy of the impinging electrons is partially or completely converted into X-ray radiation. X-rays 353 and 354 show a cone of X-rays emitted from the second coating layer through an aperture of the tube of the X-ray source (tube not shown). An X-ray filter 304 is attached to the second coating layer 303, filtering the X-ray radiation generated in the second coating layer. A portion of the impinging electrons is backscattered, as shown by electron beam 360. Another portion of the impinging electrons propagates through the second coating layer and the first coating layer. The first coating layer preferably comprises a material that is transparent to electrons. Thus, some of the electrons impinge on the substrate. The substrate comprises a material that produces X-ray radiation, such as rhenium. Thus, some of the electrons impinge on the substrate and the energy of the electrons is partially or completely converted into X-ray radiation. X-rays 355 and 356 represent a cone of X-rays produced within the substrate, which are emitted through an aperture of an X-ray tube (not shown).

[0076] FIG. 4 shows a schematic and exemplary representation of a portion of the anode shown in FIG. 3. Here again, the anode comprises a substrate 401 at least partially coated with a first coating layer 402 and a second coating layer 403. An electron beam 440 impinges on the second coating layer of the anode from an impingement angle 441. The second coating layer comprises a material that produces X-ray radiation, such as tungsten. Thus, the energy of the impinging electrons is partially or completely converted into X-ray radiation. X-rays 453 and 454 show a cone of X-rays emitted from the second coating layer through the aperture of an X-ray tube (tube not shown). An X-ray filter 404 is attached to the second coating layer 403, filtering the X-ray radiation produced in the second coating layer. A portion of the impinging electrons is backscattered, as shown by electron beam 460. Another portion of the impinging electrons propagates through the second coating layer and the first coating layer. The first coating layer preferably comprises a material that is transparent to electrons. Thus, some of the electrons impinge on the substrate. The substrate comprises a material that generates X-ray radiation, such as rhenium. Thus, some of the electrons impinge on the substrate and the energy of the electrons is partially or completely converted into X-ray radiation. X-rays 455 and 456 show a cone of X-rays generated within the substrate, which are emitted through an aperture of an X-ray tube (not shown).

[0077] In FIG. 4, electrons impinge on the anode from a smaller impingement angle 441 compared to the impingement angle 331 in FIG. 3. Due to the smaller impingement angle, the electron backscatter rate is larger in FIG. 4 compared to FIG. 3. The small impingement angle in FIG. 4 is set when a second (high) tube voltage is applied between the cathode and anode of the X-ray source, while the large impingement angle in FIG. 3 is set when a first (low) tube voltage is applied. Assuming that the tube current is constant, the power of the electron beam is larger at the second tube voltage compared to the first tube voltage. Also, the smaller impingement angle allows the power of the electron beam to be larger when the second tube voltage is applied without exceeding the power limit of the anode.

[0078] Furthermore, due to the small impact angle in FIG. 4, the propagation path through the second coating layer is longer compared to FIG. 3. Therefore, the percentage of electrons propagating through the second coating layer is smaller in FIG. 4 compared to FIG. 3. Therefore, the percentage of electrons impinging on the substrate is smaller in FIG. 4 compared to FIG. 3. Therefore, the percentage of the radiation X-rays generated in the second coating layer to the total radiation X-rays generated in the second coating layer and the substrate is higher in FIG. 4 compared to FIG. 3. That is, in FIG. 4, most of the radiation X-rays are generated in the second coating layer, and only a small portion of the radiation X-rays are generated in the substrate. On the other hand, in the case of FIG. 3, most of the radiation X-rays are generated in the substrate. Since the X-ray filter is arranged to filter the radiation X-rays generated in the second coating layer, the X-ray filter is more effective when the electron impact angle is small compared to when the electron impact angle is large. As mentioned above, the small impingement angle in FIG. 4 is set when a second (high) tube voltage is applied between the cathode and anode of the X-ray source, while the large impingement angle in FIG. 3 is set when a first (low) tube voltage is applied. Thus, the X-ray filter is more effective when the second tube voltage is applied compared to when the first tube voltage is applied. The X-ray filter includes a material such as molybdenum. Filtering the X-ray radiation generated in the second coating layer increases the measure of difference between the first energy spectrum of the X-ray radiation generated when the first tube voltage is applied and the second energy spectrum of the X-ray radiation generated when the second tube voltage is applied. For example, the X-ray filter increases the difference between the mean photon energies of the first and second energy spectra. Providing X-ray radiation having more different first and second energy spectra allows the dual energy X-ray imaging system to capture more meaningful X-ray images.

[0079] Again, assuming that the second tube voltage is applied in the scenario shown in FIG. 4 and the first tube voltage is applied in the scenario shown in FIG. 3, the X-ray filter filters a greater percentage of the emitted X-rays when the second tube voltage is applied compared to when the first tube voltage is applied. Filtering the emitted X-rays reduces the X-ray intensity. Thus, the X-ray filter provides a greater percentage of reduction in X-ray intensity when the second tube voltage is applied compared to when the first tube voltage is applied. This can reduce imbalances in X-ray intensity at the X-ray detector of the X-ray imaging system.

[0080] The thickness of the second coating layer is less than 1 micrometer. The thickness of the second coating layer is generally determined so that the difference between the first rate of radiation and the second rate of radiation is large or maximum. The first rate of radiation is the rate of radiation in the second coating layer to the total radiation in both the second coating layer and the substrate when the first tube voltage is applied. Similarly, the second rate of radiation is the rate of radiation in the second coating layer to the total radiation in both the second coating layer and the substrate when the second tube voltage is applied. The thickness of the second coating layer is selected according to the material of the second coating layer, the material of the substrate, the first and second tube voltages, and the electron collision angle.

[0081] It should be noted that the embodiments of the present invention are described with reference to different subject matter. However, a person skilled in the art can infer that, unless otherwise specified, any combination of features belonging to one type of subject matter as well as any combination of features related to different subject matters are considered to be disclosed in the present application. However, all features can be combined if they provide a synergistic effect beyond the mere collection of features.

[0082] While the invention has been illustrated and described in detail in the drawings and the 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 of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the dependent claims.

[0083] In the claims, the word "comprising" does not exclude other elements or steps, and the singular does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited 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 interpreted as limiting the scope. Further notes regarding the embodiments of the present invention are given below. (1) An x-ray source for generating x-ray radiation of a first energy spectrum and a second energy spectrum, comprising: a cathode for emitting an electron beam; an anode for at least partially converting said electron beam into emitted X-rays; an electron optical system for controlling an impact angle at which electrons of the electron beam impact the anode; a power supply that applies a first tube voltage and a second tube voltage between the cathode and the anode, the second tube voltage being higher than the first tube voltage; a control circuit operably coupled to the electron-optical system; Including, the control circuit controls the electron optical system so that, when the first tube voltage is applied, the electrons of the electron beam collide on the anode at a first average collision angle, and, when the second tube voltage is applied, the electrons of the electron beam collide on the anode at a second average collision angle; The second average impact angle is less than the first average impact angle. (2) The X-ray source described in (1), wherein the second average impact angle at which the electrons of the electron beam impact on the anode is less than 70 degrees. (3) the electrons of the electron beam impinge on a first surface section of the anode when the first tube voltage is applied and impinge on a second surface section of the anode when the second tube voltage is applied; The X-ray source of claim 1 or 2, wherein the first surface section overlaps the second surface section. (4) The X-ray source described in (3), wherein a center of the first surface section coincides with a center of the second surface section of the anode. (5) The electron optical system includes a first deflection device and a second deflection device, the first deflection device being switchable between a first state and a second state; the control circuit switches the first deflection device from the first state to the second state in response to switching the power supply from the first tube voltage to the second tube voltage; An X-ray source as described in any one of (1) to (4), wherein when the first deflection device is in the second state, the first deflection device and the second deflection device provide deflection of the electron beam. (6) The electron optical system includes a focus device and a defocus device, and the electron optical system is switchable between a third state and a fourth state; the control circuit switches the electron optical system from the third state to the fourth state in response to switching the power supply from the first tube voltage to the second tube voltage, An X-ray source as described in any of (1) to (5), wherein in the fourth state, the defocusing device first provides a broadening of the electron beam, and then the focusing device provides a focusing of the broadened electron beam. (7) The X-ray source described in (6), wherein the defocusing device provides a stronger spread of the electron beam when the electron optical system is in the fourth state compared to the third state. (8) The anode comprises a substrate at least partially coated with a first coating layer and a second coating layer, the first coating layer being disposed between the second coating layer and the substrate; the substrate includes a first material that produces x-ray radiation; the first coating layer includes a second material that is transparent to X-ray radiation and the electrons of the electron beam; the second coating layer includes a third material that produces X-ray radiation; An X-ray source as described in any one of (1) to (7), wherein an X-ray filter is attached to the second coating layer to filter the X-ray radiation generated within the second coating layer. (9) The first material of the substrate is rhenium; the second material of the first coating layer is diamond; the third material of the second coating layer is tungsten; The X-ray source according to claim 8, wherein the X-ray filter comprises molybdenum. (10) An X-ray source as described in (8) or (9), wherein the second coating layer has a thickness of less than 1 micrometer at a surface section where the electrons of the electron beam impinge on the second coating layer. (11) An X-ray source according to any one of (1) to (10); An X-ray detector; Including, The X-ray imaging system, wherein the X-ray detector is positioned to receive X-ray radiation emitted from the X-ray source. (12) A detector control circuit further comprising: the detector control circuit controls the X-ray detector to generate first image data when the first tube voltage is applied between the cathode and the anode of the X-ray source, and to generate second image data when the second tube voltage is applied; 12. The X-ray imaging system of claim 11, wherein an integration period for generating the first image data has the same time interval as an integration period for generating the second image data. (13) A method for generating X-ray radiation of a first energy spectrum and a second energy spectrum using an X-ray source including: a cathode for emitting an electron beam; an anode for at least partially converting the electron beam into X-ray radiation; a power supply for applying a tube voltage between the cathode and the anode; an electron optical system for controlling an impact angle at which electrons of the electron beam impact on the anode; and a control circuit operably coupled to the electron optical system, comprising: The step of generating radiation of a first energy spectrum includes: applying a first tube voltage by the power supply; controlling, by the control circuit, the electron optics such that the electrons of the electron beam impinge on the anode at a first average impingement angle; Including, The step of generating radiation of a second energy spectrum includes: applying a second tube voltage by the power supply, the second tube voltage being higher than the first tube voltage; controlling, by the control circuit, the electron optics such that the electrons of the electron beam impinge on the anode at a second average impingement angle, the second average impingement angle being smaller than the first average impingement angle; A method comprising: (14) A computer program comprising instructions for causing an X-ray source according to any one of (1) to (10) to perform the steps of the method according to (13). (15) A computer-readable medium having the computer program described in (14) stored thereon.

Claims

1. 1. An x-ray source for generating x-ray radiation of a first energy spectrum and a second energy spectrum, comprising: a cathode for emitting an electron beam; an anode for at least partially converting said electron beam into radiated x-rays; an electron optical system for controlling an impact angle at which electrons of the electron beam impact the anode; a power supply that applies a first tube voltage and a second tube voltage between the cathode and the anode, the second tube voltage being higher than the first tube voltage; a control circuit operably coupled to the electron-optical system; Including, the control circuit controls the electron optical system so that, when the first tube voltage is applied, the electrons of the electron beam impinge on the anode at a first average impact angle to generate radiation X-rays of the first energy spectrum, and, when the second tube voltage is applied, the electrons of the electron beam impinge on the anode at a second average impact angle to generate radiation X-rays of the second energy spectrum; the second average impact angle is less than the first average impact angle; the electron optical system includes a first deflection device and a second deflection device, the first deflection device being switchable between a first state and a second state; the control circuit switches the first deflection device from the first state to the second state in response to switching the power supply from the first tube voltage to the second tube voltage; The x-ray source, when the first deflection device is in the second state, the first deflection device and the second deflection device provide deflection of the electron beam.

2. An x-ray source for generating x-ray radiation of a first energy spectrum and a second energy spectrum, comprising: a cathode for emitting an electron beam; an anode for at least partially converting said electron beam into radiated x-rays; an electron optical system for controlling an impact angle at which electrons of the electron beam impact the anode; a power supply that applies a first tube voltage and a second tube voltage between the cathode and the anode, the second tube voltage being higher than the first tube voltage; a control circuit operably coupled to the electron-optical system; Including, the control circuit controls the electron optical system so that, when the first tube voltage is applied, the electrons of the electron beam impinge on the anode at a first average impact angle to generate radiation X-rays of the first energy spectrum, and, when the second tube voltage is applied, the electrons of the electron beam impinge on the anode at a second average impact angle to generate radiation X-rays of the second energy spectrum; the second average impact angle is less than the first average impact angle; the electrons of the electron beam impinge on a first surface section of the anode when the first tube voltage is applied and impinge on a second surface section of the anode when the second tube voltage is applied; The first surface section overlies the second surface section of the x-ray source.

3. An x-ray source for generating x-ray radiation of a first energy spectrum and a second energy spectrum, comprising: a cathode for emitting an electron beam; an anode for at least partially converting said electron beam into radiated x-rays; an electron optical system for controlling an impact angle at which electrons of the electron beam impact the anode; a power supply that applies a first tube voltage and a second tube voltage between the cathode and the anode, the second tube voltage being higher than the first tube voltage; a control circuit operably coupled to the electron-optical system; Including, the control circuit controls the electron optical system so that, when the first tube voltage is applied, the electrons of the electron beam impinge on the anode at a first average impact angle to generate radiation X-rays of the first energy spectrum, and, when the second tube voltage is applied, the electrons of the electron beam impinge on the anode at a second average impact angle to generate radiation X-rays of the second energy spectrum; the second average impact angle is less than the first average impact angle; the electron optical system includes a focus device and a defocus device, and the electron optical system is switchable between a third state and a fourth state; the control circuit switches the electron optical system from the third state to the fourth state in response to switching the power supply from the first tube voltage to the second tube voltage, An X-ray source, wherein in the fourth state, the defocusing device first provides a broadening of the electron beam, and thereafter the focusing device provides a focusing of the broadened electron beam.

4. An x-ray source for generating x-ray radiation of a first energy spectrum and a second energy spectrum, comprising: a cathode for emitting an electron beam; an anode for at least partially converting said electron beam into radiated x-rays; an electron optical system for controlling an impact angle at which electrons of the electron beam impact the anode; a power supply that applies a first tube voltage and a second tube voltage between the cathode and the anode, the second tube voltage being higher than the first tube voltage; a control circuit operably coupled to the electron-optical system; Including, the control circuit controls the electron optical system so that, when the first tube voltage is applied, the electrons of the electron beam impinge on the anode at a first average impact angle to generate radiation X-rays of the first energy spectrum, and, when the second tube voltage is applied, the electrons of the electron beam impinge on the anode at a second average impact angle to generate radiation X-rays of the second energy spectrum; the second average impact angle is less than the first average impact angle; the anode comprises a substrate at least partially coated with a first coating layer and a second coating layer, the first coating layer being disposed between the second coating layer and the substrate; the substrate includes a first material that produces x-ray radiation; the first coating layer includes a second material that is transparent to x-ray radiation and the electrons of the electron beam; the second coating layer includes a third material that produces x-ray radiation; an x-ray filter attached to the second coating layer for filtering x-ray radiation generated within the second coating layer; the first material of the substrate is rhenium; the second material of the first coating layer is diamond; the third material of the second coating layer is tungsten; The X-ray source, wherein the X-ray filter comprises molybdenum.

5. 5. The x-ray source of claim 1, wherein the second average impact angle at which the electrons of the electron beam impact on the anode is less than 70 degrees.

6. 3. The x-ray source of claim 2, wherein a center of the first surface section coincides with a center of the second surface section of the anode.

7. 4. The X-ray source of claim 3, wherein the defocusing device provides a stronger divergence of the electron beam when the electron optics is in the fourth state compared to the third state.

8. the anode comprises a substrate at least partially coated with a first coating layer and a second coating layer, the first coating layer being disposed between the second coating layer and the substrate; the substrate includes a first material that produces x-ray radiation; the first coating layer includes a second material that is transparent to x-ray radiation and the electrons of the electron beam; the second coating layer includes a third material that produces x-ray radiation; 8. An X-ray source according to claim 1 , 2 or 3, wherein an X-ray filter is attached to the second coating layer for filtering X-ray radiation generated in the second coating layer.

9. 9. An X-ray source according to claim 4 or 8, wherein the second coating layer has a thickness of less than 1 micrometer at a surface section where the electrons of the electron beam impinge on the second coating layer.

10. An X-ray source according to any one of claims 1 to 9; An X-ray detector; Including, The X-ray imaging system, wherein the X-ray detector is positioned to receive X-ray radiation emitted from the X-ray source.

11. further comprising a detector control circuit; the detector control circuit controls the X-ray detector to generate first image data when the first tube voltage is applied between the cathode and the anode of the X-ray source, and to generate second image data when the second tube voltage is applied; The x-ray imaging system of claim 10 , wherein an integration period for generating the first image data has the same time duration as an integration period for generating the second image data.

12. 1. A method for generating x-ray radiation of a first energy spectrum and a second energy spectrum using an x-ray source including: a cathode for emitting an electron beam; an anode for at least partially converting the electron beam into x-ray radiation; a power supply for applying a tube voltage between the cathode and the anode; electron optics for controlling an impact angle at which electrons of the electron beam impact on the anode; and a control circuit operably coupled to the electron optics, the method comprising: The step of generating radiation of a first energy spectrum includes: applying a first tube voltage by the power supply; controlling, by the control circuitry, the electron optics such that the electrons of the electron beam impinge on the anode at a first average impact angle to produce x-ray radiation of the first energy spectrum; Including, The step of generating radiation of a second energy spectrum includes: applying a second tube voltage by the power source, the second tube voltage being higher than the first tube voltage; controlling, by the control circuit, the electron optics such that the electrons of the electron beam impinge on the anode at a second average impact angle to generate radiation X-rays of the second energy spectrum, the second average impact angle being less than the first average impact angle; Including, the electron optical system includes a first deflection device and a second deflection device, the first deflection device being switchable between a first state and a second state; the control circuit switches the first deflection device from the first state to the second state in response to switching the power supply from the first tube voltage to the second tube voltage; The method, wherein the first deflection device and the second deflection device provide a deflection of the electron beam when the first deflection device is in the second state.

13. A computer program comprising instructions for causing an X-ray source according to any one of claims 1 to 9 to perform the steps of the method according to claim 12.

14. A computer readable medium having stored thereon the computer program of claim 13.

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