Balancing X-ray Output for Dual-Energy X-ray Imaging Systems

Abstract

An X-ray source (100) for generating X-ray radiation of a first energy spectrum and a second energy spectrum is provided, wherein an X-ray intensity imbalance between the first energy spectrum and the second energy spectrum is reduced compared to a conventional X-ray source. The reduction in the X-ray intensity imbalance is achieved by configuring a smaller electron impact angle (141) onto an anode (102) when a higher tube voltage is applied than when a lower tube voltage is applied.

Description

Technical Field

[0001] The invention relates to an X-ray source for generating X-ray radiation of a first energy spectrum 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 Art

[0002] X-ray imaging systems are used in many applications, such as medical diagnosis, airport security, material analysis, etc. For example, in a computed tomography (CT) system, an X-ray source and an X-ray detector are arranged on opposite sides of a patient. The X-ray source can generate a fan-shaped beam of X-rays. The photons of the X-ray beam will be partially absorbed by the patient's body. As a result, bone absorbs more photons than 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 anatomical structure. 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 X-ray detector rotate around the patient to capture images viewed from different viewing directions. These images can be processed by a computer system to generate a three-dimensional image of the patient's anatomical structure.

[0003] An X-ray source typically consists of a cathode and an anode arranged inside a vacuum tube. The cathode emits electrons, which are accelerated toward the anode by the tube voltage supplied by the power supply. With a tube voltage of, for example, 80 kV, the electrons are accelerated from the cathode to the anode, reaching a kinetic energy of 80 keV upon impact with the anode. This energy is fully or partially converted into X-ray radiation, which is then irradiated through an aperture in the X-ray tube housing.

[0004] The image quality provided by an X-ray imaging system can be improved by scanning a patient using X-ray radiation having multiple energy spectra. This allows for imaging of the patient using spectral material decomposition. This technique has been shown to produce improved images while simultaneously reducing the amount of toxic contrast dye (typically iodine). X-ray radiation of different energy spectra can be generated by applying different tube voltages between the cathode and anode of the X-ray tube. For example, tube voltages of 80 kV and 140 kV can be applied to generate softer and harder X-ray radiation, respectively. Summary of the Invention

[0005] It has been observed that the intensity of X-ray radiation is higher for high tube voltages than for low tube voltages. This imbalance in X-ray intensity can be at least partially compensated for at the X-ray detector of an X-ray imaging system by using a longer integration period for low tube voltages compared to high tube voltages. However, in CT systems, the X-ray source and X-ray detector typically rotate around the patient at a constant angular velocity, so using a longer integration period for low tube voltages results in reduced image resolution.

[0006] It may therefore be desirable to provide an improved X-ray source for generating X-ray radiation of a first energy spectrum and a second energy spectrum, wherein imbalances in X-ray intensities at different tube voltages are reduced.

[0007] It should be noted that any features, elements and / or functions of the X-ray source described below are equally applicable 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] According to the present disclosure, an X-ray source for generating X-ray radiation of a first energy spectrum and a second energy spectrum is provided. The X-ray source includes: a cathode for emitting an electron beam; an anode for converting the electron beam at least partially into X-ray radiation; an electron optics device configured to control an impact angle at which electrons of the electron beam impact the anode; a power supply configured to apply 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; and a control circuit operatively coupled to the electron optics device; wherein the control circuit is configured to control the electron optics device so that: when the first tube voltage is applied, the electrons of the electron beam impact the anode at a first average impact angle; and when the second tube voltage is applied, the electrons of the electron beam impact the anode at a second average impact angle; and wherein the second average impact angle is less than the first average impact angle.

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

[0010] X-ray radiation with a first energy spectrum can be generated by applying a first tube voltage between the cathode and the anode, while X-ray radiation with a second energy spectrum can be generated by applying a second tube voltage between the cathode and the anode, the second tube voltage being higher than the first tube voltage. The power supply can generate, for example, first and second tube voltages of 80 kV and 140 kV, respectively. Due to the tube voltages, electrons emitted by the cathode are accelerated toward the anode. For example, with an 80 kV tube voltage, the electrons reach a kinetic energy of 80 keV before impacting the anode. At the anode, this energy can be partially or completely converted into X-ray radiation, resulting in X-ray photons having energies up to 80 keV. In contrast, with a 140 keV tube voltage, X-ray photons reach energies up to 140 keV. Therefore, the energy spectrum of the X-ray radiation can include energies at the second tube voltage that are higher than those at the first tube voltage.

[0011] Several types of cathodes can be used for the X-ray sources of the present disclosure, including conventional thermionic cathodes, field emitter cathodes, Pierce cathodes, reserve cathodes, photocathodes, or plasma cathodes. The anode of the X-ray source can include a material for generating X-ray radiation, such as tungsten or molybdenum. The anode can be rotated so that the heat generated in the focal spot (i.e., the portion of the surface where electrons collide with the anode) is spread over a larger area than with a fixed anode. However, the present invention is not limited to the case of a rotating anode.

[0012] Electrons in the electron beam traveling from the cathode to the anode can impact the anode at various impact angles. The impact angle can range from 0° to 90°. An impact angle of 90° results in an electron propagation path perpendicular to the anode surface. The smaller the impact angle, the more coherent the electron propagation path relative to the anode surface.

[0013] The electron optics of the X-ray source are configured to control the impact angle of the electrons of the electron beam onto the anode. The electron optics may include an electrostatic device and / or a magnetic device. For example, the electron optics may include one or more dipoles, one or more quadrupoles, etc. The electron optics may be configured to form an electron beam. In one scenario, the electron optics may be configured to form a narrow electron beam so that the impact angle of the electrons impacting the anode is substantially the same for all electrons. In another scenario, the electron optics may be configured to form a wide electron beam so that the electrons impact the anode from a wide range of impact angles. In another scenario, the electron optics may deflect the electron beam to change the average impact angle of the electrons impacting the anode without changing the width of the electron beam at the impact surface area. In another scenario, the electron optics may be configured to deflect the electron beam and also form the electron beam.

[0014] The control circuitry can include analog components and / or digital components. For example, the control circuitry can include passive electronic devices (e.g., resistors, capacitors, or electronic filters), active electronic devices (e.g., amplifiers or analog-to-digital converters, microcontrollers, microprocessors, memory cells), etc. The control circuitry is operatively coupled to the electron-optical device and configured to control the electron-optical device. Additionally, the control circuitry can be operatively coupled to a power supply, and the control circuitry can be configured to control the power supply, including controlling the voltage generated by the power supply. Alternatively or additionally, the control circuitry can be configured to receive a signal from the power supply indicating the generated tube voltage, and the control circuitry can be configured to control the electron-optical device based on the signal received from the power supply.

[0015] The power supply is configured to generate a first tube voltage and a second tube voltage and apply these voltages between the cathode and the anode. The power supply can be configured to frequently switch between these tube voltages. For example, in a CT system, the tube voltage may be frequently switched to reduce imaging artifacts caused by movement of the patient (or the patient's internal organs) during the period between capturing high-energy X-ray images and low-energy X-ray images from similar directions. The power supply can be configured to switch between the first and second tube voltages at a frequency of 1 kHz, 2 kHz, 4 kHz, or 10 kHz. The voltage generated by the power supply can also be switched at a lower or higher frequency. The cathode may not be able to switch the tube current at such a high frequency, or the cathode's ability to switch at such a high frequency may be intentionally reduced. Using the X-ray tube of the present disclosure, the tube current can remain constant for the entire scan time, or at least for the period when the tube voltage is changing (particularly during the ramp-down period of the tube voltage). The tube current can be substantially the same during the period when the first tube voltage is applied and during the period when the second tube voltage is applied. Because the second tube voltage is higher than the first tube voltage, the electron beam power can be higher at the second tube voltage compared to the first tube voltage.

[0016] For tube voltages used in medical imaging, a major portion of the electron beam's power is converted into heat, and the higher power of the electron beam at a second tube voltage can exceed the heat capacity of the anode. However, according to the present disclosure, the control circuit is configured to control the electron optics so that: when the second tube voltage is applied, the electrons of the electron beam impact the anode at a second average impact angle; and when the first tube voltage is applied, the electrons of the electron beam are configured to impact the anode at a second average impact angle that is smaller than the first average impact angle. By configuring the second tube voltage with a smaller impact angle, the electron backscatter rate can be increased, thereby causing a smaller proportion of the electron beam's power to be converted into heat. Therefore, the smaller impact angle of the electrons at the second tube voltage can promote a higher power electron beam. This situation can be exploited to apply a high current when the second tube voltage is applied without exceeding the power limit of the anode.

[0017] When similar impact angles are configured for the first tube voltage and the second tube voltage, the conversion efficiency of the second tube voltage (which means the proportion of the power of the electron beam that is converted into X-ray radiation) can be higher than the conversion efficiency of the first tube voltage. In addition, the proportion of X-ray radiation absorbed by an object (e.g., a patient's body) in the medical X-ray imaging system at the first tube voltage can be higher than the proportion of X-ray radiation absorbed by the object (e.g., a patient's body) in the medical X-ray imaging system at the second tube voltage. Therefore, assuming similar impact angles and similar tube currents are configured for the first tube voltage and the second tube voltage, the X-ray intensity at the X-ray detector of the X-ray imaging system at the second tube voltage can be significantly higher than the X-ray intensity at the X-ray detector of the X-ray imaging system at the first tube voltage.

[0018] As explained above, this imbalance in X-ray intensity can be at least partially compensated for by using a longer integration period for the first tube 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 the X-ray detector typically rotate around the patient at a constant angular velocity, so that using a longer integration period for the first tube voltage results in reduced image resolution.

[0019] Alternatively, this imbalance in X-ray intensity can be mitigated by controlling the tube current emitted by the cathode so that the tube current is higher when a first tube voltage is applied than 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 compensate for the imbalance in X-ray intensity, particularly when the tube voltage is switched between each integration period. For example, a conventional thermionic cathode may allow the tube current to be switched at a frequency of 10 Hz, while the tube voltage may be switched at a frequency of 10 kHz.

[0020] Alternatively, to increase the X-ray intensity at the first tube voltage, the tube current emitted by the cathode can be increased. However, when the same high tube current is applied at the first tube voltage and the second tube voltage, the power limit of the anode may be exceeded during the period of application of the second tube voltage, thereby reducing the life of the X-ray source.

[0021] According to the present disclosure, X-ray intensity imbalance can be reduced by configuring the second tube voltage to have a smaller impact angle than the first tube voltage. Compared to larger electron impact angles, conversion efficiency is lower for smaller electron impact angles. Consequently, a smaller proportion of the electron beam's power can be converted into X-ray radiation for smaller electron impact angles. Therefore, the smaller electron impact angles at the anode for the second tube voltage can result in a reduced X-ray intensity at the X-ray detector, thereby reducing the X-ray intensity imbalance between the first and second tube voltages.

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

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

[0024] In another example, when the first tube voltage is applied, the electrons of the electron beam impact a first surface portion of the anode; and when the second tube voltage is applied, the electrons of the electron beam impact a second surface portion of the anode, wherein the first surface portion overlaps the second surface portion.

[0025] The X-ray source can be used in an X-ray imaging system. Differences between the first and second surface portions of the anode can lead to imaging artifacts, making it more difficult to compare or combine X-ray images captured for the first and second X-ray energy spectra. Therefore, in principle, it is desirable that the first and second surface portions of the anode are identical, i.e., they have approximately the same size and are located at approximately the same position relative to the interface between the X-ray source and the X-ray system. Typically, this means that the regions where electrons impinge on the anode (the first and second surface portions of the anode) substantially coincide. The first and second surface portions of the anode are considered to have approximately the same size when the difference in size between the first and second surface portions does not exceed 20%. Therefore, the control circuit can be configured to control the electron optics so that the difference between the size of the first surface portion and the size of the second surface portion is, for example, less than 20% or less than 10%. Alternatively or additionally, the control circuit can be configured to control the electron optics so that a parameter indicating the size of the first surface portion (e.g., the diameter of the first surface portion in a certain direction) differs from a corresponding parameter indicating the size of the second surface portion by, for example, less than 20% or less than 10%. In other examples, it can be expected that the second surface portion is larger than the first surface portion and / or the second surface portion overlaps the first surface portion. Then, compared with the identical first surface portion and the second surface portion, the first surface portion and the second surface portion can together span the larger part of the surface of the anode. Therefore, compared with the example with the identical first surface portion and the second surface portion, the anode surface portion that generates heat due to impact electrons can be larger. This diffusion of the heat generated on the larger anode surface portion can allow the electric power of the electron beam to increase without exceeding the electric power limit of the anode.

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

[0027] The center of the first surface portion can be defined, for example, as the average location on the anode surface where electrons impact when the first tube voltage is applied. Alternatively, the center of the first surface portion can be defined, for example, as the location on the anode surface where electrons are most likely to impact when the first tube voltage is applied. The definition of the center of the second surface portion is preferably similar to that of the center of the first surface portion. Thus, the center of the second surface portion can be defined as the average location on the anode surface where electrons impact when the second tube voltage is applied. Alternatively, the center of the second surface portion can be defined as the location on the anode surface where electrons are most likely to impact when the second tube voltage is applied.

[0028] Here and hereinafter, the center of the first surface portion is said to coincide with the center of the second surface portion when the amount of difference between the center of the first surface portion and the center of the second surface portion is negligible compared to the size of the smaller of the center of the first surface portion and the center of the second surface portion. In particular, the control circuitry may be configured to control the electron-optical device so that the difference between the center of the first surface portion and the center of the second surface portion is less than 20%, or preferably, less than 10%, relative to the diameter of the smaller of the first surface portion and the second surface portion in the direction of the difference between the center of the first surface portion and the center of the second surface portion.

[0029] In another example, the electron optical device includes a first deflection device and a second deflection device, wherein the first deflection device is switchable between a first state and a second state, wherein the control circuit is configured to switch 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, and 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.

[0030] The first deflection device and the second deflection device may each include one or more magnetic devices and / or one or more electrostatic devices, including dipoles, quadrupoles, and the like.

[0031] The control circuit may be configured to control the first deflection device so that: when the second tube voltage is applied, the first deflection device deflects the electron beam; and when the first tube voltage is applied, the first deflection device does not deflect the electron beam. Alternatively, the control circuit may be configured to control the first deflection device so that when the second tube voltage is applied, the first deflection device provides a stronger deflection of the electron beam than when the first tube voltage is applied. When the first tube voltage is applied by the power supply, the control circuit may configure the first state; and when the second tube voltage is applied by the power supply, the control circuit may configure the second state.

[0032] The second deflection device may be configured to: upon application of a first tube voltage, steer the electron beam (which may have been deflected by the first deflection device) toward a first surface portion of the anode; and upon application of a second tube voltage, steer the electron beam (which may have been deflected by the first deflection device) toward a second surface portion of the anode, wherein the first surface portion and the second surface portion of the anode may overlap, or wherein the first surface portion and the second surface portion of the anode may have coincident centers, or wherein the first surface portion and the second surface portion of the anode may be identical. The second deflection device may also be switchable, and the control circuit may be configured to control the second deflection device.

[0033] In the second state, the electron beam is deflected such that the second average impact angle is smaller than the first average impact angle. The first deflection device can deflect the electrons of the electron beam in a radial direction of the axis of rotation of the anode. Additionally or alternatively, the first deflection device can deflect the electrons of the electron beam in a direction perpendicular to both the radial direction and the axis of rotation of the rotating anode, or in a direction normal to an anode target of the fixed anode.

[0034] In another example, the electron optical device includes a focusing device and a defocusing device, wherein the electron optical device is switchable between a third state and a fourth state; wherein the control circuit is configured to switch the electron optical device 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; and wherein, in the fourth state, the defocusing device first provides a widening of the electron beam, and the focusing device subsequently provides a focusing of the widened electron beam.

[0035] The focusing device and the defocusing device may each include one or more magnetic devices and / or one or more electrostatic devices, including dipoles, quadrupoles, etc.

[0036] The control circuitry may be configured to control the defocusing device such that: when the second tube voltage is applied, the defocusing device increases the width of the electron beam; and when the first tube voltage is applied, the defocusing device does not widen the electron beam. Alternatively, the control circuitry may be configured to control the defocusing device such that when the second tube voltage is applied, the defocusing device provides a stronger widening of the electron beam than when the first tube voltage is applied.

[0037] Furthermore, the control circuitry may be configured to control the focusing system such that: when a first tube voltage is applied, the focusing system focuses the electron beam impinging on a first surface portion of the anode (which may have been widened by the defocusing device); and when a second tube voltage is applied, the focusing system focuses the electron beam impinging on a second surface portion of the anode (which may have been widened by the defocusing device), wherein the first surface portion and the second surface portion of the anode may have comparable or identical dimensions to those outlined above, and / or wherein the first surface portion and the second surface portion may overlap, and / or wherein the centre of the first surface portion of the anode may coincide with the centre of the second surface portion of the anode.

[0038] When the power supply supplies a first tube voltage, the control circuit can configure the third state, and when the power supply supplies a second tube voltage, the control circuit can configure the fourth state. In the fourth state, the electron beam is widened and subsequently focused to reduce the second average impact angle. For example, the central direction of the electron beam can be perpendicular to the anode surface. However, with a wide electron beam, electrons can also impact the anode surface at an impact angle significantly less than 90°. Therefore, with a wide electron beam, even if the central direction of the electron beam is perpendicular to the anode surface, the average impact angle of the electrons can still be significantly less than 90°.

[0039] The electron beam can be broadened in a radial direction of the axis of rotation of the rotating anode or in a normal direction to the anode target of the fixed anode. Additionally or alternatively, the electron beam can be broadened in a direction perpendicular to the axis of rotation or to the normal to the anode target of the fixed anode and perpendicular to the corresponding radial direction.

[0040] In another example, an electron-optical device includes a first deflection device, a second deflection device, a focusing device, and a defocusing device, wherein the electron-optical device is switchable between a fifth state and a sixth state, wherein the control circuit is configured to switch the electron-optical device from the fifth state to the sixth state in response to switching the power supply from the first tube voltage to the second tube voltage, and wherein, in the sixth state, the first deflection device and the second deflection device provide deflection of the electron beam, and the focusing device and the defocusing device provide widening and subsequent focusing of the electron beam.

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

[0042] The central direction of the electron beam may be orthogonal or nearly orthogonal to the anode surface.Then, widening of the electron beam by the defocusing device and subsequent focusing of the widened electron beam impinging on the anode surface portion may result in a reduced average impact angle compared to a configuration in which the electron beam is not widened.

[0043] In another example, the anode includes a substrate at least partially coated with a first coating and a second coating, the first coating being arranged between the second coating and the substrate; wherein the substrate includes a first material to generate X-ray radiation; wherein the first coating includes a second material, the second material being transparent to the X-ray radiation and the electrons of the electron beam; wherein the second coating includes a third material to generate X-ray radiation; and wherein an X-ray filter is attached to the second coating to filter the X-ray radiation generated in the second coating.

[0044] The electron beam may initially impact the second coating. When the electron beam impacts the second coating, the power of the electron beam may be partially converted into X-ray radiation. When the power supply generates the second tube voltage, the average impact angle of the electrons impacting the second surface portion of the anode is smaller than when the power supply generates the first tube voltage.

[0045] Because the average impact angle of electrons for the second tube voltage is smaller, the propagation path through the second coating layer for the second tube voltage can be, on average, longer than the propagation path through the second coating layer for the first tube voltage. Consequently, the proportion of electrons propagating through the second coating layer can be smaller at the second tube voltage compared to the first tube voltage. Consequently, the proportion of electrons impacting the substrate can be smaller at the second tube voltage compared to the first tube voltage. For this reason, the proportion of X-ray radiation generated in the second coating layer relative to the sum of X-ray radiation generated in the second coating layer and the substrate can be higher at the second tube voltage compared to the first tube voltage. In other words, when the second tube voltage is applied, most of the X-ray radiation can be generated in the second coating layer, and only a relatively small proportion can be generated in the substrate, whereas when the first tube voltage is applied, a very large proportion of the X-ray radiation can be generated in the substrate.

[0046] The X-ray filter can be arranged so that the X-ray filter filters X-ray radiation generated in the second coating layer and does not filter X-ray radiation generated in the substrate. To achieve this, the first coating layer can provide a sufficient gap between the substrate and the second coating layer so that the X-ray filter is arranged sufficiently separated from the substrate.

[0047] The X-ray filter may include a material such as molybdenum, and filtering X-ray radiation generated in the second coating may cause an increase in a measure of the difference between a first energy spectrum of X-ray radiation generated when the first tube voltage is applied and a second energy spectrum of X-ray radiation generated when the second tube voltage is applied. For example, the X-ray filter may increase the difference between the average photon energy of the first energy spectrum and the second energy spectrum. Providing X-ray radiation with more distinct first and second energy spectra may allow for the capture of more meaningful X-ray images in a dual-energy X-ray imaging system.

[0048] Furthermore, when the second tube voltage is applied, the X-ray filter can filter a larger proportion of X-ray radiation than when the first tube voltage is applied. This filtering of X-ray radiation results in a reduction in X-ray intensity. Therefore, the X-ray filter can provide a greater reduction in X-ray intensity at the second tube voltage than when the first tube voltage is applied. This can result in a reduction in X-ray intensity imbalance 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 may be diamond or a carbon-based fiber-reinforced composite material. The third material of the second coating may be tungsten. The X-ray filter may include molybdenum or another filter material that lacks 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 portion where the electrons of the electron beam impact the second coating layer.

[0051] The thickness of the second coating layer can generally be determined so that the difference between a first ratio of X-ray radiation and a second ratio of X-ray radiation is large or maximized, where the first ratio of X-ray radiation is the ratio of X-ray radiation generated in the second coating layer relative to the sum of X-ray radiation generated in the second coating layer and the substrate when the first tube voltage is applied. Similarly, the second ratio of X-ray radiation can be the ratio of X-ray radiation generated in the second coating layer relative to the sum of X-ray radiation generated in the second coating layer and the substrate when the second tube voltage is applied. The second coating layer can have a thickness of 1 micron. However, depending on the composition of the second coating layer, the composition of the substrate, the first and second tube voltages, and the electron impact angle, the second coating layer can be thinner or thicker.

[0052] According to the present disclosure, an X-ray imaging system is further provided, comprising an X-ray source according to any of the above embodiments and an X-ray detector, wherein the X-ray detector is arranged to receive X-ray radiation emitted by the X-ray source.

[0053] X-ray imaging systems can be used, for example, in medical imaging systems or airport security systems. When an X-ray source generates X-ray radiation of a first energy spectrum, an X-ray detector can capture first image data of an object. Similarly, when the X-ray source generates X-ray radiation of a second energy spectrum, the X-ray detector can capture second image data. The first energy spectrum and the second energy spectrum can be different, so that the first image data can provide different information about the object than the second image data. The first image data and the second image data can be displayed separately, or can be processed separately to, for example, detect structures or materials, or can be combined with the second image data to render an image based on both the first image data and the second image data.

[0054] In an example, the X-ray imaging system further includes a detector control circuit, wherein 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 between the cathode and the anode of the X-ray source, whereby an integration period for generating the first image data has the same duration as an integration period for generating the second image data.

[0055] Compared to conventional X-ray sources, the X-ray source of the present disclosure can provide X-ray radiation with a reduced intensity imbalance between a first tube voltage and a second tube voltage. This can be achieved by configuring the second tube voltage with an average impact angle that is smaller than the average impact angle for the first tube voltage. The reduced X-ray intensity imbalance between the first and second tube voltages can allow for the use of more similar X-ray detector integration periods when the first and second tube voltages are applied. In particular, a first X-ray detector integration period can be used when the first tube voltage is applied, and a second X-ray detector integration period can be used when the second tube voltage is applied, wherein the first integration period and the second integration period can have the same or similar duration.

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

[0057] In addition, generating X-ray radiation of the second energy spectrum includes the following steps, but these steps are not necessarily in the following order: (i) applying a second tube voltage by the power supply, wherein the second tube voltage is higher than the first tube voltage, and (ii) controlling the electron optical device by the control circuit so that the electrons of the electron beam impact the anode at a second average impact angle, wherein the second average impact angle is smaller than the first average impact angle.

[0058] According to the present disclosure, a computer program is also proposed, wherein the computer program comprises instructions for causing the X-ray source of the present disclosure to perform the steps of the method for generating X-ray radiation of a first energy spectrum and a second energy spectrum.

[0059] According to the present disclosure, a computer-readable medium is also proposed, wherein the computer-readable medium stores the computer program of the present disclosure.

[0060] 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

[0061] Exemplary embodiments of the present invention will be described hereinafter with reference to the accompanying drawings:

[0062] Figure 1 A first embodiment of an X-ray source for generating X-ray radiation of a first energy spectrum and a second energy spectrum is shown schematically and exemplarily.

[0063] Figure 2 A second embodiment of an X-ray source for generating X-ray radiation of a first energy spectrum and a second energy spectrum is shown schematically and exemplarily.

[0064] Figure 3 A section of an anode for an X-ray source is shown schematically and exemplarily, with an electron beam impinging on the anode at a large impact angle.

[0065] Figure 4 Shown Figure 3 This part of the anode has the electron beam impinging on the anode at a small impact angle. DETAILED DESCRIPTION

[0066] Figure 1 A first embodiment of an X-ray source 100 for generating X-ray radiation of a first energy spectrum and a second energy spectrum is schematically and exemplarily shown. The X-ray source comprises a cathode 101 and an anode 102. The cathode and anode are arranged inside a tube 120, which is preferably a vacuum tube. The anode can rotate about an axis 103, but this is not required for the present disclosure.

[0067] Cathode insulator 122 and anode insulator 121 allow electrical contact between the cathode and anode, respectively, enabling a tube voltage to be applied between the cathode and anode via a power supply (not shown). The power supply can be configured to supply a first tube voltage and a second tube voltage, wherein the second tube voltage is higher than the first tube voltage. For example, the power supply can be configured to supply a first tube voltage of 80 kV and a second tube voltage of 140 kV. Due to the tube voltages, electrons emitted by the cathode can be accelerated toward the anode. For example, when a first tube voltage of 80 kV is applied between the cathode and anode, the electrons can reach a kinetic energy of 80 keV before impacting the anode. When the electrons impact the anode, this energy is partially or completely converted into X-ray radiation. Therefore, when the first tube voltage is applied, X-ray photons can have energies up to 80 keV, while when the second tube voltage is applied, the X-ray photons can have energies up to 140 keV. Therefore, by applying different tube voltages, X-ray radiation with different energy spectra can be generated. The power supply can also be configured to generate more than two voltages, which can be used to generate X-ray radiation with more than two energy spectra. X-rays 151 and 152 illustrate the emission of X-ray radiation through the hole in tube 120 .

[0068] Figure 1 Also shown is an electron-optical device 110. The electron-optical device is operatively connected to a control circuit 104, which is configured to control the electron-optical device. Figure 1 In the embodiment shown, the control circuit is arranged outside the tube 120, but the control circuit can alternatively be arranged inside the tube. Figure 1 , the electron optical device 110 includes a first deflection device 111 and a second deflection device 112. The control circuit 104 can be configured to control the first deflection device so that: when the first tube voltage is applied, the electrons propagate along a path 130 from the cathode toward the anode; and when the second tube voltage is applied, the electrons propagate along a path 140. To this end, the control circuit can be configured to control the first deflection device so that: when the second tube voltage is supplied, the first deflection device provides a deflection of the electron beam; and when the first tube voltage is supplied, the first deflection device can provide a weaker deflection of the electron beam or no deflection of the electron beam. When the electrons propagate from the cathode to the anode along the path 130, the influence of the second deflection device 112 on the electron path is negligible; and when the electrons propagate from the cathode toward the anode along the path 140, the second deflection device significantly changes the direction of the electron path. In Figure 1 In FIG. 1 , a narrow electron beam is assumed, ie, when a first tube voltage is applied, the electrons propagate along approximately the same path 130 ; and when a second tube voltage is applied, the electrons propagate along approximately the same path 140 .

[0069] Figure 1The figure also shows that when the second tube voltage is applied, electrons impact the anode at a small impact angle 141, while when the first tube voltage is applied, electrons impact the anode at a large impact angle 131. Therefore, the control circuit 104 controls the electron optical device 110 so that when the first tube voltage is applied, electrons impact the anode at a first impact angle 131, and when the second tube voltage is applied, electrons impact the anode at a smaller second impact angle 141.

[0070] A major portion of the electron beam's electrical power can be converted into heat. The tube current can be the same for the first and second voltages, so the electron beam's electrical energy can be higher at the second tube voltage compared to the first tube voltage. When electrons impact the anode at an impact angle approaching 90°, the higher power of the electron beam at the second tube voltage can exceed the anode's electrical power limit. However, because the impact angle 141 decreases when the second tube voltage is applied, the electron backscatter rate increases, resulting in a smaller proportion of the electron beam's electrical power being converted into heat. Therefore, the smaller electron impact angle at the second tube voltage can promote a higher electron beam power. This situation can be exploited to apply a high current when the second tube voltage is applied without exceeding the anode's electrical power limit.

[0071] Assuming the electron impact angles are the same for the first and second tube voltages, the conversion efficiency (i.e., the proportion of the electron beam's electrical power that is converted into X-ray radiation) can be higher at the second tube voltage compared to the first tube voltage. Furthermore, the proportion of X-ray radiation absorbed by an object (e.g., a patient's body) in a medical X-ray imaging system can be smaller at the second tube voltage compared to the first tube voltage. Consequently, when similar impact angles are configured for the first and second tube voltages, the X-ray intensity at an X-ray detector of the X-ray imaging system can be significantly higher at the second tube voltage compared to the first tube voltage.

[0072] However, according to the present disclosure, a smaller impact angle is configured for the second tube voltage compared to the first tube voltage. For smaller electron impact angles, the conversion efficiency is lower than for larger electron impact angles, so a smaller proportion of the electron beam's power is converted into X-ray radiation for smaller electron impact angles. Consequently, the smaller electron impact angle for the second tube voltage results in a reduction in X-ray intensity at the X-ray detector, thereby reducing the X-ray intensity imbalance between the first and second tube voltages.

[0073] Figure 2A second embodiment of an X-ray source 200 for generating X-ray radiation of a first energy spectrum and a second energy spectrum is schematically and exemplarily shown. The X-ray source comprises a cathode 201 and an anode 202. The cathode and anode are arranged inside a tube 220, which is preferably a vacuum tube. The anode preferably rotates about an axis 203, but the present invention is also applicable to stationary anodes.

[0074] Cathode insulator 222 and anode insulator 221 allow electrical contact to the cathode and anode, respectively, enabling a tube voltage to be applied between the cathode and anode by means of a power supply (not shown). The power supply can be configured to supply a first tube voltage and a second tube voltage, wherein the second tube voltage is higher than the first tube voltage. Due to the tube voltage, electrons emitted by the cathode are accelerated toward the anode. Upon impacting the anode, the electrons' kinetic energy can be partially or completely converted into X-ray radiation. By applying different tube voltages, X-ray radiation of different energy spectra can be generated. The power supply can also be configured to supply two or more tube voltages, which can be used to generate X-ray radiation of two or more energy spectra. X-rays 251 and 252 illustrate the emission of X-ray radiation through the aperture in tube 220.

[0075] Figure 2 Also shown is an electron-optical device 210. The electron-optical device is operatively connected to a control circuit 204, which is configured to control the electron-optical device. Figure 2, the control circuit is arranged outside the tube 220, but the control circuit can alternatively be arranged inside the tube. The electron optical device 210 includes a defocusing device 213 and a focusing device 214. The control circuit 204 can be configured to control the electron optical device 210 so that: when a first tube voltage is applied, the defocusing device 213 provides weak or no defocusing of the electron beam from the cathode to the anode; and when a second tube voltage is applied, the defocusing device 213 provides strong defocusing of the electron beam from the cathode to the anode. In addition, the control circuit can be configured to control the focusing device 214 so that: when the first tube voltage is applied, the electrons are focused on a first surface portion of the anode; and when the second tube voltage is applied, the electrons are focused on a second surface portion of the anode, wherein the first surface portion and the second surface portion of the anode are the same. Alternatively (not shown), the control circuit can be configured to control the focusing device 214 so that the first surface portion overlaps the second surface portion, or so that the first surface portion is located within the second surface portion, or so that the first surface portion and the second surface portion have coinciding centers, as outlined above. Thus, when a first tube voltage is applied between the cathode and the anode, the electron beam is confined by propagation paths 232 and 234, while when a second tube voltage is applied, the electron beam is confined by propagation paths 242 and 244. In other words, when the first tube voltage is applied, the electron optics form a narrow electron beam, and when the second tube voltage is applied, the electron optics form a wide electron beam. Figure 2 Also depicted are the minimum impact angles 233 and 235 when the first tube voltage is applied. When the central direction of the electron beam is perpendicular to the anode surface and when the electron beam is symmetrical with respect to this central direction, the minimum impact angles 233 and 235 are equal. Similarly, when the second tube voltage is applied, the minimum impact angles 243 and 245 are smaller than the minimum impact angles 233 and 235. That is, when the second tube voltage is applied, the impact angles of electrons impacting the anode are spread over a wider range of angles than when the first tube voltage is applied. Therefore, the average impact angle is smaller when the second tube voltage is applied than when the first tube voltage is applied.

[0076] Since the average impact angle decreases when the second tube voltage is applied, the electron backscatter rate increases and the conversion efficiency decreases. This will cause a reduction in the X-ray intensity imbalance for low and high tube voltages as explained above.

[0077] Figure 3A portion of an anode for an X-ray source is shown schematically and illustratively. The anode comprises a substrate 301, which is at least partially coated with a first coating 302 and a second coating 303. An electron beam 330 impacts the anode's second coating at an impact angle 331 equal to 90°. The second coating comprises a material for generating X-ray radiation, such as tungsten. Thus, the energy of the impacting electrons can be partially or completely converted into X-ray radiation. X-rays 353 and 354 illustrate a cone of X-rays that radiate from the second coating through an aperture in the tube of the X-ray source (not shown). An X-ray filter 304 is attached to the second coating 303 to filter the X-ray radiation generated in the second coating. A certain proportion of the impacting electrons may be backscattered, as illustrated by electron rays 360. Another proportion of the impacting electrons propagate through the second coating and the first coating, wherein the first coating preferably comprises a material transparent to electrons. Consequently, a certain proportion of the electrons impact the substrate, which comprises a material for generating X-ray radiation, such as rhenium. Thus, a certain proportion of the electrons impinge on the substrate, where their energy may be partially or completely converted into X-ray radiation. X-rays 355 and 356 illustrate cones of X-rays generated in the substrate, where the X-rays irradiate through an aperture in an X-ray tube (not shown).

[0078] Figure 4 Schematically and exemplarily shown also in Figure 3 . Again, the anode includes a substrate 401, which is at least partially coated with a first coating 402 and a second coating 403. An electron beam 440 impacts the second coating of the anode from an impact angle 441. The second coating comprises a material for generating X-ray radiation, such as tungsten. Thus, the energy of the impacting electrons can be partially or completely converted into X-ray radiation. X-rays 453 and 454 illustrate a cone of X-rays that radiate from the second coating through an aperture in an X-ray tube (not shown in the figure). An X-ray filter 404 is attached to the second coating 403 to filter the X-ray radiation generated in the second coating. A certain proportion of the impacting electrons may be backscattered, as illustrated by electron rays 460. Another proportion of the impacting electrons propagate through the second coating and through the first coating, wherein the first coating preferably comprises a material transparent to electrons. Thus, a certain proportion of the electrons impact the substrate, which comprises a material for generating X-ray radiation, such as rhenium. Thus, a certain proportion of the electrons impinge on the substrate, where their energy may be partially or completely converted into X-ray radiation. X-rays 455 and 456 illustrate cones of X-rays generated in the substrate, where the X-rays irradiate through an aperture in an X-ray tube (not shown).

[0079] exist Figure 4 In, with Figure 3 The electrons strike the anode at a smaller impact angle 441 compared to the impact angle 331 in the anode. Figure 3 In comparison, Figure 4 When a second (high) tube voltage is applied between the cathode and anode of the X-ray source, the electron backscatter rate can be configured to be greater. Figure 4 When a first (low) tube voltage is applied between the cathode and anode of the X-ray source, it is possible to configure Figure 3 Assuming that the tube current is constant, the power of the electron beam is greater at the second tube voltage than at the first tube voltage, and the smaller impact angle can promote the electron beam to have a larger power when the second tube voltage is applied without exceeding the power limit of the anode.

[0080] In addition, due to Figure 4 The impact angle in the second coating is smaller, so the propagation path through the second coating can be smaller than Figure 3 Therefore, Figure 3 In comparison, Figure 4 The proportion of electrons that propagate through the second coating can be smaller. Figure 3 In comparison, Figure 4 In this case, the proportion of electrons impinging on the substrate can be smaller. For this reason, Figure 3 In comparison, for Figure 4 , the proportion of X-ray radiation generated in the second coating layer relative to the sum of the X-ray radiation generated in the second coating layer and the substrate can be higher. In other words, Figure 4 In the embodiment of the present invention, most of the X-ray radiation can be generated in the second coating layer, and only a relatively small proportion of the X-ray radiation can be generated in the substrate. Figure 3 In the case of a second X-ray source, a very large proportion of the X-ray radiation may be generated in the substrate. Since the X-ray filter is arranged to filter the X-ray radiation generated in the second coating, the X-ray filter is more effective when the electron impact angle is small than when the electron impact angle is large. As previously indicated, when a second (high) tube voltage is applied between the cathode and anode of the X-ray source, it is possible to configure Figure 4 When a first (low) tube voltage is applied between the cathode and anode of the X-ray source, it is possible to configure Figure 3The X-ray filter can filter the X-ray radiation generated in the second coating layer more effectively when the second tube voltage is applied than when the first tube voltage is applied. The X-ray filter can include a material such as molybdenum, and filtering the X-ray radiation generated in the second coating layer can cause an increase in a measure of the 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 can increase the difference between the average photon energy of the first energy spectrum and the second energy spectrum. Providing X-ray radiation with more distinct first and second energy spectra can allow for the capture of more meaningful X-ray images in a dual-energy X-ray imaging system.

[0081] Assume again that Figure 4 In the scenario depicted, a second tube voltage is applied and Figure 3 If the first tube voltage is applied in a scenario where the first tube voltage is applied, the X-ray filter can filter a greater proportion of X-ray radiation when the second tube voltage is applied compared to when the first tube voltage is applied. This filtering of X-ray radiation results in a reduction in X-ray intensity. Therefore, the X-ray filter can reduce X-ray intensity by a greater percentage when the second tube voltage is applied compared to when the first tube voltage is applied. This can result in a reduction in X-ray intensity imbalance at the X-ray detector of the X-ray imaging system.

[0082] The second coating layer can have a thickness of less than 1 micron. The thickness of the second coating layer can generally be determined so that the difference between a first ratio of X-ray radiation and a second ratio of X-ray radiation is large or maximized, where the first ratio of X-ray radiation is the ratio of X-ray radiation generated in the second coating layer relative to the sum of X-ray radiation generated in the second coating layer and the substrate when the first tube voltage is applied. Similarly, the second ratio of X-ray radiation can be the ratio of X-ray radiation generated in the second coating layer relative to the sum of X-ray radiation generated in the second coating layer and the substrate when the second tube voltage is applied. The thickness of the second coating layer can be selected based on the composition of the second coating layer, the composition of the substrate, the first and second tube voltages, and the electron impact angle.

[0083] It should be noted that the embodiments of the present invention are described with reference to different subject matters. However, unless otherwise indicated, a person skilled in the art will infer that, in addition to any combination of features belonging to a type of subject matter, any combination of features related to different subject matters is also considered to be disclosed in this application. However, all features can be combined to provide synergistic effects that are more than the simple sum of the features.

[0084] Although the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description should be considered illustrative or exemplary rather than restrictive. The present invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments will be understood and effected by those skilled in the art in practicing the claimed invention by studying the drawings, the disclosure, and the claims.

[0085] In the claims, the word "comprising" does not exclude other elements or steps, and the word "a" or "an" does not exclude a plurality. A single processor or other unit may perform the functions of several items recited in a claim. The 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 construed as limiting the scope.

Claims

1. An X-ray source for generating X-ray radiation of a first energy spectrum and a second energy spectrum, the X-ray source comprising: a cathode (101) for emitting an electron beam; an anode (102) for converting the electron beam at least partially into X-ray radiation; an electron optical device configured to control an impact angle (131, 141) of electrons of the electron beam impacting the anode; a power supply configured to apply 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; as well as control circuitry (104) operatively coupled to the electron-optical device; wherein the control circuit is configured to control the electron optical device so that: when the first tube voltage is applied, the electrons of the electron beam impact the anode at a first average impact angle (131) for generating X-ray radiation (151) of the first energy spectrum; and when the second tube voltage is applied, the electrons of the electron beam impact the anode at a second average impact angle (141) for generating X-ray radiation (152) of the second energy spectrum; and Wherein, the second average impact angle is smaller than the first average impact angle.

2. The X-ray source according to claim 1, in, The second average impact angle (141) at which electrons of the electron beam impact the anode (102) is less than 70 degrees.

3. The X-ray source according to claim 1 , in, When the first tube voltage is applied, the electrons of the electron beam impinge on a first surface portion of the anode (102); and when the second tube voltage is applied, the electrons of the electron beam impinge on a second surface portion of the anode; and Wherein, the first surface portion overlaps with the second surface portion.

4. The X-ray source according to claim 3, in, The center of the first surface portion of the anode (102) coincides with the center of the second surface portion of the anode.

5. The X-ray source according to any one of claims 1, 2 and 4, in, The electron optical device comprises a first deflection device (111) and a second deflection device (112), wherein the first deflection device is switchable between a first state and a second state; wherein the control circuit (104) is configured to switch 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; and Wherein, when the first deflection device is in the second state, the first deflection device and the second deflection device provide a deflection of the electron beam.

6. The X-ray source according to any one of claims 1, 2 and 4, in, The electron-optical device comprises a focusing device (214) and a defocusing device (213), wherein the electron-optical device is switchable between a third state and a fourth state; wherein the control circuit (104) is configured to switch the electronic optical device 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; and Therein, in the fourth state, the defocusing device first provides a widening of the electron beam, and the focusing device subsequently provides a focusing of the widened electron beam.

7. The X-ray source according to claim 6, in, The defocusing device (213) provides a stronger broadening of the electron beam when the electron optics is in the fourth state than when the electron optics is in the third state.

8. The X-ray source according to any one of claims 1, 2, 4 and 7, in, The anode comprises a substrate (301) at least partially coated with a first coating and a second coating, the first coating (302) being arranged between the second coating (303) and the substrate; wherein the substrate comprises a first material to generate X-ray radiation; wherein the first coating comprises a second material, the second material being transparent to X-ray radiation and the electrons of the electron beam; wherein the second coating comprises a third material to generate X-ray radiation; and wherein an X-ray filter (304) is attached to the second coating layer to filter X-ray radiation generated in the second coating layer.

9. The X-ray source according to claim 8, in, The first material of the substrate is rhenium; wherein the second material of the first coating is diamond; wherein the third material of the second coating is tungsten; and Wherein, the X-ray filter comprises molybdenum.

10. The X-ray source according to claim 8, in, The second coating layer (303) has a thickness of less than 1 micrometer at a surface portion where the electrons of the electron beam impinge on the second coating layer.

11. An X-ray imaging system comprising an X-ray source according to any one of the preceding claims and an X-ray detector, wherein: The X-ray detector is arranged to receive X-ray radiation emitted by the X-ray source.

12. The X-ray imaging system according to claim 11, further comprising a detector control circuit, in, The detector control circuit controls the X-ray detector to perform the following operations: generating first image data when the first tube voltage is applied between the cathode (101) and the anode (102) of the X-ray source; and generating second image data when the second tube voltage is applied between the cathode and the anode of the X-ray source; and Wherein, the integration period for generating the first image data has the same duration as the 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, the X-ray source comprising: a cathode (101) for emitting an electron beam; an anode (102) for converting the electron beam at least partially into X-ray radiation; a power supply for applying a tube voltage between the cathode and the anode; an electron optical device configured to control an impact angle of electrons of the electron beam onto the anode; and control circuitry (104) operatively coupled to the electro-optical device; Generating the X-ray radiation of the first energy spectrum includes: A first tube voltage is applied by the power supply, and The electron optical device is controlled by the control circuit so that the electrons of the electron beam impact the anode at a first average impact angle (131) for generating X-ray radiation (151) of the first energy spectrum; and Generating the X-ray radiation of the second energy spectrum includes: A second tube voltage is applied by the power supply, wherein the second tube voltage is higher than the first tube voltage, and The electron optical device is controlled by the control circuit so that the electrons of the electron beam impact the anode at a second average impact angle (141) to generate X-ray radiation (152) of the second energy spectrum, wherein the second average impact angle is smaller than the first average impact angle.

14. A computer program product comprising instructions for causing the X-ray source according to any one of claims 1 to 10 to execute the steps of the method according to claim 13.

15. A computer-readable medium having stored thereon the computer program product according to claim 14.

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