EP3800656A1 - Scattered electron capturing for rotating anode x-ray tubes - Google Patents

Abstract

An anode (100) for a rotating anode X-ray tube is proposed, wherein the anode comprises an inner and/or an outer electron capturing element (120, 110). The inner and/or outer electron capturing elements are configured to capture electrons backscattered from the focal track of the anode. The outer electron capturing element (110) comprises an X-ray transparent member (114) such that X-rays can propagate in a radial direction through the X-ray transparent member. The inner electron capturing element (120) is closer to the axis of rotation (103) of the anode than the focal track, whereas the outer electron capturing element (110) has a larger distance to the axis of rotation (103) than the focal track.

Description

FIELD OF THE INVENTION
• The invention relates to an anode for a rotating anode X-ray tube, a rotating anode X-ray tube, and an X-ray imaging system. The anode comprises an inner and/or an outer electron capturing element for capturing electrons backscattered from the focal track of the anode.
BACKGROUND OF THE INVENTION
• X-ray imaging systems are utilized in a number of applications such as medical diagnostics, airport security, material analysis and others. For example, in medical applications, an X-ray tube and an X-ray detector are arranged on opposite sides of a patient. The X-ray tube may generate a fan beam of X-rays. The photons of the X-ray beam are partially absorbed by the patient's body. Thereby, bones absorb more photons per volume as compared to soft tissue. The photons passing 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 computed tomography (CT) system, the X-ray source and the X-ray detector rotate about the patient to capture images from different viewing angles. These images can be processed by a computer system to reconstruct a three-dimensional image of the patient's anatomy.
• A rotating anode X-ray tube comprises a cathode and an anode, which are arranged in vacuum inside a tube envelope. The anode of a rotating anode X-ray tube rotates relative to the tube envelope. The cathode emits electrons, which are accelerated towards the anode due to a tube voltage supplied by a power supply. When electrons impinge onto the anode, which is preferably made from tungsten, rhenium, or molybdenum, their kinetic energy may be converted fully or partially to X-ray radiation. A large portion of the electrons that impinge onto the anode is backscattered from the anode.
SUMMARY OF THE INVENTION
• It has been observed that electrons, which were backscattered from the focal track of the anode, heat the tube envelope or even accumulate on the tube envelope in case it is made from insulating material, like glass or alumina. Cooling means may be necessary to avoid overheating. The bombardment of an insulating tube envelope with electrons affects its electrical potential, i.e., the electrical potential on the tube envelope may change over time. Measures for electrical isolation therefore must take into account large ranges of possible electrical potentials on the tube envelope. For this reason, large safety margins are necessary to ensure insulation under all possible charging situations. In particular, the tube envelope needs to be large. The tube envelope maybe arranged inside a housing, so also the housing of the X-ray tube needs to be large and bulky.
• Furthermore, the electrical potential on the tube envelope affects the electron beam, so that the focal track on the anode may change due to variations of this electrical potential. Especially in high-resolution CT systems, the spatial resolution of the generated image depends on the stability of the focal track on the anode. Hence, a shift of the focal track on the anode due to changes of the electrical potential on the tube envelop may cause a degradation of the spatial resolution of the CT image. For a high spatial resolution, the electrical potential on the tube envelope should be kept at a fixed value.
• It has also been observed that micro-particles, which emerge from the focal track of the anode during operation of the X-ray tube, accumulate on the inner side of the tube envelope. The tube envelope may be made of an insulating material such as glass, and an accumulation of electric current conducting micro-particles on the inner side of the tube envelope may degrade the electrical isolation properties of the envelope. In particular, the accumulation of micro-particles on the tube envelope may cause a high-voltage discharge, which is a major cause for the failure of X-ray tubes. Consequently, the accumulation of micro-particles on the tube envelope may result in a reduced lifetime of the X-ray tube.
• Hence, it may be desirable to provide an improved X-ray source with a longer lifetime and/or a more stable electrical potential on the tube envelope.
• This is achieved by the subject matter of the independent claims, wherein further embodiments are incorporated in the dependent claims and the following description. It should be noted that any feature, element, and/or function of the anode for a rotating anode X-ray tube, as described in the following, equally applies to the rotating anode X-ray tube, and the X-ray imaging system, as described in the following, and vice versa.
• According to the present invention, an anode for a rotating anode X-ray tube is presented. The anode comprises an anode body, an axis of rotation, and a focal track. The anode further comprises an inner electron capturing element and/or an outer electron capturing element, the inner and outer electron capturing elements being configured for capturing electrons backscattered from the focal track of the anode, wherein the inner electron capturing element is arranged closer to the axis of rotation than the focal track of the anode, and/or wherein the outer electron capturing element is arranged further away from the axis of rotation than the focal track of the anode, and wherein the outer electron capturing element comprises an X-ray transparent member.
• As defined above, a rotating anode X-ray tube is an X-ray tube, wherein the anode rotates relative to the tube envelope. The tube envelope may be arranged inside a housing. The housing may comprise a window for the emission of the X-ray radiation. Apart from this window, the housing may be configured to absorb X-ray radiation. Hence, leakage radiation, which is not directed towards the housing's window, may be absorbed by the housing.
• A cathode and the rotating anode may be arranged inside the tube envelope. A power supply may be configured to supply a tube voltage between the cathode and the anode. Due to the tube voltage, electrons emitted by the cathode may be accelerated towards the anode. The electrons may impinge onto a focal spot of the anode. The rotation of the anode causes a movement of the focal spot over the surface of the anode. The anode may be rotationally symmetric relative to its axis of rotation, and the locations where electrons impinge onto the anode may form a focal track, which is also rotationally symmetric relative to the axis of rotation.
• The anode body may be made of, for example, carbon. A portion of the surface of the anode, which includes the focal track, may be coated with an X-ray conversion layer. The X-ray conversion layer may be configured for converting kinetic energy of impinging electrons into X-ray radiation. The X-ray conversion layer may comprise, for example, tungsten, rhenium, and/or molybdenum
• Fractions of the electrons that impinge onto the focal track of the anode are backscattered. For example, in case of a focal track made of tungsten, more than 48% of the electrons may be backscattered, the backscattered electrons having energies up to the energy of the primary electrons. To prevent that backscattered electrons propagate to the tube envelope where they may change the envelope's electrical potential, the anode comprises an inner electron capturing element and/or an outer electron capturing element. The radial distance of the inner electron capturing element from the axis of rotation of the anode is smaller than the radial distance of the focal track from the axis of rotation. In contrast, the radial distance of the outer electron capturing element from the axis of rotation of the anode is larger than the radial distance of the focal track from the axis of rotation. Here and in the following, 'radial' means radial relative to the axis of rotation of the anode, i.e., in particular, orthogonal to this axis. Furthermore, in the following, 'axial' means axial relative to the axis of rotation of the anode, i.e., parallel to this axis.
• The inner and outer electron capturing elements may each have an annular shape that protrudes from the surface of the anode such that the focal track may lie between the inner and outer electron capturing elements. In other words, the inner and outer electron capturing elements may each be a ring, and the focal track may lie between these inner and outer electron capturing rings.
• The outer electron capturing element may be arranged between the focal spot and the window in the housing of the X-ray tube. For this reason, at least a member of the outer electron capturing element may be configured to be highly transparent for X-ray radiation so that X-ray radiation can propagate from the focal spot through the X-ray transparent member towards the window in the housing. The X-ray transparent member may have an annular shape and may be rotationally symmetric relative to the axis of rotation of the anode. Moreover, the X-ray transparent member is preferably configured such that it attenuates the intensity of X-rays traversing the X-ray transparent member in a radial direction by less than 40%, 20%, 10%, 5%, or less than 2%. Thereby, the radial thickness of the X-ray transparent member maybe more than 50%, 70%, or 90% of the corresponding radial thickness of the outer electron capturing element. The radial thickness of the X-ray transparent member maybe equal to the corresponding radial thickness of the outer electron capturing element.
• Generally, the attenuation of an electromagnetic wave when propagating through a medium depends on the thickness of the medium and on the electromagnetic attenuation coefficient. The X-ray transparent member of the outer electron capturing element may comprise or may be made of a highly X-ray transparent material. The X-ray transparent material and its radial thickness may be configured such that the outer electron capturing element withstands the operational centrifugal forces in a rotating anode X-ray tube. Furthermore, the X-ray transparent material and its thickness in the radial direction are configured such that X-rays traversing the X-ray transparent member in the radial direction are preferably attenuated by less than 40%, 20%, 10%, 5%, or less than 2%.
• The attenuation coefficient depends on the frequency of the electromagnetic wave. For a specific frequency, the attenuation coefficient may generally be defined as the fraction of the intensity of X-ray radiation removed from a plane monoenergetic wave when propagating through a unit distance of a material. The attenuation coefficient may account for absorption and scattering, including coherent scattering. For a range of X-ray frequencies, the X-ray transparent material of the X-ray transparent member may have an electromagnetic attenuation coefficient that is less than 2/cm, 1/cm, 0.5/cm, 0.2/cm, or less than 0.1/cm. Thereby, the range of X-ray frequencies may correspond to photon energies of, for example, 10 keV to 200 keV. Wider or narrower photon energy ranges including higher or lower photon energies are equally possible.
• In an example, the X-ray transparent member of the outer electron capturing element comprises an X-ray transparent material, wherein the X-ray transparent material is an element with atomic number smaller than 15 or a compound with atomic numbers smaller than 15.
• Materials with a small atomic number typically have a small attenuation coefficient for X-rays corresponding to a high transparency. Hence, an element with an atomic number smaller than 15 or a compound with atomic numbers smaller than 15 can be considered as an X-ray transparent material. For example, the X-ray transparent material of the X-ray transparent member of the outer electron capturing element may be carbon, beryllium, lithium, sodium, magnesium, aluminium, or silicon.
• For example, when the X-ray transparent material of the X-ray transparent member is carbon, the radial thickness of the X-ray transparent member may be less than 10 mm to prevent significant attenuation of the intensity of X-ray radiation.
• Alternatively, when the X-ray transparent material of the X-ray transparent member is beryllium, the radial thickness of the X-ray transparent member may be smaller than 5 mm to prevent significant attenuation of the intensity of X-ray radiation.
• In another example, the X-ray transparent member of the outer electron capturing element comprises an X-ray transparent material, and the X-ray transparent material is an electric conductor.
• The outer electron capturing element is configured to capture electrons backscattered from the focal track of the anode. The outer electron capturing element may further be configured such that captured electrons can be discharged via the anode body. In particular, the outer electron capturing element may be configured such that captured electrons can flow via the anode body to the positive terminal of the power supply. Towards this end, the X-ray transparent material of the X-ray transparent member may be an electric conductor. The electric conductivity of the X-ray transparent material of the X-ray transparent member may be higher than the electric conductivity of amorphous carbon and/or amorphous silicon. In particular, for a reference temperature of 20°C, the electric conductivity of the X-ray transparent material of the X-ray transparent member may be larger than 0.5 kS/m, 1 kS/m, 2 kS/m, 5 kS/m, 10 kS/m, 20 kS/m, 50 kS/m, or larger than 100 kS/m, where kS/m stands for kilo-Siemens per meter.
• In another example, the inner and outer electron capturing elements have an annular shape protruding from a surface of the anode.
• The inner and/or outer electron capturing elements may be rotationally symmetric relative to the axis of rotation of the anode. The inner and/or outer electron capturing elements may protrude from the surface of the anode in a direction towards the cathode of the X-ray tube. The focal track of the anode may be arranged between the inner and outer electron capturing elements.
• The inner and/or outer electron capturing elements may each have an inner and an outer surface section, wherein the inner surface section of the inner electron capturing element faces the axis of rotation of the anode, whereas the outer surface section of the inner electron capturing element faces the focal track of the anode. The inner surface section of the outer electron capturing element also faces the focal track of the anode, whereas the outer surface section of the outer electron capturing element is directed outwards away from the axis of rotation. The inner and/or outer electron capturing elements may protrude from the surface of the anode in the axial direction, i.e., parallel to the axis of rotation of the anode. Thereby, the inner and/or outer surface sections of the inner and/or outer electron capturing elements may have constant radial distances to the axis of rotation.
• However, the inner and/or outer surface sections of the inner and/or outer electron capturing elements may alternatively be angled relative to the axis of rotation, i.e., a normal vector on the inner or outer surface sections may not be orthogonal to the axis of rotation. For example, the path from the focal spot to the center of the window in the housing of the X-ray tube may not be orthogonal to the axis of rotation of the anode. The outer electron capturing element may be angled such that the path from the focal spot to the center of the window in the housing of the X-ray tube orthogonally crosses the outer electron capturing element. Thereby, the range of thicknesses of the outer electron capturing element for different directions of the emitted X-ray beam may be narrowed down.
• Alternatively, the inner and/or outer electron capturing elements may have a trapezoidal shape. In particular, the radial thickness of the inner and/or outer electron capturing element may reduce with increasing distances to the anode body.
• The outer electron capturing element may be arranged on the outer edge of the anode such that its distance to the axis of rotation is maximum. Hence, the outer electron capturing element may form an outer collar of the anode.
• In another example, a first portion of the outer electron capturing element comprises an X-ray radiation absorbing material, thereby providing a first limitation of an X-ray aperture in a direction parallel to the axis of rotation of the anode.
• The first portion of the outer electron capturing element may have an annular shape, and/or the first portion of the outer electron capturing element may be rotationally symmetric relative to the axis of rotation of the anode. Hence, the first portion of the outer electron capturing element may be an axial portion of the outer electron capturing element, i.e., a portion in the direction parallel to the axis of rotation of the anode. Furthermore, the first portion of the outer electron capturing element may be a most distal portion of the outer electron capturing element relative to the anode body.
• The first portion of the outer electron capturing element may comprise a first part of the X-ray transparent member of the outer electron capturing element. At least a section of the surface of the first part of the X-ray transparent member may be coated with the X-ray radiation absorbing material. For example, the inner surface section of the first part of the X-ray transparent member, which faces the focal track of the anode, may be coated with the X-ray radiation absorbing material. Alternatively, the outer surface section of the first part of the X-ray transparent member, which faces outwards away from the axis of rotation, maybe coated with the X-ray radiation absorbing material. The thickness of the coating with the X-ray radiation absorbing material may be 10 µm, 20 µm, 40 µm, 60 µm, or larger. Alternatively, the first portion of the outer electron capturing element may be a homogeneous block comprising the X-ray radiation absorbing material.
• The X-ray radiation absorbing material may be an element with a high atomic number, or the X-ray radiation absorbing material may be a compound with high atomic numbers. Elements of the fourth or higher periods of the periodic table of elements may be used as X-ray radiation absorbing material. For example, the X-ray radiation absorbing material may be tungsten, molybdenum, lead, or tantalum.
• The first portion of the outer electron capturing element may be configured to provide a first limitation of the generated X-ray beam in the axial direction parallel to the axis of rotation of the anode. Thereby, the first portion of the outer electron capturing element may reduce leakage radiation. In particular, the first portion of the outer electron capturing element may be configured to attenuate X-ray radiation, which is not directed towards the window in the housing of the X-ray tube.
• In another example, a second portion of the outer electron capturing element comprises a heel effect compensation filter.
• The second portion of the outer electron capturing element may have an annular shape, and/or the second portion of the outer electron capturing element may be rotationally symmetric relative to the axis of rotation of the anode. Hence, the second portion of the outer electron capturing element may form another axial portion of the outer electron capturing element. The second portion of the outer electron capturing element may be adjacent to the first portion of the outer electron capturing element. Furthermore, the second portion of the outer electron capturing element may be more proximal to the anode body than the first portion of the outer electron capturing element.
• The second portion of the outer electron capturing element may comprise a second part of the X-ray transparent member, which may comprise the X-ray transparent material. A section of the surface of the second part of the X-ray transparent member may be coated with a heel effect compensation filter. For example, the inner surface section of the second part of the X-ray transparent member, which faces the focal track of the anode, may be coated with a heel effect compensation filter. Additionally or alternatively, the outer surface section of the second part of the X-ray transparent member, which faces outwards away from the axis of the anode, may be coated with a heel effect compensation filter.
• The heel effect compensation filter may comprise the same material as the X-ray conversion layer of the anode. In other words, the heel effect compensation filter may comprise the target material on the focal track of the anode. More generally, the heel effect compensation filter may comprise a material with similar spectral properties as compared to the target material on the focal track of the anode. In particular, the heel effect compensation filter may comprise a material with a similar atomic number as compared to the target material on the focal track of the anode. Using the same X-ray absorbing material as the target material is preferred, because they share the same k-edge. Distortion of the X-ray spectrum across the X-ray fan can therefore be minimized.
• The radial thickness of the heel effect compensation filter may vary in the axial direction, i.e., in the direction parallel to the axis of rotation. At a location distal to the anode body, the heel effect compensation filter may have a larger radial thickness as compared to a location proximal to the anode body. In other words, in a rotating anode X-ray tube, the heel effect compensation filter may have a larger radial thickness at a location that is close to the cathode of the X-ray tube as compared to a location that is further away from the cathode of the X-ray tube.
• The heel effect compensation filter may be configured to provide a maximum attenuation of the X-ray radiation emitted from the focal track of the anode of about 20% at a location of maximum radial thickness. Larger or smaller maximum attenuation factors are possible.
• In another example, a third portion of the outer electron capturing element comprises an X-ray radiation absorbing material, thereby providing a second limitation of the X-ray aperture in the direction parallel to the axis of rotation of the anode.
• The third portion of the outer electron capturing element may have an annular shape, and/or the third portion of the outer electron capturing element may be rotationally symmetric relative to the axis of rotation of the anode. Hence, the third portion of the outer electron capturing element may form another axial portion of the outer electron capturing element. The third portion of the outer electron capturing element may be adjacent to the second portion of the outer electron capturing element. The third portion of the outer electron capturing element may be more proximal to the anode body than the second portion of the outer electron capturing element.
• The third portion of the outer electron capturing element may comprise a third part of the X-ray transparent member of the outer electron capturing element. At least a section of the surface of the third part of the X-ray transparent member may be coated with the X-ray radiation absorbing material. For example, the inner surface section of the third part of the X-ray transparent member, which faces the focal track of the anode, may be coated with the X-ray radiation absorbing material. Alternatively, the outer surface section of the third part of the X-ray transparent member, which faces outwards away from the axis of rotation, may be coated with the X-ray radiation absorbing material. The thickness of the coating with the X-ray radiation absorbing material may be larger than 10 µm, 20 µm, 40 µm, 60 µm, or larger. Alternatively, the third portion of the outer electron capturing element may be a homogeneous block comprising the X-ray radiation absorbing material.
• The X-ray radiation absorbing material may be an element with a high atomic number, or the X-ray radiation absorbing material may be a compound with high atomic numbers. Elements of the fourth or higher periods of the periodic table of elements may be used as X-ray radiation absorbing material. For example, the X-ray radiation absorbing material may be tungsten, molybdenum, lead, or tantalum.
• The third portion of the outer electron capturing element may be configured to provide a second limitation of the generated X-ray beam in the axial direction, i.e., in the direction parallel to the axis of rotation of the anode. Thereby, the third portion of the outer electron capturing element may reduce leakage radiation. In particular, the third portion of the outer electron capturing element may be configured to attenuate X-ray radiation, which is not directed towards the window in the housing of the X-ray tube.
• In another example, the first and/or third portions of the outer electron capturing element comprise a particle trap for capturing micro-particles expelled from the focal track of the anode.
• During operation of the anode inside a rotating anode X-ray tube, the X-ray conversion layer of the anode may erode. Micro-particles having dimensions of about 0.01 to 200 µm may be expelled from the X-ray conversion layer and in particular its focal track area. In prior art rotating anode X-ray tubes, the micro-particles may propagate in a radial direction towards the tube envelope. The tube envelope may be made of an insulator such as glass. An accumulation of electric current conducting micro-particles on the tube envelope may degrade the electrical isolation properties of the envelope. In particular, the micro-particles may cause a high-voltage discharge, which may result in a failure of the X-ray tube. This holds also for metallic tube envelopes, as, driven by electric forces, microparticles may bounce between electrodes and cause discharges.
• The first and/or third portions of the outer electron capturing element may be configured to capture micro-particles that emerged from the focal track of the anode during operation of the X-ray tube. Thereto, the first and/or third portions of the outer electron capturing element may comprise a particle trap, which preferably faces the focal track of the anode. By preventing the accumulation of micro-particles on the tube envelope, the particle trap may contribute to an increased lifetime of the X-ray tube.
• In another example, the particle trap comprises a ductile material such as tantalum or a liquid metal coated on a rigid substrate.
• Herein, a ductile material may be a material, which shows at least 5 % elongation at room temperature in a tensile test. However, higher values of elongation before failure maybe preferred, for example, 10 %, 20%, 30%, or more.
• The first and/or third portions of the outer electron capturing element may comprise first and/or third parts of the X-ray transparent member, respectively, wherein the first and/or third parts of the X-ray transparent member may comprise an X-ray transparent material. At least sections of the surface of the first and/or third parts of the X-ray transparent member may be coated with the ductile material or the liquid metal. In particular, the inner surface sections of the first and/or third parts of the X-ray transparent member, which face the focal track of the anode, may be coated with the ductile material or the liquid metal. The liquid metal may be kept in place by centrifugal forces and/or adhesion to the first and/or third parts of the X-ray transparent member.
• More generally, the first and/or third portions of the outer electron capturing element may comprise rigid substrates, which may comprise an X-ray radiation transparent or an X-ray radiation absorbing material. For example, the rigid substrates may be homogeneous blocks of an X-ray radiation absorbing material. Sections of the surfaces of the rigid substrates may be coated with the ductile material or the liquid metal. In particular, the inner surface sections of the rigid substrates, which face the focal track of the anode, may be coated with the ductile material or the liquid metal. The liquid metal may be kept in place by centrifugal forces and/or adhesion to the rigid substrates of the first and/or third portions of the outer electron capturing element.
• The ductile material and/or the liquid metal maybe X-ray radiation absorbing materials. Hence, the ductile material and/or the liquid metal may be identical to the X-ray radiation absorbing material. Alternatively, the first and/or third portions of the outer electron capturing element may comprise an X-ray radiation absorbing material in addition to a ductile material or a liquid metal. For example, sections of the surface of the rigid substrates of the first and/or third portions of the outer electron capturing element may be coated with the X-ray radiation absorbing material, the ductile material, and/or the liquid metal. For example, the inner surface sections of the rigid substrates of the first and/or third portions of the outer electron capturing element, which face the focal track of the anode, may be coated with the X-ray radiation absorbing material. The X-ray radiation absorbing layer may be coated with the ductile material or the liquid metal. Alternatively, different surface sections of the rigid substrates of the first and/or third portions of the outer electron capturing element may be coated with different materials. For example, the outer surface sections of the rigid substrates, which face outwards away from the axis of rotation, may be coated with the X-ray radiation absorbing material, whereas the inner surface sections of the rigid substrates, which face the focal track of the anode, may be coated with the ductile material or the liquid metal.
• Alternatively, the first and/or third portions of the outer electron capturing element may be homogeneous blocks comprising the ductile material.
• In another example, the particle trap comprises fins, a foam-like structure, and/or angulated microstructures.
• The fins, the foam-like structure, and/or the angulated microstructures maybe configured such that microparticles expelled from the focal track of the anode are captured by the particle trap with a high probability. In particular, the fins, the foam-like structure, and/or the angulated microstructures may be arranged facing the focal track of the anode. Furthermore, the fins, the foam-like structure, and/or the angulated microstructures may be configured such that the particle trap has a high capacity for capturing microparticles.
• In another example, the inner electron capturing element comprises a first portion for capturing electrons backscattered from the focal track of the anode, the first portion of the inner electron capturing element facing the focal track of the anode and comprising a material with a first atomic number, and the inner electron capturing element further comprises a second portion for absorbing X-ray radiation, the second portion of the inner electron capturing element comprising a material with a second atomic number larger than the first atomic number.
• The first and second portions of the inner electron capturing element may each have an annular shape, which may be rotationally symmetric relative to the axis of rotation of the anode. The inner (minimum) radius of the first portion of the inner electron capturing element may be identical with the outer (maximum) radius of the second portion of the inner electron capturing element. Hence, the first and second portions of the inner electron capturing element may form radial portions of the inner electron capturing element.
• The first portion of the inner electron capturing element, which is proximal to the focal track, may comprise a material with a low X-ray conversion rate. Hence, the first portion of the inner electron capturing element may comprise a material with low density and/or small atomic number. In particular, the first portion of the inner electron capturing element may comprise an X-ray transparent material such as an element with atomic number smaller than 15 or a compound with atomic numbers smaller than 15. For example, the first portion of the inner electron capturing element may comprise carbon, beryllium, and/or lithium. The radial thickness of the first portion of the inner electron capturing element may be configured such that the majority of electrons are being captured. For example, for a first portion of the inner electron capturing element made of carbon and for tube voltages up to 150 kV, the radial thickness of the first portion of inner electron capturing element may be larger than 20 µm, preferably larger than 50 µm.
• The second portion of the inner electron capturing element may comprise a material of high atomic number and/or high density to absorb X-ray radiation. In particular, the second portion of the inner electron capturing element may comprise an element of the fourth or higher periods of the periodic table of elements. For example, the second portion of the inner electron capturing element may comprise tungsten or molybdenum. The second portion of the inner electron capturing element may have a radial thickness of several millimeters, potentially more than 1 cm. The second portion of the inner electron capturing element may be an integral part of the anode.
• According to the present invention, also a rotating anode X-ray tube is presented. The rotating anode X-ray tube comprises a cathode for emitting an electron beam, an anode for converting the electron beam at least partly into X-ray radiation, the anode being configured as described above, and a tube envelope housing the cathode and the anode. Hence, the anode is configured to rotate relative to the tube envelope. The inner and/or outer electron capturing elements may protrude from the surface of the anode in the direction of the cathode. Thus, distal portions of the inner and/or outer electron capturing elements relative to the anode body are closer to the cathode than proximal portions of the inner and/or outer electron capturing elements.
• The inner and/or outer electron capturing elements may be configured such that their distances to the cathode remain larger than a threshold distance to prevent arcing. This threshold distance may depend on the operational tube voltages of the X-ray tube. More specifically, for large operational tube voltages, the threshold distance between the cathode and the inner and/or outer electron capturing elements should be large.
• In an example, the tube envelope is at least partly made of carbon, copper, glass, titanium, steel, aluminum, tungsten, molybdenum, or beryllium.
• The inner and/or outer electron capturing elements of the anode are configured to prevent that backscattered electrons propagate from the focal track of the anode towards the tube envelope. When the tube envelope is made of an insulator such as glass, or when a current conducting tube envelope is not in electric contact with a constant electric potential, the accumulation of electrons on the envelope may change its electric potential. As a result, a discharge may occur, which may cause the failure of the X-ray tube. In addition, the change of the electric potential on the tube envelope may result in a deflection of the focal spot, which may degrade the spatial resolution in particular for CT imaging systems, where imaging periods can be long.
• According to the present invention, also an X-ray imaging system is presented. The X-ray imaging system comprises a rotating anode X-ray tube as specified above and an X-ray detector.
• The X-ray tube and the X-ray detector may be arranged on opposite sides of an object. The X-ray tube may be configured to emit an X-ray beam in the direction of the object. The X-ray beam may partially be attenuated by the object. The X-ray detector may be configured to generate an image representing the intensity of the X-ray radiation after propagation through the object. The X-ray imaging system may further comprise a control unit for synchronizing the operation of the X-ray tube and the X-ray detector, and/or for controlling imaging parameters such as tube voltage, tube current, integration period, etc.
• It shall be understood that the anode for a rotating anode X-ray tube, the rotating anode X-ray tube, and the X-ray imaging system as defined in the claims have similar and/or identical preferred embodiments, in particular, as defined in the dependent claims. It shall be understood further that a preferred embodiment of the invention can also be any combination of the dependent claims with the respective independent claim.
• These and other aspects of the present invention will become apparent from and be elucidated with reference to the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
• Exemplary embodiments of the invention will be described in the following with reference to the accompanying drawings:
• Fig. 1 shows schematically and exemplarily a first embodiment of an anode for a rotating anode X-ray tube.
• Fig. 2 shows schematically and exemplarily a second embodiment of an anode for a rotating anode X-ray tube.
• Fig. 3 shows schematically and exemplarily a third embodiment of an anode for a rotating anode X-ray tube.
DETAILED DESCRIPTION OF EMBODIMENTS
• Fig. 1 shows schematically and exemplarily a first embodiment of an anode for a rotating anode X-ray tube. Theanode 100 comprises ananode body 101 and anX-ray conversion layer 102. The X-ray conversion layer may comprise a material such as tungsten and/or rhenium. Aprimary electron beam 104 may propagate from the cathode of the X-ray tube towards the anode and impinge onto the focal track on theX-ray conversion layer 102. When impinging onto the X-ray conversion layer, the kinetic energy of the incident electrons may be converted partly or fully into X-ray radiation. At the same time, a fraction of the electrons may be scattered back, which is represented inFig. 1 by backscatteredelectron propagation paths 131 and 132.Fig. 1 also depicts the axis ofrotation 103 of theanode 100. Theanode 100 may be rotationally symmetric relative to the axis of rotation.
• The anode further comprises an outerelectron capturing element 110. The outerelectron capturing element 110 comprises amember 114, which is configured to be transparent for X-ray radiation. Hence, the X-raytransparent member 114 may comprise or may be made of an X-ray transparent material. The outer electron capturing element further comprises afirst portion 115, asecond portion 116, and athird portion 117.
• Thefirst portion 115 of the outer electron capturing element comprises the most distal part of the X-raytransparent member 114. The inner surface section of this first part of the X-raytransparent member 114 has acoating 111 with an X-ray radiation absorbing material. The thickness of the coating with the X-ray radiation absorbing material may be 10 µm, 20 µm, 40 µm, 60 µm, or larger. The X-ray radiation absorbing material may be an element or a compound with large atomic numbers such as tungsten, molybdenum, lead, or tantalum. Thecoating 111 is configured to provide a first limitation of the X-ray aperture in the axial direction.
• Thesecond portion 116 of the outer electron capturing element comprises a second part of the X-raytransparent member 114 and a heeleffect compensation filter 112. The heel effect compensation filter may comprise a material with similar spectral characteristics as the target material of theX-ray conversion layer 102. The radial thickness of the heel effect compensation filter may be larger at a location distal to the anode body as compared to a location proximal to the anode body. At a location of maximum radial thickness, the heel effect compensation filter may be configured to attenuate X-ray radiation by about 20%. Larger or smaller maximum attenuation factors are possible.
• Similar to thefirst portion 115 of the outer electron capturing element, itsthird portion 116 comprises acoating 113 with an X-ray radiation absorbing material and a third part of the X-raytransparent member 114. Thecoating 113 comprises an X-ray radiation absorbing material, which is configured to provide a second limitation of the X-ray aperture in the axial direction. Hence, the emitted X-ray beam is limited by thecoating 111 from one side and by thecoating 113 from another side.
• The anode further comprises an innerelectron capturing element 120. The focal track on theX-ray conversion layer 102 is arranged between the inner and outerelectron capturing elements 120 and 110, respectively. The backscatteredelectron propagation paths 131 and 132 illustrate the capturing of backscattered electrons by the inner and outer electron capturing elements. Consequently, the inner and outer electron capturing rings may prevent that backscattered electrons propagate to the tube envelope and change its electric potential.
• Fig. 2 shows schematically and exemplarily a second embodiment of an anode for a rotating anode X-ray tube. Similar to theanode 100, theanode 200 comprises ananode body 201 and anX-ray conversion layer 202. Aprimary electron beam 204 may propagate from the cathode of the X-ray tube towards the anode and impinge onto the focal track on theX-ray conversion layer 202. When impinging onto the X-ray conversion layer, the kinetic energy of the incident electrons may be converted partly or fully into X-ray radiation. A fraction of the electrons maybe scattered back, which is represented inFig. 2 by backscatteredelectron propagation paths 231 and 232.Fig. 2 also illustrates the axis ofrotation 203 of theanode 200. Theanode 200 may be rotationally symmetric relative to the axis of rotation.
• The anode further comprises an outerelectron capturing element 210. The outerelectron capturing element 210 comprises an X-raytransparent member 214, afirst coating 211 for absorbing X-ray radiation, a heeleffect compensation filter 212, and asecond coating 213 for absorbing X-ray radiation.
• Fig. 2 further illustrates the emittedX-ray beam 205. The emittedX-ray beam 205 is limited in the axial direction by the X-rayradiation absorbing coating 211 from one side and by the X-rayradiation absorbing coating 213 from another side. Hence, thecoatings 211 and 213 limit the aperture for the emitted X-ray beam in the axial direction. The emitted X-ray beam propagates through the heeleffect compensation filter 212 and the X-raytransparent member 214. Moreover, not shown in the figure, the emitted X-ray beam may propagate through the tube envelope and through a window in the housing of the X-ray tube.
• InFig. 2, the innerelectron capturing element 220 comprises afirst portion 221 and asecond portion 222. The innerelectron capturing element 220 may have an annular shape and may be rotationally symmetric relative to the axis of rotation of the anode.
• The first portion of the inner electron capturing element may comprise an X-ray transparent material, so that X-ray radiation may propagate through the X-ray transparent material. The X-ray transparent material may be a material with a low density and/or small atomic numbers. In particular, the X-ray transparent material may be an element with atomic number smaller than 15 or a compound with atomic numbers smaller than 15. For example, the first portion of the inner electron capturing element may comprise carbon, beryllium, and/or lithium. The radial thickness of the first portion of the inner electron capturing element may be configured such that the majority of electrons are being captured. For example, for a first portion of the inner electron capturing element made of carbon and for tube voltages up to 150 kV, the radial thickness of the first portion of inner electron capturing element may be larger than 20 µm, preferably larger than 50 µm.
• The second portion of the inner electron capturing element may comprise an X-ray radiation absorbing material to absorb leakage radiation. The X-ray radiation absorbing material may be an element of the fourth or higher periods of the periodic table of elements. For example, the second portion of the inner electron capturing element may comprise tungsten or molybdenum. The second portion of the inner electron capturing element may have a radial thickness of several millimeters, potentially more than 1 cm.
• Fig. 3 shows schematically and exemplarily a third embodiment of an anode for a rotating anode X-ray tube. Similar to theanodes 100 and 200, theanode 300 comprises ananode body 301 and anX-ray conversion layer 302. Aprimary electron beam 304 may propagate from the cathode of the X-ray tube towards the anode and impinge onto the focal track on theX-ray conversion layer 302. When impinging onto the X-ray conversion layer, the kinetic energy of the incident electrons may be converted partly or fully into X-ray radiation. Thereby, a fraction of the electrons may be scattered back.Fig. 2 also illustrates the axis ofrotation 303 of theanode 300. Theanode 300 may be rotationally symmetric relative to the axis of rotation.
• The anode further comprises an outerelectron capturing element 310. The outerelectron capturing element 310 comprises afirst portion 315, asecond portion 316, and athird portion 317, wherein the first, second, and third portions of the outer electron capturing element comprise first, second, and third parts of the X-raytransparent member 314, respectively.
• Thefirst portion 315 of the outer electron capturing element further comprises acoating 311. Thecoating 311 may be configured to absorb X-ray radiation and/or the coating may be configured to capturemicro-particles 340a, 340b expelled from theX-ray conversion layer 302 during operation of the anode inside an X-ray tube. When thecoating 311 comprises an X-ray absorbing material, the coating may provide a first axial limitation of the aperture for the generated X-ray beam. Additionally or alternatively, thecoating 311 may serve as a particle trap for capturing micro-particles expelled from theX-ray conversion layer 302. Thereto, thecoating 311 may comprise a ductile material. For example, tantalum has a high attenuation coefficient for X-ray radiation and a high ductility, so acoating 311 made of tantalum may serve as an X-ray radiation absorbing layer and as a particle trap. Furthermore, the first portion of the outer electron capturing element comprisesfins 318 for increasing the capacity to capturemicro-particles 340a, 340b. Due to centrifugal forces, lose micro-particles propagate outwards, away from the axis of rotation. This is illustrated bymicro-particle propagation paths 341a and 341b.
• Thesecond portion 316 of the outer electron capturing element may comprise a heel effect compensation filter.
• Similar to thefirst portion 315, thethird portion 317 of the outer electron capturing element comprises acoating 313, which may be configured to absorb X-ray radiation and/or to capturelose micro-particles 340a, 340b. When thecoating 313 comprises an X-ray absorbing material, the coating may provide a second axial limitation of the aperture for the generated X-ray beam. Additionally or alternatively, thecoating 313 may serve as a particle trap for capturing micro-particles expelled from theX-ray conversion layer 302. For example, when thecoating 313 comprises tantalum, it may serve as an X-ray radiation absorbing layer and as a particle trap. Furthermore, the third portion of the outer electron capturing element comprises fins for increasing the capacity to capture micro-particles.
• For the sake of simplicity, an inner electron capturing element is not drawn inFig. 3, but theanode 300 maybe modified straightforwardly to comprise also an inner electron capturing element as illustrated inFigs. 1 and2.
• While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustrations and descriptions are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing a claimed invention, from a study of the drawings, the disclosure, and the dependent claims.
• In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfil the functions of several items re-cited in the claims. The mere fact that certain measures are re-cited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims (15)

1. An anode (100) for a rotating anode X-ray tube, the anode comprising

   - an anode body (101),

   - an axis of rotation (103), and

   - a focal track,

   wherein the anode further comprises an inner electron capturing element (120) and/or an outer electron capturing element (110), the inner and outer electron capturing elements being configured for capturing electrons backscattered from the focal track of the anode,

   wherein the inner electron capturing element is arranged closer to the axis of rotation than the focal track of the anode, and/or

   wherein the outer electron capturing element is arranged further away from the axis of rotation than the focal track of the anode, and wherein the outer electron capturing element comprises an X-ray transparent member (114).
2. The anode (100) of claim 1,
   wherein the X-ray transparent member (114) of the outer electron capturing element (110) comprises an X-ray transparent material, and
   wherein the X-ray transparent material is an element with atomic number smaller than 15 or a compound with atomic numbers smaller than 15.
3. The anode (100) of any of claims 1 or 2,
   wherein the X-ray transparent member (114) of the outer electron capturing element (110) comprises an X-ray transparent material, and
   wherein the X-ray transparent material is an electric conductor.
4. The anode (100) of any of the preceding claims,
   wherein the inner and outer electron capturing elements (120, 110) have an annular shape protruding from a surface of the anode.
5. The anode (100) of any of the preceding claims,
   wherein a first portion (115) of the outer electron capturing element (110) comprises an X-ray radiation absorbing material, thereby providing a first limitation of an X-ray aperture in a direction parallel to the axis of rotation (103) of the anode.
6. The anode (100) of claim 5,
   wherein the first portion (115) of the outer electron capturing element (110) comprises a part of the X-ray transparent member (114), and
   wherein the part of the X-ray transparent member is at least partially coated with the X-ray radiation absorbing material.
7. The anode (100) of any of the preceding claims,
   wherein a second portion (116) of the outer electron capturing element (110) comprises a heel effect compensation filter (112).
8. The anode (100) of any of the preceding claims,
   wherein a third portion (117) of the outer electron capturing element (110) comprises an X-ray radiation absorbing material, thereby providing a second limitation of the X-ray aperture in the direction parallel to the axis of rotation (103) of the anode.
9. The anode (300) of any of the preceding claims,
   wherein the first and/or third portions (315, 317) of the outer electron capturing element (310) comprise a particle trap (311, 318) for capturing micro-particles (340a, 340b) expelled from the focal track of the anode.
10. The anode (300) of claim 9,
    wherein the particle trap (311, 318) comprises a ductile material such as tantalum or a liquid metal coated on a rigid substrate.
11. The anode (300) of any of claims 9 or 10,
    wherein the particle trap (311, 318) comprises fins (318), a foam-like structure, and/or angulated microstructures.
12. The anode (200) of any of the preceding claims,
    wherein the inner electron capturing element (220) comprises a first portion (221) for capturing electrons backscattered from the focal track of the anode, the first portion of the inner electron capturing element facing the focal track of the anode and comprising a material with a first atomic number,
    wherein the inner electron capturing element further comprises a second portion (222) for absorbing X-ray radiation, the second portion of the inner electron capturing element comprising a material with a second atomic number larger than the first atomic number.
13. A rotating anode X-ray tube comprising

    - a cathode for emitting an electron beam (104),

    - an anode (100) for converting the electron beam at least partly into X-ray radiation, the anode being configured according to any of the preceding claims, and

    - a tube envelope housing the cathode and the anode.
14. The X-ray tube according to claim 13,
    wherein the tube envelope is at least partly made of carbon, copper, glass, titanium, steel, aluminum, tungsten, molybdenum, or beryllium.
15. An X-ray imaging system comprising

    - a rotating anode X-ray tube according to any of claims 13 or 14 and

    - an X-ray detector.

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