EP3389055B1 - X-ray device for generating high-energy x-ray radiation - Google Patents

Description

The invention relates to an X-ray device for generating high-energy X-rays, comprising a linear accelerator and a target. The linear accelerator is designed to generate X-rays and to generate an electron beam directed at the target, the kinetic energy of which per electron is at least 1 MeV.

X-ray systems typically have an electron beam source that delivers an accelerated electron beam to a target (also known as the target material). When the electrons strike the target, X-rays are generated in the area of the so-called focal spot. The electron beam source is usually formed by a cathode, with the emerging electrodes being accelerated by an applied acceleration field toward an anode, which in such designs forms the target. For high-energy applications, it is also known to use a linear accelerator as the electron beam source, which delivers an electron beam directed at the target.

In many applications in radioscopy or radiology, there is a need to create the smallest possible focal spot. In imaging, for example, this can achieve high spatial resolution at optical magnification or reduce the penumbra caused by the apertures that limit the X-ray field. In radiotherapy, especially intensity-modulated radiotherapy, this also allows for a more precise dose distribution of the deposited X-rays.

Out of DE 10 2012 103 974 A1 An X-ray tube is known for medical imaging such as computed tomography. It consists of a cathode and an anode. The electron beam is directed onto a target to generate X-rays. To limit the focal spot size on the target, the electron beam passes through a diaphragm channel that borders the target and is incorporated into a diaphragm body. To dissipate the heat generated by the absorption of the electrons, the area around the diaphragm channel must be as solid as possible; water cooling may be required.

GB 645 509 A discloses an electromagnetic modular waveguide of any length while maintaining a vacuum. GB 665 998 A describes the use of such a modular waveguide to accelerate charged particles.

Based on this prior art, the object of the present invention is to provide an X-ray device for generating high-energy X-rays in which the extent of the focal spot on the target can be minimized.

According to the invention, this object is achieved by an X-ray device for generating high-energy X-ray radiation of the type mentioned at the outset with the characterizing features of patent claim 1 and patent claim 13.

Advantageous embodiments of the invention are the subject of the dependent claims.

An X-ray device for generating high-energy X-rays comprises a linear accelerator and a target. The target typically consists of a target material which is used to generate X-rays by deceleration. of the accelerated electrons. The area of the target in which this conversion takes place is referred to as the focal spot. The linear accelerator is further designed and aligned to generate an electron beam directed at the target, the kinetic energy per electron of which is at least 1 MeV. According to the invention, an aperture is arranged in the beam path of the electron beam between the linear accelerator and the target, said aperture having an edge region surrounding an aperture opening, the material thickness of which, in the propagation direction of the electron beam, is less than 10% of the average range of electrons of the generated kinetic energy in the material of the edge region.

In linear accelerators, high kinetic energies are typically achieved, so that the emitted electrons have a longer average range in materials compared to the electrons generated in conventional X-ray tubes. To limit the focal spot in this energy range, the invention takes the approach of providing an aperture that is not designed to absorb the electrons of the generated energy range to any noticeable extent; rather, it is envisaged that the interaction should be essentially limited to inelastic or elastic scattering processes. For this purpose, the aperture, at least in the edge region delimiting the aperture opening, has a material thickness that is only a fraction of the average range of electrons of the generated kinetic energy in the material of the edge region. When the electron beam is transmitted through the edge region of the aperture, the peripheral electrons that penetrate the edge region are deflected and scattered. The resulting divergently propagating electrons subsequently generally no longer impact the target material. The area of the electron beam that creates the focal spot is thus essentially limited to the area of the aperture. At the same time, the energy transfer to the aperture is minimal, as this is essentially based only on inelastic scattering effects. This results, among other things, in less heat input to the aperture, which therefore does not necessarily require additional cooling.

In other words, the edge area of the aperture forms a scattering body (also known as a diffuser) for the electrons passing through it, within the energy range specified by the applied accelerating voltage. The randomly deflected electrons can be absorbed in other areas of the X-ray system and are thus no longer visible in the effective beam field of the generated X-ray radiation. Limiting the size of the focal spot on the target (also known as the target material) results, among other things, in improved image quality in imaging techniques. The acquired images exhibit less blurring and smaller penumbras because the size of the focal spot approximates that of an ideal point source. Possible fields of application include, for example, radiography, particularly the non-destructive testing of workpieces, components, or other objects; the inspection of transported goods, particularly in the context of freight inspection, where, for example, trucks or freight containers for trains or container ships are x-rayed to make their contents visible; and applications in the field of medicine, particularly in the field of radiation therapy. For example, the limitation of the focal spot provided by the invention allows for more precise dose distribution in radiotherapy, particularly in intensity-modulated radiotherapy, since the penumbra of the collimator limiting the photon beam field is smaller. Furthermore, the X-ray devices can be optimized in terms of weight, since downstream collimators for collimating the generated X-rays can be eliminated or at least limited.

In a simple embodiment, the aperture consists of a thin sheet, particularly made of steel or another transition metal or alloy. Another particularly preferred non-metallic material for the aperture is, for example, graphite.

It goes without saying that the material and thickness of the aperture, at least in the edge area surrounding the aperture opening, are matched to the kinetic energy of the electrons generated during the intended use of the X-ray device. For kinetic energies in the MeV range, the material thickness is typically in the range of one or more millimeters if it is made of a lightweight material such as graphite. Apertures made of a heavier material, particularly metal, have thinner material thicknesses, for example, in the submillimeter range, particularly in the range of approximately 1/10 mm.

In a preferred embodiment of the invention, at least the edge region of the aperture that scatters the electrons is formed by one or more foils. Such designs can be viewed as cost-effective implementations of a scattering body of sufficiently small thickness, ensuring that the interaction of the generated kinetic energy with the electrons is essentially limited to scattering processes. If the region of the aperture that is responsible for the scattering of the electrons is formed by such a foil material, the heat input is minimal. Therefore, apertures designed in this way do not necessarily need to be actively cooled during operation of the X-ray device.

The foil is preferably made of a metal. Particularly preferably, the aperture or at least the scattering edge area of the aperture is made of titanium. In other embodiments, the aperture or at least the edge area surrounding the aperture is made of stainless steel, tungsten, or copper. or from another transition metal or transition metal alloy.

The aperture, in particular the aperture described above consisting of at least one metallic foil, can be cooled by a cooling device, in particular a water cooling device, in one possible embodiment. This ensures that even the relatively low heat transfer caused by inelastic scattering processes can be reliably dissipated.

Preferably, a collimator is arranged in the beam path of the X-rays generated by the target. This serves to limit the effective beam field of the generated X-rays. If the location of X-ray generation (focal spot) is small, the penumbra at the edges of the effective beam field will also be small.

Particularly preferably, a vacuum housing surrounding at least the linear accelerator, the aperture, and the target, or a vacuum envelope surrounding these components, is provided at least in some regions with a shield suitable for absorbing X-ray radiation caused by scattered electrons that strike the vacuum housing and are thereby slowed down. The resulting X-ray radiation can be spectrally influenced by the choice of wall material and is preferably shielded locally by a shield arranged outside the vacuum housing. In other embodiments, the shield is provided inside the vacuum housing. Since the vacuum housing of the X-ray device is evacuated, the shield provided inside the vacuum housing preferably consists of a material with high vapor pressure; particularly preferably, the shield comprises elements with a low atomic number. Materials with a low vapor pressure can also be used for shielding on the outside of the vacuum housing. This shield consists, for example, of Made entirely or partially of lead. Since the scattered electrons are not absorbed by the aperture material, they propagate divergently to the direction of propagation of the electron beam and hit the vacuum enclosure, which is covered with shielding materials, where they are absorbed. Since the absorption of the electrons scattered by the aperture does not occur in a highly localized area, but rather in large areas of the vacuum enclosure, external cooling is generally not necessary here either.

In other possible embodiments of the invention, the vacuum housing of the X-ray device can be cooled by means of a fluid cooling system.

Particularly preferably, the areas provided with the shielding exhibit increased absorption for electrons of the generated kinetic energy compared to areas of the vacuum housing without shielding. In other words, it is intended to provide shielding only for those areas that are relevant for the absorption of scattered electrons. This contributes, among other things, to weight reduction.

The shielded regions preferably lie exclusively within a solid angle range emanating from the aperture and extending in the propagation direction of the electron beam. The solid angle range is preferably formed by a plurality of superimposed scattering cones, the cone apices of which lie within the edge region surrounding the aperture. In other words, the shield is arranged where the electrons scattered in the edge region of the aperture are at least highly likely to strike.

In a further development of the invention, it is provided that the solid angle range to be shielded corresponds to a medium scattering angle range of the electrons scattered in the edge region of the aperture. This refinement takes advantage of the observation that the average scattering angle depends both on the kinetic energy of the incident electrons and on the scattering body, which in this case is provided by the edge region surrounding the aperture. Depending on the acceleration voltage applied during operation and the scattering material used to limit the focal spot, it is thus possible to provide selective dimensioning of the shielding. This enables, in particular, a further reduction in weight, since only those areas of the vacuum housing in which the majority of the scattered electrons are absorbed are provided with shielding. For example, the deflection of the scattered electrons with respect to the propagation direction of the unscattered electrons is smaller at higher energies than for electrons with lower kinetic energy. As a result, the shielding in X-ray systems designed to provide higher-energy X-rays can be limited to a smaller solid angle range concentrated around the propagation direction of the unscattered electron beam.

The mean scattering angle range, as defined in this specification, is assumed to be a scattering cone centered around the mean scattering angle, whose aperture angle corresponds to a mean deviation characteristic of the scattering process, in particular a standard deviation. The mean scattering angle refers to the mean value of the angles of the scattered electrons to the acceleration axis, which corresponds to the propagation direction of the unscattered electrons.

The linear accelerator of the X-ray device is preferably designed to generate an electron beam whose kinetic energy per electron is less than 20 MeV. The X-ray device is therefore preferably suitable for the already described applications in the field of radioscopy or radiology.

The invention further relates to a method for producing an X-ray device for generating high-energy X-rays, in particular a method for producing one of the X-ray devices described above. The X-ray device comprises a linear accelerator and a target, wherein the linear accelerator is designed to generate X-rays by generating an electron beam directed at the target, the kinetic energy per electron being at least 1 MeV. According to the invention, a component is arranged in the beam path of the electron beam between the linear accelerator and the target, the material thickness of which, in the propagation direction of the electron beam, is less than 10% of the average range of electrons of the generated kinetic energy in the material of the component. A diaphragm opening is introduced into the component by exposing the component to an electron beam generated by the linear accelerator. In this sense, after the diaphragm opening has been introduced, the component forms the previously described diaphragm.

It has been shown that the electron beams generated by linear accelerators are already highly focused due to the applied electric spring, so that the particle density in the center of the electron beam is greatly increased. The invention utilizes this property to introduce the above-described aperture into the component. For this purpose, the current intensity of the accelerated electron beam provided by the linear accelerator is increased, if necessary, compared to the current intensity generated during normal operation in order to burn a hole into the component inserted in the beam path – which, for example, is formed by one or more of the above-described foils. The dimensioning of the aperture thus created corresponds to the central region of the electron beam. and thus automatically an aperture with the scattering characteristics described above for the electrons propagating away from the central region. Complex adjustment of an aperture already equipped with an aperture can be avoided, thus saving assembly and adjustment costs.

• Fig. 1: eine Röntgeneinrichtung gemäß einem ersten Ausführungsbeispiel in einer schematischen Schnittdarstellung;
• Fig. 2: eine Röntgeneinrichtung gemäß einem zweiten Ausführungsbeispiel in einer schematischen Schnittdarstellung;
• Fig. 3: mittlere Streubereiche bei der Elektronenstreuung an einem ausgewählten Streukörper.

For a further description of the invention, reference is made to the exemplary embodiments shown in the drawing figures. They show, in a schematic representation:

• Fig. 1 : an X-ray device according to a first embodiment in a schematic sectional view;
• Fig. 2 : an X-ray device according to a second embodiment in a schematic sectional view;
• Fig. 3 : mean scattering ranges in electron scattering from a selected scattering body.

Corresponding parts or reference sizes are provided with the same reference numerals in all figures.

Figure 1 shows an X-ray device 1 according to a first embodiment of the invention in a schematic sectional view. The X-ray device 1 comprises a linear accelerator 2, shown only schematically, which is designed to generate an electron beam E with a kinetic energy of at least 1 MeV per electron. The electron beam E is directed at a target 3. The target 3 emits X-ray radiation R in the region of a focal spot.

In the beam path between the linear accelerator 2 and the target 3, a diaphragm 4 is arranged, which diffuses a peripheral part of the incident primary electron beam E, so that the extent of the focal spot on the target 3 is reduced For this purpose, at least one edge region B of the aperture 4 surrounding an aperture 5 is made of a material suitable for scattering electrons of the generated kinetic energy. The edge region B of the aperture 4 has a material thickness in the propagation direction P of the electron beam E that is small compared to the range of the electrons of the generated kinetic energy in the material of the edge region B. Specifically, the material thickness of the edge region B according to the invention is less than 10% of the average range of electrons with the kinetic energy of at least 1 MeV in the material of the edge region B.

The electrons propagating away from the center of the electron beam E are diffusely scattered by the edge region B and thus distributed over a large area of the inner surface of a vacuum housing 6 of the X-ray device 1. Accordingly, the heat input caused by the absorption of these electrons is also distributed over large areas of the vacuum housing 6, so that external cooling of the vacuum housing 6 is unnecessary.

On the outside of the vacuum housing 6, a shield 7 is arranged, which in the exemplary embodiment consists of lead and extends - with the exception of the area of the target 3 - over the entire outer surface of the vacuum housing 6.

By scattering the lateral edge regions of the electron beam E away from the target 3, penumbras in images captured by the generated X-ray radiation R can be minimized. Thus, radioscopy is a suitable field of application for the X-ray device 1; other fields of application include, for example, medical radiotherapy.

In the embodiment shown, the aperture 4 is formed from a simple sheet or foil made of metal. Since the interaction of the electrons with the material of aperture 4 is essentially limited to inelastic and elastic scattering events, the heat input is also minimal. Cooling of aperture 4 is therefore not absolutely necessary.

Optionally, a cooling device 8 for fluid cooling of the aperture 4 is provided, which is shown schematically in Figure 1 is shown. In this case, the aperture 4 is designed such that a cooling fluid, for example water, can be passed through at least a portion of the aperture. In one possible embodiment, the aperture 4 is formed by two plane-parallel foils, between which a gap is formed into which the cooling fluid can be introduced.

The proportion of X-ray radiation R caused by scattered electrons can be further reduced if the X-ray radiation R emitted from the target 3 is collimated. For this purpose, a collimator 9, for example a multi-leaf collimator, is optionally arranged in the region of the emitted X-ray radiation R close to the target.

Figure 2 shows an X-ray device 1 according to a second embodiment. The second embodiment differs from the one shown in Figure 1 illustrated embodiment only with regard to the extent of the shield 7, so that reference is first made to the relevant description in order to avoid repetition.

In the Figure 2 In the second embodiment shown, the shielding 7 is limited to a partial area of the vacuum housing 6. The design of the shielding 7 is such that at least the predominant portion of the electrons scattered in the edge area B are absorbed by the shielding 7. For this purpose, a solid angle range Ω (indicated by dashed lines in the figure) emanating from the scattering edge area B must be shielded, into which on average at least the predominant A majority of electrons are scattered. The extent of the shield 7 is therefore to be designed as a function of the kinetic energy of the electrons in accordance with the mean scattering angle ϕ and the mean deviation from this mean scattering angle ϕ.

The information relevant for the design of shield 7 is in Figure 3 for a selected scattering material and for specific energy ranges between 2 MeV and 18 MeV. Shown are the mean scattering angle ϕ, which is relevant for electron scattering at the respective energy, and the mean deviation σ from it, which is represented as bars centered around the mean scattering angle ϕ. The mean deviation σ corresponds to the standard deviation, so that in the example illustrated here, assuming normally distributed scattering events, it can be assumed that approximately 68% of the scattering angles are scattered within the mean scattering angle range defined by the mean scattering angle ϕ and the mean deviation σ.

Knowledge of the mean scattering angle ranges as a function of the kinetic energy of the incident electrons can be used to specifically design and shield the X-ray device 1. The solid angle range Ω covered by the shield 7 corresponds to the sum of the mean scattering angle ranges whose scattering centers lie in the edge region B of the aperture 4, which is crucial for electron scattering. The dimensions of the shield 7 can be significantly reduced by this design.

A preferred method for producing the above-described X-ray device 1 comprises a method step in which a component, which in the final assembled state forms the aperture 4, is introduced into the beam path of the electron beam E provided by the linear accelerator 2. The aperture 5 is burned into the component by means of the electron beam E. For this purpose, if necessary an electron beam current provided by the linear accelerator 2 can be increased compared to the current generated during regular operation. Since the number of electrons is greatly increased in a central region of the electron beam E due to the focusing properties of the linear accelerator 2 and greatly decreases towards the edges, such a procedure leaves an edge region B surrounding the aperture 5 with the scattering properties described above. Edge beam regions of the electron beam E, in which the number of electrons is greatly reduced compared to the central region of the electron beam E, are thus scattered away from the target 3 during regular operation of the X-ray device 1, thus minimizing the extent of the focal spot on the target 3.

Although the invention has been illustrated and described in detail with reference to the preferred embodiment, the invention is not limited thereby. Other variations and combinations may be derived by those skilled in the art without departing from the invention, which is defined by the claims.

Claims (13)

1. X-ray device (1) for creation of high-energy x-ray radiation, comprising a linear accelerator (2) and a target (3), wherein the linear accelerator (2) for creation of x-ray radiation (R) is embodied to create an electron beam (E) directed onto the target (3), of which the kinetic energy per electron amounts to at least 1 MeV, characterised by a beam limiting device (4) arranged in the beam path of the electron beam (E) between linear accelerator (2) and target (3), which has an edge region (B) surrounding a beam limiting device opening (5), the material thickness of which in the propagation direction (P) of the electron beam (E), also referred to as primary electron beam in the following, amounts to less than 10% of the average reach of electrons of the created kinetic energy in the material of the edge region (B), wherein the edge region (B) of the beam limiting device (4) forms a scattering body for limiting the extent of the focal spot on the target (3) and wherein the beam limiting device (4) diffusely scatters a peripheral part of the incident primary electron beam (E).
2. X-ray device (1) according to claim 1, characterised in that at least the edge region (B) of the beam limiting device (4) consist of graphite.
3. X-ray device (1) according to claim 1 or 2, characterised in that at least the edge region (B) of the beam limiting device (4) is formed by at least one film.
4. X-ray device (1) according to claim 3, characterised in that the film consists of a metal.
5. X-ray device (1) according to claim 4, characterised in that the film consists at least partly of titanium, stainless steel or copper or is coated with titanium, stainless steel or copper.
6. X-ray device (1) according to one of the preceding claims, characterised in that the beam limiting device (4) is able to be cooled by means of a cooling device, in particular by means of a water cooling device.
7. X-ray device (1) according to one of the preceding claims, characterised in that a collimator (9) is arranged in the beam path of the x-rays (R) created by application of the beam to the target (3).
8. X-ray device (1) according to one of the preceding claims, characterised by a vacuum housing (6) at least surrounding the linear accelerator (2), the beam limiting device (4) and the target (3), which at least in some regions is provided with screening (7), which is suitable for absorbing x-ray radiation caused by slowing down scattered electrons.
9. X-ray device (1) according to claim 8, characterised in that the regions provided with the screening (7), compared to regions of the vacuum housing (6) without screening, exhibit an increased absorption for x-ray radiation.
10. X-ray device (1) according to claim 8 or 9, characterised in that the regions provided with the screening (7) lie exclusively within a solid angle region (Ω) emanating from the beam limiting device (4) extending in the propagation direction (P) of the electron beam (E).
11. X-ray device (1) according to claim 10, characterised in that the solid angle region (Ω) corresponds to an average solid angle region of the scattered electrons in the edge region (R) of the beam limiting device (4).
12. X-ray device (1) according to one of the preceding claims, characterised in that the kinetic energy per electron in the created electron beam (E) amounts to less than 20 MeV.
13. Method for manufacturing an x-ray device (1) for creation of high-energy x-ray radiation (R), comprising a linear accelerator (2) and a target (3), wherein the linear accelerator (2) for creation of x-ray radiation (R) is embodied so as to create an electron beam (E) directed onto the target (3), of which the kinetic energy per electron amounts to at least 1 MeV, characterised in that a component in the beam path of the electron beam (E) is arranged between linear accelerator (2) and target (3), of which the material thickness in the propagation direction (P) of the electron beam (E) amounts to less than 10% of the average reach of electrons of the created kinetic energy in the material of the component, wherein a beam limiting device opening (5) is inserted into the component by the component having an electron beam (E) created by the linear accelerator (2) applied to it.