Method for dual-energy imaging of an imaging area using an X-ray device and an X-ray device

Abstract

Method for dual-energy imaging of an imaging area (5) with an X-ray device (1) comprising an imaging arrangement with an X-ray source (3) and an X-ray detector (4) for receiving X-rays from an X-ray field (9) emitted by the X-ray source (3) in conical beam geometry, which has a central beam (10), wherein the imaging arrangement is rotated around the imaging area (5) to acquire projection data of different projection directions for two different X-ray spectra and a three-dimensional image data set of the imaging area (5) is reconstructed from the respective projection data for each X-ray spectrum. wherein projection data of both X-ray spectra are recorded during the rotation covering at least 360°, wherein for each X-ray spectrum an assigned, in particular at least substantially half of the X-ray field (9), fixed portion (24, 25) of the X-ray field (9) and a corresponding, assigned, fixed portion (28, 29) of the X-ray detector (4) is used, wherein the X-ray source (3) has two foci (43, 44) spatially separated by a focus distance (45), assigned to different X-ray spectra, which follow each other in the plane of rotation (27), characterized by that the foci (43, 44) are operated with different tube voltages and the partial beam fields (50, 51) emitted by the foci (43, 44) are separated by a shading element (52) arranged in the beam path between the foci (43, 44), in particular such that each half of the X-ray detector (4) is assigned to a partial beam field (50, 51) for illumination, that the foci (43, 44) are generated in the same X-ray tube (46) of the X-ray source (3), switching sequentially between the different tube voltages, and that the sequentially illuminated components (28, 29) of the X-ray detector (4) are read out in a common readout cycle.

Description

[0001] The invention relates to a method, particularly a computer-implemented one, for dual-energy imaging of an imaging area using an X-ray device. The device comprises an imaging arrangement with an X-ray source and an X-ray detector for receiving X-rays from a cone-beam X-ray field emitted by the X-ray source, which has a central beam. The imaging arrangement is rotated around the imaging area to acquire projection data from different projection directions for two different X-ray spectra, and a three-dimensional image data set of the imaging area is reconstructed from the respective projection data for each X-ray spectrum. The invention also relates to an X-ray device.

[0002] In X-ray imaging, for example in medical applications, it is known to reconstruct higher-dimensional image datasets from lower-dimensional projection images, in particular three-dimensional image datasets (for example as a stack of cross-sectional images) from two-dimensional projection images. For example, an imaging setup with an X-ray source and an X-ray detector can be moved around the imaging area of ​​an object to be imaged in order to acquire the projection images from different projection directions. In this process, the X-ray source moves along an imaging trajectory, for example a circular path.

[0003] While dedicated computed tomography systems are known in which the X-ray tube and, if applicable, the X-ray detector are moved within a gantry, prior art has also proposed realizing computed tomography-like imaging procedures with other X-ray equipment, for example, X-ray systems with a C-arm, as often used in angiography laboratories. This type of imaging is also referred to as "DynaCT" or, since cone-beam geometry is typically used, "Cone Beam CT" (CBCT). An overview of the dental application of Cone-Beam CT can be found, for example, in an article by William C. Scarfe and Allan F. Farman, "What is Cone-Beam CT and How Does it Work?", Dent Clin N Am 62 (2008), pages 707-730.

[0004] Dual-energy imaging uses two different X-ray spectra to acquire corresponding X-ray images. For example, a high-energy spectrum (which can be generated at a higher tube voltage) and a low-energy spectrum (which can be generated at a lower tube voltage) are used. Filters are typically employed, even with different tube voltages, to provide the desired X-ray spectrum. These filters can vary from one X-ray spectrum to another. Due to the different spectral absorption properties of various materials or material classes, such as bone and soft tissue, a combined analysis can be used to separate these materials / material classes.

[0005] The use of X-rays with different X-ray spectra is known in the prior art, e.g. from the publications DE 10 2010 062 402 A1, DE 20 2014 002 844 U1, US 2017 / 0 205 360 A1 or DE 11 2021 004 418 T5.

[0006] Several approaches to combining CBCT and dual-energy imaging have already been proposed in the prior art. For example, techniques have been developed to quickly switch between different tube voltages and filters. This allows, for instance, switching between different X-ray spectra during a single rotation to acquire projection images for both spectra. However, such a solution requires a large number of modifications to conventional X-ray equipment, particularly C-arm X-ray systems, and thus a significant technical effort.

[0007] Approaches to using so-called biplane X-ray systems are also known; these systems have two imaging arrays, each operating with different X-ray spectra. However, such solutions are only feasible in biplane systems, which are less preferred for other reasons, such as their large footprint.

[0008] One initial solution, focusing on the detector side, involves using so-called multilayer detectors, specifically two-layer detectors, which measure different X-ray spectra in the different layers. However, this results in a dose disadvantage for images without spectral analysis, necessitating the development of a new detector.

[0009] A second detector-side solution uses photon-counting detectors. However, these are technically difficult to implement as area detectors, so high effort and high costs are to be expected for their use in CBCT.

[0010] Finally, it has also been proposed to acquire the projection data for the two X-ray spectra completely sequentially, so that first the acquisition trajectory is run with one X-ray spectrum and projection data for that spectrum is acquired, after which the filter and tube voltage are slowly changed and the acquisition trajectory is run again. However, this results in a long acquisition time and the risk of motion artifacts arises, even with slow patient movements.

[0011] For spiral scans in conventional computed tomography, the use of a so-called twin-beam system has already been proposed. This involves using a filter device that divides the X-ray field into two components with different X-ray spectra. For example, DE 10 2008 056 891 A1 discloses a computed tomography device for performing a spiral scan. It comprises a rotatable X-ray source and a diametrically opposed X-ray detector with an associated evaluation unit. An X-ray filter is positioned downstream of the X-ray source, its position correlated with that of the X-ray detector. The X-ray filter generates an unfiltered and a filtered component of the radiation field, with the two components exhibiting different X-ray spectra.To operate the computed tomography device in a dual-energy mode, the evaluation unit separately evaluates a measurement signal of the unfiltered radiation component from a measurement signal of the filtered radiation component.

[0012] The invention is based on the objective of providing a simple and time-efficient method for dual-energy imaging with high-quality results for cone-beam computed tomography (CBCT).

[0013] This problem is solved according to the invention by a method, in particular a computer-implemented method, and an X-ray device according to the dependent claims. Advantageous embodiments are described in the sub-claims.

[0014] In a method of the type mentioned at the outset, it is provided according to the invention that projection data of both X-ray spectra are recorded during the rotation covering at least 360°, wherein for each X-ray spectrum an assigned, in particular at least substantially half of the X-ray field, fixed portion of the X-ray field and a corresponding, assigned, fixed portion of the X-ray detector are used.

[0015] It is proposed to perform a complete 360° rotation of the CBCT around a fixed axis of rotation with a fixed plane of rotation, i.e., with a circular path as the acquisition trajectory of the X-ray source, in order to achieve, in particular, complete coverage of the acquisition area for both X-ray spectra despite the division of the X-ray field and thus the X-ray detector for the X-ray spectra. For this purpose, it can be provided that the X-ray detector, more precisely its detection area, is divided into two sides along a central line running perpendicular to the plane of rotation through the point of impact of the central beam, with the division into the components of the X-ray spectra being antisymmetric with respect to the central line.This means that for every point on one side of the central line assigned to one of the X-ray spectra, there exists a corresponding point on the other side, at the same distance from the central line, assigned to the other X-ray spectrum. This approach exploits the fact that, during a rotation covering a 360° projection angle, each projection beam demonstrably appears twice in the cone-beam geometry. This design ensures that each projection beam is measured for each X-ray spectrum, thus providing a complete scan for both sets of projection data. Consequently, a three-dimensional image dataset can be reconstructed for each X-ray spectrum, which can then be jointly analyzed to obtain additional information based on the two X-ray spectra, particularly regarding material distribution.

[0016] The simplest way to create such antisymmetry is to define the components by bisecting the X-ray field perpendicular to the plane of rotation through the central beam. In this case, one side of the X-ray detector is irradiated with one X-ray spectrum, and the other side with the other. However, as will be shown, other divisions can also be advantageous.

[0017] During the rotation of the imaging setup around the imaging area, projection data for both X-ray spectra are acquired simultaneously, at least within a certain tolerance. Specifically, projection data for both X-ray spectra are acquired for each imaging position using the respective disjoint components of the X-ray field and the X-ray detector. This can be done either simultaneously or, as will be explained later, sequentially, but extremely rapidly, with the imaging position being defined as lying within a specific angular range during continuous movement of the imaging setup.It should be noted that, technically, to achieve 360° coverage with respect to the projection angles, a full 360° rotation is not necessary; for example, with recording positions spaced apart by an angular distance, 360° minus the angular distance is sufficient, for example, with a 2° angular distance, 358° is sufficient.

[0018] The X-ray system is specifically a C-arm X-ray system, meaning one that has a C-arm to which the X-ray source and the X-ray detector are attached opposite each other. The C-arm is rotatable, particularly around its central axis, to achieve a 360° projection angle with a constant axis and plane of rotation.

[0019] This advantageously allows dual-energy CBCT with manageable changes compared to conventional X-ray equipment, as will be explained later, thus enabling it in a simple and time-efficient manner.

[0020] In the following, two possible embodiments of the present invention are presented in more detail in order to advantageously realize the fixed division into proportions of the X-ray spectra during rotation.

[0021] In a first embodiment of the present invention, a filter device arranged between the X-ray source and the imaging area can be used to define the components. This filter device comprises a filter structure through which the X-ray field passes, with a first component for providing the first X-ray spectrum and a second component for providing the second X-ray spectrum. In this embodiment, a single focus (focus point) is used on the X-ray source, whereby the generated X-ray field is filtered separately, so that the different components for the X-ray spectra are formed in the X-ray field after passing through the filter device and at the X-ray detector. This has the advantage that both the tube voltage of an X-ray tube of the X-ray source and the filter device can remain static during the entire imaging process.In other words, with regard to the hardware, only the filter device needs to be placed in the beam path to implement this first embodiment; no other modifications are necessary.

[0022] It should also be noted that configurations are conceivable in which no or at least almost no filtering is carried out for one of the X-ray spectra.

[0023] The filter structure can also be designed, as described above, to be divided into two sides by a central line perpendicular to the plane of rotation, encompassing the point where the central beam passes through the filter structure, and to be selected and arranged such that for each point providing the first X-ray spectrum on one side of the central beam there is a point providing the second X-ray spectrum at the same distance on the other side of the central beam.

[0024] In other words, the filter structure is designed to be antisymmetric in such a way that, apart from any scattering effects and mechanical inaccuracies that may occur, all projection rays required for a complete image reconstruction are captured with both the first and second X-ray spectrums.

[0025] In a simple implementation of this first embodiment, it is conceivable that one side provides the entire first X-ray spectrum, while the other side provides the entire second X-ray spectrum. In this case, one detector half or side, relative to the central line of the X-ray detector, thus records a different X-ray spectrum than the other detector half / side. The filter arrangement can then, for example, consist of separate half-sided filters to provide the respective X-ray spectrum.

[0026] In advantageous embodiments of the present invention, it is conceivable that each component of the filter structure comprises several regions separated from one another by regions of the other component. These regions can be configured as strips extending perpendicular to the plane of rotation. All strips can have the same width, although it is particularly advantageous for the strips to have widths selected to accommodate the inverse square law. Due to the cone beam geometry, the width of the strips can thus increase outwards according to the inverse square law. This means that the width of the strips can be a function of the covered detector slits of the X-ray detector to account for the cone beam geometry. In particular, the widths increase with increasing distance from the central beam or the central line of the X-ray detector to comply with the inverse square law.In general, using two or more areas per X-ray spectrum can offer advantages in terms of correcting potential movements.

[0027] In general, the first embodiment may advantageously provide that the filter device is inserted into the beam path by means of an actuator before the start of the acquisition of the projection data. For example, the filter device may be part of a filter wheel of the X-ray device. A control unit of the X-ray device, which may be configured to carry out the method, may include a control unit to actuate the actuator for pivoting the filter device into the beam path before the start of the acquisition process.

[0028] The filter structure can be multilayered. Specifically, the multilayered filter structure can include at least one base layer covering both components. This base layer, for example a copper layer, can therefore act on both components and provide a basic filtration effect. Further layers, particularly those specific to each component, modify the X-rays spectrally to produce the desired X-ray spectrum. Suitable filter materials include, for example, gold, silver, tantalum, tungsten, and the like.

[0029] According to the invention, the X-ray source has two foci spatially separated by a focus distance, each assigned to a different X-ray spectra and positioned one after the other in the plane of rotation. The foci are operated with different tube voltages, and the partial beam fields emitted by the foci are separated by a shading element arranged in the beam path between the foci. In particular, each half of the X-ray detector is assigned to a partial beam field for illumination. Advantageously, the foci are arranged at equal distances from the (here imagined) central beam (of the entire X-ray field) along the plane of rotation. In this second embodiment, two X-ray foci are used which, in particular through different X-ray voltages (acceleration voltages), provide different emission spectra from which the X-ray spectra are derived.A shading element, also known as a central collimator element, is arranged in the beam direction between the two foci. This element absorbs the X-rays, thus separating the partial beam fields. The two X-ray foci with different tube voltages are preferably generated at two different locations within the same X-ray tube, particularly by rapid sequential switching of the tube voltage. The advantage of this design is that only a very fast switching of the tube voltage is required, which is easily accomplished, while no filter change or similar component is necessary, which is often the limiting factor in terms of duration.This is achieved by the offset foci and the shading element creating a clearly defined geometry that spatially separates the partial beam fields and thus assigns them different portions of the beam path and the X-ray detector. In other words, further processing measures for providing the X-ray spectrum in the corresponding portions of the X-ray field can also be permanently incorporated. For example, the X-ray spectra can be adjusted by X-ray spectrum-specific filters permanently installed in the beam path of the respective partial beam fields.

[0030] The focus distance is preferably kept small and can, for example, be a few millimeters. Specifically, the focus distance may be set at 0.5 to 5 mm.

[0031] It is particularly advantageous to design the shadowing element with a width smaller than the focal distance, especially such that the partial beam fields on the detector surface of the X-ray detector, particularly along its central line, are at least substantially adjacent to one another. The central collimator element can therefore be somewhat less wide than the focal distance between the X-ray foci, resulting in no or only a very small unirradiated central strip on the X-ray detector, which, for example, may be approximately 1 m away from the X-ray source, particularly the focus. It should be noted that even a small amount of crosstalk between the X-ray spectra in the central region of the X-ray detector is ultimately unproblematic, as it would only impede spectral separation in this (small) area.

[0032] As mentioned previously, the foci are generated in the same X-ray tube of the X-ray source, with sequential switching between the different tube voltages. In other words, the foci and the tube voltages are switched simultaneously. This makes it easy to achieve short focus distances and correspondingly small shadowing elements.

[0033] According to the invention, the sequentially illuminated portions of the X-ray detector are read out in a single readout cycle. In other words, the two portions, which can be understood as partial images on the X-ray detector, can be exposed very quickly and sequentially, with the X-ray detector then reading out the projection data of both portions in a single readout step. Advantageously, this variant eliminates any additional timing or readout requirements for the X-ray detector. The only component of the X-ray system that needs to switch quickly is the X-ray source, specifically its X-ray tube, which rapidly switches between one focus location with the first tube voltage and the other focus location with the second tube voltage.

[0034] In an alternative, second variant, it would also be conceivable to read out the sequentially illuminated portions of the X-ray detector by the partial beam fields one after the other, particularly using a shadow register for at least the first of the readout processes. It would thus be possible to read out the X-ray detector between the two rapidly successive X-ray pulses of the corresponding X-ray spectra, for example, by quasi-instantaneously reading out a CMOS detector into the shadow register. In this context, a particularly preferred embodiment provides that, with the readout of each portion, the part of the X-ray detector not covered by the respective partial beam field, comprising the portion of the other partial beam field, is also read out to obtain scatter radiation data that can be used for scatter radiation correction.In such a configuration, it becomes possible to use the unexposed portion of the X-ray detector for scatter radiation measurement. The scatter radiation data can then be used for image correction. For example, it can be provided that a scatter radiation image is also determined from the scatter radiation data for the exposed portion of the X-ray detector and used to correct the projection data, particularly by subtraction. In this way, improved scatter radiation correction and thus further enhanced image quality can be achieved.

[0035] In general, i.e., for both the first and second embodiments, a preferred further development of the present invention provides that projection data of the other X-ray spectrum are taken into account for the reconstruction of the image data set for one of the X-ray spectra. Even if the projection data of the other X-ray spectrum are not directly comparable with the projection data of the first spectrum, they can nevertheless contain useful information for the reconstruction, for example, regarding truncation and / or cone-beam artifacts. The projection data of the other X-ray spectrum are not directly incorporated into the reconstruction; rather, reconstruction information is derived from them, which can be included, for example, as boundary conditions, a term of an objective function, and the like.It can be shown that, within the scope of the present invention, more reconstruction information is available than, for example, with two consecutive standard CBCT scans performed with rotation in an angular interval of, say, 210°. Utilizing this additional reconstruction information thus leads to a significant increase in image quality in dual-energy CBCT. In particular, further reconstruction information is available in the case of iterative and / or multispectral image reconstruction.

[0036] A specific further development of the present invention provides that a first reconstruction of pre-image datasets is performed from the respective projection image data, material information describing, in particular, a material distribution with respect to the acquisition area is determined from the pre-image datasets, and the material information is taken into account by at least one boundary condition and / or in an objective function in at least one subsequent reconstruction from the respective projection data. Thus, pre-image datasets are initially determined separately from the respective projection data of the X-ray spectra as a first three-dimensional reconstruction. From these pre-image datasets, material information can be derived, as is generally known for dual-energy imaging. This information applies to the three-dimensional volume of the acquisition area and can provide information on how the projection images or...The result of a more accurate reconstruction would look different. Therefore, it is proposed to derive boundary conditions from the material information, especially soft material, and / or to modify an objective function if one is used, in order to achieve an improvement in at least one further reconstruction process and ultimately determine the image dataset. In this way, cone-beam artifacts, in particular, can be reduced.

[0037] Furthermore, it is possible to use projection data from both X-ray spectra to determine a truncation model, particularly a spectral one, which is then taken into account when reconstructing both image datasets. For example, techniques such as those proposed in the subsequently published German patent DE 10 2023 204 265 B4 can be employed. From the projection data and using estimation methods for incompletely covered areas outside the primary reconstruction volume, a complete truncation model of the patient in the imaging-relevant area can be determined. In truncated areas within individual projection images of the projection data, spectral X-ray absorption properties of the individual volume elements can be assumed based on a previously performed material classification.Virtual forward projection using the truncation model with the first and a second X-ray spectrum allows the truncated areas of the respective projection images to be added. Other approaches are also applicable in this context.

[0038] In addition to the method, the present invention also relates to an X-ray device comprising - a recording arrangement with an X-ray source and an X-ray detector for receiving X-rays from an X-ray field emitted by the X-ray source in conical beam geometry, which has a central beam, and - a control device designed to carry out a method according to the invention.

[0039] All descriptions of the method according to the invention can be applied to the X-ray device according to the invention and vice versa. The X-ray device is preferably a C-arm X-ray device, which has a C-arm on which the X-ray source and the X-ray detector are arranged opposite each other.

[0040] The control unit can comprise at least one processor and at least one storage medium. Functional units for carrying out steps of the method according to the invention can be formed by hardware and / or software. In particular, the control unit can comprise a recording unit for controlling the 360° rotation of the recording arrangement and the recording of the projection data, and a reconstruction unit for reconstructing the image data sets.

[0041] In the first embodiment, the X-ray device further comprises the filter device. If an actuator is provided for pivoting the filter device, the control device may include a control unit for controlling the actuator. In the second embodiment, the shading element is provided, and the receiving unit is further configured to control the X-ray tube for generating the two foci.

[0042] The method according to the invention can be implemented on the control device as a computer program which, when executed on the control device, causes it to carry out the steps of a method according to the invention. The computer program can be stored on an electronically readable data carrier.

[0043] It should be noted here that the proposed use of different, fixed portions of the X-ray field or X-ray detector during rotation is independent of any collimation of the X-ray field by a collimation device (collimator) of the X-ray system. A collimation device uses, for example, lateral lamellae to adapt the dimensions of the X-ray field to itself, whereas the present invention divides the X-ray field, i.e., the area actually to be recorded, into portions for the X-ray spectra, thereby exploiting the fact that the necessary information is still fully available despite the complete rotation.

[0044] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings. These show: Fig. 1 a general schematic diagram of an X-ray device according to the invention, Fig. 2. An embodiment of the provision of the X-ray spectra in a first embodiment of the first design, Fig. 3 a representation of a resulting recording geometry, Fig. 4A,B schematically represents simultaneous recording processes to explain the complete projection data acquisition, Fig. 5 a possible multi-layered design of the filter device in the first embodiment, Fig. 6 a schematic top view of a filter structure in a second embodiment of the first embodiment, Fig. 7 a division of the X-ray detector resulting in the second embodiment, Fig. 8 the design of the beam path in an embodiment of the second embodiment, and Fig. 9 a flowchart of an exemplary embodiment of the method according to the invention.

[0045] Fig. Figure 1 shows a schematic diagram of an X-ray device 1 according to the invention. This device has a C-arm 2 on which an X-ray source 3 and an X-ray detector 4 are arranged opposite each other. The X-ray source 3 and the X-ray detector 4 form an imaging arrangement. The C-arm 2 allows movement of this imaging arrangement in at least different rotational degrees of freedom. In this way, in particular, rotation of the imaging arrangement about an imaging area 5 of an object 7, here a patient, arranged on a patient table 6 of the X-ray device 1, is possible using a fixed axis of rotation 8 and a fixed plane of rotation (perpendicular to the image plane of the Fig. 1 by means of a central beam 10) to cover a projection angle range of at least 360° (and also more).

[0046] In this process, an X-ray field 9 can be emitted to the X-ray detector 4 by means of the X-ray source 3 in order to acquire projection data of the imaging area 5 under different projection directions, which are defined by a central ray 10 of the X-ray field 9. A conical beam geometry is used here, so that a CBCT (conical beam CT) imaging process is performed.

[0047] The operation of the X-ray unit 1 is controlled by a control unit 11, which is only shown schematically. The control unit 11 has a recording unit 12 for controlling the recording operation and a reconstruction unit 13, by means of which three-dimensional image data sets can be reconstructed from projection data from different projection directions. Furthermore, a control unit 14 is provided for controlling other components of the X-ray unit 1. Various pieces of information, for example, also recorded projection data, can be stored, at least temporarily, in a storage medium 15.

[0048] The X-ray device 1 also includes a collimator and / or filter arrangement 16, shown here in general terms, which varies in its specific design depending on the embodiment and exemplary embodiment, as will be explained in more detail below. The specific exemplary embodiments discussed below relate to various options for the simple and advantageous implementation of dual-energy CBCT on the X-ray device 1.

[0049] In all cases, the X-ray field 9, and consequently the X-ray detector, is divided into sections that are assigned to different X-ray spectra during rotation. This means that each section of the X-ray field 9 contains X-rays of a specific X-ray spectrum, with which the corresponding section of the X-ray detector is exposed – after attenuation by the detection area 5.

[0050] Fig. Figure 2 shows an embodiment of the provision of the X-ray spectra according to a first embodiment of a first configuration. Accordingly, the collimator and / or filter arrangement 16 comprises, in addition to a collimator 17 of a generally known nature, which defines the dimensions of the X-ray field 7, a filter device 18 with a filter structure 19, which in this case has a first part 20 and a second part 21, which have different filter properties. In this embodiment, the parts 20, 21 are configured as halves, since each side of a central line of the filter device 16, traversed by the central beam 10, forms a part 20, 21. The central line is perpendicular to a plane of rotation (fixed during a dual-energy CBCT extraction process), which is Fig. 2 corresponds to the image plane, formed.

[0051] From the X-ray source 3, starting from its (here exactly one) focus 22, an emission spectrum identical for both components (indicated by arrows 23) is provided, which strikes the filter device 18. Components 20 and 21 provide different filtration, so that after the filter device 18 the X-ray field 9 is divided into a first component 24 of a first X-ray spectrum and a second component 25 of a second X-ray spectrum. This division remains constant throughout the entire rotation of the dual-energy CBCT scan, which covers at least 360°.

[0052] This is achieved through Fig. Figure 3 explains this in more detail, where the acquisition trajectory 26, here a circular path, of the focus 22 during rotation of the acquisition setup in the rotation plane 27 is also shown. For the sake of clarity, the filter device 18 is not shown, but the resulting division of the X-ray field 9 into the components 24 and 25 is. Since this division remains fixed, it is also shown on the detector surface of the X-ray detector 4, as in Fig. Figure 3 shows a fixed division into a portion 28 for recording projection data of the first X-ray spectrum and a portion 29 for recording projection data of the second X-ray spectrum.

[0053] Since the recording is simultaneous, i.e., with the same X-ray pulse from the same focus 22, it therefore takes place as Fig. 4A and Fig. Figure 4B explains that two partial acquisition processes take place simultaneously: a complete 360° scan, indicated by arrow 30, of the acquisition area 5 with the first X-ray spectrum (part 24 of the X-ray field 9) and one detector half (part 28), and a complete 360° scan, indicated by arrow 30, of the acquisition area 5 with the second X-ray spectrum (part 25 of the X-ray field 9) and the other detector half (part 29). It is known that both partial acquisition processes enable a complete scan with respect to the reconstruction of three-dimensional image datasets.

[0054] The filter device 18 can, for example as part of a filter wheel, be operated by means of an actuator 31 (indicated in Fig. 2) are introduced into the beam path before the start of the dual-energy CBCT acquisition process, controlled by the control unit 14 of the control device 11.

[0055] Fig. Figure 5 shows a possible multilayer structure of the filter structure 19. This has a base layer 32, for example made of copper, which acts on both components 24, 25. Component-specific layers 33 then provide the desired X-ray spectrum.

[0056] Fig. Figure 6 shows, as a second embodiment of the first embodiment, a variant of the filter structure 19 which can be advantageous with regard to motion correction, and for which the descriptions of the first embodiment apply accordingly. Here, not the entire sides 34, 35 extending from the central line 36 of the filter device 18 are continuously assigned to the provision of an X-ray spectrum, but rather the portions 20, 21 comprise areas 37, 38, which in this case are designed as strips 39, 40 perpendicular to the plane of rotation 27. This results in a kind of comb structure. As already in the first embodiment, an antisymmetry with respect to the central line 36 is provided, meaning that for each point providing the first X-ray spectrum on one side 34, 35 of the filter structure 19, there exists, at the same distance from the central line on the other side 35, 34 of the filter structure, a point providing the second X-ray spectrum.

[0057] Fig. Figure 7 shows the corresponding division of the detector area of ​​the X-ray detector 4 into corresponding strip-shaped areas 41, 42 of the proportions 20, 21.

[0058] Fig. Figure 8 shows an embodiment of the second embodiment. Here, an X-ray source 3 is used, which has two X-ray foci 43, 44. These foci are offset outwards by an equal small distance, for example 0.25 to 2.5 mm, from an imaginary central focal point of the (here also imaginary) central beam 10 of the X-ray field 9 within an X-ray tube 46 of the X-ray source 3. The resulting focus distance 45 between the foci 43, 44 can therefore be, for example, 0.5 to 5 mm. Compared to the focus-detector distance, which can be, for example, 0.8 to 1.2 m, it is small.

[0059] The X-ray tube 46 of the X-ray source 3 can be controlled by the acquisition unit 12, enabling the foci 43, 44 to be operated with different tube voltages, for example, 70 and 140 kV. Furthermore, rapid switching between the foci 43, 44 and the tube voltages is possible, for example, with a switching time of less than 1 ms. The X-ray foci 43, 44 can thus emit different emission spectra, arrows 47, at short intervals, i.e., quasi-simultaneously, from which the different X-ray spectra in the different components 24, 25 of the X-ray field 9 are provided by means of filters 48, 49. The emission spectra, and thus the partial radiation fields 50, 51, are separated by a central shading element 52.The width of the shading element is chosen to be slightly smaller than the focal distance 45, so that the partial beam fields 50, 51 of the X-ray beam field 9 on the X-ray detector 4 connect at least substantially to each other, thus allowing each half of the X-ray detector 4 to be used as a proportion 28, 29 for measuring the projection data of the X-ray spectra, as in the upper part of the . Fig. 8 is shown. This results in a recording process as described with regard to the Fig. 3 and Fig. 4A, Fig. As already explained in section 4B, changing filters 48 and 49 is not necessary due to the shading element 52.

[0060] Two specific configurations of the X-ray detector 4 are conceivable. Firstly, it is possible that both components are read out together in a single readout cycle, despite the successive exposure by the X-ray foci 43 and 44. Alternatively, at least for the first exposure, a shadow register of the X-ray detector 4 could be used to read out the data separately for both X-ray spectra and exposures. In this case, the unilluminated portion of the X-ray detector 4, comprising components 28 and 29 of the other spectrum, is also read out to record scatter radiation data. This data is then used for scatter radiation correction.

[0061] Fig. Figure 9 finally shows a general flow chart for exemplary embodiments of the method according to the invention.

[0062] In step S1, if the filter unit 18 or the filters 48, 49 need to be swung into position, the control unit 14 is used to move them into the beam path by means of the actuator 31. If the X-ray tube 3 needs to be moved into a special operating mode with respect to several foci 43, 44, or the X-ray detector 4, for example for reading into a shadow register, this can also be done by control using the control unit 11 in this preparatory step.

[0063] In step S2, the recording unit 12 rotates the recording arrangement around the recording area 5, specifically using the recording trajectory 26, and projection data for the two energy spectra are acquired in cone-beam geometry under different projection directions, here projection angles. The rotation is performed to cover a projection angle range of at least 360°. The division of the X-ray field 9 and the X-ray detector 4 remains constant due to the rotation of the entire recording arrangement (with the collimator and / or filter arrangement 16); nevertheless, the necessary coverage for complete reconstruction is achieved, as described. If separate readout is performed in the second embodiment and / or if other shadowed areas of the X-ray detector 4 are present due to collimation by means of the collimator 17, scatter radiation data are also acquired.

[0064] In the latter case of acquiring scatter radiation data, an optional step S3 uses the scatter radiation data from the respective shaded areas to determine a scatter radiation image for each set of projection data, which is then used to correct it for scatter radiation. This can be done in a dedicated correction unit (not shown here) or directly in the reconstruction unit 13 of the control unit 11.

[0065] In reconstruction unit 13, three-dimensional image datasets are reconstructed from the projection data for both energy spectra in a single step S4. In this process, the projection data of the other energy spectrum are also considered during the reconstruction of the three-dimensional image dataset of one energy spectrum, specifically in two ways: firstly, to create a truncation model, and secondly, to improve the correction of cone-beam artifacts. Both methods can be based on iterative approaches, in which initial preliminary datasets are first reconstructed from the respective sets of projection data. These preliminary datasets provide information on the extent of the acquisition area 5 or patient 7 and / or material information. This information can then be used to estimate missing projection data due to truncation and / or to formulate boundary conditions for the subsequent reconstruction.

[0066] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples and other variations can be derived by the person skilled in the art without leaving the scope of protection of the invention.

[0067] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

Claims

[1] Method for dual-energy imaging of an imaging area (5) with an X-ray device (1) comprising an imaging arrangement with an X-ray source (3) and an X-ray detector (4) for receiving X-rays from an X-ray field (9) emitted by the X-ray source (3) in cone beam geometry, which has a central beam (10), wherein the imaging arrangement is rotated around the imaging area (5) to acquire projection data of different projection directions for two different X-ray spectra and a three-dimensional image data set of the imaging area (5) is reconstructed from the respective projection data for each X-ray spectrum. wherein projection data of both X-ray spectra are recorded during the rotation covering at least 360°, wherein for each X-ray spectrum an assigned, in particular at least substantially half of the X-ray field (9), fixed portion (24, 25) of the X-ray field (9) and a corresponding, assigned, fixed portion (28, 29) of the X-ray detector (4) is used, wherein the X-ray source (3) has two foci (43, 44) spatially separated by a focus distance (45), assigned to different X-ray spectra, which follow each other in the plane of rotation (27), characterized by , that the foci (43, 44) are operated with different tube voltages and the partial beam fields (50, 51) emitted by the foci (43, 44) are separated by a shading element (52) arranged in the beam path between the foci (43, 44), in particular such that each half of the X-ray detector (4) is assigned to a partial beam field (50, 51) for illumination, that the foci (43, 44) are generated in the same X-ray tube (46) of the X-ray source (3), switching sequentially between the different tube voltages, and that the sequentially illuminated components (28, 29) of the X-ray detector (4) are read out in a common readout cycle. [2] Method according to claim 1, characterized by, that the X-ray detector (4) is divided into two sides along a central line perpendicular to the plane of rotation (27) through the point of impact of the central beam (10), the division into the components (28, 29) of the X-ray spectra being antisymmetric with respect to the central line. [3] Method according to claim 1 or 2, characterized by , that to define the components (24, 25, 28, 29) a filter device (18) arranged between the X-ray source (3) and the recording area (5) is used, which has a filter structure (19) through which the X-ray field (9) is irradiated, with a first component (20) for providing the first X-ray spectrum and a second component (21) for providing the second X-ray spectrum. [4] Method according to claim 3, characterized by, that the components (20, 21) of the filter structure (19) each comprise several areas (37, 38) that are separated from each other by areas (38, 37) of the other component (21, 20). [5] Method according to claim 4, characterized by , that the areas (37, 38) are formed as strips (39, 40) perpendicular to the plane of rotation (27), wherein in particular all strips (39, 40) have the same width and / or the strips (39, 40) have widths chosen to take into account the inverse distance law. [6] Method according to claim 5, characterized by , that the focus distance (45) is 0.5 to 5 mm and / or the width of the shadowing element (52) is smaller than the focus distance (45), in particular such that the partial beam fields (50, 51) on the detector surface of the X-ray detector (4) are at least substantially adjacent to each other. [7] Method according to any of the preceding claims, characterized by, that for the reconstruction of the image data set for one of the X-ray spectra, projection data of the other X-ray spectrum are taken into account. [8] Method according to claim 7, characterized by , that a first reconstruction of pre-image data sets is carried out from the respective projection image data, material information describing in particular a material distribution is determined from the pre-image data sets with regard to the recording area (5) and the material information is taken into account by at least one boundary condition and / or in an objective function in at least one further reconstruction from the respective projection data. [9] Method according to claim 7 or 8, characterized by , that projection data of both X-ray spectra are used to determine a truncation model, which is taken into account when reconstructing both image datasets. [10] X-ray equipment (1) comprising - a recording arrangement with an X-ray source (3) and an X-ray detector (4) for receiving X-rays from an X-ray field (9) emitted by the X-ray source (3) in conical beam geometry, which has a central beam (10), and - a control device (11) designed to carry out a procedure according to any of the preceding claims.

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