Method for carrying out dual-energy imaging on shooting area by using X-ray machine and X-ray machine

Abstract

The invention relates to a method for dual-energy imaging of a recording region (5) with an X-ray machine (1) having a recording device comprising an X-ray radiator (3) and an X-ray detector (4) for receiving an X-ray radiation field (9) having a central beam (10), said X-ray radiation field being emitted by the X-ray radiator (3) and having a conical radiation geometry, the recording device is rotated about the recording region (5) in order to record projection data of different projection directions for two different X-spectrums and to reconstruct a three-dimensional image data set of the recording region (5) from the respective projection data for each X-spectrum, the projection data of the two X-spectrums being recorded during a rotation covering at least 360 degrees, for each X-spectrum, a corresponding stationary part (24, 25) of the X-ray radiation field (9) in rotation is used, which in particular occupies half of the X-ray radiation field (9), and a corresponding stationary part of the X-ray detector (4) is used.

Description

Field of the Invention

[0001] The present invention relates to a method for dual-energy imaging of a region of interest using an X-ray machine, in particular a method implemented by a computer. The X-ray machine has an imaging device including an X-ray emitter and an X-ray detector for receiving the X-ray radiation of an X-ray radiation field having a conical beam geometry emitted by the X-ray emitter. The X-ray radiation field has a central beam. The imaging device rotates around the region of interest to acquire projection data of different projection directions for two different X-ray spectra, and a three-dimensional image dataset of the region of interest is reconstructed from the respective projection data for each X-ray spectrum. The present invention also relates to an X-ray machine. Background Art

[0002] In X-ray machines, such as those in the medical field, it is known to reconstruct a higher-dimensional image dataset from lower-dimensional projection images, in particular to reconstruct a three-dimensional image dataset (such as a stack of sectional views) from two-dimensional projection images. Advantageously, the imaging device with the X-ray emitter and the X-ray detector can move around the region of interest of the object to be imaged to acquire a plurality of projection images from different projection directions. Here, the X-ray emitter moves along an imaging trajectory, such as a circular trajectory.

[0003] Although there are various dedicated computed tomography imaging devices known in which the X-ray emitter and, if necessary, the X-ray detector are movable in a gantry, in the prior art it was provided that computed tomography-type imaging procedures were carried out using different X-ray machines, such as an X-ray machine with a C-arm, as is commonly used in angiography laboratories. This imaging method is also referred to as "DynaCT" and, due to the generally used conical beam geometry, is also referred to as "cone-beam CT" (CBCT). For the application of cone-beam CT in the dental field, reference can be made to the 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] Two different X-ray spectra are used in dual-energy imaging to acquire the corresponding X-ray images respectively. For example, a high-energy spectrum (which can be formed, for example, at a higher X-ray tube voltage) and a low-energy spectrum (which can be formed, for example, at a lower X-ray tube voltage) can be used. Usually, filters can also be used at different X-ray tube voltages to provide the corresponding X-ray spectra in a desired form. The filter can distinguish different X-ray spectra. Given the different spectral absorption properties of different materials or material types, such as bone and soft tissue, the distinction between materials / material types can be achieved in a common evaluation.

[0005] To combine CBCT and dual-energy imaging, various methods have been proposed in the prior art. For example, various techniques have been developed for rapidly switching between different X-ray tube voltages and different filters. Thereby, for example, it is possible to vary between different X-ray spectra during rotation in order to be able to take projection images of two X-ray spectra. However, this solution presupposes a large number of changes to conventional X-ray machines, especially C-arm X-ray machines, which entails a large amount of technical costs.

[0006] It is also known to use some methods, so-called dual-plane X-ray machines, which have two imaging devices operating with different X-ray spectra respectively. However, this solution can only be applied to dual-plane systems, which are not so preferred for various reasons, such as higher space requirements.

[0007] The first detector-side solution provides for the use of so-called multi-layer detectors (currently dual-layer detectors), which measure different X-ray spectra in different layers. However, for this purpose, there are dose drawbacks in non-spectral analysis imaging, and new detectors need to be developed.

[0008] The second detector-side solution uses photon counting detectors. However, this can only be implemented technically with difficulty as a planar detector, which brings high complexity and high costs for application in CBCT.

[0009] Finally, it has been proposed to perform the projection data for the two X-ray spectra strictly sequentially in time, so that first the imaging trajectory is run with the first X-ray spectrum and the projection data for this X-ray spectrum is taken, then the filter and tube voltage are slowly changed, and the imaging trajectory is run again. However, this results in a long imaging time and even a risk of motion artifacts during slow patient movement.

[0010] For spiral scans, classical computed tomography has proposed the use of so-called dual beams. For this purpose, a filter device is used, which divides the X-ray radiation field into two parts with different X-ray spectra. For example, DE 10 2008056 891 A1 discloses a computed tomography device for performing spiral scans. It includes a rotatable X-ray radiator and an X-ray detector positioned directly opposite, together with a matching evaluation unit. Downstream of the X-ray radiator, an X-ray filter is connected, and the position of the X-ray filter is associated with the position of the X-ray detector. By means of the X-ray filter, an unfiltered and a filtered ray part of the radiation sector are formed, wherein the ray parts have different X-ray spectra. In order to operate the computed tomography device in dual-energy mode, the evaluation unit evaluates the measurement signals of the unfiltered ray part separately from the measurement signals of the filtered ray part. Summary of the Invention

[0011] The technical problem to be solved by the present invention is to provide a possible way that can be simply applied to cone beam computed tomography (CBCT) and can achieve dual - energy imaging in a time - efficient manner while ensuring high - quality quantitative results.

[0012] According to the present invention, this technical problem is solved by the method according to the parallel claims, in particular a computer - implemented method and an X - ray machine. Advantageous improvements of the present invention are given by the dependent claims.

[0013] According to the present invention, in a method of the above type, it is provided that during a rotation covering at least 360°, projection data of two X - ray spectra are acquired, wherein for each X - ray spectrum, the corresponding, in particular at least substantially half of the X - ray radiation field, fixed part during the rotation is used, and the corresponding fixed part of the X - ray detector is used.

[0014] It is proposed that the rotation of the CBCT covering 360° completely is carried out around a fixed rotation axis in a fixed rotation plane, i.e., in a circular orbit as the acquisition trajectory of the X - ray radiator, so that even if the X - ray radiation field and thus the X - ray detector are divided for the X - ray spectra, coverage of the acquisition areas of the two X - ray spectra, in particular complete coverage, can be achieved. For this purpose, in particular, it can be provided that the X - ray detector, specifically its detection surface, is divided into two sides along a center line perpendicular to the rotation plane passing through the incident point of the central beam. The division of the respective parts of the X - ray spectra is antisymmetric with respect to this center line. This means that for each point corresponding to one of the X - ray spectra on one side of the center line, there is a point corresponding to the other X - ray spectrum at the same distance from the center line on the other side. In this way, it is utilized that during the rotation covering a 360° projection angle range, each X - ray beam can be verified to appear twice in the conical radiation geometry. With this design, it is ensured that each projection beam for each X - ray spectrum is measured, and thus a complete scan for two sets of projection data exists. Thereby, three - dimensional image data sets can be reconstructed for each X - ray spectrum, and after reconstruction, a common evaluation can be performed in order to obtain additional information due to the two X - ray spectra, in particular with respect to the material distribution.

[0015] The simplest case for forming such antisymmetry is to define the parts by dividing the X - ray radiation field in half perpendicular to the rotation plane passing through the central beam. In this case, it is provided that one side of the X - ray detector is irradiated by one X - ray spectrum and the other side is irradiated by the other X - ray spectrum. Nevertheless, as mentioned above, other division methods can also be suitable.

[0016] During the revolution of the imaging device around the imaging area, projection data is acquired synchronously for two X-ray spectra, at least within certain tolerances. In particular, using corresponding non-overlapping portions of the X-ray radiation field and the X-ray detector, projection data is acquired for two X-ray spectra at each imaging position. Here, synchronous acquisition of the projection data can be carried out, or, as will be described in more detail, sequential, yet extremely rapid, successive acquisitions, where the imaging position can be defined as being within an angular range during continuous movement of the imaging device. It should be noted here that technically, for 360° coverage, it is not necessary to rotate the entire 360° in terms of the projection angle. For example, when there is an angular distance from the imaging position, it is 360° minus this angular distance. For example, when the angular distance is 2°, rotating 358° is sufficient.

[0017] The X-ray machine is in particular a C-arm X-ray machine with a C-arm, on which an X-ray radiator and an X-ray detector are fixedly arranged opposite each other. The C-arm is rotatable, in particular around its central axis, in order to achieve a rotation covering a 360° projection angle range with a constant axis of rotation and a plane of rotation.

[0018] Thereby, as will be described in more detail, in an advantageous manner, a clearly visible change compared to conventional X-ray machines is achieved, which allows dual-energy CBCT in a simple and time-efficient manner.

[0019] Two possible embodiments of the present invention are described in more detail below in order to achieve, in an advantageous manner and method, a fixed division of the X-ray spectra into multiple parts during rotation.

[0020] In a first embodiment of the present invention, it can be provided that, in order to define the parts, a filter device arranged between the X-ray radiator and the imaging area is used, the filter device having a filter structure that is penetrated by the X-ray radiation field and comprising a first part for providing a first X-ray spectrum and a second part for providing a second X-ray spectrum. In this embodiment, a single focal point (focus) is used on the X-ray radiator side, and the formed X-ray radiation field is segmented and filtered, so that different parts of multiple X-ray spectra are formed on the X-ray detector after passing through the filter device. The advantage is that neither the tube voltage of the X-ray tube of the X-ray radiator nor the filter device needs to be static during the entire imaging process. In other words, in terms of hardware, only the filter device is introduced into the optical path to implement this first embodiment; no other modifications are necessary.

[0021] It should also be noted here that various design methods can also be considered, in which no or at least hardly any filtering is carried out for one of the X-ray spectra.

[0022] The filter structure can also be designed according to the above scheme, i.e., the filter structure is divided into two sides by a center line that extends perpendicular to the rotation plane and contains the penetration points of the central beam passing through the filter structure, and is selected and arranged such that for each point that provides a first X-ray spectrum on one side of the central beam, there is a point that provides a second X-ray spectrum at the same distance on the other side of the central beam. In other words, the filter structure is constructed anti-symmetrically so that, except for possible scattering effects and mechanical errors, all projection rays required for complete image reconstruction can be captured through the first X-ray spectrum and the second X-ray spectrum respectively.

[0023] In a simple scheme of the first embodiment, it can be considered that the first X-ray spectrum is provided completely on one side, while another X-ray spectrum is provided completely on the other side. In this case, one detector half or side of the X-ray detector shoots a different X-ray spectrum from the other detector half / side with respect to the center line of the X-ray detector. The filter device can then be composed of, for example, corresponding half-side filters to provide the corresponding X-ray spectra.

[0024] However, in an advantageous improvement of the present invention, it can be considered that the parts of the filter structure each include a plurality of regions, and the regions are separated from each other by regions of other parts. Here, the regions can be constructed as strips extending perpendicular to the rotation plane. All the strips can have the same width, where, however, it is particularly advantageous that the strips have widths selected in consideration of the inverse distance law. The widths of the strips can increase outward according to the inverse distance law due to the cone beam geometry. This means that the widths of the strips can be a function of the covered detector slits of the X-ray detector in order to consider the cone radiation geometry. In particular, as the distance from the central beam or the center line of the X-ray detector increases, the width also increases, thus considering the inverse distance law. However, when using two or more regions for each X-ray spectrum, the advantage of calibrating potential movements can be achieved.

[0025] However, within the scope of the first embodiment, it can be advantageously provided that the filter device is introduced into the optical path by an actuator before starting to shoot projection data. For example, the filter device can be a part of the filter plate of an X-ray machine. The control device of the X-ray machine can be constructed to implement the method, and the control device can have a control unit to control the actuator before the start of the shooting operation so that the filter device is pivoted into the optical path.

[0026] The filter structure can be constructed in a multi-layered manner. Specifically, it can be provided here that the multi-layered filter device has at least one base layer covering two parts. Thus, the base layer, such as a copper layer, can act on two parts and provide the filtration that should basically be used. Other layers, especially layers specific to the parts, modify the X-ray radiation spectrally in order to provide the desired X spectrum. Suitable filter materials include, for example, gold, silver, tantalum, tungsten, etc.

[0027] In an alternative second embodiment of the method according to the invention, it can be provided that the X-ray radiator has two foci that are spatially separated from each other by a focal distance and correspond to different X spectra, and the foci follow each other in the rotation plane, where the foci are operated with different tube voltages, and the partial radiation fields emitted by the foci are separated by a shielding element arranged between the foci in the optical path, in particular such that the halves of the X-ray detector each correspond to a partial radiation field for irradiation. Advantageously, the foci are arranged in the rotation plane at the same distance from the central beam (here imaginary) of the (entire X-ray radiation field). In the second embodiment, two X-ray foci are used, which in particular provide different emission spectra through different X-ray voltages (acceleration voltages), and the X spectra are provided by the emission spectra. A shielding element is arranged between the two foci in the radiation direction, and the shielding element can be referred to as a central collimator element, and the central collimator element absorbs the X-ray radiation and thus is responsible for separating the partial radiation fields. Two X-ray foci with different tube voltages are preferably formed at two different positions of the same X-ray tube, in particular by a rapid sequential switching of the tube voltage. The advantage of this type of design is that only a very rapid conversion of the tube voltage is required, which is easily achievable, while there is no need to replace filters or similar devices, which are often limiting factors for the duration. This is achieved in that a clearly defined geometry is formed by the offset foci and the shielding element, and the geometry spatially separates the partial radiation fields from each other, and thus different parts of the optical path and different parts of the X-ray detector are fixedly configured for them. In other words, further processing measures for providing the X spectrum can also be fixedly specified for the corresponding parts of the X-ray radiation field. For example, it can be provided that the X spectrum is adjusted by filters specific to the X spectrum fixedly introduced into the optical path of the corresponding partial radiation field.

[0028] The focal distance is preferably kept small and can be, for example, a few millimeters. Specifically, it can be provided that the focal distance is 0.5 to 5 mm.

[0029] It can be particularly advantageously provided that the width of the shielding element is less than the focal distance, in particular that partial radiation fields are at least substantially adjacent to one another on the detector surface of the X-ray detector, in particular on its center line. The central collimator element can be less wide than the focal distance between the X-ray foci, such that there is no unirradiated central strip or only a very small unirradiated central strip on the X-ray detector, which central strip can be, for example, approximately 1 m away from the X-ray radiator, in particular from the focus. It should be noted here that in the central region of the X-ray detector, even if there is a small amount of crosstalk between the X-ray spectra, this ultimately does not pose a problem, since only the spectral separation in this (small) region is prevented.

[0030] As previously mentioned, the foci are preferably formed in the same X-ray tube of the X-ray radiator, wherein the switching is sequential between different tube voltages. In other words, the foci and the tube voltage are switched synchronously. This also enables the easy implementation of a shorter focal distance and a correspondingly smaller shielding element.

[0031] In a first variant of the second embodiment using a single X-ray tube, it can be provided that the parts of the X-ray detector irradiated sequentially by the X-ray spectra are read in a common read cycle. In other words, this means that these two parts can be understood as sub-images on the X-ray detector, which two parts can be irradiated rapidly in sequence, wherein the X-ray detector can read the projection data of these two parts in a common read step. Advantageously, in this variant, there are no additional timing or read requirements for the X-ray detector. The only component of the X-ray machine that must be able to be switched rapidly is in this case the X-ray radiator, specifically the X-ray tube, in which there is a rapid switching between a focal position with a first tube voltage and another focal position with a second tube voltage.

[0032] However, in an alternative second variant, it can be considered that the parts of the X-ray detector sequentially irradiated by the partial radiation field are read out successively, in particular using a shadow register (German: Schattenregister) at least for the first readout process. It is also feasible to read the X-ray detector between two rapidly successive X-ray pulses of the corresponding X-ray spectrum, for example by means of an almost instantaneous readout of a CMOS detector into a shadow register. In this case, a particularly preferred improvement provides that with each partial readout, the parts of the X-ray detector not covered by the respective partial radiation field (including parts of another partial radiation field) are also read out together in order to obtain scatter radiation data, which are used for scatter radiation correction. In this design, the non-irradiated parts of the X-ray detector can be used for scatter radiation measurement. The scatter radiation data can then be applied to image correction. For example, it can be provided that from the scatter radiation data, a scatter radiation image can also be determined for the irradiated parts of the X-ray detector and, in particular, used for the correction of projection data by means of subtraction. In this way, improved scatter radiation correction and thus further improved image quality can be achieved.

[0033] Overall, for both the first and second embodiments, a preferred improvement of the invention provides that for the reconstruction of the image data set for one of the X-ray spectra, the projection data of another X-ray spectrum are taken into account. And even if the projection data of the other X-ray spectrum are not directly analogous to the projection data of this spectrum, they can still contain data useful for reconstruction, for example with regard to truncation and / or cone beam artifacts. Here, the projection data of the other X-ray spectrum are not directly incorporated into the reconstruction, but reconstruction information is derived from the projection data, which can be invoked, for example, as a boundary condition, a term of an objective function, etc. It can be shown that more reconstruction information is provided within the scope of the present invention than, for example, in the case of two successively performed standard CBCT scans rotated in an angular interval of, for example, 210°. Therefore, the use of this additional reconstruction information results in a significant improvement in the image quality in dual-energy CBCT. Especially in the case of iterative and / or multi-spectrum image reconstruction, more reconstruction information is provided.

[0034] Specific improvement measures of the present invention may provide that a first reconstruction of a preliminary data set is performed from corresponding projection image data, material information is determined from this preliminary data set in terms of the imaging range, in particular the material information describes the material distribution, and the material information is taken into account by at least one boundary condition and / or in the objective function during at least one further reconstruction from the corresponding projection data. Thus, a preliminary data set as a first three-dimensional reconstruction is first separated from the corresponding projection data of the X-ray spectrum. Generally speaking, material information can be derived from this preliminary data set, as is basically known for dual-energy imaging. The material information applies to the three-dimensional volume of the imaging range and can indicate how the projection image or the result of a more accurate reconstruction should look. Therefore, it is proposed to derive particularly flexible boundary conditions from the material information and / or modify them when using the objective function, in order to subsequently achieve improvements during at least one further reconstruction process and ultimately determine the image data set. In this way, artifacts of the cone-beam geometry (Cone-Beam-Artefakte) can be particularly reduced.

[0035] Furthermore, it is also feasible to use projection data of two X-ray spectra to determine a truncation model, in particular a spectral truncation model, which is taken into account during the reconstruction of two image data sets. For example, various techniques can be used, as proposed in the later published German patent application DE 10 2023 204 265.7. From the projection data, and in the case of using an estimation method for regions outside the main reconstruction volume that are not fully covered, the overall truncation model of the patient in the imaging-related region can be determined. In the truncated region of a single projection image of the projection data, the X-ray spectral absorption properties of individual voxels can be set based on previously performed material classification. By means of virtual forward projection using the truncation model with the first X-ray spectrum and the second X-ray spectrum, the truncated region of the corresponding projection image can be supplemented. Other methods can also be used in this context.

[0036] In addition to the above method, the present invention also relates to an X-ray machine, which has:

[0037] - An imaging device, which includes an X-ray radiator and an X-ray detector for receiving the X-ray radiation of the X-ray radiation field emitted by the X-ray radiator in a cone-beam geometry, the X-ray radiation field having a central beam, and

[0038] - A control device, which is designed to implement the method according to the present invention.

[0039] All embodiments in terms of the method according to the present invention can be transferred to the X-ray machine according to the present invention, and vice versa. The X-ray machine is preferably a C-arm X-ray machine with a C-arm, on which the X-ray radiator and the X-ray detector are arranged opposite each other.

[0040] The control device may have at least one processor and at least one memory. Functional units for carrying out the steps of the method according to the invention may be formed by hardware and / or software. In particular, the control device may have an imaging unit for controlling the 360° rotation of the imaging device and the capture of projection data, and a reconstruction unit for reconstructing the image dataset.

[0041] In the case of a first embodiment, the X-ray machine further has a filter device. In the case of an actuator provided for swinging the filter device, the control device may have a control unit for controlling the actuator. In the case of a second embodiment, an occlusion element is provided, and the imaging unit is also designed to control the X-ray radiator to form two focal points.

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

[0043] It should also be noted here that the use of different fixed parts of the X-ray radiation field or the X-ray detector during the rotation process proposed here is independent of the substantially feasible collimation of the X-ray radiation field achieved by the collimation device (collimator) of the X-ray machine. The collimation device uses, for example, lateral plates to adjust the size of the X-ray radiation field itself, while the present invention divides the interior of the X-ray radiation field, i.e., the actual area to be imaged, into multiple regions for multiple spectra and utilizes this. All necessary information still exists through the complete rotation. Description of the Drawings

[0044] Other advantages and details of the invention result from the embodiments described below and with the aid of the drawings. In the drawings:

[0045] Figure 1 A schematic diagram of the principle of an X-ray machine according to the invention is generally shown.

[0046] Figure 2 The design for providing an X-ray spectrum in the first embodiment of the first embodiment is shown.

[0047] Figure 3 A view showing the formed imaging geometry is shown.

[0048] Figure 4A 、 Figure 4B A view schematically showing the simultaneously occurring imaging process to illustrate the complete capture of projection data.

[0049] Figure 5 The possible multi-layer design of the filter device in the first embodiment is shown.

[0050] Figure 6 Schematic top view of the filter structure in the second embodiment showing the first embodiment

[0051] Figure 7 Shows the division of the X-ray detector formed in the second embodiment

[0052] Figure 8 Shows the design method of the optical path in an embodiment of the second embodiment, and

[0053] Figure 9 Shows a flowchart of an embodiment of the method according to the present invention. Detailed description of specific embodiments

[0054] Figure 1 Shows a schematic diagram of the principle of the X-ray machine 1 according to the present invention. The X-ray machine has a C-arm 2, and an X-ray radiator 3 and an X-ray detector 4 are arranged opposite to each other on the C-arm. The X-ray radiator 3 and the X-ray detector 4 constitute an imaging device. The C-arm 2 allows the imaging device to move at least in different rotational degrees of freedom. In this way, the rotation of the imaging device can be carried out especially around the imaging area 5 of the examination object 7 (here a patient) arranged on the hospital bed 6 of the X-ray machine 1 using a fixed rotation axis 8 and a fixed rotation plane (perpendicular to Figure 1 the drawing plane passing through the central beam 10) to cover a projection angle range of at least 360° (and even larger).

[0055] Here, an X-ray radiation field 9 can be emitted from the X-ray radiator 3 towards the X-ray detector 4 in order to take or collect projection data of the imaging area 5 in different projection directions, and the projection directions are defined by the central beam 10 of the X-ray radiation field 9. A cone beam radiation geometry is used here, so as to carry out a CBCT (cone beam CT) imaging process.

[0056] The operation of the X-ray machine 1 is controlled by a control device 11 which is only schematically shown. The control device 11 has an imaging unit 12 for controlling the imaging operation and has a reconstruction unit 13, by means of which a three-dimensional image dataset can be reconstructed from the projection data in different projection directions. In addition, a control unit 14 for controlling other components of the X-ray machine 1 is also provided. Different information, such as the taken projection data, is at least temporarily stored in the storage device 15.

[0057] Here, the X-ray machine 1 also includes a collimator and / or filter device 16 which is also roughly shown here, and its specific structure varies according to the embodiments and examples, as will be described in detail below. Here, the specific examples discussed below relate to different alternative ways for simplifying and advantageously implementing dual-energy CBCT on the X-ray machine 1.

[0058] In any case, the X-ray radiation field 9 and the X-ray detector here are divided into a plurality of parts, and the plurality of parts are fixed corresponding to different X-ray spectra during rotation. This means that X-rays of each X-ray spectrum are present in the parts of the X-ray radiation field 9, and the corresponding parts of the X-ray detector are irradiated with the X-ray spectrum (attenuated after passing through the imaging area 5).

[0059] Figure 2 Shows the design for providing X-ray spectra in the first embodiment according to the first embodiment. Accordingly, the collimator and / or filter device 16 includes, in addition to the substantially known collimator 17 (which sets the size of the X-ray radiation field 9), a filter device 18 with a filter structure 19, and this filter structure here includes a first part 20 and a second part 21 with different filter properties. In this embodiment, these parts 20, 21 are formed as half parts, so that on each side of the center line traversed by the central beam 10 of the filter device 16, a part 20, 21 is formed. Here, the center line is perpendicular to the rotation plane (fixed during the dual-energy CBCT imaging process), and the rotation plane is equivalent to the drawing plane in Figure 2 is equivalent to the drawing plane.

[0060] The X-ray radiator 3 provides the same emission spectrum (represented by the arrow 23) for the two parts from its (here exactly one) focal point 22, and the emission spectrum is projected onto the filter device 18. These parts 20, 21 are responsible for different filtering, so that after the filter device 18, the X-ray radiation field 9 is divided into a first part 24 of the first X-ray spectrum and a second part 25 of the second X-ray spectrum. This division is fixed during the entire rotation process of at least 360° in the dual-energy CBCT imaging process.

[0061] By Figure 3 Explaining this in detail again, the imaging trajectory 26 of the focal point 22 (here a circular orbit) when the imaging device rotates in the rotation plane 27 is also shown there. For the sake of simplicity, the filter device 18 is not shown, but nevertheless the division of the X-ray radiation field 9 into the parts 24, 25 is shown. Since this division is fixed, as Figure 3 shown, a fixed division is also formed on the detector surface of the X-ray detector 4, divided into a part 28 for imaging the projection data of the first X-ray spectrum and a part 29 for imaging the projection data of the second X-ray spectrum.

[0062] Since the imaging is synchronous, that is, with the same X-ray pulse of the same focal point 22, as Figure 4A and Figure 4BAs shown, two acquisition subprocesses are performed simultaneously: a complete 360° scan of the acquisition region 5 is shown by arrow 30 using a first X-ray spectrum (portion 24 of the X-ray radiation field 9) and one detector half (portion 28), and a complete 360° scan of the acquisition region 5 is shown by arrow 30 using a second X-ray spectrum (portion 25 of the X-ray radiation field 9) and another detector half (portion 29). It is known that the two acquisition subprocesses enable a complete scan with respect to the reconstruction of a three-dimensional image data set.

[0063] The filter device 18 can be provided, for example, as part of a filter plate by means of ( Figure 2 The actuator 31 (shown) is introduced into the optical path before the dual-energy CBCT imaging process begins and is controlled by the control unit 14 of the control device 11.

[0064] Figure 5 A possible multilayer structure of the filter structure 19 is shown. The filter structure has a base layer 32, for example made of copper, which acts on the two parts 24, 25. The part-specific layers 33 then provide the desired X-ray spectrum.

[0065] Figure 6 A variation of the filter structure 19 of the second exemplary embodiment as the first embodiment is shown, which can be advantageous in terms of motion correction, and the description of the first exemplary embodiment applies accordingly. Here, not all sides 34, 35 starting from the center line 36 of the filter device 18 correspond to the provision of the X-ray spectrum in a coherent manner, but the parts 20, 21 include regions 37, 38 of strips 39, 40 which are constructed perpendicular to the rotation plane 27. Thus, a type of comb-like structure is formed. As already described in the first exemplary embodiment, an antisymmetry about the center line 36 is set, which means that for each point on one of the sides 34, 35 of the filter structure 19 that provides the first X-ray spectrum, there are points on the other side 35, 34 of the filter structure at the same distance from the center line that provide the second X-ray spectrum.

[0066] Figure 7 The respective strip-shaped regions 41 , 42 which divide the detector surface of the x-ray detector 4 into the sections 20 , 21 are shown.

[0067] Figure 8An embodiment of the second embodiment is shown. Here, an X-ray radiator 3 is used, which has two focal points 43, 44 that are offset outward from the (hypothetical) central ray 10 of the X-ray radiation field 9 by the same small distance, for example 0.25 to 2.5 mm, relative to the equally hypothetical central focal point in the X-ray tube 46 of the X-ray radiator 3. The focal point spacing 45 formed between the thus formed focal points 43, 44 can be, for example, 0.5 to 5 mm. This is smaller compared to the focal point - detector spacing (for example 0.8 to 1.2 m).

[0068] The X-ray tube 46 of the X-ray radiator 3 can be controlled by the imaging unit 12 so that the focal points 43, 44 operate at different tube voltages, for example 70 kV and 140 kV. In addition, rapid switching can also be carried out between the focal points 43, 44 and between different tube voltages, for example with a switching time of less than 1 ms. The X-ray focal points 43, 44 can emit different emission spectra at short time intervals, almost simultaneously, as shown by the arrows 47. Different X-ray spectra are provided in different parts 24, 25 of the X-ray radiation field 9 by means of the filters 48, 49 from the emission spectra. Here, the emission spectra and thus the partial radiation fields 50, 51 are separated by a central shielding element 52. The width of the shielding element can be selected to be slightly less than the focal point spacing 45, so that the partial radiation fields 50, 51 of the X-ray radiation field 9 can be at least substantially adjacent to each other on the X-ray detector 4, that is, each half of the X-ray detector 4 can be used as parts 28, 29 for measuring the projection data of the X-ray spectra, as Figure 8 shown in the upper region. Thus, the imaging process already explained in Figure 3 and Figure 4A 、 4B is formed. Due to the shielding element 52, the transformation of the filters 48, 49 is unnecessary.

[0069] Here, two specific design methods for the X-ray detector 4 can be considered. On the one hand, it is feasible that the two parts can be read together in the read cycle even when irradiated by the X-ray focal points 43, 44 gradually. However, it is also feasible to use the shadow register of the X-ray detector 4 at least for the first irradiation process in order to read separately for the two X-ray spectra and irradiations. In this case, the corresponding non-irradiated part of the X-ray detector 4 (including parts 28, 29 of other spectra) can also be read in order to image the scattered radiation data. The scattered radiation data is used for scattered radiation correction.

[0070] Finally, Figure 9 a general flow of an embodiment of the method according to the invention is shown.

[0071] Here, in step S1, when the filter device 18, or rather the filters 48, 49, has to be pivoted in, the control unit 14 is used to introduce it into the optical path by means of the actuator 31. If the X-ray radiator 3 has to be changed with respect to a plurality of focal points 43, 44 or the X-ray detector 4, for example, has to be switched to a specific operating mode for reading in a shadow register, this can also be done in this preparatory step by means of the control by the control device 11.

[0072] In step S2, the imaging device is rotated by means of the imaging unit 12, in particular around the imaging area 5 when using the imaging trajectory 26, and projection data are taken in a conical radiation geometry for two energy spectra in different projection directions, here the projection angles. Here, the rotation is carried out to cover a projection angle range of at least 360°. Since the entire imaging device (with collimator and / or filter device 16) rotates, the division of the X-ray radiation field 9 and the X-ray detector 4 always remains the same; nevertheless, the coverage necessary for a complete reconstruction is achieved, as described above. If separate readings are to be carried out in the second embodiment and / or there are other occluded areas of the X-ray detector 4 by means of collimation by means of the collimator 17, scattered radiation data can also be taken.

[0073] For the latter case of taking scattered radiation data, in an optional step S3, the scattered radiation data from the corresponding occluded areas are used to determine a scattered radiation image for each set of projection data, which scattered radiation image is used for its correction with respect to scattered radiation. This can be done in particular in a dedicated correction unit (not shown in detail) or directly in the reconstruction unit 13 of the control device 11.

[0074] In step S4, a three-dimensional image dataset is reconstructed from the projection data for two energy spectra in the reconstruction unit 13. Here, the projection data of the corresponding other energy spectrum can also be taken into account in the reconstruction of the three-dimensional image dataset of an energy spectrum, and specifically, two types are involved here. On the one hand, it is for establishing a truncated model, and on the other hand, it is for improving the correction of cone beam artifacts. These two types can be based on iterative methods, where first a first preliminary dataset is reconstructed from the corresponding set of projection data, and information about the size of the imaging area 5 or the patient 7 and / or material information can be derived from the first preliminary dataset. This can be used to estimate the missing projection data due to truncation and / or to express boundary conditions for subsequent reconstruction.

[0075] Although the details of the present invention have been illustrated and described in detail by means of preferred embodiments, the present invention is not limited by the disclosed examples, and those skilled in the art can derive other different variants therefrom as long as they do not depart from the scope of protection of the present invention.

Claims

1. A method for dual-energy imaging of an imaging region (5) using an X-ray machine (1), the X-ray machine having an imaging device, the imaging device comprising an X-ray radiator (3) and an X-ray detector (4) for receiving X-ray radiation of an X-ray radiation field (9) with a conical radiation geometry emitted by the X-ray radiator (3), the X-ray radiation field having a central beam (10), wherein: The photographing device rotates around the photographing region (5) to photograph projection data in different projection directions for two different X-ray spectra, and reconstructs a three-dimensional image data set of the photographing region (5) from the corresponding projection data for each X-ray spectrum. It is characterized in that projection data of two X-ray spectra are acquired during a rotation covering at least 360°, wherein for each X-ray spectrum, a corresponding fixed part (24, 25) of the X-ray radiation field (9) in rotation is used, which fixed part in particular at least substantially occupies half of the X-ray radiation field (9), and a corresponding corresponding fixed part (28, 29) of the X-ray detector (4) is used.

2. The method according to claim 1, characterized in that The X-ray detector (4) is divided into two sides along a center line extending perpendicularly to the rotation plane (27) through the incident point of the central beam (10), wherein the X-ray spectrum is divided into a plurality of parts (28, 29) in an anti-symmetrical manner with respect to the center line.

3. The method according to claim 1 or 2, characterized in that: To define the portions (24, 25, 28, 29), a filter device (18) is used which is arranged between the X-ray radiator (3) and the recording region (5), the filter device having a filter structure (19) which is transmitted by the X-ray radiation field (9) and comprises a first portion (20) for providing a first X-ray spectrum and a second portion (21) for providing a second X-ray spectrum.

4. The method according to claim 3, characterized in that Parts (20, 21) of the filter structure (19) each include a plurality of regions (37, 38) which are separated from one another by regions (38, 37) of other parts (21, 20).

5. The method according to claim 4, characterized in that The regions (37, 38) are designed as strips (39, 40) extending perpendicularly to the rotation plane (27), wherein in particular all strips (39, 40) have the same width and / or the strips (39, 40) have a width selected to take into account the inverse distance law.

6. The method according to claim 1 or 2, characterized in that: The X-ray radiator (3) has two focal spots (43, 44) which are spatially spaced apart from each other by a focal distance (45) and correspond to different X-ray spectra, the focal spots following each other in a rotation plane (27), the focal spots (43, 44) being operated with different tube voltages, and the partial radiation fields (50, 51) emitted by the focal spots (43, 44) being separated by a shielding element (52) arranged between the focal spots (43, 44) in the beam path, in particular so that one half of the X-ray detector (4) is respectively assigned to a partial radiation field (50, 51) for irradiation.

7. The method according to claim 6, characterized in that The focal distance (45) is 0.5 to 5 mm and / or the width of the shielding element (52) is smaller than the focal distance (45), in particular so that the partial radiation fields (50, 51) at least substantially abut one another on the detector surface of the X-ray detector (4).

8. The method according to claim 6 or 7, characterized in that: The focal spots (43, 44) are formed in the same x-ray tube (46) of the x-ray radiator (3), wherein switching is performed sequentially between different tube voltages.

9. The method according to claim 8, characterized in that The parts (28, 29) of the X-ray detector (4) which are sequentially irradiated with the X-ray spectrum are read out in a common read-out cycle.

10. The method according to claim 8, characterized in that Parts (28, 29) of an x-ray detector (4) which are sequentially irradiated by partial radiation fields (50, 51) are read out one after the other, in particular using a shadow register for at least a first readout process.

11. The method according to claim 10, characterized in that As each portion (28, 29) is read, portions of the X-ray detector (4) not covered by the corresponding partial radiation field (50, 51), and portions (29, 28) including another partial radiation field (51, 50) are also read to obtain scattered radiation data, which is used for scattered radiation correction.

12. The method according to any one of the preceding claims, characterized in that In each case, the image data set of one X-ray spectrum is reconstructed while the projection data of the other spectrum are taken into account.

13. The method according to claim 12, characterized in that A first reconstruction of a preliminary data set is carried out from the corresponding projection image data, from which material information is determined with respect to the acquisition region (5), which material information in particular describes the material distribution and is taken into account via at least one boundary condition and / or in a target function during at least one new reconstruction from the corresponding projection data.

14. The method according to claim 12 or 13, characterized in that The projection data of the two X-ray spectra are used to determine a truncation model which is taken into account in the reconstruction of the two image data sets.

15. An X-ray machine (1) having - a recording device comprising an X-ray radiator (3) and an X-ray detector (4) for receiving X-ray radiation emitted by the X-ray radiator (3) in an X-ray radiation field (9) with a conical radiation geometry, the X-ray radiation field having a central beam (10), and - A control unit (11) which is designed to carry out the method according to any of the preceding claims.

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