Method for recording projection images for dual-energy imaging and x-ray device

Abstract

The invention relates to a method for recording projection images for dual-energy imaging, at least one pair of projection images of an examination object, which are associated with one another, are recorded using different X-ray spectra, and an X-ray device is used having a recording device which comprises an X-ray radiator and an X-ray detector, in order to capture each pair of projection images associated with one another, two focal points of the X-ray radiator, which are spaced apart in the middle of the movement plane, are used, recording a first projection image of the pair at a first radiator position of the X-ray radiator using a first X-ray spectrum emitted from the first focal point, moving the X-ray radiator into a second radiator position such that the second focal point lies at least within a tolerance range with respect to the position of the first focal point in the first radiator position, and-taking a second projection image of the pair at a second radiator position using a second X-ray spectrum emanating from a second focal point.

Description

Technical Field

[0001] The invention relates to a method for recording projection images for dual-energy imaging, wherein at least one pair of associated projection images of an object under examination is recorded by means of different X-ray spectra and an X-ray device is used which has a recording device comprising an X-ray radiator and an X-ray detector in a manner that they are movable relative to each other and can be moved together in a motion plane, in particular can be rotated in a rotation plane. The invention also relates to an X-ray device. Background Art

[0002] In X-ray dual-energy imaging, projection images are taken using different X-ray spectra, but usually using the same projection geometry. The X-ray spectrum may include, for example, a high-energy spectrum and a low-energy spectrum. Possible image processing measures for such a pair of projection images include a combination of projection images, in particular a linear and / or weighted combination. Dual-energy imaging is also often used in computed tomography (CT), in which a high-dimensional image data set is reconstructed from low-dimensional projection images of different projection geometries (in particular, at different projection angles in the circular shooting trajectory of the X-ray radiator). In many applications, image processing of the projection image pair also occurs first before reconstruction.

[0003] In computed tomography, dual-energy imaging approaches that gradually acquire projection images of different X-ray spectra after a continuous movement of the X-ray emitter (and usually also the X-ray detector) usually along an acquisition trajectory present special challenges.

[0004] Basically, there are different approaches to dual-energy imaging. Thus, on the one hand, it is conceivable to generate different X-ray spectra continuously with an X-ray radiator and record projection images. It is also possible to use different spectral filters continuously, i.e. to introduce them into the beam path. Here, it has been proposed to use techniques for fast switching of the tube voltage and / or for fast switching of filters in order to generate X-ray pulses of different X-ray spectra in a chronological order at least during the movement of the X-ray radiator. However, in this case, there is an offset between the two projection images, in particular a rotation-related offset, which makes the evaluation more difficult and produces additional artifacts.

[0005] In the prior art, some solutions have been proposed to solve this problem. Thus, a double-layer detector is able to take projection images of two different X-ray spectra in one shot, so there is no problem of rotational offset. However, there is the problem of poor spectral separation. In addition, the dose efficiency is low.

[0006] In the case of photon counting detectors, it is also possible to record projection images of both X-ray spectra in one shot, but such detectors (particularly with regard to the use of X-ray devices with a C-arm) are expensive and challenging to manufacture.

[0007] It is also recommended to carry out two separate, consecutive exposure operations along the same exposure trajectory, once with the first X-ray spectrum and once with the second X-ray spectrum. However, this method is extremely sensitive to movements of the object under examination, in particular the patient, and to deformations of the anatomy. When using a C-arm, problems may arise with regard to the exact reproducibility of the exposure trajectory. Finally, long breath holds are required. Therefore, this method can only be used for at least almost static scenes, such as in neurology.

[0008] However, if a rapid change of the X-ray spectrum is used (e.g. a rapid switching of the tube voltage and / or a rapid switching of the filters) in order to record pairs of projection images in chronological order during a rotation, it is also advisable to first perform a three-dimensional image reconstruction, optionally using a model-based and / or iterative method, in order to avoid a direct combination or joint image processing of the pairs of projection images, in particular pixel by pixel. A separate reconstruction of the X-ray spectrum is also conceivable in order to subsequently combine the results. Additionally or alternatively, a 2D motion compensation can be performed between the two projection images. However, between the recording of the projection images, the focus of the X-ray radiator has moved considerably, for example by several millimeters. Therefore, the motion compensation may not be optimal, since the X-rays have taken physically different paths through the examination object.

[0009] In another context, a concept has been proposed in which, in spiral CT, two different focal spots with the same X-ray spectrum are used in a direction perpendicular to the direction of rotation (longitudinal direction of the spiral, z-direction) so that two offset spirals are ultimately scanned. In this way, the resolution in the longitudinal direction of the spiral can be increased, the dose efficiency can be increased, and spiral artifacts can be reduced, for example in neuro computed tomography. In this regard, purely as an example, please refer to the lecture "Double Z Sampling: How Does It Work and What Does It Do?" by MR Bruesewitz et al., RSNA 2005, available at https: / / www.mayo.edu / research / documents / rsna2005-doublez-samplingpdf / DOC-10027306. Summary of the invention

[0010] The object of the present invention is to provide a variant for recording projection images for dual-energy imaging, in particular within the context of computed tomography.

[0011] In order to solve this technical problem, according to the invention, a method and an X-ray device, in particular a computer-implemented method, are provided.

[0012] A method of the type mentioned at the outset provides two focal points of an X-ray radiator which are spaced apart from one another in a plane of movement, in particular a plane of rotation, for recording each pair of associated projection images, wherein:

[0013] - taking a first projection image of the pair at a first radiator position of the X-ray radiator using a first X-ray spectrum emitted from a first focus,

[0014] - by moving, in particular rotating, the recording device in a plane of movement, in particular a plane of rotation, the X-ray radiator is moved into the second radiator position in such a way that a second of the focal points is at least within a tolerance range with respect to the position of the first focal point in the first radiator position, and

[0015] - recording a second projection image of the pair at a second radiator position of the X-ray radiator using a second X-ray spectrum emitted from the second focus.

[0016] In this case, the X-ray emitter is moved in particular along a circular path in the projection plane, and in particular when only one pair is recorded, a correction of the position of the X-ray emitter in the projection direction is also conceivable. However, in principle, other recording trajectories are also conceivable, along which the X-ray emitter can be moved, in particular when it is moved continuously to record several pairs of projection images.

[0017] Preferably, during recording operation, in particular during a computed tomography recording, a plurality of pairs of mutually associated projection images are recorded for different projection geometries along a single recording trajectory of the X-ray radiator, in particular at a constant movement speed of the X-ray radiator.

[0018] The invention is particularly preferably used for a computed tomography examination of an examination object, in particular a patient, wherein an X-ray emitter, in particular an X-ray detector, is continuously moved along a recording trajectory. In this case, the recording trajectory is at least substantially circular and / or covers a projection angle range of a sector angle of more than 180°. In this case, an X-ray device with a C-arm can be used, on which the X-ray emitter and the X-ray detector are arranged opposite each other. Alternatively, the X-ray device can also be a computed tomography device, in which the X-ray emitter and the X-ray detector are guided in a gantry.

[0019] In principle, however, other fields of application are also conceivable with other forms of acquisition trajectories, such as tomosynthesis, in particular tomosynthesis of the female breast, wherein, for example, linear acquisition trajectories are also conceivable and, if necessary, the second focus can even take the exact position of the first focus. Although the invention relates primarily to circular acquisition trajectories, it is not restricted thereto.

[0020] Therefore, a basic idea of ​​the invention is to use two focal points of an X-ray radiator, which are however offset in the direction of movement, i.e. in a rotational movement perpendicular to the projection direction (direction of the central beam) and the rotation axis. The focal points are physically offset relative to each other on the X-ray radiator so that the focal distance between the focal points exists in the moving coordinate system (of the X-ray radiator) even if the X-ray radiator moves. Each focal point is used to emit one of the two X-ray spectra. In this case, specific recording parameters, in particular the spatial distance of the focal points (focal distance) and / or the movement speed, in particular the rotation speed (angular speed of the X-ray radiator) and / or the time interval between the X-ray pulses of the energy spectrum, are selected so that due to the movement of the X-ray radiator, when an X-ray pulse of the second energy spectrum is emitted from the second focal point, the second focal point is at least substantially located at the position (in the fixed coordinate system relative to the examination object) where the first focal point was when the X-ray pulse of the first energy spectrum was emitted. Therefore, in the fixed coordinate system (of the X-ray device / examination object), due to the movement of the X-ray radiator, the second focal point for imaging is located at the position of the first focal point (for its imaging).

[0021] Here, it is preferred to use known techniques for fast switching of focus and X-ray spectrum, such as are known from z-overscanning in spiral CT, in particular fast switching of the tube voltage and / or fast switching of filters, such as are known from CT techniques using a single focus.

[0022] Therefore, fast focus switching is used to generate two temporally consecutive and spatially separated focus positions, for example on a rotating anode of an X-ray radiator. A different X-ray spectrum is generated at each focus, in particular by using different tube voltages and / or spectral filters. The two focal spots are positioned along the direction of movement, in particular the direction of rotation. Against the background of other acquisition parameters, in particular the focal spacing, the temporal course of the application of the X-ray pulses is selected such that the deviation between the projection geometry of the two focal spots at the acquisition time is small, in particular minimal, or even at least almost eliminated.

[0023] Thus, an X-ray radiator, in particular an X-ray radiator comprising an X-ray tube, has the ability to switch between two focal points in a time sequence and also to select the used X-ray spectrum in a focus-specific manner, for example by adjusting the tube voltage. For example, in the case of an X-ray radiator with a rotating anode, two defined paths on the rotating anode can be provided for the focal points. In order to generate electron beams for different focal points, different radiators and / or field generators can be used to generate electric fields that influence the paths of electron movement. Alternatively or additionally, for example, when two paths are used on a rotating anode, they can be provided with different materials.

[0024] One of the key points of the invention is the target-oriented selection of imaging parameters, in particular the relationship between the focus position, the direction of movement and the time course of the X-ray pulses.

[0025] This scheme of using a variable focus during the shooting process, especially during the continuous movement of the X-ray radiator along the shooting trajectory, is also called a flying focal spot. In this regard, the concept of the present invention can be called a flying focus with different X-ray spectra in the direction of movement. The motion offset, especially the rotation offset, i.e. the angular offset, between temporally adjacent X-ray pulses of two different X-ray spectra used to generate a pair of X-ray images is reduced or eliminated. This means that, despite the continuous and continuous movement, especially rotation, of the X-ray radiator in the coordinate system of the examination object (such as a patient or a phantom), a pair of continuous X-ray pulses of different X-ray spectra (especially high energy spectrum and low energy spectrum) can be generated at least approximately at the same physical position in the fixed coordinate system (of the X-ray device / examination object).

[0026] In this way, a simpler dual energy evaluation for dual energy imaging is achieved, in particular in computed tomography, preferably using a C-arm.Computed tomography with a C-arm in cone beam geometry is also known as cone beam computed tomography (CBCT - cone beam computed tomography).

[0027] The object of the invention is to obtain an optimally matched projection geometry for a pair of projection images. In this context, it is particularly important to note that, in the case of a rotational movement and conventional design of the X-ray radiator and the X-ray detector, an exact correspondence of the positions of the focal points at the corresponding acquisition times is not possible. However, if the best possible agreement is sought, a secondary goal may also be considered, which will be discussed in more detail below.

[0028] It has generally proven to be expedient if the tolerance range is defined (of course in a fixed coordinate system) such that the distance between the second focus of the second radiator position and the first focus of the first radiator position is less than 35%, in particular less than 10%, of the displacement distance of the X-ray radiator between the first radiator position and the second radiator position. In this way, a significant improvement is provided with regard to the coprocessability of the paired projection images, in particular compared to solutions with a single focus and a continuously moving rapid switching of the X-ray spectrum. The consistency of the position of the focus is more important in the direction of movement (in particular during rotation) than in the direction of projection, so that it can be particularly advantageous to provide that the distance between the second focus in the second radiator position and the first focus in the first radiator position in the direction of movement, in particular in the direction of rotation, is less than in the direction of projection, in particular at most half. For example, it can be provided that the distance between the second focus in the second radiator position and the first focus in the first radiator position in the direction of movement is less than 5%.

[0029] In an advantageous development of the invention, provision can be made that, with the X-ray radiator continuously moving at an at least partially constant speed of movement, in particular at an at least partially constant rotational speed, and with the focal distance of the focal points of the X-ray radiator being preset, the time interval between the output of the first and second X-ray pulses for each pair of projection images is selected such that the second focal point is at least within a tolerance range with respect to the position of the first focal point in the first radiator position. In particular, by suitable selection of the time sequence, i.e. the timing, of the X-ray pulses at a predetermined movement speed and a predetermined focal distance, a desired close positioning of the focal positions relative to one another can be achieved at the respective recording times. Further parameters can also be entered. In particular, within the scope of the rotation, the distance of the X-ray radiator from the isocenter of the X-ray device, i.e. the center about which it rotates, is also reduced here.

[0030] This will be explained in more detail using an example. If it is assumed that the distance between the focus of the X-ray radiator and the isocenter is 650 mm, the length of the circular recording trajectory is, for example, over 200°, 2270 mm. For example, during such a rotation, a total of 500 projection images, i.e. 250 pairs, are recorded. If the focal distance is 4.5 mm, a series of recording times can be determined with a known rotation speed so that the X-ray radiator moves by said 4.5 mm between two projection images (at least within the tolerance range).

[0031] In general, for a rotational movement, it can be said that the focal distance, the distance of the X-ray radiator (or more specifically, the focus) from the isocenter, the rotation speed and the time interval between the pulses are matched to one another in such a way that the second focus is at least within a tolerance range with respect to the position of the first focus in the first radiator position.

[0032] In one embodiment, it can be provided that the movement of the recording device, in particular the rotational movement and / or the recording parameters describing the recording of the first and second projection images and / or the focal distance are selected in the optimization operation such that the distance of the second focal point of the second radiator position from the first focal point of the first radiator position is minimized. Thus, in this embodiment, an ideal solution is determined, wherein the optimization algorithm can also be used for more complex situations. Here, boundary conditions can also be formulated depending on which variables exactly form the parameters to be optimized. In general, an advantageous embodiment can be provided in which, during the optimization, as a boundary condition, a displacement of the X-ray detector in the image plane of one of the projection images due to the movement of the recording device corresponds to an integer multiple of the pixel size of the pixels of the X-ray detector. In this way, a movement of the X-ray detector between the recording of the projection images (even already occurring) can be taken into account particularly easily, which leads to a displacement of the image information corresponding to each other in the coordinate system of the X-ray detector.

[0033] Before discussing this in more detail, it should also be pointed out that it is generally advantageous to carry out an adaptation of the image content within each pair in such a way that the same part of the examination object is displayed whenever the area of ​​the examination object covered by the radiation field changes due to a movement of the X-ray detector. In particular, it can be provided that image content contained in only one of the pair of projection images due to a movement of the recording device is discarded, in particular at least for the joint evaluation of the pair of projection images.

[0034] In order to prepare a joint evaluation of each pair of projection images, it can also be provided that each pair of projection images is shifted relative to one another by a displacement of the X-ray detector in the image plane of one of the projection images in order to compensate for the movement of the X-ray detector. In particular, in the case of a joint rotational movement of the X-ray radiator and the X-ray detector, the X-ray detector is also moved with the X-ray radiator at least substantially shifting the focal length. The displacement thus occurring can be compensated by a corresponding displacement of one of the projection images, so that the same pixel at least substantially corresponds to the same beam path. As described above, a particularly simple correction possibility is obtained if the displacement of the X-ray detector in the image plane due to the movement of the recording device corresponds to an integer multiple of the pixel size of a pixel of the X-ray detector. In other cases, more complex algorithms, in particular interpolation algorithms, can also be used.

[0035] It should be noted here that during the rotational movement, the X-ray detector may also tilt. However, in computed tomography and other applications, in which a pair of projection images are captured briefly in succession by rapid switching, this tilt is so small that it does not require any special correction, especially when the tilt between the capture of the projection images is less than 0.5°.

[0036] However, embodiments of the invention, in particular in the case of tilt angles greater than 0.5°, may provide that, in the case of rotational motion, the tilt of the X-ray detector is also compensated for the rotational motion by applying a compensation algorithm to each pair of projection images. In other words, for example, more complex image distortion methods may be applied to compensate for (usually small) rotations of the X-ray detector.

[0037] It should be noted that in exemplary embodiments it is also conceivable to perform a general compensation of detector position variations by applying a registration algorithm.

[0038] As already mentioned, the joint evaluation of a pair of projection images can in particular provide that during the evaluation an evaluation image is determined from each pair of projection images by a combination of the projection images (in particular linearly and / or weighted and / or pixel-by-pixel). Thus, in particular, the shift is performed pixel-by-pixel, which can be performed in a simplified manner by a corresponding prior compensation of the displacement of the X-ray detector. The pixel-by-pixel combination can in particular be a subtraction, but other combinations, in particular a weighted linear combination, are also conceivable.

[0039] In an advantageous refinement of the invention, a spectral filter of the X-ray radiator can be provided for at least one focal point of the X-ray radiator. By means of the spectral filter, the X-ray spectrum can be modified to achieve a desired shape. Although methods for quickly changing filters are basically known, it is particularly advantageous to fix the spectral filter on the X-ray radiator so that it only acts on the corresponding focal point. It can therefore be specifically provided that at least one spectral filter is fixedly assigned to the corresponding focal point, in particular fixedly mounted in the X-ray radiator. For example, the fixedly mounted spectral filter can be arranged on the outlet window of the X-ray radiator or can extend in an annular manner on a rotating anode so as to rotate together with the rotating anode.

[0040] In a particularly advantageous refinement, in particular with regard to the X-ray device, provision can be made that an X-ray radiator is used which is rotatable about the projection direction and, in order to record the projection image, is rotated or kept rotating in such a way that the focal points follow each other in the direction of movement, in particular the direction of rotation. In particular, this makes it possible to use the same X-ray device to also carry out other applications in which the focal points should follow each other in other directions. Thus, for example, it can be provided that in the second rotational position the focal points follow each other in the z direction (longitudinal direction) perpendicularly to the projection direction and the direction of rotation, so that projection images displaced relative to each other in the z direction can be recorded, in particular using the same X-ray spectrum and / or within the framework of a spiral CT. The corresponding examination feasibility has already been described at the beginning. Thus, a multi-purpose X-ray device is obtained.

[0041] As mentioned above, in addition to the method, the present invention also relates to an X-ray device, which includes an image acquisition device, which includes an X-ray radiator and an X-ray detector in a manner that is opposite to each other, in particular on a C-arm and can be moved together in a movement plane, in particular rotated in a rotation plane, and a control device designed to carry out the method according to the invention. All statements about the method according to the invention can be transferred analogously to the X-ray device according to the invention, whereby the advantages already mentioned can also be obtained.

[0042] In particular, the control device has at least one storage device and at least one processor. The functional units can be formed by hardware and / or software in order to implement the steps of the method according to the invention and, if necessary, also its embodiments. In particular, the control device can include, for example, a basically known recording unit, which can control the recording activity of the recording device and its movement. In particular, when two focal spots of the X-ray radiator are used to record each pair of associated projection images, the recording unit can also record the first projection image at the first radiator position using the first focal spot and the second projection image at the second radiator position using the second focal spot (with the corresponding first and second X-ray spectra), and the recording device is moved in the movement plane of the X-ray radiator to the second radiator position so that the second focal spot in the focal spot is at least within the tolerance range of the first focal spot position with respect to the first radiator position. The control device can also include a determination unit for recording parameters, in particular the X-ray pulse time course, so that when the recording parameters are used, a corresponding recording process is generated for each pair of projection images. In addition, an image processing unit can also be provided, in particular for compensating for detector movements between the recording moments of a pair of projection images. Here, in particular, as already described, the offset ("shifting") is corrected. An evaluation unit for joint evaluation, in particular combining the projection image pairs, may also be provided. With regard to the rotatability of the X-ray radiator, in particular the z-shift of the projection images, an adjustment unit for rotating the X-ray radiator may also be provided.

[0043] The method according to the invention can also be implemented by a computer program having program means which, when the computer program is executed in a control device of an X-ray device, cause the X-ray device to carry out the steps of the method according to the invention. The computer program can be stored on an electronically readable data carrier, which thus has control information stored thereon, which includes at least one such computer program and is configured in such a way that, when the data carrier is used in the control device of an X-ray device, the latter is configured for carrying out the method according to the invention. The data carrier can in particular be a non-transitory data carrier, for example a CD-ROM. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Further advantages and details can be derived from the exemplary embodiments described below and from the drawings. In the drawings:

[0045] Figure 1 shows a flow chart of an exemplary embodiment of a method according to the invention,

[0046] Figure 2 A projection geometry for capturing a pair of first projection images is shown;

[0047] Figure 3 showing a projection geometry for capturing a pair of second projection images;

[0048] Figure 4 A schematic diagram showing an X-ray device according to the present invention; and

[0049] Figure 5 The functional structure of a control device of an X-ray device is shown. DETAILED DESCRIPTION

[0050] Figure 1 A flow chart of an exemplary embodiment of a method according to the invention for recording paired projection images for dual-energy imaging with an X-ray device is shown. Here, as an example, a computed tomography scan is performed, in particular using an X-ray device with a C-arm (cone beam CT or CBCT), wherein the X-ray radiator is continuously moved at a certain movement speed, in particular an angular speed, on a circular recording trajectory, which can cover a sector angle of, for example, 200° or generally more than 180°. During this rotational movement of the X-ray radiator, a plurality of pairs, for example 250 pairs of projection images will be recorded, wherein the first projection image will be recorded with a first X-ray spectrum (for example a low-energy spectrum) and the second projection image will be recorded with a second X-ray spectrum (for example a high-energy spectrum). The exemplary embodiment described here provides the possibility that despite the continuous movement of the X-ray radiator and the time offset between the X-ray pulses of the two X-ray spectra, the first and second projection images produce similar, in particular at least substantially identical, projection geometries.

[0051] For this purpose, an X-ray radiator is used, wherein two separate focal points can be used, which are arranged offset in a plane of motion (in which the recording device is movable, in particular rotatable) (in particular in the direction of motion, in this case the direction of rotation), wherein the focal distance in the (jointly moving) coordinate system of the X-ray radiator and the recording parameters, in particular the parameters describing the motion and the X-ray pulse time course are adapted to one another so that the second focal point at the time of emission of the X-ray pulse of the second projection image (second radiator position) is located—with respect to the fixed coordinate system of the examination object (or the X-ray device)—at least within a tolerance range with respect to the position of the first focal point (first radiator position) at the time of emission of the X-ray pulse of the first projection image (first radiator position). For this purpose, suitable recording parameters can be preset, but can also be optionally determined in step S1, for example if other recording parameters can be selected in a targeted manner. In this case, it is in particular the time interval between the emission of the X-ray pulse of the first projection image and the emission of the X-ray pulse of the second projection image, i.e. the recording time adjusted according to the remaining preset parameters. Thus, during the continuous rotational movement along the recording trajectory, the X-ray pulse time course is appropriately selected or predetermined.

[0052] Figure 2 and Figure 3 The basic idea of ​​the invention is explained in the form of corresponding schematic diagrams of a first radiator position and a second radiator position.

[0053] Figure 2 The schematic diagram shows a recording device of an X-ray device with an X-ray radiator 1 and an X-ray detector 2 arranged opposite to each other. The object to be examined, for example a patient or a phantom, is arranged between the X-ray radiator and the X-ray detector. The recording device can be moved according to arrow 4. Figure 2 and Figure 3 As the plane of movement, the image plane is rotated about the isocenter 5 of the X-ray device. Also shown is the radiation field 6 emitted in the cone beam geometry at the first radiator position, which starts from the first focus 7 of the X-ray radiator. A first X-ray spectrum is used here.

[0054] However, if Figure 2As shown, the X-ray radiator 1 also has a further second focal point 8, which is physically spaced apart from the first focal point 7 in the coordinate system of the X-ray radiator 1, which moves with the X-ray radiator 1 during the movement of the X-ray radiator 1 and is rotationally offset relative to the first focal point 7 in the plane of movement, in particular in the direction of movement. The two focal points 7, 8, which are separated by a focal distance of, for example, 3 to 6 mm, can be realized, for example, by different focusing paths on the rotating anode of the X-ray radiator 1. Here, a fixed spectral filter 9 is also associated with the second focal point 8, which can also be arranged on the rotating anode, then annular, but can also cover the corresponding part of the exit window of the X-ray radiator 1 used by the second focal point 8. In order to generate different X-ray spectra, in addition to using spectral filters, different radiators and / or different materials can also be used on the rotating anode. The filter is also designed to be quickly replaceable.

[0055] Figure 3 A second radiator position is shown, in which the recording arrangement consisting of the X-ray radiator 1 and the X-ray detector 2 has been rotated further according to the arrow 4. It should firstly be noted that for reasons of representation a significantly larger rotation angle of 6° is used here, since usually a significantly smaller rotation angle of, for example, less than 0.5° is given due to the possibility of rapid switching between the focal spots 7, 8 and the X-ray spectrum and is not suitable for a clear representation.

[0056] For better understanding, Figure 3 It also shows Figure 2 The positions 10 and 11 of the X-ray radiator 1 and the X-ray detector 2, and the radiation zone 6 are shown in FIG. Figure 3 The second radiator position now generates a radiation field 12 starting from the second focus 8, which is obviously located according to Figure 2 The position of the first focus 7 in the first radiator position of the embodiment of the present invention or even at the position of the first focus 7 in a fixed coordinate system, whereby the filter 9 is additionally used. In addition, the image acquisition here is carried out with a second x-ray spectrum.

[0057] In other words, the focal distance and the recording parameters, in particular the X-ray pulse time course, are selected in such a way that the second focus 8 is moved by rotation into a previous position of the first focus 7 relative to the examination object 3 and the fixed coordinate system of the X-ray device, and the first projection image is recorded from the first focus 7 with a first X-ray spectrum and the second projection image is recorded from the second focus 8 with a second X-ray spectrum with at least approximately the same projection geometry.

[0058] At the second radiator position in the fixed coordinate system, the second focus is at least within a tolerance range in the fixed coordinate system with respect to the position of the first focus in the first radiator position. The tolerance range can be defined here so that the distance between the second focus in the second radiator position and the first focus in the first radiator position (in the fixed coordinate system) is less than 35%, here less than 10%, of the movement distance of the X-ray radiator between the first radiator position and the second radiator position (in the fixed coordinate system), the second radiator position being used as a reference. Since the similarity of the projection directions is more important, in the fixed coordinate system the distance between the second focus of the second radiator position and the first focus of the first radiator position in the rotation direction is less than in the projection direction. In other exemplary embodiments, for example in tomosynthesis, it is even conceivable that the second focus takes up exactly the position of the first focus.

[0059] When determining or presetting the acquisition parameters, in particular the X-ray pulse time course (which can take place during an optimization process), in addition to minimizing the distance between the position of the second focus in the second radiator position and the position of the first focus in the first radiator position, boundary conditions, in particular displacements of the X-ray detector 2 (e.g. Figure 3 ) is a multiple of the pixel width of the X-ray detector 2 (pixel detector). This will be discussed in more detail below.

[0060] Back to Figure 1 , the general recording process starts in step S2. This means that the X-ray radiator 1 and the X-ray detector 2 start their continuous rotational movement with the recording trajectory, and the X-ray pulse time course is used to define the recording time of multiple pairs of projection images. Here, the X-ray spectrum is always alternating, and the interval between a single projection image pair does not necessarily correspond to the time interval between a pair of projection images, but can be shorter or preferably longer.

[0061] Thus, for a pair of projection images, in which a first projection image is recorded with a first focus 7 in a first X-ray spectrum and a second projection image is recorded with a second focus 8 in a previous position as close as possible to the first focus 7, the first projection image is first recorded in a first radiator position in step S3. Then, in step S4, in particular without further necessary control measures, a further movement of the X-ray radiator 1 (and in parallel to the X-ray detector 2) to the second radiator position is continuously carried out. After a corresponding time interval that is appropriately predetermined or determined has elapsed, in step S5, a second projection image is recorded with a second X-ray spectrum from the second focus 8 in a position as close as possible to the first focus in the first radiator position.

[0062] In step S6, it is checked whether the recording process has now been completed or whether more pairs of projection images need to be recorded. If the latter is the case, the process returns to step S3, in which the next first projection image is recorded after the time interval between the multiple pairs predetermined by the X-ray pulse time course has expired. Otherwise, the recording process ends.

[0063] In step S7, image processing of the recorded pair is then performed in such a way that at least the displacement of the X-ray detector from the first radiator position to the second radiator position is corrected so that the projection images of the pair can be offset against each other pixel by pixel in a simple manner. This is easy to implement in particular when the displacement of the X-ray detector 2 is a multiple of the pixel size. In particular, if the X-ray detector 2 has tilted from the first radiator position to the second radiator position by more than 0.5°, compensation of this tilt can also be provided. In each case, image content contained in only one of the projection images of the pair is removed from the further evaluation and discarded.

[0064] The projection image pairs prepared in this way are then evaluated in step S, wherein within the scope of the evaluation a pair of projection images are combined in a pixel-by-pixel weighted linear manner to form at least one evaluation image, which is possible because similar or even identical projection geometries and image processing are used in step S7 without any problems and with reasonable results.

[0065] Figure 4 A schematic diagram of an exemplary embodiment of an X-ray device 13 according to the invention is shown. The X-ray device 13 has a C-arm 14 on which an X-ray radiator 1 and an X-ray detector 2 are arranged opposite one another. The C-arm 14 is mounted on a support 15 and can be rotated according to arrow 16 at least about an axis of rotation that passes through the isocenter 5.

[0066] At this point it should also be noted that the X-ray radiator 1 can also be arranged rotatably on the C-arm 14 according to arrow 17. In this case, a corresponding rotary actuator can be provided, which can be driven by the control device of the X-ray device. The rotatability of the X-ray radiator 1 allows the focal spots 7, 8 to follow each other in directions other than the direction of rotation, for example along Figure 4 Then, for example, a spiral CT image with an active focus can be taken. Figure 1 Then, in step S2, before the photographing operation starts, a reverse rotation must be performed so that the focal points follow each other in the rotation direction again.

[0067] Of course, further degrees of freedom of movement can also be provided in the X-ray device.

[0068] Figure 5The functional structure of the control device 18 is shown in more detail. It firstly has a storage device 19, for example for storing recording parameters, projection images, etc. An optional determination unit may be provided to perform the optional step S1.

[0069] In the recording unit 21, the recording operation of the X-ray device, in particular the movements occurring during the recording operation, are controlled. The recording unit is therefore used in particular to carry out steps S2 to S6. The image processing according to step S7 can be carried out in the image processing unit 22, and the evaluation of step S8 can be carried out in the evaluation unit 23. With regard to the rotation of the X-ray radiator 1 about its own axis according to the arrow 17, a control unit, which is not shown in greater detail here, can also be provided.

[0070] Although the present invention has been shown and described in more detail by means of preferred embodiments, the present invention is not limited to the disclosed embodiments and other variations may be derived therefrom by those skilled in the art without departing from the scope of protection of the present invention.

Claims

1. A method for capturing a projection image of dual-energy imaging, wherein: At least one pair of mutually associated projection images of an object to be examined (5) is recorded using different X-ray spectra and using an X-ray device (13) having a recording device, which comprises an X-ray emitter (1) and an X-ray detector (2) which are arranged opposite to each other and can be moved together in a plane of movement, in particular can be rotated in a plane of rotation, It is characterized in that, for recording each pair of mutually associated projection images, two focal spots (7, 8) of the X-ray radiator (1) which are spaced apart from each other in the plane of movement are used, wherein: - taking a first projection image of the pair at a first radiator position of the X-ray radiator (1) by using a first X-ray spectrum emitted from a first focus (7), - the X-ray radiator (1) is moved to a second radiator position by moving the recording device in the movement plane, so that a second focus (7, 8) of the foci (7, 8) is at least within a tolerance range with respect to the position of the first focus (7) in the first radiator position, and - recording a second projection image of the pair at a second radiator position of the X-ray radiator (1) by using a second X-ray spectrum emitted from the second focus (8).

2. The method according to claim 1, characterized in that: In a recording operation, in particular in a computed tomography recording, a plurality of pairs of mutually associated projection images for different projection geometries are recorded along a single recording trajectory of an X-ray radiator (1), in particular at an at least partially constant rotational speed of the X-ray radiator (1).

3. The method according to any one of the preceding claims, characterized in that The tolerance range is defined such that the distance between the second focus (8) in the second radiator position and the first focus (7) in the first radiator position is less than 35%, in particular less than 10%, of the movement distance of the X-ray radiator (1) between the first radiator position and the second radiator position, and / or the distance between the second focus (7) in the second radiator position and the first focus (8) in the first radiator position in the direction of movement, in particular in the direction of rotation, is less than the distance in the projection direction, in particular is at most half the distance in the projection direction.

4. The method according to any one of the preceding claims, characterized in that With continuous movement of the X-ray radiator (1) at an at least partially constant movement speed, in particular at an at least partially constant rotational speed, and with a preset focal distance between the focal points (7, 8) of the X-ray radiator (1), the time interval between the X-ray pulse outputs of each pair of first and second projection images is selected such that the second focal point (8) is at least within a tolerance range with respect to the position of the first focal point (7) in the first radiator position.

5. The method according to any one of the preceding claims, characterized in that Recording parameters describing the movement of the recording device, in particular the rotational movement and / or the recording of the first and second projection images and / or the focal distance are selected in an optimization operation so that the distance between the second focal point (8) in the second radiator position and the first focal point (7) in the first radiator position is minimized.

6. The method according to claim 5, characterized in that During the optimization process, as a boundary condition, it is required that the displacement of the X-ray detector (2) in the image plane of one of the projected images due to the movement of the recording device corresponds to an integer multiple of the pixel size of the pixels of the X-ray detector (2).

7. The method according to any one of the preceding claims, characterized in that In order to compensate for the movement of the X-ray detector (2), each pair of projection images is moved relative to each other by a displacement of the X-ray detector (2) in the image plane of one of the projection images.

8. The method according to claim 7, characterized in that During the rotational movement, the tilt of the X-ray detector (2) is also compensated for by the rotational movement by applying a compensation algorithm to each pair of projection images.

9. The method according to claim 1, characterized in that During the evaluation, an evaluation image is determined from each pair of projection images by a combination of the projection images, in particular a linear and / or weighted and / or pixel-by-pixel combination.

10. The method according to any one of the preceding claims, characterized in that A spectral filter (9) of the X-ray radiator (1) is used for at least one focus of the X-ray radiator (1).

11. The method according to any one of the preceding claims, characterized in that An X-ray radiator (1) is used which is rotatable about a projection direction and which is rotated or kept rotating to obtain a projection image so that the focal spots (7, 8) follow one another in a direction of movement, in particular a direction of rotation.

12. An X-ray device (13), comprising a recording device, which includes an X-ray radiator (1) and an X-ray detector (2), which are arranged opposite to each other, in particular on a C-arm (14) and can move together in a movement plane, in particular can rotate in a rotation plane, and the recording device includes a control device (18) suitable for executing a method according to one of the preceding claims.

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