Improving image quality by using a detector area that is not directly irradiated in X-ray imaging

Abstract

The invention relates to a method for performing X-ray imaging based on X-rays by - Detection (S1) of primary radiation superimposed with secondary radiation in an area (13) of a detector (4) directly irradiated by the X-rays, - Detection (S2) of secondary radiation in an outer area (14) of the detector (4) outside the directly irradiated area (13), - automatic determination (S3) of an irradiation parameter depending on the detected primary radiation and the detected secondary radiation and - Performing X-ray imaging (S5) based on the radiation parameter.

Description

[0001] The present invention relates to a method for performing X-ray imaging based on X-rays by detecting primary radiation superimposed with secondary radiation in an area of ​​a detector directly irradiated by the X-rays. Furthermore, the present invention relates to a computer program, an image processing device, and an X-ray-based imaging modality.

[0002] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

[0003] In X-ray-based imaging, particularly in medical imaging, scatter radiation can be a significant cause of image quality degradation. Therefore, when using imaging techniques such as computed tomography and / or fluoroscopy, scatter radiation correction can play a crucial role in image processing and / or workflow optimization.

[0004] In fluoroscopy techniques such as fluoroscopy or (serial) radiography, an X-ray source and a detector are used, with the object positioned between the X-ray source and the detector. Due to the partial absorption of X-rays by the object, which typically varies depending on the material, the directly transmitted X-rays (also referred to as primary radiation) can be representative of the object's internal structure. However, due to radiation scattering, scattered radiation (also referred to as secondary radiation) also reaches the detector, thus impairing image quality. A collimator is usually used to illuminate only a specific area of ​​an object, such as part of a patient. In many cases, the collimator illuminates only a portion of the detector. The collimator can reduce the radiation exposure of the object as well as the amount of scattered radiation.Nevertheless, some scattered radiation, especially radiation scattered within the object, usually still hits the aforementioned part of the detector and therefore impairs the image quality.

[0005] In computed tomography (CT), a primary X-ray beam, generated by an X-ray source, is directed at an object within a scan area and detected by an X-ray detector on the opposite side of the scan area. Depending on the object's properties, portions of the primary X-ray beam are absorbed, resulting in object-dependent attenuation. The attenuation of the primary X-ray beam allows the object's structures to be determined. By varying the projection angle of the primary X-ray beam onto the object, a three-dimensional image can be generated. However, some of the X-ray radiation is scattered, which can lead to the X-ray detector capturing this scattered radiation. Scattered radiation detected by the CT detector can, in turn, cause image artifacts and thus reduce image quality.

[0006] One measure to address the problem of scatter radiation in CT imaging, fluoroscopy, or (serial) radiography is the use of an anti-scatter grid. Anti-scatter grids are generally based on the assumption that the angle of incidence of the primary X-ray beam on the detector is constant for every projection angle. While anti-scatter grids can be a useful measure in typical third-generation CT systems with a rotating gantry, they are generally not useful in CT scanner geometries with distributed, especially static, non-rotating X-ray sources. With distributed X-ray sources, the orientation of the source and detector, and thus the angle of incidence of the X-rays within the primary X-ray beam, differs for each source in the distributed source field.The assumption that the angle of incidence of the primary X-ray beam on the detector is constant for every projection angle is therefore no longer valid, which generally means that corresponding scatter beam grids are not feasible. Consequently, scatter radiation can pose a problem, particularly with scanner geometries featuring distributed X-ray sources.

[0007] To address the problem of scatter radiation in CT, it has been proposed to provide scatter estimation and correction by simulating the physical process of X-ray interaction, e.g., by Monte Carlo simulations, and / or to use scattering kernels to directly estimate the scatter radiation for the primary X-ray beam. Corresponding correction and estimation methods are described, for example, by Rührnschopf, EP, & Klingenbeck, K. in “A general framework and review of scatter correction methods in x-ray conebeam computerized tomography” (2011), Part 1: Scatter Compensation Approaches, Medizinische Physik, 38(7), 4296–4311; and Part 2: Approaches to Scatter Estimation, Medizinische Physik, 38(9), 5186–5199. Although direct Monte Carlo-based methods have the potential to be quite accurate, these methods have the disadvantage of being computationally intensive and time-consuming.This usually prevents their direct use in clinical CT image reconstruction.

[0008] Image quality in X-ray imaging is degraded by secondary / scattered radiation. Every object (patient) through which the X-ray beam passes generates secondary radiation due to physical scattering processes of the primary X-rays. This secondary radiation is registered by the X-ray detector in addition to the primary signal. Anti-scatter grids are often used to reduce the detection of this secondary radiation. In two-dimensional imaging, these are predominantly grids with lamellae that reduce the scattered signal perpendicular to their orientation. Grids with a two-dimensional structure are also used for 3D imaging. Furthermore, in some clinical applications, increasing the distance to the scattering object (patient) can reduce the amount of scattered radiation reaching the detector (airgap technique).

[0009] The object of the present invention is to solve at least some of the problems mentioned above, in particular to offer a way to reduce the influence of scattered radiation on image data from x-ray-based imaging.

[0010] This problem is solved by a method, a computer program, an image processing device, and an X-ray-based imaging modality according to the independent claims. Further advantages and features will become apparent from the dependent claims, the description, and the accompanying figures.

[0011] According to the invention, a method for performing X-ray imaging based on X-rays is provided. - Detecting primary radiation superimposed with secondary radiation in an area of ​​a detector directly irradiated by the X-rays, - Detection of secondary radiation in an outer area of ​​the detector outside the directly irradiated area, - Automatic determination of an irradiation parameter depending on the detected primary radiation and the detected secondary radiation and - Performing X-ray imaging based on the radiation parameter.

[0012] Detecting primary or secondary radiation in a region of the detector means that the respective radiation strikes a specific area of ​​the detector and is detected there by one or more detector elements. Detection can also include preprocessing of the detection signals. Automatically determining an irradiation parameter based on the detected primary and secondary radiation can involve automatic calculation and / or automatic assignment. For example, automatic determination can be implemented using an algorithm, and in particular, a machine learning algorithm.

[0013] Performing X-ray imaging based on the radiation parameter means, for example, that the radiation parameter is set accordingly in an X-ray imaging modality. For instance, the radiation parameter might be the X-ray dose or a corresponding tube voltage and / or tube current in the imaging modality. Alternatively, the radiation parameter could also indicate the presence of a filter or an anti-scatter grid, etc.

[0014] An X-ray imaging modality may include an examination area between an X-ray source and an X-ray detector. The examination area is generally designed to allow the placement of an object to be examined or imaged. The object may be part of a larger object or subject. A subject may be, for example, a human being, such as a patient, or an animal. The object may be, for example, an anatomical part of the subject, such as a limb or organ. The object may be any other type of object, such as luggage. For example, a table, particularly a patient table, may be provided that is, or can be, at least partially located within the examination area. The table may be suitable for placing the object and / or the person on it. The table may be movable, for example,to move the person or the object that is part of the person into the area of ​​investigation.

[0015] The X-ray detector (or simply detector) can consist of detector elements. Detector elements can also be called detector pixels or be representative of detector pixels. The detector resolution and / or the detector grid size can be (partially) determined by the detector elements and their respective spacing.

[0016] The (X-ray) imaging modality comprises at least one X-ray source. Therefore, an X-ray-based imaging modality is one that uses X-rays to generate images. Examples of such imaging modalities include computed tomography systems, fluoroscopes, and projection radiographs. The at least one X-ray source, in particular the X-ray sources, can be configured to emit the primary X-ray beam. An X-ray source can also be referred to as an X-ray generator. The at least one X-ray source is preferably directed toward the examination area and the X-ray detector, with at least part of the X-ray detector being located outside the examination area. For example, the imaging modality can be based on an X-ray tube, as is known in the prior art.

[0017] The directly irradiated area (hereinafter also referred to as the sub-area) comprises a portion of the detection range of the X-ray detector. Therefore, a part of the X-ray detector (outer area) is not directly illuminated by the primary X-ray beam. "Direct illumination" can be understood as X-rays that directly illuminate the object are transmitted directly through or around the object (e.g., depending on the object's size and shape) to the detector. Scattered radiation, on the other hand, is not part of the primary X-ray beam that directly illuminates the sub-area. Typically, however, some scattered radiation also reaches the directly illuminated sub-area. Therefore, the detected signal of this portion of the detector's detection range is typically based on the radiation from the primary X-ray beam (primary radiation) and on scattered radiation (secondary radiation). The primary X-ray beam can, for example, be a cone X-ray beam.The scattered radiation contains no spatial information about the object under investigation in the spatial frequency range that the detector pixels provide based on their sampling. Therefore, a certain degree of combining (binning) of the pixel signals of the scattered signal may be useful.

[0018] In the context of this disclosure, the X-ray radiation data recorded by the detector within the sub-area are referred to as internal radiation data, i.e., radiation data captured within the sub-area. The scattered radiation captured by the detector within the sub-area may be referred to as internal scattered radiation data. In the context of this application, "X-ray radiation" may also generally be referred to simply as "radiation".

[0019] Radiation can be scattered within the object or, in some cases, by other scattering fields. External scatter data can be based on measurements of the entire detector area outside the directly irradiated sub-area. Alternatively, external scatter data can be based on only a portion of the detector area outside the sub-area. For example, external scatter data can be based only on the portion of the detector area outside the sub-area that is closest to the sub-area. Thus, the area outside the sub-area where scatter radiation is detected can be chosen differently for the same irradiation geometry and detector. Since the detector area outside the sub-area is not directly illuminated by the primary X-ray beam, this external detector area can only be reached through scatter radiation.Accordingly, it can be assumed that all radiation data reaching this outer detector region consists essentially, and in particular exclusively, of scattered radiation. For the purposes of this disclosure, the X-rays detected outside this region are referred to as "external scattered radiation data." Preferably, the internal radiation data and the external scattered radiation data are acquired substantially simultaneously and / or during the same measurement. It may also be possible in general to acquire this data separately, e.g., in successive measurements. However, it is usually more advantageous to acquire the data simultaneously in order to save time and reduce the applied X-ray dose.

[0020] According to one embodiment, the imaging modality is a computed tomography (CT) system, in particular a static computed tomography system, wherein the X-ray detector comprises an arrangement of detector elements that may be arranged at least partially around an examination area, the directly irradiated sub-area comprising a subset of the detector elements, and the data of the external scatter radiation being acquired with at least some of the detector elements outside the directly irradiated sub-area. The examination area of ​​the computed tomography system can generally be designed such that the object can be placed within the examination area. The computed tomography system can include a gantry, the gantry comprising the at least one X-ray source and the detector elements of the X-ray detector. The examination area can, in particular, be arranged at least partially within the gantry.In connection with this invention, a static computed tomography system is, in particular, a computed tomography system with X-ray sources and detector elements that do not rotate during a computed tomography examination. The gantry can be configured so that it does not rotate around the examination area during a computed tomography measurement. Accordingly, the X-ray detector can be a static X-ray detector. Advantageously, rotational forces can be avoided in a static computed tomography system. The detector elements of the X-ray detector are arranged at least partially around the examination area. The arrangement of the detector elements partially around the examination area can be understood as meaning that the detector elements are only located at parts of the circumference, e.g.,The detector elements are distributed around the perimeter of the examination area in an annular shape and / or have a gap in the perimeter of the examination area, wherein the gap has no or fewer detector elements than other parts of the perimeter. The detector elements can, for example, be arranged in an arc and / or a semi-annular shape. The detector elements can also be distributed around the examination area in a circular or semi-circular arrangement. Preferably, an X-ray detector is used in which the arrangement of detector elements has an extent that is, in particular, at least 5%, preferably at least 10%, larger than the directly illuminated portion of the detector. In particular, the detector elements can be distributed over an angle greater than the angle of the conical shape of the primary X-ray beam. Preferably, the detector elements can be distributed over 360°.In other words, the detector elements can be arranged completely around the area under investigation.

[0021] In connection with this invention, the direction perpendicular to the arrangement of the detector elements of the CT system can be referred to as the axial direction. In particular, the axial direction can be perpendicular to a region spanned by the arrangement of the detector elements. The axial direction can be the axial direction of a (partially) annular arrangement of the detector elements. The axial direction can correspond to a z-direction in Cartesian coordinates. An axial distribution can also be present when referring to a general shape such as an annular or ring-shaped arrangement of detector elements. A direction perpendicular to the axial direction can be referred to as the radial direction. In particular, the radial direction can be the direction from the center of the examination area to one of the detector elements. The radial direction can correspond to an x,y direction.Furthermore, a circumferential direction can be defined. The circumferential direction is a direction that follows the arrangement of the detector elements around the area under investigation. In particular, the circumferential direction of a ring-shaped detector can follow the ring shape of the ring.

[0022] The arrangement of detector elements in a computed tomography system can comprise rows and / or columns of detector elements. Rows of detector elements can be arranged one behind the other in the circumferential direction. Columns of detector elements can be arranged one behind the other in the axial direction. Therefore, the distribution of detector elements can extend in the axial direction.

[0023] The computed tomography system can comprise at least one X-ray source, preferably several X-ray sources. The at least one X-ray source, and in particular the X-ray sources, can be configured to emit the primary X-ray beam. For example, distributed X-ray sources can be used, in which several X-ray sources are distributed around the examination area in a circular or partially circular arrangement. The X-ray sources of the computed tomography system can be distributed around the examination area. For example, the X-ray sources can be distributed over 360°. The X-ray sources can be part of an X-ray source field. The X-ray source field can be annular. The X-ray source field can be a static X-ray source field, in particular corresponding to the static computed tomography system. Therefore, the X-ray sources can be stationary during operation.In other words, in the case of a static computed tomography system, the X-ray sources are not rotated around the scan area during a scan. Preferably, both the X-ray sources of the computed tomography system and the detector elements can be distributed around the scan area. The X-ray sources can be directed towards the detector elements, particularly those on the opposite side of the scan area. Specifically, the X-ray sources, the scan area, and the X-ray detector can be arranged such that the primary X-ray beam can reach the detector elements of the X-ray detector in a straight line through the scan area. Rotational scanning of the scan area can be achieved by selectively activating X-ray sources. For example, the X-ray sources can be selectively activated by electronic switching.

[0024] The sub-area comprises a subset of the detector elements. Therefore, some of the detector elements are not directly illuminated by the primary X-ray beam. "Direct illumination" can be understood as X-rays that directly illuminate the object are transmitted directly through or around the object (e.g., depending on the object's size and shape) to the detector elements. Scattered radiation, on the other hand, is not part of the primary X-ray beam that directly illuminates the sub-area. However, scattered radiation typically also reaches the directly illuminated sub-area. Therefore, the detected signal of the subset of detector elements typically includes radiation from the primary X-ray beam and scattered radiation. The primary X-ray beam may, in particular, be a cone X-ray beam.

[0025] According to one embodiment, the partial area of ​​the X-ray detector is a coherent arc-shaped section of the arrangement of detector elements, in particular corresponding to a cone angle of a cone-shaped primary X-ray beam. Therefore, the angle of the cone can define the partial area of ​​the X-ray detector that is directly illuminated. The external scatter radiation data can be based on measurements of all detector elements outside the partial area. Alternatively, the external scatter radiation data can be based only on a subset of the detector elements outside the partial area. For example, the external scatter radiation data can be based only on those detector elements outside the partial area that are closest to it.

[0026] According to one embodiment, the detector elements of the CT system are arranged in a ring around the examination area, particularly in the form of a 360° detector. A 360° detector is, in particular, an X-ray detector in which the detector elements are arranged around the examination area. A 360° detector can be advantageous because more scatter radiation, i.e., from a larger total detector area, can be used to determine the scatter radiation within the sub-area. With more scatter radiation information, the scatter correction can potentially be determined more accurately. In particular, a 360° detector can provide information about the shape of the object and its scattering properties. The scattering properties can include information about the internal structure and materials within the object that influence the scattering of the radiation.

[0027] According to one embodiment, the imaging modality is a fluoroscope comprising a collimator, wherein the collimation via the collimator defines the partial area on the X-ray detector. The fluoroscope can be or include a C-arm, in particular a mobile C-arm. The fluoroscope can include an X-ray source and an X-ray detector. The X-ray source and the X-ray detector can be located at opposite ends of the C-arm. The collimator can be located between the X-ray source and the X-ray detector, in particular in a straight line between the X-ray source and the X-ray detector. The direct line between the X-ray source and the X-ray detector can be defined as the longitudinal direction within the scope of this invention. The collimator can be located in front of the X-ray source. In particular, the collimator can be arranged between the X-ray source and the examination area.The collimator can be configured so that a collimation window is adjustable. Optionally, the collimator can be set for this procedure so that the portion of the detector directly illuminated by the X-rays is smaller than the remaining area of ​​the detector not directly illuminated. The degree of collimation may depend on the zoom format used for a particular fluoroscopy examination. Collimation can be set automatically or manually. While collimation can prevent X-rays from directly striking the detector outside the partial area (i.e., outside the collimation window), scattered radiation, especially radiation scattered by the object being examined (e.g., part of a patient), can still reach the area outside the partial area.Advantageously, using information from outside the sub-area allows for a reasonable estimation of scattered radiation within the sub-area. This can lead to improved image processing and / or workflow optimization.

[0028] In a flat-panel detector, the directly irradiated (partial) area can be a rectangular region of the detector surface, and the surrounding area can be a frame or partial frame around the directly irradiated area. The frame can be formed on all four sides of the rectangular area, and the partial frame on only three, two, or one side of the rectangular area. Alternatively, a circular collimator can be used, resulting in a circular directly irradiated area and a surrounding area that at least partially surrounds the area. Another alternative is to use a rectangular collimator at an angle, resulting in a directly irradiated area in the shape of a parallelogram and a surrounding area that at least partially surrounds the area. Furthermore, other collimator shapes and orientations are possible, resulting in corresponding directly irradiated areas and surrounding areas.

[0029] According to one embodiment, estimating the portion of the internal radiation data attributable to scattered radiation involves applying a computational model and / or algorithm that derives the scattered radiation within the subregion from the external scattered radiation data. Advantageously, the computational model or algorithm can be relatively simple, i.e., require relatively little computation time, since, due to the provided external scattered radiation data, it is not necessary to completely re-simulate the scattering; rather, the external scattered radiation data can be used as a basis.

[0030] According to one embodiment, the scattered radiation within the subregion is determined by interpolation from the data of the external scattered radiation. For example, linear interpolation can be applied to at least some of the data of the external scattered radiation. In general, a second-order or higher-order polynomial may also be possible or necessary for the interpolation. The interpolation, in particular linear interpolation, can be based on the detector part, e.g., the detector elements, outside the subregion that is closest to it. In other words, the interpolation can be performed using the measured scattered radiation intensities of the detector part that lies just outside the illuminated primary X-ray beam. Advantageously, the application of interpolation can provide a relatively simple way to use the data of the external scattered radiation for determining the scattering correction.

[0031] X-ray imaging is performed based on an irradiation parameter. In one embodiment, the irradiation parameter can refer to an X-ray dose. In this case, the X-ray dose is automatically determined depending on the detected primary radiation and, if applicable, the detected secondary radiation (e.g., by calculation, allocation, and the like).

[0032] According to another embodiment, the X-ray dose for X-ray imaging is controlled by an automatic dose control system. A key feature of this system is its feedback mechanism. For example, the X-ray dose is controlled based on an image quality measure (e.g., contrast). To achieve this, a measured actual quality measure is compared with a corresponding target quality measure, and the X-ray dose is adjusted until the deviation between the target and actual values ​​is minimal according to the selected quality criteria.

[0033] In another embodiment, the irradiation parameter relates to the presence (and absence) of an anti-scatter grid or filter in an X-ray beam path, and the automatic determination of the irradiation parameter takes into account the effect of the anti-scatter grid or filter on the X-rays. In this case, the irradiation parameter is a binary value indicating whether the anti-scatter grid or filter is present in the beam path or not. For example, "1" signifies the presence and "0" the absence of the anti-scatter grid (hereinafter also used to represent the filter) in the X-ray beam path. If the anti-scatter grid is present in the beam path, it typically affects the X-rays.In particular, the scatter radiation grid affects the ratio or relationship between primary and secondary radiation. This effect can be measured, providing information about whether the presence of the scatter radiation grid in the beam path is beneficial. For example, this information is obtained by estimating image quality based on the ratio between primary and secondary radiation. If a corresponding image quality measure does not exceed or fall below a predetermined threshold, the presence or absence of the scatter radiation grid is considered advantageous. Accordingly, the presence or absence of the scatter radiation grid is automatically determined as an irradiation parameter, i.e., recommended for further X-ray imaging.For example, a scatter radiation grid is disadvantageous for small objects, as these generate little scatter radiation. For larger objects, however, a scatter radiation grid can be advantageous. Furthermore, the benefit of the scatter radiation grid can also depend on the material, and therefore, depending on the material of the object being examined, it can be automatically determined whether the presence of the scatter radiation grid is beneficial. A corresponding irradiation parameter regarding its presence or absence can then be output. The presence of a filter can affect the spectrum and thus the scattering, and ultimately also the ratio between primary and secondary radiation.

[0034] According to a further embodiment, the irradiation parameter is determined iteratively in such a way as to optimize image quality (e.g., contrast) in X-ray imaging. The steps of the method according to the invention can therefore be repeated multiple times in an iterative loop. The iteration can then be terminated when an image quality parameter has reached or exceeded a certain value. For example, the X-ray dose, as an irradiation parameter, can be increased until a certain image quality is achieved. Alternatively, the X-ray dose can also be reduced until a certain image quality parameter is reached or falls below a certain threshold. In the latter case, an optimization with respect to radiation exposure is thus performed.

[0035] In another embodiment, the irradiation parameter is automatically determined such that the ratio between the detected primary radiation and the detected secondary radiation is optimized or maximized with respect to an optimization criterion. As already indicated above, the goal of automatically determining the irradiation parameter can be to optimize or maximize (or minimize) the ratio between the detected primary radiation and the detected secondary radiation. For example, the ratio can be driven to a local or global maximum or minimum. Here, as in other examples, the "detected primary radiation" can (unless otherwise specified) be the primary radiation superimposed with the secondary radiation, or the pure primary radiation, which is determined, for example, by subtracting an estimated secondary radiation component.

[0036] According to a further embodiment, for the automatic determination of the irradiation parameter, a pure component of the primary radiation is determined by interpolating the secondary radiation for the directly irradiated area from the detected secondary radiation outside the directly irradiated area and subtracting the interpolated secondary radiation from the detected, superimposed primary radiation. In the present example, the pure component of the primary radiation is thus used to determine the irradiation parameter, and not the primary radiation superimposed with the secondary radiation. The interpolation of the secondary radiation can be based on a single measurement point or pixel in the outer area of ​​the detector, where only the secondary radiation is detected. Preferably, however, the interpolation is based on several support points arranged around the directly irradiated area.For example, two support points can be arranged on opposite sides of the directly irradiated area. Thus, two opposing frame sections, which are part of the detector's exterior and between which the directly irradiated area is located, can be used for interpolation. Of course, interpolation can also be performed two-dimensionally based on correspondingly two-dimensionally distributed support points. The interpolated secondary radiation or scatter radiation distribution can then be subtracted from the primary radiation (or primary radiation distribution) superimposed with the secondary radiation to obtain the pure primary radiation component.

[0037] In another embodiment, the pure component of primary radiation constitutes the controlled variable in automatic dose control. In this example, the X-ray dose is controlled, and a certain setpoint can be specified for the primary radiation. According to a further embodiment, it is provided that when performing X-ray imaging, an X-ray image is obtained solely based on the pure component of primary radiation. This means that the X-ray image is obtained without the effect of secondary radiation. The X-ray image is thus corrected for the interfering component of secondary radiation, or scattered radiation. Such a correction allows, for example, the entire brightness range, or black-and-white range, to be better utilized, and thus the contrast increased or maximized.

[0038] In another embodiment, the secondary radiation is detected exclusively in a sub-region of the detector's exterior. This results in a local detection of the secondary radiation; it is not measured globally across the entire detector. Therefore, irradiation optimization can be based solely on the local area where the secondary radiation is detected.

[0039] According to another embodiment, the automatic determination of the irradiation parameter is carried out using a machine learning algorithm. For example, a neural network can be trained to determine the irradiation parameter based on measured values ​​of the primary and secondary radiation. If the irradiation parameter is part of a multidimensional irradiation parameter vector, the use of the machine learning algorithm is particularly advantageous, as individual irradiation parameters (i.e., vector components) can then be calculated easily.

[0040] In another embodiment, the dimensions of an object generating secondary radiation are determined based on the detected primary and secondary radiation. For example, the object's thickness can be determined very reliably in this way. This results in a dual use of the detected primary and secondary radiation, as not only the irradiation parameter but also the object's dimensions can be determined.

[0041] According to the invention, a computer program is also provided which contains instructions that, when executed by a computer, cause the computer to perform the method described above. The computer may, for example, be a control station of an imaging modality. The computer may have one or more processors and one or more memory units. The program may be stored in the memory unit(s).

[0042] The above problem is also solved according to the invention by an X-ray-based imaging modality, in particular a computed tomography system or a flat-panel detector X-ray unit with a processing circuit configured to perform the method as described above. The flat-panel detector X-ray unit can be implemented as a fluoroscope, a radioscope, and in particular as a C-arm unit. The advantages and further development possibilities described above in connection with the method according to the invention also apply mutatis mutandis to the X-ray-based imaging modality. Accordingly, the described method features can be interpreted as functional features of the imaging modality.

[0043] In one embodiment of the X-ray-based imaging modality, an output unit is provided for displaying the irradiation parameter. For example, the irradiation parameter can be displayed optically on a screen. If the irradiation parameter is, for instance, the X-ray dose, a user can monitor this dose. If the irradiation parameter includes, for example, a recommendation as to whether or not an anti-scatter grid is advisable, the user can follow this recommendation and insert the anti-scatter grid accordingly into the beam path. Alternatively, the output unit can be designed to output the irradiation parameter electronically, so that it can be used, for example, for subsequent control.

[0044] The present invention will now be explained in more detail with reference to the accompanying drawings, which show: Fig. 1 a C-arm angiography system with an industrial robot as a support device; Fig. 2. An X-ray image showing a directly irradiated area and a non-directly irradiated area; Fig. 3 the signal strength of the X-ray image of Fig. 3 along a line; and Fig. 4 a schematic procedure flow of an embodiment of the present invention.

[0045] The exemplary embodiments described in more detail below represent preferred embodiments of the present invention.

[0046] The Fig. Figure 1 shows an example of a monoplane X-ray system with a C-arm 2 held by a stand 1 in the form of a six-axis industrial or articulated robot. An X-ray source, for example an X-ray tube 3 with a collimator, and an X-ray image detector 4 are mounted at the ends of the C-arm. The implementation of the X-ray diagnostic system is not limited to the industrial robot; conventional C-arm devices can also be used.

[0047] In the beam path of the X-ray tube 3, a patient 6 or a technical object is positioned on a table 5 of a patient positioning table. A control unit 7 with a computer 8 for image processing is connected to the X-ray diagnostic device. This computer receives and processes the image signals from the X-ray image detector 4 (operating elements are not shown). The X-ray images can then be viewed on displays of a monitor 9. The monitor 9 can be supported by a ceiling-mounted, longitudinally movable, swiveling, rotating, and height-adjustable support system 10 with a boom and a lowerable support arm. The control unit 7 may also include an output unit 11 for displaying an irradiation parameter.

[0048] According to the invention, a method for performing X-ray imaging is provided in which, in one embodiment, the X-ray dose can be controlled or regulated. For the sake of simplicity, only the term "dose control" will be used hereafter to represent the two terms "dose control" and "dose regulation." While the defining characteristic of regulation is the closed-loop process (feedback), in which the controlled variable continuously influences itself within the control loop, regulation is simply a process in which an input variable influences an output variable according to a specific rule (compare DIN IEC 60050-351).

[0049] Dose control can be achieved based on an integral detector signal P', which is composed of the sum of a primary signal P and a secondary signal S. In the detection range of an X-ray detector, pure primary radiation P is superimposed with the secondary radiation S, so that only the superimposed signal P' can be measured as the detector signal. Each pixel element of such a detector measures this detector signal P'.

[0050] According to the in Fig. In the example shown in Figure 2, the X-ray detector (detector 4 for short) has a detector area 12 that is larger than the directly irradiated area 13. The directly irradiated area 13 is the area that X-rays from an X-ray source strike without scattering. An outer area 14 on the detector area 13 completely surrounds the directly irradiated area 13. Only secondary radiation, which is generated by scattering of X-rays from an object being examined (e.g., patient 6) and is omnidirectional, strikes this outer area 14.

[0051] The outer area 14 can be created, for example, by placing a collimator in the beam path that at least substantially blocks the X-rays from the outer area 14. In the present example of Fig. 2. The aperture can have a rectangular or square opening. This aperture ensures that not the entire detector area 12 is irradiated with primary radiation (X-rays from the radiation source attenuated by the object). However, an area 14 not irradiated by primary radiation can also result from the fact that the detector (especially a flat-panel detector) is already structurally larger than the maximum clinically usable area of ​​the system.

[0052] The signal in the outer area 14 (i.e., the pixel signals there), outside the directly irradiated area 13, is therefore primarily due to scattered radiation or secondary radiation S. Any primary transparency of the aperture (i.e., the collimator) can be eliminated from the signal experimentally or theoretically, if necessary.

[0053] The outer area 14 can be located on only one side of the directly irradiated area 13. However, it can also be located on several sides, e.g., on two opposite sides of the directly irradiated area 13. Furthermore, it can also be located on three or four sides, whereby in Fig. 2 the latter example is shown.

[0054] The pixels in the outer area 14 can be used to interpolate the signal pattern of the scattered radiation across the entire detector area. Fig. Figure 3 shows an example. The pixels on line 15 are evaluated here. Fig. 2. In the directly irradiated area 13, the signal strength is above the position shown in curve 16. In the area not directly irradiated, or outside area 14, the signal strength follows curve 17.

[0055] In the present example, curve 17 has two curve components: one in the left half of the image and one in the right half. Both curve components can be used to determine the scattered radiation along the entire line 15 by interpolation. For example, the interpolation can be performed using a polynomial.

[0056] In the example image of Fig. 2. A scattering body was used as a phantom. This results in Fig. 3. The arc-shaped curve 18 shows a scatter radiation maximum approximately in the center of the image. Since the scatter radiation has such an arc-shaped intensity profile, the signal profile of the primary radiation P superimposed with the secondary radiation S is also arc-shaped, averaged according to curve 16. Without scatter radiation, the profile of curve 16 could be flat or linear, since the primary radiation signal is usually calibrated accordingly so that contrast values ​​(e.g., of bones) at the image edge and in the image center are essentially the same. Fig. 3 can thus be seen that in the directly irradiated area 13 the primary radiation S is superimposed with the secondary radiation S.

[0057] The scattered radiation signal, i.e., the signal resulting from the secondary radiation S in the outer area 14, can be compared to the signal from the directly irradiated area 13. This allows the ratio between primary and scattered radiation to be optimized. For example, the ratio can be maximized within the technical limits. This can, for instance, also maximize the image contrast.

[0058] Using the detected primary radiation (possibly superimposed with secondary radiation) and the detected secondary radiation in the surrounding area, dose control for X-ray imaging can be implemented. For example, scattered radiation can be used to correct the signals in the directly irradiated area, thus optimizing dose control solely with respect to the primary radiation signal. In this way, for instance, a true water quality value can be determined.

[0059] The scattered radiation signal (short: scatter signal; composed of many pixel signals) can be evaluated locally for dose control. This means that the entire outer area 14, which is not directly irradiated, does not need to be evaluated. For example, only the upper left corner of the outer area 14 is evaluated. Fig. 2 is evaluated with respect to the scattering signal, so that the dose control can be optimized accordingly. The outer area 14 forms the example of Fig. 2 a “scattering frame” around the primary image or the directly irradiated area 13. This scattering frame contains “long-range” spatial information about the scattering intensity, since the projection location of a scattering center can be far away from the actual detection location of the scattered radiation.

[0060] As in connection with Fig. As shown in Figure 3, the "long-range" scattering signal can be incorporated into the image homogenization. A global or location-dependent correction of the scattering or secondary radiation can be performed. This can increase the image contrast.

[0061] Knowing the primary radiation P and the secondary radiation S, as well as the technical properties of an available scattering grid or filter, it is possible to calculate whether the use of the grid / filter will result in an image improvement, possibly depending on the radiation dose.

[0062] From the recorded signal values ​​for primary and secondary radiation, an irradiation parameter can be automatically determined, which can be interpreted as a recommendation as to whether or not an anti-scatter grid is useful. This decision, whether the grid improves image quality or not, depends on the thickness of the object or patient. Especially in pediatric or extremity applications, it may be beneficial to remove or expand the grid.

[0063] Fig.Figure 4 shows an embodiment of a process sequence according to the invention in a block diagram. In a first step S1, primary radiation superimposed with secondary radiation is detected in an area of ​​the detector directly irradiated by the X-rays. Detection means that corresponding signal values ​​caused by the primary radiation are measured or stored. In a further step S2, secondary radiation is detected in an outer area of ​​the detector outside the directly irradiated area. Here, too, corresponding signal values ​​with respect to the secondary radiation are measured, received, or recorded. In a subsequent step S3, an irradiation parameter is automatically determined as a function of the detected primary radiation and the detected secondary radiation.The radiation parameter can refer to any physical value of an X-ray imaging modality that can influence the radiation or the resulting image. In an optional step S4, the radiation parameter can be optimized. For example, if the radiation parameter is an X-ray dose, it can be optimized or minimized to a reasonable limit. Finally, in a further step S5, the X-ray imaging is performed based on the radiation parameter. Here, an X-ray image is acquired based on the automatically determined or optimized radiation parameter.

[0064] According to the invention, it is advantageously possible to adapt and optimize a dose control with regard to the resulting image quality or the resulting image contrast.

[0065] Another advantage of separately recording primary and secondary radiation in the form of two values ​​is that, in addition to automatically determining an irradiation parameter, a reliable estimate of the true object or patient thickness is also made possible.

[0066] Another advantage is that the described method can also be applied to non-collimated images, provided that despite the clinically maximally enlarged image area, a (small) edge area still remains at the detector, which only sees the scatter signal.

[0067] Overall, a key advantage of the invention is that a specific X-ray dose can be utilized more effectively. Furthermore, image post-processing can be optimized because more reliable primary absorption values ​​(water values) can be used. Additionally, contrast and / or noise reduction can be optimized for specific signal values ​​(grayscale ranges). QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited non-patent literature

[0000] Rührnschopf, EP, & Klingenbeck, K. in “A general framework and review of scatter correction methods in x-ray conebeam computerized tomography” (2011), Part 1: Scatter Compensation Approaches, described. Medical Physics, 38(7), 4296-4311

[0007] Approaches to scatter estimation. Medical Physics, 38(9), 5186-5199

[0007]

Claims

[1] Methods for performing X-ray imaging based on X-rays by - Detection (S1) of primary radiation superimposed with secondary radiation in an area (13) of a detector (4) directly irradiated by the X-rays, - Detection (S2) of secondary radiation in an outer area (14) of the detector (4) outside the directly irradiated area (13), - automatic determination (S3) of an irradiation parameter depending on the detected primary radiation and the detected secondary radiation and - Performing X-ray imaging (S5) based on the radiation parameter. [2] Method according to claim 1, wherein the irradiation parameter relates to an X-ray dose. [3] Method according to claim 2, wherein the X-ray dose for X-ray imaging is controlled by an automatic dose control. [4] Method according to claim 1, wherein the irradiation parameter relates to the presence of a scattering grid in a beam path of the X-rays, and in the automatic determination of the irradiation parameter, an effect of the scattering grid on the X-rays is taken into account. [5] Method according to one of the preceding claims, wherein the irradiation parameter is determined iteratively (S3) such that image quality is optimized in X-ray imaging. [6] Method according to one of the preceding claims, wherein the irradiation parameter is automatically determined (S3) such that a ratio between the detected primary radiation and the detected secondary radiation is optimized or maximized with respect to an optimization criterion. [7] Method according to one of the preceding claims, wherein for the automatic determination of the irradiation parameter (S3) a pure component of the primary radiation is determined by interpolating the secondary radiation for the directly irradiated area (13) from the detected secondary radiation in the outer area (14) outside the directly irradiated area (13) and subtracting the interpolated secondary radiation from the detected, superimposed primary radiation. [8] Method according to claims 3 and 7, wherein the pure component of the primary radiation represents the control variable in the automatic dose control. [9] Method according to claim 7 or 8, wherein when performing the X-ray imaging (S5) an X-ray image is obtained only on the basis of the pure component of the primary radiation. [10] Method according to one of the preceding claims, wherein the secondary radiation is detected exclusively in a partial area of ​​the outer region (14) of the detector (4). [11] Method according to one of the preceding claims, wherein the automatic determination of the irradiation parameter (S3) is carried out using a machine learning algorithm. [12] Method according to one of the preceding claims, wherein a dimension of an object (6) causing the secondary radiation is determined as a function of the detected primary radiation and the detected secondary radiation. [13] Computer program containing instructions which, when the program is executed by a computer, cause the computer to perform the method according to any of the preceding claims. [14] X-ray-based imaging modality, in particular a computed tomography system or a flat-panel detector X-ray unit, comprising a processing circuit configured to perform the method according to any one of claims 1 to 12. [15] X-ray-based imaging modality according to claim 14, comprising an output unit (11) to output the irradiation parameter.

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