Method for locating a tool in an X-ray image, data processing device, X-ray imaging system and computer program product

Abstract

Method for locating a tool (2) in an X-ray image, wherein the tool (2) has an X-ray absorption strength (12) that is spatially variable along a predetermined longitudinal direction of the tool (2) according to a predetermined absorption characteristic, and wherein - a spatial frequency spectrum of a partial area (6) of the X-ray image is determined; - checking whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2); and - if the inspection reveals that the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2), the partial area (6) of the X-ray image is determined as the position of the tool (2), whereby - according to the absorption characteristic, the spatial change in the X-ray absorption strength (12) along the longitudinal direction has a characteristic frequency at which a reference spatial frequency spectrum (13) of the spatial change in the X-ray absorption strength (12) along the longitudinal direction has a peak value; and - to check whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2), it is checked whether the spatial frequency spectrum has a peak value at a point which corresponds to the characteristic frequency within a predetermined tolerance range.

Description

The present invention relates to a method for locating a tool in an x-ray image, to an associated data processing device for carrying out such a method, to an x-ray imaging system having such a data processing device, and to a corresponding computer program product.In medical interventions, in particular minimally invasive interventions, fluoroscopy techniques can be used, for example, in which the intervention can be visually displayed and tracked on the basis of X-ray-based imaging. During such interventions, tools are introduced into the patient's body, which tools are likewise visible on the fluoroscopic x-ray images and can accordingly be tracked by the attending person. In the field of angiography, such tools are, for example, catheters which are introduced into hollow organs, in particular blood vessels.It can occur, in particular in the case of very small catheters or catheters with very small diameters, for example so-called micro-catheters with tip diameters of less than 2 mm, in some cases less than 1 mm, that the catheters are difficult or hardly visible to a viewer in the X-ray images.In order to improve the recognizability of such tools in X-ray images, the X-ray dose used could be increased and / or the image rate during the recording of the X-ray images could be increased. However, this would be associated with an increased radiation exposure for the patient and for other persons in his environment.It would also be possible to perform digital subtraction angiography in order to suppress irrelevant backgrounds in the X-ray images if necessary and accordingly, among other things, to emphasize the tool more strongly. However, this results in a considerable increase in the cost of image processing.The use of dual-energy X-ray imaging methods ("dual-energy acquisition methods") would also be conceivable, but would likewise be associated with increased outlay in image processing and, under certain circumstances, increased radiation loading.US 2009 / 0 171 196 A1 discloses an imaging system comprising an image recording device and an interventional device with a marker, wherein the marker is formed from a material detectable by the image recording device, which is arranged in an identifying pattern. The publication US 2008 / 0 048 040 A1 discloses a system and method for determining the position of a movable component on the basis of a movable rod on the basis of a barcode on the surface thereof. U.S. Pat. No. 6,351,513 B1 discloses a method and a device for providing a high-quality representation of a volume with a real-time reconstruction of a movement of an object, wherein the real-time movement of the object is determined by means of a representation of a part of the volume of lower quality. The publication DE 10 2005 030 607 A1 discloses an interventional instrument with at least one marking element, which can be identified in an x-ray image, enables a differentiation of different instruments and a position determination of the instrument. The publication DE 10 2005 022 901 A1 discloses a method for determining the orientation of an instrument on the basis of markings on or on a section of the instrument, wherein the markings can have a different X-ray density along the section compared to the material of the section.It is an object of the present invention to simplify the locating of a tool in an x-ray image, in particular without increasing a radiation load.This object is achieved by the subject matter of the independent claim. Advantageous refinements and preferred embodiments are the subject matter of the dependent claims.The invention is based on the concept of providing a tool, in particular a catheter, which has a spatially variable X-ray absorption strength along a predefined longitudinal direction of the tool in accordance with a predefined absorption characteristic. By analyzing the corresponding spatial frequency spectrum in the resulting x-ray image, it is possible to establish where the tool is located in the x-ray image or whether the tool is located in a specific sub-region of the x-ray image.According to one aspect of the invention, a method, in particular a computer-implemented method, for locating a tool in an x-ray image is specified. In this case, the tool has an X-ray absorption strength that is spatially variable along a predefined longitudinal direction of the tool according to a predefined absorption characteristic. In particular, the tool is predetermined or provided in such a way that it has the spatially variable X-ray absorption intensity according to the predetermined absorption characteristic.A spatial frequency spectrum of a partial region of the x-ray image is determined, for example automatically, in particular by means of at least one arithmetic unit. It is checked, in particular by means of the at least one arithmetic unit, for example automatically, whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool. If it is determined during the check that the spatial frequency spectrum corresponds to the absorption characteristic of the tool, the partial region of the x-ray image is determined as the position of the tool, in particular by means of the at least one arithmetic unit, for example automatically.Unless otherwise stated, all steps of the computer-implemented method can be carried out by a data processing device which has the at least one arithmetic logic unit. In particular, the at least one computing unit is configured or adapted to carry out the steps of the computer-implemented method. For this purpose, the at least one arithmetic unit can store, for example, a computer program which contains instructions which, when executed by the at least one arithmetic unit, cause the at least one arithmetic unit to execute the computer-implemented method.For each embodiment of the computer-implemented method, a corresponding embodiment of a method for locating a tool is immediately obtained, which is not purely computer-implemented by recording a method step according to which the x-ray image is generated.The at least one computing unit can be part of an X-ray imaging system, in particular an X-ray angiography system, for example.For example, the x-ray image can represent the tool and an organ, for example a hollow organ, for example a blood vessel or a vascular tree, of the patient. The tool can be, for example, a catheter, in particular a micro-catheter. The catheter can then be inserted in particular into the hollow organ on the x-ray image.However, it should be noted that, if appropriate, interventional steps, such as, for example, the introduction of the tool into the body of the patient or the guidance of the tool within the body of the patient, are not part of the method according to the invention.The x-ray image is in particular a two-dimensional x-ray image or an x-ray projection image. Such x-ray images have the advantage that, in contrast to three-dimensional reconstructions, they can be generated quickly during the intervention even with small radiation doses. When using small radiation doses, however, the recognizability of the tool in the X-ray image naturally suffers, so that the invention has a particularly advantageous effect here.The x-ray image corresponds to a spatial, in particular two-dimensional, distribution of an image intensity or image brightness, which is given by the corresponding pixel values of the x-ray image. In other words, the x-ray image thus reproduces an intensity in the spatial space. An analog representation, although less intuitive for human viewers, results in the so-called frequency space or spatial frequency space, which can be viewed as the reciprocal space of the spatial space. The spatial space and the frequency space are linked to one another in particular by a Fourier transformation.In other words, brightness fluctuations in the x-ray image can also be represented by corresponding frequency components of a spectrum in the frequency space. The invention now uses the spatially variable X-ray absorption strength of the tool along the longitudinal direction in order to detect it in the frequency space of the representation. Since the spatial frequency spectrum is determined for a given partial area of the x-ray image in the present case, by identifying the tool on the basis of its absorption characteristic in the frequency space, its position in the spatial space can also be limited, namely to the examined or analyzed partial area of the x-ray image. Locating the tool can thus consist in determining a specific sub-region of the x-ray image within which the spatial frequency spectrum corresponds to the absorption characteristic of the tool.There are various possibilities for determining the spatial frequency spectrum not for the entire x-ray image, but only for the sub-region which is in particular smaller than the entire x-ray image. For this purpose, the x-ray image can be divided into predefined cells or regions of interest, for example, wherein each region of interest corresponds to a corresponding partial region of the x-ray image and the spatial frequency spectrum is determined accordingly for the individual regions of interest and is matched to the absorption characteristic of the tool.It is also possible, during the Fourier transformation from the spatial domain into the frequency domain, to convolute the corresponding Fourier operator with a filter operator, in particular a local filter operator. The filter operator then has the effect that, at a given position of the filter operator in the x-ray image, only image information from a predefined region contributes or significantly contributes to the Fourier transformation or, in other words, image information outside the corresponding region is greatly suppressed. It can be a Gaussian filter or the like, for example.In particular, the tool, for example the catheter, has a rigid region which extends along the longitudinal direction. The spatially variable X-ray absorption strength is accordingly present in the rigid part. The fact that the rigid part extends along the longitudinal direction can be understood in particular in such a way that a spatial extent of the rigid part along the longitudinal direction is very much greater than a spatial extent perpendicular to the longitudinal direction, wherein very much greater can correspond, for example, to a factor of at least 10 or at least 50 or at least 100, for example a factor of 10 to 200 or the like. In other words, a spatial extent of the tool in the region of the rigid part along any direction perpendicular to the longitudinal direction is smaller than a length, i.e. a spatial extent of the rigid part along the longitudinal direction, by at least the said factor.The spatially variable X-ray absorption strength can be achieved, for example, by using different materials for the tool or by applying different materials on a surface of the tool. For example, a material composition of the tool can vary accordingly along the longitudinal direction. A tool, such as a catheter in particular, may include various metals, metal fabrics, composites, composites and / or composite fiber materials, and so forth. The composite materials, composite fabrics, and composite fiber materials may include metals, plastics, and / or carbon fibers, and so forth. By targeted modification or modulation of the material composition along the longitudinal direction, a corresponding absorption characteristic of the tool can be predefined.The absorption characteristic of the tool therefore corresponds, for example, to a reference location frequency spectrum of the spatial change of the X-ray absorption strength along the longitudinal direction. In order to check whether the spatial frequency spectrum of the sub-region corresponds to the absorption characteristic of the tool, the absorption characteristic, i.e. in particular the reference spatial frequency spectrum, can be matched to the spatial frequency spectrum of the sub-region.In particular, specific frequencies can be selectively emphasized in the reference location frequency spectrum by the X-ray absorption strength of the tool being predetermined periodically or quasi-periodically for two or more period durations along the longitudinal direction. In a simple, non-limiting illustrative example, the absorption characteristic of the tool could correspond to a periodic change with a single defined frequency, for example a sinusoidal curve or a curve of a rectangular curve, and so on. In this case, a pronounced peak, also referred to as a peak value, would be seen in the reference location frequency spectrum at the corresponding fundamental frequency and, if appropriate, for example in the case of a periodic square-wave signal, at corresponding higher harmonic frequencies.If the tool is located in the examined sub-region, corresponding peaks will also result in the spatial frequency spectrum of the sub-region, so that the comparison can be positive in this case, whereas a corresponding match cannot be found in other sub-regions. It should be noted that the absolute position of the peak in the spatial frequency spectrum of the sub-region does not necessarily exactly correspond to the corresponding peak in the reference spatial frequency spectrum, even if the tool is located in the sub-region.This can be the case in particular if the longitudinal direction does not run parallel to the X-ray projection plane of the X-ray image, but encloses an angle with it that is different from zero. Such angular deviations can lead to shifts in the frequency spectrum. For example, however, the orientation of the tool in three-dimensional space can optionally be estimated or at least narrowed down by further information, so that a corresponding adjustment is nevertheless possible. This can be done, for example, on the basis of pre-operative three-dimensional CT reconstructions or the like or on the basis of anatomical boundary conditions which lead to the tool being able to be located only in specific orientations or at least approximately in such orientations.The X-ray projection plane is a plane which runs parallel to a detector plane of an X-ray detector with which the X-ray image was generated and is perpendicular to the central X-ray projection beam. Although only the orientation of the X-ray projection plane is thus unambiguously defined, its position cannot be selected as desired, since here and in the following always only the orientation of the X-ray projection plane is important.Furthermore, it is possible, in particular when using cone beam geometries, for the peaks to shift with the distance of the tool from the X-ray source.Moreover, by a corresponding selection of the frequency ranges in which peaks are found in the absorption characteristic, a clear identification can nevertheless be made possible if, for example, no other peaks are to be expected in the corresponding frequency range. In addition, the absorption characteristic can also be more complex and define, for example, a sequence of two or more peaks in the reference location frequency spectrum. If a sequence of a corresponding number of two or more peaks is now likewise found in the spatial frequency spectrum of the subregion, this can serve for locating the tool even if the exact positions of the peaks and optionally their distances differ by different orientations of the tool with respect to the X-ray projection plane.It is also possible to estimate the angle by analyzing two or more X-ray images having different X-ray projection directions and determining differences in peak positions or the like in different X-ray images. Based thereon, the angle can then be estimated. Optionally, the angle can also be corrected or the position of the X-ray imaging system, for example an X-ray source and an X-ray detector of the X-ray imaging system, can be adjusted in a targeted manner in order to reduce the angle.The partial region is in particular a two-dimensional partial region of the x-ray image. The spatial frequency spectrum can accordingly also be a two-dimensional spatial frequency spectrum. The corresponding absorption characteristic of the tool can then be matched to the two-dimensional spatial frequency spectrum. For this purpose, it is in particular not necessary to know in advance how the longitudinal direction or its projection is aligned within the X-ray projection plane. However, it is also possible for the spatial frequency spectrum to be calculated as a one-dimensional spatial frequency spectrum and to be analyzed accordingly for different directions.The invention makes use of the specific spatial variability of the X-ray absorption strength of the tool along the longitudinal direction in order to localize it in the X-ray image. As a result, the measures mentioned at the beginning for improving the visibility can be dispensed with, which overcomes the disadvantages mentioned. However, it is also possible to combine the method according to the invention with one or more of the measures mentioned at the beginning in order to ensure even better visibility. The disadvantages mentioned at the beginning can then optionally be at least partially overcome or the recognizability can be further improved compared to known approaches.According to at least one embodiment, according to the absorption characteristic, the spatial change of the X-ray absorption strength along the longitudinal direction repeats or repeats except for a different amplitude.The fact that the spatial change is repeated can be understood in particular in such a way that the X-ray absorption strength is identical in each case in at least two successive length sections along the longitudinal direction. The fact that the spatial change of the X-ray absorption strength repeats up to a different amplitude can be understood in such a way that the X-ray absorption strength during two successive length segments are identical up to a constant factor in the amplitude or a linearly location-dependent factor or an otherwise modulated factor modulated in a defined manner.In particular, the tool is provided in such a way that the spatial change of the X-ray absorption strength along the longitudinal direction is repeated according to the absorption characteristic or is repeated apart from the deviating amplitude.As a result of the repetition, in particular at least apart from the deviating amplitude, it is achieved that marked peaks are present at corresponding points in the reference spatial frequency spectrum, which facilitates a comparison with the spatial frequency spectrum of the sub-region.According to a first variant of the method for locating a tool in an x-ray image, the spatial change of the x-ray absorption intensity along the longitudinal direction according to the absorption characteristic has a characteristic frequency at which the reference location frequency spectrum of the spatial change of the x-ray absorption intensity along the longitudinal direction has a peak value. In order to check whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool, it is checked whether the spatial frequency spectrum has a peak value at a location which corresponds to the characteristic frequency within a predefined tolerance range.In particular, the tool is provided such that the spatial variation of the X-ray absorption intensity along the longitudinal direction according to the absorption characteristic has the characteristic frequency.A peak value can also be referred to as a peak, as already mentioned above. Known algorithms for peak finding can be used to check the presence of corresponding peak values in the spatial frequency spectrum of the sub-region. For this purpose, suitable parameters to be varied, for example a peak shape, peak height, peak width and so forth, can be specified.As likewise already mentioned above, the characteristic frequency is not necessarily identical to the location in the spatial frequency spectrum at which the peak value is to be expected when the tool is located in the subregion. This is partly due to the mentioned orientation of the longitudinal direction with respect to the X-ray projection plane. Such deviations can be taken into account by the tolerance range in that they are selected in particular on the basis of empirical values or other boundary conditions. If the orientation of the longitudinal direction can be restricted to a specific angular range, for example based on preoperative imaging or other boundary conditions, the tolerance range can be correspondingly restricted.According to at least one embodiment, a measure of similarity between the spatial frequency spectrum and the reference spatial frequency spectrum is calculated in order to check whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool.The calculated value of the similarity measure is compared, for example, with a predefined threshold value. If the calculated value of the similarity measure is greater than the threshold value, it can be established, for example, that the spatial frequency spectrum corresponds to the absorption characteristic of the tool.The similarity measure can be calculated, for example, as a cross-correlation or other overlap integral.Further embodiments of the method also result from the embodiment mentioned for further characteristic frequencies which can be treated analogously.The tolerance range can correspond, for example, to a range [f0 / (1+cos(α)), f0 / (1-cos(α'))]. f0 corresponds to the characteristic frequency according to the absorption characteristic and the angles α, α' correspond to the maximum expected deviations of the longitudinal direction from the X-ray projection plane.In this way, an automated simple matching of the spatial frequency spectrum with the absorption characteristic can be achieved.According to at least one embodiment, the spatial change of the X-ray absorption intensity along the longitudinal direction according to the absorption characteristic has two characteristic frequencies at which the reference location frequency spectrum of the spatial change of the X-ray absorption intensity along the longitudinal direction has a peak value in each case. In order to check whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool, it is checked whether the spatial frequency spectrum has a peak value in each case at two points, the distance between which corresponds to a distance between the two characteristic frequencies within a predefined tolerance range.Such embodiments may be extended by further peak values and corresponding distances therebetween, where appropriate. It is also possible to combine such embodiments in which the distance of the characteristic frequencies is analyzed with the above-mentioned embodiments in which the location of the characteristic frequency is analyzed.Analogously to the above, the tolerance range for the distance df is also obtained, for example, from the angles α and α', which are expected at most between the longitudinal direction and the X-ray projection direction, such that [df / (1+cos(α)), df / (1-cos(α'))].In particular, the tool is provided such that the spatial variation of the X-ray absorption intensity along the longitudinal direction according to the absorption characteristic has the two characteristic frequencies.In this way, an automated simple matching of the spatial frequency spectrum with the absorption characteristic can be achieved.According to at least one embodiment, in order to check whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool, it is checked whether a ratio of the peak values at the two points of the spatial frequency spectrum corresponds to a ratio of the peak values at the two characteristic frequencies of the reference spatial frequency spectrum.Therefore, not only the relative position of the two characteristic frequencies is checked, but also their relative amplitudes with respect to one another. It is also possible that in other embodiments only the ratio of the peak values is checked, not the distance between the characteristic frequencies.When checking whether the ratio of the peak values of the spatial frequency spectrum corresponds to the ratio of the peak values of the characteristic frequencies of the reference spatial frequency spectrum, a predefined or predetermined modulation transfer function, MTF, can be taken into account, for example. The modulation transfer function describes, in particular for a given X-ray imaging system or a complete image chain of the X-ray source used, or the spectrum of the emitted X-ray radiation, via the object, the X-ray detector, the signal processing and the image display how a given frequency in the object space is transmitted or imaged to a frequency in the image space in the X-ray image. The MTF is a characteristic of the X-ray imaging system or the image chain used.Typically, higher frequencies are attenuated or suppressed more strongly along the image chain than lower frequencies. This can be expressed by a corresponding MTF that has a lower value at higher frequencies than at lower frequencies. The MTF can be determined by calibration measurements or the like. The reference location frequency spectrum of the spatial change of the X-ray absorption strength can then be given, for example, as a convolution of the modulation transfer frequency with an original reference spectrum, wherein the original reference spectrum results from the Fourier transformation of the spatial change of the X-ray absorption strength without taking into account the MTF.However, the MTF can also be taken into account indirectly by, for example, creating reference images of the tool against a neutral background and determining the reference location frequency spectrum by means of corresponding Fourier transformation.According to a second variant of the method for locating a tool in an x-ray image, a further x-ray image is generated, which displays the tool in a calibration environment, i.e. in particular against a neutral background, as mentioned. The absorption characteristic, in particular the reference location frequency spectrum, is determined based on the further X-ray image.In this way, the corresponding influences of the image chain or of the MTF can be taken into account indirectly in a simple manner, which leads to an increased reliability of the method.According to at least one embodiment, an angle β which the longitudinal direction of the tool encloses during the generation of the x-ray image with a predetermined x-ray projection plane, in particular an x-ray projection plane corresponding to the x-ray image during the generation thereof, is obtained or determined, in particular by means of the at least one arithmetic unit. The check as to whether the spatial frequency spectrum of the subregion corresponds to the absorption characteristic of the tool is carried out as a function of the angle β.If, for example, in the case that the longitudinal direction is parallel to the X-ray projection plane, a peak or a distance of two peaks from one another is expected according to the absorption characteristic, a shift of the peak or peaks f→cos(β)*f or a compression of the spectrum A(f) results, so that A(f)→ A(cos(β)*f).According to at least one embodiment, the x-ray image images a hollow organ, wherein the tool is arranged in the hollow organ.The hollow organ is in particular a vessel or a vascular system, for example a blood vessel or a blood vessel system. The tool is in particular a vascular catheter, in particular a so-called micro-catheter.According to at least one embodiment, a three-dimensional representation of the hollow organ is obtained or determined, in particular before the generation of the x-ray image. The angle that the longitudinal direction of the tool encloses with the X-ray projection plane is determined depending on an orientation of the three-dimensional representation with respect to the X-ray projection plane and a position of the tool in the three-dimensional representation.The three-dimensional representation can thus be, in particular, a preoperative or preventive 3D reconstruction or a corresponding 3D model. The three-dimensional representation can be generated in particular by a CT acquisition.In the three-dimensional representation, the hollow organ can be segmented in particular. A user, in particular medical personnel, can optionally estimate at which position of the three-dimensional representation or at which corresponding position of the hollow organ the tool is approximately located. The position of the tool in the three-dimensional representation is thus approximately determined. Furthermore, the position of the three-dimensional representation with respect to the X-ray projection plane is also known, so that the angle of the tool or the longitudinal direction can be estimated with the X-ray projection plane. Thus, more reliable localization can be achieved.In particular, the three-dimensional representation of the hollow organ and the x-ray image are registered with respect to one another or the three-dimensional representation of the hollow organ and the x-ray image are obtained registered with respect to one another.According to at least one embodiment, the partial region is a region of interest of a plurality of predefined, and in particular non-overlapping, regions of interest of the x-ray image.The plurality of predefined regions of interest can be, for example, a plurality of grid cells or grid cells of the x-ray image. The x-ray image is thus divided in particular into the plurality of regions of interest. The described method steps can be carried out for different regions of interest, in particular until that region of interest in which the tool can be localized as described is identified.By dividing the x-ray image into the plurality of predefined regions of interest, the required computing effort can be reduced.According to at least one embodiment, the spatial frequency spectrum of the sub-region is determined using a local filter, wherein a position of the local filter defines a position of the sub-region in the x-ray image.The width or extent of the local filter parallel to the X-ray projection plane thus determines or defines the size of the sub-region. The isotropy or anisotropy of the local filter defines the shape of the sub-region. The local filter can be, for example, a Gaussian filter or another local filter. In particular, the local filter can be convoluted with an operator for Fourier transformation in order to determine the spatial frequency spectrum of the sub-region.The use of such a local filter makes it possible to dispense with a previously performed division into regions of interest according to a corresponding grid or the like. However, it is also possible to combine the two approaches with one another.According to at least one embodiment, a modified x-ray image based on the x-ray image is displayed on a display device, wherein the partial region in the modified x-ray image is visually emphasized compared to the x-ray image if it is determined during the checking that the spatial frequency spectrum corresponds to the absorption characteristic of the tool.The visual emphasis can be effected, for example, by a modified brightness and / or color and / or a frame or another visual emphasis. In this way, the viewer can be quickly and intuitively alerted to the position of interest in the x-ray image during the intervention.According to at least one embodiment, the tool is designed as a catheter for insertion into a blood vessel or as a catheter for insertion into another hollow organ. The catheter can be introduced directly into the hollow organ or blood vessel or, for example, through a further catheter. In the latter case, the tool is designed in particular as a so-called micro-catheter. The further catheter can be introduced into the hollow organ or blood vessel and the micro-catheter can be guided through the further catheter.According to a further aspect of the invention, a data processing device is provided. The data processing device has at least one arithmetic unit which is adapted to carry out a method according to the invention for locating a tool in an X-ray image.A computing unit can be understood in particular as a data processing device which contains a processing circuit. The computing unit can therefore process data in particular for carrying out computing operations. This also includes operations to perform indexed accesses to a data structure, for example a look-up table (LUT).The computing unit can in particular contain one or more computers, one or more microcontrollers and / or one or more integrated circuits, for example one or more application-specific integrated circuits, ASICs (application-specific integrated circuit), one or more field programmable gate arrays, FPGAs, and / or one or more single-chip systems, SoCs (system on a chip). The computing unit may also contain one or more processors, for example one or more microprocessors, one or more central processing units, CPUs (central processing units), one or more graphics processing units, GPUs (graphics processing units) and / or one or more signal processors, in particular one or more digital signal processors, DSPs. The computing unit may also include a physical or virtual group of computers or other of the aforementioned units.In various exemplary embodiments, the computing unit contains one or more hardware and / or software interfaces and / or one or more memory units.A memory unit can be used as volatile data memory, for example as dynamic random access memory, DRAM (dynamic random access memory) or static random access memory, SRAM (static random access memory), or as nonvolatile data memory, for example as read-only memory, ROM (read-only memory), as programmable read-only memory, PROM (programmable read-only memory), as erasable programmable read-only memory, EPROM (erasable programmable read-only memory), as electrically erasable programmable read-only memory, EEPROM (electrically erasable programmable read-only memory), as flash memory or flash EEPROM, as ferroelectric random access memory, FRAM (ferroelectric random access memory), as magnetoresistive random access memory, MRAM (magnetoresistive random access memory) or as phase-change random access memory, PCRAM (phase-change random access memory).According to a further aspect of the invention, an X-ray imaging system is specified, which has a data processing device according to the invention.The X-ray imaging system can be configured in particular as an X-ray angiography system.The X-ray imaging system has, in particular, an X-ray source and an X-ray detector, and the at least one arithmetic unit. The at least one arithmetic unit can control the X-ray source to generate X-ray radiation and to emit it in the direction of a region for arranging an object or patient to be examined. The X-ray detector is arranged to detect portions of the X-ray radiation that have passed through the object and can generate corresponding detector signals and transmit them to the at least one arithmetic unit. The at least one arithmetic unit is configured to generate the x-ray image based on the detector signals.According to various embodiments of the X-ray imaging system, the latter also has the tool.Further embodiments of the X-ray imaging system according to the invention follow directly from the various embodiments of the method according to the invention and vice versa. In particular, individual features and corresponding explanations and advantages with respect to the various embodiments can be transferred analogously to corresponding embodiments of the X-ray imaging system according to the invention in relation to the method according to the invention. In particular, the X-ray imaging system according to the invention is designed or programmed for carrying out a method according to the invention. In particular, the inventive X-ray imaging system carries out the inventive method.According to a further aspect of the invention, a tool for a medical intervention on a hollow organ is specified. The tool has a spatially variable X-ray absorption strength along a predefined longitudinal direction of the tool according to a predefined absorption characteristic.In various embodiments, the tool is configured in particular as a catheter, for example as a micro-catheter.Further embodiments of the tool according to the invention result from the embodiments of the method according to the invention for locating a tool and the associated explanations.According to a further aspect of the invention, a computer program comprising instructions is specified. When the commands are executed by a data processing device, in particular a data processing device according to the invention, the commands cause the data processing device to carry out a method according to the invention for locating a tool in an x-ray image.The instructions can be present, for example, as program code. The program code can be provided, for example, as binary code or assembler and / or as source code of a programming language, for example C, and / or as a program script, for example Python.According to a further aspect of the invention, a computer-readable storage medium is specified which stores a computer program according to the invention.The computer program and the computer-readable storage medium may each be referred to as a computer program product having the instructions.Further features of the invention are evident from the claims, the figures and the description of the figures. The features and combinations of features mentioned above in the description and the features and combinations of features mentioned below in the description of the figures and / or shown in the figures can be included by the invention not only in the respectively specified combination but also in other combinations. Embodiments and combinations of features may also be included in the invention, which do not have all features of an originally formulated claim. The invention may also include embodiments and combinations of features that go beyond or depart from the combinations of features set forth in the appended claims.The invention is explained in more detail below with reference to specific exemplary embodiments and associated schematic drawings. In the figures, identical or functionally identical elements can be provided with the same reference numerals. The description of identical or functionally identical elements may not necessarily be repeated with respect to different figures.The figures show: FIG. 1 is a schematic block diagram of an exemplary embodiment of an X-ray imaging system according to the invention; FIG. 2 schematically shows an exemplary representation of a spatial change of the X-ray absorption strength of a tool; FIG. 3 shows a schematic illustration of a reference location frequency spectrum corresponding to the X-ray absorption strength from FIG. 2 ; FIG. 4 schematically shows an exemplary representation of a spatial change of the X-ray absorption strength of a further tool; and FIG. 5 schematically shows an exemplary representation of a spatial change of the X-ray absorption strength of a further tool.FIG. 1 schematically illustrates an exemplary embodiment of an X-ray imaging system 1 according to the invention, which has an exemplary embodiment of a data processing device 7 according to the invention having at least one arithmetic unit. The at least one computing unit is configured to carry out an exemplary embodiment of a method according to the invention for locating a tool 2.The X-ray imaging system 1 can in particular have an X-ray source 8 and an X-ray detector 9, which are arranged on different opposite sides of a recording region for placing a patient 3. The X-ray imaging system 1 can furthermore have a display device 10 for displaying X-ray images and / or other information.During an intervention, a tool 2 can be introduced into the body of the patient 3, for example. The tool 2 can be, for example, a micro-catheter, which is inserted into a blood vessel or another hollow organ 4 of the patient 3. In this state, the at least one arithmetic unit 7 can control the X-ray source 8 to generate X-ray radiation and correspondingly obtain detector signals generated by the X-ray detector 9 in order to generate an X-ray image based thereon, which represents the tool 2 and, for example, the hollow organ 4 into which the tool 2 has been introduced. The at least one computing unit can identify a partial region 6 of the x-ray image in which the tool 2 is located. For example, the at least one arithmetic unit can display a modified x-ray image 11 on the display device 10 in that the subregion 6 is visually highlighted in comparison with the original x-ray image.In order to determine the sub-region 6 as the position of the tool 2, the at least one computing unit can determine in particular a spatial frequency spectrum of the sub-region 6. The at least one arithmetic unit can then check whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool, wherein the absorption characteristic of the tool defines a spatially variable X-ray absorption intensity 12 of the tool along a predefined longitudinal direction. If it is determined that the spatial frequency spectrum of the subregion 6 corresponds to the absorption characteristic of the tool 2, the subregion 6 can be determined as the position of the tool 2.For example, a grid 5 can be specified, which divides the x-ray image into a plurality of regions of interest, and the partial region 6 can be one of these regions of interest. The at least one arithmetic unit can check the individual regions of interest of the grid 5 as described in order to identify the subregion 6 in which the tool is arranged. In addition to or instead of the grid 5, a local filter, for example a Gaussian filter, can also be used to spatially limit the involved Fourier transformation for determining the spatial frequency spectrum of the subregion 6.The X-ray imaging system 1 is configured in particular as a 2D X-ray imaging system, for example as a C-base-based X-ray imaging system 1. However, this is not necessarily required for the reaction of the process according to the invention.For example, a modulation transfer function can be provided which describes how spatial frequencies of the material absorption in the object space are transferred to spatial frequencies of the image brightness in the image space.The tool 2, in particular the micro-catheter, is provided in such a way that a variable X-ray absorption intensity 12 is present in the longitudinal direction, for example a periodic or repetitive change of the X-ray absorption intensity 12. For example, a different metal concentration can be used in corresponding composite materials or the surface of the tool 2 can be coated with highly absorbing metals or the like in different regions and / or a proportion of plastic materials or more or less X-ray transparent fibers and so on can be modulated in a corresponding composite.A non-limiting example of a spatially variable x-ray absorption intensity 12 is schematically illustrated in FIG. 2. This is in particular a superposition of two harmonic oscillations with frequencies of 2 or 3 in arbitrary units over a length section of 4π. The oscillation with the higher frequency of 3 thus occurs six times in the length section shown, the oscillation with the lower frequency of 2 occurs four times. A corresponding reference location frequency spectrum 13 with peaks at the points 2 and 3 is schematically shown in FIG. 3.FIGS. 4 and 5 schematically show other exemplary periodic curves of the X-ray absorption intensity 12, which are present in the form of discrete steps. In the example of FIG. 2, the profile of the X-ray absorption strength 12 is represented by a simple square-wave signal with a duty cycle of more than 50%, for example of approximately 75%. In the example of FIG. 5, a somewhat more complex sequence of square-wave pulses with two different pulse durations and period durations is shown.Depending on the size of the available space along the longitudinal direction in order to implement the variable X-ray absorption intensity, more or less complex curves, preferably periodic curves, of the X-ray absorption intensity 12 can thus be achieved in order to enable the most unambiguous and nevertheless simple localization of the tool 2 possible.It can be assumed, for example, that the longitudinal direction of the tool 2 is substantially perpendicular to the X-ray projection direction, i.e. lies in the X-ray projection plane or parallel thereto. However, deviations of up to 30° or even up to 45° for the angle which the longitudinal direction encloses with the X-ray projection plane can also be accepted. The corresponding frequency characteristics or positions of peaks in the reference location frequency spectrum are likewise subject to tolerances or deviations accordingly.The spatial frequency spectrum in the sub-region 6 has, when the tool 2 is located there, a characteristic which is easy to identify, for example having one or more peaks, preferably two or more peaks having defined distances. This enables a very sensitive localization of the tool 2 in the x-ray image. The MTF of the image chain can likewise be taken into account, in particular in order to balance the relative heights of the various peaks and / or their position with respect to one another and / or their shape in the frequency space.Depending on how accurately the angle that the longitudinal direction encloses with the X-ray projection plane is known or can be narrowed down, the more accurately the peaks in the frequency spectrum can be narrowed down.The orientation of the tool 2 to the X-ray projection plane can be estimated, for example, on the basis of preoperative CT volumes. It is also possible to generate a first and a second x-ray image and to carry out the described analysis in both x-ray images. The first and the second x-ray image then correspond to different x-ray projection directions, preferably x-ray projection directions perpendicular to one another. By shifting the peak positions or changing the distances of the individual peaks and / or changing the shape of the peaks in the respective spatial frequency spectra according to the first and the second x-ray image, the orientation of the longitudinal direction with respect to the x-ray projection plane can be estimated. For example, the X-ray projection plane can be set on the basis thereof such that the longitudinal direction lies in the X-ray projection plane and a third X-ray image can accordingly be recorded in order to repeat the precise localization.Regardless of the grammatical sex of a certain term, individuals with male, female or other sex identity are included.

Claims

Method for locating a tool (2) in an X-ray image, wherein the tool (2) has an X-ray absorption intensity (12) which varies spatially along a predetermined longitudinal direction of the tool (2) in accordance with a predetermined absorption characteristic, and wherein - a spatial frequency spectrum of a partial region (6) of the X-ray image is determined; - a check is made as to whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2); and - if it is determined during the check that the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2), the partial region (6) of the X-ray image is determined as the position of the tool (2), wherein - in accordance with the absorption characteristic the spatial variation of the X-ray absorption intensity (12) along the longitudinal direction has a characteristic frequency at which a reference spatial frequency spectrum (13) of the spatial variation of the X-ray absorption intensity (12) along the longitudinal direction has a peak value; and for checking whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2), it is checked whether the spatial frequency spectrum has a peak value at a location which corresponds to the characteristic frequency within a predetermined tolerance range.Method for locating a tool (2) in an X-ray image, wherein the tool (2) has an X-ray absorption strength (12) which varies spatially along a predetermined longitudinal direction of the tool (2) in accordance with a predetermined absorption characteristic, and wherein - a spatial frequency spectrum of a partial region (6) of the X-ray image is determined; - a check is made as to whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2); and - if it is determined during the check that the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2), the partial region (6) of the X-ray image is determined as the position of the tool (2), wherein - a further X-ray image is generated which represents the tool (2) in a calibration environment; and - the absorption characteristic is determined on the basis of the further X-ray image.Method according to claim 1 or 2, wherein according to the absorption characteristic the spatial variation of the X-ray absorption strength (12) repeats along the longitudinal direction or repeats up to a different amplitude.Method according to one of the preceding claims, wherein - according to the absorption characteristic, the spatial change of the X-ray absorption intensity (12) along the longitudinal direction has two characteristic frequencies at which a reference spatial frequency spectrum (13) of the spatial change of the X-ray absorption intensity (12) along the longitudinal direction has a respective peak value; and - for checking whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2), it is checked whether the spatial frequency spectrum has a respective peak value at two points whose distance from one another within a predefined tolerance range corresponds to a distance of the two characteristic frequencies from one another.Method according to claim 4, wherein for checking whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2), it is checked whether a ratio of the peak values at the two locations of the spatial frequency spectrum corresponds to a ratio of the peak values at the two characteristic frequencies of the reference spatial frequency spectrum (13).Method according to one of the preceding claims, wherein the check as to whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2) is carried out as a function of a predefined modulation transfer function.Method according to one of the preceding claims, wherein - an angle which the longitudinal direction of the tool (2) encloses with a predetermined X-ray projection plane during the generation of the X-ray image is obtained or determined; and - the checking of whether the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2) is carried out as a function of the angle.Method according to claim 7, wherein - the X-ray image images a hollow organ (4), wherein the tool (2) is arranged in the hollow organ (4); - a three-dimensional representation of the hollow organ (4) is obtained or determined; and - the angle that the longitudinal direction of the tool (2) encloses with the X-ray projection plane is determined depending on an orientation of the three-dimensional representation with respect to the X-ray projection plane and a position of the tool (2) in the three-dimensional representation.Method according to one of the preceding claims, wherein - the sub-region (6) is an area of interest of a plurality of predetermined areas of interest of the X-ray image; or - the spatial frequency spectrum of the sub-region (6) is determined using a local filter, wherein a position of the local filter defines a position of the sub-region (6) in the X-ray image.Method according to one of the preceding claims, wherein a modified X-ray image (11) based on the X-ray image is displayed on a display device (10), wherein the partial region (6) is visually emphasized if it is determined during the checking that the spatial frequency spectrum corresponds to the absorption characteristic of the tool (2).Method according to one of the preceding claims, wherein the tool (2) is designed as a catheter for insertion into a blood vessel (4) or as a catheter for insertion into another hollow organ (4).Data processing device (7) having at least one arithmetic unit which is adapted to carry out a method according to one of the preceding claims.An X-ray imaging system (1) comprising a data processing apparatus (7) according to claim 12.A computer program product comprising instructions which, when executed by a data processing device (7), cause the data processing device (7) to perform a method according to any one of claims 1 to 11.

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