Spectrally resolved X-ray imaging

Abstract

X-ray system (1), in particular computed tomography system (1), for acquiring projection measurement data (P1, P2) of an object under investigation (2), comprising: - an X-ray array with an X-ray source (3) for emitting X-rays (10, RS1, RS2), - a photon-counting X-ray detector (4) with at least one spatially and / or temporally variable detection threshold (E1, E2) in one and the same measurement for spectrally resolved detection of the X-rays (10, RS1, RS2), wherein the photon-counting X-ray detector (4) has at least a first sub-area (4a) with a first detection threshold (E1) and at least a second sub-area (4b) arranged spatially alternating with the first sub-area (4a) with a second detection threshold (E2) which differs from the first detection threshold (E1), and wherein the photon-counting X-ray detector (4) is configured to determine the detection thresholds (E1,E2) of the first (4a) and the second sub-area (4b) to be exchanged at predetermined time intervals.

Description

The invention relates to an X-ray system for acquiring projection measurement data of an examination object. The X-ray system has an X-ray radiator arrangement with at least one X-ray radiation source for emitting X-ray radiation. The invention further relates to a method for image reconstruction by means of an X-ray system.Modern imaging methods are often used to generate two- or three-dimensional image data which can be used for visualizing an imaged examination object and, in addition, also for further applications.The imaging methods are frequently based on the detection of X-ray radiation, so-called projection measurement data being generated. For example, projection measurement data can be acquired with the aid of a computed tomography (CT) system. In CT systems, a combination of X-ray source and X-ray detector arranged opposite one another and arranged on a gantry usually circulates around a measurement space in which the examination object (which is referred to below as a patient without limiting generality) is located. The center of rotation (also called "isocenter") coincides with a so-called system axis z. During one or more revolutions, the patient is irradiated with X-ray radiation from the X-ray radiation source, wherein projection measurement data are acquired with the aid of the opposing X-ray detector.The generated projection measurement data depends in particular on the type of X-ray detector. X-ray detectors usually have a plurality of detection units, which are usually arranged in the form of a regular pixel array. The detection units each generate a detection signal for X-ray radiation impinging on the detection units, which detection signal is analyzed at specific points in time with regard to intensity and spectral distribution of the X-ray radiation in order to obtain conclusions about the examination object and to generate projection measurement data.To acquire the projection measurement data, X-ray detectors have generally been used up to now, which integrate X-ray radiation for the acquisition of measurement data over the entire energy range, the so-called total spectrum. In order to achieve a certain energy resolution, these can also be arranged in two layers as a "dual-layer X-ray detector", wherein the lower energy X-ray quanta are predominantly detected in the first layer and the remaining higher energy X-ray quanta are detected in the second layer.For the acquisition of measurement data over the entire energy range in a CT system, solid-state X-ray detectors are generally used, which convert the arriving X-ray quanta into visible light. The visible light is in turn converted into a digital measurement signal with the aid of photodiodes and suitable evaluation electronics. In this case, the energy of the arriving X-ray quanta is not differentiated, and only an energy-integrated signal is measured.Basically, two approaches are possible for generating spectrally resolved image data: on the one hand, the emitted spectrum can be regulated via the X-ray source, and on the other hand, the X-ray detector can be designed, for example by filters, and optionally be adjustable in such a way that it only detects defined regions of a relatively wide incoming spectrum. For the spectral separation, the spectral sensitivity distribution is an indicator which is given by the product of the spectrum and the sensitivity of the X-ray detector.In order to generate spectrally resolved CT images, it is therefore necessary, in combination with such a conventional X-ray detector, to change the X-ray spectrum at the X-ray radiation source in order to enable spectral material separation. As a technical solution, this can be done in the simplest form by two independent scans over the same object with different tube voltages.A further variant is the "split filter", the prefilter of which consists of two different materials which are transmissive for different X-ray spectra. The x-ray radiation is then detected in each case only in the x-ray detector units or x-ray detector subareas of an x-ray detector which are assigned to the corresponding prefilter. In this variant, also characterized by the English word "TwinBeam", the spectrum is selectively changed at the X-ray source by a selective material filter (for example tin and gold), so that a part of the X-ray detector measures a spectrum filtered by a first filter material, for example a "gold" spectrum, and a separate part of the X-ray detector detects a spectrum filtered by a second filter material, for example a "tin" spectrum. Alternatively, different filters can be introduced into the beam path of the X-ray source in a manner offset in time.In so-called "kV switching", the tube voltage, also called acceleration voltage, is varied in short time intervals over one or more readout cycles, so that the electrons absorb different energies, which ultimately result as bremsstrahlung radiation in different X-ray spectra. For example, the tube voltage is varied very quickly over one revolution of the CT system, i.e. between e.g. 80 kV and 140 kV every 0.5 ms with a rotation time of e.g. 0.5 seconds.Spectrally resolved image data can also be obtained with the aid of dual-source systems, which make it possible by means of two X-ray tubes and two X-ray detectors to set the tube voltages and currents independently also in combination with additional prefiltering, in order in this way to obtain X-rays with different spectra.The methods described here for generating x-rays with different spectrum, such as e.g. dual source, kV switching and the use of filters, can also be combined with one another as desired.In contrast, photon counting X-ray detectors measure the input spectrum in a spectrally resolved manner. In this case, depending on the number of detection thresholds or energy thresholds implemented, a plurality of spectrally different data sets are generated. However, the spectral separation of the individual data sets can be less good due to the only one input spectrum than in the previously described multi-energy approaches, in which the spectrum of the emitted x-ray radiation is varied. Counting CT X-ray detectors make it possible to individually acquire the energy of each incident X-ray quantum. The X-ray detector signal resulting in this way usually consists of a count rate for each individual X-ray detector pixel per individual projection (~0.25 ms duration). This information is available with spectral resolution by being simultaneously fed back for a plurality of detection thresholds, e.g. 20 keV / 40 keV / 65 keV / 80 keV. CT images from the data of the lowest threshold (e.g. 20 keV) thus correspond approximately to a classic CT x-ray detector at the same tube voltage with all x-ray quanta, while a CT image from an upper threshold (e.g. 65 keV) corresponds to a classic image at a higher tube voltage, since only the high-energy measured quanta contribute to the reconstruction. In this way, a counting x-ray detector provides spectrally resolved information intrinsically, which enables spectral image reconstruction (iodine images, virtual non-contrast images, monoenergetic images, material parameter images).Conventionally, spectral separation, which can be generated in this way with a counting X-ray detector, is a limiting factor. The spectral separation achieved lies in a range which is comparable to two independent CT scans at 80 kV and 140 kV. Although this is sufficient for a multiplicity of applications, it does not achieve the spectral separation of two different recordings at, for example, 70 kV and 150 kV with additional tin filtering of the high spectrum, which can be achieved with a classic dual-source dual-energy system.Furthermore, counting detectors with a plurality of detection thresholds simultaneously generate very extensive amounts of data, the transmission of which requires sufficient data transmission capacity to be present. However, the data transmission capacity of the data link, in particular of a CT system, is limited by a "bottleneck", e.g. the slip ring, which forms the interface for the data transmission between the rotating part of a CT system and the stationary part of a CT system. Therefore, it is necessary to keep the amount of projection measurement data generated by the X-ray detector as small as possible.The X-ray system is known from the document DE 10 2015 226 489 A1 that, for the acquisition of projection data of an examination object, an X-ray generator arrangement with a number of X-ray radiation sources and a number of photon-counting detectors with at least two detection thresholds comprises. In this case, the number of X-ray sources is designed such that it generates X-ray radiation with at least two X-ray radiation spectra.A CT system is known from document US 2008 / 0 205 585 A1 that comprises a detector element which is segmented into a plurality of subpixels. Each sub-pixel has at least two threshold values and counting channels, wherein the second threshold value varies for each sub-pixel across the detector element.It is therefore an object of the present invention to specify an X-ray system and a method for acquiring projection measurement data on the basis of a counting X-ray detector and with improved spectral separation and / or reduced data volume than has been achieved hitherto in the application of counting X-ray detectors.This object is achieved by an X-ray system according to claim 1 and a method for image reconstruction according to claim 11. The X-ray system mentioned at the outset, preferably a computed tomography system, comprises, in addition to the X-ray emitter arrangement already described at the outset, for acquisition of projection measurement data of an examination object, according to the invention a photon-counting X-ray detector having at least one detection threshold, which is spatially variable and / or temporally variable in one and the same measurement, for spectrally resolved detection of the X-ray radiation.It should be expressly mentioned at this point that the indefinite article "a" or "an" does not mean any restriction with respect to the number of claimed articles, but rather it also comprises a plurality thereof. In particular, the X-ray system according to the invention can also comprise a plurality of X-ray radiation sources and a plurality of X-ray detectors. However, in the present patent application, an X-ray system having a single X-ray source and a single X-ray detector is preferably considered.The term "X-ray system" is preferably understood to mean a computed tomography system, but it can also comprise a simple projection X-ray device, a C-arm X-ray device, an angiography device, or other devices of X-ray-based imaging. In the following, therefore, without limiting generality, reference is also made to a computed tomography system. The object to be examined can be an object or an animal, but is preferably a human patient.The X-ray source of the X-ray source arrangement comprises an X-ray tube and thus represents the radiation-generating unit with which X-ray radiation with a predetermined spectrum is emitted. This unit can also comprise further elements such as, for example, diaphragms, filters, shutters or the like. The term "spectrum" or X-ray spectrum corresponds to emitted radiation with a distribution of different energies or wavelengths, which is generated in an X-ray radiation source as bremsstrahlung radiation. In this case, the spectrum is frequently characterized by the voltage present in the X-ray tube, for example 80 kV or 140 kV or the like.Projection measurement data of the examination object are acquired by the X-ray detector. These thus indicate, in a spatially resolved manner, the intensity and the energy distribution of the X-ray radiation which are projected as a defined spectrum from the X-ray radiation source in the main irradiation direction through the examination object onto the X-ray detector. In this case, the x-ray radiation can be absorbed and / or scattered locally differently by the examination object depending on the thickness and the material of the examination object. As a result, the projection measurement data contains local information about the examination object in the logical reversal. The energy distribution of the x-ray radiation arriving at the x-ray detector behind the examination object, i.e. the energy distribution of the projection, is measured via the detection thresholds of the location- and energy-resolving x-ray detector. The measurement data to be understood as projection measurement data can thus form a basis for generating image data of the examination object. In this case, the projection measurement data can be assigned to the spectrum emitted by the X-ray source. The projection measurement data thus comprise information about the used source-side spectrum, location information, information about the main irradiation direction and about the energy of the individual photons deposited in the X-ray detector.The photon-counting X-ray detector in turn preferably comprises a sensor array of semiconductor sensors, e.g. made of silicon (Si), cadmium telluride (CdTe) or cadmium zinc telluride (CZT), as X-ray detector units or X-ray detector pixels which convert the incident photons directly into an electrical signal.Photon-counting X-ray detectors, also referred to as quantum-counting X-ray detectors, generally have an arrangement of a plurality of detector elements (pixels) made of a directly converting semiconductor material. Incident x-ray photons or x-ray quanta generate an electrical signal in the semiconductor material. A detected radiation quantum generates in the respective detector element of an exemplary photon-counting detector a charge pulse which is converted by the detector electronics into a measurement voltage which is compared in one or more comparators of the exemplary photon-counting detector with threshold voltages representing different energy levels. In this way, a specific energy can be assigned to a detected photon and the photon can be counted accordingly.Since the magnitude of the charge pulse or of the measurement voltage generated therefrom depends on the energy of the incident radiation quantum, spectral selection of the counted radiation quanta can be achieved by adjusting the electrical threshold height or the threshold value of the comparator. Only the radiation quanta are counted which, due to their energy, generate an electrical signal which exceeds the threshold value of the comparator. This represents a detection threshold, wherein only signals of X-ray quanta are detected, the energy of which lies above the detection threshold.In the example photon-counting X-ray detector, one or more detection thresholds, in particular a plurality of adjustable detection thresholds, can be provided. This enables the detection of X-ray quanta in a plurality of energy ranges (also referred to as "energy windows" or "energy bin" or "bin") which are defined or adjustable by the detection thresholds.The photons are thus preferably detected locally via the sensor field, i.e. detected in a locally resolved manner, and at the same time sorted into so-called bins depending on their energy, i.e. detected in an energy-resolved manner. The semiconductor sensors can thus separately record the light quanta for at least two different energy ranges. For this sorting, detection thresholds can be established as required and set electronically via a control device. As a result, the individual X-ray detector units can be adapted to the requirements of the examination. The electrical signals, the totality of which represents the projection measurement data, are then forwarded to an evaluation unit. This can be included by the control device and implemented, for example, in a computer unit.In the simplest case, the photon-counting X-ray detector has only a single detection threshold which varies over time or locally. The detection threshold specifies a threshold value or minimum value of an energy of an X-ray quantum that is required in order to be able to detect the X-ray quantum with a counting X-ray detector or a sensor unit of the X-ray detector. Due to the spatial and / or temporal variation of the detection threshold, it is now possible to reduce the number of detection thresholds per pixel, optimally to a single detection threshold, and due to the variation of the energy value of this individual detection threshold, whether it be spatially and / or temporally, nevertheless to obtain spectrally differentiated or resolved projection measurement data from an examination object. In the case of a spatial variation of the detection thresholds, therefore, different regions of one and the same X-ray detector, in particular different sensor pixels, are controlled in such a way that they have different detector thresholds with different energy ranges at one and the same point in time. In the case of a temporal variation of the detector thresholds, on the other hand, the detector thresholds of an X-ray detector change as a function of time during one and the same measurement. The temporal variation is preferably carried out alternately at short temporal intervals, in particular at temporal intervals, which comprise the rotation time of the X-ray detector system or a multiple thereof. The difference from already known X-ray systems with photon-counting X-ray detectors is thus the special control of the X-ray detector during one and the same measurement. In this case, projection measurement data with different detection thresholds are recorded in different temporal sections or different detector regions.By "one and the same measurement" is meant a single acquisition or image acquisition, which is also typically referred to as "scan". The term "one and the same measurement" thus denotes a set of individual projections consisting of the respective detector measurement values (typically a few thousand projections), which represent a complete acquisition and from which a complete image volume is calculated at the end. The typical acquisition time of such a single scan is between 1 and 30 seconds.This flexible type of control can now be used advantageously for reducing the number of detection thresholds simultaneously required per pixel, since the threshold values of the detection thresholds are adapted to the respective incident specific spectrum or the X-ray radiation incident with the respective specific spectrum. By reducing the number of detection thresholds, the necessary data rate during the transmission of the projection measurement data from the X-ray detector, for example via a slip ring, to a stationary control device is advantageously reduced.If the X-ray detector has a plurality of different detection thresholds per X-ray pixel, the spatially and / or temporally spectrally variable X-ray radiation can be separated more precisely than is possible in an X-ray system with spatially and temporally invariable threshold values for one and the same measurement or only one variable detection threshold per X-ray detector unit. For example, two detection thresholds can be selected such that projection measurement data with low energy is acquired with the X-ray detector or X-ray detector pixel above the first detection threshold and below the second detection threshold and projection measurement data with particularly high energy is acquired separately therefrom above the second detection threshold, whereby the spectral separation of the irradiated X-ray spectrum and thus also the image quality of possible image data constructed on the basis of the spectral projection measurement data is improved. It should be noted that the second detection threshold cannot be selected to be as high as desired, because the number of X-ray quanta detected above the second detection threshold then decreases so much that the statistical error becomes too large for reliable measurement. If necessary, the value for the individual detection thresholds can be determined in an optimized manner by a simulation program, for example as a function of the examination object and the X-ray spectrum of the irradiated X-ray radiation, and the image quality of image data of a subsequent image reconstruction can thus be optimized. To implement the X-ray system according to the invention, the firmware for defining and selecting the thresholds of different X-ray detector subareas, in particular different X-ray detector pixels, can be modified accordingly in order to enable a selection of the threshold values from their detection thresholds that is individually different in time or space.The method mentioned at the beginning for image reconstruction by means of an X-ray system comprises the following steps:In a first step, X-ray radiation is generated and emitted by means of an X-ray emitter arrangement with an X-ray radiation source.In a second step, an energy-resolved detection of at least the X-ray radiation urgent through the examination object is carried out as projection measurement data by a photon-counting X-ray detector having at least one spatially and / or temporally variable detection threshold for spectrally resolved detection of the X-ray radiation.In a third step, an image is reconstructed on the basis of the projection measurement data of the examination object.An X-ray system according to the invention is preferably used to carry out the method. The X-ray system can be automatically controlled by a control device, for example on the basis of a specified examination protocol, and / or settings of the individual parameters required for the examination can be carried out by an operator via an input interface. In particular, the x-ray spectra used and the detection thresholds can be spatially variably defined during one and the same measurement or can be varied in time during the measurement. After the acquisition, the projection measurement data can be transmitted directly to an evaluation unit and / or to a buffer memory in which the projection measurement data is stored as projection measurement data. The step of image reconstruction can therefore take place immediately or at a later point in time, as required. In the course of the reconstruction, an image of the examination object is generated on the basis of the projection measurement data of the examination object, if appropriate, in accordance with known image generation methods modified for a finer spectral separation.This can be an image of the examination object, e.g. in the form of sectional images, 3D images or even 4D image data (with a temporal component). The image can be displayed in color in gray levels or also in a manner highlighted according to a material or fabric composition, wherein the composition is determined on the basis of the projection measurement data. This is particularly advantageous for the detection of calcifications of blood vessels, contrasted objects and / or tumors.The method according to the invention for image reconstruction by means of an X-ray system shares the advantages of the X-ray system according to the invention.A large part of the aforementioned components of the X-ray system can be realized wholly or partly in the form of software modules in a processor of a corresponding computing system, for example by a control device of an X-ray system. A realization largely through software has the advantage that even previously used computing systems, in particular control devices, can be easily retrofitted with a software update in order to operate in the manner according to the invention. In this respect, the object is also achieved by a corresponding computer program product having a computer program which can be loaded directly into a storage device of a control device of an X-ray system, having program sections in order to carry out all steps of the method according to the invention when the program is executed in the control device. In addition to the computer program, such a computer program product can optionally comprise additional components, such as documentation and / or additional components, also hardware components, such as hardware keys (dongles, etc.) for using the software.For transport to the control device and / or for storage on or in the control device, a computer-readable medium, for example a memory stick, a hard disk or another transportable or permanently installed data carrier, can be used, on which the program sections of the computer program that can be read in and executed by a computer unit of the control device are stored. For this purpose, the computer unit can have, for example, one or more cooperating microprocessors or the like.Further, particularly advantageous embodiments and developments of the invention are evident from the dependent claims and the following description, wherein the independent claims of one claim category can also be developed analogously to the dependent claims of another claim category and in particular individual features of different exemplary embodiments or variants can also be combined to form new exemplary embodiments or variants.In a variant of the X-ray system according to the invention, the photon-counting X-ray detector has at least two spatially and / or temporally variable detection thresholds for spectrally resolved detection of the X-ray radiation. In this variant, the X-ray spectrum can be segmented, whereby the spectral separation is improved.In a preferred embodiment of the X-ray system according to the invention, the photon-counting X-ray detector has at least four, preferably at least six, detection thresholds. By means of a higher number of detection thresholds, the X-ray spectrum can be further segmented, which additionally improves the spectral separation. For this purpose, however, the maximum data transmission rate must be increased accordingly in comparison with a variant with only one or two detection thresholds.In a particularly preferred variant of the X-ray system according to the invention, the X-ray emitter arrangement is designed to generate X-ray radiation with at least two spectra, which means that either a single X-ray radiation source can generate at least two, preferably different, spectra, or a plurality of X-ray radiation sources preferably generate different spectra. Thus, in the case of a plurality of X-ray sources as well, each individual X-ray source can be configured to generate different spectra.Advantageously, the x-ray system according to the invention thus combines the known multi-energy techniques with a photon-counting x-ray detector and not with energy-integrating x-ray detectors, wherein the detection thresholds of the entire x-ray detector or individual x-ray detector fields or x-ray detector units of the photon-counting x-ray detector are configured to be variable in time or space.The photon-counting X-ray detector is preferably configured such that the detection thresholds of the X-ray detector are adapted to a temporal or spatial variation of the X-ray spectrum occurring during one and the same measurement. This makes it possible to achieve a better spectral separation, i.e. a finer segmentation of the X-ray spectrum acquired for the imaging, since the spectrum is matched to one another, divided in regions or time-dependent fashion both on the source side during the emission and on the X-ray detector side during the acquisition. This ultimately has an effect in improved imaging.For generating different spectra, the X-ray radiation source of the X-ray system according to the invention preferably comprises at least one prefilter for dividing the X-ray beam into at least two spatially separated X-ray beam sections having a different X-ray spectrum. The prefilter can be designed, for example, as a plate made of light or heavy metal in order to absorb the soft and medium-soft rays. A prefilter of this shape therefore substantially cures the beam, since it primarily filters out the X-ray radiation with fewer penetrating, larger wavelengths. It is thus possible, already by means of a prefilter, to generate two different X-ray spectra, namely the spectrum actually generated by the X-ray tube and the spectrum modified by the prefilter. Two possible applications of this effect are described in the subvariants below. Such a filter is also referred to as a split filter, since it splits the x-ray beam into two spatially separated partial beams with different x-ray spectra. An X-ray beam generated by the split filter is referred to as a "twin beam" and a "twin beam", respectively.In a special variant, the at least one prefilter of the X-ray system according to the invention is preferably configured to spatially distribute regions of an X-ray spectrum to defined angular ranges of the photon-counting X-ray detector. In this variant, the detection thresholds of the photon-counting X-ray detector have different values in different angular ranges depending on the ranges of the X-ray spectrum, in order to be able to resolve the partial beams having different spectrum particularly exactly in spectral terms.For this purpose, the prefilter has filter regions made of preferably different materials, which are transmissive for different spectra. The radiation emitted by this split filter is accordingly emitted via the filter regions under certain angular ranges, so that it finally also impinges on dedicated regions of the sensor array of the X-ray detector which are assigned to the respective spectrum by virtue of their arrangement. If, for example, an arrangement of x-ray source and x-ray detector is now rotated on a circular path around the examination object and all filter regions lie on the circular path, complete projection measurement data of the examination object are acquired for all spectra generated by the prefilter. In the case of filter regions arranged orthogonally to the circular path, that is to say in the direction of the axis of rotation, the projection measurement data can be acquired, for example, in different revolutions if the respective region of the examination object is covered a plurality of times in accordance with the filter regions under axial displacement during a spiral scan. Particularly preferably, the prefilter comprises two filter regions.In order to split the X-ray spectrum, the at least one prefilter preferably has a first sub-filter with a first filter material and a second sub-filter with a second filter material different from the first filter material. Advantageously, different spectral components are absorbed by sub-filters with different filter materials, so that the previously mentioned spatially divided sub-beams with different spectra are produced.Particularly preferably, the first filter material of the prefilter of the X-ray system according to the invention comprises gold and the second filter material comprises tin. Advantageously, X-rays with a low energy X-ray spectrum are generated with the gold filter and X-rays with a high energy X-ray spectrum are generated with the tin filter.In a variant of the X-ray system according to the invention, the at least one prefilter is configured to modify the X-ray spectrum in time, which is detected in a synchronized manner by the photon-counting X-ray detector.In this second subvariant of the X-ray system according to the invention, the at least one prefilter modifies the X-ray spectrum in time, which is detected in a synchronized manner by the photon-counting X-ray detector. As a result, the spectrum is varied over time here. This can be achieved, for example, by periodic, rapid introduction and removal of the prefilter into or from the beam path. Alternatively, it is also possible to switch between different prefilters. In this variant, a switching of the detection thresholds of the X-ray detector is synchronized with the switching between the different X-ray spectra.In principle, the two subvariants described above can also be combined with one another. Thus, in principle, for example, by suitable prefilters arranged in series, spatially separated and temporally modified spectra can be generated at the same time in order to achieve an even finer energetic segmentation of the spectra.In a very practicable variant of the X-ray system according to the invention, the X-ray radiation source is designed such that it switches between different acceleration voltages under the control of a control device. In addition, the photon-counting X-ray detector is configured to detect the X-ray radiation in a synchronized manner with the control of the X-ray radiation source. In this variant, the selection of the detection thresholds of the X-ray detector is synchronized with the X-ray spectrum emitted at this time.Thus, the x-ray radiation is detected by the photon-counting x-ray detector synchronously with the change of the acceleration voltages in each case with one or more detection thresholds that are particularly suitable for the specific x-ray spectrum. In this variant, the so-called kV switching, the spectrum of the emitted x-ray radiation can therefore already be modified during its generation solely by regulating the tube voltage. Preferably, the tube voltage alternates at high frequency, e.g., 500 or 1000 Hz, between two fixed values, e.g., 80 kV and 140 kV. For the synchronization of the detection and for the comparison with the originally emitted spectra, the data can be used here advantageously which are recorded from regions in which the X-ray radiation does not pass through the examination object. This is because these data essentially represent, i.e. apart from the influence of the ambient air, exactly the spectrum emitted by the X-ray source.In the above-described variant of the X-ray system according to the invention with "kV switching", the photon-counting X-ray detector is therefore preferably configured to switch back and forth between at least two adapted sets of counter thresholds or detection thresholds synchronized with the switching of the X-ray source under the control of the X-ray system. Advantageously, the counter thresholds are adapted to the incident X-ray spectrum, whereby the resolution of the different X-ray spectra emitted at different times is improved.Particularly preferably, the photon-counting X-ray detector of the X-ray system according to the invention is configured to read out and transmit only a portion of the counter thresholds in each case under the control of the control device synchronized with the switching of the X-ray source. Spectral resolution can advantageously be achieved despite a technically induced restriction of the data transmission rate for the transmission of data between the rotating x-ray detector via a slip ring to a stationary evaluation unit. In phases of high voltage, for example at 140 kV, only a higher detection threshold, for example 55 keV, is thus read out and transmitted to the evaluation unit, and at low voltage, for example 80 kV, only a low detection threshold, for example 20 keV, is accordingly read out and transmitted to the evaluation unit.A further variant of the X-ray system according to the invention comprises a control device which is designed such that it controls an X-ray source and the photon-counting X-ray detector such that projection measurement data with different X-ray spectra are acquired in at least two different recordings. In this case, the different X-ray spectra are generated by different acceleration voltages of the X-ray source. In this variant called "double spiral", the X-ray spectrum of an X-ray tube is thus varied over the tube voltage in a similar manner to the case of kV switching. Unlike in kV switching, however, the voltage is not changed at a high frequency, but remains constant, e.g. 80 kV, for a scanning operation, and is controlled to a different constant value, e.g. 140 kV, for the at least one sequential scan. In this variant, too, the detection thresholds are adjusted in synchronization with the change in the tube voltage. Such a variation of the tube voltage between different recordings can be combined, for example, with a slow temporal variation of the detection thresholds and a spatial variation of the detection thresholds.In a further variant of the X-ray system according to the invention, the X-ray radiator arrangement has at least two X-ray radiation sources which are designed such that they generate different X-ray spectra. The different spectra of the at least two X-ray radiation sources can be generated by different acceleration voltages and / or different filters. It is indeed possible for only one large X-ray detector with detection regions which are assigned to the individual X-ray sources to be formed for the at least two X-ray sources. Preferably, however, each X-ray source is assigned its own X-ray detector. The X-ray system particularly preferably has exactly two X-ray radiation sources, preferably arranged approximately orthogonally to one another, with a respectively assigned X-ray detector. This variant advantageously makes it possible to generate completely separated X-ray spectra and also to record these using separate X-ray detectors. Here, too, detection thresholds are varied in time or space on the detector side in order, if appropriate, to improve the acquisition of the different spectra or, if appropriate, to reduce the data transmission rate during the transmission of projection measurement data between the X-ray detector and an evaluation unit.In a preferred embodiment of a method according to the invention for image reconstruction, the projection measurement data are assigned to the respective X-ray spectrum. This means that on the basis of the energy values per se and / or the spatial separation and / or temporal synchronization, the spectrum emitted by the X-ray source can be linked to the values detected by the X-ray detector in the projection measurement data. It can thus be understood on the basis of the projection measurement data with which emitted spectrum the acquired energy values or intensity values were generated.In the reconstruction of the image according to the invention, the projection measurement data assigned to the different X-ray spectra are preferably combined to form an optimized image. The image can be optimized on the basis of different criteria. For example, specific regions can be selected from the projection measurement data of the examination object, in which regions e.g. blood vessels, bones or organs are located, which are optimally represented using projection measurement data from a specific spectral region. The optimization can also be effected with regard to parameters such as contrast, noise or the like.In one embodiment of the X-ray system according to the invention, the X-ray system comprises a control device which is designed such that it controls an X-ray radiation source and the photon-counting X-ray detector such that at least two sets of projection measurement data with different X-ray spectra are acquired.The photon-counting X-ray detector of the X-ray system according to the invention has at least one first subregion having at least one first detection threshold and a second subregion, arranged spatially alternately to the first subregion, having at least one second detection threshold, the threshold value of which differs from a threshold value of the first detection threshold.In this variant, spectral images can be generated, although only one detection threshold is available in the respective subareas. This variant is particularly preferably used in so-called spectral O'CLOCK imaging ("O'CLOCK" stands for "ultra high resolution", which means "ultra high resolution" in German). In this case, a checkerboard-like arrangement of X-ray detector fields or X-ray detector pixels is formed, wherein fields directly adjacent to the respective edges of a field have different detection thresholds or threshold values and diagonally adjacent fields have the same detection thresholds or threshold values. In order to obtain data for the respective gaps of the spectral image data for an ultra-high-resolution image representation, interpolation is carried out on the basis of the projection measurement data of the adjacent fields of the same spectrum, whereby the maximum accessible sharpness of the image data obtained on the basis of the projection measurement data is reduced compared to image data which are based on projection measurement data of all fields of the X-ray detector. However, the sampling is uniform during the course of the entire data recording without any appreciable spatial or temporal offset and the potential artifacts resulting therefrom.Particularly preferably, the first sub-region and the second sub-region form a checkerboard-like pattern. Advantageously, missing measured values of fields lying between two fields with the same detection threshold can be interpolated easily and precisely. This interpolation can be carried out quasi "three-dimensionally", since, in addition to the neighbors in the line direction and channel direction, a temporal neighbor is also available in the next projection with the target spectrum, since the detector pixel has then switched over. That is, each pixel to be interpolated has 6 neighbors in total at each time (2 x channel + 2 x row + 2 x adjacent projection).The first and the second sub-region are configured to exchange their detection thresholds or threshold values after each projection. In this case, projection measurement data with maximum resolution are obtained for both spectra. Because only a single image is produced, the spectral information is completely ignored and a mixed image is simply calculated on the basis of the measured attenuation, which mixed image contains the full original resolution without effects of the interpolation.The variants described above, in which alternating partial regions of an X-ray detector are exposed to different threshold voltages, can also be combined with the above variants, in which the X-ray radiation is divided into at least two spatially separated X-ray sections having a different X-ray spectrum ("split filters").In the so-called threshold mixing (too-to-be-german mixing of count values of a plurality of detection thresholds), sensor data of a plurality of detection thresholds are combined. For example, there is conventionally a technique in which four thresholds E 1,... E4 are recorded on the X-ray detector, but are then combined to two new thresholds before transmission, i.e. E1*=2*E1+E2 and E2*=E3+E4 are transmitted. In this conventional threshold mixing, no spatial or temporal variation is generated. In a combination of the above-described variants WATCH scan and TwinBeam, i.e. a preferably checkerboard-like division of the sensor field and a spatial and spectral division of the X-ray beam, it is possible that the basic arrangement of the "checkerboard" of the WATCH scan remains, but first and second thresholds E 1 and E 2 are used on the side covered by a first filter, in particular gold, and third and fourth thresholds E 3 and E 4 different therefrom are used on the side covered by a second filter, in particular tin, which respectively generate this checkerboard arrangement and at the same time produce an improved spectral separation by the difference of E 1, E 2 and E 3, E 4.A combination of the variants in which the X-ray radiation is divided into at least two spatially separated X-ray sections with a different X-ray spectrum ("split filter"), with the variants in which the X-ray source switches between different acceleration voltages (kV switching), can also be realized. Here too, particularly good spectral separation can be achieved by suitable selection of the different spectra of the emitted X-rays and a suitable selection of the filters.The invention is explained in more detail below with reference to the attached figures on the basis of exemplary embodiments. In this case, identical components are provided with identical reference numerals in the different figures. The figures are generally not to scale. The following are shown: FIG. 1 shows a schematic illustration of an X-ray system with split filter according to an exemplary embodiment of the invention, FIG. 2 shows a schematic illustration of an X-ray system with kV switching according to an exemplary embodiment of the invention, FIG. 3 shows a schematic representation of an X-ray detector of an X-ray system with spectral X-ray detection and ultra-high resolution, FIG. 4 shows a flow chart which illustrates the method according to the invention for image reconstruction by means of an X-ray system according to an exemplary embodiment of the invention.FIG. 1 shows a computed tomography system 1 according to an exemplary embodiment of the invention with a split filter 9.The use of a split filter 9 enables the recording of spectrally resolved images with only one X-ray source 3.Since a computed tomography system 1 represents the preferred embodiment of the X-ray system according to the invention, the following explanations relate, without limiting generality, to a computed tomography system 1. The computed tomography system 1 here comprises an X-ray emitter arrangement having an X-ray radiation source 3, an X-ray detector 4 and a control device 5. The X-ray source 3 and the X-ray detector 4 are movable and arranged diametrically with respect to one another on a circular path 6. They are therefore in a fixed positional relationship with respect to one another in which the X-ray detector 4 detects the radiation emitted by the X-ray source 3 and thus form a first source-X-ray detector arrangement. A patient 2 is located in the center of the circular path 6 as the examination object. The X-ray source 3 comprises an X-ray tube 7 and a diaphragm 8, the diaphragm 8 being arranged at a slight distance from the X-ray tube 7 on a side of the X-ray tube 7 facing the patient 2. Via it, an exit angle of x-ray radiation 10 emitted by the x-ray tube 7 during operation can be adjusted.A prefilter in the form of a split filter 9 is introduced between the X-ray tube 7 and the diaphragm 8. This filter section has two filter sections 9 a, 9 bwith materials with different properties with regard to their X-ray absorption, for example gold and tin. A first filter section 9 a, which is shown on the left-hand side in FIG. 1, has gold as the filter material in this specific exemplary embodiment, and a second filter section 9 b, which is shown on the right-hand side in FIG. 1, has tin as the filter material. The x-ray radiation generated by the x-ray tube 7 is thus filtered differently depending on the material and exits from the x-ray radiation source 3 in the form of two different x-ray spectra. These are spatially separated along a separation line 11 corresponding to the boundary between the materials of the split filter 9 into a low-energy and a high-energy spectrum. In addition, the x-ray radiation 10 of the two spectra projected by the patient 2 onto the x-ray detector 4 is also detected spatially separately in different regions of the x-ray detector 4 and can thus be assigned to the respectively emitted spectrum. For this purpose, the X-ray detector 4 has two different sub-regions 4 a, 4 b, wherein the first sub-region 4 a(shown on the left in FIG. 1 ) has detector fields with detection thresholds which are adapted to the detection of low-energy X-ray radiation, and the second sub-region 4 b(shown on the right in FIG. 1 ) has detector fields with detection thresholds which are adapted to the detection of high-energy X-ray radiation. Alternatively, the split filter 9 can also be arranged rotated through 90°, so that the separating line 11 divides the different spectra quasi in the image plane. In this alternative, the projection measurement data for both spectra for the regions of the patient 2 to be recorded are acquired sequentially with the table feed. In this alternative, the detector is also divided into different sub-regions 4 a, 4 bby a separation line lying in the image plane.In operation, for acquisition of projection measurement data, the X-ray source 3 and the X-ray detector 4 are rotated on the circular path 6 around the patient 2. The acquired projection measurement data can then be transmitted to an evaluation unit located, for example, in the control device 5 and reconstructed there to form an image B of the patient 2 (see also step 4.III in FIG. 4 ). In order to acquire projection measurement data from further regions of the patient 2, the patient 2 can be displaced perpendicular to the plane of the circular path 6 relative to the computed tomography system 1, for example by means of a positionable patient table (not shown here). In the case of so-called spiral CT, the acquisition takes place continuously with likewise continuous table feed.In an X-ray detector 4 with 64 X-ray detector rows, rows 1 to 32 acquire the X-ray spectrum which has been filtered with the gold filter 9 aand rows 33 to 64 acquire the X-ray spectrum which has been filtered with the tin filter 9 b. The projection measurement data acquisition is preferably carried out in a spiral mode with a low pitch of, for example, 0.4, as a result of which independent image reconstruction is separately made possible in two volumes on the basis of projection measurement data of the first 32 lines and of the last 32 lines, images being generated in a spectrally resolved manner. The detection thresholds of the individual subareas 4 a, 4 bof the X-ray detector 4, which are assigned to X-rays with different X-ray spectra, are adapted to the different X-ray spectra in the arrangement shown in FIG. 1. For example, with a fixed tube voltage of 140 keV, detection thresholds of 20 keV and 35 keV are set up for the first 32 rows of the X-ray detector 4, i.e. the first sub-region 4 aof the X-ray detector 4, which records the low-energy X-ray radiation that was transmitted through the gold filter 9 a, and correspondingly higher detection thresholds of 25 keV and 60 keV are set up for the last 32 rows of the X-ray detector 4, i.e. the second sub-region 4 bof the X-ray detector 4, which records the high-energy X-ray radiation that was transmitted through the tin filter 9 b. Alternatively, the necessary data rate can also be halved by only providing a single detection threshold for each sub-region 4 a, 4 bof the X-ray detector 4. In this variant, a detection threshold of 20 keV would be used for the first 32 rows and a detection threshold of 50 keV would be used for the second 32 rows.In contrast to the CT system shown in FIG. 1, FIG. 2 shows a computed tomography system 1, in which the acceleration voltage of the X-ray source 3 is varied over time by alternating, for example in a step-like manner regulated by the control device 5, between two values of 80 kV and 140 kV with a frequency of, for example, 1000 Hz or 500 Hz, which is also referred to as "kV switching". In a typical CT acquisition with 2000 projections per revolution, the tube voltage or acceleration voltage of the X-ray source 3 is thus switched over every 2 to 4 projections.The acceleration voltage thus alternates rapidly compared to the rotational movement of the X-ray detector 4 and the X-ray source 3, which takes place on the circular path at a frequency of typically at most about 4 Hz. The alternating acceleration voltage generates different X-ray spectra in the X-ray tube 7, in particular low energy and high energy X-ray spectra. These penetrate through the patient 2 as X-ray radiation 10 at the exit angle defined via the aperture 8. Projection measurement data are thus acquired from different angular positions relative to the patient 2, which can be assigned in time to the spectrum emitted by the X-ray tube 7 of the X-ray source 3.The recorded projection measurement data are then divided into their high and low energy projections for reconstruction and a separate reconstruction is carried out. The spectral separation of the projection measurement data is improved in that the threshold values of the detection thresholds of the X-ray detector 4 are changed in synchronization with the change in the energy spectrum of the X-ray source 3. The threshold values of the detection thresholds of the X-ray detector 4 thus jump back and forth between two settings in time. If it is desired to reduce the data transmission rate, then only projection measurement data can be transmitted from a single detection threshold in each case, wherein a higher threshold is read out at 55 keV, for example, in phases of high voltage (e.g. 140 kV) and an energy-lower threshold at 20 keV is read out in phases of low voltage (e.g. 80 kV) and transmitted to an evaluation unit or the control device 5.FIG. 3 illustrates a schematic illustration of an X-ray detector 4 of an X-ray system with spectral X-ray detection and ultra-high resolution. In this variant, much smaller areas are used for the individual X-ray detector pixels or detector fields F 11,..., F nm of the X-ray detector 4 than is the case with conventional X-ray detectors. In the so-called S1 layout, an increased resolution can be achieved by the greater number of X-ray detector pixels per area, for example 2752 channels (channels corresponding to columns) and 288 rows. However, for the reasons already mentioned, the data transmission is not designed for a correspondingly high data rate, so that the resolution, in particular when recording moving images, is reduced by combining a total of four pixels of two adjacent lines. For example, in the so-called M4 layout, projection measurement data of only 1376 channels with only 144 lines are thus transmitted.In the so-called OHR mode, the number of lines is reduced to 120 lines and, in the case of recording a moving heart, conventionally projection measurement data of only a threshold of 20 keV are transmitted, because the data transmission rate is limited by the hardware.In order to enable the acquisition of spectrally resolved projection measurement data, two different detection thresholds E 1, E 2 with different threshold values are now defined depending on the position, said thresholds alternating over the channels (columns) and rows in a checkerboard pattern. For example, the detector fields left white in FIG. 3 are operated with an energy of 20 keV with a first detection threshold E 1 and the shaded detector fields are operated with an energy of 50 keV with a detection threshold E 2. Spectrally separated images are then reconstructed separately on the basis of the projection measurement data of the detector fields of different field types or detector fields (white, shaded) with different detection thresholds E 1, E 2. The necessary interpolation of the gaps between the detector fields does reduce the maximum possible sharpness and resolution of the images, but the scanning is uniform without any appreciable spatial and temporal offset and the resulting potential artifacts during the course of the entire acquisition of the projection measurement data.In order to generate high-resolution images, the assignment of the detection thresholds E 1, E 2 between the fields of different field types (white, shaded) can be interchanged in the case of successive projections or frames of one and the same measurement. In this case, the images are reconstructed without taking into account the detection thresholds E 1, E 2 as mixed images with complete or maximum resolution.FIG. 4 shows, by way of example, a sequence of a method according to the invention for image reconstruction as a flow diagram 400. In a first step 4.I of the method, in a computed tomography system 1 according to the invention, X-ray radiation RS 1 having a first defined spectrum and X-ray radiation RS 2 having a second defined spectrum different from the first defined spectrum are generated, as has already been described above. That is to say that the radiation RS 1 has an energy distribution which differs from the radiation RS 2.This radiation RS 1, RS 2 penetrates at least partially through an object under examination 2 and is acquired as a projection of this object under examination 2 by an X-ray detector 4 with temporal and / or local variation of the threshold values of the detection thresholds E 1, E 2 in the second step 4.II. Here, for the radiation RS 1 with a first defined spectrum, the detection takes place spatially and / or temporally separately from the detection of the radiation RS 2 with a second defined spectrum. Unlike in previously used systems with spectral acquisition of X-rays, the detection thresholds E 1, E 2 of the X-ray detector 4 are adapted in temporal and / or spatial coordination to the respective X-ray spectrum of the radiation RS 1, RS 2 within one and the same scan. By adapting the detection thresholds E 1, E 2 to the respective X-ray spectrum of the radiation RS 1, RS 2, either an improved quality of the spectral images can be achieved, or by using a smaller number of adapted detection thresholds, the amount of data which is transmitted from the rotating X-ray detector 4 via a slip ring to the control device 5 can be reduced. Although the transmission of the projection measurement data P 1, P 2 from the moving x-ray detector 4 to the stationary control device 5 is time-critical and is limited due to the properties of the slip ring, a spectrally resolved image of an examination object can be generated by reducing the threshold number despite the limitation of the data rate.Accordingly, the projection measurement data P 1 of the X-ray radiation S 1 is assigned to the first defined spectrum. The method is analogously carried out using the projection measurement data P 2 which are assigned to the second defined spectrum.During the acquisition, the projection measurement data P 1 and P 2 are possibly segmented even further, namely on the basis of the energy distribution of the projection P 1, P 2 into energy-resolved projection measurement data P 11, P 12,..., P 1 i, P 21, P 2,..., P 2 i. The projection measurement data P 11, P 12,..., P 1 iare assigned to the first spectrum and the projection measurement data P 21, P 212,..., P 2 iare assigned to the second spectrum. The index i indicates the number of detection thresholds used for the detection of the X-rays having the respective different spectrum.The projection measurement data P 11 is furthermore the data of the first spectrum from a defined energy range of the X-ray radiation projected through the examination object 2, namely the energy range which is acquired in a first bin of the energy-selective X-ray detector 4. The same applies analogously up to the projection measurement data P 2 ithat are acquired in the i-th bin and are assigned to the second spectrum. The bins of the X-ray detector 4 thus each record data of a defined energy range of the projection. The limits of the energy ranges of the bins can be defined, for example, by a control protocol or by an operator, and set with the aid of the control device 5. In particular, the limits of the energy ranges for the acquisition of projection measurement data of different spectra are adapted to the profile of these spectra. As already described in detail, this process can comprise a temporal change of the energy ranges or a spatial variation of the energy ranges. Further steps of the acquisition are carried out analogously to the methods already established in computed tomography.Thus, the projection measurement data P1, P2 contains information about the generating spectrum and about the energy distribution present in the projection. Images can be generated from these spectrally separated projection measurement data P 1, P 2 in the third step 4.III for the individual energy ranges in each case using known reconstruction algorithms. These can then be mixed with one another as required in order to display certain materials or fabrics in a emphasized manner as desired and to optimize them with regard to contrast and / or noise and / or contrast-to-noise ratio. Finally, the method according to the invention provides an improved representation of the reconstructed image B due to the better spectral separation.Finally, it is pointed out once again that the apparatuses and methods described in detail above are merely exemplary embodiments which can be modified in a wide variety of ways by the person skilled in the art without departing from the scope of the invention. Furthermore, the use of the indefinite article "a" or "an" does not exclude that the features in question can also be present multiple times. Likewise, the term "element" does not exclude that the component in question consists of a plurality of interacting subcomponents, which may optionally also be spatially distributed. Regardless of the grammatical sex of a certain term, individuals with male, female or other sex identity are included.

Claims

X-ray system (1), in particular computed tomography system (1), for acquiring projection measurement data (P1, P2) of an examination object (2), comprising: - an X-ray emitter arrangement having an X-ray radiation source (3) for emitting X-ray radiation (10, RS1, RS2), - a photon-counting X-ray detector (4) having at least one detection threshold (E1, E2) which is spatially and / or temporally variable in one and the same measurement for spectrally resolved detection of the X-ray radiation (10, RS1, RS2), wherein the photon-counting X-ray detector (4) has at least one first subregion (4a) having a first detection threshold (E1) and at least one second subregion (4b) which is spatially arranged alternately with the first subregion (4a) and has a second detection threshold (E2) which differs from the first detection threshold (E1), and wherein the photon counting X-ray detector (4) is configured to exchange the detection thresholds (E1, E2) of the first (4a) and the second sub-region (4b) at predetermined time intervals.X-ray system according to claim 1, wherein the x-ray radiation source (3) is configured to emit spatially and / or in one and the same measurement of temporally spectrally variable x-ray radiation (10).X-ray system according to claim 2, wherein the photon-counting x-ray detector (4) has at least one detection threshold (E1, E2) for spectrally resolved detection of the x-ray radiation (10) that can be adjusted spatially and / or spectrally variable in time in one and the same measurement.X-ray system according to one of the preceding claims, wherein the x-ray radiation source (3) has a prefilter (9) for splitting the x-ray radiation (10) into at least two spatially separated x-ray sections having a different x-ray spectrum.X-ray system according to claim 4, wherein the prefilter (9) is configured to distribute different regions of an x-ray spectrum of the emitted x-ray radiation (10) to spatially defined angular regions (11) of the photon-counting x-ray detector (4), wherein the detection thresholds (E1, E2) of the photon-counting x-ray detector (4) have different values in different angular regions (11) depending on the regions of the x-ray spectrum.X-ray system according to one of claims 4 or 5, wherein the prefilter (9) for dividing the x-ray beam has a first sub-filter (9a) with a first filter material and a second sub-filter (9b) with a second filter material different from the first filter material.The X-ray system of claim 6, wherein the first filter material comprises gold and the second filter material comprises tin.X-ray system according to one of the preceding claims, wherein - the x-ray radiation source (3) is designed such that it switches back and forth between different acceleration voltages under the control of a control device (5) and - the photon-counting x-ray detector (4) has at least one time-variable detection threshold (E1, E2) and is designed to detect the x-ray radiation (10, RS1, RS2) in a synchronized manner with the control of the x-ray radiation source (3), wherein the synchronization comprises a coordination of the time-variable detection threshold (E1, E1) to the value of the different acceleration voltages.X-ray system according to claim 8, wherein the photon-counting x-ray detector (4) is configured to read out and transmit only a part of the detection thresholds (E1, E2) in each case under the control of the control device (5) synchronized with the switching of the x-ray source (3).The X-ray system according to claim 1, wherein the first portion (4a) and the second portion (4b) form a checkerboard-like pattern.Method for image reconstruction by means of an X-ray system (1), in particular according to one of the preceding claims, having the following steps: - generation and emission of X-ray radiation (10, RS1, RS2), - energy-resolved detection of at least the X-ray radiation (10, RS1, RS2) which is urgent through the examination object (2) as projection measurement data (P1, P2) by a photon-counting X-ray detector (4) having a detection threshold (E1, E2) which is spatially and / or temporally variable in one and the same measurement for spectrally resolved detection of the X-ray radiation (10, RS1, RS2), - reconstruction of an image (B) on the basis of the projection measurement data (P1, P2) of the examination object (2), wherein the photon-counting x-ray detector (4) has at least one first subregion (4a) with a first detection threshold (E1) and at least one second subregion (4b) arranged spatially alternating with the first subregion (4a) with a second detection threshold (E2) which differs from the first detection threshold (E1), and wherein the photon-counting x-ray detector (4) is configured to exchange the detection thresholds (E1, E2) of the first (4a) and of the second subregion (4b) at predetermined time intervals.Computer program product having a computer program which can be loaded directly into a storage device of a control device (5) of an X-ray system (1), having program sections for carrying out all the steps of the method according to Claim 11 when the computer program is executed in the control device (5) of the X-ray system (1).Computer-readable medium on which program sections readable and executable by a computer unit are stored in order to carry out all the steps of the method according to Claim 11 when the program sections are executed by the computer unit.

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