Estimating ratio of random coincidence in counted x-ray detector

Abstract

The invention relates to a method for estimating the ratio of random coincidence in a counted X-ray detector having a plurality of detector elements, the method comprising the steps of: detecting an X-ray signal by the X-ray detector and converting the X-ray signal into an electrical signal; at least some of the electrical signals are forwarded to a signal input of the coincidence unit, the signal input comprising a first signal input and at least one further signal input, a signal for the first signal input being detected in a first detector element, a signal for at least one further signal input is detected in each other of the detector elements that are not directly adjacent to the first of the detector elements, and / or wherein the detector elements are arranged in the detector elements, the signal for the at least one further signal input or the signal for the first signal input is temporally offset in the circuit by a defined time interval; the coincidence of the signals forwarded to the coincidence unit is counted and the ratio of random coincidence is estimated.

Description

Technical Field

[0001] The invention relates to a method for estimating the ratio of random coincidences in a counting X-ray detector, a method for estimating real coincidences, a method for recording an X-ray image data set, a counting X-ray detector and a medical imaging device. Background Art

[0002] These include persons of both masculine and feminine identities, regardless of the grammatical gender of a particular term.

[0003] Photon-counting X-ray detectors are used in many different applications, particularly in imaging. For example, photon-counting detectors are increasingly being used in computed tomography (CT) systems. Photon-counting X-ray detectors are generally based on the principle that incident X-ray signals or X-ray photons are converted by a converter into electrical signals that can then be recorded and evaluated. X-ray detectors with adjustable energy thresholds can be used, enabling energy-resolved detection of X-ray signals.

[0004] To achieve high spatial resolution on the one hand and limit the count rate in individual detector elements on the other, detector elements or pixel elements are increasingly being designed to be very small. However, this has the disadvantage that the entire energy of an X-ray quantum is typically not deposited in a single pixel, but rather distributed across two or more (usually adjacent) detector elements or pixels, since the charge cloud generated in the detector extends over more than one detector element. This can result in photons being counted multiple times in adjacent detector elements. This can limit both the spatial resolution and the energy resolution of the detector system. Double occurrences can be referred to as coincidences, true coincidences, or real coincidences.

[0005] A solution to this problem is to include, for each detector element, a coincidence counter in addition to a counter for the count rate of detected photons. This coincidence counter counts events in which at least one adjacent detector element is recorded simultaneously with the observed detector element. The count rates of the counter and the coincidence counter can be monitored, in particular, relative to a given energy threshold, so that the coincidence counter counts events in which at least one adjacent pixel exceeds the energy threshold simultaneously with the observed pixel.

[0006] Approaches for detecting overlap are known in the prior art and are described, for example, in DE 10 2012 224 209 A1, EP 3 839 577 A1 and EP 3 839 576 A1.

[0007] However, such coincidence counters generally have the tendency to overestimate the frequency of X-ray quanta whose signals are divided over a number of pixels. This is because the coincidence counter is incremented not only in the case of a true coincidence, but also when a second independent X-ray quantum deposits its energy in one of the adjacent pixels simultaneously and randomly with the X-ray quantum incident in the observed pixel (also known as random coincidence). In the event of random coincidences, the use of a coincidence counter can lead to overcorrection. In general, the size of the error caused by not taking random coincidences into account increases significantly with higher photon fluxes. This is because the number of these events, in a first approximation, results from the product of the count rate in the pixel, the count rate in the adjacent pixels, the number of adjacent pixels and the length of the coincidence time window, while a true coincidence represents only a certain proportion of the count rate in the observed pixel.

[0008] Estimating the number of random coincidences based on the individual count rates of all pixels is only possible to a limited extent, since for this the exact proportion of real coincidences in the individual count rates must be known, which is usually not the case in CT systems since this proportion is again spectrally dependent. Summary of the Invention

[0009] It is therefore an object of the present invention to provide a method which can at least reduce the problem of overestimation of the true overlap in the case of counting X-ray detectors or provide a better estimate of the true overlap.

[0010] This object is achieved by the method according to the invention, the counting X-ray detector according to the invention, and the medical imaging device according to the invention. Further features and advantages are apparent from the description and the drawings.

[0011] According to a first aspect of the present invention, a method for estimating the rate of random coincidence in a counted X-ray detector is provided. The X-ray detector comprises a plurality of detector elements. The method comprises the following steps:

[0012] (a) detecting an X-ray signal by an X-ray detector and converting the X-ray signal into an electrical signal at a detector element;

[0013] (b) forwarding at least some of the electrical signals to a signal input of the coincidence unit, wherein the signal input comprises a first signal input and at least one further signal input,

[0014] wherein a signal at the first signal input is detected in a first of the detector elements,

[0015] wherein the signal of at least one further signal input is detected in each of the other detector elements which are not directly adjacent to the first detector element,

[0016] and / or

[0017] wherein the signal for the at least one further signal input or the signal for the first signal input is temporally offset by a defined time interval in the circuit before being forwarded to the coincidence unit;

[0018] (c) counting the coincidences of the signals forwarded to the coincidence unit in order to determine at least one count rate of detected random coincidences;

[0019] (d) estimating a rate of random coincidence based on the determined at least one count rate.

[0020] Advantageously, the method according to the invention provides a relatively reliable estimation of the probability of random coincidences. This makes it possible, for example, to extend the usable area for the coincidence counter to higher X-ray fluxes, by making it possible to distinguish between real coincidences and contributions from random coincidences through knowledge of the random coincidences.

[0021] Random coincidence can also be referred to as independent coincidence, false coincidence, or random coincidence. The term "random coincidence" should be understood to be distinct from the term "true coincidence." Random coincidence occurs when two primary X-ray quanta randomly strike different detector elements simultaneously, particularly adjacent detector elements. Various factors, such as the X-ray flux and the width of the detector elements, can influence the rate of random coincidence. True coincidence can also be referred to as true coincidence. True coincidence occurs when the same event, particularly a single X-ray quantum, simultaneously triggers signals in adjacent detector elements. Counting X-ray detectors attempt to detect the correct number of X-ray quanta and their correct energy. Detecting coincidences allows inferences to be drawn when individual photons or X-ray quanta trigger multiple signals, thus enabling a better determination of the correct number of X-ray quanta. However, this determination can be impaired by random coincidence, resulting in overcorrection by counting more true coincidences than are actually present. Advantageously, the method according to the present invention allows for the determination of random coincidence, making it possible to more accurately determine the true coincidence or correct the corresponding count rate, particularly in the case of high X-ray fluxes.

[0022] A counting X-ray detector can, for example, be a counting X-ray detector in a computed tomography system. A counting X-ray detector can also be referred to as a photon-counting X-ray detector. Such X-ray detectors are generally designed to detect and count individual X-ray photons, particularly in a positionally and / or temporally differentiated manner. Within the scope of the present invention, the term X-ray signal should be understood in a broad sense. An X-ray signal can generally refer to incident X-rays or X-ray photons. A counting X-ray detector typically includes an X-ray converter, in which incident X-rays generate movable charge carriers, particularly electron-hole pairs. Electrical contacts are typically attached to the X-ray converter as electrodes, and a voltage is applied to these contacts. This voltage transmits the generated charge carriers to the contacts and connected readout electronics, where they are typically amplified, the signal magnitude compared to a threshold, and, when the threshold is exceeded, converted into an electrical signal to be output, particularly a logic and / or digital electrical signal. Within the scope of the present invention, the electrical signal may also be simply referred to as a signal. The contacts, together with at least part of the connected readout electronics, are part of the detector element. The input signals can be counted by a counter in the readout electronics. In counting X-ray detectors, particularly in the context of computed tomography, multiple thresholds are often used. The detector elements typically have comparators, which can be used to determine the minimum energy that must be input in order to count the signal. The principle of using threshold values ​​and counters can be used to generally count incident photons and to count coincidences. It can be provided that a coincidence unit is provided for only one, for example, the lowest, threshold, or for a dedicated, separate threshold. However, it is also conceivable to provide a coincidence unit for one or more thresholds, or even for a combination of different thresholds.

[0023] The X-ray detector includes a plurality of detector elements. The detector elements can be arranged in a matrix. In particular, multiple detector elements can be assigned to a sub-detector. The detector elements can be evenly distributed. Alternatively, the detector elements can be arranged in groups. The detector elements can also be named using other names, such as pixels, pixel elements, image point elements, etc. In general, the term detector element should be interpreted broadly within the scope of the present invention. Apart from the explicitly mentioned features and required functions, the design of the detector elements can be selected relatively freely. Thus, one, several, or all comparators or counters, for example, can be part of a detector element or implemented separately.

[0024] For example, the overlap unit may also be referred to as an overlap circuit. Within the scope of the present invention, the overlap unit for determining random overlap may also be referred to as a random overlap unit. The overlap unit has multiple signal inputs and is particularly designed to simultaneously detect input signals as overlaps and generate corresponding count signals for counting overlaps. The signal inputs are particularly configured to input electrical signals generated by X-ray radiation. The electrical signals can be filtered based on threshold values. For example, provision may be made to direct only electrical signals with a defined minimum intensity to the first signal input and / or the second signal input. The signal detected by a first detector element among the detector elements is directed to the first signal input. For example, the first detector element among the detector elements may be a detector element that detects the ratio of random overlaps. Alternatively, the first detector element among the detector elements may be a detector element different from the detector element that detects the ratio of random overlaps. For example, the first detector element among the detector elements may be a detector element arranged near the detector element that detects the ratio of random overlaps. "Nearby" may, for example, mean an adjacent or next-next detector element. Within the scope of the present invention, "adjacent" may particularly mean that a detector element is the closest detector element to the output detector element in a specific direction. In particular, no detector element is directly between two adjacent detector elements. In a rectangular matrix of detector elements, detector elements are also considered adjacent if they are adjacent at an angle. In other words, detector elements are also considered adjacent if they are not adjacent along the sides of the rectangle, but rather are adjacent at an angle to the sides. Within the context of the present invention, "at an angle" particularly refers to the diagonals of the rectangular matrix.

[0025] According to an alternative, it can be provided that the signal of the first signal input or the further signal input is delayed. This can be achieved in particular by a delay in the circuit. Preferably, the delay is greater than the signal processing time of the detector element. The delay can exclude the possibility that the signal is triggered by a single photon. Preferably, the delay is chosen to be so small that the change in the X-ray flux during the time period of the delay is statistically negligible. Since the time series of the signals of a plurality of X-ray quanta are uncorrelated, the count rate of random coincidences does not change when the signal path is delayed, as long as the delay is short compared to a scan in an X-ray device, for example in a computed tomography device. Since it can be assumed that the statistics remain approximately unchanged during the time period of the delay, a good measure of random coincidences can be determined, wherein real coincidences can at the same time be essentially excluded by the delay.

[0026] According to a further alternative, the signals for the first signal input and for at least one further signal input come from detector elements that are not directly adjacent to one another. Preferably, the signals for the further signal inputs can come from a detector element that is the respective next detector element relative to the first detector element. In other words, it can be provided that exactly one further detector element is arranged between the first detector element and the detector element for the further signal input. Since the detector elements are not directly adjacent, it can be largely excluded that the signals detected there come from the same X-ray photons. Advantageously, in this variant, a delay circuit is not necessarily required. If necessary, the delays of multiple further signal inputs do not have to be coordinated and balanced. As a result, the cost and / or required space for the circuits can be reduced. Random overlap can thus be achieved via spatial separation.

[0027] Optionally, it is preferred to provide a maximum of nine, particularly preferably a maximum of five, signal inputs for the overlap unit. For example, it is very particularly preferred to provide exactly five or exactly three signal inputs. From the signal inputs, a signal input can be provided in particular for the signal of the detector element for which the random overlap is to be determined, and the remaining signal inputs can be provided for the signals of the other detector elements. In some embodiments, it may be particularly preferred to provide a maximum of four, particularly preferably exactly two, signal inputs for the overlap unit. In these embodiments, it can be provided in particular that all the signals of the signal inputs do not come from the detector element for which the random overlap is to be determined, but from the other detector elements. Since the number of detector elements used for the estimation is therefore very small, the occurrence of random overlaps counted can be kept very small. As a result, the counter does not rise too strongly with the X-ray flux and can therefore advantageously provide information about a larger extended flux range. It has been shown that using two signal inputs can generally also produce statistics that are good enough for estimating the ratio of random overlaps.

[0028] Optionally, both of the described alternatives can also be combined.

[0029] The detected coincidences can then be counted using the coincidence unit in order to determine at least one count rate of the detected random coincidences. Thus, the number of random coincidences can be unambiguously measured.

[0030] The random overlap ratio can be estimated based on the counted random overlaps. Generally speaking, the estimate can be more accurate when more electrical signals are used. On the other hand, if fewer electrical signals are considered, the circuit cost can be reduced. The random overlap ratio can be scaled accordingly based on the actual setting.

[0031] Optionally, provision can be made for not all recorded electrical signals (also regardless of their intensity) to be used for counting coincidences. For example, coincidences can be determined only as an example for selecting electrical signals. This can reduce circuit costs. When estimating the rate of random coincidences, a statistical calculation can be used to take fewer signals into account.

[0032] According to one embodiment, the signal for at least one additional signal input is detected in each of the detector elements, in particular, in each of the detector elements adjacent to the first detector element. The signal for the at least one additional signal input or the signal for the first signal input is temporally offset by a defined time interval in the circuit before being forwarded to the coincidence unit. Preferably, this time interval exceeds the maximum propagation time difference of the analog and / or digital signal, preferably both signals, in the circuit. Advantageously, a true coincidence cannot cause the counter to increment. For example, the time interval can be in the range of 50 ns to 10 μs, preferably in the range of 80 ns to 300 ns, and even more preferably in the range of 100 ns to 200 ns. These ranges are particularly advantageous because the propagation time differences of X-ray detectors are typically lower than these times, and typical integration times of computed tomography systems are typically significantly higher. A range exceeding 100 ns is particularly suitable for distinguishing from typical propagation times and, therefore, from true coincidence. An upper limit of 300 ns is particularly suitable for keeping the influence of the time-varying X-ray flux, for example due to further movement of the detector, to a negligible level. A time interval of up to 200 ns is more advantageous, as this transmission time allows further reductions in circuit complexity and the required power of the X-ray detector. Using adjacent detector elements can be particularly advantageous, as this allows for a configuration that is very similar to, or even largely identical to, a true coincidence.

[0033] According to one embodiment, the signal for at least one further signal input is also detected in a first of the detector elements, wherein the signal for the at least one further signal input or the signal for the first signal input is temporally staggered in the circuit by a defined time interval before being forwarded to the coincidence unit. This time interval can correspond to the time interval of an embodiment with different detector elements. This embodiment can have the advantage that the number of connections between the different detector elements can be reduced.

[0034] According to an embodiment, a plurality of overlap units are provided, wherein at least one overlap unit is assigned to a subgroup of detector elements, in particular a subgroup consisting of detector elements arranged spatially close to each other, wherein the method is applied to each overlap unit of the plurality of overlap units, wherein the ratio of random overlap of the detector elements for the associated subgroup is estimated using each overlap unit of at least one overlap unit of the plurality of overlap units.

[0035] According to an embodiment, the ratio of random overlaps for a detector element to be estimated (whose electrical signals themselves are not fed into the overlap unit) is estimated using at least one overlap unit, wherein the signal for the first signal input and the signal for the at least one further signal input come from detector elements that are respectively adjacent to the detector element to be estimated. Advantageously, the random overlap can thereby be derived from detector elements that are spatially closer to the detector element under consideration, so that spatial variations in the count rate at the detector can have a lesser influence on the value of the random overlap. Preferably, there is precisely one further signal input of the overlap unit, and the signals for the first signal input and for the one further signal input come in particular from a total of two adjacent detector elements. Preferably, the detector elements are arranged in a rectangular matrix, and a total of two adjacent detector elements for the first signal input and for the one further signal input are arranged obliquely adjacent to the detector element to be estimated.

[0036] According to an embodiment, at least one coincidence unit is assigned to a subgroup of detector elements, in particular, a subgroup consisting of detector elements arranged spatially adjacent to one another, wherein the at least one coincidence unit is used to estimate the random coincidence rate of the detector elements of the associated subgroup. Using a coincidence unit for the entire subgroup can reduce circuit complexity. However, multiple coincidence units, for example, two to four coincidence units, can also be provided for the subgroup. In particular, it can be provided that the circuit for measuring random coincidence is designed for only a portion of the detector elements. For example, a subgroup can include 2 to 200, preferably 4 to 100, and particularly preferably 10 to 50 detector elements. The subgroup can be arranged, for example, in a rectangular shape, with the side length of the rectangle being defined by the number of detector elements in the subgroup. For example, the side length of the rectangle can each be in the range of 2 to 10 detector elements. A range of 2 to 10 detector elements in each direction can be particularly advantageous because, within this range, the count rates generally do not differ significantly and, at the same time, significant circuit complexity can be saved. Preferably, one overlap unit is provided for every 2 to 36 detector elements, particularly preferably one overlap unit is provided for every 6 to 24 detector elements.

[0037] Preferably, only one pair of detector elements is used to determine random overlap within a subgroup. Particularly preferably, a coincidence unit is provided for each subgroup, wherein the coincidence unit has exactly two signal inputs for the signals of two detector elements. Preferably, 4 to 100, particularly preferably 9 to 36, detector elements can be provided for each subgroup. The detector elements can be arranged in a rectangular matrix. The matrix can have an N×M form, where N and M are the number of detector elements along a side of the rectangular matrix. N and M can each have values ​​of 2 to 10, preferably 3 to 6, for example. For example, the detector elements can be arranged in a rectangular 4×6 matrix. It has been shown that in this embodiment, using such a matrix can achieve particularly good results while simultaneously saving circuitry. Preferably, the detector elements for the signal inputs of the detector units are arranged in the subgroup so that every other detector element is at least one of the two detector elements' next-next neighbors.

[0038] According to an embodiment, the detector elements are divided into subgroups of detector elements, wherein intermediate spaces exist between the subgroups. At least one overlap unit, particularly one overlap unit or multiple overlap units, is arranged in the intermediate spaces between the subgroups. In particular, at least one overlap unit can be arranged in a region of the intermediate space that is in the shadow of the Anti-Scatter Grid. Advantageously, in this embodiment, the circuitry for the overlap units can be moved from the typically limited space directly below or on the detector elements to the region between the detector elements. The space directly below or on the detector elements is typically limited because the electronic circuitry for processing the detector element signals is typically already located there. For example, provision can be made to alternately direct the overlap units of the side-by-side subgroups to different sides, particularly so that two overlap units always share a common intermediate space. This can be particularly advantageous, for example, for further processing of count signals of random overlap. This variant can be combined with other variants described herein, particularly those involving subgroups. In particular, a variant having a pair of detector elements for determining random overlap within a subgroup can be advantageously combined with this embodiment.

[0039] Another aspect of the present invention is a method for estimating the true coincidence of two X-ray signals detected simultaneously according to a predetermined criterion on adjacent detector elements of a counting X-ray detector. The method comprises the following steps:

[0040] - performing the method for estimating the rate of random coincidence as described herein; and

[0041] - determining the coincidence of two x-ray signals detected simultaneously according to a predetermined criterion on adjacent detector elements in order to determine an uncorrected coincidence, in particular by counting the number of coincidence count signals for each detector element using at least one adjacent detector element, optionally using each adjacent detector element;

[0042] - Correcting the uncorrected coincidence based on the ratio of random coincidences in order to estimate the true coincidence. The predetermined criterion may, for example, include a time period within which events are evaluated as coincident or simultaneous. The true coincidence can in particular be detected and / or transmitted as a count value.

[0043] All advantages and features of the method for estimating the ratio of random coincidences in a counted X-ray detector can be similarly transferred to the method for estimating real coincidences, and vice versa. Advantageously, the contribution of real coincidences from random coincidences can be distinguished in the coincidence counter by determining the ratio based on explicit measurements, in particular. This allows the usable measurement range for the coincidence counter to be extended, for example, to significantly higher X-ray fluxes, thereby also yielding the advantages of the coincidence circuit. Possible overcorrections caused by an overestimation of the real coincidence can be counteracted. In particular, this can be achieved without relying on model assumptions or empirical corrections.

[0044] For example, the overlap of two X-ray signals detected simultaneously according to a predetermined criterion on adjacent detector elements can be determined according to methods known in the prior art. For example, electrical signals detected simultaneously in an overlapping manner, i.e., within a specific tolerance, can be recorded and counted in adjacent detector elements using a coincidence counter. In order to record the overlap, for example, a coincidence unit can be provided, which detects the situation in which the electrical signal of the corresponding threshold input of a detector element is input simultaneously with the electrical signal of the threshold of one or more adjacent detector elements in a specific time frame. For this purpose, both the electrical signal of the detector element and the electrical signal of the adjacent detector element can be directed to the coincidence unit or the signal input of the coincidence unit. The coincidence counter can then count the number of coincidence events in which at least one adjacent detector element exceeds the threshold at the same time as the observed detector element.

[0045] According to an embodiment, at least one coincidence unit is used to estimate the rate of random coincidence for a detector element to be estimated (whose electrical signal itself is not fed into the coincidence unit), wherein the signal for the first signal input and the signal for the at least one further signal input originate from detector elements adjacent to the detector element to be estimated. Preferably, the detector elements from which the signals for the signal inputs originate are not adjacent to one another. Preferably, there is precisely one further signal input, and the signals for the first signal input and the further signal input originate from a total of two adjacent detector elements. Preferably, the detector elements are arranged in a rectangular matrix, and the total of two adjacent detector elements for the first signal input and the further signal input are arranged obliquely adjacent to the detector element to be estimated. Preferably, an uncorrected coincidence is performed based on the detector element to be estimated and four adjacent detector elements, wherein the four adjacent detector elements are particularly preferably not the two adjacent detector elements for the first signal input and the further signal input. In particular, the four detector elements used to determine the uncorrected coincidence can be arranged adjacent to the detector element to be estimated along a rectangular side of the rectangular matrix.

[0046] According to one embodiment, a coincidence unit is provided for determining the uncorrected coincidence and for estimating the random coincidence, wherein the same number of signal inputs for the electrical signals is provided for both coincidence units, and the coincidences of the signals are counted. This embodiment allows for particularly simple adaptation of the uncorrected coincidence, in particular because the ratio of the random coincidence generally does not need to be scaled.

[0047] According to an embodiment, identical detector elements are provided for determining uncorrected overlap and for estimating random overlap, wherein in particular, the detector element (whose true overlap is to be determined) and its adjacent detector elements are provided. This allows for a particularly precise estimation of random overlap. In particular, the signal of the detector element (whose true overlap is to be determined) or the signals of adjacent detector elements are delayed and then directed to the overlap unit for estimating random overlap. Random overlap can be determined based on a temporal offset within a defined time interval. In other words, in this embodiment, the digital overlap circuit for each detector element can be doubled, and two overlap values ​​can be counted for each detector element. Here, the uncorrected overlap value is based on the overlap of the signal of the detector element and the undelayed signals of its adjacent detector elements. This value particularly represents the sum of true overlap and random overlap. For the second signal path, the signal of the detector element or the signals of all adjacent detector elements can be delayed. This count value particularly represents the number of random overlaps.

[0048] According to one embodiment, for determining the uncorrected overlap and for estimating the random overlap, signals are provided for the signal inputs of the corresponding overlap units of the detector element whose real overlap is to be determined and for one or more detector elements adjacent thereto, wherein only a subset of the adjacent detector elements is provided for determining the uncorrected overlap and / or for estimating the random overlap. According to an alternative embodiment, for determining the random overlap, signals are provided for the signal inputs of the overlap units for estimating the random overlap for the detector element for which the real overlap is to be determined and for the detector element immediately below it, wherein only a subset of the immediately below detector elements is provided for estimating the random overlap and / or only a subset of the adjacent detector elements is provided for determining the uncorrected overlap.

[0049] By using only one subgroup, the circuit outlay can advantageously be reduced.A subgroup can optionally consist of only one detector element.

[0050] In addition, the number of overlaps counted in this way can be reduced. As a result, information about random overlaps can be estimated with respect to an extended X-ray flux range. When using this subgroup-based value to correct the count value to estimate the true overlap, the count value or the ratio of random overlaps can be scaled with the ratio of the number of adjacent detector elements used to determine the uncorrected overlap. Optionally, subgroups can also be used to determine the uncorrected overlap, for example subgroups of the same size or subgroups with a different number of detector elements. For example, obliquely adjacent detector elements generally contribute less to the true overlap, and therefore, when these detector elements are omitted, no significant errors are usually introduced. It can also be meaningful to limit the circuits at the edges of the detector to fewer signal inputs of the corresponding overlap units.

[0051] According to an embodiment, true coincidences are transmitted as count values, wherein, in addition to true coincidences, random coincidences and / or uncorrected coincidences are also transmitted as additional count values. Transmitting random coincidences and / or uncorrected coincidences also allows these coincidences to be used for further evaluation and / or subsequent verification of true coincidences. For example, the transmitted count values ​​for random coincidences can be compared with other count values, such as uncorrected coincidences, in a reconstruction computer, in an integrated circuit, particularly an FPGA (field programmable gate array), in the detector's internal infrastructure, and / or in the detector's external circuitry infrastructure. Advantageously, this allows, for example, to determine which corrections to use later, for example based on the actual count values. Optionally, the corrected degree of mixing can be a function of one or more additional parameters. These additional parameters can include, for example, the count rate, the number of uncorrected coincidences or true coincidences, the number of random coincidences, and / or other parameters that are particularly dependent on the X-ray flux. For example, with very high X-ray fluxes, linear pile-up effects can accumulate. Based on this, for example, a smooth transition from full application of the correction to discarding the count value can be controlled depending on the degree of mixing. For example, the degree of mixing can be a factor of 1 and, in the case of particularly high X-ray fluxes, a factor of 0. In an alternative embodiment, it can also be provided that only uncorrected coincidences and random coincidences are transmitted. In this case, it can be provided that the true coincidence is subsequently determined. In another alternative embodiment, it can also be provided that only true coincidences are transmitted, and in particular, random coincidences and uncorrected coincidences are not transmitted.

[0052] According to an embodiment, the count rate of random overlap is used to monitor and, if necessary, correct the paralysis of other counters of the signal, in particular the paralysis of other counters of the signal whose random overlap is estimated at this count rate. The other counters can, for example, be counters for counting incoming X-ray photons. In the case of an excessively high X-ray flux, at least some of the counters can be paralyzed because they can no longer detect further photons. This can even cause the counter to record lower values ​​starting from a certain ratio of incident photons. This can lead to ambiguity, as it is unclear whether the counter has just recorded an actually low X-ray flux or whether the counter has been paralyzed. Counters with random overlap are paralyzed less often because the overlap of events usually occurs less frequently than individual events. A paralyzable counter can, in particular, be a counter with a low energy threshold, i.e., a counter that records the majority of incoming photons. Such a counter is called a paralyzable counter. Advantageously, the count signal of the random overlap counter can be used to linearize the paralyzable counter, in particular to achieve a monotonically rising, non-paralyzable behavior.

[0053] According to an embodiment, correction for uncorrected overlap is performed based on the ratio of random overlap in the front end of the X-ray detector. This embodiment is relatively easy to implement because only a simple difference in count values ​​must be generated. Scaling the count values ​​can be relatively easily achieved in the form of a simple multiplication. When the X-ray detector is divided into multiple subgroups, such as in a computed tomography system, there are usually some edges and corners where the number of detector elements that contribute not only to true overlap but also to random overlap is smaller than the number of other detector elements. This difference can be taken into account during scaling and, if necessary, by implementing different embodiments. It is precisely for such situations that calculation or correction in the front end can be particularly advantageous. For example, the precise wiring at each location, that is, the corresponding number of detector elements used, can be known and taken into account in a targeted manner.

[0054] According to an embodiment, at least some of the electrical signals converted by the detector elements are mixed with pulse signals. In particular, the signal with the mixed pulse signal is applied to at most one of the signal inputs of the corresponding overlap unit. The pulse signal can, for example, be provided for calibration, dead time measurement and / or for preventing paralysis of the circuit in the event of a high X-ray flux. The pulse signal can, for example, be mixed as a clock signal. By regularly feeding in the clock signal, the signals can be counted together and thus non-paralysis can be created. The pulse signals are generally completely or partially correlated with each other over a large detector range. By applying the signal with the mixed pulse signal to at most one of the signal inputs of the corresponding overlap unit, systematic effects can be prevented which could distort the count values ​​due to artificially generated overlaps.

[0055] Another aspect of the present invention is a method for recording an X-ray image dataset, in particular a computed tomography image dataset, of an object using an X-ray system, in particular a computed tomography system, having a counting X-ray detector. The X-ray detector includes a plurality of detector elements. The method comprises the following steps:

[0056] - counting at least a certain number of count signals from the incident X-ray radiation in each detector element; and

[0057] - performing the method for estimating true coincidence as described herein;

[0058] - generating an X-ray image data set based on at least a certain number of count signals counted in each detector element and an estimated true coincidence.

[0059] All advantages and features of the method for estimating the rate of random overlap in a counted X-ray detector and the method for estimating true overlap can be similarly transferred to the method for acquiring X-ray images, and vice versa. When generating an X-ray image data set, image values, in particular voxel values ​​or pixel values, can be provided based on at least a certain number of count signals and an estimated true overlap. The number of count signals can, in particular, be corrected or adapted based on the true overlap. In particular, image reconstruction can be based on the adapted at least a certain number of count signals. For example, the number of overlapping count signals can be subtracted from at least a certain number of count signals. For example, a weighted subtraction can be provided, in particular by multiplying the number of overlapping count signals by a mixing factor before the subtraction. In other words, only a portion or multiple of at least a certain number of overlapping count signals can be subtracted or added. A threshold can be used to count at least a certain number of count signals so that signals below the threshold are not counted. If multiple numbers of count signals are counted for a detector element, a threshold can be used for each of the numbers, with different thresholds being used for different numbers of detector elements. Overlap can be determined based on one of the thresholds for the number of count signals. For example, one of the thresholds may be the lowest threshold among the thresholds. Using only one threshold can advantageously reduce circuit costs. However, it is also conceivable to set multiple thresholds or a combination of different thresholds for the overlap unit. Preferably, the same threshold or multiple identical thresholds are used to determine uncorrected overlap and random overlap.

[0060] According to one embodiment, only X-ray signals whose energy exceeds a first minimum threshold are detected to count the number of count signals in each detector element, and only X-ray signals whose energy exceeds a second minimum threshold are detected to determine random coincidences, wherein the first and second minimum thresholds are in particular different. Thus, a dedicated, dedicated threshold can be used for coincidence measurements.

[0061] Another aspect of the present invention is a counting X-ray detector, particularly for use in a computed tomography system, for recording X-ray image data sets of an object irradiated by X-ray radiation. The X-ray detector comprises a plurality of detector elements and at least one circuit having at least one coincidence unit, wherein the X-ray detector is configured to implement the method described herein. The at least one circuit can be at least partially part of a detector element. For example, each detector element can include a circuit. All advantages and features of the method for estimating the rate of random coincidence in a counting X-ray detector, the method for estimating true coincidence, and the method for recording X-ray images are similarly transferable to a counting X-ray detector, and vice versa. The X-ray detector may include an X-ray converter in which incident X-rays generate mobile charge carriers. The detector elements may be designed, in particular, to detect the mobile charge carriers generated by the X-ray converter and to process them as electrical signals. Each detector element may include at least one comparator. The comparator may include a settable signal threshold. The detector elements may include a counter for counting the electrical signals, in particular as described herein.

[0062] Another aspect of the present invention is a medical imaging device, in particular a computed tomography system, having a counting X-ray detector, in particular an X-ray detector as described herein, and a control module, wherein the medical imaging device, in particular the computed tomography system, is configured to implement the method as described herein. In particular, the control module can be designed to control the implementation of the method. The control module can be implemented, for example, in the form of a computer, a microcontroller or an integrated circuit or be part of one thereof. The control module can have hardware components and / or software components. The control module can optionally be a collection of computers or a cloud or be part of one thereof. All advantages and features of the method for estimating the ratio of random overlap in a counting X-ray detector, the method for estimating real overlap, the method for recording X-ray images, and the counting X-ray detector can be similarly transferred to a computed tomography system, and vice versa.

[0063] Unless explicitly stated otherwise, all embodiments described herein can be combined with each other. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] Hereinafter, embodiments will be described with reference to the accompanying drawings.

[0065] Figure 1 A flow chart showing a method for estimating a ratio of random coincidence in an X-ray detector comprising counts of a plurality of detector elements according to an embodiment of the present invention,

[0066] Figure 2A flow chart showing a method for estimating the true coincidence of two X-ray signals detected simultaneously according to a predetermined criterion on adjacent detector elements of a counting X-ray detector according to an embodiment of the present invention is provided.

[0067] Figure 3 A flow chart showing a method according to an embodiment of the present invention for recording an X-ray image, in particular a computed tomography image, of an object using an X-ray system, in particular a computed tomography system, is provided.

[0068] Figure 4 shows a circuit example of a detector element according to an embodiment of the present invention,

[0069] Figure 5 shows a circuit example of a detector element according to another embodiment of the present invention,

[0070] Figure 6 shows a circuit example of a detector element according to another embodiment of the present invention,

[0071] Figure 7 Show Figure 6 The embodiment shown is a variant according to the invention with regard to the selection options for determining the overlapping detector elements.

[0072] Figure 8 Show Figure 6 The embodiment shown is a variant according to the invention with regard to the selection options for the detector elements for determining the overlap.

[0073] Figure 9 Show Figure 6 The embodiment shown is a variant according to the invention with regard to the selection options for determining the overlapping detector elements.

[0074] Figure 10 A further variant according to the invention is shown with regard to the selection possibilities of detector elements for determining overlap.

[0075] Figure 11 A further variant according to the invention is shown with regard to the selection possibilities of detector elements for determining overlap.

[0076] Figure 12 shows a circuit example of a detector element according to another embodiment of the present invention,

[0077] Figure 13 Show Figure 12 The embodiment shown differs according to the invention in terms of the selection options for determining the overlapping detector elements.

[0078] Figure 14A further variant according to the invention is shown with regard to the selection possibilities of detector elements for determining overlap.

[0079] Figure 15 A further variant according to the invention is shown with regard to the selection possibilities of detector elements for determining overlap.

[0080] Figure 16 A further variant according to the invention is shown with regard to the selection possibilities of detector elements for determining overlap.

[0081] Figure 17 shows a variant of the placement of a circuit for determining random coincidence according to an embodiment of the present invention, and

[0082] Figure 18 A computed tomography system according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0083] Figure 1A flow chart shows a method for estimating the rate of random coincidence in an X-ray detector 31 comprising a plurality of detector elements, according to an embodiment of the present invention. In a first step 101, an X-ray signal is detected by the X-ray detector 31 and converted into an electrical signal at the detector element. This can be accomplished, for example, by converting the X-ray radiation into mobile charge carriers in an X-ray converter and then feeding these into the detector elements at corresponding locations via an applied voltage, where they are amplified and compared with a threshold value, thereby outputting an electrical signal when the threshold value is exceeded. In a further step 102, at least some of the electrical signals are forwarded to signal inputs of a coincidence unit. The signal inputs include a first signal input and at least one additional signal input. A signal for the first signal input is detected in the first detector element. Signals for the at least one additional signal input are detected in each of the other detector elements that are not directly adjacent to the first detector element. Detecting the signals for the at least one additional signal input in the non-directly adjacent detector elements largely ensures that the signals do not originate from the same X-ray photons as the signal for the first signal input. Additionally or alternatively, the signal for at least one further signal input or the signal for the first signal input is temporally staggered in a defined time interval in the circuit before being forwarded to the coincidence unit. It can also be achieved by the temporal offset that the signals of the first signal input and at least one further signal input do not come from the same X-ray photon. In a further step 103, the coincidences of the signals forwarded to the coincidence unit are counted in order to determine at least one count rate of the detected random coincidences. In a further step 104, the ratio of random coincidences is estimated based on the count rate. The ratio of random coincidences can be derived directly from the count rate of the detected random coincidences. Alternatively, the count rate can be adapted, in particular scaled, in order to estimate the ratio of random coincidences.

[0084] Figure 2 A flow chart of a method for estimating the true coincidence of two X-ray signals detected simultaneously according to a predetermined criterion on adjacent detector elements of a counting X-ray detector 31 according to an embodiment of the present invention is shown. The method for estimating the ratio of random coincidence is performed in a superordinate step 210. The individual steps 211-214 of the superordinate step 210 can correspond, for example, to the method described in reference to Figure 1Steps 101-104 are shown. In a further, higher-level step 220, a method for determining the overlap of two X-ray signals detected simultaneously according to a predetermined criterion on adjacent detector elements is performed. This is done by determining an uncorrected overlap, in particular by counting the number of overlap count signals for at least some of the detector elements using at least one detector element adjacent thereto. For example, the number of overlap count signals can be counted in each detector element or in a portion of the detector elements. This count is based on the signal directly input into the corresponding detector element and on the signal occurring in the overlap of at least one adjacent detector element. Preferably, the two steps 210, 220 are performed substantially simultaneously. In a further step 230, the uncorrected overlap is corrected based on the ratio of random overlaps in order to estimate the actual overlap.

[0085] Figure 3 A flow chart of a method for recording an X-ray image, in particular a computed tomography image, of an object using an X-ray system, in particular a computed tomography system, according to an embodiment of the present invention is shown. Steps 310-330 correspond to the steps in reference Figure 2 Steps 210-230 of the described method are performed. In a further step 340, at least a certain number of count signals are counted in each detector element in response to the incident X-ray radiation. Steps 310 and 320, which relate to the method for estimating the ratio of random coincidences and the method for determining the coincidence of two simultaneously detected X-ray signals, can be performed substantially simultaneously with the further step 340 of counting at least a certain number of count signals in each detector element in response to the incident X-ray radiation. In a further step 350, an X-ray image data set is generated based on the at least certain number of count signals counted in each detector element and based on the estimated actual coincidence.

[0086] Figure 4An example circuit diagram of a detector element according to an embodiment of the present invention is shown. The circuit includes an input 7 at which an electrical signal converted from an incident X-ray signal is received. The electrical signal is amplified and filtered using a plurality of comparators 11, 12, and 13, which determine N different threshold values ​​(threshold 1, ... threshold i, ... threshold N). This allows for setting the minimum energy that a recorded X-ray signal must have in order to be counted. The circuit diagram shows a circuit tree for one of the threshold values ​​12. The electrical signal is counted by a counter 4. It may happen that not all of the energy of an X-ray quantum is deposited in one detector element, but rather is distributed across two or more (usually adjacent) detector elements. This may be due, for example, to a charge cloud generated in the detector that extends beyond one pixel. To account for and correct this effect, a coincidence unit 3 is provided. The coincidence unit 3 detects situations in which the electrical signal input to the corresponding threshold 12 of a detector element enters simultaneously with the electrical signal of one or more adjacent detector elements within a specific time frame. To this end, not only the electrical signal of this detector element but also the electrical signal 21 of the adjacent detector element is fed to the coincidence unit 3. A coincidence counter 6 then counts the number of coincidence events in which at least one adjacent detector element exceeds a threshold value simultaneously with the observed detector element. Because the coincidence counter 6 is incremented not only in the event of a true coincidence, but also when an independent second X-ray quantum randomly deposits its energy in one of the adjacent detector elements simultaneously with the incident X-ray quantum in the observed detector element, overcorrection can occur because more true coincidences are counted than there are actually true coincidences. According to this embodiment of the invention, an additional coincidence unit 2 (hereinafter also referred to as random coincidence unit 2) is provided for random coincidences. In this embodiment, the random coincidence unit 2 counts additional coincidences, i.e., additional coincidences between the delayed signal of the detector element and the signal 21 of the adjacent detector element. Accordingly, the signal 22 of this detector element is fed to the random coincidence units 2 of the other detector elements. In this case, the delay is implemented by a delay unit 8 integrated into the random coincidence unit 2. This delay unit is placed upstream of the actual coincidence detection and temporally offsets the incoming signals by a defined time interval. The time interval of the delay is selected so that it exceeds the maximum propagation time difference between analog and digital signals in the circuit. This ensures that true coincidences can never cause the random coincidence counters 5 to increment. Therefore, only random coincidences are counted by the random coincidence counters 5. Because the temporal sequences of the signals of multiple X-ray quanta are uncorrelated, the count rate of random coincidences does not change if the signal path is delayed, as long as the delay is short compared to the scanning process in the CT system.In this embodiment, the digital coincidence circuit for each detector element is effectively doubled, so that two coincidences are counted for each detector element. The first coincidence is counted using the coincidence unit 3 and the coincidence counter 6 and the coincidence is detected without delay from the signal of the detector element and the signal of the neighbor. This value represents the sum of real coincidences and random coincidences. For the second signal path, the digital signal of the detector element is delayed and the coincidence with the adjacent detector element is detected using the random coincidence unit 2 and the random coincidence counter 5. Alternatively, for example, the signal 21 of the adjacent detector element can also be delayed. This count value of the random coincidence counter 5 represents the number of random coincidences. Subsequently, for example, the number of random coincidences detected by the random coincidence counter 5 can be subtracted from the number of total coincidences detected by the coincidence counter 6 to obtain the number of real coincidences. As shown here, the same number of signal inputs for electrical signals (whose coincidences are counted) can be provided for the two coincidence units 2 and 3.

[0087] For example, in order to minimize the circuit cost, the circuit can be simplified by only partially constructing a random overlap overlap tree. Therefore, it can be optionally provided that not all adjacent detector elements, but only a portion of the adjacent detector elements or even only a single adjacent detector element are considered for the measurement of random overlap. In these optional variants, in order to correct the count value of the real overlap, the count value of the random overlap can be scaled by the ratio of the number of uncorrected overlapping adjacent detector elements of the overlap unit 3. It can also be optionally provided for the overlap unit 3 that the signals of all adjacent detector elements are not taken into account. For example, the neighbors at the corners usually make little contribution to the real overlap and therefore these neighbors can be appropriately omitted. It can also make sense to limit the circuit to fewer input terminals at the edge of the detector.

[0088] When describing the following embodiments, the differences between the respective embodiments will be discussed first, and in particular, common features will not be explained again for each figure.

[0089] Figure 5 An example circuit diagram of a detector element according to another embodiment of the present invention is shown. In this embodiment, in the random coincidence unit 2, the electrical signal of the detector element is correlated with a delayed copy of the signal itself. This value can also be scaled with the number of detector elements whose signals are used in the coincidence unit to control a suitable correction of the count value of the coincidence counter 6. Optionally, it can be provided that the circuit for measuring random coincidence is designed only for a portion of the detector elements, for example, for one or two detector elements in an N×M subgroup of detector elements, wherein N and M preferably have values ​​between 2 and 10. This advantageously further reduces circuit complexity.

[0090] Figure 6 An example of a circuit of a detector element according to another embodiment of the present invention is shown. In this embodiment, random coincidences are detected by correlation of spatially distant detector elements. Here, in the random coincidence unit 2, the coincidences of the signal 23 of a detector element with the next or even further detector element are counted. Since the detector elements are clearly separated in space, so that real coincidences do not actually occur, only random coincidences are counted here. Advantageously, for this purpose, it is not necessary to install an additional delay unit 8, which is usually relatively expensive. For example, in this embodiment, the coincidence circuit for each detector element can also be doubled, so that the number of signals input to the coincidence unit 3 and the number of signals input to the random coincidence unit 2 are the same. In this embodiment, in order to measure the coincidence, the signal 21 from the directly adjacent detector element also enters the coincidence unit 3, whereby all types of coincidence (random coincidence and real coincidence) are measured. Since the random coincidence is determined by the random coincidence counter 5, the real coincidence can also be calculated here. In Figure 6 In the embodiment shown in FIG, the two coincidence units 2 and 3 may optionally be provided with the same number of signal input terminals for electrical signals, the coincidences of which are counted.

[0091] Figure 7-Figure 9 Show Figure 6 The embodiments shown differ according to the invention in terms of the selection possibilities of the detector elements for determining the overlap. A subgroup of detector elements is shown, wherein a detector element is represented by a single square. In this case, P_xy denotes the detector element whose real overlap is to be determined. The letter C identifies the detector element whose signal 21 is fed into the overlap unit 3 for determining the uncorrected overlap. The letter R identifies the detector element whose signal 23 is fed into the random overlap unit 2 for determining the random overlap. Of course, in Figure 6 Other configurations are also conceivable within the scope of the embodiment. Different variants can also be mixed within the system, for example for edges and corners of the detector segment. Figure 7 In a variant of , all adjacent detector elements C are taken into account for determining the (uncorrected) overlap. In addition, the same number of detector elements (however in the form of the next-next neighbors R) are taken into account for determining the random overlap. In this example, all next-next neighbors are taken into account which are not arranged in a corner and not centrally on an edge of the rectangle in the next-next neighbor's rectangle. Figure 8In , the detector element P_xy whose true coincidence is to be determined is located in a corner of the subgroup of detector elements. Accordingly, this detector element has 3 neighbors C, which are used to determine the (uncorrected) coincidence. The 3 next neighbors R are accordingly taken into account for determining the random coincidence. In this example, the 3 next neighbors that are not located at the edge of the subgroup are used. Figure 9 In one embodiment, the four next neighbors C are considered for determining the (uncorrected) overlap and the four next neighbors R are considered for determining the random overlap. In this case, only neighbors C or next neighbors R are considered which, within the rectangular subset of detector elements, lie on the same horizontal or vertical line as the detector element P_xy whose overlap is determined.

[0092] Figure 10 and Figure 11 Different further variants of the invention are shown with regard to the selection possibilities of the detector elements for determining the overlap. The designations of the detector elements correspond to Figures 7 to 9 In this case, a smaller number of detector elements is used to determine the random overlap than to determine the (uncorrected) overlap. In particular, only a small subset of the next detector elements is provided to determine the random overlap. Figure 7 As in the variant shown in Figure 10 and Figure 11 In the embodiment of , all adjacent detector elements C are taken into account for determining the (uncorrected) overlap. However, only the two next neighbors R are taken into account in each case for determining the random overlap. Figure 10 In the embodiment shown, two further detector elements R are used in the X direction. Figure 11 In the embodiment shown in , the two next detector elements R in the Y direction are used. Preferably, when applying one of these two variants, the measured values ​​of the random overlap are scaled by a factor K / 2, where K is the number of detector elements used in the overlap unit 3. Thus, in this example, K has the value 8.

[0093] Figure 12 An example circuit diagram of a detector element according to another embodiment of the present invention is shown. In this embodiment, the electrical signal of the detector element whose random overlap ratio is being estimated is not used to determine the random overlap. The signal 21 directed to the random overlap unit 2 originates from detector elements adjacent to the detector element to be estimated. The signal 21 from the adjacent detector element may enter only the overlap unit 3, only the random overlap unit 2, or both. The detector elements whose signals enter the random overlap unit are not adjacent to each other.

[0094] Figure 13 Show Figure 12The embodiments shown represent different variants of the invention with regard to the selection options for the detector elements used to determine the overlap. The designations of the detector elements correspond to Figures 7 to 11 . In a variant, either the diagonal, the left and right neighbors, the upper and lower neighbors or the 4 corners of the shown subgroup of detector elements are taken into account for measuring the random coincidence. In the upper row, all neighbors C, in the lower row only the direct neighbors C in the cross, i.e. not the tilted neighbors, are used for the (uncorrected) coincidence unit 3. In principle, other combinations are also possible. Optionally, it can be provided that a plurality of variants are used in the detector. Since a different number of detector elements contributes to the random coincidence than to the uncorrected coincidence, these random coincidences can preferably be scaled accordingly before they are taken into account for correction. In Figure 12 and 13 The variant shown in has the advantage that the random overlap is derived from detector elements that are spatially closer to the detector element to be estimated, so that spatial variations in the count rate at the detector can have a smaller influence on the value of the random overlap. The variant shown in the lower left is particularly advantageous because it represents the lowest circuit complexity with minimal loss of important information. In this variant, the two next-next neighbors R lying on the diagonal are used to measure the random overlap, and only the next-next neighbors C that are not diagonal neighbors are used for the (uncorrected) overlap unit 3.

[0095] Figures 14 to 16 Different further variants of the invention are shown for selecting the detector elements for determining the overlap. M×N subgroups of detector elements are shown. Here, the circuit for measuring the random overlap is not designed separately for each detector element, but rather only one or a few detector elements are used for measuring the random overlap for the M×N subgroup. This can further reduce the circuit complexity. Figure 14 In the variant shown, two separate random coincidence counters 5 are constructed for random coincidences for a subgroup of 4×6 detector elements, which represent all 24 detector elements. The random coincidence unit 2 of the random coincidence counter 5 receives the signals of two detector elements (R22 and R34 or R33 and R25) respectively. The sum of the two count values ​​of random coincidences can be scaled by a factor K / 2 as a correction value, where K is the number of adjacent detector elements entered into the coincidence logic for (uncorrected) coincidences. Figure 15 In the variant shown, in a 2×3 subgroup, a random coincidence counter 5 is provided for all detector elements of the subgroup, wherein the random coincidence unit 2 of the random coincidence counter 5 receives the signals of two detector elements (R11 and R23). Figure 16In the variant shown in , in a 4×6 subgroup, four random coincidence counters 5 are set for all detector elements of the subgroup, wherein the random coincidence unit 2 of the random coincidence counter 5 respectively obtains the signals of two detector elements (R22 and R41 or R14 and R33 or R23 and R35 or R45 and R26).

[0096] Figure 17 A variant of the placement of the circuit for determining random overlap according to an embodiment of the present invention is shown. Here, the detector elements represented by rectangles are divided into subgroups of detector elements, two of which can be seen here. In this embodiment, two distant central detector elements are taken into account in each of two adjacent subgroups to determine the random overlap. There is an intermediate space between the subgroups. The random overlap units 2 of the two subgroups shown are arranged in the intermediate space between the subgroups. Further random overlap units 2 of further subgroups can be arranged accordingly. When a plurality of 4×6 subgroups are constructed in a circuit, for example an ASIC, it can be provided that the random overlap units are derived alternately to the left and right. In this case, the circuit is moved from the limited space under the detector elements to the otherwise possibly unused areas between the subgroups of detector elements. Advantageously, the random overlap circuit is thus placed outside the actual matrix of detector elements. As a result, the detector elements can be implemented in the same manner and the entire space at the detector elements is available for the circuit, which must be present for all detector elements. The division and arrangement of the detector elements can also be similar to that shown, for example Figures 14 to 16 The difference shown in .

[0097] Figure 18 The computer tomography system according to an embodiment of the present invention is shown. The computer tomography system comprises a control module 33, an X-ray source 32 and a counting X-ray detector 31 having a plurality of detector elements and a circuit as described herein, for example as in Figures 4 to 17 The computed tomography system is configured for example to Figures 1 to 3 The X-ray source 32 and the X-ray detector 31 are rotatably arranged in a gantry 34 .

Claims

1. A method for estimating the ratio of random coincidences in a counted X-ray detector (31), wherein: The X-ray detector (31) comprises a plurality of detector elements, wherein the method comprises the following steps: (a) detecting an X-ray signal by the X-ray detector (31) and converting the X-ray signal into an electrical signal at the detector element; (b) forwarding at least some of the electrical signals to a signal input of a coincidence unit (2), wherein the signal input comprises a first signal input and at least one further signal input, wherein a signal for the first signal input is detected in a first of the detector elements, wherein the signal for the at least one further signal input is detected in each of the further detector elements which are not directly adjacent to the first detector element, and / or wherein the signal for the at least one further signal input or the signal for the first signal input is temporally offset by a defined time interval in the circuit before being forwarded to the coincidence unit (2); (c) counting the coincidences of the signals forwarded to the coincidence unit (2) in order to determine at least one count rate of detected random coincidences; (d) estimating a rate of random coincidence based on the determined at least one count rate.

2. The method according to claim 1, in, The signal for the at least one further signal input is respectively detected in a further one of the detector elements, in particular in a detector element adjacent to the first detector element, The signal for the at least one further signal input or the signal for the first signal input is temporally offset by a defined time interval in the circuit before being forwarded to the overlap unit (2).

3. The method according to claim 1, in, The signal for the at least one further signal input is each likewise detected in the first of the detector elements, The signal for the at least one further signal input or the signal for the first signal input is temporally offset by a defined time interval in the circuit before being forwarded to the overlap unit (2).

4. The method according to any one of the preceding claims, in, A plurality of overlap units (2) are provided, wherein at least one overlap unit (2) of the overlap units is assigned to a subgroup of detector elements, in particular to a subgroup consisting of detector elements arranged close to each other. wherein the method is applied to each of the plurality of overlapping units (2), The random overlap ratio of the detector elements for the associated subgroup is estimated using each overlap unit of at least one overlap unit of the plurality of overlap units (2).

5. The method according to any one of the preceding claims, in, estimating the ratio of random coincidences for the detector element to be estimated by means of at least one of the coincidence units (2), the electrical signals of the detector element to be estimated not being directed into the coincidence unit (2), The signal for the first signal input and the signal for the at least one further signal input come from detector elements respectively adjacent to the detector element to be evaluated.

6. The method according to any one of the preceding claims, in, The detector elements are divided into a plurality of subgroups of detector elements, wherein intermediate spaces are present between the subgroups, One overlapping unit (2) or a plurality of overlapping units (2) are arranged in the intermediate space between the subgroups.

7. A method for estimating the true coincidence of two X-ray signals detected simultaneously according to a predetermined criterion on adjacent detector elements of a counting X-ray detector (31), in, The X-ray detector (31) comprises a plurality of detector elements, wherein the method comprises the following steps: - performing the method for estimating the rate of random coincidence according to any one of the preceding claims; - determining the coincidence of two X-ray signals detected simultaneously according to a predetermined criterion on adjacent detector elements in order to determine an uncorrected coincidence, in particular by counting the number of coincidence count signals for each detector element using at least one adjacent detector element, optionally using each adjacent detector element; - correcting the uncorrected coincidence based on the ratio of random coincidences in order to estimate the true coincidence.

8. The method according to claim 7, in, To determine the uncorrected overlap and to estimate the random overlap, a respective overlap unit (2, 3) is provided, wherein the same number of signal inputs for electrical signals are provided for each of the two overlap units (2, 3), the overlaps of which are counted.

9. The method according to claim 8, in, In order to determine the uncorrected overlap and to estimate the random overlap, signals of the signal inputs of the corresponding overlap units (2, 3) are provided for the detector element for which the real overlap is to be determined and for one or more detector elements adjacent to the detector element for which the real overlap is to be determined. Therein, only a subset of the adjacent detector elements is provided for determining the uncorrected overlap and / or estimating the random overlap.

10. The method according to claim 8, in, In order to determine the random overlap, signals are provided for the signal input of the overlap unit (2) for estimating the random overlap, for the detector element for which the real overlap is to be determined and for the detector element following this detector element. Therein, only a subset of the adjacent detector elements is provided for determining the uncorrected overlap and / or only a subset of the next detector elements is provided for estimating the random overlap.

11. The method according to any one of claims 7 to 10, in, The true coincidence is transmitted as a count value, In this case, in addition to the actual overlap, the random overlap and / or the uncorrected overlap are also transmitted as additional count values.

12. The method according to any one of claims 7 to 11, in, The count rate of the random coincidences is used to monitor and, if necessary, correct the breakdown of other counters of the signal, in particular the breakdown of other counters of the signal whose random coincidences are estimated at the count rate.

13. A method for recording an X-ray image data set, in particular a computed tomography image data set, of an object using an X-ray system, in particular a computed tomography system, the X-ray system having a counting X-ray detector (31), in, The X-ray detector (31) comprises a plurality of detector elements, The method comprises the following steps: - counting at least a certain number of count signals from the incident X-ray radiation in each detector element; - performing a method according to any one of claims 7 to 12; - generating an X-ray image data set based on at least a certain number of count signals counted in each detector element and an estimated true coincidence.

14. A counting X-ray detector (31), in particular for a computer tomography system, for recording X-ray image data sets of an object irradiated by X-ray radiation. in, The X-ray detector (31) comprises a plurality of detector elements and at least one circuit having at least one coincidence unit (2), Therein, the X-ray detector (31) is configured to implement the method according to any of the preceding claims.

15. A medical imaging device, in particular a computed tomography system, having a counting X-ray detector (31), in particular an X-ray detector (31) according to claim 14, and a control module (33), wherein: The medical imaging device is configured to implement the method according to any one of claims 1 to 13 .

Images