Recalibration of radiation detectors

Abstract

A method for calibrating a radiation detector having a plurality of pixels is described. In the method, a radiation spectrum of a radiation source is determined by the radiation detector, for each pixel of the radiation detector, a plurality of energy-dependent count rates for different signal threshold values of an evaluation unit of the radiation detector are measured and based on the different energy threshold values and count rates an updated signal threshold count value pair is determined. By comparing the updated signal threshold count value pair with a previously calibrated pixel-individual reference signal threshold count value pair determined at an earlier point in time for the radiation detector, a pixel-individual shift value for the signal threshold is determined pixel by pixel. By adding the shift value pixel by pixel to the signal threshold of the previously calibrated reference signal threshold signal energy value pair, a recalibrated signal threshold signal energy value pair is determined pixel by pixel. The radiation detector is recalibrated with the help of the recalibrated signal threshold signal energy value pair. A recalibration device, a radiation detector and an imaging system are also described.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for recalibrating a radiation detector having a plurality of pixels. The present invention also relates to a calibration device. Furthermore, the present invention relates to a radiation detector. The present invention also relates to an imaging system. BACKGROUND

[0002] With modern imaging methods, usually two-dimensional or three-dimensional image data are generated, which can be used using the image data to examine an examination object and furthermore also for other applications. In order to obtain image information from the examination object, systems which are usually used are based on detecting electromagnetic waves which interact with the examination object.

[0003] One method for obtaining image data from within such an examination object is based on X-ray imaging technology. An X-ray system for this purpose comprises an X-ray source for generating X-rays and an X-ray detector for detecting the X-rays. The X-ray detector comprises a detection unit which generates a detection signal for X-rays which hit the detection unit and an evaluation unit which determines a set of count rates for the X-rays which hit the detection unit on the basis of the detection signal.

[0004] In order to obtain three-dimensional data from the examination object, a computed tomography system (CT system for short) is often used, with which projection measurement data from the examination object are acquired. In a CT system, a combination of an X-ray source which is arranged on a gantry and an X-ray detector which is arranged opposite it is usually rotated around a measurement room in which the examination object, which is hereinafter referred to without loss of generality as a patient, is located. The centre of rotation, also referred to as the "isocentre", coincides with the so-called system axis z. During one or more rotations, the patient is irradiated with X-rays from the X-ray source, wherein projection measurement data or X-ray projection data are detected by means of the oppositely arranged X-ray detector.

[0005] The generated projection measurement data depend in particular on the construction type of the X-ray detector. The X-ray detector usually has a plurality of detection units which are usually arranged in the form of a regular pixel array. The detection units each produce a detection signal for X-rays which hit the detection unit, which detection signal is analysed at a specific point in time with regard to the intensity and spectral distribution of the X-rays in order to obtain conclusions about the object to be examined and to generate the projection measurement data.

[0006] So-called quantum-counting X-ray detectors have recently been used to detect projection measurement data. In the case of such quantum-counting or photon-counting X-ray detectors, the detection signals for the X-rays are analyzed in terms of their intensity and spectral distribution in the form of a count rate. The count rate can be provided as output data of so-called detector channels, which are each associated with one detection cell. In the case of quantum- or photon-counting detectors with a plurality of energy thresholds, a set of count rates is typically generated on the basis of the respective detection signals of the detection cells per projection. The set of count rates can include count rates for a plurality of different, in particular simultaneously checked, energy thresholds. The energy thresholds and the number of energy thresholds associated with each energy threshold are typically specified as signal analysis parameters for the detection of the projections.

[0007] The control and signal analysis in the evaluation unit of such an X-ray detector is carried out by means of externally provided register values, which are converted into control voltages via digital-to-analog converters, shortly DACs. In this context, the digital settings are also referred to as DAC values or signal thresholds in the following, which by the naming of the detector parameters illustrate the control function. For example, the DAC values correspond to control voltages of the X-ray detector, which determine the electronic threshold values of the comparators in the signal evaluation unit, respectively. The electronic threshold values, for example voltage values, of the comparators in turn correspond to the energy thresholds, by means of which incident X-ray quanta can be measured and distinguished in terms of their energy. The energy threshold defines the range of energy values in which the comparator detects and, in turn, counts the X-ray quanta. The X-ray detector typically has a plurality of electronic threshold values per channel, which are controlled with different DAC values or signal thresholds. In order to be able to measure the energy of the incident X-ray quanta absolutely, the absolute energy scale must be calibrated. This means that the link between the electronic signal threshold of the comparator and the corresponding energy threshold must be determined for the incident X-ray quanta, by means of which the signal energy value can be determined. Thus, when measuring the X-ray quanta in accordance with the knowledge of the aforementioned relationship and based on the knowledge of the measurement signal threshold, the measurement signal threshold is associated with the signal energy value.

[0008] For this purpose, the calibration, also referred to as tuning, is typically carried out at initial tuning during production of the X-ray detector or of a CT system having such an X-ray detector. Furthermore, in order to increase the accuracy, the X-ray detector must be recalibrated if necessary even after delivery to the customer.

[0009] However, setting the energy threshold, or the corresponding electronic threshold, DAC value, or signal threshold at a fixed level, is only possible at a certain granularity. For example, a DAC might correspond to an energy level of 0.5 keV. Therefore, the electronic threshold cannot be precisely adjusted but is binned or rounded to an integer value before creating the final table. If the sensor signal changes very little over time, it may be necessary to update or optimize this setting.

[0010] If image quality artifacts occur, it indicates a spectral change in the X-ray detector, which usually requires a complete readjustment. However, this process is time-consuming, during which technicians operate the equipment for a full workday, and radiologists cannot use the equipment.

[0011] Furthermore, noise effects often hinder the accurate determination of the characteristic structure of a feature X-ray spectrum. Due to these noise effects, a certain number of pixels are typically combined for fine-tuning of the spectrum, as spectral structure can only be identified by combining and averaging multiple pixels.

[0012] Through this readjustment, the energy threshold, or the electron threshold assigned to it, is completely remeasured and set to its defined value. However, measurement and evaluation noise (including rounding / sampling errors) is fully incorporated into the table through this step. Therefore, all subsequent adjustments to the table will become invalid and must be made accordingly. Summary of the Invention

[0013] Summary of the Invention The purpose of this invention is to provide a method for calibrating radiation detectors, particularly X-ray detectors in CT systems, which is simpler and, in particular, more time-saving than conventional methods for calibrating radiation detectors.

[0014] This objective is achieved by means of the method, calibration apparatus, radiation detector and imaging system for recalibrating a radiation detector according to the present invention.

[0015] In the method of the present invention for recalibrating a radiation detector having multiple pixels, the radiation detector first detects the radiation spectrum of a radiation source, preferably an X-ray spectrum, and particularly preferably a characteristic X-ray spectrum of an X-ray source. The radiation spectrum represents the energy distribution of the radiation emitted by the radiation source. Importantly, the radiation spectrum used in the recalibration should be as similar as possible to an older radiation spectrum measured in a previous calibration or recalibration, and this older radiation spectrum and its associated calibration or recalibration application serve as a reference for the current recalibration. High similarity can be achieved by activating the radiation source identically in both the reference measurement and the current recalibration. In this respect, the radiation detector should be considered a device for detecting electromagnetic waves or particle radiation. Therefore, the radiation source used in the recalibration should be specifically the same as the radiation source used in the older calibration.

[0016] In the re-calibration measurement, a plurality of energy-dependent count rates of each pixel of the radiation detector is measured for different signal thresholds or electronic thresholds of the evaluation unit of the radiation detector, and based on the measured count rates and their associated signal thresholds or DAC values, an updated signal threshold count rate pair is determined.

[0017] These DAC values or signal thresholds are threshold values of the electronic thresholds of a digital-to-analog converter that generates a digital value based on a sensor signal. By comparing the updated signal threshold count rate pair with the reference signal threshold count rate pair of a previous reference measurement, now mathematically the shift of the signal threshold of the updated signal threshold count rate pair relative to the older signal threshold of the reference signal threshold count rate pair is determined, which comparison preferably includes a full calibration.

[0018] In addition to the characterization of the signal threshold-count rate relationship by the reference signal threshold count rate pair, the previous reference measurement is also associated with a signal energy value-signal threshold relationship with reference signal energy values and reference signal thresholds. The latter relationship can be derived based on the signal threshold-count rate relationship and the knowledge of the characteristics of the radiation spectrum on which the reference measurement is based.

[0019] As explained in more detail later, the shift of the signal threshold is preferably first determined as a continuous value for correcting the signal energy value-signal threshold relationship of the reference measurement. Only at the end of the re-calibration, the thus determined corrected signal energy value-signal threshold relationship is rounded to discrete DAC values or signal thresholds.

[0020] Thus, by comparing the updated pixel-individual signal threshold count rate pair with the previously calibrated, pixel-individual reference signal threshold count rate pair of the radiation detector determined at an earlier point in time, a pixel-individual shift value for the signal threshold is determined pixel by pixel.

[0021] Finally, the re-calibrated signal threshold is determined by adding the pixel-individual shift value to the signal threshold of the pixel-individual correlation between the reference signal threshold and the reference signal energy value of the respective pixel pixel by pixel. In other words, the re-calibrated signal threshold signal energy value pair is determined by adding the pixel-individual shift value to the signal threshold of the reference signal threshold signal energy value pair of the respective pixel-individual of the previously calibrated respective pixel pixel by pixel.

[0022] Furthermore, the radiation detector is re-calibrated by the re-calibrated signal threshold signal energy value pair. In the context of the re-calibration, the signal threshold signal energy value pair stored in the data memory of the radiation detector is changed based on the determined signal threshold signal energy value pair and provided for the correction of future measurements.

[0023] Advantageously, the reference signal threshold value of the determined pair of signal threshold value and signal energy value at the initial calibration can be corrected pixel by pixel without the need to re-determine the signal energy value. Since the shift of the determined signal threshold value is usually very small, the respective signal threshold value does not change for most pixels due to the rounding of the signal threshold value to an integer value. Only for pixels for which the shift results in a change of the rounded signal threshold value, the correlation between the rounded signal threshold value and the signal energy value has to be changed. These pixels are usually the pixels for which the non-rounded signal threshold value is very close to the rounding threshold.

[0024] Thus, the effort for the recalibration is reduced compared to a complete new calibration, since there is no need to measure the signal energy values again and the rounded signal threshold value, i.e. the DAC value, does not have to be changed for each pixel.

[0025] Furthermore, the recalibration can be performed without a detailed analysis of the detected count rate spectrum with respect to the unique spectral lines and their associated energy values. Since such an analysis is usually hampered by severe noise, a usual new calibration increases the uncertainty in defining the single energy threshold or signal threshold signal energy value pair. Usually, in a conventional calibration, multiple pixels are combined and a common energy threshold or signal threshold signal energy value pair is defined for them in order to reduce the noise influence and to reliably determine the unique spectral lines to some extent. Advantageously, the method of the present invention can perform a pixel by pixel correction compared to the traditional new calibration and can suppress the noise influence since the spectral information items are not needed for the recalibration right now. Thus, the resolution of the detector can be improved by the recalibration compared to the conventional method. Since the resolution of the accurate detection of the X-ray radiation is improved, the image quality of a radiation detector, in particular such an X-ray detector, recalibrated using the method of the present invention can be improved overall.

[0026] The recalibration device for a radiation detector of the present invention has a data reception interface for detecting a radiation spectrum of a radiation source, preferably an X-ray spectrum of an X-ray source, by the radiation detector. For each pixel of the radiation detector, a plurality of energy-dependent count rates of different signal thresholds of an evaluation unit of the radiation detector are received and an updated signal threshold count rate pair is determined based on the different signal thresholds and their associated count rates.

[0027] A comparison unit is also part of the recalibration device of the present invention, which comparison unit is used to determine a shift value of the pixel individual of the signal threshold pixel by pixel by a comparison between the updated signal threshold count rate pair and a previously calibrated, pixel individual reference signal threshold count rate pair of the radiation detector determined at an earlier point in time.

[0028] The recalibration device according to the application further has a recalibration unit for determining a recalibrated signal threshold signal energy value pair pixel by pixel by adding the shift value of the respective pixel individual to the signal threshold of the reference signal threshold signal energy value pair of the respective pixel individual of the previously calibrated respective pixel and for recalibrating the radiation detector by means of the recalibrated signal threshold signal energy value pair. The recalibration device according to the application shares the advantages of the method according to the application for recalibrating a radiation detector having a plurality of pixels.

[0029] The radiation detector according to the application, preferably an X-ray detector, has a detection unit which generates a detection signal for radiation, in particular X-ray radiation, hitting the detection unit. The radiation detector according to the application further has an evaluation unit which determines a set of measurement values for the X-ray radiation hitting the detection unit on the basis of the detection signal. For example, the measurement values can relate to a count rate of count events detected by means of the radiation detector. They can also relate to time intervals between individual count events, count values, intensity values in the detector pixels or photon rates. As mentioned in the introduction, the set of measurement values usually comprises a plurality of count rates determined simultaneously for individual detector channels. The count rates determined simultaneously for individual detector channels, in particular for a plurality of energy thresholds, are determined with respectively associated mutually different energy thresholds. In addition, the radiation detector according to the application further comprises a recalibration device according to the application.

[0030] Such a radiation detector can be embodied, for example, as a quantum-counting or photon-counting X-ray detector. In this case, the detection unit of the X-ray detector according to the application can in particular comprise a detection region in order to generate, for example, charge pulses as detection signals for the X-ray radiation absorbed by the detection unit. The detection signals, in particular the charge pulses, are therefore particularly suitable for determining the energy of the X-ray radiation absorbed.

[0031] The evaluation unit can in particular be integrated in or realized in the structure of a so-called application-specific integrated circuit (ASIC). In particular, the detection unit and at least some of the evaluation unit can be combined in the detector channels. The radiation detector according to the application has the advantages of the recalibration device according to the application.

[0032] The application further proposes an imaging system, preferably an X-ray imaging system, in particular a CT system, having a radiation source, preferably an X-ray source for emitting X-ray radiation, and a radiation detector according to the application, preferably an X-ray detector for detecting radiation emitted by the radiation source. The imaging system according to the application has the advantages of the radiation detector according to the application.

[0033] Most of the aforementioned components of the recalibration apparatus according to the invention can be implemented, in whole or in part, as software modules, such as control devices for a radiation detector or an imaging system having the radiation detector, within the processor of a corresponding computing system. This particularly relates to the comparison unit and the recalibration unit. The advantage of primarily software implementation is that even previously used computing systems can be easily reconfigured via software updates to operate in accordance with the invention. In this respect, this objective is also achieved by a corresponding computer program product having a computer program that can be directly loaded into a computing system, the computer program having program segments for performing the steps of the method according to the invention, performing at least computer-executable steps when the program is executed in the computing system, for determining the shift value of individual pixels pixel by pixel, for determining the recalibration signal threshold signal energy value pair pixel by pixel, and for recalibrating the radiation detector using the recalibration signal threshold pair. In addition to the computer program, the computer program product may optionally include additional components, such as documentation and / or additional components, and hardware components, such as hardware keys (dongles, etc.), for use with the software.

[0034] In particular, the recalibration device according to the invention can be part of the user terminal or control device of an imaging system, preferably an X-ray imaging system, especially a CT system.

[0035] Computer-readable media, such as memory sticks, hard disks, or other transportable or permanently mounted data carriers, can be used to transport to or within a computing system or control device and / or for storage on or within the computing system or control device, on which program segments of a computer program are stored, which can be read and executed by the computing system. For this purpose, the computing system may have one or more cooperating microprocessors, etc.

[0036] Furthermore, particularly advantageous embodiments and modifications of the present invention can be found in the following description.

[0037] In one embodiment of the method according to the invention, the radiation detector includes one of the following types of detectors:

[0038] -X-ray detector

[0039] - Quantum counting X-ray detector

[0040] - Photodetector.

[0041] Advantageously, the method according to the invention can be applied to different types of radiation detectors. However, it is particularly preferred to apply the method according to the invention to quantum counting X-ray detectors.

[0042] In a preferred embodiment of the method according to the application for recalibrating a radiation detector having a plurality of pixels, the shift values of the individual pixels are determined pixel by pixel based on one of the following types of comparison methods:

[0043] - performing a regression analysis based on the updated signal threshold count value pairs and the previously calibrated, at an earlier point in time determined, reference signal threshold count value pairs of the individual pixels of the radiation detector,

[0044] - applying an artificial intelligence based method to the updated signal threshold count value pairs and the previously calibrated, at an earlier point in time determined, reference signal threshold count value pairs of the individual pixels of the radiation detector.

[0045] Advantageously, by adding the entire signal threshold scale or DAC value scale for determining the shift, the statistical error, in particular the noise influence, can be reduced.

[0046] In an embodiment of the method according to the application for recalibrating a radiation detector having a plurality of pixels, the shift value of each individual pixel comprises an offset value. In the step of determining the recalibrated signal threshold signal energy value pairs pixel by pixel, the offset value is added to the signal threshold of the previously calibrated reference signal threshold signal energy value pairs of the respective individual pixel of the respective pixel. This recalibration corresponds to a zero order recalibration, which can be carried out in particular easily and without effort.

[0047] If the recalibration should be more accurate, it can also be carried out in an order higher than said zero order. A fitting curve between the signal threshold values determined in the recalibration and their associated older calibrated reference signal threshold values is determined. The shift value is generated based on a determined parameter value of the fitting curve and optionally depends on the respective signal threshold. The fitting curve preferably comprises a polynomial of a higher order than the zero order.

[0048] For example, in a first order recalibration, the updated, recalibrated pixel related shift values are calculated from the offset value and the gradient change of the fitting curve, which is preferably determined during the regression analysis, respectively. The fitting curve reproduces the relationship between the signal threshold values measured in the recalibration and the reference measurements or older calibrated signal threshold values.

[0049] The gradient change of the fitting curve is due to its gradient change compared to the reference value, preferably the value 1. The shift value is derived as the sum of the offset value and the product of the determined gradient change of the fitting curve and the signal threshold of the reference signal threshold count rate pair. Advantageously, compared to the zero order recalibration, which only considers the offset value, the accuracy of the recalibration is further improved due to the consideration of the gradient change.

[0050] In the step of recalibrating the signal threshold signal energy values with the aid of the recalibration, the determined shift can be applied to a plurality of different calibration tables based on signal thresholds. This is also possible, in particular because no new noise influences are generated in the recalibration compared to a complete new calibration. As explained in detail later, in addition to the correlation between the reproduced signal threshold and the signal energy value or the energy adjustment table which provides a correction value for this correlation, there are also other types of calibration tables which are based on signal thresholds and provide a correction for the measured values of the radiation detector. Advantageously, during the recalibration of a plurality of different types of calibration tables, the effort of the recalibration is thus significantly reduced compared to a corresponding complete new calibration of these calibration tables.

[0051] The different calibration tables preferably comprise at least one of the following types:

[0052] - an energy adjustment table,

[0053] - a linear correction table,

[0054] - an offset correction table,

[0055] - a gain correction table,

[0056] - a defective pixel correction table,

[0057] - a hardening correction table.

[0058] The energy adjustment table comprises a relationship between the DAC values, i.e. the signal thresholds, and the signal energy values, which has already been described in the introduction.

[0059] The linear correction table serves to correct for pile-up effects and general non-linear effects of the measurement scale. Pile-up effects are related to the measurement of events which occur so close in time that they cannot or can hardly be seen as single events. This leads to a loss of counts.

[0060] The offset correction table enables a zero-point correction for pixel individual differences in the measurement of X-ray radiation.

[0061] The gain correction table enables a proportionality correction for pixel individual differences in the measurement of X-ray radiation.

[0062] The defective pixel correction table enables the identification of insufficiently functioning pixels and the generation of interpolation values as a replacement.

[0063] The hardening correction table allows a pixel individual correction in the spectral range for the physical properties of the measurement object.

[0064] Advantageously, the shift values can be applied to different calibration tables separately. Thus, a single table does not need to be calibrated or adjusted separately again by a separate calibration measurement. Thus, the method of the present application saves a large amount of time and significantly reduces the measurement and calculation effort in the case of a corresponding number of different calibration tables.

[0065] In a preferred embodiment of the method according to the application for recalibrating a radiation detector having a plurality of pixels, the pixel-individual reference signal threshold count rate pair is associated with a consecutive reference signal threshold scale and has a consecutive reference signal threshold.

[0066] For determining the recalibrated signal threshold, the determined pixel-individual shift value is added to the consecutive reference signal threshold, resulting in a consecutive signal threshold, and by rounding the consecutive signal threshold to an integer signal threshold or DAC value, the recalibrated signal threshold of the recalibrated signal threshold count rate pair is determined.

[0067] Thus, preferably, only pixels whose previously calibrated signal threshold is close to a value that lies an odd multiple of half of a previously defined increment on the signal threshold scale are corrected. For example, the increment can lie at the value 1. Correction is only carried out when the rounded recalibrated signal threshold falls below or exceeds a rounding limit of 0.5, 1.5, 2.5, etc., i.e. the signal threshold to be rounded is close to an odd multiple of half of the increment, or the rounding of the value to a different multiple than the previous reference signal threshold takes place as a result of the recalibration.

[0068] Thus, the shift is first applied to the consecutive scale and rounded to a "DAC scale" having integer DAC values during the adjustment. However, the method according to the application is also applicable to a continuous scale.

[0069] If the method according to the application is carried out several times at different points in time and if the consecutive signal thresholds are stored with each recalibration, a trend analysis is thus preferably subsequently carried out on the basis of the consecutive signal thresholds determined at the different points in time, on the basis of which an updated recalibrated signal threshold signal energy value pair can be approximately determined even without carrying out the method according to the application again. Advantageously, the recalibration effort is further reduced.

[0070] Also preferably, it is determined on the basis of the trend analysis at which point in time the recalibration method according to the application must be carried out again. If the shift exceeds a predetermined threshold value, for example, recalibration according to the method of the application should be carried out. Thus, the point in time can be estimated in advance by means of the trend analysis, so that the number of recalibrations is limited to a minimum number, thereby further reducing the calibration effort.

[0071] For obtaining particularly reliable calibration results, the recalibration according to the application is preferably verified on the basis of at least one error criterion, preferably on the basis of a plurality of such error criteria. This or these error criteria are determined on the basis of the measurements made for the recalibration.

[0072] On the basis of the verification, it can be determined, inter alia, that the selected pixels are not recalibrated, or only partially recalibrated, when an excessively pronounced shift has occurred. An excessively pronounced shift is to be regarded as a reason for a complete readjustment. Advantageously, the recalibration can be applied purposefully to cases in which an improvement on the previous calibration is made.

[0073] Preferably, the at least one error criterion comprises at least one of the following types of criteria:

[0074] - a shift value occurs which exceeds a predetermined threshold value,

[0075] - a signal threshold signal energy value for the recalibration is exceeded by a predetermined proportion of the total number of pixels of the radiation detector,

[0076] - the number of pixels which are graded as defective exceeds a predetermined number.

[0077] The predetermined threshold value for the shift depends on the granularity of the signal threshold scale used. The predetermined threshold value for a shift with a granularity of 0.5 keV is, for example, preferably two DAC values.

[0078] The aforementioned checks are, in alternative embodiments, carried out by means of an AI-based method (AI here stands for "artificial intelligence"). Advantageously, different criteria can be taken into account in a particularly individually adjusted manner.

[0079] If the verification results are negative, a warning message is preferably issued. Advantageously, the user is informed that no recalibration has been made and is warned that the measurement can be biased and inaccurate.

[0080] In a particularly advantageous variant of the method according to the application, different electronic threshold values of the same detector pixel are mapped to a common DAC scale, i.e. to an electronic signal threshold scale. In the case of an X-ray detector, a plurality of electronic signal thresholds are associated with a single pixel. These can be used to cover the entire bandwidth of the X-ray energy spectrum. If all signal thresholds are to require a common DAC scale, the individual signal threshold scales must be coordinated with one another. This can be achieved, for example, by shifting, compressing or lengthening the individual scales. Advantageously, in the case of such a universal DAC scale, all measurement count rates of the detector pixels can be represented in a diagram. Advantageously, by means of the method of the application, it is possible to bring the common DAC scale into the correct relationship with the energy scale.

[0081] In a particularly effective variant of the method according to the application, the detector pixels are divided into a first group and a second group. The first group of detector pixels is selected for the recalibration of the radiation detector. The detector pixels contained in the first group are selected according to a specific classification criterion, with which particularly suitable detector pixels are selected for the recalibration of the radiation detector. The remaining detector pixels then form the second group. It is advantageous to carry out the calibration on the basis of suitable detector pixels, so that more accurate results are obtained. The classification criterion used can include, for example, that detector pixels located near a scatter radiation grid or the sensor edge or near a feed cable are not taken into account. The detector pixels assigned to the first group are also referred to hereinafter as "clean" pixels, which do not experience shading since, in this specific case, they are not located near a scatter radiation grid. Using this sub-group of available pixels makes it possible to determine unaltered measurement values, for example count values, which can be calibrated with the energy scale by means of a signal threshold scale obtained beforehand. In this way, the frequently occurring problem of a poorly focused or possibly poorly collimated orientation is solved.

[0082] It is particularly preferred that an ensemble of detector pixels having a sufficiently similar responsiveness to the individual detector pixels is formed for the recalibration. The ensemble of detector pixels is preferably formed on the basis of a plurality of adjacent detector pixels which have a sufficiently similar responsiveness to the individual detector pixels. The value of the statistical evaluation based on the ensemble is then used as an input variable for the recalibration instead of the measurement values of the individual detector pixels.

[0083] For example, an ensemble average of the measurement values of the individual detector pixels of a pixel group is associated with the ensemble of detector pixels. The measurement signals of these pixels can be combined, for example by means of addition, in order to obtain an improved statistical significance compared to the measurements of the individual pixels. The grouping of the pixels in a pixel group significantly improves the signal-to-noise ratio of the measurement values obtained during the test measurement, so that the quality and reliability of the recalibration are improved. In addition, the computational effort for the recalibration is reduced since the total number of sensor sub-units to be recalibrated is reduced compared to the recalibration of the individual pixels.

[0084] A single threshold value of an individual detector pixel can also be oriented as a common scale within the pixels of this pixel group. This orientation is less sensitive to the limited statistics of the individual measurement values of the underlying test measurement, so that the measurement values of the pixel group can be combined first.

[0085] In order to also take into account detector pixels which do not belong to the first group of detector pixels and thus do not exhibit approximately the same responsivity, the equalization step is preferably carried out. This step is also referred to in the literature as the homogenization method. During the equalization step, the responsivity of the first and second group of detector pixels is brought into line with the responsivity of the first group of detector pixels. Methods for equalizing the responsivity of the detector pixels are described, for example, in DE 102006022596 Al and DE 102011080656 B4.

[0086] In connection with the method according to the application, the equalization measure produced in the homogenization process can be used to determine a shift in the correlation between the DAC values or signal threshold values and the signal energy values, since adjacent pixels can have a similar shift in the first order and an additional pixel-specific shift component in a higher order.

[0087] In the method for equalizing the responsivity of the detector pixels described in DE 102006022596 Al, a first measurement is carried out with radiation having a first energy spectrum, wherein for each detector pixel there is a first energy threshold value which lies within the first energy spectrum. For each detector pixel, a first count rate of the X-ray radiation which is determined in the process below or above the first energy threshold value is detected. Subsequently, x-ray radiation having a second energy spectrum is emitted, wherein the first energy threshold value remains unchanged. In turn, for each detector pixel, a second count rate of the X-ray radiation below or above the first energy threshold value is detected. Subsequently, a quotient is formed from the first count rate and the second count rate of each detector pixel. The first energy threshold value of the detector pixels is subsequently changed such that the quotient of the respective detector pixels is equalized. Advantageously, the recalibration measurement of the method according to the application can now be used simultaneously for equalizing the responsivity of the detector pixels. As a result of the double use of the measurement data of the recalibration measurement, effort and time can be saved in the method according to the application.

[0088] In order to be able to carry out the described calibration steps of the detector pixels, the method according to the application preferably comprises a measurement in addition to the recalibration measurement, and this measurement is carried out with a different photon flow spectrum than the recalibration measurement. This second measurement value then corresponds to the measurement value with the second energy spectrum described in the calibration method described in DE 102006022596 Al.

[0089] The X-ray detector according to the application can in particular comprise a detection region or a detection surface which has a semiconductor material which directly captures or absorbs the X-ray radiation (direct converter).

[0090] It is also conceivable that the X-ray detector according to the application comprises a detection region having a scintillator material which converts the X-ray radiation into radiation of other spectral ranges, in particular in the visible spectral range. The converted radiation can be detected with a semiconductor detector, for example a photodiode or a silicon photomultiplier, which is arranged in the radiation path following the scintillator and is likewise contained in the detection unit. The photodiode, the silicon photomultiplier or the semiconductor detector then generates a detection signal which can likewise be analyzed in terms of the spectral distribution and the intensity of the X-ray radiation detected by the scintillator. BRIEF DESCRIPTION OF DRAWINGS

[0091] The application is explained in more detail below using embodiments with reference to the accompanying drawings. In different figures, identical parts have identical reference signs. Herein:

[0092] Figure 1 A diagram showing an example of measuring a characteristic X-ray spectrum of an X-ray source according to the prior art to illustrate a drift of an X-ray detector is shown;

[0093] Figure 2 A diagram showing a first and a second measurement curve of pairs of signal threshold count rates recorded at different points in time is shown,

[0094] Figure 3 A diagram illustrating updated signal thresholds plotted in dependence on a reference signal threshold is shown,

[0095] Figure 4 A diagram showing a pixel distribution with a shift for a zeroth order shift describing four electronic energy thresholds is shown,

[0096] Figure 5 A diagram showing a pixel distribution with a shift for a first order shift representing four electronic energy thresholds is shown,

[0097] Figure 6 A flowchart illustrating a method for recalibrating a radiation detector having a plurality of pixels according to a first embodiment of the application is shown,

[0098] Figure 7 A flowchart illustrating a method for recalibrating a radiation detector having a plurality of pixels according to a first embodiment of the application is shown, Figure 6 A flowchart of step 6.IV of the method shown,

[0099] Figure 8 A block diagram showing a calibration device according to an embodiment of the application is shown,

[0100] Figure 9 A block diagram showing an X-ray detector having a calibration device according to an embodiment of the application is shown. DETAILED DESCRIPTION

[0101] Figure 1 Figure 10, based on existing technology, illustrates the drift of an X-ray detector using an example of measuring the characteristic X-ray spectrum of an X-ray source. The first measurement curve m1 shows the normalized spectral response nsr plotted on the signal energy value E [keV], illustrating the measurement characteristics of the X-ray detector at a first time point. The second measurement curve m2, slightly off-center from the first measurement curve m1, illustrates the measurement characteristics of the X-ray detector at a second time point later than the first time point. Figure 1 It also showed several significant energy values ​​in the X-ray spectrum, such as 23 keV (fluorescence from the X-ray detector), 59 keV (fluorescence from the X-ray tube), and 68 keV. Figure 1 The graph assigns maximum values ​​to these values. These maximum values, or the energy values ​​associated with them, can be used as direction values ​​in conventional calibration. However, noise effects often make it difficult to accurately determine the maximum values. Due to the aforementioned noise effects, it is traditionally necessary to combine a certain number of pixels for spectral fine-tuning, as only by combining and averaging multiple pixels can the desired result be obtained. Figure 1 The spectral structure shown is shown.

[0102] Figure 2 Figure 11 shows the first measurement curve M1 and the second measurement curve M2 of the signal threshold count rate pair recorded at different time points. It can be clearly seen that, for the same count rate value ZR, there is a shift in the associated signal threshold DAC.

[0103] Figure 3 Figure 12 shows the threshold of the reference signal DAC. ref The updated signal threshold DAC is plotted above. Therefore, Figure 2 The curves shown are figuratively described as sliding on top of each other, and the DAC value or signal threshold associated with the same count rate will be... ref The DACs are compared to each other. The angle bisector represents the reference signal threshold of the DAC. ref A shiftless mapping to the updated energy threshold DAC. From Figure 3 As can be seen, the new signal threshold DAC is partially different from the reference signal threshold DAC. ref There is a slight deviation. Averaging at order 0, this deviation is V = 0.148 DAC. This means that during recalibration, the shift is 0.148 DAC.

[0104] Figure 4 Figure 13 shows the distribution ZP of pixels P with shifts V or ΔDAC for the four electron energy thresholds th1, ..., th4 for a zero-order shift V. The shifts are characterized by shift values ​​between 2 and +1.

[0105] Figure 5Figure 14 is shown, illustrating the pixel distribution ZP for a first-order shift with shifts V or ΔDAC having first to fourth energy thresholds th1, th2, th3, and th4, in two sub-charts. The offset value of shift V can be seen in the left sub-chart. The shift is characterized by offset values ​​between -5 and +4. Figure 5 The reference energy threshold DAC is shown in the chart on the right. ref The deviation ΔS of the slope of the regression line between the newly measured energy threshold DAC and the new energy threshold DAC.

[0106] Figure 6 A flowchart 600 is shown, illustrating a method for recalibrating a radiation detector 1 having a plurality of pixels P according to a first embodiment of the present invention.

[0107] In step 6.I, radiation detector 1 detects the characteristic X-ray spectrum of the X-ray source. For each pixel P of radiation detector 1, multiple energy-dependent count rates ZR are measured for different signal thresholds DAC used in the evaluation unit of radiation detector 1.

[0108] In step 6.II, the updated signal threshold count value pair SZP is determined by associating the signal threshold DAC with the spectral count rate ZR based on knowledge of the characteristic X-ray spectrum.

[0109] In step 6.III, the signal threshold count value of SZP is compared with the reference signal threshold count value of the pixel individual determined at an earlier time point by the previously calibrated signal threshold count value of radiation detector 1. ref The comparison between the two is used to determine the individual pixel shift value V of the signal threshold DAC pixel by pixel. i .

[0110] In step 6.IV, the shift value V of the corresponding individual pixel is... i Added pixel by pixel to the corresponding pixel P i The reference signal threshold signal energy value of the corresponding pixel individual for SEP ref Signal threshold DAC ref Determine the signal threshold signal energy value for recalibration of SEP neu Furthermore, the SEP was evaluated using the recalibrated signal threshold signal energy value. neu To recalibrate radiation detector 1.

[0111] Figure 7 A flowchart illustrating step 6.IV according to a first embodiment of the present invention is shown.

[0112] In step 6.IVa, the reference signal threshold signal energy value of each pixel is compared with the SEP. refassociated with the consecutive reference signal threshold scales such that they have consecutive reference signal threshold DACs Ref_kont .

[0113] In step 6.IVb, the determined pixel individual shift values V i are added to the consecutive reference signal threshold DACs Ref_kont , thereby generating consecutive signal threshold DACs neu_kont .

[0114] Subsequently, in step 6.IVc, the recalibrated signal threshold DACs neu_kont are determined by rounding the consecutive signal threshold DACs ganz to integer signal threshold DACs neu . These integer recalibrated signal threshold DACs neu are parts of the recalibrated signal threshold signal energy value pairs SEP neu , respectively.

[0115] Figure 8 A recalibration device 50 for a radiation detector 1 (cf. Figure 9 ) according to an embodiment of the present application is schematically shown. The recalibration device 50 comprises a data reception interface 51 for receiving data of a characteristic X-ray spectrum of an X-ray source. For each pixel P of the radiation detector 1, a plurality of energy-dependent count rates ZR for different signal threshold DACs of an evaluation unit of the radiation detector 1 is measured.

[0116] Part of the recalibration device 50 is a correlation unit 52 which is set up to determine, by correlating the signal threshold DACs with the spectral count rates ZR based on the characteristic X-rays, updated signal threshold count value pairs SZPspectra.

[0117] The recalibration device 50 further comprises a comparison unit 53 for determining, pixel by pixel, pixel individual shift values V ref for the signal threshold DACs by comparison, based on a regression analysis, between the signal threshold count value pairs EZP and the previously calibrated, pixel individual reference signal threshold count value pairs SZP i of the radiation detector 1 determined at an earlier point in time.

[0118] The recalibration device 50 according to the present application further has a recalibration unit 54 for determining, pixel by pixel, recalibrated signal threshold signal energy value pairs SEP neu by adding the respective pixel individual shift values V i pixel by pixel to the signal threshold DACs ref of the respective pixel individual reference signal threshold count value pairs SZP i of the respective pixels P zef .

[0119] The determined recalibrated signal threshold signal energy value to the SEP neu is finally transferred to the output interface 55, which outputs the recalibration data SEP neu to a database DB (see Figure 9 ).

[0120] Figure 9 A radiation detector 1 in the form of a semiconductor detector is shown schematically, which is used as a direct converter. Such a semiconductor detector 1 is used, for example, in computed tomography apparatuses. The semiconductor detector 1 is designed as a planar pixel detector. It comprises a semiconductor material 2 in the form of a single crystal as a detector material. One side of the semiconductor material 2 is covered by a metal surface, which forms a backside electrical contact 3. A voltage HV is applied at the back contact 3.

[0121] The opposite side of the single crystal 2 is covered by a structured metallization layer, whose respective facets are designed as pixel contacts 4 and form a pixel matrix as a whole. The size and the pitch of the pixel contacts 4 together with further material parameters of the semiconductor material 2 determine the maximum spatial resolution capability of the semiconductor detector 1. The resolution is typically in the range of 10 pm to 500 pm. The respective pixel contacts 4 are connected to individual readout electronics 5, with which the measurement signals s1, s2, s3, s4 are detected, respectively. Comparators installed in the readout electronics 5 can be adjusted using an electronic threshold voltage to detect certain spectral components of the X-rays.

[0122] Furthermore, the radiation detector 1 comprises a summing element 6, with which the measurement signals s1, s2, s3, s4 are combined into a group signal G. The summing element 6 can also weight the individual measurement signals or pixel signals before they are added. In order to correctly add the individual signals s1, s2, s3, s4, the electronic threshold values of the comparators associated therewith must be set in such a way that they correspond to exactly the same energy threshold value. Figure 8 The recalibration device 50 shown is used for this purpose. In Figure 6 and Figure 7In the context of the illustrated recalibration method, for this purpose, the recalibration device 50 sends threshold setting commands SEB to the comparators 5 in order to change or finally set their comparator thresholds and obtains the measurement signals ZR from the individual comparators 5 (only the signal link of the comparator sending the fourth signal s4 is drawn, but the signal links are similarly present for the other comparators as well). In addition, the recalibration device 50 can also optionally actuate the X-ray tube R in the context of the recalibration by means of control commands SB, wherein the X-ray stream is changed accordingly for the individual measurements. This method can be useful if the pixels need to be equalized or homogenized. After the calibration has been carried out, the determined calibration data KD are transferred to the database DB. For example, when processing the count rates ZR of the individual pixels in the addition element 6, the calibration data KD can be used in order to correctly add the count rates ZR to the correct associated energy values.

[0123] It is noted that Figure 9 The arrangement 1 in Fig. 1 is shown in cross section, so that Figure 9 The arrangement 1 in Fig. 1 comprises a total of 16 pixel contacts. For greater clarity, only 4 pixel contacts are drawn per pixel group according to the cross-sectional view. As already mentioned, each pixel group usually has 16 or another number of pixel contacts.

[0124] Figure 9 The function of the illustrated semiconductor detector 1 is as follows: X-rays hitting the semiconductor detector 1 cause an interaction of the X-ray quanta with the semiconductor material 2 of the semiconductor detector 1, whereby electron-hole pairs are generated. The voltage HV applied to the contacts of the detector 1 generates an electric field which moves the generated charge carriers towards the electrical contacts, in particular towards the pixel contacts 4. The charge movement in the sensor material between the electrodes or pixel contacts 4 of the semiconductor detector 1 generates an electrical charge pulse which is proportional to the absorption energy of the X-rays. This charge pulse is read out by the connected readout electronics 5. The pixel signals or measurement signals s1, s2, s3, s4 detected by the readout electronics 5 are forwarded to the addition element 6, which combines the measurement signals s1, s2, s3, s4 (in fact, the measurement signals s1 to s 16 ) into a group signal G. The number of four different measurement signals s1, s2, s3, s4 is merely an example and can be replaced by an arbitrary number n of measurement signals.

[0125] Finally, it should be noted that all features of the embodiments or developments disclosed in the drawings can be used in any combination. Finally, it is also pointed out that the above-described X-ray detector, X-ray imaging system, calibration device and method for recalibrating a radiation detector are merely embodiments, which can be modified in various ways by a person skilled in the art without departing from the scope of the present application. For example, the X-ray detector can also be designed as a ring-shaped detector, which completely surrounds the measurement space of the imaging system in one direction. Furthermore, the use of the indefinite article "a" or "an" does not exclude the fact that the features discussed can appear several times. Likewise, the term "unit" does not exclude that the component discussed consists of a plurality of interacting sub-components, which can also be spatially distributed. Regardless of the grammatical gender of a particular term, it includes persons with male, female gender identity.

Claims

1. A method for recalibrating multiple pixels (P) i A method for a radiation detector (1) comprising the following steps: The radiation spectrum of the radiation source is detected by the radiation detector (1), wherein for each pixel (P) of the radiation detector (1) i Multiple energy-dependent count rates (ZR) for different signal thresholds (DAC) of the evaluation unit (5) of the radiation detector (1) are measured, and updated signal threshold count pairs (SZP) are determined based on the different signal thresholds (DAC) and the count rates (ZR) associated with the signal thresholds. By comparing the updated signal threshold count pair (SZP) with the reference signal threshold count pair (SZP) of the previously calibrated pixel individual determined at an earlier time point by the radiation detector (1),... ref The comparison between the values ​​determines, pixel by pixel, the shift value (V) for each individual pixel in the signal threshold (DAC). i ), By shifting the corresponding pixel value (V) i Added pixel-by-pixel to the corresponding previously calibrated pixel (P i The reference signal threshold signal energy value pair (SEP) of the corresponding pixel individual. ref ) signal threshold (DAC) ref ), determine the recalibrated signal threshold signal energy value pair (SEP) pixel by pixel. neu ), Using the recalibrated signal threshold signal energy value pair (SEP) neu (1) to recalibrate the radiation detector.

2. The method of claim 1, wherein the shift value (V) of the plurality of individual pixels is determined pixel by pixel. i It is based on one of the following types of comparison methods: Based on the updated signal threshold count pair (SZP) and the previously calibrated reference signal threshold count pair (SZP) of the radiation detector (1) for the individual pixels determined at an earlier time point. ref To perform regression analysis, or The artificial intelligence-based method is applied to the updated signal threshold count pair (SZP) and the previously calibrated reference signal threshold count pair (SZP) of the radiation detector (1) for the individual pixels determined at an earlier time point. ref ).

3. The method of claim 1, wherein the shift value (V) of the plurality of individual pixels i Each includes an offset value (ΔDAC), which is determined in the pixel-by-pixel recalibration signal threshold signal energy value pair (SEP). neu In the step of adding to the corresponding pixel (P) of the previous calibration, i The reference signal threshold signal energy value pair (SEP) of the corresponding pixel individual. ref ) signal threshold (DAC) ref ).

4. The method of claim 1, wherein the pixel-related shift value (V) i The signal threshold (DAC) and the reference signal threshold count pair (SZP) are respectively based on the updated signal threshold count pair (SZP). ref The signal threshold (DAC) ref Functions of order higher than zero that have been determined between () are determined.

5. The method of claim 1, wherein the signal energy value pair (SEP) is obtained by means of the recalibrated signal threshold signal energy value pair (SEP). neu In the recalibration step, based on the determined shift value (V) i The calibration is performed on several different calibration tables based on the signal threshold (DAC).

6. The method according to claim 1, wherein The reference signal threshold count pairs (SZP) of the individual pixels ref It is associated with a continuous reference signal threshold scale and has a continuous reference signal threshold (DAC). Ref_kont ), First, the determined shift value (V) of the individual pixels is... i ) added to the continuous reference signal threshold (DAC) Ref_kont ), where the continuous signal threshold (DAC) neu_kont ) is generated, and The recalibrated signal threshold signal energy value pair (SEP) neu The recalibrated signal threshold (DAC) neu By using the continuous signal threshold (DAC) neu_kont Rounding to integer threshold (DAC) ganz It was determined that...

7. The method of claim 6, wherein the method is performed multiple times at different time points, and the continuous signal threshold (DAC) is stored at each recalibration. neu_kont ), and subsequently based on the continuous signal threshold (DAC) determined at different time points. neu_kont Perform trend analysis. Based on this, the updated, recalibrated signal threshold signal energy value pair (SEP) neu It can also be approximately determined without having to re-execute the method according to any one of claims 1 to 5. and / or Based on this, it is determined at what point in time the method according to any one of claims 1 to 5 must be re-executed.

8. The method of claim 1, wherein the verification of the recalibration is based on at least one error criterion, the error criterion being based on measurements performed for recalibration.

9. The method of claim 8, wherein the at least one error criterion comprises at least one of the following criterion types: A shift value (V, V) exceeding a predetermined threshold occurs. i ), For the recalibrated signal threshold signal energy value pair (SEP) neu The pixels that must be generated (P) i The number of (Z) i A predetermined portion (A) exceeding the total number of pixels (P) of the radiation detector (1). max ), Pixels classified as defective (P) k The number of (Z) k Exceeding the predetermined number (Z) max ).

10. The method of claim 8, wherein a warning message is issued if the verification result is negative.

11. A recalibration device (50) for a radiation detector (1), comprising: A data receiving interface (51) is used to detect the characteristic radiation spectrum of the radiation source by the radiation detector (1), wherein multiple energy-dependent count rates (ZR) of different signal thresholds (DAC) for the evaluation unit (5) of the radiation detector (1) are received, and updated signal threshold count pairs (SZP) are determined based on the different signal thresholds (DAC) and the count rates (ZR) associated with the signal thresholds. Comparison unit (53) for comparing the updated signal threshold count pair (SZP) with the reference signal threshold count pair (SZP) of the previously calibrated pixel individual determined at an earlier time point by the radiation detector (1). ref The comparison between the values ​​determines, pixel by pixel, the shift value (V) for each individual pixel in the signal threshold (DAC). i ), The recalibration unit (54) is used to adjust the shift value (V) of the corresponding individual pixel by... i Added pixel-by-pixel to the corresponding previously calibrated pixel (P i The corresponding pixel individual reference signal threshold signal energy value pair (SEP) ref ) signal threshold (DAC) ref ), determine the recalibrated signal threshold signal energy value pair (SEP) pixel by pixel. neu ), and used with the aid of the recalibrated signal threshold signal energy value pair (SEP) neu (1) to recalibrate the radiation detector.

12. A radiation detector (1), comprising: Detection unit (2), the detection unit generates a detection signal in response to radiation hitting the detection unit (2), Evaluation unit (5) determines multiple energy-dependent count rates (ZR) for different signal thresholds (DAC) based on the probe signal. The recalibration device (50) for a radiation detector (1) according to claim 11 is designed to recalibrate the radiation detector (1).

13. The radiation detector (1) according to claim 12, wherein the radiation detector is an X-ray detector, wherein the detection unit generates a detection signal in response to X-rays hitting the detection unit (2).

14. An imaging system having a radiation source (R) and a radiation detector (1) according to claim 12.

15. The imaging system of claim 14, wherein the imaging system is an X-ray imaging system.

16. A computer program product comprising instructions that, when a computer runs the program, cause to perform the steps of the method according to any one of claims 1 to 10, for: determining, pixel-by-pixel, a shift value (V) for individual pixels. i ), determine the recalibrated signal threshold signal energy value pair (SEP) pixel by pixel. neu ), and the signal energy value pair (SEP) obtained by means of the recalibrated signal threshold. neu Recalibrate the radiation detector (1).

17. A computer-readable storage medium comprising instructions that, when a computer executes a program, cause to perform the steps of the method according to any one of claims 1 to 10, for: determining, pixel-by-pixel, a shift value (V0) for an individual plurality of pixels. i ), determine the recalibrated signal threshold signal energy value pair (SEP) pixel by pixel. neu ), and the signal energy value pair (SEP) obtained by means of the recalibrated signal threshold. neu Recalibrate the radiation detector (1).

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