Recalibration of radiation detector

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 a radiation detector, for each pixel of the radiation detector a plurality of energy-dependent count rates for different signal thresholds of an evaluation unit of the radiation detector are measured and updated signal threshold count value pairs are determined based on the different energy thresholds and the count rates. Shift values for the individual pixels for the signal threshold are determined pixel by pixel by comparison between the updated pair of signal threshold counts and a previously calibrated pair of reference signal threshold counts for the individual pixels of the radiation detector determined at an earlier point in time. A recalibrated signal threshold signal energy value pair is determined pixel by pixel by adding the offset value to a signal threshold of a previously calibrated reference signal threshold signal energy value pair. The radiation detector is recalibrated by means of the recalibrated signal threshold signal energy value pair. A recalibration apparatus, 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 Art

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

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

[0004] To obtain three-dimensional data from an examination object, a computed tomography system (abbreviated as CT system) is often used, and projection measurement data is acquired from the examination object using this system. In a CT system, a combination of an X-ray source arranged on a gantry and an X-ray detector arranged opposite thereto usually rotates around a measurement chamber where an examination object (hereinafter generically referred to as a patient) is located. The center of rotation (also referred to as the "isocenter") coincides with a so-called system axis z. During one or more rotations, the patient is irradiated with X-rays from the X-ray source, and projection measurement data or X-ray projection data is detected by means of the opposing X-ray detector.

[0005] The generated projection measurement data depends particularly on the structural 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. Each detection unit generates a detection signal for the X-rays hitting the detection unit, and the detection signal is analyzed at a specific time point in terms of the intensity and spectral distribution of the X-rays in order to draw conclusions about the object to be examined and generate projection measurement data.

[0006] The so-called quantum counting X-ray detector has recently been used to detect projection measurement data. In the case of such a quantum counting or photon counting X-ray detector, the detection signal for X-rays is analyzed in terms of the intensity and spectral distribution of the X-rays in the form of a counting rate. The counting rate can be provided as the output data of so-called detector channels, which are each associated with a detection unit. In the case of a quantum or photon counting detector with multiple energy thresholds, each detector channel typically generates a set of counting rates based on the respective detection signal of the projection detection unit each time. The set of counting rates can include counting rates for multiple different, in particular simultaneously examined, energy thresholds. The energy thresholds and the number of energy thresholds associated with each individual energy threshold are typically specified as signal analysis parameters for the detection of projections.

[0007] The control and signal analysis in the evaluation unit of such an X-ray detector are performed by externally provided register values, which are converted into control voltages via a digital-to-analog converter (abbreviated as DAC). In this context, the digital settings are also abbreviated as DAC values or signal thresholds hereinafter, and the control function is described by the naming of the detector parameters. For example, the DAC values correspond respectively to the control voltages of the X-ray detector, which determine the electronic threshold of the comparator in the signal evaluation unit. The electronic threshold of the comparator, such as a voltage value, in turn corresponds to the energy threshold, by means of which the incident X-ray quanta can be measured and differentiated in terms of their energy. The energy threshold defines the range of energy values at which the comparator detects X-ray quanta and thus counts the X-ray quanta. The X-ray detector typically has multiple electronic thresholds per channel, which are controlled by 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 relationship between the electronic signal threshold of the comparator and the corresponding energy threshold for the incident X-ray quanta must be determined, from which the signal energy value can be determined. Thus, when measuring X-ray quanta based on the knowledge of the aforementioned relationship and based on the knowledge of the measured signal threshold, the measured signal threshold is associated with the signal energy value.

[0008] For this purpose, the calibration, also known as tuning, is typically performed during initial tuning during the production of the X-ray detector or a CT system having such an X-ray detector. In addition, in order to improve accuracy, the X-ray detector must also be recalibrated if necessary even after being delivered to the customer.

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

[0010] If image quality artifacts occur, indicating spectral changes in the X-ray detector, a complete readjustment is usually performed. However, this process takes a significant amount of time, during which a technician operates the device for, for example, a full working day, and the radiologist cannot use the device.

[0011] In addition, noise effects usually prevent the accurate determination of the characteristic structure of the characteristic X-ray spectrum. Due to these noise effects, a certain number of pixels are usually combined for spectral fine-tuning because only by combining and averaging multiple pixels can the spectral structure be identified.

[0012] Through this readjustment, the energy threshold or the electronic threshold assigned to the energy threshold is completely re-measured and set to a defined value. However, the measurement and evaluation of noise (including rounding / sampling errors) are fully incorporated into the table through this step. Therefore, all subsequent adjustment tables will also become invalid and must be adjusted accordingly. Summary of the Invention

[0013] Summary of the Invention The object of the present invention is to provide a method for calibrating a radiation detector, in particular an X-ray detector in a CT system, which is simpler and in particular more time-saving than conventional methods for calibrating radiation detectors.

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

[0015] In the method according to the present invention for recalibrating a radiation detector having a plurality of pixels, first the radiation detector detects the radiation spectrum of a radiation source, preferably an X-ray spectrum, particularly preferably the characteristic X-ray spectrum of an X-ray source. The radiation spectrum represents the energy distribution of the radiation emitted by the radiation source. Here, it is important that the radiation spectrum used in the recalibration should be as similar as possible to the older radiation spectrum measured in a previous calibration or recalibration, and this older radiation spectrum and its associated calibration or recalibration should be used as a reference for the current recalibration. A high degree of similarity can be achieved by starting the radiation source identically in the reference measurement and the current recalibration. In this regard, the radiation detector should be regarded as a device for detecting electromagnetic waves or particle radiation. Therefore, the radiation source used in the recalibration should particularly be the same as the radiation source used in the older calibration.

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

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

[0018] In addition to characterizing 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 having a reference signal energy value and a reference signal threshold. The latter relationship can be derived based on the signal threshold - count rate relationship and knowledge of the characteristics of the radiation spectrum on which the reference measurement is based.

[0019] As will be 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 recalibration is the thus determined corrected signal energy value - signal threshold relationship rounded to discrete DAC values or signal thresholds.

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

[0021] Finally, the recalibrated signal threshold is determined by adding the shift value of the corresponding individual pixel pixel by pixel to the signal threshold of the correlation of the reference signal threshold and the reference signal energy value of the corresponding pixel. In other words, the recalibrated signal threshold signal energy value pair is determined by adding the shift value of the corresponding individual pixel pixel by pixel to the signal threshold of the corresponding individual pixel's reference signal threshold signal energy value pair of the previously calibrated corresponding pixel.

[0022] In addition, the radiation detector is recalibrated by the recalibrated signal threshold signal energy value pair. In the context of the recalibration, 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 is provided for correction of future measurements.

[0023] Advantageously, the determined pair of reference signal thresholds of the signal threshold and the signal energy value can be corrected pixel by pixel during initial calibration without having to re-determine the signal energy value. Since the shift of the determined signal threshold is usually very small, for most pixels, the corresponding signal threshold does not change due to the signal threshold being rounded to an integer value. Only for pixels where the shift causes a change in the rounded signal threshold is it necessary to change the correlation between the rounded signal threshold and the signal energy value. These pixels are typically those where the unrounded signal threshold is very close to the rounded threshold.

[0024] Therefore, compared to a complete new calibration, the workload of recalibration is reduced because it is not necessary to measure the signal energy value again and it is not necessary to change the rounded signal threshold, i.e., the DAC value, for each pixel.

[0025] Furthermore, recalibration can be performed without having to perform a detailed analysis of the detected count rate spectrum in terms of the unique spectral lines and their associated energy values. Since such an analysis is usually hindered by severe noise effects, a conventional new calibration usually increases the uncertainty when defining a single energy threshold or a pair of signal threshold signal energy values. Usually, in a conventional calibration, multiple pixels are combined and an energy threshold or a pair of signal threshold signal energy values is defined for them jointly in order to reduce the noise effects and to reliably determine the unique spectral lines to a certain extent. Advantageously, compared to a conventional new calibration, the method according to the invention allows for pixel-by-pixel correction and can suppress the noise effects because exactly this spectral information is not needed for recalibration now. Therefore, compared to a conventional method, recalibration can improve the resolution of the detector. Due to the improved resolution for accurately detecting X-ray radiation, the image quality of a radiation detector recalibrated using the method according to the invention, in particular such an X-ray detector, can be improved overall.

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

[0027] A comparison unit is also part of the recalibration device according to the invention, which comparison unit is used to determine pixel by pixel the shift value of the signal threshold for an individual pixel by comparing the updated pair of signal threshold count rates with the previously calibrated pair of reference signal threshold count rates of the individual pixel of the radiation detector determined at an earlier time point.

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

[0029] The radiation detector of the present invention, preferably an X-ray detector, has a detection unit that generates a detection signal for the radiation hitting the detection unit, in particular X-ray radiation. The radiation detector of the present invention further has an evaluation unit that determines a set of measurement values of the X-ray radiation hitting the detection unit based on the detection signal. For example, the measurement values may relate to the counting rate of the counting events detected by means of the radiation detector. They may also relate to the time interval between individual counting events, the count value, the intensity value in the detector pixel, or the photon rate. As mentioned in the introduction, the set of measurement values typically includes a plurality of counting rates determined simultaneously for individual detector channels. The counting rates determined simultaneously for individual detector channels, in particular for a plurality of energy thresholds, are determined using respectively associated mutually different energy thresholds. Additionally, the radiation detector of the present invention further includes the recalibration device of the present invention.

[0030] Such a radiation detector can be implemented as, for example, a quantum counting or photon counting X-ray detector. In this case, the detection unit of the X-ray detector of the present invention may particularly include a detection region for generating, for example, a charge pulse as a detection signal for the X-ray radiation absorbed by the detection unit. Therefore, the detection signal, in particular the charge pulse, is particularly suitable for determining the energy of the absorbed X-ray radiation.

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

[0032] The present invention also 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 present invention, preferably an X-ray detector for detecting the radiation emitted by the radiation source. The imaging system according to the present invention has the advantages of the radiation detector according to the present invention.

[0033] Most of the above components of the recalibration device according to the invention can be implemented in whole or in part in the form of software modules, such as a control device of a radiation detector or an imaging system having such a radiation detector, in a processor of a corresponding computing system. This particularly relates to the comparison unit and the recalibration unit. The advantage of mainly implementing in software is that even a previously used computing system can be easily reconfigured by means of a software update so as to operate in accordance with the invention. In this regard, the object 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 steps executable by a computer when the program is executed in the computing system, for determining the shift value of a pixel individual pixel-by-pixel, for determining the recalibrated signal threshold signal energy value pair pixel-by-pixel, and for recalibrating a radiation detector by means of the recalibrated signal threshold. In addition to the computer program, the computer program product may optionally include additional components, such as documentation and / or additional components, as well as hardware components, such as a hardware key (dongle, etc.), for using the software.

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

[0035] A computer-readable medium, such as a storage stick, a hard disk or other transportable or permanently installed data carrier, can be used for transporting to a computing system or a control device and / or for storing on or in a computing system or a control device, and program segments of a computer program are stored on the medium, which can be read and executed by the computing system. For this purpose, the computing system can have one or more microprocessors working together, etc.

[0036] In addition, particularly advantageous embodiments and improvements of the invention can be found in the following description.

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

[0038] - an X-ray detector,

[0039] - a quantum counting X-ray detector,

[0040] - a 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 a quantum counting X-ray detector.

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

[0043] - A regression analysis is performed based on the updated signal threshold count value pair and the reference signal threshold count value pair of the pixel individual previously calibrated for the radiation detector and determined at an earlier time point.

[0044] - An artificial intelligence-based method is applied to the updated signal threshold count value pair and the reference signal threshold count value pair of the pixel individual previously calibrated for the radiation detector and determined at an earlier time point.

[0045] Advantageously, by adding the entire signal threshold scale or DAC value scale to determine the shift, statistical errors, in particular noise effects, can be reduced.

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

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

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

[0049] The gradient change of the fitting curve is due to its gradient change compared to the reference value (preferably value 1). The shift value is obtained 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 that only considers the offset value, the accuracy of the recalibration is further improved due to the consideration of the change in the gradient.

[0050] In the step of recalibrating using recalibrated signal threshold-signal energy value pairs, the determined shift can be applied to a plurality of different calibration tables based on the signal threshold. This is also possible, especially since no new noise effects are generated during recalibration as compared to a complete new calibration. As will be explained in detail later, in addition to the energy adjustment table that reproduces the correlation between the signal threshold and the signal energy value or provides a correction value for this correlation, there are other types of calibration tables that are based on the signal threshold and provide corrections for the measured values of the radiation detector. Advantageously, during the recalibration of a plurality of different types of calibration tables, the amount of work for recalibration is thus significantly reduced as compared to the corresponding complete new calibration of these calibration tables.

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

[0052] - Energy adjustment table,

[0053] - Linear correction table,

[0054] - Offset correction table,

[0055] - Gain correction table,

[0056] - Defective pixel correction table,

[0057] - Hardening correction table.

[0058] The energy adjustment table includes the relationship between the DAC value (i.e., the signal threshold) and the signal energy value, which has been described in the introduction.

[0059] The linear correction table is used to correct the pile-up effect and general non-linear effects of the measurement scale. The pile-up effect is related to the measurement of events that occur so close in time that they are not or are hardly distinguishable as individual events. This will result in count losses.

[0060] The offset correction table is capable of performing zero-point correction for individual pixel differences in X-ray radiation measurement.

[0061] The gain correction table is capable of performing proportional correction for individual pixel differences in X-ray radiation measurement.

[0062] The defective pixel correction table enables the identification of inadequately functioning pixels and generates interpolation values as an alternative.

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

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

[0065] In a preferred embodiment of the method according to the invention for recalibrating a radiation detector having a plurality of pixels, the reference signal threshold count rate pairs of the individual pixels are associated with a continuous reference signal threshold scale and have a continuous reference signal threshold.

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

[0067] Thus, preferably, pixels are only recalibrated whose previously calibrated signal threshold is close to a value that is an odd multiple of half of a previously defined increment on the signal threshold scale. For example, the increment can be at the value 1. Correction is only made if the rounded signal threshold after recalibration is below or above a rounding limit such as 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 due to recalibration, the value is rounded to a different multiple than the previous reference signal threshold.

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

[0069] If the method according to the invention is performed multiple times at different time points and if the continuous signal threshold is stored with each recalibration, then trend analysis is preferably subsequently performed based on the continuous signal thresholds determined at different time points. Based on this analysis, even without performing the method according to the invention again, an updated recalibrated signal threshold signal energy value pair can be approximately determined. Advantageously, the workload of recalibration is further reduced.

[0070] Also preferably, based on trend analysis, it is determined at which time point the recalibration method according to the invention must be performed again. If the shift exceeds, for example, a predetermined threshold, then recalibration should be performed using the method according to the invention. Thus, this moment can be estimated in advance by trend analysis, and thus the number of recalibrations is limited to a minimum, thereby further reducing the calibration work.

[0071] In order to obtain particularly reliable calibration results, it is preferred to verify the recalibration according to the invention based on at least one error criterion, preferably based on a plurality of such equal-error criteria. This or these error criteria are determined based on the measurements made for the recalibration.

[0072] Based on the verification, it can in particular be determined that when an overly significant shift occurs, the selected pixels are not recalibrated, or only a part of the selected pixels are recalibrated. An overly significant shift should be regarded as a reason for a complete readjustment. Advantageously, the recalibration can be purposefully applied to situations that constitute an improvement over the previous calibration.

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

[0074] - An offset value exceeding a predetermined threshold occurs,

[0075] - The number of pixels for which a signal threshold signal energy value pair must be generated for the recalibration exceeds a predetermined part of the total number of pixels of the radiation detector,

[0076] - The number of pixels classified as defective exceeds a predetermined number.

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

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

[0079] If the verification result is negative, it is preferably to issue a warning message. Advantageously, the user is informed that no recalibration has been carried out and is warned that the measurement may be biased and inaccurate.

[0080] In a particularly advantageous variant of the method according to the invention, different electronic thresholds of the same detector pixel are mapped to a common DAC scale, namely the electronic signal threshold scale. In the case of an X-ray detector, multiple 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 require a common DAC scale, the individual signal threshold scales must be coordinated with each other. For example, this can be achieved by shifting, compressing or extending the individual scales. Advantageously, in the case of such a common DAC scale, all measurement count rates of the detector pixels can be represented in a graph. Advantageously, by means of the method according to the invention, the common DAC scale can be made to have the correct relationship with the energy scale.

[0081] In a particularly advantageous variant of the method according to the invention, the detector pixels are divided into a first group and a second group. The first group of detector pixels is selected for recalibration of the radiation detector. The detector pixels comprised in the first group are selected according to a specific classification criterion, using which particularly suitable detector pixels are selected for recalibration of the radiation detector. The remaining detector pixels then form the second group. Advantageously, calibration is based on suitable detector pixels, thus obtaining more accurate results. The classification criterion used may include, for example, not taking into account detector pixels located near the scatter radiation grid or the sensor edge or the feed cable. The detector pixels assigned to the first group are also referred to hereinafter as "clean" pixels, which do not experience shadowing, since in this particular case they are not located near the scatter radiation grid. Using this subgroup of available pixels enables the determination of an unchanged measured value, such as a count value, which, with the aid of previously obtained parameters, can be used to calibrate the signal threshold scale with an energy scale. In this way, the frequently occurring problem of poorly focused or possibly poorly collimated orientation is solved.

[0082] Particularly preferably, a whole of detector pixels is formed that has a responsiveness sufficiently similar to that of an individual detector pixel for recalibration. The whole of detector pixels is preferably formed on the basis of a plurality of adjacent detector pixels that have a responsiveness sufficiently similar to that of the individual detector pixels. The value of the statistical evaluation based on the whole is subsequently used as an input variable for recalibration instead of the measured value of an individual detector pixel.

[0083] For example, the overall mean value of the measured values of the individual detector pixels of a pixel group is associated with the whole of detector pixels. The measured signals of these pixels can be combined, for example, by addition, in order to obtain improved statistical significance compared to the measurement of an individual pixel. The pixel grouping in the pixel group significantly increases the signal-to-noise ratio of the measured values obtained during the test measurement, thus improving the quality and reliability of the recalibration. In addition, since the total number of sensor subunits to be recalibrated is reduced compared to the recalibration of individual pixels, the computational effort for the recalibration is reduced.

[0084] The individual threshold of an individual detector pixel can also be oriented to a common scale within the pixels of the pixel group. This orientation is less sensitive to the limited statistics of the individual measured values of the underlying test measurement and thus enables the measured values of the pixel group to be combined first.

[0085] To also take into account detector pixels that do not belong to the first group of detector pixels and thus do not exhibit approximately the same responsiveness, an equalization step is preferably performed. This step is also known in the literature as a homogenization method. During the equalization step, the responsiveness of the first and second groups of detector pixels is made consistent with the responsiveness of the first group of detector pixels. For example, methods for equalizing detector pixels are described in DE102006022596A1 and DE 102011080656 B4.

[0086] In combination with the method according to the invention, the equalization metric generated during the homogenization process can be used to determine the shift in the correlation between the DAC value or the signal threshold and the signal energy value, since adjacent pixels may have a similar shift in the first order and an additional pixel - individual shift component in higher orders.

[0087] In the method for equalizing the responsiveness of detector pixels described in DE 102006022596 A1, a first measurement is made using radiation having a first energy spectrum, where for each detector pixel there is a first energy threshold located within the first energy spectrum. For each detector pixel, the first count rate of X - ray radiation determined during processing that is below or above the first energy threshold is detected. Subsequently, X - ray radiation having a second energy spectrum is emitted, where the first energy threshold remains unchanged. In turn, for each detector pixel, the second count rate of X - ray radiation below or above the first energy threshold 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 of the detector pixel is then changed such that the quotients of the individual detector pixels are equalized. Advantageously, the recalibration measurement according to the method of the invention can now be used simultaneously to equalize the responsiveness of the detector pixels. Due to the dual use of the measurement data of the recalibration measurement, effort and time can be saved in the method according to the invention.

[0088] To be able to perform the described calibration step of the detector pixels, the method according to the invention preferably includes a measurement in addition to the recalibration measurement, and this measurement is performed with a photon - flux spectrum different from the recalibration measurement. This second measurement value then corresponds to the measurement value with the second energy spectrum as described in the calibration method of DE 102006022596A1.

[0089] The X - ray detector according to the invention can in particular include a detection area or detection surface having a semiconductor material (direct converter) that directly captures or absorbs X - ray radiation.

[0090] It is also conceivable that an X-ray detector according to the invention comprises a detection area with a scintillator material which converts X-ray radiation into radiation in another spectral range (in particular in the visible spectral range). The converted radiation can be detected by a semiconductor detector (such as a photodiode or a silicon photomultiplier), which is arranged in the radiation path following the scintillator and is generally also included in the detection unit. The photodiode, silicon photomultiplier or semiconductor detector then generates a detection signal which can likewise be analyzed with respect to the spectral distribution and intensity of the X-ray radiation detected by the scintillator. Description of the Drawings

[0091] The invention will now be explained in more detail with reference to the drawings using embodiments. In the different figures, identical components have the same reference numerals. Here:

[0092] Figure 1 A graph is shown which illustrates the drift of an X-ray detector using an example of measuring the characteristic X-ray spectrum of an X-ray source according to the prior art;

[0093] Figure 2 A graph is shown which shows the first and second measurement curves of the signal threshold count rate pairs recorded at different time points,

[0094] Figure 3 A graph is shown which illustrates the updated signal threshold plotted according to a reference signal threshold,

[0095] Figure 4 A graph is shown which depicts the pixel distribution with shift describing four electron energy thresholds for a zero-order shift,

[0096] Figure 5 A graph is shown which depicts the pixel distribution with shift representing four electron energy thresholds for a first-order shift,

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

[0098] Figure 7 A flowchart is shown which illustrates step 6.IV of the method shown according to a first embodiment of the invention Figure 6 of the method shown,

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

[0100] Figure 9 A block diagram is shown which shows an X-ray detector having a calibration device according to an embodiment of the invention. Detailed Description of the Invention

[0101] Figure 1 Figure 10 shows, according to the prior art, an example of using the characteristic X-ray spectrum of a measurement X-ray source to illustrate the drift of an X-ray detector. The first measurement curve m1 shows the normalized spectral response nsr plotted against the signal energy value E [keV], which shows the measurement characteristics of the X-ray detector at the first time point. The second measurement curve m2 is slightly offset relative to the first measurement curve m1, which shows the measurement characteristics of the X-ray detector at a second time point later than the first time point. Figure 1 Also shown are a number of significant energy values of the X-ray spectrum, such as 23 keV (fluorescence of the X-ray detector), 59 keV (fluorescence of the X-ray tube), and 68 keV, for which maximum values are assigned in the Figure 1 chart. These maximum values or the energy values associated with them can be used as orientation values in traditional calibration. However, noise effects often make it difficult to accurately determine the maximum values. Due to the mentioned noise effects, it has traditionally been necessary to combine a certain number of pixels for spectral fine-tuning, because only by combining and averaging multiple pixels can the Figure 1 spectral structure shown in

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

[0103] Figure 3 Figure 12 shows an updated signal threshold DAC plotted against a reference signal threshold DAC ref . For this purpose, the curves shown in Figure 2 are slid over each other graphically, and the DAC values or signal threshold DACs ref associated with the same count rate are compared with each other. The angular bisector symbolizes the shift-free mapping of the reference signal threshold DAC ref to the updated energy threshold DAC. As can be seen from Figure 3 , the new signal threshold DAC is partially slightly deviated from the reference signal threshold DAC ref . Averaged to the zero order, the value of this deviation is V = 0.148 DAC. This means that in recalibration, the shift is 0.148 DAC values.

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

[0105] Figure 5Shows the graph 14, which shows the distribution ZP of pixels of the shift V or ΔDAC with the first to fourth energy thresholds th1, th2, th3, th4 for a first-order shift in two subgraphs. The offset value of the shift V can be seen in the left subgraph. The shift is characterized in that the offset value is between -5 and +4. Figure 5 The right graph in ref shows the deviation ΔS of the slope of the regression line between the reference energy threshold DAC and the newly measured energy threshold DAC.

[0106] Figure 6 Shows the flowchart 600, which shows 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, the radiation detector 1 detects the characteristic X-ray spectrum of the X-ray source. For each pixel P of the radiation detector 1, a plurality of energy-dependent count rates ZR for different signal thresholds DAC of the evaluation unit for the radiation detector 1 are measured.

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

[0109] In step 6.III, by comparing between the signal threshold count value pairs SZP and the previously calibrated reference signal threshold count value pairs SZP of the individual pixels of the radiation detector 1 determined at an earlier time point ref the individual shift value V of the signal threshold DAC is determined pixel by pixel i .

[0110] In step 6.IV, by adding the individual shift value V of the corresponding pixel i pixel by pixel to the corresponding reference signal threshold signal energy value pair SEP of the corresponding pixel P i the recalibrated signal threshold signal energy value pair SEP is determined ref for the signal threshold DAC ref . Furthermore, the recalibrated signal threshold signal energy value pair SEP neu is used to recalibrate the radiation detector 1. neu

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

[0112] In step 6.IVa, the reference signal threshold signal energy value pair SEP of the individual pixel refassociated with successive reference signal threshold scales such that they have a successive reference signal threshold DAC Ref_kont 。

[0113] In step 6.IVb, the shift value V of the determined pixel individual is i added to the successive reference signal threshold DAC Ref_kont to generate a successive signal threshold DAC neu_kont 。

[0114] Subsequently, in step 6.IVc, the successive signal threshold DAC neu_kont is rounded to an integer signal threshold DAC ganz to determine the recalibrated signal threshold DAC neu 。These integer recalibrated signal threshold DACs neu are respectively parts of the recalibrated signal threshold signal energy value pairs SEP neu 。

[0115] Figure 8 Schematically shows a recalibration device 50 for a radiation detector 1 (see Figure 9 ) according to an embodiment of the present invention. The recalibration device 50 includes a data receiving interface 51 for receiving data of the 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 of different signal threshold DACs of the evaluation unit for the radiation detector 1 are measured.

[0116] A part of the recalibration device 50 is an association unit 52, which is arranged to determine an updated signal threshold count value pair SZP spectrum by associating the signal threshold DAC with the spectral count rate ZR based on the characteristic X-ray.

[0117] The recalibration device 50 further includes a comparison unit 53 for determining, pixel by pixel, a shift value V of the pixel individual for the signal threshold DAC by comparing between the signal threshold count value pair EZP and the reference signal threshold count value pair SZP of the pixel individual of the radiation detector 1 calibrated previously at an earlier time point based on regression analysis ref 。 i 。

[0118] The recalibration device 50 according to the present invention further has a recalibration unit 54 for determining, pixel by pixel, a recalibrated signal threshold signal energy value pair SEP i by adding the corresponding pixel individual shift value V i to the signal threshold DAC of the corresponding pixel individual reference signal threshold count value pair SZP zef of the corresponding pixel P ref 。 neu 。

[0119] The determined recalibrated signal threshold signal energy value pair SEP neu is finally transmitted 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 schematically shown, which serves as a direct converter. Such a semiconductor detector 1 is used, for example, in a computed tomography device. The semiconductor detector 1 is designed as a planar pixel detector. It includes a semiconductor material 2 in the form of a single crystal as the detector material. One side of the semiconductor material 2 is covered with a metal surface, which forms a back electrical contact 3. A voltage HV is applied at the back contact 3.

[0121] The opposite side of the single crystal 2 is covered with a structured metallization layer, the respective sub-areas of which are designed as pixel contact points 4 and together form a pixel matrix. The size and pitch of the pixel contact points 4, together with the other 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 μm to 500 μm. The respective pixel contact points 4 are connected to separate readout electronic circuits 5, and measurement signals s 1 、s 2 、s 3 、s 4 are detected by means of the readout electronic circuits 5. A comparator installed in the readout electronic circuit 5 can be adjusted using an electronic threshold voltage to detect certain spectral components of the X-rays.

[0122] In addition, the radiation detector 1 includes an adder element 6, by means of which the measurement signals s 1 、s 2 、s 3 、s 4 are combined into a group signal G. The adder element 6 can also weight the individual signals to be measured or pixel signals before adding them. In order to correctly add the individual signals s 1 、s 2 、s 3 、s 4 、s Figure 8 The recalibration device 50 shown serves this purpose. In Figure 6 and Figure 7In the context of the recalibration method shown, 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 to obtain measurement signals ZR from the individual comparators 5 (only the signal chain of the comparator that sends the fourth signal s4 is drawn, but the signal chain also exists similarly for the other comparators). In addition, the recalibration device 50 can also optionally drive the X-ray tube R within the context of the recalibration by means of control commands SB, wherein the X-ray flux 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 transmitted to a database DB. For example, when processing the count rates ZR of the individual pixels in the adding element 6, the calibration data KD can be used in order to correctly add the count rates ZR correspondingly with the correctly associated energy values.

[0123] Notice, Figure 9 The arrangement 1 is shown as a cross section, so Figure 9 The arrangement 1 in FIG. 1 includes 16 pixel contact points in total. For greater clarity, only 4 pixel contact points are drawn for each pixel group according to the cross-sectional view. As already mentioned, each pixel group usually has 16 or other number of pixel contact points.

[0124] Figure 9 The function of the semiconductor detector 1 shown is as follows: X-rays striking 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. A voltage HV applied to the contact points 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 a charge pulse which is proportional to the absorbed energy of the X-rays. This charge pulse is read out by the connected readout electronics 5. The pixel signal or measurement signal s detected by the readout electronics 5 1 、s 2 、s 3 、s 4 is forwarded to the summing element 6, which adds the measured signal s 1 、s 2 、s 3 、s 4 (actually the measurement signal s 1 to 16 ) is combined into a group signal G. Four different measurement signals s 1 、s 2 、s 3 、s 4 The number is only an example and can be replaced by any arbitrary number n of measurement signals.

[0125] Finally, it should be noted that all the embodiments or developed features disclosed in the drawings can be used in any combination. Finally, it is also pointed out that the X-ray detector, X-ray imaging system, calibration device, and method for recalibrating a radiation detector described in detail above are merely embodiments, which can be modified in various ways by those skilled in the art without departing from the scope of the present invention. For example, the X-ray detector can also be designed as an annular detector that completely surrounds the measurement space of the imaging system in one direction. In addition, the use of the indefinite article "a" or "an" does not exclude the fact that the features under discussion can occur multiple times. Similarly, the term "unit" does not exclude the fact that the components under discussion are composed of multiple interacting sub-components, which can also be spatially distributed. Regardless of the grammatical gender of a particular term, it includes people with male and female gender identities.

Claims

1. A method for recalibrating a plurality of pixels (P i ) of the invention, comprising the following steps: The radiation spectrum of the radiation source is detected by the radiation detector (1), wherein for each pixel (P i ), a plurality of energy-dependent count rates (ZR) for different signal thresholds (DAC) of an evaluation unit (5) of the radiation detector (1) are measured, and updated signal threshold count value 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 value pairs (SZP) with reference signal threshold count value pairs (SZP) of the radiation detector (1) determined at an earlier point in time for individual pixels of the previous calibration ref ) to determine the pixel-by-pixel shift value (V i ), By transforming the corresponding individual pixel shift value (V i ) is added pixel by pixel to the corresponding pixel (P i ) of 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 ), By means of the recalibrated signal threshold signal energy value pair (SEP neu ) to recalibrate the radiation detector (1).

2. The method according to claim 1, wherein the shift value (V i ) is performed based on one of the following types of comparison methods: Based on the updated signal threshold count value pair (SZP) and the reference signal threshold count value pair (SZP) of the pixel individual determined at an earlier point in time from a previous calibration of the radiation detector (1), ref ), to perform regression analysis, or applying an artificial intelligence based method to the updated signal threshold count value pairs (SZP) and previously calibrated reference signal threshold count value pairs (SZP) of the radiation detector (1) for the individual pixels determined at an earlier point in time; ref ).

3. The method according to claim 1 or 2, wherein the shift value (V i ) respectively include an offset value (ΔDAC) which determines the recalibrated signal threshold signal energy value pair (SEP) pixel by pixel. neu ) is added to the corresponding pixel (P i ) of the corresponding pixel individual reference signal threshold signal energy value pair (SEP ref ) signal threshold (DAC ref ).

4. A method according to any one of the preceding claims, wherein the pixel-dependent shift value (V i ) are respectively based on the signal threshold value (DAC) and the reference signal threshold value count value pair (SZP) in the updated signal threshold value count value pair (SZP ref ) of the signal threshold (DAC ref ) is determined by a function of order higher than zero that has been determined between them.

5. The method according to claim 1 , wherein the recalibrated signal threshold signal energy value pair (SEP neu ) is performed in a recalibration step, a plurality of different calibration tables based on a signal threshold (DAC) are used to determine the offset value (V i ) is corrected.

6. A method according to any one of the preceding claims, wherein The reference signal threshold count value pair (SZP ref ) 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 i ) is 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 ) of the recalibrated signal threshold (DAC neu ) by setting the continuous signal threshold (DAC neu_kont ) rounded to integer signal threshold (DAC ganz ) and was determined.

7. The method according to 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 then based on the continuous signal threshold (DAC) determined at different time points neu_kont ) to perform trend analysis, On this basis, the updated and recalibrated signal threshold signal energy value pair (SEP neu ) can also be approximately determined without having to re-execute the method according to any one of claims 1 to 6, and / or On this basis, it is determined at which point in time the method according to any one of claims 1 to 6 must be performed again.

8. The method according to any of the preceding claims, wherein the verification of the recalibration is performed on the basis of at least one error criterion, preferably on the basis of a plurality of such error criteria, which are based on the measurements performed for the recalibration.

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

10. The method according to claim 8 or 9, wherein if the verification result is negative, a warning message is issued.

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

12. A radiation detector (1), preferably an X-ray detector, comprising: a detection unit (2) which generates a detection signal for radiation, preferably X-rays, which strike the detection unit (2), an evaluation unit (5) which determines a plurality of energy-dependent count rates (ZR) for different signal thresholds (DAC) based on the detection signal, A recalibration device (50) for recalibrating the radiation detector (1) according to claim 11.

13. An imaging system, preferably an X-ray imaging system, in particular a CT system, having a radiation source (R) and a radiation detector (1) according to claim 12.

14. A computer program product, comprising instructions, which, when executed by a computer, cause the execution of the steps of the method according to any one of claims 1 to 10, for: determining pixel-by-pixel the shift value (V i ), determine the recalibrated signal threshold signal energy value pair (SEP) pixel by pixel neu ), and by means of the recalibrated signal threshold signal energy value pair (SEP neu ) recalibrate the radiation detector (1).

15. A computer-readable storage medium, comprising instructions, which, when executed by a computer, cause the execution of the steps of the method according to any one of claims 1 to 10, for: determining the shift value (V) of an individual pixel pixel by pixel i ), determine the recalibrated signal threshold signal energy value pair (SEP) pixel by pixel neu ), and by means of the recalibrated signal threshold signal energy value pair (SEP neu ) recalibrate the radiation detector (1).

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