Total Time Over Threshold (TTOT) Processing for Photon Counting X-ray Detectors

Abstract

The present invention provides a circuit (502; 503; 504) configured to work with a multi-bin photon counting X-ray detector (20) having multiple energy thresholds, wherein the circuit (502; 503; 504) is configured to obtain or generate a plurality of total time over threshold (TTOT) signals corresponding to a plurality of different energy thresholds, and to provide energy integration information based on the plurality of TTOT signals.

Description

[0001] The project for which this patent application is filed has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 830294. Technical Field

[0002] The proposed technology relates to a measurement method to be performed, for example, in an X-ray imaging system. The proposed technology also relates to corresponding circuits, devices and / or systems, as well as related computer programs and computer program products. Background Art

[0003] Radiographic imaging, such as X-ray imaging, has been used for non-destructive testing in medical applications for many years.

[0004] Typically, an X-ray imaging system includes an X-ray source and an X-ray detector array, which consists of multiple detectors, each containing one or more detector elements (independent devices that measure X-ray intensity / energy density). The X-ray source emits X-rays, which pass through the subject or object to be imaged and are then recorded by the detector array. Because some materials absorb a greater portion of the X-rays than others, an image of the subject or object is formed.

[0005] The challenge facing X-ray imaging detectors is to extract maximum information from the detected X-rays to provide input for an image of an object or subject, wherein the object or subject is characterized in terms of density, composition, and structure.

[0006] In a typical medical X-ray imaging system, X-rays are generated by an X-ray tube. Typical medical X-ray tubes have a wide energy spectrum, ranging from 0 to 160 keV. Therefore, detectors typically detect X-rays with varying energies.

[0007] refer to Figure 1 It may be useful to briefly summarize an illustrative overall X-ray imaging system. In this illustrative, non-limiting example, the X-ray imaging system 100 generally includes an X-ray source 10, an X-ray detector system 20, and an associated image processing system or device 30. Generally speaking, the X-ray detector system 20 is configured to record radiation from the X-ray source 10, which radiation has optionally been focused by optional X-ray optics and has passed through an object, subject, or portion thereof. The X-ray detector system 20 can be connected to the image processing system 30 via suitable analog and readout electronics that are at least partially integrated into the X-ray detector system 20 to enable the image processing system 30 to perform image processing and / or image reconstruction.

[0008] There is a general need to improve the performance of X-ray detectors and / or X-ray imaging systems. In particular, it is desirable to be able to make optimal use of the photon interaction information from the X-ray detector. Summary of the Invention

[0009] A general object of the present invention is to improve the performance of an X-ray detector and / or an X-ray imaging system.

[0010] For example, it is desirable to provide new useful signal information from multi-bin photon counting X-ray detectors.

[0011] It may also be desirable to improve the performance of multi-bin photon counting detectors, especially at high photon rates.

[0012] It is a specific object of the present invention to provide a general circuit configured to operate with a multi-bin photon counting X-ray detector.

[0013] Another object is to provide a total time over threshold (TTOT) logic circuit configured for operation with a multi-bin photon counting X-ray detector.

[0014] Yet another object is to provide a digital processing circuit configured to operate with a multi-bin photon counting X-ray detector.

[0015] Yet another object is to provide a measurement circuit for a photon counting X-ray detector.

[0016] It is also an object of the present invention to provide an overall X-ray imaging system comprising such a circuit system.

[0017] Another object is to provide a system configured to work with a multi-bin photon counting X-ray detector.

[0018] Yet another object is to provide a method of obtaining energy integration information from a multi-bin photon counting X-ray detector.

[0019] It is also an object of the present invention to provide a corresponding computer program and / or computer program product.

[0020] These and other objects can be achieved by one or more embodiments of the proposed technology.

[0021] According to a first aspect, a circuit is provided for operating with a multi-bin photon counting X-ray detector having multiple energy thresholds. The circuit is configured to obtain or generate a plurality of total time above threshold (TTOT) signals corresponding to a plurality of different energy thresholds and to provide energy integration information based on the plurality of TTOT signals.

[0022] According to a second aspect, a total time above threshold (TTOT) logic circuit is provided that is configured to work with a multi-bin photon counting X-ray detector having multiple energy thresholds, wherein the TTOT logic circuit is configured to generate a number of total time above threshold (TTOT) signals corresponding to a number of different energy thresholds, and to provide energy integration information based on the number of TTOT signals.

[0023] According to a third aspect, there is provided a digital processing circuit configured to work with a multi-bin photon counting X-ray detector having multiple energy thresholds, wherein the digital processing circuit is configured to obtain more than one total time above threshold (TTOT) signals corresponding to more than one energy threshold and to provide energy integration information based on the more than one TTOT signals.

[0024] According to a fourth aspect, there is provided a measurement circuit for a photon counting X-ray detector, comprising the TTOT logic circuit of the second aspect and / or the digital processing circuit of the third aspect.

[0025] According to a fifth aspect, an X-ray imaging system is provided, comprising the circuit according to any one of the first aspect, the second aspect, the third aspect and / or the fourth aspect.

[0026] According to a sixth aspect, there is provided a system configured to work with a multi-bin photon counting X-ray detector having multiple energy thresholds, wherein the system is configured to generate more than one total time above threshold (TTOT) signals based on the output of more than one comparator from the multi-bin photon counting detector.

[0027] According to the seventh aspect, a method for obtaining energy integration information from a multi-bin photon counting X-ray detector is provided, wherein the method includes: providing or generating a signal representing or approximating an energy integration signal based on a total time over threshold (TTOT) signal of several energy thresholds set at different energies in the multi-bin photon counting detector.

[0028] According to an eighth aspect, there is provided a computer program comprising instructions which, when executed by a processor, cause the processor to perform the method of the seventh aspect.

[0029] According to a ninth aspect, there is provided a computer program product comprising a non-transitory computer readable medium having stored thereon the computer program of the eighth aspect.

[0030] In this way, a signal representing or approximating an energy integration signal can be obtained based on the total time-over-threshold (TTOT) signal for several energy thresholds set at different energies in a multi-bin photon counting X-ray detector. This signal can be called a digital energy integration signal.

[0031] The inventors have recognized that a signal formed or represented by several TTOT signals has a significantly more linear relationship to the input photon rate and allows the overall X-ray imaging system to maintain dose efficiency also at higher rates.

[0032] The specific non-limiting example of the proposed method and structural configuration for obtaining a digital energy-integrated signal has the advantage over prior art techniques for obtaining an energy-integrated signal in that it does not require dedicated analog circuitry for integrating / accumulating the signal. Instead, the inventors have recognized that it is feasible to utilize the output of a digital comparator that already exists as part of the capabilities of a multi-bin photon counting detector.

[0033] Another benefit of the proposed technique is that the signal formed or represented by several TTOT signals includes spectral (photon energy) information that can be used for spectral imaging and also for imaging situations that suffer from high pulse pile-up. Such imaging tasks may be able to be performed in combination with photon counting signals.

[0034] In other words, the proposed technique relates to Total Time Over Threshold (TTOT) processing for photon counting X-ray detectors.

[0035] Other advantages will be appreciated upon reading the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The embodiments, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which:

[0037] Figure 1 is a schematic diagram showing an example of an overall X-ray imaging system.

[0038] Figure 2 is a schematic diagram showing another example of an X-ray imaging system.

[0039] Figure 3 is a schematic block diagram of a CT system as an illustrative example of an X-ray imaging system.

[0040] Figure 4 is a schematic diagram showing another example of relevant parts of an X-ray imaging system.

[0041] Figure 5 is a schematic diagram of a photon counting circuit and / or device according to the prior art.

[0042] Figure 6 is a diagram showing an example of how the pulse voltage changes over time and the corresponding comparator output for each clock cycle.

[0043] Figure 7is a schematic diagram illustrating one example of measurement circuitry including at least total time over threshold (TTOT) logic applied directly to a comparator output.

[0044] Figure 8A is a schematic diagram illustrating one example of a total time above threshold (TTOT) circuit system for generating several total time above threshold (TTOT) signals corresponding to several different energy thresholds.

[0045] Figure 8B is a schematic diagram illustrating another example of a system / circuitry for obtaining and / or generating more than one TTOT signal.

[0046] Figure 9 is a schematic diagram illustrating one example of digital processing circuitry for generating at least one combined value from more than one TTOT signals and, optionally, one or more photon counting signals.

[0047] FIG. 10A to FIG. 10B is a schematic diagram showing an example of a voltage pulse for two photons arriving close in time.

[0048] Figures 11A to 11B is a schematic diagram showing an example of a TTOT signal originating from two photons.

[0049] Figure 12 is a diagram showing one example of how a photon counting signal saturates (loses signal) at a lower photon rate than a TTOT signal, which in turn saturates at a lower photon rate than a digital energy integration signal.

[0050] Figure 13 is a schematic diagram showing an example of the behavior of the TTOT signal as the photon rate increases.

[0051] Figure 14 is a schematic diagram showing an example of a method for obtaining energy integration information from a multi-bin photon counting X-ray detector.

[0052] Figure 15 is a schematic diagram illustrating an example of a computer implementation according to one embodiment. DETAILED DESCRIPTION

[0053] For a better understanding, it may be useful to continue with the introductory description of a non-limiting example of an overall X-ray imaging system.

[0054] Figure 21 is a schematic diagram illustrating an example of an X-ray imaging system 100, which includes an X-ray source 10 that emits X-rays; an X-ray detector system 20 having an X-ray detector that detects the X-rays after they have passed through an object; an analog processing circuit system 25 that processes and digitizes the raw electrical signals from the X-ray detector; a digital processing circuit system 40 that can perform further processing operations on the measurement data, such as applying corrections, temporarily storing, or filtering; and a computer 50 that stores the processed data and can perform further post-processing and / or image reconstruction. According to the present invention, all or part of the analog processing circuit system 25 can be implemented in the X-ray detector system 20.

[0055] The entire X-ray detector may be considered an X-ray detector system 20 , or the X-ray detector system 20 in combination with associated analog processing circuitry 25 .

[0056] The digital portion, including the digital processing circuitry 40 and / or the computer 50, can be considered the image processing system 30, which performs image reconstruction based on image data from the X-ray detector. Thus, the image processing system 30 can be considered the computer 50, or alternatively a combination of the digital processing circuitry 40 and the computer 50, or, if the digital processing circuitry 40 is further specialized for image processing and / or reconstruction, the digital processing circuitry itself.

[0057] An example of a commonly used X-ray imaging system is an X-ray computed tomography (CT) system, which may include an X-ray tube that produces a fan or cone beam of X-rays and an opposing X-ray detector array that measures the fraction of X-rays that pass through a patient or object. The X-ray tube and detector array are mounted in a gantry that rotates around the imaging object.

[0058] Figure 3FIG2 is a schematic block diagram of a CT system, which serves as an illustrative example of an X-ray imaging system. The CT system includes a computer 50 that receives commands and scanning parameters from an operator via an operator console 60, which may have a display and some form of operator interface, such as a keyboard and mouse. The commands and parameters provided by the operator are then used by computer 50 to provide control signals to an X-ray controller 41, a gantry controller 42, and a table controller 43. Specifically, the X-ray controller 41 provides power and timing signals to the X-ray source 10 to control the emission of X-rays onto an object or patient positioned on a table 12. The gantry controller 42 controls the rotational speed and position of the gantry 11, which includes the X-ray source 10 and the X-ray detector 20. For example, the X-ray detector may be a photon counting X-ray detector. The table controller 43 controls and determines the position of the patient table 12 and the scan coverage of the patient. A detector controller 44 is also provided, which is configured to control and / or receive data from the detector 20.

[0059] In one embodiment, the computer 50 also performs post-processing and image reconstruction on the image data output from the X-ray detector. Figure 1 and Figure 2 An image processing system 30 is shown. An associated display allows an operator to observe reconstructed images and other data from the computer.

[0060] An X-ray source 10 disposed in a gantry 11 emits X-rays. An X-ray detector 20 (e.g., in the form of a photon counting detector) detects the X-rays after they have passed through the patient. The X-ray detector 20 can, for example, be formed from a plurality of pixels (also referred to as sensors or detector elements) and associated processing circuitry (such as an ASIC) disposed in a detector module. A portion of the analog processing portion can be implemented in the pixels, while any remaining processing portion is implemented, for example, in the ASIC. In one embodiment, the processing circuitry (ASIC) digitizes the analog signals from the pixels. The processing circuitry (ASIC) can also include a digital processing portion that can perform further processing operations on the measurement data, such as applying corrections, temporarily storing, and / or filtering. During scanning to acquire X-ray projection data, the gantry and components mounted thereon rotate about the isocenter.

[0061] Modern X-ray detectors typically require the conversion of incident X-rays into electrons, which often occurs through the photoelectric effect or Compton interaction, with the resulting electrons typically producing secondary visible light until the energy is lost and this light is detected by a photosensitive material. There are also detectors based on semiconductors, for which the X-ray interaction results in the release of electron-hole pairs that are collected by an applied electric field.

[0062] There are detectors that operate in energy integrating mode, in the sense that they provide an integrated signal from a large number of X-rays. The output signal is proportional to the total energy deposited by the detected X-rays.

[0063] X-ray detectors with photon counting and energy-resolving capabilities are increasingly common in medical X-ray applications. Photon counting detectors offer advantages because, in principle, they can measure the energy of each X-ray photon, which yields additional information about the composition of the object. This information can be used to improve image quality and / or reduce radiation dose.

[0064] Generally speaking, photon-counting X-ray detectors determine a photon's energy by comparing the height of the electrical pulse generated by the photon's interaction in the detector material with a set of comparator voltages. These comparator voltages are also known as energy thresholds. Typically, the analog voltage in the comparator is set by a digital-to-analog converter (DAC). The DAC converts the digital setting sent by the controller into an analog voltage against which the photon pulse's height can be compared.

[0065] A photon counting detector counts the number of photons that have interacted in the detector during the measurement time. A new photon is typically identified by the height of an electrical pulse exceeding the comparator voltage of at least one comparator. When a photon is identified, the event is stored by incrementing a digital counter associated with the channel.

[0066] When several different thresholds are used, a so-called energy-resolving photon counting detector is obtained, in which the detected photons can be sorted into energy bins corresponding to the various thresholds. Sometimes, this type of photon counting detector is also called a multi-bin detector. Generally speaking, energy information allows the creation of new kinds of images, in which new information is available and image artifacts inherent to conventional techniques can be removed. In other words, for an energy-resolving photon counting detector, the pulse height is compared with multiple programmable thresholds (T1-TN) in a comparator and sorted according to the pulse height, which in turn is proportional to the energy. In other words, a photon counting detector comprising more than one comparator is referred to herein as a multi-bin photon counting detector. In the case of a multi-bin photon counting detector, the photon counts are stored in a set of counters, typically with each counter corresponding to an energy threshold. For example, a counter can be assigned to correspond to the highest energy threshold that a photon pulse has exceeded. In another example, a counter tracks the number of times a photon pulse crosses each energy threshold.

[0067] As an example, "side-on facing" is a special non-limiting design of a photon counting detector in which X-ray sensors (such as X-ray detector elements or pixels) are oriented sideways toward incoming X-rays.

[0068] For example, such a photon counting detector can have pixels in at least two directions, where one of the two directions facing sideways toward the photon counting detector has a component in the direction of the X-ray. Such sideways facing photon counting detectors are sometimes referred to as depth-segmented photon counting detectors, which have two or more pixel depth segments in the direction of the incoming X-ray.

[0069] Alternatively, the pixels can be arranged in an array (non-depth segmented) in a direction substantially perpendicular to the incoming X-rays, and each pixel can be oriented sideways toward the incoming X-rays. In other words, the photon counting detector can be non-depth segmented while still being arranged sideways toward the incoming X-rays.

[0070] To improve the absorption efficiency, the side-facing photon counting detector can be arranged accordingly to be side-facing, in which case the absorption depth can be chosen to be any length and the side-facing photon counting detector can still be fully depleted without reaching very high voltages.

[0071] The conventional mechanism for detecting X-ray photons using direct semiconductor detectors essentially works as follows. The energy of the X-ray interaction with the detector material is converted into electron-hole pairs within the semiconductor detector, where the number of electron-hole pairs is generally proportional to the photon energy. The electrons and holes drift toward the detector electrodes and back surface (or vice versa). During this drift, the electrons and holes induce a current in the electrodes, which can be measured.

[0072] like Figure 4 As shown, signals are routed 22 from the detector elements 21 of the X-ray detector to the inputs of a parallel processing circuit (e.g., an ASIC) 25. It should be understood that the term "application-specific integrated circuit (ASIC)" should be broadly interpreted as any general-purpose circuit used and configured for a specific application. The ASIC processes the charge generated from each X-ray and converts it into digital data that can be used to obtain measurement data, such as photon counts and / or estimated energy. The ASIC is configured to connect to digital data processing circuitry so that the digital data can be sent to further digital data processing 40 and / or one or more memories 45, and ultimately this data will be input to image processing 50 to generate a reconstructed image.

[0073] Since the number of electrons and holes from an X-ray event is proportional to the energy of the X-ray photon, the total charge in an induced current pulse is proportional to this energy. After the filtering step in the ASIC, the pulse amplitude is proportional to the total charge in the current pulse and, therefore, to the X-ray energy. The pulse amplitude can then be measured by comparing the value of the pulse amplitude to one or several threshold values ​​(THR) in one or more comparators (COMP), and a counter is introduced to record the number of pulses greater than the threshold value. In this way, the number of X-ray photons with an energy exceeding the energy corresponding to the corresponding threshold value (THR) detected within a certain time frame can be counted and / or recorded.

[0074] The ASIC typically samples the analog photon pulse once per clock cycle and records the comparator output. Depending on whether the analog signal is above or below the comparator voltage, the comparator (threshold) outputs a one or zero. The available information at each sample is, for example, a one or zero for each comparator, indicating whether the comparator has been triggered (the photon pulse is above the threshold) or not.

[0075] In a photon counting detector, there is typically a photon counting logic component that determines whether a new photon has been recorded and records the photon in a counter. In the case of a multi-bin photon counting detector, there are typically several counters, for example one counter for each comparator, and the photon count is recorded in these counters based on an estimate of the photon energy. This logic component can be implemented in several different ways. Two of the most common categories of photon counting logic components are the so-called non-paralyzable counting mode and the paralyzable counting mode

[29] . Other photon counting logic components include, for example, local maximum detection, which counts the local maxima detected in the voltage pulse and may also record its pulse height

[28] .

[0076] The following is an example of a non-paralyzable counting mode: 1) If the threshold is triggered, a new photon is detected; 2) If a new photon is recorded, a dead time is started during which the maximum trigger threshold is recorded; 3) After the dead time ends, the count corresponding to the maximum trigger threshold is recorded in the counter; 4) After the dead time ends, the channel is open to new photons. For the non-paralyzable counting mode, the number of counts recorded reaches the maximum value: N max = measurement time / dead time.

[0077] As an example of a paralyzable counting mode, take the previous example of a non-paralyzable counting mode and add the following condition: whenever a photon pulse hits any of the thresholds, the duration of the dead time is extended. The result of this change is that for very high photon count rates, the number of counts recorded drops to zero.

[0078] The situation when the number of incoming photons cannot be resolved by the photon counting channel is called pulse pile-up, which refers to the situation where photon pulses increase and merge together and cannot be distinguished from each other. Pulse pile-up can be a serious problem that limits the performance of photon counting detectors [1].

[0079] Photon-counting detectors have many benefits, including but not limited to high spatial resolution, low electronic noise, energy resolution, and material separation (spectral imaging). However, energy-integrating detectors offer the advantage of high count rate tolerance. This count rate tolerance stems from the fact / understanding that, since the total energy of photons is measured, adding one additional photon will always increase the output signal (within reasonable limits), regardless of the number of photons currently recorded by the detector. This key advantage is one of the main reasons why energy-integrating detectors have become the standard for medical CT today.

[0080] Several attempts have been made to combine the benefits of photon counting detectors and energy integrating detectors.

[0081] An ASIC (CIX chip) has been developed that can simultaneously perform photon counting (single threshold) and energy integration functions. The incoming signal is replicated and sent to both the energy integration channel and the photon counting channel ([2], [3], [4]). The dual energy imaging capability of the ASIC has also been evaluated, which is attributed to the difference in energy response between the photon counting signal and the energy integration signal [5]. It has also been proposed to use the energy integration channel as a backup channel in the event that the photon counting channel is saturated during the measurement time [6]. In addition, an ASIC has been developed in which both photon counting and energy integration acquisition are available (but not simultaneously). This allows the detector to meet the requirements of a wide variety of X-ray experiments ([7], [8]).

[0082] Several patents

[25]

[26]

[27] relate to devices having the following components: two parallel channels per detector element: a counting channel and an energy integrating channel (a measure of the total charge collected); and a processing unit in which the signals are combined to determine the amount of X-ray absorption.

[0083] Several other techniques have been developed for obtaining measurements from both energy-integrating and photon-counting detectors. For example, a dual-detector system with an energy-integrating and photon-counting detector has been developed and its imaging performance evaluated [9]. Patent US 2012 / 0085915A1

[24] describes a detector having a detector element comprising a photon-counting portion and an energy-integrating portion.

[0084] Another concept that has been proposed involves using a detector with interleaved energy-integrating and photon-counting pixel elements (i.e., each pixel is either a photon-counting pixel or an energy-integrating pixel)

[10] . Yet another approach is to combine thresholding of the signal to obtain energy resolution with charge integration, thereby simplifying the electronics

[11] .

[0085] Measuring the time above threshold (TOT), i.e., the duration that a pulse is above the comparator threshold, has been widely used as a means of measuring the energy of individual particles (

[12] ,

[13] ,

[14] ,

[15] ,

[16] ,

[17] ). In order to improve the resolution of the energy and arrival time of the detected particles, it has been proposed to analyze the TOT signals of several thresholds at different voltages simultaneously. The so-called multiple time above threshold (MTOT) technique has been used in various applications such as photomultiplier tube signal processing, neutrino telescopes, positron emission tomography (PET), and cosmic ray detection (

[18] ,

[19] ,

[20] ,

[21] ).

[0086] In reference

[22] , it has been demonstrated that a total overthreshold time (TTOT) readout can be used to increase the dynamic range of a photon counting detector; the TTOT signal saturates more slowly than the photon counting signal. The detector described in this reference can operate in an overthreshold time mode, but not simultaneously with the counting mode. The TOT mode has a specific circuit implementation, and a command can be sent to the ASIC to switch to this mode. A patent describes a method for improving high count rate performance using the total overthreshold time (TTOT) in conjunction with a paralyzable photon counting detector

[23] . The described method obtains the total overthreshold time value with the aid of a separate high-throughput electronic circuit. The electronic circuit can be configured to integrate an analog voltage signal that is switched on when the voltage pulse exceeds the threshold and vice versa. The electronic TTOT measurement circuit can also be implemented by associating a counter with a comparator and incrementing the counter on each clock cycle when the comparator is triggered.

[0087] In order to better understand the proposed measurement method, it may be useful to start with a brief system overview and / or analysis of the technical issues. For this purpose, ref. Figure 5 , which provides a schematic diagram of a photon counting circuit and / or device according to the prior art.

[0088] When photons interact in a semiconductor material, a cloud of electron-hole pairs is generated. By applying an electric field across the detector material, the charge carriers are collected by electrodes attached to the detector material. The signal is routed from the detector elements to the input of a parallel processing circuit (e.g., an ASIC). It should be understood that the term "application-specific integrated circuit (ASIC)" should be broadly interpreted as any general-purpose circuit used and configured for a specific application. The ASIC processes the charge generated from each X-ray and converts it into digital data that can be used to obtain measurement data, such as photon counts and / or estimated energy. In one example, the ASIC can process the charge so as to produce a voltage pulse whose maximum height is proportional to the amount of energy deposited by the photon in the detector material.

[0089] The ASIC may include a set of comparators 302, each of which compares the magnitude of an input voltage pulse to a reference voltage (corresponding to an energy threshold). The comparator output is typically zero or one (0 / 1), depending on which of the two voltages being compared is larger. Here, we assume that if the voltage pulse is higher than the reference voltage, the comparator output is one (1); if the reference voltage is higher than the voltage pulse, the comparator output is zero (0). A digital-to-analog converter (DAC) 301 can be used to convert a digital setting that may be provided by a user or a control program into a reference voltage that can be used by the comparators 302. If the height of the voltage pulse exceeds the reference voltage for a particular comparator, then we will refer to that comparator as being "triggered." Each comparator is typically associated with a digital counter 303, which increments based on the comparator output according to the photon counting logic.

[0090] For reference, Figure 6 An example of a signal analyzed by an ASIC is shown. The ASIC typically includes an ASIC clock that determines the rate at which the comparator outputs are sampled. The sampling interval is referred to herein as a clock cycle 402, and the length of one clock cycle is typically about 10ns. During each clock cycle, the output of each comparator is sampled by the ASIC. For example, if a channel includes 5 comparators, the ASIC receives a binary number for each comparator at each clock cycle, which indicates whether the corresponding comparator is triggered. In other words, at each clock cycle, the ASIC receives information about which thresholds the voltage pulse 403 currently exceeds. Figure 6 In this example, the ASIC receives information according to the following table: A set of ones and / or zeros for each threshold (THR1, THR2, THR3) indicating whether the voltage pulse 403 exceeds the threshold.

[0091] Conventionally, it is not practical to read out the output from the comparator for every clock cycle due to limitations in the data transmission chain. Instead, the ASIC aggregates a representation of the comparator output during a measurement time 401. The measurement time is typically about 100 μs. As an example, for a clock period of 10 ns and a measurement time of 100 μs, there are 10'000 clock cycles per measurement time. Figure 6 In this example, the length of the measurement time is 11 clock cycles. The aggregate representation of the comparator output can be, for example, a photon count recorded according to a photon counting logic component. Another example is to record each time a voltage pulse crosses a threshold level in an upward direction (i.e., the comparator output switches from zero (0) to one (1)). Figure 6 In this example, the voltage pulse crosses THR1 twice in the upward direction and crosses THR2 once.

[0092] FIG. 10A to FIG. 10B is a diagram showing an example of a voltage pulse for two photons arriving close in time. Figure 10A In the case of Figure 10B In a photon-counting detector, two pulses add to form a single, larger voltage pulse. This phenomenon is known as pulse pile-up, and for photon-counting detectors, it degrades the signal in two ways: first, the event is recorded as one count instead of two, resulting in a statistical loss that negatively impacts the system dose efficiency; second, the event is recorded at the wrong energy, which compromises the spectral fidelity of the measurement and impairs the system's spectral imaging capabilities.

[0093] According to a first aspect, a circuit is provided for operating with a multi-bin photon counting X-ray detector having multiple energy thresholds. The circuit is configured to obtain or generate a plurality of total time above threshold (TTOT) signals corresponding to a plurality of different energy thresholds and to provide energy integration information based on the plurality of TTOT signals.

[0094] This corresponds to a general circuit system that can be implemented in various forms (e.g., as one or more separate circuits) and / or can include a set of separate circuits and / or various sub-circuits, a non-limiting example of which is shown in FIG. Figure 7 and Figures 8A to 8B and / or Figure 9 Schematically shown in .

[0095] For example, the circuit may be configured to provide energy integration information formed or represented by the plurality of TTOT signals.

[0096] In one specific example, the circuit is configured to form a signal that approximates or represents an energy integration signal based on the TTOT signal at the several energy thresholds set at different energies.

[0097] For example, the circuit may be configured to form or generate the digital energy integration signal by a (weighted) summation or a linear or non-linear combination of the several TTOT signals.

[0098] Alternatively, or in addition, the circuit may be configured to output energy integration information via the several TTOT signals, eg for more or less direct use in image reconstruction.

[0099] Interestingly, the aggregation and / or combination of the several TTOT signals may include spectral energy information.

[0100] In one specific example, the circuit is configured to be applied directly to the comparator output of the multi-bin photon counting X-ray detector, e.g. Figure 7 and Figures 8A to 8B Schematically shown in .

[0101] For example, the circuit may be configured to generate or obtain the TTOT signals corresponding to several different energy thresholds based on comparator outputs from corresponding comparators of a multi-bin photon counting X-ray detector as inputs (see, for example, Figure 7 and Figures 8A to 8B ).

[0102] See again Figure 5 In a specific example, each comparator can be configured to operate based on a respective energy threshold.

[0103] The multi-bin photon counting X-ray detector may have a set of comparators 302, each comparator configured to compare the magnitude of an input voltage pulse from one or more detector elements with a reference voltage corresponding to a respective energy threshold to produce a comparator output.

[0104] For example, the circuit can be configured to generate or obtain each of the TTOT signals based on an input comprising a total number of clock cycles during which an input voltage pulse exceeds a reference voltage during a predetermined measurement time or a subset of the predetermined measurement time in the corresponding comparator.

[0105] In one specific example, the circuit may be configured to generate or derive each of the TTOT signals from the comparator output by summing the number of clock cycles during which the corresponding comparator is triggered during a predetermined measurement time or a predetermined measurement time subset.

[0106] Optionally, the circuit may be configured to sum the comparator outputs of more than one (ie several) comparators during a measurement time or a subset of the measurement time, or

[0107] The circuit can be configured to calculate the average value of the comparator output during the measurement time or a subset of the measurement time, or

[0108] The circuit may be configured to calculate a sum of clock cycles during which each comparator is the highest triggering comparator.

[0109] The circuit may further be configured to generate or obtain the number of TTOT signals for a subset of available energy thresholds and / or for a subset of detector elements of the X-ray detector.

[0110] For example, the circuit includes a total time over threshold (TTOT) logic circuit 502 and / or a digital processing circuit 504 .

[0111] According to a second aspect, a total time above threshold (TTOT) logic circuit is provided that is configured to work with a multi-bin photon counting X-ray detector having multiple energy thresholds, wherein the TTOT logic circuit is configured to generate a number of total time above threshold (TTOT) signals corresponding to a number of different energy thresholds, and to provide energy integration information based on the number of TTOT signals.

[0112] As an example, the TTOT logic circuit may be configured to provide energy integration information formed or represented by the plurality of TTOT signals.

[0113] As a preferred example, the TTOT logic circuit can be configured to be directly applied to the comparator output of a multi-bin photon counting X-ray detector.

[0114] For example, the TTOT logic circuit may be configured to sample a comparator output of a photon counting X-ray detector at a sampling interval.

[0115] In one specific example, the TTOT logic circuit is configured to generate the TTOT signals corresponding to several different energy thresholds based on comparator outputs from corresponding comparators of a multi-bin photon counting X-ray detector as inputs.

[0116] According to a third aspect, there is provided a digital processing circuit configured to work with a multi-bin photon counting X-ray detector having multiple energy thresholds, wherein the digital processing circuit is configured to obtain more than one total time above threshold (TTOT) signals corresponding to more than one energy threshold and to provide energy integration information based on the more than one TTOT signals.

[0117] For example, the digital processing circuit may be configured to receive the more than one TTOT signals and to form a combined value representing at least the energy integration information based on the more than one TTOT signals.

[0118] In one specific example, the digital processing circuit is configured to combine the TTOT signal from the low energy threshold and the TTOT signal from the high energy threshold to form the combined value.

[0119] For example, the digital processing circuit may be configured to receive the more than one TTOT signals and the at least one photon counting signal, and to form a combined value representing energy integration information and photon counting information by combining the signals.

[0120] As an example, the digital processing circuit may be configured to form the combined value by combining signals that depend on the rate of incoming X-ray photons.

[0121] Optionally, the digital processing circuit can be configured to form the combined value by assigning increased weight to the at least one photon counting signal in the signal combination at a photon rate below a predetermined threshold rate and assigning increased weight to the more than one TTOT signals in the signal combination at a photon rate above the threshold rate.

[0122] According to a fourth aspect, there is provided a measurement circuit for a photon counting X-ray detector, comprising the TTOT logic circuit of the second aspect and / or the digital processing circuit of the third aspect.

[0123] Optionally, the measurement circuit further comprises a photon counting logic circuit, such as Figure 7 Schematically shown in .

[0124] In one specific example, the measurement circuit is configured to sample a comparator output of a photon counting X-ray detector at a sampling interval.

[0125] According to a fifth aspect, an X-ray imaging system is provided, comprising the circuit according to any one of the first aspect, the second aspect, the third aspect and / or the fourth aspect.

[0126] For example, the X-ray imaging system may be configured to perform material-specific imaging of an object to be imaged based on TTOT signals from several energy thresholds as spectral information.

[0127] For example, the X-ray imaging system may be configured to perform material-specific imaging of an object to be imaged based on a combination of the TTOT signal and a photon counting signal and / or a digital energy integration signal formed from the TTOT signal.

[0128] According to a sixth aspect, there is provided a system configured to work with a multi-bin photon counting X-ray detector having multiple energy thresholds, wherein the system is configured to generate more than one total time above threshold (TTOT) signals based on the output of more than one comparator from the multi-bin photon counting detector.

[0129] As mentioned, a signal representing or approximating an energy integration signal can be obtained based on the total time-over-threshold (TTOT) signal for several energy thresholds set at different energies in a multi-bin photon counting X-ray detector. This signal can be referred to as a digital energy integration signal.

[0130] The inventors have recognized that a signal formed or represented by several TTOT signals has a significantly more linear relationship to the input photon rate and allows the overall X-ray imaging system to maintain dose efficiency also at higher rates.

[0131] The specific non-limiting example of the proposed method and structural configuration for obtaining a digital energy-integrated signal has the advantage over prior art techniques for obtaining an energy-integrated signal in that it does not require dedicated analog circuitry for integrating / accumulating the signal. Instead, the inventors have recognized that it is feasible to utilize the output of a digital comparator that already exists as part of the capabilities of a multi-bin photon counting detector.

[0132] In the following, non-limiting examples will be described in more detail.

[0133] In a specific embodiment, the present invention relates to obtaining a signal representing or approximating an energy integration signal based on a total time-over-threshold (TTOT) signal 601 for several energy thresholds set at different energies in a multi-bin photon counting detector. The obtained signal will be referred to herein as a digital energy integration signal.

[0134] The digital energy integration signal may be formed, for example, by the summation or linear or nonlinear combination of TTOT signals corresponding to several energy thresholds.The energy integration information may also be indirectly transferred to the image reconstruction process via several TTOT signals.

[0135] The proposed method of obtaining a digital energy-integrated signal has the advantage over existing techniques for obtaining an energy-integrated signal in that it does not require dedicated analog circuitry for integrating / accumulating the signal. Instead, the method utilizes the output of a digital comparator that already exists as part of the capabilities of a multi-bin photon counting detector.

[0136] A signal formed or represented by several TTOT signals has a significantly more linear relationship with the input photon rate and maintains dose efficiency at higher rates.

[0137] The benefit of the proposed method is that several TTOT signals contain spectral (photon energy) information that can be used for spectral imaging as well as for imaging situations that suffer from high pulse pile-up.

[0138] Furthermore, the present invention relates to using TTOT signals from several energy thresholds as spectral information to perform material separation of the imaged object (material specific imaging). This operation can be combined with photon counting signals and / or digital energy integration signals.

[0139] In an exemplary embodiment, the photon counting logic is not paralyzable.

[0140] A particular object of the present invention may be to improve the performance of multi-bin photon counting detectors at high photon rates. Figure 12 , which shows that the photon counting signal 602 (here using non-destructible photon counting logic) saturates (loses signal) at a lower photon rate than the TTOT signal 601, which in turn saturates at a lower photon rate than the digital energy integration signal 603. The main reason for this behavior is that the TTOT signal does not utilize dead time, during which no signal can be recorded. Therefore, adding additional photon pulses generally also increases the measured TTOT signal in the presence of pulse pileup. The digital energy integration signal increases proportionally with the number of photons up to very high photon rates.

[0141] In one embodiment of the invention, the (each) total time over threshold (TTOT) signal 601 is based on the total number of clock cycles 402 during which the input voltage pulse exceeds the reference voltage during the measurement time 401. The basic idea is to extract the TTOT signal from the comparator output by, for example, summing the number of clock cycles during which the comparator is triggered during the measurement time. Figure 6 , during the measurement time, the sum of the clock cycles triggered is six (6) for THR1 and two (2) for THR2.

[0142] The proposed method for extracting the TTOT signal has the advantage over the prior art in that it does not require dedicated analog electronic circuitry to estimate the TTOT signal. Instead, the total over-threshold time counting logic is applied directly to the comparator output. Figure 7 , where the output from comparator 302 is sent to a measurement circuit that includes a total overthreshold time logic component 502 and, in one exemplary embodiment, an (optional) photon counting logic component 501. In one exemplary embodiment, the outputs from the TTOT logic component and the photon counting logic component optionally undergo a digital processing step 504 before being read out from the measurement circuit 503.

[0143] According to another aspect, an exemplary embodiment of the present invention also relates to a total over-threshold time counting logic component 502 and corresponding devices and / or systems. In an exemplary embodiment, the total over-threshold time (TTOT) logic component can be implemented as the sum of the comparator outputs of more than one (i.e., several) comparators during the measurement time. In an alternative embodiment, the TTOT logic component calculates the mean of the comparator outputs during the measurement time. In another embodiment, the TTOT logic component calculates the sum of the clock cycles during which each comparator is the highest trigger comparator during the clock cycle. Reference Figure 6 , the latter implementation will return 4 for THR1 and 2 for THR2.

[0144] According to yet another aspect, an exemplary embodiment of the present invention further relates to a measurement circuit 503 comprising both a photon counting logic component 501 and a total over-threshold time logic component 502, such as Figure 7 The measurement circuit 503 can be implemented to output a photon counting output and a TTOT output, or any combination thereof, via a digital processing step 504. It should be understood that the present invention is not limited by the specific implementation of the photon counting logic component. For example, the photon counting logic component can be paralyzable or non-paralyzable.

[0145] See also Figure 8A The present invention also relates to a method / system for obtaining and / or generating more than one TTOT signal 601 based on the outputs of more than one comparator 302 in a multi-bin photon counting detector.

[0146] In one exemplary embodiment, the TTOT signal is obtained for a subset of available energy thresholds 601. In another embodiment, the TTOT signal is obtained for a subset of detector elements 601. In another embodiment, the TTOT signal is measured during a subset of measurement times.

[0147] Figure 8B is a schematic diagram illustrating another example of a system / circuitry system for obtaining and / or generating more than one TTOT signal. In this particular example, in addition to the TTOT logic circuit 502, the system / circuitry system may optionally include a digital processing circuit 504 to generate the TTOT signal and / or combined value or signal.

[0148] Figure 9 is a schematic diagram illustrating one example of digital processing circuitry for generating at least one combined value from more than one TTOT signals and, optionally, one or more photon counting signals.

[0149] The present invention also relates to a method / system / circuitry for forming at least one combined value 603 from more than one TTOT signal 601. The combined value is, for example, a weighted sum of the TTOT values. The combined value may also include a photon counting signal 602 in an attempt to obtain the beneficial effects of both signals. The combined value may be an approximation of the energy-integrated signal, for example, obtained via a weighted sum of the TTOT values, where the weight represents the distance between two adjacent comparator voltages. The signal combining method selected may, for example, depend on the rate of incoming X-ray photons. For example, at low photon rates (i.e., below a given threshold rate), the photon counting signal outperforms the TTOT signal, and therefore, it is beneficial to assign an increased weight to the photon counting signal. On the other hand, at high photon rates or fluxes (i.e., above a given threshold rate), the TTOT signal outperforms the photon counting signal, and therefore, an increased weight may be assigned to the TTOT signal.

[0150] Figures 11A to 11B is a schematic diagram showing an example of a TTOT signal originating from two photons.

[0151] Figure 11A An example of a TTOT signal 601 from a measurement comprising two photon pulses separated in time is shown. In this example, the TTOT signal for each threshold is formed by the sum of the number of clock cycles during which the threshold is triggered. Figure 11A In the , the two photon pulses are clearly separated, while in Figure 11B In the example above, two photon pulses are added together to form a single, larger voltage pulse. For each case, the TTOT signal 601 is shown in the table on the right side of the figure. It can be seen that the TTOT signal for low energy thresholds (THR1 and THR2) decreases due to pulse pile-up. On the other hand, for high energy thresholds (THR3, THR4, and THR5), the TTOT signal increases.

[0152] According to yet another aspect, an exemplary embodiment of the present invention relates to a method for forming a combined value from TTOT signals corresponding to several thresholds at different energies. In one specific example, the method involves forming a combined value that represents or approximates the integral of a voltage pulse during a measurement time. In an exemplary embodiment, the combined value is formed by calculating a weighted sum of several TTOT signals. The weights can be selected so that the combined value represents or approximates the integral of the voltage pulse. In this example of Figure 11, consider the weights: [3, 2, 2, 2, 2] for TTOT signals corresponding to thresholds 1, 2, 3, 4, 5, respectively. The weights correspond to the distance between the energy thresholds. In this case, the weighted sum will be equal to the integral of the bar graph in Figure 11, which is a very close approximation of a true voltage pulse. In this case, the weighted sum will approximate the integral of the voltage pulse. In this specific example of Figure 11, for case A, the weighted sum is equal to 102, and for case B, the weighted sum is equal to 103, which shows that the combined signal is relatively insensitive to pulse pile-up, as desired. An alternative weighting scheme is to calculate, for each threshold, the integral of the voltage pulse at clock cycle N. i Period threshold i is the clock period N of the highest trigger threshold i If threshold i is the highest trigger threshold, the average height of the pulse is H i , then for all thresholds (i), the integral of the voltage pulse can be approximated as sum(N i ×H i ). H i The value of can be estimated based on the knowledge of the energy threshold position, while the value of Ni can be calculated directly from the comparator value in the TTOT logic component, or by calculating Ni = TTOT i –TTOT i+1 To calculate, where TTOT i is the total number of clock cycles during which threshold i is triggered.

[0153] The benefit of measuring the TTOT signal at several thresholds at different energy levels is that these TTOT signals saturate (below the ideal signal) at different rates. The higher the threshold voltage, the slower the signal saturates because there are fewer photons with high energy. In the case of pulse pileup, the TTOT signal at the low energy threshold 601 begins to saturate. The TTOT signal at the high energy threshold 601 may increase due to pulse pileup. Figure 13The behavior of the TTOT signal as the photon rate increases is demonstrated. This is due to the fact that the combined pulse of two or more photons has a higher amplitude than each individual pulse and therefore more often triggers a higher threshold, as can be seen from Figure 11. In a sense, the reduced TTOT signal at low thresholds due to pulse pile-up is compensated by the increased TTOT signal at higher thresholds. In other words, as the photon rate increases, there are two competing characteristics in the signal: the average pulse length decreases, while the average pulse height increases. At low energies, the decrease in average pulse length dominates, while at high energies, the increase in average pulse height dominates. On the other hand, the total energy, which is proportional to the integral of the voltage pulse, scales linearly with increasing photon rate.

[0154] To understand why the comparator's TTOT signal saturates as the photon rate increases at low energies, consider a case in which two photon pulses arrive at the channel very close in time (closer together than the widths of the individual pulses). These two pulses form a combined pulse whose length is longer than that of each individual pulse, but shorter than the sum of the lengths of the two pulses. Therefore, the TTOT signal does not scale linearly with the increasing photon rate, but rather increases more slowly than linearly. At very high count rates, the TTOT signal approaches a maximum value equal to the measurement time.

[0155] If the TTOT signals 601 from the low energy threshold and the high energy threshold are combined, a combined value 603 can be formed that is linearly related to the rate of incoming photons, such as Figure 13 This linear relationship results from the configuration and / or design of combining the high and low TTOT signals 601 to form a signal that is proportional to the total deposited energy during the measurement time. The linear signal is similar to the signal from an energy integrating detector, except that the integration is performed on a digitized signal rather than an analog signal.

[0156] In order to have good photon detection efficiency, that is, most photons are recorded, it is desirable to have at least one threshold set at a low energy so that low-energy photons are also recorded. If only one threshold is used, it will typically be set at a relatively low energy. Therefore, the TTOT signal of a single comparator will saturate at relatively low photon count rates.

[0157] The output from each detector element (photon counting signal and TTOT signal) can be read out from the X-ray imaging system and analyzed after readout, or analyzed in a processing unit, eg located in an ASIC or a field programmable gate array (FPGA).

[0158] This method can be used to simultaneously obtain spectral photon counting data and energy integrated data without the need to use two separate channels for each pixel.

[0159] Compared to the TTOT method using a single threshold, the combined value 603 based on several TTOT signals 601 can be configured to have a higher count rate impedance. Moreover, having several TTOT signals can improve the spectral imaging capability at higher count rates.

[0160] According to another aspect, a multi-bin photon counting detector is provided, wherein at least a subset of the detector elements can obtain a photon counting signal and a TTOT signal.

[0161] According to another aspect, an X-ray imaging system is provided that includes an X-ray source and an X-ray detector, the X-ray detector including a photon counting detector array configured to obtain a TTOT signal and a photon counting signal for at least a subset of detector elements.

[0162] According to a complementary aspect, an X-ray detector and an X-ray imaging system are provided, comprising a photon counting silicon side-facing detector configured to obtain a TTOT signal and a photon counting signal for at least a subset of detector elements.

[0163] Figure 14 is a schematic diagram showing an example of a basic method for obtaining energy integration information from a multi-bin photon counting X-ray detector.

[0164] Basically, the method comprises step S1 of providing or generating a signal representing or approximating an energy integration signal based on total time above threshold (TTOT) signals for several energy thresholds set at different energies in the multi-bin photon counting detector.

[0165] It will be appreciated that the mechanisms and arrangements described herein can be implemented, combined and rearranged in various ways.

[0166] For example, embodiments may be implemented in hardware, or at least partly in software executed by appropriate processing circuitry, or a combination of the above.

[0167] The steps, functions, processes and / or blocks described herein may be implemented in hardware (including general purpose electronic circuitry and application specific circuitry) using any conventional technology, such as discrete circuitry or integrated circuit technology.

[0168] Alternatively, or in addition, at least some of the steps, functions, processes and / or blocks described herein can be implemented in software, such as a computer program executed by appropriate processing circuitry (such as one or more processors or processing units).

[0169] According to a supplementary aspect, a corresponding computer program and computer program product are provided.

[0170] In particular, a computer program is provided comprising instructions which, when executed by a processor, cause the processor to carry out the method as described herein.

[0171] For example, a computer program product comprising a non-transitory computer-readable medium having such a computer program stored thereon may also be provided.

[0172] Figure 15 2 is a schematic diagram illustrating an example of a computer implementation according to one embodiment. In this particular example, system 200 includes a processor 210 and a memory 220, the memory including instructions that can be executed by the processor, whereby the processor is operable to perform the steps and / or actions described herein. These instructions are typically organized as computer programs 225, 235, which can be pre-configured in the memory 220 or downloaded from an external memory device 230. Optionally, system 200 includes an input / output interface 240 that can be interconnected to the processor 210 and / or the memory 220 to enable input and / or output of relevant data (such as input parameters and / or resulting output parameters).

[0173] In one particular example, the memory includes a set of instructions executable by the processor, whereby the processor is operable to determine an estimate or measurement of charge spreading and estimate a starting point of an interaction along the thickness of the detector submodule based on the determined estimate of charge spreading.

[0174] The term "processor" should be interpreted in a general sense as any system or device capable of executing program code or computer program instructions to perform specific processing, determination or computing tasks.

[0175] Thus, processing circuitry including one or more processors is configured to perform well-defined processing tasks (such as those described herein) when executing a computer program.

[0176] The processing circuitry need not be dedicated solely to performing the steps, functions, processes and / or blocks described above, but may also perform other tasks.

[0177] The proposed technology also provides a computer program product comprising a computer readable medium 220, 230 having such a computer program stored thereon.

[0178] By way of example, the software or computer program 225, 235 may be implemented as a computer program product typically carried or stored on a computer-readable medium 220, 230 (particularly a non-volatile medium). The computer-readable medium may include one or more removable or non-removable memory devices, including but not limited to: read-only memory (ROM), random access memory (RAM), compact disc (CD), digital versatile disc (DVD), Blu-ray disc, universal serial bus (USB) memory, hard disk drive (HDD) storage device, flash memory, magnetic tape, or any other conventional memory device. Thus, the computer program may be loaded into the operating memory of a computer or equivalent processing device for execution by its processing circuitry.

[0179] The method flow, when executed by one or more processors, can be considered as a computer action flow. The corresponding device, system and / or apparatus can be defined as a set of functional modules, wherein each step performed by the processor corresponds to a functional module. In this case, the functional module is implemented as a computer program running on the processor. Therefore, the device, system and / or apparatus can alternatively be defined as a set of functional modules, wherein these functional modules are implemented as a computer program running on at least one processor.

[0180] The computer program residing in the memory may thus be organized into appropriate functional modules that are configured to perform at least part of the steps and / or tasks described herein when the computer program is executed by a processor.

[0181] Alternatively, these modules may be implemented primarily through hardware modules, or alternatively through hardware. The degree of software versus hardware is purely an implementation choice.

[0182] When the proposed technique is used to perform material-specific imaging based on the spectral information contained in several TTOT signals, basic material decomposition techniques can be exploited.

[0183] Basis material decomposition exploits the fact that all substances composed of low atomic number elements (such as human tissue) have a linear attenuation coefficient μ(E) whose energy dependence can be well approximated as a linear combination of two (or more) basis functions:

[0184] μ(E)=a1f1(E)+a2f2(E).

[0185] where f i is a basis function, and a iis the corresponding basis coefficient. If one or more elements with high atomic numbers are present in the imaging volume, high enough to exhibit a k-absorption edge in the energy range used for imaging, a basis function must be added for each such element. In medical imaging, such k-edge elements are typically iodine or gadolinium, substances used as contrast agents.

[0186] In general, the basis material decomposition is described in Alvarez and Macovski, "Energy-selective reconstructions in X-ray computerised tomography", Phys. Med. Biol. 21, 733. In the basis material decomposition, each base coefficient a i The line integral A i is based on each projection ray from the source to the detector element The line integral A is inferred from the measured data. i It can be expressed as:

[0187] For i=1,...,N,

[0188] Where N is the number of basis functions. In one embodiment, the basis material decomposition is performed by first expressing the expected number of records in each energy bin as A i Typically, such a function can take the following form:

[0189]

[0190] Here, λ i is the expected number of counts in energy bin i, E is the energy, S i is a response function that depends on the shape of the spectrum incident on the imaged object, the quantum efficiency of the detector, and the sensitivity of energy bin i to X-rays of energy E. Although the term "energy bin" is most commonly used for photon counting detectors, this formula can also describe other energy-resolving X-ray systems, such as multi-slice detectors or kVp switching sources.

[0191] Then, under the assumption that the number of counts in each bin is a Poisson-distributed random variable, we can use maximum likelihood to estimate A iThis is achieved by minimizing the negative log-likelihood function, see Roessl and Proksa, K-edge imaging in x-ray computed tomography using multi-bin photon counting detectors, Phys. Med. Biol. 52 (2007), 4679-4696:

[0192]

[0193] where m i is the number of counts measured in energy bin i, and M b is the number of energy bins.

[0194] From the line integral A, a tomographic reconstruction can be performed to obtain the basis coefficient a i This process step can be considered as a separate tomographic reconstruction, or can alternatively be considered as part of an overall basis decomposition.

[0195] When the resulting estimated basis coefficients for each projection line are integrated When arranged into an image matrix, the result is a material-specific projection image for each basis i, also called a basis image. This basis image can be viewed directly (e.g. in projection X-ray imaging) or used as a basis coefficient a for forming the interior of an object. i Input to a reconstruction algorithm for mapping (eg in CT).In any case, the result of the basis decomposition can be viewed as one or more basis image representations, such as basis coefficient line integrals or the basis coefficients themselves.

[0196] The above embodiments are given as examples only, and it should be understood that the technology proposed is not limited thereto. Those skilled in the art will understand that various modifications, combinations, and variations may be made to these embodiments without departing from the scope of the present invention as defined by the appended claims. In particular, different partial solutions in different embodiments may be combined in other configurations, where technically possible.

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Claims

1. A circuit (502; 503; 504), the circuit being configured to operate with a multi-bin photon counting X-ray detector (20), the multi-bin photon counting X-ray detector (20) being configured to count the number of photons that have interacted in the multi-bin photon counting X-ray detector during a measurement time, the multi-bin photon counting X-ray detector (20) having a plurality of energy thresholds and being configured to operate based on a corresponding comparator, wherein the circuit (502; 503; 504) is configured to obtain or generate a plurality of total over-threshold time (TTOT) signals corresponding to a plurality of different energy thresholds based on a comparator output from the corresponding comparator as an input, wherein the circuit (502; 503; 504) is configured to generate or obtain each of the TTOT signals based on an input, the input comprising a total number of clock cycles during which an input voltage pulse exceeds a reference voltage during a clock cycle in the corresponding comparator during the measurement time, and / or the circuit is configured to generate or obtain each of the TTOT signals from a comparator output by summing the number of clock cycles during which the corresponding comparator is triggered during the measurement time, and wherein the circuit (502; 503; 504) is configured to provide energy integration information as a digital energy integration signal based on aggregation and / or combination of the several TTOT signals.

2. The circuit (502; 503; 504) according to claim 1, wherein the circuit (502; 503; 504) is configured to provide energy integration information formed or represented by the several TTOT signals, and / or the circuit is configured to form a signal that approximates or represents an energy integration signal based on the TTOT signals of the several energy thresholds set with different energies.

3. The circuit (502; 503; 504) according to any one of claims 1 to 2, wherein the circuit is configured to form or generate a digital energy integration signal by summing or linearly or nonlinearly combining the several TTOT signals, and the circuit is configured to output the energy integration information via the several TTOT signals.

4. The circuit (502; 503; 504) according to any one of claims 1 to 2, wherein the number of TTOT signals comprises spectral energy information.

5. The circuit (502; 503; 504) according to any one of claims 1 to 2, wherein the circuit is configured to be applied directly to a comparator output of the multi-bin photon counting X-ray detector (20).

6. The circuit (502; 503; 504) of claim 1, wherein the multi-bin photon counting X-ray detector (20) has a set of comparators (302), and each comparator is configured to compare the magnitude of an input voltage pulse from one or more detector elements with a reference voltage corresponding to a respective energy threshold to generate a comparator output.

7. A total time over threshold (TTOT) logic circuit (502), comprising the circuit according to any one of claims 1 to 6.

8. A digital processing circuit (504) comprising the circuit according to any one of claims 1 to 6.

9. A measurement circuit (503) for a photon counting X-ray detector (20), the measurement circuit comprising the TTOT logic circuit (502) of claim 7 and / or the digital processing circuit (504) of claim 8.

10. An X-ray imaging system (100), comprising the circuit according to any one of claims 1 to 6.

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