Calibration of material discrimination for X-ray imaging systems

Abstract

To provide an X-ray imaging system, such as an X-ray CT, that can be used with a calibration phantom and performs calibration of material discrimination.SOLUTION: An X-ray imaging system includes an X-ray source configured to emit X-rays, and an X-ray detector disposed in an X-ray beam path and configured to generate detector data. A calibration phantom is disposed in the X-ray beam path. The X-ray imaging system further includes an X-ray beam limiting apparatus that includes at least one calibration element disposed in the X-ray beam path. The X-ray imaging system also includes an image processing circuit unit configured to acquire projection data of a set of projections based on the detector data, and to determine a path length through at least one material of the at least one calibration element and at least one material of the calibration phantom at least partially based on the acquired projection data to perform calibration of material discrimination.SELECTED DRAWING: Figure 11

Description

[Technical Field]

[0001] The proposed technology relates to X-ray technology and X-ray imaging, and more specifically to the calibration of X-ray imaging systems (including calibration phantoms) and corresponding calibration procedures. In particular, the proposed technology relates to X-ray imaging systems (such as computed tomography (CT) imaging systems) configured for material discrimination calibration, and corresponding methods for material discrimination calibration of CT imaging systems.

[0002] Radiation imaging (such as CT imaging systems and other common X-ray imaging systems) has been used for many years in medical applications (such as medical diagnosis and treatment).

[0003] A typical X-ray imaging system (such as a CT imaging system) includes an X-ray source, an X-ray detector, and an associated image processing system. The X-ray detector may consist of multiple detector modules, each containing one or more detector elements, allowing for separate measurement of X-ray intensity. The X-ray source emits X-rays, which pass through the subject or object being imaged and are received by the X-ray detector. The X-ray source and detector are typically arranged to rotate around the subject or object on a rotating member of a gantry. The emitted X-rays attenuate as they pass through the subject or object, and the resulting transmitted X-rays are measured by the X-ray detector. The X-ray detector is coupled to a digital acquisition system (DAS), and the measured X-ray data is transferred to an image processing system to reconstruct an image of the subject or object.

[0004] Referring to Figure 1A, it is considered effective to briefly describe an exemplary and general overview of a prior art X-ray imaging system. In this exemplary embodiment, the X-ray imaging system 100 includes an X-ray source 10, an X-ray detector 20, and an associated image processing system 30. Generally, the X-ray detector 20 is configured to record radiation from the X-ray source 10, which is focused by an optional X-ray optical system or collimator and passes through an object, subject, or part thereof. The X-ray detector 20 is connectable to the image processing system 30 via a suitable readout electronic circuit, which is at least partially integrated into the X-ray detector 20, and the image processing system 30 can perform image processing and / or image reconstruction.

[0005] For example, a conventional CT imaging system includes an X-ray source and X-ray detectors positioned to acquire projected images of a subject or object at different viewing angles covering at least 180 degrees. This is most commonly achieved by mounting the X-ray source and detectors on a support (e.g., a rotating member of a gantry) that can rotate around the subject or object. An image containing projections recorded on different detector elements for different viewing angles is called a sinogram. Hereafter, a set of projections recorded on different detector elements for different viewing angles will be referred to as a sinogram even if the detector is two-dimensional, and the sinogram will be a three-dimensional image.

[0006] Figure 1B is a schematic diagram showing an example of a conventional X-ray imaging system setup, illustrating the projection line from the X-ray source through the object to the X-ray detector.

[0007] A further development of X-ray imaging is energy-resolved X-ray imaging, also known as spectral X-ray imaging, in which X-ray transmittance is measured for several different energy levels. This can be achieved by using two or more X-ray sources that emit different X-ray spectra by rapidly switching the X-ray source between two different emission spectra, or by using an energy-discriminating detector that measures incident radiation at two or more energy levels. An example of such a detector is a multi-bin photon counting detector, in which each recorded photon generates a current pulse, and this current pulse is compared to a set of thresholds, thereby counting the number of photons incident on each of several energy bins.

[0008] Spectral X-ray projection measurements yield projection images at each energy level. By calculating the weighted sum of these projection images, the contrast-to-noise ratio (CNR) can be optimized for a given imaging task, as described in “SNR and DQE analysis of broad spectrum X-ray imaging”, Tapiovaara and Wagner, Phys. Med. Biol. 30, 519.

[0009] Another technique made possible by energy-resolved X-ray imaging is reference material discrimination. This method takes advantage of the fact that all materials composed of elements with small atomic numbers (such as human tissue) have a linear attenuation coefficient that can be approximately well expressed by a linear combination of two (or more) basis functions, i.e., the following equation. μ(E) = a1f1(E) + a2f2(E) Here, f1 and f2 are basis functions, and a1 and a2 are the corresponding basis coefficients. To generalize further, f i is a basis function, a i∫ is the corresponding basis coefficient, where i=1,...,N, and N is the total number of basis functions. If one or more elements with high atomic numbers are present in the imaged volume and the K absorption edge appearing in the energy range used for imaging is sufficiently high, then one basis function must be added for each such element. In the field of medical imaging, such K absorption edge elements can typically be iodine or gadolinium, which are substances used as contrast agents.

[0010] Reference substance discrimination is described in “Energy-selective reconstructions in X-ray computerized tomography”, Alvarez, Macovski, Phys. Med. Biol. 1976; 21(5):733-744. In reference substance discrimination, the integral of each basis coefficient, expressed by the following formula, is estimated from the measurement data of each projection line l (L) from the source to the detector element.

number

number

[0011] Next, under the assumption that the count value of each bin is a Poisson-distributed random variable, A i can be estimated using the maximum likelihood method. This is achieved by minimizing the negative log-likelihood function (see, for example, “K-edge imaging in X-ray computed tomography using multi-bin photon counting detectors”, Roessl and Proksa, Phys. Med. Biol. 52 (2007), 4679-4696). [Number] Here, m i is the measured count value of energy bin i, and M b is the number of energy bins.

[0012] When the estimated basis coefficient line integral [Number] of each projection line obtained as a result is arranged in the image matrix, a substance-specific projection image (also called a basis image) for each basis i is obtained. This basis image can be directly seen (e.g., in projection X-ray imaging) or taken as an input to a reconstruction algorithm (e.g., in CT imaging) to form a map of the basis coefficient a i inside the subject. In either case, the result of basis discrimination can be regarded as one or more basis image representations (such as line integrals of basis coefficients or the basis coefficients themselves).

[0013] Standard calibration procedures for X-ray imaging systems are not adapted to, and cannot correspond to, material discrimination, and thus it is difficult to ensure the robust operation of an X-ray imaging system that functions based on material discrimination.

[0014] Furthermore, calibration is typically required to be valid over a long period. In practice, this typically means that the calibration procedure should have the robustness and operability to be automated or semi-automated to minimize human intervention. This advantage is to shorten the machine handling time and minimize errors that humans are prone to make.

[0015] Therefore, there still exists a general requirement regarding improving the calibration and operation of X-ray imaging systems (such as CT imaging systems).

Summary of the Invention

[0016] This summary is intended to introduce concepts of the content that will be described in more detail in the form of implementing the invention. This summary should not be used to identify the essential features of the claimed subject matter, nor should it be used to limit the scope of the claimed subject matter.

[0017] According to one aspect, there is provided an X-ray imaging system configured for calibration of material discrimination, which can be used together with a calibration phantom. The X-ray imaging system includes an X-ray source configured to emit X-rays, and an X-ray detector disposed in the X-ray beam path and configured to generate detector data. The calibration phantom is disposed in the X-ray beam path between the X-ray source and the X-ray detector.

[0018] The X-ray imaging system further includes an X-ray beam limiting device disposed in the X-ray beam path near the X-ray source, and the X-ray beam limiting device includes at least one calibration element disposed in the X-ray beam path. The X-ray imaging system acquires projection data of a set of projections based on the detector data, and includes an image processing circuit configured to determine the path lengths through at least one material of the at least one calibration element and at least one material of the calibration phantom, at least partially based on the acquired projection data, for performing calibration of material discrimination.

[0019] Another aspect is a method for calibrating material discrimination of an X-ray imaging system. The X-ray imaging system has an X-ray source, an X-ray detector, an X-ray beam limiting device disposed in the X-ray beam path near the X-ray source, and an image processing circuit unit. The X-ray beam limiting device includes at least one calibration element.

[0020] The method includes placing the calibration phantom in the X-ray beam path of the X-ray imaging system between the X-ray beam limiting device and the X-ray detector, starting a calibration sequence, and acquiring projection data of a set of projections based on the output of the X-ray detector. The method further includes determining, at least partially based on the acquired projection data, the path lengths through at least one material of the at least one calibration element and at least one material of the calibration phantom, and performing calibration of material discrimination based at least partially on the determined path lengths.

[0021] In the proposed technique, for the purpose of a new calibration procedure, a calibration phantom and an X-ray beam limiting device including one or more calibration elements can be used in combination, and the image processing circuit unit of the X-ray imaging system is configured to acquire projection data of a set of projections and determine the path lengths through at least one material of the calibration element and at least one material of the calibration phantom in order to perform calibration of material discrimination. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Various aspects of the present disclosure can be better understood by referring to the accompanying drawings and reading the following detailed description of the invention. [Figure 1A] It is a schematic diagram showing an exemplary X-ray imaging system. [Figure 1B] It is a schematic diagram showing an exemplary X-ray imaging system. [Figure 2]This is a schematic diagram showing other examples of X-ray imaging systems (such as CT imaging systems). [Figure 3] This is a schematic block diagram of a CT imaging system as an example of an X-ray imaging system. [Figure 4] This is a schematic diagram showing other examples of related components for X-ray imaging systems (such as CT imaging systems). [Figure 5] This is a schematic diagram of a photon counting circuit and / or apparatus according to an exemplary embodiment. [Figure 6] This is a schematic diagram showing an example of a semiconductor detector submodule according to an exemplary embodiment. [Figure 7] This is a schematic diagram showing an example of a semiconductor detector submodule according to another exemplary embodiment. [Figure 8A] This is a schematic diagram showing an example of a semiconductor detector submodule according to yet another exemplary embodiment. [Figure 8B] This is a schematic diagram showing an example of a set of arranged detector submodules, where each detector submodule is segmented in the depth direction, and an application-specific integrated circuit (ASIC) or corresponding circuit is located beneath the detector submodule as viewed from the direction of the incident X-rays. [Figure 9] This is a schematic diagram showing an example of a CT imaging system. [Figure 10] This is a schematic diagram showing an example design of an X-ray source and X-ray detector system. [Figure 11] This is a schematic diagram showing an example of an X-ray imaging system (such as a CT imaging system) that is suitable for calibration of material discrimination. [Figure 12] This is a schematic diagram showing an example of a specific related component of a CT imaging system, with the calibration phantom positioned for material discrimination calibration. [Figure 13A] This is a schematic diagram showing a cross-section of an example of a calibration phantom. [Figure 13B] Figure 13A is a schematic diagram showing a perspective view of the calibration phantom. [Figure 14]This is a schematic diagram showing a cross-section of another example of a calibration phantom. [Figure 15A-B] This is a schematic diagram showing a cross-section of an example of a pair of calibration phantoms. [Figure 16A-C] This is a schematic diagram illustrating an example of a specific relevant part of a CT imaging system, with the calibration phantom positioned for material discrimination calibration. [Figure 17] This is a schematic diagram showing an example of a specific relevant part of a CT imaging system adapted for material discrimination according to an exemplary embodiment. [Figure 18A-B] This is a schematic diagram showing an example of at least one calibration element. [Figure 19A-C] This is a schematic diagram showing an example of at least one calibration element. [Figure 20A] This is a schematic diagram showing an example of at least one calibration element and at least one filter. [Figure 20B] This is a schematic diagram showing an example of at least one calibration element. [Figure 21] This is a schematic flowchart illustrating an example of a calibration method for material discrimination in a CT imaging system. [Figure 22] This is a schematic diagram showing an example of a computer implementation according to one embodiment. [Modes for carrying out the invention]

[0023] Next, embodiments of the present disclosure will be described with reference to the figures, using examples.

[0024] For a better understanding, it is useful to continue with an introductory explanation of a non-limiting example of an entire X-ray imaging system capable of performing data processing and transfer in accordance with the concepts of the present invention.

[0025] Figure 2 is a schematic diagram showing an example of an X-ray imaging system 100 (such as a CT imaging system), which includes an X-ray source 10 that emits X-rays, an X-ray detector 20 that detects X-rays after they have passed through an object, an analog processing circuit 25 that processes and digitizes the raw electrical signals from the X-ray detector, a digital processing circuit 40 that can perform other processing operations on the measured data (such as applying corrections, temporarily storing data, and filtering), and a computer 50 that can store the processed data and perform further post-processing and / or image reconstruction. The digital processing circuit 40 may include a digital processor. According to an exemplary embodiment, all or part of the analog processing circuit 25 can be mounted on the X-ray detector 20. The X-ray source and X-ray detector can be coupled to a rotating member of the gantry 11 of the CT imaging system 100.

[0026] The entire X-ray detector can be considered as an X-ray detector system 20, or as an X-ray detector 20 combined with an associated analog processing circuit section 25.

[0027] The image processing system 30 communicates with and is electrically coupled to the analog processing circuit unit 25, and may include a digital processing circuit unit 40 and / or a computer 50, and may be configured to perform image reconstruction based on image data from the X-ray detector. Therefore, the image processing system 30 may be viewed as a computer 50, as a system combining the digital processing circuit unit 40 and the computer 50, or as the digital processing circuit unit 40 itself if the digital processing circuit unit is further specialized for image processing and / or reconstruction.

[0028] A commonly used example of an X-ray imaging system is a CT imaging system, which may include an X-ray source or X-ray tube that generates a fan beam or cone beam of X-rays, and an array of opposing X-ray detectors that measure the percentage of X-rays that have passed through a patient or object. The X-ray source or X-ray tube and the X-ray detectors are mounted on a gantry 11 that can rotate around the object being imaged.

[0029] Figure 3 schematically shows a CT imaging system 100 as an example of an X-ray imaging system. The CT imaging system includes a computer 50. The computer 50 receives commands and scanning parameters from the operator through an operator console 60 which may have a display 62 and some form of operator interface (e.g., a keyboard, mouse, joystick, touchscreen, or other input device). The commands and parameters supplied by the operator are used by the computer 50 to supply control signals to the X-ray controller 41, the gantry controller 42, and the table controller 43. Specifically, the X-ray controller 41 supplies power and timing signals to the X-ray source 10 to control the X-rays irradiated onto an object lying on the table 12 or a patient. The gantry controller 42 controls the rotation speed and rotation position of the gantry 11, which includes the X-ray source 10 and the X-ray detector 20. As an example, the X-ray detector 20 may be a photon counting type X-ray detector. The table controller 43 controls and determines the position of the patient table 12 and the patient scanning range. The system also includes a detector controller 44, which is configured to control the X-ray detector 20 and / or to receive data from the X-ray detector 20.

[0030] In one embodiment, the computer 50 also performs post-processing and image reconstruction of the image data output from the X-ray detector 20. Therefore, the computer 50 corresponds to the image processing system 30 shown in Figures 1 and 2. The operator can view the reconstructed image and other data from the computer 50 via the associated display 62.

[0031] An X-ray source 10 positioned in the gantry 11 emits X-rays. The X-ray detector 20 can be in the form of a photon-counting type X-ray detector and detects X-rays after they have passed through an object or patient. The X-ray detector 20 can be formed, for example, by a plurality of pixels (also called sensors or detector elements) and associated image processing circuitry (such as an application-specific integrated circuit (ASIC)) positioned in the detector module. Some of the analog processing is implemented in the pixels, and the remaining processing is implemented, for example, in the ASIC. In one embodiment, the image processing circuitry (ASIC) digitizes the analog signals from the pixels. The image processing circuitry (ASIC) may also include a digital processing unit that can perform further processing operations on the measurement data (such as applying corrections, temporarily storing the measurement data, and / or filtering it). During scanning to acquire X-ray projection data, the gantry and components attached to the gantry rotate around the isocenter 13.

[0032] Current X-ray detectors typically require the incident X-rays to be converted into electrons, usually by the photoelectric effect or Compton interaction. The resulting electrons then produce secondary visible light, which is detected by a photosensitive material until their energy is lost. There are also semiconductor detectors, in which case the electrons generated by the X-rays create electron-hole pairs, which are collected by applying an electric field.

[0033] There is a detector that operates in energy-integrating mode, which provides an integrated signal of a large number of X-rays. The output signal is proportional to the total energy imparted by the detected X-rays.

[0034] In medical X-ray applications, X-ray detectors with photon counting ability and energy resolution are becoming common. Photon counting detectors have the advantage of being able to measure the energy of each X-ray, thus providing additional information about the composition of an object. This information can be used to improve image quality and / or reduce radiation dose.

[0035] Generally, photon-counting X-ray detectors determine the energy of photons by comparing the height of the electrical pulses generated by photon interactions within the detector material to a set of comparator voltages. These comparator voltages are also called energy thresholds. Typically, the analog voltages of the comparator are set by a digital-to-analog converter (DAC). The DAC converts the digital settings transmitted from the controller into analog voltages that can be compared to the height of the photon pulses.

[0036] A photon counting detector counts the number of photons that interact with the detector during a measurement period. A new photon is generally identified by the fact that the height of an electrical pulse exceeds the comparator voltage of at least one comparator. Once a photon is identified, the event is recorded by incrementing a digital counter associated with the channel.

[0037] When multiple different thresholds are used, an energy-discriminating photon count detector is obtained. In an energy-discriminating photon count detector, detected photons can be classified into energy bins corresponding to various thresholds. This type of photon count detector is sometimes called a multi-bin detector. Generally, energy information can be used to create a new type of image, in which new information is available and image artifacts inherent in the prior art can be removed. In other words, in an energy-discriminating photon count detector, the pulse height is compared to N thresholds (T1-TN) programmable by a comparator and classified according to the pulse height. This pulse height is proportional to the energy. In other words, a photon count detector containing two or more comparators is called a multi-bin photon count detector. In the case of a multi-bin photon count detector, the photon count values ​​are stored in a set of counters (typically one counter for each energy threshold). For example, one count value can be assigned to the highest energy threshold that a photon pulse exceeds. In another example, the counters record the number of times the photon pulse exceeds each energy threshold.

[0038] For example, edge-on refers to a special, non-restrictive design for photon counting detectors, where the edges of the X-ray sensor (such as X-ray detector elements or pixels) are oriented towards the incident X-rays.

[0039] For example, such a photon counting detector may have pixels aligned in at least two directions, and one of the at least two directions of the edge-on photon counting detector has a component in the direction of X-rays. Such an edge-on photon counting detector may be called a depth-segmented photon counting detector, having two or more depth segments of pixels in the direction of incident X-rays. Note that one detector element may correspond to one pixel, and / or multiple detector elements may correspond to one pixel, and / or data signals from multiple detector elements may be used for one pixel.

[0040] Alternatively, the pixels may be arranged as an array substantially orthogonal to the direction of the incident X-rays (not segmented in the depth direction), and each pixel of the array may be positioned so that its edge is oriented toward the incident X-rays. In other words, the photon counting detector may not be segmented in the depth direction, but may be positioned so that its edge is oriented toward the incident X-rays.

[0041] By positioning the edge-on photon counting detector at the edge, absorption efficiency can be increased, the absorption depth can be selected to any length, and the edge-on photon counting detector can be completely depleted without requiring very high voltages.

[0042] The conventional mechanism for detecting X-ray photons using direct semiconductor detectors basically works as follows: The energy of the X-ray interaction in the detector material is converted into electron-hole pairs within the semiconductor detector, and the number of electron-hole pairs is roughly proportional to the photon energy. Electrons and holes drift toward (or backward from) the detector electrodes and backside. During this drift, electrons and holes induce an electric current in the electrodes, and this current can be measured.

[0043] As shown in Figure 4, the signal is transmitted through a path 26 from the detector element 22 of the X-ray detector to the input of the analog processing circuit (e.g., ASIC) 25. It should be understood that the term Application-Specific Integrated Circuit (ASIC) should be broadly interpreted as a general-purpose circuit used and configured for a specific application. The ASIC processes the charge generated from each X-ray, converts the processed charge into digital data, and uses the digital data to obtain measurement data (such as photon counts and / or estimated energy). Since the ASIC is configured to connect to the digital processing circuit, the digital data is sent to the digital processing circuit 40 and / or one or more memory circuits or components 45, and finally the data becomes the input to the image processing circuit 30 or computer 50 in Figure 2 to generate a reconstructed image.

[0044] Since the number of electrons and holes generated from a single X-ray event is proportional to the energy of the X-ray photon, the total charge of a single induced current pulse is proportional to that energy. After the filtering step in the ASIC, the pulse amplitude is proportional to the total charge of the current pulse and therefore proportional to the X-ray energy. The pulse amplitude can be measured by comparing its value to one or more thresholds (THRs) of one or more comparators (COMPs), and a counter can be used to record the number of times the pulse is greater than the threshold. In this way, the number of X-ray photons detected within a certain time frame that have energy exceeding the energy corresponding to each threshold (THR) can be counted and / or recorded.

[0045] ASICs typically sample an analog photon pulse once for each clock cycle and record the output of a comparator. The comparator (threshold) outputs either 1 or 0 depending on whether the analog signal is above or below the comparator voltage. The information available for each sample is, for example, the 1 or 0 of each comparator, which indicates whether the comparator was triggered (the photon pulse was greater than the threshold).

[0046] In photon counting detectors, there is typically a photon counting logic that determines whether a new photon has been recorded and records the photon in a counter. In multi-bin photon counting detectors, there are typically multiple counters, for example, one counter for each comparator, and the photon count value is recorded in the counter according to an estimate of the photon energy. The logic can be implemented in several different ways. Two of the most common categories of photon counting logic are non-paralyzed counting mode and paralyzed counting mode. Other photon counting logics include, for example, maximal detection, which counts the detected maximal in a voltage pulse and, in some cases, also records the pulse height of the detected maximal.

[0047] Photon counting detectors have many advantages (including, but not limited to, high spatial resolution, low sensitivity to electronic noise, excellent energy resolution, and material resolution (spectral imaging ability)). However, energy-integrating detectors have the advantage of high count rate tolerance. This count rate tolerance comes from the fact / recognition that because the total energy of the photons is measured, adding one photon always increases the output signal (within a reasonable range), regardless of the amount of photons currently recorded by the detector. This advantage is one of the main reasons why energy-integrating detectors have become the standard in medical CT today.

[0048] Figure 5 shows a schematic diagram of a photon counting circuit and / or photon counting device according to an exemplary embodiment.

[0049] When photons interact in a semiconductor material, a cloud of electron-hole pairs is generated. When an electric field is applied to the detector material, charge carriers are collected by electrodes attached to the detector material. The signal is transmitted from the detector element to the input of a parallel processing circuit (e.g., an ASIC). For example, the ASIC can process the charge such that it generates a voltage pulse with a maximum height proportional to the amount of energy imparted by the photons in the detector material.

[0050] The ASIC may include a set of comparators 302, each comparator 302 comparing the magnitude of a voltage pulse to a reference voltage. The comparator output is typically (0 / 1) depending on which of the two voltages being compared is greater. Here, the comparator output is 1 if the voltage pulse is higher than the reference voltage, and 0 if the reference voltage is higher than the voltage pulse. A digital-to-analog converter (DAC) 301 can be used to convert a digital setting, which can be supplied by the user or a control program, into a reference voltage that can be used by the comparators 302. We say that a comparator is triggered when the height of a voltage pulse exceeds the reference voltage of a particular comparator. Each comparator is generally associated with a digital counter 303, which is incremented based on the comparator output according to photon counting logic.

[0051] As mentioned above, the estimated basis coefficient line integral values ​​obtained for each projection line

number

[0052] It is understood that the mechanisms and configurations described herein can be implemented, combined, and rearranged in various ways.

[0053] For example, the embodiment may be implemented in hardware, or at least partially in software and executed by a suitable image processing circuit, or a combination thereof.

[0054] The steps, functions, procedures, and / or blocks described herein can be implemented in hardware using prior art (such as discrete circuit or integrated circuit technology, including both general-purpose and application-specific circuit components).

[0055] Alternatively or complementary, at least some of the steps, functions, procedures, and / or blocks described herein can be implemented in software, such as a computer program, executed by a suitable image processing circuit (one or more processors or processing units).

[0056] The following describes non-limiting examples of specific detector module implementations. More specifically, these examples represent edge-on oriented detector modules and depth-segmented detector modules. Other types of detectors and detector modules are also possible.

[0057] Figure 6 is a schematic diagram showing an example of a semiconductor detector submodule according to an exemplary embodiment. This is an example of a detector module 21 comprising a semiconductor sensor having multiple detector elements or pixels 22, each detector element (or pixel) typically based on a diode having a charge collection electrode as its main component. X-rays are incident from the edge of the detector module.

[0058] Figure 7 is a schematic diagram showing an example of a semiconductor detector submodule according to another exemplary embodiment. In this example, the detector module 21 having a semiconductor sensor is also divided into multiple depth segments or detector elements 22 in the depth direction, assuming that X-rays are incident from the edge of the detector module.

[0059] Typically, detector elements are individual X-ray-sensitive sub-elements of a detector. Generally, photon interactions occur within the detector elements, and the resulting charges are collected by the corresponding electrodes of the detector elements.

[0060] Each detector element typically measures the incident X-ray beam as a series of frames. A frame is a set of measurement data at a specified time interval called the frame time.

[0061] Depending on the detector topology, particularly if the detector is a flat-panel detector, one detector element may correspond to one pixel. A depth-segmented detector can be considered to have several detector strips, each strip having several depth segments. In such a depth-segmented detector, each depth segment can be considered a separate detector element, especially if each of the multiple depth segments is associated with its own separate charge collection electrode.

[0062] Detector strips in depth-segmented detectors are sometimes called pixel strips because they typically correspond to pixels in a standard flat-panel detector. However, depth-segmented detectors can also be viewed as three-dimensional pixel arrays, in which case each pixel corresponds to an individual depth segment / detector element.

[0063] A semiconductor sensor can be implemented as a so-called multi-chip module (MCM) in the sense that it is used as a base substrate for electrical wiring and, preferably, as a base substrate for multiple ASICs mounted by so-called flip-chip technology. The wiring includes signal connections from each pixel or detector element to the input of the ASIC, and connections from the ASIC to external memory and / or digital data processing. Power to the ASIC can be supplied through similar wiring, taking into consideration increasing the cross-sectional area to allow high currents to flow through these connections, although this power may be supplied through separate connections. The ASIC can be positioned alongside the active sensor, which means that the ASIC can be protected from incident X-rays by placing an absorbent cover on top of the ASIC, and also from scattered X-rays by placing an absorber in that direction to protect against scattered X-rays from the side.

[0064] Figure 8A is a schematic diagram showing a detector module implemented as an MCM similar to the embodiment of U.S. Patent No. 8,183,535. This example shows how the semiconductor sensor 21 can also function as the substrate of the MCM. The signal is transmitted by a path 23 from the detector element 22 to the input of a parallel processing circuit 24 (e.g., an ASIC) located next to the active sensor area. The ASIC processes the charge generated from each X-ray and converts the processed charge into digital data. The digital data can be used to detect photons and / or estimate the energy of photons. The ASIC may have its own digital processing circuit and memory to perform simple tasks. The ASIC can then be configured to connect to a digital processing circuit and / or memory circuit or component located outside the MCM, and finally, the data is used as input for reconstructing the image.

[0065] However, employing depth segmentation presents two significant challenges for silicon-based photon counting detectors. First, a large number of ASIC channels must be used to process the data supplied from the relevant detector segments. The small pixel size and the use of depth segmentation increase the number of channels, and the multi-energy bins further increase the data size. Second, because the count of a given X-ray input is divided into small pixels, segments, and energy bins, the signal in each bin is very low, and calibration / correction of the detector requires several orders of magnitude more calibration data to minimize statistical uncertainty.

[0066] Naturally, as data size increases by several orders of magnitude, in addition to requiring larger computing resources, hard disks, memory, and central processing units (CPUs) or graphics processing units (GPUs), both data handling and preprocessing become slower. For example, if the data size increases from 10 megabytes to 10 gigabytes, the data handling time (reading and writing) can increase by 1000 times.

[0067] A problem with counting-type X-ray photon detectors is pile-up. When the flux rate of X-ray photons is high, there can be problems distinguishing between two consecutive charge pulses. As mentioned earlier, the filtered pulse length depends on the shaping time. If this pulse length is longer than the time between two charge pulses induced by the X-ray photons, the pulse becomes one, and the two photons cannot be distinguished and are counted as one pulse. This is called pile-up. Therefore, one way to avoid pile-up at high flux is to use a short shaping time or to use depth segmentation.

[0068] To generate pile-up calibration vectors, the pile-up calibration data must be preprocessed for spit correction. When generating material discrimination vectors, it is preferable to preprocess the material discrimination data for both spit correction and pile-up correction. In the case of patient scan data, this data must be preprocessed for spit, pile-up, and material discrimination before image reconstruction. Note that these examples are simplified examples to illustrate preprocessing, and actual preprocessing steps may include several other calibration steps (such as reference normalization and air calibration) as needed. The term "processing" may refer only to each calibration vector generation process or the final step in a patient scan, but this term may be used interchangeably in some cases.

[0069] Figure 8B is a schematic diagram showing an example of an arranged set of detector submodules, each of which is a detector submodule segmented in the depth direction, and the ASIC or corresponding circuit 24 is positioned below the detector element 22 as viewed from the direction of the incident X-rays, and a path 23 from the detector element 22 to the parallel processing circuit 24 (e.g., ASIC) can be defined in the space between the detector element 22 and the detector element.

[0070] Figure 9 is a schematic diagram showing an overall example of a CT imaging system. In this schematic example, the CT imaging system 100 includes a gantry 111 and a patient table 112. The patient table 112 can be inserted into the opening 114 of the gantry 111 during patient scanning and / or calibration scanning. The z-direction represents the direction of the axis of rotation of the rotating member of the gantry around the subject or patient being imaged. The x-direction represents the angular direction of the CT imaging system, and the y-direction represents the direction of the incident X-rays.

[0071] However, it should be understood that the rotating and stationary members of the gantry do not necessarily have to be part of the CT system and may be in other arrangements and / or configurations (for example, moving linearly and / or translationally relative to each other without rotation). For example, the combination of the X-ray source and detector can be moved linearly and / or translationally relative to the stationary members of the entire gantry. For example, the X-ray source and detector can be moved together as a single assembly unit along the table axis (commonly called the z axis). Alternatively, the patient table may move, but the combination of the X-ray source and detector may remain stationary, where relative movement is important. This includes, for example, geometric system configurations that allow the patient to stand in a so-called telephone booth type scanner.

[0072] Figure 10 is a schematic diagram showing an example of an overall design of an X-ray source-detector system. This example shows a schematic diagram of an X-ray detector containing multiple detector modules and an X-ray source that emits X-rays. Each detector module may have a set of detector elements that define a corresponding pixel. For example, the detector modules may be arranged in a row, with their edges facing the X-ray source, making them edge-on detector modules, and they may be arranged in a slightly curved manner overall. As mentioned above, the direction of the incident X-rays is called the Y direction. Multiple detector pixels in the rotation axis direction of the gantry (called the z direction) can be used to acquire multi-slice images. Also, multiple detector pixels in the angular direction (called the x direction) can be used to simultaneously measure multiple projections on the same plane, which is applied in fan / cone-beam CT. The x direction is sometimes called the channel direction. Most detectors have detector pixels arranged in both the slice (z) direction and the angular (x) direction.

[0073] Figure 11 is a schematic diagram showing an example of an X-ray imaging system (such as a CT imaging system adapted for calibration of material discrimination according to an exemplary embodiment). In this example, the X-ray imaging system 100 includes an X-ray source 110 configured to emit X-rays and an X-ray detector 120 positioned in the X-ray beam path, configured to generate detector data. A calibration phantom 150 is positioned in the X-ray beam path between the X-ray source 110 and the X-ray detector 120.

[0074] The X-ray imaging system 100 further includes an X-ray beam limiter 130 positioned in the X-ray beam path near the X-ray source 110, the X-ray beam limiter 130 including at least one calibration element 135 positioned in the X-ray beam path. The at least one calibration element 135 may include a calibration slab similar to the slab used in the calibration phantom. The at least one calibration element 135 may include a material present in the calibration phantom 150 and / or a material not present in the calibration phantom 150. The material of the at least one calibration element 135 and / or the calibration phantom 150 may be a single atom (such as iodine) or a combination of different atoms. Thus, the material can be a composition of atoms having specific properties. For example, the material may be a composition of multiple atoms that mimic the properties of a single atom. This can be a more convenient and / or cheaper alternative than using a rare and expensive single-atom material.

[0075] The X-ray imaging system 100 also includes an image processing circuit unit 140. In this embodiment, the image processing circuit unit 140 includes a material discrimination (MD) calibration module 142, which is configured and / or pre-programmed to acquire projection data of a set of projections based on detector data, determine the path length through at least one material of at least one calibration element and at least one material of the calibration phantom, at least in part based on the acquired projection data, and perform material discrimination calibration.

[0076] As an example, the X-ray imaging system may be a CT imaging system (for example, a CT imaging system schematically shown in Figure 12). Figure 12 is a schematic diagram showing an example of a specific related component of a CT imaging system, with a calibration phantom shown for material discrimination calibration. As previously mentioned, the CT imaging system includes an X-ray source 110 and an X-ray detector 120 arranged to acquire projected images of a subject or object at different field angles. This is most commonly achieved by mounting the X-ray source 110 and the X-ray detector 120 on a support (e.g., a rotating member of a gantry) that can rotate around the subject or object (a calibration phantom 150 in this example).

[0077] For example, the X-ray beam limiting device 130 can be part of a pre-collimator positioned relative to the X-ray source 110.

[0078] In certain embodiments, at least one calibration element 135 of the X-ray beam limiting device 130 and the calibration phantom 150 include at least two different materials through which at least some X-rays pass, so that material discrimination calibration can be performed.

[0079] Optionally, at least one calibration element 135 includes at least one surface. This example will be explained in detail later.

[0080] For example, at least one calibration element 135 includes a first portion having a first predetermined thickness T1 and a second portion having a second predetermined thickness T2.

[0081] In a particular example, at least one calibration element includes a first calibration element having a first material M1, and the image processing circuit is configured to determine the path length passing through the first material M1.

[0082] Preferably, the X-ray imaging system 100 and / or the X-ray beam limiting device 130 further includes a second calibration element having a second material M2, wherein the second material M2 is different from the first material M1, and the image processing circuit is configured to determine the path length passing through the second material M2.

[0083] In non-limiting examples, the calibration element 135 may include a high-density material (e.g., iodine). This makes it possible to provide a convenient, efficient, and / or versatile setup for material discrimination using the X-ray imaging system according to the present invention.

[0084] Optionally, the X-ray beam limiting device 130 includes a motor configured to move at least one calibration element relative to the X-ray beam path.

[0085] As an example, the X-ray beam limiting device 130 further includes a bowtie filter and / or a hardened filter. Thus, the X-ray beam limiting device may include a filter and at least one calibration element 135.

[0086] In a practical exemplary embodiment, the X-ray imaging system 100 is configured such that, during calibration, if the calibration phantom 150 is placed in the X-ray beam path between the X-ray beam limiter 130 and the X-ray detector 120, the emitted X-rays are irradiated onto the calibration phantom 150.

[0087] In another non-limiting example, the X-ray imaging system 100 is used with multiple calibration phantoms 150. For example, two calibration phantoms can be placed in the X-ray beam path and moved / rotated relative to the X-ray beam path. For example, by placing one of the two calibration phantoms on top of the first calibration phantom and / or using it independently, material space can be sampled more accurately for combinations that occur in imaging tasks of small objects (infants, heads, etc.).

[0088] In actual embodiments, the X-ray imaging system 100 can also be considered to include a calibration phantom 150.

[0089] For example, calibration phantom 150 may contain a first phantom material PM1, the first phantom material PM1 being different from at least one material of at least one calibration element.

[0090] As another example, a calibration phantom may include a first phantom substance PM1 and a second phantom substance PM2, and at least one calibration element may include a first substance M1 and a second substance M2, where the second substance M2 is the same as the first phantom substance PM1 or the second phantom substance PM2.

[0091] For example, the calibration phantom 150 includes a combination of geometric objects of at least two different shapes and / or materials, the combination of which includes a first geometric object located in the center and containing a first phantom material PM1, and a plurality of second geometric objects arranged around the first geometric object. At least one subset of the plurality of second geometric objects contains a second phantom material PM2 different from the first phantom material PM1, and the first geometric object is relatively larger than the second geometric objects.

[0092] Figure 13A is a schematic diagram showing a cross-section of an example of a calibration phantom. Figure 13B is a schematic diagram showing a perspective view of the calibration phantom of Figure 13A. The calibration phantom may include a column having a circular, oval, or elliptical cross-section. The calibration phantom may include at least one rod having a diameter in the range of 10 cm to 30 cm (preferably 15 cm to 25 cm, more preferably 20 cm). The calibration phantom may also include multiple rods surrounding at least one rod, having a diameter in the range of 1 cm to 9 cm (preferably 3 cm to 7 cm, more preferably 5 cm).

[0093] In certain non-limiting examples, the calibration phantom 150 may include a first phantom material PM1, the first phantom material PM1 may be the same as at least one material of at least one calibration element (e.g., different in shape and / or thickness).

[0094] Figure 14 is a schematic diagram showing another example of a calibration phantom in cross-section. In this specific, non-limiting example, the calibration phantom 150 may include a combination of geometric objects of at least three different shapes and / or materials. For example, the calibration may include a plurality of geometric objects 150c (third geometric objects) arranged around at least one geometric object 150a (first geometric object) and / or around at least one subset of another plurality of geometric objects 150b (second geometric objects). At least one subset of the plurality of third geometric objects 150c includes a third phantom material PM3 that is different from the first phantom material PM1 and the second phantom material PM2. The third geometric objects 150c are relatively smaller than the second geometric objects 150b. The first, second, and third geometric objects may be prisms having circular, oval, or elliptical cross-sections.

[0095] Optionally, the X-ray imaging system further includes a movable platform positioned in the X-ray beam path between the X-ray beam limiter and the X-ray detector, the platform being configured to hold the calibration phantom 150.

[0096] Furthermore, the X-ray system can be configured to perform material discrimination calibration based on a mapping between i) determining the path length passing through at least one calibration element and calibration phantom 150, and ii) the corresponding detector response of the X-ray detector.

[0097] As an example, the X-ray imaging system may be a computed tomography (CT) system including a movable assembly, in which the X-ray source 110, X-ray detector 120, and X-ray beam limiter 130 are located in the movable assembly.

[0098] Refer again to Figure 12. An example of a specific relevant component of a CT imaging system is shown, with a calibration phantom positioned for calibration. For example, such a CT imaging system allows for material discrimination calibration based on a mapping between i) determining the path length through at least one calibration element 135 and calibration phantom 150 for each of the multiple rotation angles of the movable assembly of the CT imaging system and each of the multiple detector elements of the X-ray detector 120, and ii) the corresponding detector response of the X-ray detector 120.

[0099] For example, a CT imaging system can be configured to generate detector data at multiple angles, and a movable assembly can be configured to move to a set of predetermined angles, stop at each angle, and generate detector data at each stopping angle.

[0100] Optionally, the X-ray detector can be a photon-counting type multi-energy bin X-ray detector.

[0101] Figure 15A is a schematic diagram showing an example of a cross-section of a pair of calibration phantoms. In this example, the calibration phantom 150 is used in an X-ray imaging system configured for material discrimination calibration according to the present invention. The calibration phantom 150 includes a first geometric object 150a, which is centrally located and surrounded by a plurality of second geometric objects 150b, the plurality of second geometric objects 150b arranged around the first geometric object 150a. The plurality of second geometric objects 150b can be smaller than the first geometric object 150a. Thus, each second geometric object 150b provides a shorter path length than the first geometric object 150a. The first and second geometric objects can be prisms having a circular, oval, or elliptical cross-section. The first geometric object 150a contains a first phantom material PM1. The multiple second geometric objects 150b may contain the first phantom substance PM1 and / or the second phantom substance PM2. For example, one of the multiple second geometric objects 150b may contain the first phantom substance PM1, and another of the multiple second geometric objects 150b may contain the second phantom substance PM2. PM2 may have a higher density than PM1, for example.

[0102] In Figure 15A, the calibration phantom 150 includes a first geometric object 150a containing a first phantom substance PM1, and a plurality of second geometric objects 150b containing either the first phantom substance PM1 or the second phantom substance PM2. Specifically, the plurality of second geometric objects 150b are arranged around the first geometric object 150a, with every other second geometric object 150b containing the first phantom substance PM1 and the remaining second geometric objects 150b containing the second phantom substance PM2. The first phantom substance PM1 can be polyethylene (PE). The second phantom substance PM2 can be polyvinyl chloride (PVC). In alternative embodiments, the plurality of second geometric objects 150b may not contain the first phantom substance PM1, all second geometric objects 150b may contain the second phantom substance PM2, or all second geometric objects 150b may contain the first phantom substance PM1. It should be understood that the second geometric object 150b may contain one or more different substances.

[0103] Figure 15B is a schematic diagram showing an example of a cross-section of a pair of calibration phantoms. In this example, the calibration phantom 150 is used in an X-ray imaging system configured for material discrimination calibration. The calibration phantom 150 in Figure 15B has several features in common with the calibration phantom 150 shown in Figure 15A. In Figure 15B, one or more of the second geometric objects, including the first geometric object 150a and / or a plurality of second geometric objects 150b, have an oval and / or elliptical shape. The first and second geometric objects can be prisms having a circular, oval, or elliptical cross-section. The oval and / or elliptical shapes of the geometric objects are similar to those of the human body, and therefore this shape can produce similar X-ray scattering. The calibration phantom 150 shown in Figure 15B can provide good and / or uniform coverage. It should be understood that the second geometric objects 150b can include one or more different shapes and / or materials.

[0104] Figures 16A, 16B, and 16C are schematic diagrams showing examples of specific relevant components of a CT imaging system, with a calibration phantom positioned for calibration. The CT imaging system 100 includes an X-ray source 110, an X-ray detector 120, and an X-ray beam limiter 130, the X-ray source 110, the X-ray detector 120, and the X-ray beam limiter 130, which are housed in a movable assembly. The movable assembly of the X-ray imaging system can be a movable assembly of a gantry. The CT imaging system 100 performs material discrimination calibration based on a mapping between i) determining the path length through at least one calibration element (not shown) and a calibration phantom 150 for each of the multiple rotation angles of the movable assembly of the CT imaging system 100 and each of the multiple detector elements of the X-ray detector, and ii) the corresponding detector response of the X-ray detector 120. Therefore, the X-ray source 110, the X-ray detector 120, and the X-ray beam limiter 130 can be moved in a synchronized manner so that detector data is generated at different rotation angles around the calibration phantom 150. The movement of the movable assembly, and thus the movement of the X-ray source 110, the X-ray detector 120, and the X-ray beam limiter 130, can be performed in a controlled manner. This movement can be controlled, for example, by the image processing circuit section of the CT imaging system 100 and / or any computer included in the CT imaging system 100.

[0105] During calibration, the X-ray source 110, the X-ray beam limiter 130, and the X-ray detector 120 rotate as they do during normal scanning, rotating each position as much as possible to collect a large number of statistics. For each detector element, different combinations of calibration element and calibration phantom material path lengths are used at different field angles or rotation angles.

[0106] In Figure 16A, the CT imaging system 100 is configured to continuously generate detector data, and the movable assembly is configured to move continuously, so the CT imaging system can continuously generate detector data from different rotation angles of the movable assembly.

[0107] In Figure 16B, the CT imaging system 100 is configured to generate detector data at multiple angles, and the movable assembly is configured to move to a set of predetermined angles, stop at each angle, and generate detector data at each stop angle. In other words, the X-ray source 110, X-ray detector 120, and X-ray beam limiter 130 can move stepwise to multiple positions and acquire projection data at each step. The X-ray source 110, X-ray detector 120, and X-ray beam limiter 130 stop at each position, and the X-ray source 110 irradiates the calibration phantom 150 with X-rays. The X-ray beam path passes through at least one calibration element (not shown), and the X-ray detector 120 generates a detector response. The step size can be equal angles (e.g., every 6 degrees) and / or predetermined angles optimized for the shape and position of the calibration phantom 150. At each angle, data from multiple views can be collected and / or averaged to reduce statistical variability.

[0108] In Figure 16C, the CT imaging system 100 can be used with two or more calibration phantoms 150. The CT imaging system can be configured to irradiate two or more calibration phantoms 150. Here in Figure 16C, two calibration phantoms 150 are being imaged by the CT imaging system 100. The two calibration phantoms 150 may be different in material, size, and / or shape, or they may be the same.

[0109] Figure 17 is a schematic diagram showing an example of specific relevant components of a CT imaging system adapted to material discrimination according to an exemplary embodiment, in which a calibration phantom 150 is positioned to calibrate the CT imaging system 100. In this example, the calibration phantom 150 can be moved relative to the X-ray beam path in the horizontal and / or vertical plane, and the detector elements of the X-ray detector 120 can acquire X-rays with varying path lengths passing through the material of the calibration phantom 150 and at least one calibration element 135. In other words, the calibration phantom 150 can be moved toward the periphery of the X-ray beam path (i.e., the field of view), so that the edge detector elements have varying path lengths for different rotation angles. To move the calibration phantom 150, the X-ray imaging system 100 may include a mechanism configured to shift / move the calibration phantom 150 in the horizontal and / or vertical plane. The mechanism performing the movement may be a movement mechanism incorporated into the patient / table of the CT imaging system or may be another mechanical mechanism.

[0110] If the calibration phantom 150 is smaller than the full field of view, as schematically shown in Figure 17, only air will be present at the edge detector if the phantom is positioned only at the isocenter. To address this, the movement mechanism of the CT imaging system 100 can move / shift the calibration phantom 150.

[0111] Furthermore, the calibration phantom 150 can be rotated relative to the X-ray source 110, the X-ray detector 120, and the X-ray beam limiter 130. For example, the calibration phantom 150 can be rotated around an axis passing through its center and / or around an axis outside of it. This allows the X-ray imaging system 100 to acquire projection data at different angles without moving the X-ray source 110, the X-ray detector 120, and / or the X-ray beam limiter 130. In other words, instead of the X-ray source, X-ray detector, and X-ray beam limiter, the X-ray imaging system can acquire projection data by rotating the calibration phantom 150 to a predetermined number of angles, stopping the calibration phantom 150 at each angle, and at each stopping angle, the X-ray detector generates a detector response. The acquired projection data is at least partially based on the detector response.

[0112] Figures 18A and 18B are schematic diagrams showing an example of at least one calibration element 135. At least one calibration element 135 can be placed in an X-ray beam limiting device and may include different parts or components having different shapes, thicknesses, and / or materials. Calibration element 135 may include a first part 135-1 having a first predetermined thickness T1, a second part 135-2 having a second predetermined thickness T2, and a third part 135-3 having a third predetermined thickness T3. The first part 135-1, the second part 135-2, and the third part 135-3 may have different thicknesses, shapes, and / or materials. In this example, T1, T2, and T3 are different from each other.

[0113] The calibration element 135 can be moved in and out of the X-ray beam path. For example, the calibration element 135 can be moved relative to the X-ray beam limiter and the X-ray beam path. The movement of the calibration element 135 relative to the X-ray beam limiter and the X-ray beam path can be performed by a motor located within the X-ray beam limiter or a motor connected to the X-ray beam limiter.

[0114] In Figure 18A, calibration elements 135-1, 135-2, and 135-3 are located outside the X-ray beam path. Therefore, no X-rays pass through calibration elements 135-1, 135-2, and 135-3.

[0115] In Figure 18B, calibration element 135-2 is moved along the X-ray beam path, and only the second portion 135-2 is present in the X-ray beam path. As a result, the X-rays pass through the second portion 135-2 and not through the first portion 134-1 and the third portion 135-3. Calibration element 135 can move in and out of the X-ray beam path so that one of the first portion 135-1, the second portion 135-2, and the third portion 135-3 is present in the X-ray beam path. Thus, the CT imaging system can be controlled and tunable to allow X-rays to pass through components of different thicknesses. All parts of the calibration element can be moved as a single assembly. The first portion 135-1, the second portion 135-2, and the third portion 135-3 may each contain the same material, or they may each contain different materials.

[0116] Figures 19A, 19B, and 19C are schematic diagrams illustrating examples of calibration elements. At least one calibration element 135 can include multiple calibration elements 135. For example, Figures 19A, 19B, and 19C show multiple calibration elements including a first calibration element 135a, a second calibration element 135b, and a third calibration element 135c. Each component of the multiple calibration elements 135a, 135b, and 135c can consist of multiple different materials, thicknesses, and / or shapes. For example, the first calibration element 135a may contain a first material, the second calibration element 135b may contain a second material different from the first material, and the third calibration element 137c may contain a third material different from the first and second materials. In other exemplary embodiments, the first, second, and third materials may all be the same material or may consist of two different materials. As an example, at least one calibration element may contain a high-density material (e.g., iodine), at least one calibration element may contain polyethylene (PE), and at least one calibration element may contain polyvinyl chloride (PVC). As another example, at least one calibration element may have different thicknesses; for example, at least one calibration element may have a first thickness, at least one calibration element may have a second thickness, and at least one calibration element may have a third thickness. The first, second, and third thicknesses may be the same or different. The third calibration element 135c may contain the first material, but may have a different thickness than the first calibration element 135a. The different calibration elements 135a, 135b, and 135c can be moved in and out of the X-ray beam path. This allows for the option of changing the X-ray path length by different materials, thicknesses, and / or shapes. In Figure 19A, only the third calibration element 135c is present in the X-ray beam path. In Figure 19B, only the second calibration element 135b and the third calibration element 135c are present in the X-ray beam path. In Figure 19C, only the first calibration element 135a and the third calibration element 135c are present in the X-ray beam path.

[0117] Figure 20A is a schematic diagram showing an example of at least one calibration element 135 and at least one filter 137. In the example of Figure 20A, the X-ray beam limiting device includes a filter 137 (e.g., a bowtie filter), and at least one calibration element 135 includes a first calibration element 135a, a second calibration element 135b, and a third calibration element 135c. The calibration elements 135a, 135b, and 135c may differ in material, thickness, and / or shape. Each of the calibration elements 135a, 135b, and 135c may have a thickness corresponding to each calibration element and a similar shape. The first calibration element 135a and the third calibration element 135c may include a first material. The second calibration element 135b may include a second material different from the first material. The filter 137 includes a filter material different from the first and second materials. The material and shape of filter 137 are selected to modulate the incident X-ray beam as a function of the angle of the X-ray beam with respect to the object / subject, thereby balancing the X-ray photon flux in the X-ray detector.

[0118] Figure 20B is a schematic diagram showing an example of at least one calibration element 135. At least one calibration element 135 includes a first calibration element 135a, a second calibration element 135b, and a third calibration element 135c, each calibration element including a curved surface 136. The curved surfaces 136 of calibration elements 135a, 135b, and 135c can be configured to contribute equally to the path length for all detector pixels. This is considered particularly advantageous for CT imaging systems utilizing fan beams.

[0119] Figure 21 is a schematic flowchart illustrating an example of a method for calibrating material discrimination in a CT imaging system. The CT imaging system comprises an X-ray source configured to emit X-rays, an X-ray detector, an X-ray beam limiter positioned in the X-ray beam path near the X-ray source, and an image processing circuit. The X-ray beam limiter includes at least one calibration element. The method in Figure 21 includes a first step S1 of placing a calibration phantom in the X-ray beam path of the CT imaging system between the X-ray beam limiter and the X-ray detector. A second step S2 is to start the calibration sequence. A third step S3 is to acquire projection data of a set of projections based on the output of the X-ray detector. A fourth step S4 is to determine the path length through at least one material of at least one calibration element and at least one material of the calibration phantom, at least partially based on the acquired projection data. A fifth step S5 is to perform material discrimination calibration, at least partially based on the determined path length.

[0120] As an example, the step of determining the path length includes determining the path length through at least one first material of the calibration element, the second material and the third material of the calibration phantom for each rotation angle of a plurality of rotation angles and each detector element of a plurality of detector elements of the X-ray detector.

[0121] In certain examples, the calibration step for material discrimination includes generating a mapping between path length and the detector response of the X-ray detector.

[0122] For example, mapping is used for calibrated image reconstruction.

[0123] Optionally, the X-ray detector is a photon-counting multi-bin X-ray detector, and the execution step includes determining an intrinsic mapping of the detector elements between the path lengths of different materials and the recorded photon count values ​​of the photon-counting multi-energy-bin X-ray detector corresponding to the path lengths of the different materials.

[0124] In certain embodiments, the step of acquiring projection data includes moving the X-ray source, X-ray detector, and X-ray beam limiter to multiple positions, stopping the X-ray source, X-ray detector, and X-ray beam limiter at each position, and allowing the X-ray detector to generate a detector response at each stopping position. The acquired projection data is at least partially based on the average and / or cumulative values ​​of the detector responses. The advantage of this is that the input data size for material discrimination calibration is reduced, and therefore the execution time for material discrimination calibration is shortened. Furthermore, this step-and-shoot method has the advantage of being a static scan and unaffected by rotational motion.

[0125] Alternatively, the step of acquiring projection data may involve, instead of using an X-ray source, X-ray detector, and X-ray beam limiter, rotating a calibration phantom to a predetermined number of angles, stopping the calibration phantom at each angle, and causing the X-ray detector to generate a detector response at each stopping angle. The acquired projection data is at least partially based on the detector response. If there is focal movement in the X-ray tube and the detector has its own angle-dependent movement, and this movement has a significant impact on the scan, this can be resolved by rotating the phantom.

[0126] As described above, at least some of the steps, functions, procedures, and / or blocks described herein can be implemented in software (such as a computer program) and executed by a suitable image processing circuit (such as one or more processors or processing units).

[0127] Figure 22 is a schematic diagram showing an example of a computer implementation according to one embodiment. In this particular example, system 200 includes a processor 210 and memory 220, the memory containing instructions that can be executed by the processor, thereby enabling the processor to operate to perform the steps and / or operations described herein. The instructions are typically configured as computer programs 225;235 and may be pre-configured in memory 220 or downloaded from an external memory device 230. Optionally, system 200 includes an input / output interface 240 that can be connected to the processor 210 and / or memory 220 to allow relevant data (such as input parameters and / or resulting output parameters) to be input and / or output.

[0128] In certain embodiments, the memory 220 includes a set of instructions executable by the processor, thereby enabling the processor to operate to perform the steps and / or operations described herein.

[0129] The term "processor" should be interpreted in a general sense as a system or device capable of executing program code or computer program instructions to perform a specific processing, decision, or computational task.

[0130] Therefore, an image processing circuit unit including one or more processors is configured to perform clearly defined processing tasks, such as those described herein, when a computer program is executed.

[0131] The image processing circuit does not need to be specialized solely for executing the steps, functions, procedures, and / or blocks described above, and may perform other tasks.

[0132] Furthermore, this technology also provides a computer program product that includes a computer-readable medium 220;230 on which such a computer program is stored.

[0133] For example, software or computer programs 225;235 can be implemented as computer program products, typically supported or stored on computer-readable media 220;230 (in particular, non-volatile media). Computer-readable media include, but are not limited to, one or more removable or non-removable memory devices (including, but not limited to, read-only memory (ROM), random-access memory (RAM), compact discs (CDs), digital versatile discs (DVDs), Blu-ray discs, universal serial bus (USB) memory, hard disk drives (HDDs), flash memory, magnetic tape, or any other conventional memory devices). Thus, computer programs can be loaded into the operating memory of a computer or equivalent processing unit and executed by its image processing circuitry.

[0134] A flow of a method can be considered a flow of computer operation when it is executed by one or more processors. A corresponding device, system, and / or machine can be defined as a group of functional modules, where each step executed by a processor corresponds to a functional module. In this case, the functional modules are implemented as computer programs executed by the processors. Therefore, a device, system, and / or machine can, alternatively, be defined as a group of functional modules, where each functional module can be implemented as a computer program executed by at least one processor.

[0135] Therefore, a computer program residing in memory can be organized into a suitable functional module configured to perform at least some of the steps and / or tasks described herein when executed by a processor.

[0136] Alternatively, the majority of the module could be implemented using hardware modules, or even replaced entirely with hardware. The choice between software and hardware is purely an implementation choice.

[0137] The embodiments of this disclosure shown in the drawings and described above are illustrative embodiments and are not intended to limit the scope of the claims (including equivalents included in the claims). Those skilled in the art will understand that various modifications, combinations, and changes can be made to the embodiments without departing from the scope defined by the claims. Any combination of non-exclusive features described herein is intended to be within the scope of the invention. That is, features of the described embodiments can be combined with any suitable embodiment described above, and any feature of one embodiment can be combined with another suitable embodiment. Similarly, features described in multiple dependent claims can be combined with non-exclusive features of other dependent claims, particularly when the multiple dependent claims depend on the same independent claim. While single-claim dependency has been used in practice and some jurisdictions require single-claim dependency, this does not mean that the features of multiple dependent claims are mutually exclusive.

[0138] Furthermore, it should be noted that the concepts of the present invention relate to all possible combinations of features unless otherwise explicitly stated. In particular, different partial solutions in different embodiments can be combined in other configurations, where technically possible. [Explanation of symbols]

[0139] 10 X-ray source 11 Gantry 13 Isocenter 23 routes 25 Analog Processing Circuit Section 25ASIC 26 routes 40 Digital Processing Circuit Section 41 X-ray controller 42 Gantry Controller 43 Table Controller 44 Detector Controller 45 components 50 Computers 60 Operator Console 62 displays 110 X-ray source 111 Gantry 112 Patient Table 114 Opening 120 X-ray detectors 130 X-ray beam limiting device 134 Part 1 136 Curved surface 137 Filters 140 Image Processing Circuit Section 142 Calibration Module 150 Calibration Phantom 200 Systems 210 processors 225 Computer Programs 230 External memory devices 240 output interfaces 301 Digital-to-Analog Converter (DAC) 303 Digital Counter

Claims

1. An X-ray imaging system configured for calibration of material discrimination and usable with a calibration phantom, wherein the X-ray imaging system is An X-ray source configured to emit X-rays, X-ray detectors positioned in the X-ray beam path and configured to generate detector data. Includes, The calibration phantom is positioned between the X-ray source and the X-ray detector, and is placed in the X-ray beam path. The aforementioned X-ray imaging system is An X-ray beam limiting device positioned near the X-ray source and in the X-ray beam path, wherein the X-ray beam limiting device includes at least one calibration element positioned in the X-ray beam path. An image processing circuit is configured to acquire projection data of a set of projections based on the detector data and to determine the path length passing through at least one material of the at least one calibration element and at least one material of the calibration phantom, at least partially based on the acquired projection data, in order to perform material discrimination calibration. X-ray imaging system including [specific component].

2. The X-ray imaging system according to claim 1, wherein the X-ray beam limiting device is part of a pre-collimator positioned relative to the X-ray source.

3. The X-ray imaging system according to claim 1, wherein at least one calibration element of the X-ray beam limiting device and the calibration phantom include at least two different materials through which at least some X-rays pass, so as to enable material discrimination calibration.

4. The X-ray imaging system according to claim 1, wherein the at least one calibration element includes at least one curved surface.

5. The at least one calibration element has a first predetermined thickness T １ A first part having a second predetermined thickness T ２ The X-ray imaging system according to claim 1, further comprising a second part having a

6. The at least one calibration element is a first substance M １ The image processing circuit includes a first calibration element having the first substance M １ The X-ray imaging system according to claim 1, configured to determine the path length through which the X-ray passes.

7. Second substance M ２ The present invention further includes a second calibration element having the second substance M ２ is the first substance M １ Unlike the above, the image processing circuit section uses the second substance M ２ The X-ray imaging system according to claim 6, configured to determine the path length through which the X-ray passes.

8. The X-ray imaging system according to claim 1, wherein the X-ray beam limiting device includes a motor configured to move the at least one calibration element relative to the X-ray beam path.

9. The X-ray imaging system according to claim 1, wherein the X-ray beam limiting device further includes a bowtie filter and / or a hardened filter.

10. The X-ray imaging system according to claim 1, wherein the X-ray imaging system is configured to irradiate the calibration phantom, which is located in the X-ray beam path between the X-ray beam limiter and the X-ray detector, with X-rays during calibration.

11. The X-ray imaging system according to claim 10, wherein the X-ray imaging system includes the calibration phantom.

12. The calibration phantom includes a first phantom substance PM １ and the first phantom substance PM １ is different from at least one substance of the at least one calibration element, the X-ray imaging system according to claim 11.

13. The calibration phantom includes a combination of geometric objects of at least two different shapes and / or materials, the combination being: A first geometric object located in the center, which is the first phantom material PM １ A first geometric object including, A plurality of second geometric objects arranged around the first geometric object, wherein at least one subset of the plurality of second geometric objects is a second phantom material PM different from the first phantom material PM1. ２ A plurality of second geometric objects, wherein the first geometric object is relatively larger than the second geometric object. The X-ray imaging system according to claim 12, including the following:

14. The calibration phantom includes a combination of geometric objects of at least three different shapes and / or materials, the combination being: A plurality of third geometric objects arranged around the first geometric object and / or around at least one subset of the second geometric objects, wherein at least one subset of the third geometric objects is the first phantom material PM １ and the second phantom material PM ２ A third phantom substance PM, which is different from the above. ３ The third geometric object includes a plurality of third geometric objects that are relatively smaller than the second geometric object. The X-ray imaging system according to claim 13, including the following:

15. The X-ray imaging system according to claim 1, further comprising a movable platform positioned in the X-ray beam path between the X-ray beam limiting device and the X-ray detector, wherein the movable platform is configured to hold the calibration phantom.

16. The X-ray imaging system according to claim 1, wherein the X-ray imaging system is configured to perform material discrimination calibration based on a mapping between the determination of the path length passing through the at least one calibration element and the calibration phantom and the corresponding detector response of the X-ray detector.

17. The X-ray imaging system is a computed tomography (CT) imaging system including a movable assembly, wherein the X-ray source, the X-ray detector, and the X-ray beam limiting device are arranged in the movable assembly, according to claim 1.

18. The X-ray imaging system according to claim 17, wherein the CT imaging system is configured to perform material discrimination calibration based on a mapping between the rotation angles of each of the plurality of rotation angles of the movable assembly of the CT imaging system and each of the plurality of detector elements of the X-ray detector, the determination of the path length passing through the at least one calibration element and the calibration phantom, and the corresponding detector response of the X-ray detector.

19. The X-ray imaging system according to claim 17, wherein the CT imaging system is configured to generate detector data at multiple angles, and the movable assembly is configured to move to a set of multiple predetermined angles, stop at each angle, and generate detector data at each stopping angle.

20. The X-ray imaging system according to claim 1, wherein the X-ray detector is a photon-counting multi-energy bin X-ray detector.

21. A method for calibrating material discrimination in an X-ray imaging system comprising an X-ray source configured to emit X-rays, an X-ray detector, an X-ray beam limiting device positioned in the X-ray beam path near the X-ray source, and an image processing circuit unit, wherein the X-ray beam limiting device includes at least one calibration element, and the method is: Between the X-ray beam limiting device and the X-ray detector, a calibration phantom is placed in the X-ray beam path of the X-ray imaging system. Start the calibration sequence. To acquire projection data of a set of projections based on the output of the X-ray detector, Based at least partially on the acquired projection data, determine the path length through at least one material of the at least one calibration element and at least one material of the calibration phantom, and Perform material discrimination calibration based at least partially on the determined path length. Methods that include...

22. The method according to claim 21, wherein determining the path length includes determining the path length through the first material of the at least one calibration element, the second material of the calibration phantom, and the third material for each rotation angle of the plurality of rotation angles and each detector element of the plurality of detector elements of the X-ray detector.

23. The method according to claim 21, wherein performing the material discrimination includes generating a mapping between the path length and the detector response of the X-ray detector.

24. The method according to claim 23, wherein the mapping is used for calibrated image reconstruction.

25. The method according to claim 21, wherein the X-ray detector is a photon-counting multi-energy-bin X-ray detector, and the act of performing the act includes determining an intrinsic mapping of the detector elements between the path lengths of different materials and the recorded photon count values ​​of the photon-counting multi-energy-bin X-ray detector corresponding to the path lengths of different materials.

26. Acquiring the aforementioned projection data means This includes moving the X-ray source, the X-ray detector, and the X-ray beam limiting device to multiple positions, stopping the X-ray source, the X-ray detector, and the X-ray beam limiting device at each position, and having the X-ray detector generate a detector response at each stopping position. The method according to claim 21, wherein the acquired projection data is at least partially based on the mean and / or cumulative values ​​of the detector response.

27. Acquiring the aforementioned projection data means Instead of the aforementioned X-ray source, X-ray detector, and X-ray beam limiting device, the calibration phantom is rotated to a predetermined number of angles, the calibration phantom is stopped at each angle, and the X-ray detector generates a detector response at each stopping angle. The method according to claim 21, wherein the acquired projection data is at least partially based on the detector response.

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