Calibration of material discrimination for x-ray imaging system

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 technique relates to X-ray technology and X-ray imaging, and more particularly to the calibration of X-ray imaging systems (including calibration phantoms) and corresponding calibration procedures. In particular, the proposed technique relates to X-ray imaging systems (such as computed tomography (CT) imaging systems) configured for material decomposition calibration, and corresponding methods of material decomposition calibration of CT imaging systems.

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

[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 includes multiple detector modules, each containing one or more detector elements, such that the X-ray intensity can be measured separately. The X-ray source emits X-rays, which pass through a subject or object being imaged and are received by the X-ray detector. The X-ray source and the X-ray detector are typically arranged to rotate around the subject or object on a rotating member of a gantry. The emitted X-rays are attenuated 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] It may be useful to provide a brief overview of an exemplary typical X-ray imaging system according to the prior art with reference to FIG. 1A. 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. In general, the X-ray detector 20 is configured to record radiation from the X-ray source 10, which is focused by optional X-ray optics or a collimator and passes through an object, subject, or part thereof. The X-ray detector 20 can be connected to the image processing system 30 through suitable readout electronics, at least partially integrated in the X-ray detector 20, which can perform image processing and / or image reconstruction.

[0005] As an example, a conventional CT imaging system includes an X-ray source and an X-ray detector arranged to obtain projection images of a subject or object at different view angles covering at least 180 degrees. This is most commonly achieved by mounting the X-ray source and the detector on a support (e.g., a rotating member of a gantry) that can rotate around the subject or object. An image containing the projections recorded on different detector elements for different view angles is called a sinogram. In the following, the collection of projections recorded on different detector elements for different view angles is called a sinogram even though the detector is two-dimensional, resulting in a three-dimensional image.

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

[0007] A further development of X-ray imaging is energy resolved X-ray imaging, also known as spectral X-ray imaging, where the X-ray transmission is measured for several different energy levels. This can be achieved by using two or more X-ray sources emitting 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 the incident radiation at two or more energy levels. One example of such a detector is a multi-bin photon counting detector, where each recorded photon generates a current pulse that is compared to a set of thresholds to count the number of photons incident on each of several energy bins.

[0008] Spectral X-ray projection measurements provide projection images for each energy level. A weighted sum of these projection images can be calculated to optimize the contrast-to-noise ratio (CNR) 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 enabled by energy-resolved X-ray imaging is basis material decomposition, which exploits the fact that all materials composed of elements with low atomic numbers (such as human tissue) have linear attenuation coefficients whose energy dependence can be approximately well represented as a linear combination of two (or more) basis functions, i.e. μ(E)=a1f1(E)+a2f2(E) where f1 and f2 are basis functions, and a1 and a2 are the corresponding basis coefficients. More generally, f i is the basis function, a iare the corresponding basis coefficients, where i=1,...,N, and N is the total number of basis functions. If there are one or more elements with high atomic numbers in the imaged volume, and their K-edges are high enough to appear in the energy range used for imaging, then one basis function needs to be added for each such element. In the field of medical imaging, such K-edge elements can typically be iodine or gadolinium, which are substances used as contrast agents.

[0010] Reference material decomposition is described in "Energy-selective reconstructions in X-ray computerized tomography", Alvarez, Macovski, Phys. Med. Biol. 1976; 21(5):733-744. In reference material decomposition, the integral of each basis coefficient, given by:

number

number

[0011] Next, under the assumption that the counts of each bin are random variables with a Poisson distribution, we use the maximum likelihood method to find A i can be estimated. This is accomplished by minimizing the negative log-likelihood function (see, e.g., “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

[0012] The resulting estimated basis coefficient line integrals for each projection line

number

[0013] Standard calibration procedures for X-ray imaging systems are not adapted and cannot accommodate material decomposition, and therefore it is difficult to ensure robust operation of X-ray imaging systems that function based on material decomposition.

[0014] Furthermore, calibrations are usually required to be valid over time. In practice, this typically means that the calibration procedure should be robust and easy to operate, such that it can be automated or semi-automated to minimize human intervention. The advantage of this is that it reduces machine handling time and minimizes human-prone errors.

[0015] As such, there remains a general need for improved 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 subject matter that are more fully described in the Detailed Description. This summary is not intended to identify 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, an X-ray imaging system configured for calibration of material decomposition is provided that can be used 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 an 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, the X-ray beam limiting device including at least one calibration element disposed in the X-ray beam path. The X-ray imaging system also includes image processing circuitry configured to acquire projection data for 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 based at least in part on the acquired projection data to perform a material decomposition calibration.

[0019] Another aspect is a method for calibration of material decomposition in an X-ray imaging system having an X-ray source, an X-ray detector, an X-ray beam limiting device disposed in the X-ray beam path proximate the X-ray source, and image processing circuitry, the X-ray beam limiting device including at least one calibration element.

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

[0021] In the proposed technique, for the purpose of a novel calibration procedure, a calibration phantom can be used in combination with an X-ray beam limiting device including one or more calibration elements, and an image processing circuitry of the X-ray imaging system is configured to acquire projection data for a set of projections and determine a path length through at least one material of the calibration element and at least one material of the calibration phantom to perform a material decomposition calibration. [Brief description of the drawings]

[0022] The various aspects of the present disclosure can be better understood by reference to the accompanying drawings and by reading the following detailed description. [Figure 1A] FIG. 1 is a schematic diagram illustrating an exemplary X-ray imaging system. [Figure 1B] FIG. 1 is a schematic diagram illustrating an exemplary X-ray imaging system. [Diagram 2]FIG. 1 is a schematic diagram illustrating another example of an X-ray imaging system (such as a CT imaging system). [Diagram 3] FIG. 1 is a schematic block diagram of a CT imaging system as an example of an X-ray imaging system. [Figure 4] FIG. 2 is a schematic diagram illustrating another example of relevant components of an X-ray imaging system (such as a CT imaging system). [Diagram 5] 1 is a schematic diagram of a photon counting circuit and / or device according to an exemplary embodiment. [Figure 6] FIG. 2 is a schematic diagram illustrating an example of a solid-state detector sub-module in accordance with an exemplary embodiment. [Figure 7] FIG. 13 is a schematic diagram illustrating an example of a solid-state detector sub-module according to another exemplary embodiment. [Figure 8A] FIG. 13 is a schematic diagram illustrating an example of a solid-state detector sub-module according to yet another exemplary embodiment. [Figure 8B] FIG. 1 is a schematic diagram showing an example of a set of arrayed detector submodules, each of which is a depth-segmented detector submodule, with an application specific integrated circuit (ASIC) or corresponding circuitry located below the detector submodule when viewed from the direction of incident X-rays. [Figure 9] FIG. 1 is a schematic diagram illustrating an example of a CT imaging system. [Figure 10] FIG. 1 is a schematic diagram illustrating a design example of an X-ray source and X-ray detector system. [Figure 11] FIG. 1 is a schematic diagram illustrating an example of an X-ray imaging system (such as a CT imaging system) suitable for material decomposition calibration. [Figure 12] FIG. 1 is a schematic diagram illustrating an example of certain relevant components of a CT imaging system, with a calibration phantom shown in position for performing a calibration of material decomposition. [Figure 13A] FIG. 2 is a schematic diagram showing a cross section of an example calibration phantom. [Figure 13B] FIG. 13B is a schematic diagram showing a perspective view of the calibration phantom of FIG. 13A. [Figure 14]FIG. 13 is a schematic diagram showing a cross section of another example of a calibration phantom. [Figure 15A-B] FIG. 1 is a schematic diagram showing a cross section of an example pair of calibration phantoms. [Figure 16A-C] FIG. 1 is a schematic diagram illustrating an example of certain relevant parts of a CT imaging system, with a calibration phantom shown in position for performing a calibration of material decomposition. [Figure 17] FIG. 1 is a schematic diagram illustrating an example of particular relevant portions of a CT imaging system adapted for material decomposition in accordance with an exemplary embodiment. [Figure 18A-B] FIG. 2 is a schematic diagram illustrating an example of at least one calibration element. [Figure 19A-C] FIG. 2 is a schematic diagram illustrating an example of at least one calibration element. [Figure 20A] FIG. 2 is a schematic diagram illustrating an example of at least one calibration element and at least one filter. [Figure 20B] FIG. 2 is a schematic diagram illustrating an example of at least one calibration element. [Figure 21] FIG. 1 is a schematic flow diagram illustrating an example method for calibrating material decomposition in a CT imaging system. [Figure 22] FIG. 1 is a schematic diagram illustrating an example implementation of a computer according to one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Embodiments of the present disclosure will now be described, by way of example only, with reference to the figures.

[0024] For a better understanding, it is useful to follow with an introductory description of a non-limiting example of an overall X-ray imaging system in which data processing and transfer in accordance with the concepts of the present invention can be implemented.

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

[0026] The entire X-ray detector may be considered as an X-ray detector system 20, or may be considered as an X-ray detector 20 in combination with associated analog processing circuitry 25.

[0027] The image processing system 30 is in communication with and electrically coupled to the analog processing circuitry 25, and may include a digital processing circuitry 40 and / or a computer 50, and may be configured to perform image reconstruction based on image data from the X-ray detector. Thus, the image processing system 30 may be viewed as the computer 50, as a system combining the digital processing circuitry 40 and the computer 50, or as the digital processing circuitry 40 itself, where the digital processing circuitry is further dedicated to image processing and / or reconstruction.

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

[0029] FIG. 3 shows a schematic of 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 an operator through an operator console 60, which may have a display 62 and some form of operator interface (e.g., keyboard, mouse, joystick, touch screen, or other input device). The commands and parameters provided by the operator are used by the computer 50 to provide control signals to an X-ray controller 41, a gantry controller 42, and a table controller 43. In particular, the X-ray controller 41 provides power and timing signals to the X-ray source 10 to control the X-rays emitted to an object or patient lying on a table 12. The gantry controller 42 controls the rotational speed and rotational position of the gantry 11, which includes the X-ray source 10 and the X-ray detector 20. By way of example, the X-ray detector 20 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 area of ​​the patient. Also included is a detector controller 44 configured to control and / or receive data from the X-ray detector 20 .

[0030] In one embodiment, computer 50 also performs image reconstruction and post-processing of image data output from x-ray detector 20. Computer 50 thus corresponds to image processing system 30 shown in Figures 1 and 2. An associated display 62 allows an operator to view the reconstructed image and other data from computer 50.

[0031] An X-ray source 10 disposed on a gantry 11 emits X-rays. An X-ray detector 20, which may be in the form of a photon-counting X-ray detector, detects the X-rays after passing through an object or patient. The X-ray detector 20 may be formed, for example, by a number of pixels (also called sensors or detector elements) and associated image processing circuitry (such as an application specific integrated circuit (ASIC)) disposed on a detector module. Part 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 may perform further processing operations on the measurement data (such as applying corrections, temporarily storing and / or filtering the measurement data). During a scan to acquire X-ray projection data, the gantry and the components attached to the gantry rotate about an isocenter 13.

[0032] Current X-ray detectors usually require the conversion of incident X-rays into electrons, typically by the photoelectric effect or by Compton interaction, and the resulting electrons usually produce secondary visible light until they lose their energy, which is detected by a photosensitive material. There are also semiconductor-based detectors, where the electrons generated by the X-rays create electron-hole pair charges that are collected by applying an electric field.

[0033] Some detectors operate in an energy-integrating mode, providing an integrated signal of multiple x-rays, with an output signal proportional to the total energy deposited by the detected x-rays.

[0034] X-ray detectors with photon counting capability and energy resolution are becoming more common in medical X-ray applications. Photon counting detectors have the advantage that, in principle, the energy of each X-ray can be measured, providing additional information about the composition of the object. This information can be used to improve image quality and / or reduce radiation dose.

[0035] Typically, photon counting X-ray detectors determine the energy of a photon by comparing the height of the electrical pulse produced by the photon interaction in the detector material to a set of comparator voltages. These comparator voltages are also called energy thresholds. Typically, the analog voltage of the comparator is set by a digital-to-analog converter (DAC). The DAC converts the digital setting sent from the controller into an analog voltage that can be compared to the height of the photon pulse.

[0036] Photon counting detectors count the number of photons that have interacted with the detector during a measurement time. A new photon is typically 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 stored by incrementing a digital counter associated with the channel.

[0037] When multiple different thresholds are used, an energy-discriminating photon counting detector is obtained. In an energy-discriminating photon counting detector, the detected photons can be classified into energy bins corresponding to the various thresholds. This type of photon counting detector is sometimes called a multi-bin detector. In general, the energy information allows a new type of image to be created, in which new information is available and image artifacts inherent in the prior art can be eliminated. In other words, in an energy-discriminating photon counting detector, the pulse height is compared with N programmable thresholds (T1-TN) in the comparator and classified according to the pulse height. This pulse height is proportional to the energy. In other words, a photon counting detector that includes two or more comparators is called a multi-bin photon counting detector. For a multi-bin photon counting 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 exceeded by the photon pulse. In another example, the counter records the number of times the photon pulse exceeds each energy threshold.

[0038] As an example, edge-on refers to a special non-limiting design for photon counting detectors, where the edge of the x-ray sensor (eg, x-ray detector element or pixel) faces the incident x-rays.

[0039] For example, such photon counting detectors may have pixels aligned in at least two directions, with one of the at least two directions of the edge-on photon counting detector having a component in the direction of the x-rays. Such edge-on photon counting detectors may be referred to as depth-segmented photon counting detectors, having two or more depth segments of pixels in the direction of the incident x-rays. It should be noted 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 plurality of pixels may be arranged in an array in a direction substantially perpendicular to the direction of the incident x-rays (not segmented in depth), and each pixel of the plurality of pixels may be arranged such that it is edge-oriented toward the incident x-rays. In other words, the photon-counting detector may not be segmented in depth, but may be arranged such that it is edge-oriented toward the incident x-rays.

[0041] The absorption efficiency can be increased by placing the edge-on photon counting detector on edge, in which case the absorption depth can be chosen to be any length and the edge-on photon counting detector can be fully depleted without requiring very high voltages.

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

[0043] As shown in FIG. 4, signals are transmitted from the detector elements 22 of the X-ray detector to an input of an analog processing circuitry (e.g., ASIC) 25 via a path 26. It should be understood that the term application specific integrated circuit (ASIC) should be broadly interpreted as a general circuit that is used and configured for a specific application. The ASIC processes the charge generated from each X-ray, converts the processed charge to digital data, and uses the digital data to obtain measurement data (such as photon counts and / or estimated energy). The ASIC is configured to couple to the digital processing circuitry so that the digital data is sent to the digital processing circuitry 40 and / or one or more memory circuits or components 45, and finally, the data is input to the image processing circuitry 30 of FIG. 2 or a computer 50, which generates a reconstructed image.

[0044] The number of electrons and holes generated from an X-ray event is proportional to the energy of the X-ray photon, and therefore the total charge of an induced current pulse is proportional to said energy. After a filtering step in the ASIC, the pulse amplitude is proportional to the total charge of the current pulse and thus to the X-ray energy. The pulse amplitude can be measured by comparing its value with one or more thresholds (THR) of one or more comparators (COMP), 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 with an energy greater than the energy corresponding to the respective threshold (THR) can be counted and / or recorded.

[0045] The ASIC typically samples the analog photon pulse once for every clock cycle and records the outputs of the comparators. The comparators (thresholds) output a 1 or 0 depending on whether the analog signal is above or below the comparator voltage. The information available at each sample is, for example, a 1 or 0 for each comparator representing whether the comparator was triggered (the photon pulse was greater than the threshold) or not.

[0046] In photon counting detectors, there is typically a photon counting logic that determines if a new photon has been recorded and records the photon in a counter. In multi-bin photon counting detectors, there are typically multiple counters, e.g., 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 logic, for example, local maxima detection, which counts detected local maxima in a voltage pulse and sometimes also records the pulse height of the detected local maxima.

[0047] Photon counting detectors have many advantages (including but not limited to high spatial resolution, low sensitivity to electronic noise, good energy resolution, and material separation (spectral imaging)). However, energy integrating detectors have the advantage of high count-rate tolerance. Count-rate tolerance comes from the fact / realization that since the total energy of the photons is measured, adding one additional photon will always increase the output signal (within reasonable limits), regardless of the amount of photons currently registered by the detector. This advantage is one of the main reasons why energy integrating detectors have become the standard in medical CT today.

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

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

[0050] The ASIC may include a set of comparators 302, each of which compares the magnitude of the voltage pulse with a reference voltage. The comparator output is typically (0 / 1) depending on which of the two compared voltages 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 may be used to convert a digital setting, which may be provided by a user or a control program, into a reference voltage that may be used by the comparators 302. If the height of the voltage pulse exceeds the reference voltage of a particular comparator, that comparator is said to be triggered. Each comparator is typically associated with a digital counter 303, which is incremented based on the comparator output according to a photon counting logic.

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

number

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

[0053] For example, embodiments may be implemented in hardware, or at least partially implemented in software and performed by suitable image processing circuitry, or a combination thereof.

[0054] The steps, functions, procedures, and / or blocks described herein may be implemented in hardware using conventional techniques (such as discrete or integrated circuit technologies, including both general purpose electronic circuitry and application specific circuitry).

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

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

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

[0058] 7 is a schematic diagram illustrating an example of a semiconductor detector sub-module according to another exemplary embodiment, in which a detector module 21 having a semiconductor sensor is also divided into a number of depth segments or detector elements 22 in the depth direction, assuming that X-rays are incident from the end of the detector module.

[0059] Typically, a detector element is an individual X-ray sensitive sub-element of the detector. In general, photon interaction occurs with the detector element and the charge thus generated is collected by a corresponding electrode of the detector element.

[0060] Each detector element typically measures the incident x-ray flux as a series of frames, where a frame is a specified time interval of measurement data called the frame time.

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

[0062] The detector strips of a depth-segmented detector usually correspond to the pixels of a typical flat panel detector and are therefore sometimes referred to as pixel strips, however, a depth-segmented detector can also be considered as a three-dimensional pixel array, where each pixel corresponds to an individual depth segment / detector element.

[0063] The 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 for several ASICs, which are preferably attached by the so-called flip-chip technique. The wiring includes connection lines for the signals from each pixel or detector element to the input of the ASIC and connection lines from the ASIC to an external memory and / or a digital data processing unit. The power to the ASIC can be supplied through similar wiring, taking into account the large cross-sectional area in order to pass high currents through these connection lines, but this power can also be supplied through a separate connection line. The ASIC can be arranged next to the active sensor, which means that the ASIC can be protected from incident X-rays by placing an absorbent cover on top of it and also from scattered X-rays by placing an absorber in this direction for scattered X-rays from the side.

[0064] FIG. 8A is a schematic diagram showing a detector module implemented as an MCM similar to the embodiment of US Pat. No. 8,183,535. In this example, it is shown how a semiconductor sensor 21 can also have the function of the substrate of the MCM. Signals are transmitted by paths 23 from the detector elements 22 to the inputs 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 that can be used to detect photons and / or estimate the energy of the photons. The ASIC can have its own digital processing circuitry and memory to perform simple tasks. The ASIC can then be configured to connect to digital processing circuitry and / or memory circuits or components located outside the MCM, and finally the data is used as input to reconstruct the image.

[0065] However, the adoption of depth segmentation poses two significant challenges for silicon-based photon-counting detectors. First, a large number of ASIC channels must be used to process the data provided by the associated detector segments. In addition to the increased channel count due to both the small pixel size and the use of depth segmentation, the data size is further increased by multi-energy bins. Second, because the counts for a given x-ray input are split into small pixels, segments and energy bins, the signal in each bin is very low and detector calibration / correction requires calibration data over several orders of magnitude to minimize statistical uncertainty.

[0066] Naturally, when data sizes increase by several orders of magnitude, both data handling and pre-processing become slower, in addition to requiring larger computing resources, hard disks, memory, and central processing units (CPUs) or graphics processing units (GPUs). For example, when data sizes become 10 gigabytes instead of 10 megabytes, data handling times (reading and writing) increase by 1000 times.

[0067] A problem with counting X-ray photon detectors is pile-up. At high X-ray photon flux rates, there can be problems distinguishing between two successive charge pulses. As mentioned before, the pulse length after filtering depends on the shaping time. If this pulse length is longer than the time between the two charge pulses induced by the X-ray photons, the pulses will merge and the two photons cannot be distinguished and will be 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 a pile-up calibration vector, the pile-up calibration data needs to be pre-processed for spit correction. To generate a material decomposition vector, it is preferable to pre-process the material decomposition data for both spit correction and pile-up correction. In the case of patient scan data, the data needs to be pre-processed for spit, pile-up, and material decomposition before image reconstruction. Note that these examples are simplified examples to explain the pre-processing, and the actual pre-processing steps may include several other calibration steps (such as reference normalization and air calibration) as necessary. The term processing may refer only to the final step in each generation process of a calibration vector or in a patient scan, but the term is sometimes used interchangeably.

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

[0070] 9 is a schematic diagram of an example of a general CT imaging system. In this schematic example, CT imaging system 100 includes a gantry 111 and a patient table 112. The patient table 112 can be inserted into an opening 114 in the gantry 111 during a patient scan and / or a calibration scan. The direction of the axis of rotation of the rotating members of the gantry around the object or patient being imaged is represented as the z-direction. The angular orientation of the CT imaging system is represented as the x-direction, and the direction of the incident x-rays is represented as the y-direction.

[0071] However, it should be understood that the rotating and stationary members of the gantry need not be part of the CT system and may be in other arrangements and / or configurations (e.g., linear and / or translational relative movement without rotation). As an example, the x-ray source and detector combination may be moved linearly and / or translationally relative to the stationary members of the entire gantry. For example, the x-ray source and detector may move together as an integral assembly unit along the table axis (commonly referred to as the z-axis). Alternatively, the patient table may move but the x-ray source and detector combination may be stationary, which is where the relative motion becomes important. This includes, for example, a geometric system configuration in which the patient may be standing in a so-called telephone booth style scanner.

[0072] FIG. 10 is a schematic diagram showing an example of the overall design of an X-ray source-detector system. In this example, a schematic diagram of an X-ray detector including multiple detector modules and an X-ray source emitting X-rays is shown. Each detector module can have a set of detector elements that define corresponding pixels. For example, the detector modules can be edge-on detector modules arranged side by side, with edges facing the X-ray source, and arranged to be slightly curved overall. As mentioned above, the direction of the incident X-rays is called the Y direction. Multiple detector pixels in the axial direction of the gantry rotation (called the z direction) allow multi-slice images to be acquired. Multiple detector pixels in the angular direction (called the x direction) allow multiple projections of the same plane to be measured simultaneously, 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) and angular (x) directions.

[0073] 11 is a schematic diagram illustrating an example of an X-ray imaging system (such as a CT imaging system adapted for calibration of material decomposition according to an example 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 disposed in an X-ray beam path and configured to generate detector data. A calibration phantom 150 is adapted to be disposed 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 limiting device 130 disposed in the X-ray beam path near the X-ray source 110, the X-ray beam limiting device 130 including at least one calibration element 135 disposed 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 absent 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 may be a composition of atoms with certain properties. For example, the material may be a composition of multiple atoms that mimics the properties of a single atom. This may be a more convenient and / or cheaper alternative to using rare and expensive single atom materials.

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

[0076] As an example, the X-ray imaging system may be a CT imaging system (e.g., a CT imaging system as shown diagrammatically in FIG. 12). FIG. 12 is a schematic diagram showing an example of certain relevant components of a CT imaging system, and a calibration phantom is shown for calibrating material decomposition. As mentioned above, a CT imaging system includes an X-ray source 110 and an X-ray detector 120 arranged to obtain projection images of a subject or object at different viewing 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 (calibration phantom 150 in this use case).

[0077] As an example, the x-ray beam limiting device 130 may 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 to allow calibration of material decomposition.

[0079] Optionally, the at least one calibration element 135 includes at least one curved surface, an example of which is described in more detail below.

[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, the at least one calibration element includes a first calibration element having a first material M1, and the image processing circuitry is configured to determine a path length 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, the second material M2 being different from the first material M1, and the image processing circuitry is (further) configured to determine a path length through the second material M2.

[0083] In a non-limiting example, the calibration element 135 can include a high density material (e.g., iodine), which can provide a convenient, efficient, and / or versatile setup for material decomposition using an X-ray imaging system according to the present invention.

[0084] Optionally, the X-ray beam limiting device 130 includes a motor configured to move the 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 hardening filter. Thus, the X-ray beam limiting device can 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, when the calibration phantom 150 is placed in the X-ray beam path between the X-ray beam limiting device 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. As an example, one of the two calibration phantoms can be placed above the first calibration phantom and / or used independently to more accurately sample material space for combinations that arise in imaging tasks of small objects (infants, heads, etc.).

[0088] In a practical embodiment, the X-ray imaging system 100 may also be considered to include a calibration phantom 150 .

[0089] By way of example, the calibration phantom 150 can include a first phantom material PM1, which is different from at least one material of the at least one calibration element.

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

[0091] For example, the calibration phantom 150 may include a combination of at least two geometric objects of different shapes and / or materials, including a centrally located first geometric object that includes a first phantom material PM1, and a plurality of second geometric objects arranged around the first geometric object, at least a subset of the plurality of second geometric objects includes a second phantom material PM2 that is different from the first phantom material PM1, and the first geometric objects are relatively larger than the second geometric objects.

[0092] FIG. 13A is a schematic diagram showing a cross section of an example of a calibration phantom. FIG. 13B is a schematic diagram showing a perspective view of the calibration phantom of FIG. 13A. The calibration phantom may include a cylinder 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 a plurality of rods surrounding the at least one rod and having a diameter in the range of 1 cm to 9 cm (preferably 3 cm to 7 cm, more preferably 5 cm).

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

[0094] FIG. 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 can include a combination of geometric objects of at least three different shapes and / or materials. For example, the calibration can 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 a subset of another plurality of geometric objects 150b (second geometric objects). At least a subset of the plurality of third geometric objects 150c includes a third phantom material PM3 different from the first phantom material PM1 and the second phantom material PM2. The third geometric object 150c is relatively smaller than the second geometric object 150b. The first, second, and third geometric objects can be cylinders with a circular, oval, or elliptical cross section.

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

[0096] Further, the X-ray system may be configured to allow calibration of material decomposition based on a mapping between i) a determination of the path length through at least one calibration element and the calibration phantom 150, and ii) a corresponding detector response of the X-ray detector.

[0097] As an example, the X-ray imaging system may be a computed tomography (CT) system that includes a moveable assembly, in which the X-ray source 110, the X-ray detector 120, and the X-ray beam limiting device 130 are disposed on the moveable assembly.

[0098] Referring again to FIG. 12, an example of certain relevant components of a CT imaging system is shown, with a calibration phantom shown in position for performing calibration. For example, such a CT imaging system allows for calibration of material decomposition based on i) a determination of a path length through at least one calibration element 135 and the calibration phantom 150 for each of a plurality of rotation angles of a moveable assembly of the CT imaging system and each of a plurality of detector elements of the X-ray detector 120, and ii) a corresponding detector response of the X-ray detector 120.

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

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

[0101] FIG. 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 calibration of material decomposition 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, which are arranged around the first geometric object 150a. The plurality of second geometric objects 150b may be smaller than the first geometric object 150a. Thus, each of the second geometric objects 150b provides a shorter path length than the first geometric object 150a. The first and second geometric objects may be cylinders with a circular, oval, or elliptical cross-section. The first geometric object 150a includes a first phantom material PM1. The plurality of second geometric objects 150b may include a first phantom material PM1 and / or a second phantom material PM2. For example, one geometric object of the plurality of second geometric objects 150b may include a first phantom material PM1, and another geometric object of the plurality of second geometric objects 150b may include a second phantom material PM2. PM2 may, for example, be denser than PM1.

[0102] In FIG. 15A, the calibration phantom 150 includes a first geometric object 150a including a first phantom material PM1 and a plurality of second geometric objects 150b including either the first phantom material PM1 or the second phantom material PM2. Specifically, the plurality of second geometric objects 150b are arranged around the first geometric object 150a, and every other one of the second geometric objects 150b includes the first phantom material PM1, and the remaining second geometric objects 150b include the second phantom material PM2. The first phantom material PM1 can be polyethylene (PE). The second phantom material PM2 can be polyvinyl chloride (PVC). In an alternative embodiment, the plurality of second geometric objects 150b may not include the first phantom material PM1, and all of the second geometric objects 150b may include the second phantom material PM2, or all of the second geometric objects 150b may include the first phantom material PM1. It should be appreciated that the second geometric object 150b may comprise one material or a number of different materials.

[0103] FIG. 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 calibration of material decomposition. The calibration phantom 150 of FIG. 15B has some features in common with the calibration phantom 150 shown in FIG. 15A. In FIG. 15B, the first geometric object 150a and / or one or more second geometric objects of the plurality of second geometric objects 150b have an oval and / or elliptical shape. The first and second geometric objects may be cylinders with a circular, oval, or elliptical cross-section. The oval and / or elliptical shape of the geometric objects is similar to the human body, and therefore this shape may produce similar X-ray scattering. The calibration phantom 150 shown in FIG. 15B may provide good and / or uniform coverage. It should be understood that the second geometric object 150b may include one or more different shapes and / or materials.

[0104] 16A, 16B, and 16C are schematic diagrams showing examples of certain relevant components of a CT imaging system, with a calibration phantom shown in a position for performing calibration. The CT imaging system 100 includes an X-ray source 110, an X-ray detector 120, and an X-ray beam limiting device 130, which are arranged 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 a calibration of the material decomposition based on i) determining a path length through at least one calibration element (not shown) and the calibration phantom 150 for each rotation angle of a plurality of rotation angles of the movable assembly of the CT imaging system 100 and each detector element of a plurality of detector elements of the X-ray detector, and ii) a mapping between a corresponding detector response of the X-ray detector 120. Thus, the X-ray source 110, the X-ray detector 120 and the X-ray beam limiting device 130 can be moved in a synchronized manner such that detector data is generated at different rotation angles around the calibration phantom 150. The movement of the movable assembly, and thus the X-ray source 110, the X-ray detector 120 and the X-ray beam limiting device 130, can be performed in a controlled manner. This movement can be controlled, for example, by image processing circuitry 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, x-ray beam limiting device 130, and x-ray detector 120 rotate as they would during a normal scan, rotating through each position as many times as possible to gather as many statistics as possible. At different viewing angles or rotation angles, each detector element has a different combination of calibration element and path length of the calibration phantom material.

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

[0107] In FIG. 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 multiple predetermined angles, stop at each angle, and generate detector data at each stop angle. In other words, the X-ray source 110, the X-ray detector 120, and the X-ray beam limiting device 130 can move in steps to multiple positions and acquire projection data at each step. The X-ray source 110, the X-ray detector 120, and the X-ray beam limiting device 130 stop at each position, 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 a predetermined angle 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 Fig. 16C, the CT imaging system 100 can be used with two or more calibration phantoms 150. The CT imaging system can be configured to illuminate two or more calibration phantoms 150. Here, in Fig. 16C, two calibration phantoms 150 are imaged by the CT imaging system 100. The two calibration phantoms 150 may be different in material, size and / or shape, or may be the same.

[0109] FIG. 17 is a schematic diagram illustrating an example of certain relevant components of a CT imaging system adapted for material decomposition according to an exemplary embodiment, in which a calibration phantom 150 is placed in a position for calibrating the CT imaging system 100. In this example, the calibration phantom 150 can be moved relative to the x-ray beam path in a 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 materials of the calibration phantom 150 and the 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., field of view), such that the edge detector elements experience varying path lengths for different rotation angles. To move the calibration phantom 150, the x-ray imaging system 100 can include a mechanism configured to shift / move the calibration phantom 150 in a horizontal and / or vertical plane. The mechanism for performing the movement can be a moving mechanism built into the couch / table of the CT imaging system or can be another mechanical mechanism.

[0110] If the calibration phantom 150 is smaller than the full field of view, then there will only be air at the edge detector if the phantom is placed only at the isocenter, as shown diagrammatically in Figure 17. To handle this, the motion 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 limiting device 130. For example, the calibration phantom 150 can be rotated around an axis passing through the center of the calibration phantom 150 and / or around an axis outside the calibration phantom 150. 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 limiting device 130. In other words, the X-ray imaging system can acquire projection data by rotating the calibration phantom 150 to a plurality of predetermined angles instead of the X-ray source, the X-ray detector, and the X-ray beam limiting device, stopping the calibration phantom 150 at each angle, and at each stop angle, the X-ray detector generates a detector response. The acquired projection data is based at least in part on the detector response.

[0112] 18A and 18B are schematic diagrams illustrating an example of at least one calibration element 135. The at least one calibration element 135 may be disposed in the X-ray beam limiting device and may include different portions or components having different shapes, thicknesses, and / or materials. The calibration element 135 may include a first portion 135-1 having a first predetermined thickness T1, a second portion 135-2 having a second predetermined thickness T2, and a third portion 135-3 having a third predetermined thickness T3. The first portion 135-1, the second portion 135-2, and the third portion 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 limiting device and the X-ray beam path. The movement of the calibration element 135 relative to the X-ray beam limiting device and the X-ray beam path can be performed by a motor located in or connected to the X-ray beam limiting device.

[0114] In Figure 18A, the calibration elements 135-1, 135-2, and 135-3 are positioned outside the x-ray beam path, so that no x-rays pass through the calibration elements 135-1, 135-2, and 135-3.

[0115] In FIG. 18B, the calibration element 135-2 has been moved into the X-ray beam path, and only the second portion 135-2 is present in the X-ray beam path. As a result, X-rays pass through the second portion 135-2 and do not pass through the first portion 134-1 and the third portion 135-3. The calibration element 135 can be moved in and out of the X-ray beam path such 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 cause X-rays to pass through components of different thicknesses in a controllable and adjustable manner. All portions 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 can each include the same material or each portion can include different materials.

[0116] 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, FIGS. 19A, 19B, and 19C illustrate 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 be configured with multiple different materials, thicknesses, and / or shapes. For example, the first calibration element 135a can include a first material, the second calibration element 135b can include a second material different from the first material, and the third calibration element 137c can include a third material different from the first and second materials. In other exemplary embodiments, the first material, the second material, and the third material can all be the same material or can be configured with two different materials. As an example, at least one calibration element may include a high density material (e.g., iodine), at least one calibration element may include polyethylene (PE), or at least one calibration element may include polyvinyl chloride (PVC). As another example, at least one calibration element may have different thicknesses, e.g., 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 thickness, the second thickness, and the third thickness may be the same thickness or different thicknesses. The third calibration element 135c may include the first material, but may have a different thickness than the first calibration element 135a. The different calibration elements 135a, 135b, 135c may be moved in and out of the X-ray beam path. This allows the option of changing the path length of the X-rays due to different materials, thicknesses, and / or shapes. In FIG. 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 in the x-ray beam path, and in Figure 19C, only the first calibration element 135a and the third calibration element 135c are in the x-ray beam path.

[0117] FIG. 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 FIG. 20A, the X-ray beam limiting device includes a filter 137 (e.g., a bowtie filter), and the 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 can have different materials, thicknesses, and / or shapes. Each calibration element of the calibration elements 135a, 135b, and 135c can have a similar shape with a thickness corresponding to each calibration element. The first calibration element 135a and the third calibration element 135c can include a first material. The second calibration element 135b can 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 the filter 137 are selected to modulate the incident X-ray beam as a function of the angle of the X-ray beam relative to the object / subject, and to balance the X-ray photon flux at the X-ray detector.

[0118] 20B is a schematic diagram illustrating an example of at least one calibration element 135. The at least one calibration element 135 includes a first calibration element 135a, a second calibration element 135b, and a third calibration element 135c, each including a curved surface 136. The curved surfaces 136 of the calibration elements 135a, 135b, 135c may be such that all detector pixels contribute the same amount to the path length. This may be particularly advantageous for CT imaging systems that utilize fan beams.

[0119] FIG. 21 is a schematic flow diagram illustrating an example of a method for calibration of material decomposition of a CT imaging system. The CT imaging system includes an X-ray source configured to emit X-rays, an X-ray detector, an X-ray beam limiting device arranged in the X-ray beam path near the X-ray source, and image processing circuitry. The X-ray beam limiting device includes at least one calibration element. The method of FIG. 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 limiting device and the X-ray detector. A second step S2 initiates a calibration sequence. A third step S3 acquires projection data of a set of projections based on an output of the X-ray detector. A fourth step S4 determines a path length through at least one material of the at least one calibration element and at least one material of the calibration phantom based at least in part on the acquired projection data. A fifth step S5 performs a calibration of the material decomposition based at least in part on the determined path length.

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

[0121] In a particular example, the material decomposition calibration step includes generating a mapping between path length and detector response of the X-ray detector.

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

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

[0124] In a particular embodiment, acquiring the projection data includes moving the X-ray source, the X-ray detector, and the X-ray beam limiting device to a plurality of positions, stopping the X-ray source, the X-ray detector, and the X-ray beam limiting device at each position, and the X-ray detector generating a detector response at each stop position. The acquired projection data is based at least in part on an average value of the detector responses and / or an accumulated value of the detector responses. This has the advantage that the input data size for the material decomposition calibration is reduced, and therefore the time required to perform the material decomposition calibration is reduced. Furthermore, this step-and-shoot method has the advantage that it is a static scan and is not affected by rotational motion.

[0125] Alternatively, the step of acquiring the projection data includes rotating a calibration phantom to a plurality of predetermined angles, instead of an x-ray source, an x-ray detector, and an x-ray beam limiting device, stopping the calibration phantom at each angle, and causing the x-ray detector to generate a detector response at each stop angle. The acquired projection data is based at least in part on the detector response. If the x-ray tube has focal spot motion and the detector has its own motion with respect to angle, and the motion has a significant effect on the scan, this can be resolved by rotating the phantom.

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

[0127] FIG. 22 is a schematic diagram illustrating an example of a computer implementation according to one embodiment. In this particular example, the system 200 includes a processor 210 and a memory 220, the memory including instructions executable by the processor, such that the processor is operable 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 the memory 220 or downloaded from an external memory device 230. Optionally, the system 200 includes an input / output interface 240 that may be connected to the processor 210 and / or the memory 220 to allow relevant data (such as input parameters and / or resulting output parameters) to be input and / or output.

[0128] In particular embodiments, memory 220 includes a set of instructions executable by a processor, such that the processor is operable to perform the steps and / or operations described herein.

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

[0130] Thus, the image processing circuitry, which may include one or more processors, is configured, when executing a computer program, to perform well-defined processing tasks as described herein.

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

[0132] The present technology also provides a computer program product comprising a computer readable medium 220; 230 having such a computer program stored thereon.

[0133] As an example, the software or computer program 225;235 may be realized as a computer program product, which is typically carried or stored on a computer-readable medium 220;230 (particularly a non-volatile medium). The computer-readable medium may include, but is not limited to, one or more removable or non-removable memory devices, including read-only memory (ROM), random access memory (RAM), compact disks (CD), digital versatile disks (DVD), Blu-ray disks, universal serial bus (USB) memories, hard disk drive (HDD) storage devices, flash memories, magnetic tapes, or any other conventional memory devices. Thus, the computer program may be loaded into the operating memory of a computer or equivalent processing device and executed by its image processing circuitry.

[0134] The flow of the method can be considered as a flow of computer operations when executed by one or more processors. The corresponding apparatus, system and / or machine can be defined as a group of functional modules, and each step executed by a processor corresponds to a functional module. In this case, the functional module is implemented as a computer program executed by a processor. Thus, the apparatus, system and / or machine can alternatively be defined as a group of functional modules, and the functional module can be implemented as a computer program executed by at least one processor.

[0135] Thus, the computer programs resident in the memory may be organized as appropriate functional modules configured to perform at least a portion of the steps and / or tasks described herein when executed by the processor.

[0136] Alternatively, most of the modules may be implemented as hardware modules or may be substituted with hardware - software or hardware is purely an implementation choice.

[0137] The embodiments of the present disclosure shown in the drawings and described above are merely exemplary embodiments and are not intended to limit the scope of the claims (including equivalents thereto). Those skilled in the art will appreciate that various modifications, combinations, and changes can be made to the embodiments without departing from the scope of the present disclosure defined by the claims. Any combination of non-mutually exclusive features described herein is intended to be within the scope of the present invention. That is, features of the described embodiments can be combined with any suitable aspect described above, and any feature of one aspect can be combined with other suitable aspects. Similarly, features described in multiple dependent claims can be combined with non-mutually exclusive features of other dependent claims, especially when multiple dependent claims are dependent on the same independent claim. Although 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] It should further be noted that the inventive concept relates to all possible combinations of features, unless expressly stated otherwise, and in particular different part solutions in the 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 Computer 60 Operator Console 62 Display 110 X-ray source 111 Gantry 112 Patient Table 114 Opening 120 X-ray detector 130 X-ray beam limiting device 134 First Part 136 Curved surface 137 Filters 140 Image processing circuit section 142 Calibration Module 150 Calibration Phantom 200 Systems 210 Processor 225 Computer Programs 230 External Memory Device 240 Output Interface 301 Digital-to-Analog Converter (DAC) 303 Digital Counter

Claims

1. 1. An x-ray imaging system configured for calibration of material decomposition and usable with a calibration phantom, the x-ray imaging system comprising: an x-ray source configured to emit x-rays; an x-ray detector disposed in the x-ray beam path and configured to generate detector data; Including, the calibration phantom is disposed in the x-ray beam path between the x-ray source and the x-ray detector; The X-ray imaging system includes: an X-ray beam limiting device disposed in the X-ray beam path near the X-ray source, the X-ray beam limiting device including at least one calibration element disposed in the X-ray beam path; image processing circuitry configured to acquire projection data for a set of projections based on the detector data and determine, based at least in part on the acquired projection data, a path length through at least one material of the at least one calibration element and at least one material of the calibration phantom to perform a material decomposition calibration.

1. An X-ray imaging system comprising:

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

3. 2. The x-ray imaging system of 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 to allow calibration of material decomposition.

4. The x-ray imaging system of 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 and a second predetermined thickness T ２ and a second portion having the x-ray imaging system of claim 1 .

6. The at least one calibration element comprises a first substance M １ and the image processing circuitry includes a first calibration element having the first material M １ 13. The x-ray imaging system of claim 1, configured to determine a path length through

7. Second substance M ２ and a second calibration element having a second material M ２ is the first substance M １ Unlike the second material M ２ 7. The x-ray imaging system of claim 6, configured to determine a path length through

8. The X-ray imaging system of 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 of claim 1 , wherein the X-ray beam limiting device further comprises a bowtie filter and / or a hardening filter.

10. 2. The X-ray imaging system of claim 1, wherein the X-ray imaging system is configured to irradiate the calibration phantom, the calibration phantom being disposed in an X-ray beam path between the X-ray beam limiting device and the X-ray detector, during calibration with X-rays.

11. The x-ray imaging system of claim 10 , wherein the x-ray imaging system includes the calibration phantom.

12. The calibration phantom is a first phantom material PM １ The first phantom material PM １ 12. The x-ray imaging system of claim 11, wherein is different from the at least one material of the at least one calibration element.

13. The calibration phantom comprises a combination of geometric objects of at least two different shapes and / or materials, the combination comprising: A first geometric object located in the center, the first phantom material PM １ a first geometric object comprising: a plurality of second geometric objects arranged around the first geometric object, at least a subset of the plurality of second geometric objects being oriented in the first phantom material PM; ２ A second phantom material PM different from ２ a plurality of second geometric objects, the first geometric object being relatively larger than the second geometric object; 13. The x-ray imaging system of claim 12, comprising:

14. The calibration phantom comprises a combination of geometric objects of at least three different shapes and / or materials, the combination comprising: a plurality of third geometric objects arranged around the first geometric object and / or around at least a subset of the second geometric objects, the at least a subset of the third geometric objects being oriented in a direction parallel to the first phantom material PM; １ and the second phantom material PM ２ A third phantom material PM different from ３ a plurality of third geometric objects, the third geometric objects being relatively smaller than the second geometric objects; 14. The x-ray imaging system of claim 13, comprising:

15. 2. The x-ray imaging system of claim 1, further comprising a movable platform disposed in the x-ray beam path between the x-ray beam limiting device and the x-ray detector, the platform configured to hold the calibration phantom.

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

17. 2. The X-ray imaging system of claim 1, wherein the X-ray imaging system is a computed tomography (CT) imaging system including a movable assembly, and the X-ray source, the X-ray detector, and the X-ray beam limiting device are disposed on the movable assembly.

18. 18. The X-ray imaging system of claim 17, wherein the CT imaging system is capable of calibrating material decomposition based on a mapping between a determination of a path length through the at least one calibration element and the calibration phantom and a corresponding detector response of the X-ray detector for each rotation angle of a plurality of rotation angles of the moveable assembly of the CT imaging system and each detector element of a plurality of detector elements of the X-ray detector.

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

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

21. 1. A method for calibration of material decomposition in an X-ray imaging system having an X-ray source configured to emit X-rays, an X-ray detector, an X-ray beam limiting device disposed in the X-ray beam path proximate the X-ray source, and image processing circuitry, the X-ray beam limiting device including at least one calibration element, the method comprising: placing the calibration phantom in an x-ray beam path of the x-ray imaging system between the x-ray beam limiting device and the x-ray detector; initiating a calibration sequence; acquiring projection data for a set of projections based on an output of the x-ray detector; determining a path length through at least one material of the at least one calibration element and at least one material of the calibration phantom based at least in part on the acquired projection data; and performing a calibration of the material decomposition based at least in part on the determined path length. A method comprising:

22. 22. The method of claim 21 , wherein determining the path length comprises determining a path length through each of a first material of the at least one calibration element, a second material of the calibration phantom, and a 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.

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

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

25. 22. The method of claim 21 , wherein the X-ray detector is a photon counting multi-bin X-ray detector, and said performing comprises determining a detector element-specific mapping between path lengths of different materials and recorded photon count values ​​of the photon counting multi-energy bin X-ray detector corresponding to the path lengths of the different materials.

26. The acquiring of the projection data includes: moving the X-ray source, the X-ray detector, and the X-ray beam limiting device to a plurality of positions, and at each position, deactivating the X-ray source, the X-ray detector, and the X-ray beam limiting device; and at each deactivation position, the X-ray detector generates a detector response; 22. The method of claim 21, wherein the acquired projection data is based at least in part on an average value of the detector response and / or an accumulated value of the detector response.

27. The acquiring of the projection data includes: rotating the calibration phantom, instead of the X-ray source, the X-ray detector, and the X-ray beam limiting device, to a plurality of predetermined angles, stopping the calibration phantom at each angle, and causing the X-ray detector to generate a detector response at each stop angle; The method of claim 21 , wherein acquired projection data is based at least in part on the detector response.

Images