Material decomposition calibration for x-ray imaging systems

Abstract

The invention relates to material decomposition calibration for an X-ray imaging system. An X-ray imaging system, such as a computed tomography (CT) imaging system, is provided for material decomposition calibration and is intended for use with a calibration phantom. The X-ray imaging system includes an X-ray source configured to emit X-rays and an X-ray detector arranged in an X-ray beam path configured to generate detector data. The calibration phantom is located in the X-ray beam path. The X-ray imaging system also includes an X-ray beam confinement device comprising at least one calibration element 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 determining a path length through the at least one material of the at least one calibration element and the at least one material of the calibration phantom based at least in part on the acquired projection data for performing a material decomposition calibration.

Description

Background Art

[0001] The proposed technology relates to X-ray technology and X-ray imaging, and more particularly to calibration of an X-ray imaging system including a calibration phantom and a corresponding calibration process. In particular, the proposed technology relates to an X-ray imaging system configured for material decomposition calibration, such as a computed tomography (CT) imaging system, and a corresponding method for material decomposition calibration of a CT imaging system.

[0002] Radiographic imaging, such as CT imaging systems and other more general purpose X-ray imaging systems have 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 includes a plurality of detector modules, which include one or more detector elements for independently measuring X-ray intensity. The X-ray source emits X-rays, which pass through the imaged subject or object 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 by the subject or object as they pass through, and the resulting transmitted X-rays are measured by the X-ray detector. The X-ray detector is connected to a digital acquisition system (DAS), and the measured X-ray data is transmitted to an image processing system to reconstruct an image of the subject or object.

[0004] refer to Figure 1A It may be useful to briefly summarize an illustrative, general X-ray imaging system according to the prior art. In this illustrative example, 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 speaking, the X-ray detector 20 is configured to record radiation from the X-ray source 10, which radiation has optionally been focused by an optional X-ray optics or collimator and has passed through an object, subject, or portion thereof. The X-ray detector 20 may be connected to the image processing system 30 via suitable readout electronics that are at least partially integrated in the X-ray detector 20 to enable the image processing system 30 to perform image processing and / or image reconstruction.

[0005] By way of example, a conventional CT imaging system comprises an X-ray source and an X-ray detector arranged so that projection images of a subject or object can be acquired at different viewing angles covering at least 180 degrees. This is most commonly achieved by mounting the 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 projections recorded in different detector elements for different viewing angles is called a sinogram. In the following, the collection of projections recorded in different detector elements for different viewing angles will be called a sinogram, even if the detector is two-dimensional (2D), making the sinogram a three-dimensional (3D) image.

[0006] Figure 1B is a schematic diagram illustrating an example of an X-ray imaging system setup according to the prior art, showing projection lines 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 called spectral X-ray imaging, in which the X-ray transmission is measured for several different energy levels. This can be achieved by rapidly switching the source between two different emission spectra, by using two or more X-ray sources emitting different X-ray spectra, or by using an energy-resolving detector that measures the 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 that is compared to a set of thresholds, thereby counting the number of photons incident in each of a plurality of energy bins.

[0008] Spectral X-ray projection measurement produces projection images for each energy level. A weighted sum of these projection images can be performed to optimize the contrast-to-noise ratio (CNR) for a given imaging task, as described in "SNR and DQE analysis of broadspectrum 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. This technique exploits the fact that all substances composed of low atomic number elements (such as human tissue) have a linear attenuation coefficient whose energy dependence can be well approximated as a linear combination of two (or more) basis functions:

[0010] μ(E)=a1f1(E)+a2f2(E)

[0011] where f1 and f2 are basis functions, and a1 and a2 are the corresponding basis coefficients. More generally, f i is a basis function, and ai are the corresponding basis coefficients, where i = 1, …, N, where N is the total number of basis functions. If one or more elements with high atomic numbers are present in the imaging volume, high enough to have a K absorption edge in the energy range used for imaging, one basis function must 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.

[0012] Basis material decomposition has been described in "Energy-selective reconstructions in X-ray computerized tomography", Alvarez, Macovski, Phys Med Biol. 1976; 21(5): 733-744. In basis material decomposition, the integral of each of the basis coefficients (A i =∫ l a i dl, where i = 1, ..., N, where N is the number of basis functions) is inferred from the measured data in each projection ray l from the source to the detector element. In one specific implementation, this is done by first expressing the expected number of recorded counts in each energy bin as A i This is achieved by the function:

[0013]

[0014] where λ i is the expected number of counts in energy bin i, E is the energy, S i is a response function that depends on the shape of the spectrum incident on the imaged object, the quantum efficiency of the detector, and the sensitivity of energy bin i to X-rays of energy E. Although the term "energy bin" is most commonly used for photon counting detectors, this equation can also describe other energy-resolving X-ray imaging systems, such as multi-slice detectors, kVp switching sources, or multiple source systems.

[0015] Then, under the assumption that the number of counts in each bin is a Poisson-distributed random variable, we can use maximum likelihood to estimate A i 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:

[0016]

[0017] Where m i is the number of counts measured in energy bin i, and M b is the number of energy bins.

[0018] When the estimated basis coefficients obtained from each projection line are integrated When arranged into an image matrix, the result is a material-specific projection image for each basis i, also called a basis image. This basis image can be viewed directly (e.g., in projection X-ray imaging) or used as input to a reconstruction algorithm to form the basis coefficients a inside the object i In either case, the result of the basis decomposition can be viewed as one or more basis image representations, such as basis coefficient line integrals or the basis coefficients themselves.

[0019] Standard calibration procedures for X-ray imaging systems neither accommodate nor cater for material decomposition and therefore make it difficult to ensure robust operation of X-ray imaging systems that work based on material decomposition.

[0020] Furthermore, calibration generally needs to be maintainable. In practice, this generally means that the calibration process should be robust and operable, which allows the calibration process to be automated or semi-automated in order to minimize human interaction. The beneficial effects will be reduced service time and minimized human-influenced errors.

[0021]

[0006] Therefore, there remains a general need for improvements regarding the calibration and operation of X-ray imaging systems, such as CT imaging systems. Summary of the invention

[0022] This summary introduces concepts that are described in more detail in the detailed description. It should not be used to identify essential features of the claimed subject matter, nor should it be used to limit the scope of the claimed subject matter.

[0023] According to one aspect, an X-ray imaging system is provided, the X-ray imaging system being configured for material decomposition calibration and intended for use with a calibration phantom. The X-ray imaging system comprises an X-ray source configured to emit X-rays, and an X-ray detector arranged in an X-ray beam path and configured to generate detector data. The calibration phantom is located in the X-ray beam path between the X-ray source and the X-ray detector.

[0024] The X-ray imaging system further comprises an X-ray beam limiting device disposed in the X-ray beam path proximate to the X-ray source, wherein the X-ray beam limiting device comprises at least one calibration element for being disposed in the X-ray beam path. The X-ray imaging system further comprises an image processing circuit system configured to acquire projection data of a set of projections based on the detector data, and to determine a path length of at least one material through 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 for performing a material decomposition calibration.

[0025] According to another aspect, a method for material decomposition calibration of an X-ray imaging system. The X-ray imaging system has an X-ray source configured to emit X-rays, an X-ray detector, an X-ray beam limiting device disposed in an X-ray beam path proximate to the X-ray source, and image processing circuitry. The X-ray beam limiting device includes at least one calibration element.

[0026] The method includes placing a calibration phantom in an X-ray beam path of an X-ray imaging system between an X-ray beam limiting device and an X-ray detector, initiating a calibration sequence, and acquiring projection data of a set of projections based on an output of the X-ray detector. The method also includes determining a path length through at least one material in the at least one calibration element and at least one material in 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.

[0027] The proposed technology enables the combined use of a calibration phantom and an X-ray beam limiting device, which includes one or more calibration elements for the purpose of a novel calibration process, whereby the image processing circuit system of the X-ray imaging system is configured to acquire projection data of a set of projections and determine the path length through at least one material of the calibration element and at least one material of the calibration phantom for performing material decomposition calibration. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Aspects of the present disclosure may be better understood after referring to the drawings and reading the detailed description.

[0029] Figure 1A and Figure 1B is a schematic diagram illustrating an example of an X-ray imaging system.

[0030] Figure 2 is a schematic diagram illustrating another example of an X-ray imaging system such as a CT imaging system.

[0031] Figure 3 is a schematic block diagram of a CT imaging system as an illustrative example of an X-ray imaging system.

[0032] Figure 4 is a schematic diagram illustrating another example of relevant components of an X-ray imaging system, such as a CT imaging system.

[0033] Figure 5 is a schematic diagram of a photon counting circuit and / or device according to an exemplary embodiment.

[0034] Figure 6 is a schematic diagram illustrating an example of a semiconductor detector submodule according to an exemplary embodiment.

[0035] Figure 7 is a schematic diagram illustrating an example of a semiconductor detector submodule according to another exemplary embodiment.

[0036] Fig. 8A is a schematic diagram illustrating an example of a semiconductor detector sub-module according to yet another exemplary embodiment.

[0037] Figure 8B is a schematic diagram showing an example of a set of tiled detector submodules, wherein each detector submodule is a depth-segmented detector submodule and an application-specific integrated circuit (ASIC) or corresponding circuit system is arranged below the detector submodule as can be seen from the direction of the incident X-ray.

[0038] Fig. 9 is a schematic diagram illustrating an example of a CT imaging system.

[0039] Fig.10 is a schematic diagram showing an example of the design of an X-ray source and an X-ray detector system.

[0040] Fig.11 is a schematic diagram illustrating an example of an X-ray imaging system, such as a CT imaging system, suitable for material decomposition calibration.

[0041] Fig.12 is a schematic diagram illustrating an example of certain relevant components of a CT imaging system, in which a calibration phantom is shown in position for material decomposition calibration.

[0042] Fig.13A is a schematic diagram showing an example of a calibration phantom in cross-section.

[0043] Fig. 13B It is shown Fig.13A Schematic diagram of a perspective view of the calibration phantom.

[0044] Fig.14 is a schematic diagram showing another example of a calibration phantom in cross-section.

[0045] Fig.15Ais a schematic diagram showing an example of a pair of calibration phantoms in cross-section.

[0046] Fig. 15B is a schematic diagram showing an example of a pair of calibration phantoms in cross-section.

[0047] Fig.16A is a schematic diagram illustrating an example of certain relevant components of a CT imaging system, in which a calibration phantom is shown in position for material decomposition calibration.

[0048] Fig. 16B is a schematic diagram illustrating an example of certain relevant components of a CT imaging system, in which a calibration phantom is shown in position for material decomposition calibration.

[0049] Fig. 16C is a schematic diagram illustrating an example of certain relevant components of a CT imaging system, in which a calibration phantom is shown in position for material decomposition calibration.

[0050] Fig.17 is a schematic diagram illustrating an example of certain relevant components of a CT imaging system suitable for material decomposition according to an exemplary embodiment.

[0051] Fig.18A is a schematic diagram illustrating an example of at least one calibration element.

[0052] Fig.18B is a schematic diagram illustrating an example of at least one calibration element.

[0053] Fig.19A is a schematic diagram illustrating an example of at least one calibration element.

[0054] Fig.19B is a schematic diagram illustrating an example of at least one calibration element.

[0055] Fig.19C is a schematic diagram illustrating an example of at least one calibration element.

[0056] Fig. 20A is a schematic diagram illustrating an example of at least one calibration element and at least one filter.

[0057] Fig. 20B is a schematic diagram illustrating an example of at least one calibration element.

[0058] Fig.21 is a schematic flow chart illustrating an example of a method for material decomposition calibration of a CT imaging system.

[0059] Fig. 22 is a schematic diagram illustrating an example of a computer implementation according to one embodiment. DETAILED DESCRIPTION

[0060] Embodiments of the present disclosure will now be described by way of example with reference to the accompanying drawings.

[0061] For a better understanding, it may be useful to continue with the introductory description of a non-limiting example of an overall X-ray imaging system in which data processing and transmission according to the inventive concept may be implemented.

[0062] Figure 2 1 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 has an X-ray detector that detects the X-rays after they have passed through an object; an analog processing circuit system 25 that processes and digitizes the raw electrical signals from the X-ray detector; a digital processing circuit system 40 that can perform further processing operations on the measurement data, such as applying corrections, temporary storage, or filtering; and a computer 50 that stores the processed data and can perform further post-processing and / or image reconstruction. The digital processing circuit system 40 may include a digital processor. According to an exemplary embodiment, all or part of the analog processing circuit system 25 may be implemented in the X-ray detector 20. The X-ray source and the X-ray detector may be coupled to a rotating member of a gantry 11 of the CT imaging system 100.

[0063] The entire X-ray detector may be considered an X-ray detector system 20 , or an X-ray detector 20 in combination with associated analog processing circuitry 25 .

[0064] The image processing system 30 communicates with and is electrically coupled to the analog processing circuit system 25, and may include a digital processing circuit system 40 and / or a computer 50, which 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, or alternatively as a combination of the digital processing circuit system 40 and the computer 50, or, if the digital processing circuit system 40 is further specialized for image processing and / or reconstruction, it may be viewed as the digital processing circuit system itself.

[0065] An example of a commonly used 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 fraction of X-rays that pass through a patient or object. The X-ray source or X-ray tube and the X-ray detectors are mounted in a gantry 11 that can rotate around the imaging object.

[0066] Figure 3 A CT imaging system 100 is schematically shown as an illustrative example of an X-ray imaging system. The CT imaging system includes a computer 50 that receives commands and scanning parameters from an operator via an operator console 60, which may have a display 62 and some form of operator interface, such as a keyboard, mouse, joystick, touch screen, or other input device. The commands and parameters provided by the operator are then used by the computer 50 to provide control signals to an X-ray controller 41, a gantry controller 42, and a table controller 43. Specifically, the X-ray controller 41 provides power and timing signals to the X-ray source 10 to control the emission of X-rays to an object or patient located on the table 12. The gantry controller 42 controls the rotation speed and position of the gantry 11 including the X-ray source 10 and the X-ray detector 20. For 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 coverage of the patient. There is also a detector controller 44, which is configured to control and / or receive data from the X-ray detector 20.

[0067] In one embodiment, the computer 50 also performs post-processing and image reconstruction on the image data output from the X-ray detector 20. Thus, the computer 50 corresponds to FIG. 1 and FIG. Figure 2 The image processing system 30 is shown. An associated display 62 allows an operator to observe the reconstructed image and other data from the computer 50.

[0068] An X-ray source 10 arranged in 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 they have passed through an object or patient. The X-ray detector 20 may be formed, for example, of a plurality of pixels (also referred to as sensors or detector elements) and associated image processing circuitry (such as an application specific integrated circuit (ASIC)) arranged in a detector module. A portion of the analog processing may be implemented in the pixels, while any 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 digital processing that may perform further processing operations on the measurement data, such as applying corrections, temporary storage, and / or filtering. During scanning to acquire X-ray projection data, the gantry and components mounted thereon rotate about an isocenter 13.

[0069] Modern X-ray detectors generally require the conversion of incident X-rays into electrons, which often occurs via the photoelectric effect or Compton interaction, and the resulting electrons generally produce secondary visible light until their energy is lost and this light is in turn detected by a photosensitive material. Semiconductor-based detectors also exist, and in this case the electrons generated by the X-rays generate a charge from electron-hole pairs that are collected by an applied electric field.

[0070] There are detectors that work in energy integrating mode, in the sense that they provide an integrated signal from a large number of X-rays. The output signal is proportional to the total energy deposited by the detected X-rays.

[0071] X-ray detectors with photon counting and energy resolving capabilities are increasingly being used in medical X-ray applications. Photon counting detectors have the advantage that in principle the energy of each X-ray can be measured, which yields additional information about the composition of the object. This information can be used to improve image quality and / or reduce radiation dose.

[0072] Generally speaking, photon counting X-ray detectors determine the energy of a photon by comparing the height of an electrical pulse generated by the interaction of a photon in the detector material with a set of comparator voltages. These comparator voltages are also called energy thresholds. Generally speaking, the analog voltage in the comparator is set by a digital-to-analog converter (DAC). The DAC converts the digital setting sent by the controller into an analog voltage to which the height of the photon pulse can be compared.

[0073] Photon counting detectors count the number of photons that have interacted in the detector during the measurement time. New photons are generally identified by the fact that the height of the electrical pulse exceeds the comparator voltage of at least one comparator. When a photon is identified, the event is stored by incrementing a digital counter associated with the channel.

[0074] When several different thresholds are used, an energy-resolving photon counting detector is obtained, in which the detected photons can be classified into energy bins corresponding to various thresholds. Sometimes, this type of photon counting detector is also called a multi-bin detector. In general, energy information allows the creation of new kinds of images, in which new information is available and image artifacts inherent to conventional techniques can be removed. In other words, for an energy-resolving photon counting detector, the pulse height is compared with multiple programmable thresholds (T1-TN) in a comparator and classified according to the pulse height, which is in turn proportional to the energy. In other words, a photon counting detector comprising more than one comparator is referred to herein as a multi-bin photon counting detector. In the case of a multi-bin photon counting detector, the photon counts are stored in a set of counters, typically each counter corresponding to an energy threshold. For example, a count can be assigned to the highest energy threshold that a photon pulse has exceeded. In another example, a counter tracks the number of times a photon pulse crosses each energy threshold.

[0075] For example, "side-on facing" is a special non-limiting design of a photon counting detector in which the X-ray sensors (such as X-ray detector elements or pixels) are oriented sideways toward incoming X-rays.

[0076] For example, such a photon counting detector may have pixels in at least two directions, wherein one of the two directions facing the sideways photon counting detector has a component in the direction of the X-ray. Such a sideways photon counting detector is sometimes referred to as a depth segmented photon counting detector having two or more pixel depth segments in the direction of the incoming X-ray. 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.

[0077] Alternatively, the pixels may be arranged in an array (non-depth segmented) in a direction substantially perpendicular to the incident X-rays, and each pixel may be oriented sideways toward the incident X-rays. In other words, the photon counting detector may be non-depth segmented while still being arranged sideways toward the incoming X-rays.

[0078] The absorption efficiency can be improved by arranging the side-facing photon counting detector to face sideways, in which case the absorption depth can be chosen to be any length and the side-facing photon counting detector can still be fully depleted without reaching very high voltages.

[0079] The conventional mechanism for detecting X-ray photons by direct semiconductor detectors works essentially as follows. The energy of the X-ray interaction in the detector material is converted into electron-hole pairs inside the semiconductor detector, where the number of electron-hole pairs is generally proportional to the photon energy. The electrons and holes drift toward the detector electrodes and back (or vice versa). During this drift, the electrons and holes induce a current in the electrodes, which can be measured.

[0080] like Figure 4 As shown in , the signals are routed via wiring paths 26 of the detector elements 22 of the X-ray detector to the input of an analog processing circuit system (e.g., an ASIC) 25. It should be understood that the term "application specific integrated circuit (ASIC)" should be broadly interpreted as any general purpose circuit used and configured for a particular application. The ASIC processes the charge generated from each X-ray and converts it into digital data, which can be used to obtain measurement data, such as photon counts and / or estimated energy. The ASIC is configured to be connected to the digital processing circuit system so that the digital data can be sent to the digital processing circuit system 40 and / or one or more memory circuits or components 45, and ultimately the data will be provided for Figure 2 The image processing circuitry 30 or computer 50 in generates input for reconstructing the image.

[0081] Since the number of electrons and holes from one X-ray event is proportional to the energy of the X-ray photon, the total charge in one induced current pulse is proportional to this energy. After the filtering step in the ASIC, the pulse amplitude is proportional to the total charge in the current pulse and therefore proportional to the X-ray energy. The pulse amplitude can then be measured by comparing the value of the pulse amplitude with one or more thresholds (THR) in one or more comparators (COMP) and introducing a counter by which the number of cases where the pulse is greater than the threshold can be recorded. In this way, the number of X-ray photons whose energy exceeds the energy corresponding to the corresponding threshold (THR) that has been detected within a certain time frame can be counted and / or recorded.

[0082] The ASIC typically samples the analog photon pulse once per clock cycle and records the output of the comparator. Depending on whether the analog signal is above or below the comparator voltage, the comparator (threshold) outputs a one or zero. The available information at each sample is, for example, a one or zero for each comparator, which indicates whether the comparator has been triggered (the photon pulse is above the threshold) or not triggered.

[0083] In a photon counting detector, there is typically a photon counting logic component that determines whether a new photon has been recorded and records the photon in a counter. In the case of a multi-bin photon counting detector, there are typically several counters, such as one counter for each comparator, and the photon counts are recorded in these counters based on an estimate of the photon energy. This logic component can be implemented in several different ways. Two of the most common categories of photon counting logic components are non-paralyzable counting mode and paralyzable counting mode. Other photon counting logic components include, for example, local maximum detection, which counts local maxima detected in a voltage pulse and may also record its pulse height.

[0084] Photon counting detectors have many benefits, including but not limited to: high spatial resolution, low sensitivity to electronic noise, good energy resolution; and material separation capabilities (spectral imaging capabilities). However, energy integrating detectors have the advantage of high count rate tolerance. The count rate tolerance comes from the fact / recognition 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 being recorded by the detector. This advantage is one of the main reasons why energy integrating detectors have become the standard for medical CT today.

[0085] Figure 5 A schematic diagram of a photon counting circuit and / or device according to an exemplary embodiment is shown.

[0086] When photons interact in a semiconductor material, a cloud of electron-hole pairs is generated. The charge carriers are collected by electrodes attached to the detector material by applying an electric field across the detector material. The signal is routed from the detector element to the input of a parallel processing circuit (e.g., an ASIC). In one example, the ASIC can process the charge so that a voltage pulse is generated whose maximum height is proportional to the amount of energy deposited by the photon in the detector material.

[0087] The ASIC may include a set of comparators 302, each of which compares the magnitude of the voltage pulse to a reference voltage. The comparator output is typically zero or one (0 / 1), depending on which of the two voltages being compared is larger. Here, we assume that if the voltage pulse is higher than the reference voltage, the comparator output is one (1); if the reference voltage is higher than the voltage pulse, the comparator output is zero (0). A digital-to-analog converter (DAC) 301 can be used to convert a digital setting that may be provided by a user or a control program into a reference voltage that may be used by the comparators 302. If the height of the voltage pulse exceeds the reference voltage of a particular comparator, we will refer to that comparator as being "triggered." Each comparator is typically associated with a digital counter 303, which increments based on the comparator output according to the photon counting logic component.

[0088] As mentioned earlier, when the estimated basis coefficients obtained for each projection line are integrated When arranged into an image matrix, the result is a material-specific projection image for each basis i, also called a basis image. This basis image can be viewed directly (e.g., in projection X-ray imaging) or used as input to a reconstruction algorithm to form the basis coefficients a inside the object i In any case, the result of the basis decomposition can be viewed as one or more basis image representations, such as basis coefficient line integrals or the basis coefficients themselves.

[0089] It will be appreciated that the mechanisms and arrangements described herein can be implemented, combined and rearranged in various ways.

[0090] For example, embodiments may be implemented in hardware, or at least partially in software executed by appropriate image processing circuitry, or a combination thereof.

[0091] The steps, functions, processes and / or blocks described herein may be implemented in hardware (including both general purpose electronic circuitry and application specific circuitry) using any conventional technology such as discrete circuitry or integrated circuit technology.

[0092] Alternatively, or in addition, at least some of the steps, functions, processes and / or blocks described herein can be implemented in software, such as a computer program executed by appropriate image processing circuitry (such as one or more processors or processing units).

[0093] In the following, non-limiting examples of specific detector module implementations will be discussed. More specifically, these examples refer to edge-oriented detector modules and depth-segmented detector modules. Other types of detectors and detector modules are also possible.

[0094] Figure 6 is a schematic diagram illustrating an example of a semiconductor detector submodule according to an exemplary embodiment. This is an example of a detector module 21 in which the semiconductor sensor has a plurality of detector elements or pixels 22, wherein each detector element (or pixel) is typically based on a diode having a charge collection electrode as a key component. X-rays enter through the edge of the detector module.

[0095] Figure 7 is a schematic diagram illustrating an example of a semiconductor detector submodule according to another exemplary embodiment. In this example, again assuming that X-rays enter through the edge of the detector module, the detector module 21 with semiconductor sensors is also divided into a plurality of depth segments or detector elements 22 in the depth direction.

[0096] Typically, a detector element is a single X-ray sensitive sub-element of a detector. Generally speaking, photon interactions occur in a detector element and the charge generated thereby is collected by corresponding electrodes of the detector element.

[0097] Each detector element typically measures the incident X-ray flux as a sequence of frames. A frame is the data measured during a specified time interval, called a frame time.

[0098] Depending on the detector topology, a detector element may correspond to a pixel, especially when the detector is a flat panel detector. A depth segmented detector may be viewed as having multiple detector strips, each strip having multiple depth segments. For such a depth segmented detector, each depth segment may be viewed as a separate detector element, especially if each of the depth segments is associated with its own separate charge collection electrode.

[0099] The detector strips of a depth segmented detector usually correspond to pixels of a common flat panel detector and are therefore sometimes also called pixel strips. However, a depth segmented detector can also be viewed as a three-dimensional pixel array, where each pixel corresponds to a separate depth segmenter / detector element.

[0100] The semiconductor sensor can be implemented as a so-called multi-chip module (MCM), in the sense that it serves as a base substrate for electrical wiring and multiple ASICs, which are preferably attached by so-called flip-chip technology. The wiring will include signal connections from each pixel or detector element to the ASIC input and connections from the ASIC to external memory and / or digital data processing. The power to the ASIC can be supplied by similar wiring, but also by separate connections, taking into account the increase in the cross-section required for the high currents in these connections. The ASIC can be positioned to the side of the active sensor, which means that it can be protected from incident X-rays if an absorber cover is placed on top, and it can also be protected from scattered X-rays coming from the side by positioning the absorber in this direction as well.

[0101] Fig. 8A8,183,535 is a schematic diagram showing a detector module implemented as an MCM similar to the embodiment in U.S. Patent No. 8,183,535. In this example, it is shown how the semiconductor sensor 21 can also have the function of the substrate in the MCM. The signal is routed from the detector element 22 to the input of the parallel processing circuit 24 (e.g., ASIC) positioned next to the active sensor area via the wiring path 23. The ASIC processes the charge generated from each X-ray and converts it into digital data, which can be used to detect photons and / or estimate the energy of the photons. The ASIC can have their own digital processing circuit system and memory for small tasks. In addition, the ASIC can be configured to connect to the digital processing circuit system and / or memory circuit or component located outside the MCM, and ultimately use the data as input for reconstructing the image.

[0102] However, the adoption of deep segmentation also brings two notable challenges to silicon-based photon counting detectors. First, a large number of ASIC channels must be employed to process the data fed from the associated detector segments. In addition to the increased number of channels due to both smaller pixel size and deep segmentation, multiple energy bins also increase the data size. Second, since a given X-ray input count is divided into smaller pixels, segments, and energy bins, each with a much lower signal, detector calibration / correction requires several orders of magnitude more calibration data to minimize statistical uncertainty.

[0103] Naturally, in addition to requiring larger computing resources, hard drives, memory, and central processing units (CPUs) or graphics processing units (GPUs), the data size that is several orders of magnitude larger also slows down both data processing and preprocessing. For example, when the data size is 10GB instead of 10MB, the processing time for reading and writing data may be as long as 1000 times.

[0104] A problem in any detector that counts X-ray photons is the pile-up problem. When the flux rate of X-ray photons is high, problems may arise in distinguishing two subsequent charge pulses. As mentioned above, the pulse length after the filter depends on the shaping time. If this pulse length is greater than the time between two X-ray photon induced charge pulses, the pulses will grow together and the two photons will be indistinguishable and can be counted as one pulse. This is called pile-up. Therefore, one way to avoid pile-up at high photon fluxes is to use a small shaping time, or to use deep segmentation.

[0105] For pile-up calibration vector generation, the pile-up calibration data needs to be pre-processed for jet correction. For material decomposition vector generation, the material decomposition data should preferably be pre-processed for both jet correction and pile-up correction. For patient scan data, the data needs to be pre-processed for jet, pile-up, and material decomposition before image reconstruction ensue. These are simplified examples to explain pre-processing, as actual pre-processing steps may include several other calibration steps such as reference normalization and air calibration as needed. The term processing may only indicate the last step of each calibration vector generation or patient scan, but may be used interchangeably in some cases.

[0106] Figure 8B is a schematic diagram showing an example of a set of tiled detector submodules, wherein each detector submodule is a deeply segmented detector submodule and an ASIC or corresponding circuit system 24 is arranged below the detector elements 22 as can be seen from the direction of the incoming X-rays, thereby allowing wiring paths 23 from the detector elements 22 to parallel processing circuits 24 (e.g., ASICs) in the space between the detector elements.

[0107] Fig. 9 1 is a schematic diagram illustrating an outline example of a CT imaging system. In this schematic example, the overall CT imaging system 100 includes a gantry 111, a patient table 112, which can be inserted into an opening 114 of the gantry 111 during a patient scan and / or a calibration scan. The direction of the rotation axis of the rotating member of the gantry around the imaged object or patient is denoted as the z-direction. The angular direction of the CT imaging system is denoted as the x-direction, and the direction of the incident X-ray is referred to as the y-direction.

[0108] It should be understood, however, that the rotating and fixed components of the gantry need not be part of the CT imaging system, but may be arranged and / or configured in other ways, such as for linear and / or translational relative movement without rotation. As an example, the X-ray source and detector combination may move in a linear and / or translational manner relative to the fixed components of the entire gantry. For example, the X-ray source and detector may move together as a combined assembly unit along a table axis (commonly referred to as the z-axis). Alternatively, the patient table moves while the X-ray source and detector combination is stationary; relative motion is key. This also includes geometric system configurations in which the patient can stand, such as in a so-called phone booth scanner.

[0109] Fig.10is a schematic diagram showing an example of an overall design of an X-ray source-detector system. In this example, a schematic diagram of an X-ray detector including a plurality of detector modules and an X-ray source emitting X-rays is shown. Each detector module may have a set of detector elements defining corresponding pixels. For example, the detector modules may be arranged in parallel and oriented with the side facing the detector module pointing to the X-ray source, and they may be arranged in a slightly curved overall configuration. As described above, the direction of the incident X-rays is referred to as the y-direction. Multiple detector pixels in the direction of the rotation axis of the gantry (referred to as the z-direction) enable multi-plane image acquisition. Multiple detector pixels in the angular direction (referred to as the x-direction) enable simultaneous measurement of multiple projections in the same plane, and this is applied to fan / cone beam CT. The x-direction is sometimes also referred to as the channel direction. Most detectors have detector pixels in both the plane (z) direction and the angular (x) direction.

[0110] Fig.11 1 is a schematic diagram showing an example of an X-ray imaging system, such as a CT imaging system, suitable for material decomposition calibration 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 arranged in an X-ray beam path and configured to generate detector data. A calibration phantom 150 is intended to be located in the X-ray beam path between the X-ray source 110 and the X-ray detector 120.

[0111] The X-ray imaging system 100 also includes an X-ray beam limiting device 130, which is arranged in the X-ray beam path near the X-ray source 110, wherein the X-ray beam limiting device 130 includes at least one calibration element 135 for being arranged in the X-ray beam path. The at least one calibration element 135 may include a calibration plate, similar to the plate 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 atomic element, such as iodine, or a combination of different atomic elements. Thus, the material may be a composition of atomic elements having certain properties. For example, the material may be a composition of multiple atomic elements that simulate the properties of a single atomic element. This may be a more convenient and / or cheaper alternative than using a material containing a single atomic element that may be more rare / expensive.

[0112] The X-ray imaging system 100 also includes an image processing circuit system 140. In this example, the image processing circuit system 140 includes a material decomposition (MD) calibration module 142, which is configured and / or preprogrammed to acquire projection data of a set of projections based on the detector data, and determine a path length of 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 for performing a material decomposition calibration.

[0113] For example, the X-ray imaging system may be a CT imaging system, such as Fig.12 Schematically shown in . Fig.12 is a schematic diagram showing an example of certain relevant components of a CT imaging system, wherein a calibration phantom is shown in a position for material decomposition calibration. As mentioned, the CT imaging system comprises an X-ray source 110 and an X-ray detector 120, which are arranged in such a way that projection images of a subject or object can be acquired from different viewing angles. This is most commonly achieved by mounting the X-ray source 110 and the X-ray detector 120 on a support that can rotate around the subject or object (e.g., a rotating member of a gantry), or in this use case, on a calibration phantom 150.

[0114] By way of example, the X-ray beam limiting device 130 may be part of a pre-patient collimator arranged in connection with the X-ray source 110 .

[0115] In a specific example, at least one calibration element 135 in the X-ray beam limiting device 130 and the calibration phantom 150 together include at least two different materials through which at least some of the X-rays will travel to achieve material decomposition calibration.

[0116] Optionally, at least one calibration element 135 includes at least one curved surface, examples of which will be described in more detail below.

[0117] For example, at least one calibration element 135 includes a first section having a first predetermined thickness T1 and a second section having a second predetermined thickness T2.

[0118] In a particular example, the at least one calibration element includes a first calibration element having a first material M1 , wherein the image processing circuitry is configured to determine a path length through the first material M1 .

[0119] Preferably, the X-ray imaging system 100 and / or the X-ray beam limiting device 130 further comprises a second calibration element comprising a second material M2, wherein the second material M2 is different from the first material M1, and wherein the image processing circuit system is configured to determine (also) the path length through the second material M2.

[0120] In a non-limiting example, the calibration element 135 may include a high-density material, such as iodine. This may provide a more convenient, efficient and / or versatile arrangement for material decomposition of an X-ray imaging system according to the present invention.

[0121] Optionally, the X-ray beam limiting device 130 comprises a motor configured to move the at least one calibration element relative to the X-ray beam path.

[0122] For example, the X-ray beam limiting device 130 further comprises a butterfly filter and / or a hardening filter. Thus, the X-ray beam limiting device may comprise a filter and at least one calibration element 135 .

[0123] In an exemplary practical embodiment, the X-ray imaging system 100 is configured to illuminate the calibration phantom 150 with emitted X-rays during calibration when the calibration phantom is arranged in the X-ray beam path between the X-ray beam limiting device 130 and the X-ray detector 120.

[0124] In another non-limiting example, the X-ray imaging system 100 is intended for use with more than one calibration phantom 150. For example, two calibration phantoms may be arranged in the X-ray beam path and moved / rotated relative to the X-ray beam path. For example, one of the two calibration phantoms may be placed on top of the first calibration phantom and / or used independently to more accurately sample the material space for combinations that occur in imaging tasks of smaller objects (infants, heads, etc.).

[0125] In a practical embodiment, the X-ray imaging system 100 may be considered to include a calibration phantom 150 .

[0126] By way of example, the calibration phantom 150 may include a first phantom material PM1 , wherein the first phantom material PM1 is different from at least one material of the at least one calibration element.

[0127] 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, wherein the second material M2 is the same as the first phantom material PM1 or the second phantom material PM2.

[0128] For example, the calibration phantom 150 may include a combination of at least two geometric objects of different shapes and / or materials, including: a first geometric object, which is located in the middle and includes a first phantom material PM1; and a plurality of second geometric objects, which are arranged around the periphery of the first geometric object, wherein at least a subset of the second geometric objects includes a second phantom material PM2 different from the first phantom material PM1, and wherein the first geometric objects are relatively larger than the second geometric objects.

[0129] Fig.13Ais a schematic diagram showing an example of a calibration phantom in cross-section. Fig. 13B It is shown Fig.13A The calibration phantom may comprise a cylinder with a circular, oval or elliptical cross section. The calibration phantom may comprise at least one rod, for example wherein the diameter is in the interval of 10 cm to 30 cm, preferably 15 cm to 25 cm, more preferably 20 cm. The calibration phantom may also comprise a plurality of rods surrounding the at least one rod, for example wherein the diameter is in the interval of 1 cm to 9 cm, preferably 3 cm to 7 cm, more preferably 5 cm.

[0130] In a specific non-limiting example, the calibration phantom 150 may include a first phantom material PM1 , wherein the first phantom material PM1 may be the same as at least one material of at least one calibration element, for example having a different shape and / or thickness.

[0131] 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 may include a combination of at least three geometric objects of different shapes and / or materials. For example, the calibration may include a plurality of geometric objects 150c (third geometric objects) arranged around the periphery of at least one geometric object 150a (first geometric object) and / or around the periphery of at least a subset of another plurality of geometric objects 150b (second geometric objects), wherein at least a subset of the third geometric objects 150c includes a third phantom material PM3 different from the first phantom material PM1 and the second phantom material PM2, wherein the third geometric object 150c is relatively smaller than the second geometric object 150b. The first geometric object, the second geometric object, and the third geometric object may be cylinders with circular, oval, or elliptical cross-sections.

[0132] Optionally, the X-ray imaging system further comprises a movable platform arranged in the X-ray beam path between the X-ray beam limiting device and the X-ray detector, wherein the platform is configured to hold the calibration phantom 150 .

[0133] Furthermore, the X-ray system may be configured such that the material decomposition can be calibrated based on a mapping between i) path length determinations through the at least one calibration element and the calibration phantom 150 and ii) corresponding detector responses of the X-ray detector.

[0134] For example, the X-ray imaging system may be a computed tomography (CT) system comprising a moving assembly, wherein the X-ray source 110, the X-ray detector 120 and the X-ray beam limiting device 130 are arranged on the moving assembly.

[0135] See again Fig.12, shows an example of certain relevant components of a CT imaging system, wherein a calibration phantom is shown in a position for calibration. For example, such a CT imaging system may be designed to enable calibration of material decomposition based on a mapping between: i) for each of a plurality of rotation angles of a moving component of the CT imaging system and each of a plurality of detector elements of an X-ray detector 120, a path length determination through at least one calibration element 135 and a calibration phantom 150; and ii) a corresponding detector response of the X-ray detector 120.

[0136] By way of example, a CT imaging system may be configured to generate detector data at multiple angles, wherein the moving assembly is configured to move to a predetermined set of angles, stop at each angle, and generate detector data at each stop.

[0137] Optionally, the X-ray detector may be a photon counting multi-energy bin X-ray detector.

[0138] Fig.15A Schematic diagram showing an example of a pair of calibration phantoms in cross section. In this example, the calibration phantom 150 is intended for use in an X-ray imaging system configured for material decomposition calibration according to the present invention. The calibration phantom 150 includes a first geometric object 150a, which is centrally located between a plurality of second geometric objects 150b arranged around the first geometric object 150a. The plurality of second geometric objects 150b may be smaller than the first geometric object 150a. Therefore, each second geometric object 150b provides a smaller path length than the first geometric object 150a. The first geometric object and the second geometric object 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 of the second geometric objects 150b may include the first phantom material PM1, and another of the second geometric objects 150b may include a second phantom material PM2, wherein PM2 may, for example, have a higher density than PM1.

[0139] exist Fig.15AIn the calibration phantom 150, the first geometric object 150a of the first phantom material PM1 and a plurality of second geometric objects 150b of the first phantom material PM1 or the second phantom material PM2 are arranged around the periphery of the first geometric object 150a, and every other second geometric object 150b includes the first phantom material PM1, while the remaining second geometric objects 150b include the second phantom material PM2. The first phantom material PM1 may be polyethylene (PE). The second phantom material PM2 may 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 the second geometric objects 150b may include the second phantom material PM2, or all the second geometric objects 150b may include the first phantom material PM1. It should be understood that the second geometric objects 150b may include one, two, three or more different materials.

[0140] Fig. 15B is a schematic diagram showing an example of a pair of calibration phantoms in cross-section. In this example, the calibration phantom 150 is intended for use in an X-ray imaging system configured for material decomposition calibration. Fig. 15B The calibration phantom 150 has Fig.15A The calibration phantom 150 shown in FIG. Fig. 15B In the embodiment of the present invention, the first geometric object 150a and / or one or more of the second geometric objects 150b have an elliptical and / or oval shape. The first geometric object and the second geometric object may be cylinders with a circular, oval or elliptical cross section. Because the oval and / or elliptical shape of the geometric object is more similar to the human body, it may produce more similar X-ray scattering. Fig. 15B The calibration phantom 150 shown in FIG. 1 may provide better and / or more uniform coverage. It should be understood that the second geometric object 150b may include one, two, three or more different shapes and / or materials.

[0141] Fig.16A , Fig. 16B and Fig. 16C1 is a schematic diagram showing an example of certain relevant components of a CT imaging system, wherein a calibration phantom is shown in a position for 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, wherein the X-ray source 110, the X-ray detector 120, and the X-ray beam limiting device 130 are arranged on a moving assembly. The moving assembly of the X-ray imaging system may be a moving assembly of a gantry. The CT imaging system 100 is intended to enable calibration of material decomposition based on a mapping between: i) for each of a plurality of rotation angles of the moving assembly of the CT imaging system 100 and each of a plurality of detector elements of the X-ray detector, determined by a path length of at least one calibration element (not shown) and the calibration phantom 150; and ii) 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 may be moved in a synchronized manner such that detector data are generated at different rotation angles around the calibration phantom 150. The movement of the moving assembly and thus the X-ray source 110 , the X-ray detector 120 and the X-ray beam limiting device 130 may be performed in a controlled manner. For example, the movement may be controlled by the image processing circuitry of the CT imaging system 100 and / or any computer included in the CT imaging system 100 .

[0142] During calibration, the X-ray source 110, X-ray beam limiting device 130 and X-ray detector 120 may be rotated as during a regular scan, with more rotations at each position to collect more statistics. Each detector element will see a different path length combination of the calibration element and the material in the calibration phantom at a different viewing angle or rotation angle.

[0143] exist Fig.16A In the embodiment, the CT imaging system 100 is configured to continuously generate detector data, wherein the moving component is configured to continuously move, thereby allowing the CT imaging system to continuously generate detector data from different rotation angles of the moving component.

[0144] exist Fig. 16BIn the present invention, the CT imaging system 100 is configured to generate detector data at multiple angles, wherein the moving assembly is configured to move to a set of predetermined angles, stop at each angle and generate detector data at each stop. In other words, the X-ray source 110, the X-ray detector 120 and the X-ray beam limiting device 130 are moved stepwise to multiple positions for collecting projection data at each step. At each position, they stop and the X-ray source 110 irradiates the calibration phantom 150, wherein 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 six degrees) and / or predefined angles optimized for the shape and position of the calibration phantom 150. At each angle, multiple views of the data can be collected and / or averaged to reduce statistical variations.

[0145] exist Fig. 16C In the present invention, the CT imaging system 100 is intended for use with two or more calibration phantoms 150. The CT imaging system can be configured to illuminate the two or more calibration phantoms 150. Fig. 16C In FIG. 1 , two calibration phantoms 150 are imaged by the CT imaging system 100. The two calibration phantoms 150 may be different or identical in material, size and / or shape.

[0146] Fig.17 1 is a schematic diagram showing an example of certain relevant components of a CT imaging system suitable for material decomposition according to an exemplary embodiment, wherein a calibration phantom 150 is located in a position for calibration of the CT imaging system 100. In this example, the calibration phantom 150 can be moved in a horizontal plane and / or in a vertical plane relative to the X-ray beam path, thereby allowing the detector elements of the X-ray detector 120 to collect X-rays with varying path lengths 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 (i.e., the field of view) of the X-ray beam path, thereby allowing edge detector elements to see varying path lengths for different rotation angles. In order to perform the movement of the calibration phantom 150, the X-ray imaging system 100 may include a mechanism configured to shift / move the calibration phantom 150 in a horizontal plane and / or in a vertical plane. The mechanism that performs the movement may be a built-in examination table / examination table movement of the CT imaging system, or some other mechanical mechanism.

[0147] If the calibration phantom 150 is smaller than the full field of view, then the edge detector will only see air if the phantom is placed only at the isocenter, e.g. Fig.17 To handle this, the movement mechanism of the CT imaging system 100 may move / displace the calibration phantom 150 .

[0148] In addition, 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 having to move 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, rather than the X-ray source and the X-ray detector and the X-ray beam limiting device, to a plurality of predetermined angles at which it stops, and allowing the X-ray detector to generate a detector response at each stop, wherein the acquired projection data is based at least in part on the detector response.

[0149] Fig.18A and Fig.18B is a schematic diagram showing an example of at least one calibration element 135. At least one calibration element 135 may be arranged within an X-ray beam limiting device and may include different sections or components having different shapes, thicknesses and / or materials. The calibration element 135 may include a first section 135-1 having a first predetermined thickness T1, a second section 135-2 having a second predetermined thickness T2, and a third section 135-3 having a third predetermined thickness T3. The first section 135-1, the second section 135-2, and the third section 135-3 may have different thicknesses, shapes, and / or materials. In this example, T1, T2, and T3 are different from each other.

[0150] The calibration element 135 can be moved into 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 arranged in or connected to the X-ray beam limiting device.

[0151] exist Fig.18A In the embodiment, the calibration elements 135-1, 135-2 and 135-3 are arranged outside the X-ray beam path. Therefore, there are no X-rays extending through the calibration elements 135-1, 135-2 and 135-3.

[0152] exist Fig.18BIn the embodiment of the present invention, the calibration element 135-2 has been moved into the X-ray beam path so that only the second section 135-2 is located in the X-ray beam path. Therefore, the X-rays travel through the second section 135-2, but not the first section 135-1 or the third section 135-3. The calibration element 135 can be moved in and out of the X-ray beam path so that one of the first section 135-1, the second section 135-2, and the third section 135-3 is located in the X-ray beam path. Therefore, the CT imaging system can allow the X-rays to travel through different thicknesses of the calibration element in a controllable and adjustable manner. All sections of the calibration element can be moved as a single component. The first section 135-1, the second section 135-2, and the third section 135-3 can include the same material in each section, or different materials in each section.

[0153] Fig.19A , Fig.19B and Fig.19C is a schematic diagram showing an example of a calibration element in each figure. The at least one calibration element 135 may include a plurality of calibration elements 135. For example, Fig.19A , Fig.19B and Fig.19C A plurality of calibration elements are shown, including a first calibration element 135a, a second calibration element 135b, and a third calibration element 135c. Each of the plurality of calibration elements 135a, 135b, 135c may be composed of a variety of different materials, thicknesses, and / or shapes. For example, the first calibration element 135a may include a first material, the second calibration element 135b may include a second material different from the first material, and the third calibration element 137c may include a third material different from the first material and the second material. In other exemplary embodiments, the first material, the second material, and the third material may all be the same, or may be composed of two different materials. For example, at least one calibration element may include a high-density material, such as iodine, at least one calibration element may include polyethylene (PE), or at least one calibration element may include polyvinyl chloride (PVC). For another example, at least one calibration element may include different thicknesses, for example, at least one calibration element may include a first thickness, or at least one calibration element may include a second thickness, or at least one calibration element may include a third thickness. The first thickness, the second thickness, and the third thickness may be the same thickness, or they may be different thicknesses. The third calibration element 135c may include a first material, but have a different thickness than the first calibration element 135a. Different calibration elements 135a, 135b, 135c can be moved in and out of the X-ray beam path. This provides the option of changing the path length of the X-rays through different materials, thicknesses and / or shapes. Fig.19A In , only the third calibration element 135c is located in the X-ray beam path. Fig.19BIn FIG. 1 , only the second calibration element 135b and the third calibration element 135c are located in the X-ray beam path. Fig.19C In the embodiment, only the first calibration element 135a and the third calibration element 135c are located in the X-ray beam path.

[0154] Fig. 20A is a schematic diagram showing an example of at least one calibration element 135 and at least one filter 137. Fig. 20A In an example of, the X-ray beam limiting device includes a filter 137, such as a butterfly 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, 135c may be different in material, thickness and / or shape. Each of the calibration elements 135a, 135b, 135c may include a corresponding thickness 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 material and the second material. The material and shape of the filter 137 are selected to modulate the incident X-ray beam according to the angle of the X-ray beam relative to the object / subject to balance the X-ray photon flux on the X-ray detector.

[0155] Fig. 20B is a schematic diagram showing 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, wherein each calibration element includes a curved surface 136. The curved surfaces 136 of the calibration elements 135a, 135b, 135c can make the contribution of all detector pixels to the path length the same. This may be particularly advantageous for CT imaging systems utilizing fan beam applications.

[0156] Fig.21 The present invention is a schematic flow chart showing an example of a method for material decomposition calibration of a CT imaging system. The CT imaging system has 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 an image processing circuit system. The X-ray beam limiting device includes at least one calibration element. Fig.21The method in the invention comprises a first step S1 of placing a calibration phantom in an X-ray beam path of a CT imaging system between an X-ray beam limiting device and an X-ray detector. A second step S2 of starting a calibration sequence. A third step S3 of acquiring projection data of a set of projections based on an output of the X-ray detector. A fourth step S4 of determining a path length through at least one material in at least one calibration element and at least one material in the calibration phantom based at least in part on the acquired projection data. A fifth step S5 of performing a material decomposition calibration based at least in part on the determined path length.

[0157] By way of example, the step of determining the path length includes determining the path length of a first material through at least one calibration element and each of a second material and a third material of a calibration phantom for each of a plurality of rotation angles and each of a plurality of detector elements of the X-ray detector.

[0158] In a specific example, performing the material decomposition calibration step includes generating a mapping between path length and detector response of the X-ray detector.

[0159] For example, the mapping can be used for calibrated image reconstruction.

[0160] Optionally, the X-ray detector is a photon counting multi-bin X-ray detector and the performing step comprises determining a detector element specific mapping of path lengths of different materials to corresponding recorded photon counts of the photon counting multi-energy bin X-ray detector.

[0161] In a specific example, the step of acquiring projection data includes moving the X-ray source, the X-ray detector, and the X-ray beam limiting device to a plurality of positions where they stop, and allowing the X-ray detector to generate a detector response at each stop. The acquired projection data is based at least in part on an average and / or accumulated detector response. This is advantageous because it reduces the input data size for material decomposition calibration and therefore reduces the time spent performing material decomposition calibration. Furthermore, an advantage of this step-and-shoot method is that it is a static scan and is not affected by rotational motion.

[0162] Alternatively, the step of acquiring projection data comprises rotating the calibration phantom, rather than the X-ray source and the X-ray detector and the X-ray beam limiting device, to a plurality of predetermined angles at which it stops, and allowing the X-ray detector to generate a detector response at each stop. The acquired projection data is based at least in part on the detector response. When the X-ray tube has focal spot motion and the detector has its own motion according to angle, rotating the phantom may be a solution if this motion significantly affects the scan.

[0163] As mentioned, at least some of the steps, functions, processes and / or blocks described herein may be implemented in software, such as a computer program, for execution by appropriate image processing circuitry, such as one or more processors or processing units.

[0164] Fig. 22 2 is a schematic diagram illustrating an example of a computer-specific 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, whereby the processor is operable to perform the steps and / or actions described herein. The instructions are typically organized as a computer program 225, 235, which 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, which may be interconnected to the processor 210 and / or the memory 220 to enable input and / or output of relevant data such as input parameters and / or resulting output parameters.

[0165] In a particular example, the memory 220 includes a set of instructions executable by the processor, whereby the processor is operable to perform the steps and / or actions described herein.

[0166] The term "processor" should be interpreted in a general sense as any system or device capable of executing program code or computer program instructions to perform specific processing, determination or computing tasks.

[0167] Thus, image processing circuitry including one or more processors is configured to perform well-defined processing tasks (such as those described herein) when executing a computer program.

[0168] The image processing circuit system does not have to be dedicated only to performing the above-mentioned steps, functions, processes and / or blocks, but may also perform other tasks.

[0169] The proposed technology also provides a computer program product comprising a computer readable medium 220, 230 having such a computer program stored thereon.

[0170] By way of example, the software or computer program 225, 235 may be implemented as a computer program product, which is typically carried or stored on a computer readable medium 220, 230, specifically a non-volatile medium. The computer readable medium may include one or more removable or non-removable memory devices, including but not limited to: read-only memory (ROM), random access memory (RAM), compact disc (CD), digital versatile disc (DVD), Blu-ray disc, universal serial bus (USB) memory, hard disk drive (HDD) storage device, flash memory, magnetic tape, or any other conventional memory device. Thus, the computer program may be loaded into the operating memory of a computer or equivalent processing device for execution by its image processing circuit system.

[0171] The method flow, when executed by one or more processors, can be regarded as a computer action flow. The corresponding device, system and / or apparatus can be defined as a set of functional modules, wherein each step executed by the processor corresponds to a functional module. In this case, the functional module is implemented as a computer program running on the processor. Therefore, the device, system and / or apparatus can alternatively be defined as a set of functional modules, wherein these functional modules are implemented as computer programs running on at least one processor.

[0172] The computer program resident in the memory may therefore be organized into appropriate functional modules that are configured to perform at least part of the steps and / or tasks described herein when the computer program is executed by a processor.

[0173] Alternatively, these modules may be implemented primarily through hardware modules, or alternatively through hardware. The degree of software versus hardware is purely a choice of the particular implementation.

[0174] The embodiments of the present disclosure shown in the drawings and described above are exemplary embodiments only and are not intended to limit the scope of the appended claims, including any equivalents included within the scope of the claims. Those skilled in the art will appreciate that various modifications, combinations and changes may be made to these embodiments without departing from the scope of the invention as defined by the appended claims. It is intended that any combination of non-mutually exclusive features described herein is within the scope of the invention. That is, the features of the embodiments may be combined with any appropriate aspect described above, and the optional features of any one aspect may be combined with any other appropriate aspect. Similarly, the features listed in the dependent claims may be combined with non-mutually exclusive features of other dependent claims, particularly where the dependent claims are dependent on the same independent claim. In some jurisdictions that require single claim dependencies, these dependencies may have been used as practice, but this should not be taken to mean that the features in the dependent claims are mutually exclusive.

[0175] It should also be noted that, unless explicitly stated otherwise, the concept of the present invention relates to all possible feature combinations. In particular, where technically possible, different partial solutions in different embodiments can be combined in other configurations.

Claims

1. An X-ray imaging system configured for material decomposition calibration and intended for use 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; wherein the calibration phantom is located in the X-ray beam path between the X-ray source and the X-ray detector; an X-ray beam limiting device arranged in the X-ray beam path close to the X-ray source, wherein the X-ray beam limiting device comprises at least one calibration element for arrangement in the X-ray beam path; as well as An image processing circuit system is 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 based at least in part on the acquired projection data for performing material decomposition calibration.

2. The X-ray imaging system of claim 1, wherein the X-ray beam limiting device is part of a pre-patient collimator arranged in connection with the X-ray source.

3. An X-ray imaging system according to claim 1, wherein the at least one calibration element in the X-ray beam limiting device and the calibration phantom together include at least two different materials, and at least some of the X-rays will pass through the at least two different materials to achieve material decomposition calibration.

4. The X-ray imaging system of claim 1, wherein the at least one calibration element comprises at least one curved surface. 5 . The X-ray imaging system of claim 1 , wherein the at least one calibration element comprises a first section having a first predetermined thickness T1 and a second section having a second predetermined thickness T2 . 6 . The X-ray imaging system of claim 1 , wherein the at least one calibration element comprises a first calibration element comprising a first material M1 , wherein the image processing circuitry is configured to determine a path length through the first material M1 .

7. An X-ray imaging system according to claim 6, further comprising a second calibration element having a second material M2, wherein the second material M2 is different from the first material M1, and wherein the image processing circuit system is configured to determine the path length through the second material M2.

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

10. The X-ray imaging system of claim 1, wherein the X-ray imaging system is configured to illuminate the calibration phantom during calibration, the calibration phantom being arranged in the X-ray beam path between the X-ray beam limiting device and the X-ray detector.

11. The X-ray imaging system of claim 10, wherein the X-ray imaging system comprises the calibration phantom. 12 . The X-ray imaging system of claim 11 , wherein the calibration phantom comprises a first phantom material PM1 , wherein the first phantom material PM1 is different from at least one material of the at least one calibration element.

13. The X-ray imaging system according to claim 12, wherein the calibration phantom comprises a combination of at least two geometric objects of different shapes and / or materials, the geometric objects comprising: a first geometric object, which is located in the middle and includes a first phantom material PM1; A plurality of second geometric objects are arranged around the periphery of the first geometric object, at least a subset of the second geometric objects comprises a second phantom material PM2 different from the first phantom material PM2, wherein the first geometric objects are relatively larger than the second geometric objects.

14. The X-ray imaging system according to claim 13, wherein the calibration phantom comprises a combination of at least three geometric objects of different shapes and / or materials, the geometric objects further comprising: a plurality of third geometric objects, the plurality of third geometric objects being arranged around a periphery of the first geometric object and / or around a periphery of at least a subset of the second geometric objects, the at least a subset of the third geometric objects comprising a third phantom material PM3 different from the first phantom material PM1 and the second phantom material PM2, wherein the third geometric objects are relatively smaller than the second geometric objects.

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

16. An X-ray imaging system according to claim 1, wherein the X-ray system is configured to achieve calibration for material decomposition based on a mapping between: path length determination through the at least one calibration element and the calibration phantom; and a corresponding detector response of the X-ray detector.

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

18. An X-ray imaging system according to claim 17, wherein the CT imaging system is intended to achieve calibration for material decomposition based on a mapping between: for each of the multiple rotation angles of the moving component of the CT imaging system and each of the multiple detector elements of the X-ray detector, determined by the path length of 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 of claim 17, wherein the CT imaging system is configured to generate detector data at multiple angles, wherein the moving assembly is configured to move to a predetermined set of angles, stop at each angle and generate detector data at each stop.

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

21. A method for material decomposition calibration of an X-ray imaging system, the 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 to the X-ray source, and an image processing circuit system, wherein the X-ray beam limiting device includes at least one calibration element, the method comprising: placing a 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; Start the calibration sequence; collecting projection data of a set of projections based on outputs of the X-ray detector; determining a path length through at least one material in the at least one calibration element and at least one material in the calibration phantom based at least in part on the acquired projection data; as well as A material decomposition calibration is performed based at least in part on the determined path length.

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

23. The method of claim 21, wherein performing a material decomposition step 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. The method of claim 21, wherein the X-ray detector is a photon counting multi-bin X-ray detector and the executing step comprises determining a detector element specific mapping of the path lengths of the different materials to corresponding recorded photon counts of the photon counting multi-energy bin X-ray detector.

26. The method of claim 21, wherein the step of acquiring projection data comprises: moving the X-ray source, the X-ray detector, and the X-ray beam limiting device to a plurality of positions where they stop, and allowing the X-ray detector to generate a detector response at each stop; Wherein the acquired projection data is based at least in part on an average and / or accumulated detector response.

27. The method according to claim 21, wherein the step of collecting projection data comprises: rotating the calibration phantom, but not the X-ray source, the X-ray detector, and the X-ray beam limiting device, to a plurality of predetermined angles at which they stop, and allowing the X-ray detector to generate a detector response at each stop; Wherein the acquired projection data is based at least in part on the detector response.

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