Method for reducing dependency on focal spot size in material density calibration

Abstract

Methods and systems are provided for calibrating a photon counting computed tomography (PCCT) system (100, 200). In order to reduce the amount of calibration data stored in a memory of the PCCT system (100, 200), and in order to reduce the time taken to calibrate the PCCT system (100, 200), a method (600, 700) is provided for correcting projection data acquired at different focal spot sizes using the PCCT system using a material decomposition (MD) calibration vector generated for a first focal spot size. To compensate for spectral differences due to focal spot sizes, the projection data is corrected and normalized by air calibration vectors generated for each different focal spot size.

Description

Technical Field

[0001] The embodiments of the subject matter disclosed herein relate to imaging systems and methods, and more specifically, to the calibration of computed tomography (CT) imaging systems. Background Technology

[0002] In a computed tomography (CT) imaging system, an electron beam generated by a cathode is directed at a target within an X-ray tube. A fan-shaped or cone-shaped X-ray beam, produced by electrons colliding with the target, is then directed at the subject, such as a patient. After being attenuated by the object, the X-rays strike an array of X-ray detectors, thus generating a CT image. The quality of CT images can be improved by using photon-counting CT (PCCT), where the X-ray detectors are photon-counting detectors, and photons are counted to provide spectral information.

[0003] To ensure the accuracy of images reconstructed from PCCT scans, the CT system can be calibrated periodically and / or periodically. During calibration, various calibration vectors are generated to compensate for variations in detector response, which may depend on the scanning protocol, spot size, bowtie filter used, or selected X-ray peak energy. Calibration vectors can be applied to the detector response to adjust photon counts and improve image quality. However, generating these calibration vectors can be a time-consuming process, during which the CT system may be unavailable to the patient. Summary of the Invention

[0004] This disclosure addresses at least one or more of the aforementioned problems in part by providing a method for a photon-counting computed tomography (PCCT) system, the method comprising: performing a scan using the PCCT system with a first focal spot of a first size; applying a material decomposition (MD) calibration vector to correct projection data acquired during the scan, the material decomposition (MD) calibration vector being generated for a second focal spot of a second size, different from the first size; reconstructing an image from the corrected projection data; and displaying the image on a display device.

[0005] The above-described advantages, as well as other advantages and features, of this specification will become apparent from the following detailed description when considered alone or in conjunction with the accompanying drawings. It should be understood that the above summary is provided to present a simplified version of a series of concepts further described in the detailed description. This is not intended to identify key or essential features of the claimed subject matter, the scope of which is uniquely defined by the claims following the detailed description. Furthermore, the claimed subject matter is not limited to specific implementations that address any of the shortcomings mentioned above or in any part of this disclosure. Attached Figure Description

[0006] A better understanding of the various aspects of this disclosure can be achieved by reading the following detailed description and referring to the accompanying drawings, in which:

[0007] Figure 1 A painting view of a photon counting computational computed tomography (PCCT) imaging system according to one or more embodiments of the present disclosure is shown;

[0008] Figure 2 A schematic block diagram of an exemplary PCCT imaging system according to one or more embodiments of the present disclosure is shown;

[0009] Figure 3A This is a schematic diagram of an exemplary X-ray tube according to one or more embodiments of the present disclosure;

[0010] Figure 3B This is a schematic diagram of a focal spot on a target according to one or more embodiments of the present disclosure;

[0011] Figure 4 The distribution of electrons in an electron beam of a PCCT system focused on four discrete focal spots according to one or more embodiments of the present disclosure is shown.

[0012] Figure 5 This is a flowchart illustrating a method for calibrating a PCCT system according to one or more embodiments of the present disclosure;

[0013] Figure 6 This is a flowchart illustrating a method for correcting projection data acquired via a PCCT system using a calibration vector generated during the calibration of the PCCT system, according to one or more embodiments of the present disclosure.

[0014] Figure 7 This is a flowchart illustrating a method for applying a calibration vector to projection data acquired via a PCCT system according to one or more embodiments of the present disclosure;

[0015] Figure 8 This is a first exemplary image of a phantom after the projection data has been first corrected using a calibration vector, according to one or more embodiments of this disclosure;

[0016] Figure 9 This is a second exemplary image of the phantom after the projection data has been second-corrected using a calibration vector, according to one or more embodiments of this disclosure; and

[0017] Figure 10 This is a third exemplary image of a phantom after the projection data has been corrected using a calibration vector, according to one or more embodiments of this disclosure.

[0018] The accompanying drawings illustrate specific aspects of the systems and methods described herein. Together with the following description, the drawings illustrate and explain the structures, methods, and principles described herein. In the drawings, the dimensions of components may be enlarged or otherwise modified for clarity. Well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the components, systems, and methods described. Detailed Implementation

[0019] This specification and embodiments of the subject matter disclosed herein relate to methods and systems for reducing the amount of time spent calibrating photon-counting computed tomography (PCCT) systems. Typically, in a computed tomography (CT) imaging system, an X-ray source or X-ray tube emits a fan-shaped or cone-shaped beam toward a subject such as a patient. Typically, in a CT system, the X-ray source and detector array rotate around a gantry within the imaging plane and around the patient, and images are generated from projection data at multiple views at different angles. For example, for one rotation of the X-ray source, 1000 views may be generated by the CT system. The beam strikes the radiation detector array after being attenuated by the patient. The X-ray detector or detector array typically includes a collimator for collimating the X-ray beam received at the detector, a scintillator disposed adjacent to the collimator for converting the X-rays into light energy, and a photodiode for receiving the light energy from the adjacent scintillator and generating an electrical signal therefrom. The intensity of the attenuated X-ray beam radiation received at the detector array typically depends on the patient's attenuation of the X-ray beam. Each detector element in the detector array generates a separate electrical signal, which indicates the attenuation beam received by each detector element. These electrical signals are transmitted to a data processing system for analysis. The data processing system processes the electrical signals to facilitate the generation of an image.

[0020] Conventional CT imaging systems utilize detectors that convert X-ray energy into electrical signals. These signals are integrated over a period of time, then measured and ultimately digitized. However, a drawback of such detectors is that they cannot provide data or feedback regarding the number and / or energy of the detected photons. In contrast, PCCT detectors can provide photon counting and / or energy discrimination feedback with high spatial resolution. PCCT detectors can operate in X-ray counting mode, energy measurement mode per X-ray event, or both.

[0021] However, it can be difficult to accurately measure the PCCT detector response for various reasons. One reason is photon count "stacking," a phenomenon that occurs when the source flux at the detector is so high that there is a non-negligible possibility that two or more X-ray photons deposit their energy in a single pixel at temporal closeness, causing their signals to overlap. There are two general types of stacking, and they produce somewhat different effects. In the first type, two or more events are separated by enough time that they are identified as distinct events, but the signals overlap, reducing the accuracy of measuring the energy of one or more X-rays arriving later. This type of stacking leads to a decrease in the system's energy resolution. In the second type of stacking, two or more events arrive at times close enough that the system cannot resolve them as distinct events. In this case, these events are identified as a single event with the sum of their energies, and these events are shifted to higher energies in the spectrum. Furthermore, stacking causes a more or less significant decrease in the count at high X-ray fluxes, resulting in a loss of detector quantum efficiency (DQE).

[0022] Therefore, the output of a photon counting detector is typically corrected for stacking effects. Depending on the size and density of the anatomical region being scanned, stacking can occur at different detectors. To correct for stacking effects, the photon count measured at each detector is adjusted by applying a stacking calibration vector. The stacking calibration vector includes a value that indicates how much the detector response should be adjusted to account for stacking for each individual detector element (e.g., pixel).

[0023] PCCT detector responses can also vary and require correction for other reasons. For example, the amount of tube current supplied by the X-ray tube during a scan may not precisely match the amount of tube current requested for the scan. To correct for variations in tube current, a tube current calibration vector can be applied to the detector response. The tube current calibration vector serves as a lookup table that indicates how the tube current should be adjusted for each detector based on the requested amount of tube current.

[0024] Furthermore, the span of each detector element in the detector array can vary. To obtain accurate projection data, signals from multiple detector elements can be normalized pixel-by-pixel by applying a third air calibration vector. A fourth material decomposition (MD) calibration vector can be applied to adjust the photon count based on the ratio of component materials in the scanned anatomical structure. Other types of calibration vectors can also be applied.

[0025] During the calibration phase of a PCCT system prior to a scan, stacking, tube current, air, MD, and other calibration vectors are typically generated or updated against a given set of protocols. During the calibration phase, various scans can be performed on one or more phantoms according to various protocols. Each protocol specifies a set of desired parameters for the scan, such as focal spot size, tube current, collimation parameters, etc. After each scan or a set of scans, calibration vectors can be created based on the detector output during the scan.

[0026] Therefore, obtaining the desired image quality from a PCCT system typically requires generating sets of calibration vectors, which can be selectively applied to the PCCT system detector output to compensate for and correct pixel-wise variations in the detector response. Multiple sets of calibration vectors are generated during periodic (e.g., annual) calibrations that can be performed on a PCCT system. The generation of calibration vectors during annual calibrations can be performed in a detailed manner, potentially requiring numerous acquisition scans on many different phantoms using various combinations of scan parameters. When subsequent scans are performed on the subject, appropriate calibration vectors are selected and applied based on the protocol used for the scan and the physical properties of the scanned anatomy. The more phantoms and combinations used, the more calibration vectors can be created, potentially leading to an overall improvement in the quality of the images reconstructed using the PCCT system.

[0027] However, as the number of scans increases, calibration consumes more time. Therefore, there is a trade-off between generating the desired number of scan / calibration vectors to meet image quality goals and keeping the total calibration time within the desired range. Currently, calibration can take several days per year, resulting in greater-than-expected customer downtime.

[0028] The calibration vector can be additionally regenerated or updated during periodic calibrations performed on a PCCT system. During periodic calibrations, scans can be performed using fewer combinations of scan parameters than in annual calibrations, reducing calibration time. For example, periodic calibrations can be performed daily, where daily calibrations may take 30 to 60 minutes.

[0029] During periodic and scheduled calibrations, PCCT system resources may be unavailable for performing scans on patients. Therefore, to increase the usability and availability of PCCT systems, it is desirable to reduce the amount of time spent performing calibrations while still generating a sufficient number of calibration vectors to achieve the desired image quality for the intended patient scanning protocol.

[0030] While some types of calibration, such as stacking calibration, are independent of one or more protocol settings, others require repetition for each protocol. For example, the detector response for air, tube current, and MD calibrations depends on the focal size, primarily due to X-ray beam width, its intensity variations, and detector architecture. Focal sizes range from ultra-small (XS) to small (S) to large (L) to ultra-large (XL). The repetition of those focal size-dependent calibrations can be very costly for some calibrations in terms of both calibration data time and processing time. For instance, air and tube current calibrations require only one scan per focal size, but for MD, a set of multiple scans is necessary per focal size.

[0031] In particular, generating MD calibration vectors can be more complex and time-consuming than other types of calibration vectors because MD calibration is based on scans of different combinations of two or more reference plate materials (such as polyethylene (PE) and aluminum (Al) or PE and polyvinyl chloride (PVC)). PE represents soft tissue in the human body, and Al is used to represent bone. For example, five different thicknesses of PE and four different thicknesses of Al can be used to cover human X-ray attenuation. For a single focal spot size MD vector, combining these different thicknesses may require up to 20 calibration scans (e.g., 5 × 4). For four focal spot sizes, this could be as many as 20 × 4 = 100 scans. The number of scans can increase rapidly depending on the number of plates and their combinations. Additionally, for thicker combinations of reference plates that result in high X-ray attenuation, the calibration scan time during which the X-rays are on can be increased to collect a sufficient number of X-ray counts, which can increase the data size and its disposal time, and reduce the X-ray tube life. Therefore, a large portion of the total time spent calibrating a PCCT system is likely spent generating MD calibration vectors, while a smaller portion of that total time is spent generating other types of calibration vectors.

[0032] To reduce the total time spent calibrating a PCCT system, this paper discloses a system and method for reducing the time spent generating MD calibration vectors during calibration. Specifically, the disclosed method reduces the total number of MD calibration scans and corresponding vectors that the PCCT system relies on for material density decomposition during image reconstruction. Instead of relying on separate sets of calibration vectors for each focal size, one or more common sets of MD calibration vectors can be generated that can be applied during image reconstruction for various focal sizes. By using common sets of MD calibration vectors for projection data acquired at each focal size, the time spent performing calibration vector generation and the amount of MD calibration data collected can be reduced by up to 60%, while maintaining image quality within desired limits. By reducing the time spent performing calibration, the availability of the PCCT system for patient scans can be increased. Furthermore, the amount of computational and memory resources consumed by the PCCT system can be reduced, thereby increasing the availability of resources for other tasks and increasing the efficiency of PCCT system usage.

[0033] Figure 1 An exemplary PCCT system 100 configured for CT imaging using a photon counting detector is illustrated. Specifically, the PCCT system 100 is configured to image a subject 112 (such as a patient, inanimate object, one or more manufactured parts) and / or foreign objects (such as dental implants, stents, and / or contrast agents present in the body). In one embodiment, the PCCT system 100 includes a gantry 102, which may further include at least one X-ray source 104 configured to project an X-ray radiation beam 106 (see [link to documentation]). Figure 2 This is used to image the subject 112 lying on the examination table 114. Specifically, the X-ray source 104 is configured to project an X-ray radiation beam 106 toward a detector array 108 positioned on the opposite side of the gantry 102. Although Figure 1 A single X-ray source 104 is depicted, but in some embodiments, multiple X-ray sources and detectors may be employed to project multiple X-ray radiation beams for acquiring projection data at different energy levels corresponding to the patient. In some embodiments, the X-ray source 104 can achieve dual-energy gemstone spectral imaging (GSI) via rapid peak kilovolt (kVp) voltage switching. In the embodiments described herein, the X-ray detector employed is a photon counting detector capable of distinguishing X-ray photons of different energies.

[0034] In some embodiments, the PCCT system 100 further includes an image processor unit 110 configured to reconstruct an image of the target volume of the subject 112 using iterative or analytical image reconstruction methods. For example, the image processor unit 110 may use analytical image reconstruction methods such as filtered back projection (FBP) to reconstruct an image of the patient's target volume. Alternatively, the image processor unit 110 may use iterative image reconstruction methods such as advanced statistical iterative reconstruction (ASIR), conjugate gradient (CG), maximum likelihood expectation maximization (MLEM), model-based iterative reconstruction (MBIR), etc., to reconstruct an image of the target volume of the subject 112. As further described herein, in some examples, in addition to iterative image reconstruction methods, the image processor unit 110 may also use analytical image reconstruction methods (such as FBP).

[0035] In some CT imaging system configurations, an X-ray source projects a cone-shaped X-ray beam, which is collimated to lie in the XYZ plane of a Cartesian coordinate system and is often referred to as the "imaging plane." The X-ray beam passes through the object being imaged, such as a patient or subject. After being attenuated by the object, the X-ray beam strikes an array of detector elements. The intensity of the attenuated X-ray beam received at the detector array depends on the attenuation of the X-ray beam by the object. Each detector element in the array generates a separate electrical signal, which is a measurement of the X-ray beam attenuation at the detector location. Attenuation measurements from all detector elements are acquired individually to produce the transmission distribution.

[0036] In some CT systems, a gantry is used to rotate the X-ray source and detector array around the object being imaged within the imaging plane, causing the angle at which the X-ray beam intersects the object to continuously change. A set of X-ray radiation attenuation measurements (e.g., projection data) from the detector array at a given gantry angle is called a "view." A "scan" of the object comprises a set of views taken at different gantry angles or viewing angles during one rotation of the X-ray source and detector.

[0037] Figure 2 Examples similar to Figure 1 An exemplary imaging system 200 of a PCCT system 100. According to various aspects of this disclosure, the imaging system 200 is configured to image a subject 204 (e.g., ...). Figure 1 Imaging is performed on the subject 112. In one embodiment, the imaging system 200 includes a detector array 108 (see [link to image processing system]). Figure 1 The detector array 108 also includes a plurality of detector elements 202 that together sense an X-ray radiation beam 106 passing through the subject 204 (such as a patient) (see [link]). Figure 2To acquire corresponding projection data. In some embodiments, the detector array 108 may be manufactured as a multi-slice configuration including multiple rows of cells or detector elements 202, wherein one or more additional rows of detector elements 202 are arranged in a parallel configuration for acquiring projection data.

[0038] In some embodiments, the imaging system 200 is configured to traverse different angular positions around the subject 204 to acquire desired projection data. Therefore, the gantry 102 and the components mounted thereon may be configured to rotate about a center of rotation 206 to acquire projection data, for example, at different energy levels. Alternatively, in embodiments where the projection angle relative to the subject 204 varies over time, the mounted components may be configured to move along a generally curved path rather than along a circumference.

[0039] Therefore, as the X-ray source 104 and detector array 108 rotate, detector array 108 collects data of the attenuated X-ray beam. The data collected by detector array 108 then undergoes preprocessing and calibration to adjust the data to represent the line integral of the attenuation coefficient of the scanned subject 204. The processed data is typically referred to as a projection. In some examples, individual detectors or detector elements 202 in detector array 108 may include photon counting detectors that record the interactions of individual photons into one or more energy bins.

[0040] The acquired projection dataset can be used for Base Material Decomposition (BMD). During BMD, the measured projections are converted into a set of material density projections by applying MD calibration vectors. The material density projections can be reconstructed to form a pair or set of material density maps or images (such as bone maps, soft tissue maps, and / or contrast agent maps) for each corresponding base material. The density maps or images can then be correlated to form a 3D volumetric image of the base material (e.g., bone, soft tissue, and / or contrast agent) in the imaging volume.

[0041] Once reconstructed, the base material image generated by imaging system 200 reveals the internal features of subject 204 represented by the densities of the two base materials. Density images can be displayed to illustrate these features. In traditional methods of diagnosing medical conditions (such as disease states), and more generally, medical events, radiologists or physicians will consider a hard copy or display of the density image to identify features of interest. Such features may include lesions, size, and shape of specific anatomical structures or organs, as well as other features that should be identifiable in the image based on the individual practitioner's skill and knowledge.

[0042] In one embodiment, the imaging system 200 includes a control mechanism 208 to control the movement of components, such as the rotation of the gantry 102 and the operation of the X-ray source 104. In some embodiments, the control mechanism 208 further includes an X-ray controller 210 configured to provide power and timing signals to the X-ray source 104. Additionally, the control mechanism 208 includes a gantry motor controller 212 configured to control the rotational speed and / or position of the gantry 102 based on imaging requirements.

[0043] In some embodiments, control unit 208 further includes a data acquisition system (DAS) 214 configured to sample analog data received from detector element 202 and convert the analog data into digital signals for subsequent processing. DAS 214 may also be configured to selectively aggregate a subset of analog data from detector element 202 into a so-called macro detector, as further described herein. The data sampled and digitized by DAS 214 is sent to a computer or computing device 216. In one example, computing device 216 stores the data in storage device 218. For example, storage device 218 can be any type of non-transitory memory and may include hard disk drives, floppy disk drives, optical disc read / write (CD-R / W) drives, digital versatile optical disc (DVD) drives, flash memory drives, and / or solid-state storage drives.

[0044] Additionally, computing device 216 provides commands and parameters to one or more of the DAS 214, X-ray controller 210, and rack motor controller 212 for system operation, such as data acquisition and / or processing. In some embodiments, computing device 216 controls system operation based on operator input. Computing device 216 receives operator input, such as commands and / or scan parameters, via an operator console 220 operatively coupled to computing device 216. Operator console 220 may include a keyboard (not shown) or a touchscreen to allow the operator to specify commands and / or scan parameters.

[0045] Although Figure 2 An operator console 220 is illustrated, but more than one operator console may be coupled to the imaging system 200, for example, to input or output system parameters, request checks, plot data, and / or view images. Furthermore, in some embodiments, the imaging system 200 may be coupled via one or more configurable wired and / or wireless networks (such as the Internet and / or VPNs, wireless telephone networks, wireless LANs, wired LANs, wireless WANs, wired WANs, etc.) to multiple displays, printers, workstations, and / or similar devices, located locally or remotely, either within an institution or hospital or in entirely different locations.

[0046] In one implementation, for example, imaging system 200 includes or is coupled to a Picture Archiving and Communication System (PACS) 224. In an exemplary specific implementation, PACS 224 is further coupled to a remote system (such as a radiology information system, a hospital information system) and / or to an internal or external network (not shown) to allow operators in different locations to supply commands and parameters and / or obtain access to image data.

[0047] The computing device 216 operates the inspection table motor controller 226 using operator-provided and / or system-defined commands and parameters. This inspection table motor controller, in turn, controls the inspection table 114, which may be an electric inspection table. Specifically, the inspection table motor controller 226 can move the inspection table 114 to properly position the subject 204 within the rack 102 to acquire projection data corresponding to the target volume of the subject 204.

[0048] As previously described, the DAS214 samples and digitizes the projection data acquired by detector element 202. Subsequently, image reconstructor 230 uses the sampled and digitized X-ray data to perform high-speed reconstruction. Although Figure 2 Image reconstructor 230 is illustrated as a separate entity, but in some embodiments, image reconstructor 230 may be part of computing device 216. Alternatively, image reconstructor 230 may not be present in imaging system 200, and alternatively, computing device 216 may perform one or more functions of image reconstructor 230. Furthermore, image reconstructor 230 may be located locally or remotely and may be operatively connected to imaging system 200 using wired or wireless networks. Specifically, one exemplary embodiment may use computing resources in a "cloud" network cluster for image reconstructor 230.

[0049] In one embodiment, image reconstructor 230 stores reconstructed images in storage device 218. Alternatively, image reconstructor 230 may send the reconstructed images to computing device 216 to generate usable patient information for diagnosis and evaluation. In some embodiments, computing device 216 may transmit the reconstructed images and / or patient information to a display or display device 232 communicatively coupled to computing device 216 and / or image reconstructor 230. In some embodiments, reconstructed images may be transferred from computing device 216 or image reconstructor 230 to storage device 218 for short-term or long-term storage.

[0050] Now for reference Figure 3A An exemplary X-ray tube 300 is shown. In one embodiment, the X-ray tube 300 may be an X-ray source 104 (see [link to documentation]). Figures 1 to 2In the illustrated embodiment, the X-ray tube 300 includes an exemplary cathode 302 and an anode 303 disposed within a tube housing 306. The cathode may include one or more emitters 308. In this example, the cathode 302, and in particular the one or more emitters 308, can be directly heated by passing an electric current through the one or more emitters 308, which may be supplied by a voltage source 310. In one embodiment, a current of about 10 amperes (A) may be passed through the one or more emitters 308. The one or more emitters 308 may emit an electron beam 312 due to being heated by the current supplied by the voltage source 310. As used herein, the term "electron beam" may be used to refer to a stream of electrons having substantially similar velocities.

[0051] An electron beam 312 can be directed toward a target 304 to generate X-rays 314. More specifically, the electron beam 312 can be accelerated from the emitter 308 toward the target 304 by applying a potential difference between one or more emitters 308 and anode 303. In one embodiment, a high voltage in the range of about 40 kV to about 450 kV can be applied to establish a potential difference between one or more emitters 308 and anode 303, thereby generating one or more electric fields 320 in the X-ray tube 300.

[0052] Electron beam 312 may strike target 304 at a focal spot 332. The focal spot 332 may have an orientation indicated by coordinate axis 349. When electron beam 312 strikes target 304, heat may be generated in target 304 at the location of focal spot 332, and this heat may be sufficient to melt target 304. In various embodiments, a rotating target may be used to circumvent the problem of heat generation in target 304. For example, target 304 may be configured to rotate such that the focal spot 332 generated by electron beam 312 striking target 304 does not always strike target 304 at the same location, thus preventing target 304 from melting. In various embodiments, target 304 may comprise a material such as, but not limited to, tungsten or molybdenum.

[0053] Brief Reference Figure 3B The image shows a front view 350 of target 304, including a focal spot 332. The focal spot 332 may have dimensions indicated by coordinate axes 352. Figure 3B The orientation of the target 304 is as described above. The target 304 can be a circular target that rotates such that focal spots 332 are generated at different locations on the surface of the target 304 as it rotates. By generating focal spots 332 at different locations on the surface of the target 304, the amount of heat absorbed at those locations on the target 304 can be minimized.

[0054] return Figure 3AThe size of the focal spot on target 304 can be adjusted to reduce the amount of heat generated in target 304, where a smaller focal spot can generate a greater amount of heat at a specific location. An electron collector 329, maintained at the same potential as target 304, acts as a collector for electrons bounced back from the surface of target 304 during the initial impact, reducing the chance of those same electrons re-bombing the target. Collecting backscattered electrons in this manner further reduces target heating.

[0055] X-ray tube 300 may include one or more focusing electrodes 316 disposed adjacent to emitter 308, such that the one or more focusing electrodes 316 focus electron beam 312 toward target 304. As used herein, the term “adjacent” means close in space or location. To focus electron beam 312, a voltage may be applied to one or more focusing electrodes 316 to generate one or more electric fields 321. The voltage may be different for each of the one or more focusing electrodes 316. In some embodiments, a first portion of focusing electrode 316 may be used to deflect electron beam 312, and a second portion of focusing electrode 316 may be used to focus electron beam 312. In this way, a voltage may be selectively applied by a controller of control electronics module 322 to generate one or more specific electric fields that focus electron beam 312 into a desired shape and deflect electron beam 312 to a desired location. Additionally, X-ray tube 300 may include one or more extraction electrodes 318 that may be used to additionally control electron beam 312 and focus it toward anode 303.

[0056] When the electron beam 312 is focused onto the target 304, the electrons can form a Gaussian distribution. For the purposes of this disclosure, the Gaussian distribution can be an approximate Gaussian distribution. The Gaussian distribution of the electrons in the electron beam 312 can be narrowed or parallelized, wherein electrons colliding with the target 304 at each side of the Gaussian distribution can be guided toward the center of the Gaussian distribution. In other words, the electron distribution at each side of the Gaussian distribution can be reversed, thereby producing a rectangular or batwing-shaped focal spot.

[0057] Additionally, the X-ray tube 300 may include one or more magnets 324 for focusing and / or positioning and deflecting the electron beam 312 onto the target 304. In various embodiments, one or more magnets 324 may be disposed between the cathode 302 and the target 304.

[0058] When the properties of the electron beam current and voltage change, the electrostatic focusing of the electron beam 312 will change accordingly. To maintain a stable focal spot size, shape, and other properties, or to rapidly modify the focal spot size and / or shape according to system requirements, one or more magnets 324 can provide a magnetic field with controllable performance for a wide range of focal spot sizes and shapes from steady state to sub-30 microsecond time scales. Once the electron beam 312 has been focused and positioned, it strikes the target 304 at the focal spot 332 to generate X-rays 314. The X-rays 314 generated by the collision of the electron beam 312 with the target 304, which can be guided from the X-ray tube 300, pass through an opening in the tube housing 306 at the X-ray window 337 toward the object 328.

[0059] As the electron beam 312 collides with the target 304 at the focal spot 332, a set of X-rays 336 is generated and guided toward the object 328 through the X-ray window 337. This set of X-rays 336 may intersect the object 328 at the effective focal spot 340. The effective focal spot may have a width (in the X dimension, as indicated by coordinate axis 348) and a length (in the Z dimension, as indicated by coordinate axis 348).

[0060] As described above, X-ray tubes can be designed to produce multiple discrete focal spots that can be individually selected for scanning. For example, a CT system can support four different focal spot sizes: ultra-small focal spot, small focal spot, large focal spot, and ultra-large focal spot, such as... Figure 4 As shown.

[0061] Now for reference Figure 4 Figure 400 illustrates the electron distribution of an electron beam (e.g., electron beam 312) focused on a first focal spot 402 of a first size, a second focal spot 404 of a second size, a third focal spot 406 of a third size, and a fourth focal spot 408 of a fourth size in a PCCT system. These focal spots can each be focused on a target 304. Figure 3A The focal spot 332 is the same as or similar to the focal spot 402. In various embodiments, focal spots 402, 404, 406 and 407 may be examples of discrete focal spot sizes supported by the PCCT system, where the PCCT system may not support other focal spot sizes.

[0062] The size and shape of focal spots 402, 404, 406, and 407 may depend on one or more electric or electromagnetic fields (e.g., one or more electric fields 321) generated by one or more focusing electrodes (e.g., one or more focusing electrodes 316). For example, one or more electric fields may perform a first focusing of the electron beam (e.g., electron beam 312) to generate a first focal spot 402. One or more electric fields may perform a second focusing of the electron beam to generate a second focal spot 404. One or more electric fields may perform a third focusing of the electron beam to generate a third focal spot 406. One or more electric fields may perform a fourth focusing of the electron beam to generate a fourth focal spot 408. In some embodiments, the size and shape of focal spots 402, 404, 406, and 407 may also depend on one or more magnetic fields (e.g., magnetic field 323) generated by one or more magnets (e.g., one or more magnets 324), which may further focus the electron beam after focusing by one or more electric fields.

[0063] The distribution of electrons generating focal spots 402, 404, 406, and 407 is depicted as having a rectangular shape. Although a rectangular shape can be approximated on the target due to the configuration of one or more electric fields, one or more magnetic fields, and various shielding or barriers of the PCCT system, it should be understood that the electron distribution throughout the focal spots may not be uniform, but rather a combination of distributions generated by the configuration of one or more electric fields and / or one or more magnetic fields.

[0064] The first focal spot 402 has a length 420 and a width 409 as measured along the X-axis of the first focal spot 402. Therefore, the first focal spot 402 has a first dimension based on the length 420 and the width 409. The first focal spot 402 may be referred to as an ultra-small (XS) focal spot. The second focal spot 404 may have a different length 422 and a different width 410, wherein the width 410 is greater than the width 409, and the length 422 is greater than the length 420. The second focal spot 404 may be referred to as a small (S) focal spot. Similarly, the third focal spot 406 may have a width 412 and a length 424 that are respectively greater than the width 410 and the length 420; this third focal spot may be referred to as a large (L) focal spot. The fourth focal spot 408 may have a width 414 and a length 426 that are respectively greater than the width 412 and the length 424.

[0065] The sizes of the focal spots 402, 404, 406, and 408 can be adjusted in various ways. When the emitter is activated with a predetermined amount of current, a first amount of energy (e.g., current) delivered to one or more focusing electrodes and a second amount of energy delivered to one or more magnets can be independently controlled to focus the electron beam onto the target to achieve the desired size.

[0066] The quality of an image reconstructed by a PCCT system can depend on the selected focal spot size used for the corresponding acquisition. In other words, while other scan parameters remain constant, using a first focal spot 402 produces a first image of first quality; using a second focal spot 404 produces a second image of second quality; using a third focal spot 406 produces a third image of third quality; and using a fourth focal spot 408 produces a fourth image of fourth quality, where the first, second, third, and fourth qualities can be different. The scan protocol can specify the focal spot size to use for a given acquisition. Typical focal spot sizes range from 0.3 mm to 2 mm. The smaller the focal spot size, the better the desired spatial resolution. However, smaller focal spot sizes limit the scan mA range because they may heat and potentially damage small X-ray target areas. After acquiring projection data using a selected focal spot size / shape, one or more calibration vectors indicated by the scan protocol can be applied to the projection data to correct for variations in detector response.

[0067] One or more calibration vectors may include an air calibration vector and an MD calibration vector, which may depend on the selected focal spot size. Typically, when using a first focal spot 402, a first MD calibration vector is applied to a combination of scan parameters; when using a second focal spot 404, a different second MD calibration vector is applied to that combination; when using a third focal spot 406, a different third MD calibration vector is applied to that combination; and when using a fourth focal spot 408, a fourth different MD calibration vector is applied to that combination, wherein during the calibration of the PCCT system, the first MD calibration vector, the second MD calibration vector, the third MD calibration vector, and the fourth MD calibration vector are generated and stored in the PCCT system's memory.

[0068] However, since different MD calibration vectors can be selected for each of the many combinations of other scan parameters, the amount of MD calibration vectors stored in memory can be large, and the amount of time spent generating that amount of MD calibration vectors during the calibration phase can be long. To reduce the amount of different MD calibration vectors stored in memory, and to reduce the amount of time spent generating MD calibration vectors, based on... Figures 5 to 7 The method shown presents an alternative procedure for using calibration vectors to correct projection data.

[0069] Now for reference Figure 5 This illustrates the acquisition of multiple calibration vectors for calibration using a PCCT system (such as...). Figure 1 and Figure 2 Method 500 involves acquiring projection data using PCCT systems 100 and 200. Calibration vectors can be acquired during periodic or scheduled calibration procedures performed on the PCCT system. Method 500 and other methods described herein can be performed using the computing devices of the PCCT system (such as…). Figure 2 The processor of the computing device 216 executes the commands.

[0070] Method 500 begins at 502, wherein method 500 includes setting the scan parameter for the focal spot size to ultra-large (XL) and performing multiple acquisitions to generate corresponding multiple calibration vectors. These multiple calibration vectors may include stacked calibration vectors, tube current calibration vectors, air calibration vectors, and MD calibration vectors, as well as other types of calibration vectors. Multiple acquisitions can be performed for multiple combinations of scan parameters, including aperture size, tube voltage, bowtie shape, and focal spot size. For example, a first acquisition can be performed using an XL focal spot, a first aperture size, a first tube voltage, and a first bowtie shape parameter; then a second acquisition can be performed using an XL focal spot, a second aperture size, a second tube voltage, and a second bowtie shape parameter; then a third acquisition can be performed using an XL focal spot, a third aperture size, a third tube voltage, and a third bowtie shape parameter; and so on.

[0071] In various implementations, the aperture size, tube voltage, and bowtie shape parameters can be adjusted in increments for each acquisition within a predetermined range for each parameter type. In this way, each acquisition can use a different set of parameter settings until the parameter range has been covered, acquiring a set of calibration vectors corresponding to the different combinations of parameter settings. For example, four different aperture sizes can be considered (e.g., 5 mm, 20 mm, 40 mm, 80 mm); three different tube voltages can be considered (e.g., 80 kVp, 120 kVp, 140 kVp); two different bowtie shape parameters can be considered (e.g., small, large); and four different focal spots (XS, S, L, XL). Since the focal spot size may account for the largest variations in detector response, the focal spot size can be set first (e.g., initially XL), and calibration vectors for each combination of other scan parameters can be generated before changing to different focal spot sizes.

[0072] At 504, method 500 includes setting the scan parameters for the focal spot size to large (L) and performing multiple acquisitions as described in step 502 to generate multiple calibration vectors corresponding to different combinations of parameter settings for the L focal spot. However, for the L focal spot, acquisitions can be performed to acquire the air calibration vector and the tube current calibration vector, but the acquisition of the stacked calibration vector and the MD calibration vector may not be performed. (Stacked calibration is independent of focal spot size.) By not performing the acquisition of the stacked calibration vector and the MD calibration vector, the amount of calibration data collected by the PCCT system can be reduced. Therefore, the first amount of time spent performing step 504 can be significantly less than the second amount of time spent performing step 502, and the total calibration time can be reduced. In this case, the XL focal spot MD can be used for image reconstruction of L or other smaller focal spot scan data.

[0073] At 506, method 500 includes setting the scan parameters for the focal spot size to small (S) and performing multiple acquisitions as described in step 504 to generate multiple calibration vectors corresponding to different combinations of parameter settings for the S focal spot. In some embodiments, for the S focal spot, acquisitions may be performed to acquire air calibration vectors and tube current calibration vectors, but acquisitions of MD calibration vectors may not be performed to reduce the amount of time spent performing step 506 and the amount of MD calibration data collected during acquisitions. However, in other embodiments, at 508, a scan may be performed to generate MD calibration vectors as described above with reference to step 502, which can improve the quality of the image reconstructed from the projection data when using a smaller focal spot (e.g., XS).

[0074] At 510, method 500 includes setting the scan parameters for the focal spot size to ultra-small (XS) and performing multiple acquisitions as described in step 504 to generate multiple calibration vectors corresponding to different combinations of parameter settings for the S focal spot. As in step 504, acquisitions can be performed to acquire the air calibration vector and the tube current calibration vector, but acquisitions of the MD calibration vector can be omitted to reduce the amount of time spent performing step 506 and to reduce the amount of MD calibration data collected during acquisition.

[0075] At 512, method 500 includes storing calibration vectors of multiple combinations of scanning parameters for each of the XL, L, S, and XS focal spot sizes in the memory of the PCCT system. The calibration vectors can be retrieved during the scanning of a subject using the PCCT system and applied to projection data acquired during the scan, as described below. Figure 6 As described in Method 600. Method 500 ends.

[0076] Now for reference Figure 6 This illustrates a method for correcting a subject's use of a PCCT system (such as a PCCT system) by applying one or more calibration vectors stored in the PCCT system's memory to the projection data. Figure 1 and Figure 2 Method 600 for acquiring projection data using PCCT systems 100 and 200.

[0077] Method 600 begins at 602, wherein method 600 includes performing multiple calibration scans on one or more phantoms to generate one or more calibration vectors. The calibration vectors may be generated and stored in the memory of the PCCT system before scanning the subject, for example during a previous periodic or scheduled calibration procedure performed on the PCCT system, as referenced above. Figure 5 Method 500 describes this. For example, as part of daily calibration, one or more calibration vectors may be acquired and stored during a calibration process performed on the PCCT system on the morning of the scanning day.

[0078] Multiple sets of calibration scans can be performed at each of the four focal size ranges using various combinations of parameter settings. Specifically, a first set of calibration scans can be performed using a first focal size, including a calibration scan for generating a stacked calibration vector, an MD calibration vector, an air calibration vector, and a tube current calibration vector to correct for variations in detector output under various combinations of parameter settings. Additional sets of calibration scans can be performed using different focal size ranges, including calibration scans for generating air and tube current calibration vectors for various combinations of parameter settings, but excluding calibration scans for generating the MD calibration vector. By not performing calibration scans for generating the MD calibration vector at each focal size range, the amount of calibration data stored in the PCCT system's memory is reduced, thereby increasing the memory's availability for other data and tasks. Additionally, the time spent performing calibration scans for generating the MD calibration vector at each focal size range is saved, thus shortening the calibration phase and increasing the PCCT system's usability for use on patients.

[0079] At 604, method 600 includes receiving a scanning protocol and setting multiple scanning parameters of the PCCT system based on the scanning protocol to perform a scan on the subject. In various embodiments, the scanning protocol can be selected by the operator of the PCCT system.

[0080] At 606, method 600 includes performing a scan of the subject based on multiple scanning parameters. The multiple scanning parameters may include the size of the focal spot of the PCCT system. Additionally, the multiple scanning parameters may include one or more of the following: aperture size, tube current, collimation parameters, and / or other parameters of the PCCT system. Projection data of the subject is acquired during the execution of the scan.

[0081] At 608, method 600 includes correcting for variations in detector output during scanning by applying a set of calibration vectors to the acquired projection data. This set of calibration vectors may include a stacking calibration vector, a tube current calibration vector, an air calibration vector, and an MD calibration vector. This set of calibration vectors can be retrieved from the PCCT system's memory using one or more lookup tables. These one or more lookup tables may indicate the set of calibration vectors to be applied to the acquired projection data based on multiple scan parameters used to perform the scan.

[0082] First, a stacking correction independent of the focal spot size is applied to the acquired projection data. Second, a tube current and air calibration vector corresponding to a first focal spot size of the projection data are applied. Using a matched air calibration vector, the acquired projection data is compensated for focal spot size differences in the MD vector. Essentially, the input projection data for MD calibration is corrected for focal spot size differences or MD calibration reduction in the detector response. Third, based on a first focal spot size used for scanning, MD calibration is applied to the acquired projection data, where the indicated MD calibration vector may have already been generated for a second focal spot size different from the first size. (Reference) Figure 7 This procedure for retrieving and applying one or more calibration vectors is described in more detail.

[0083] Because MD correction is dependent on focal spot size, using MD calibration vectors generated for focal spots of different sizes may reduce the effectiveness of MD correction. Therefore, at 610, method 600 optionally includes using an air calibration vector to correct and normalize the projection data to compensate for the effects of MD reduction. This correction and normalization can occur before MD correction. During normalization using the air calibration vector, for each pixel, the air calibration correction value is multiplied by the photon count at each energy box of the corresponding detector.

[0084] In some implementations, air calibration vector normalization can be performed based on the sum of air calibration correction values ​​from each energy box of each detector. That is, for each energy box I out of a total of n energy boxes for a given pixel, the projection data of the energy box can be divided by the sum of the air calibration correction values ​​from each energy box of the detector, as described in the following expression 1:

[0085]

[0086] In other implementations, the projection data can be normalized on a box-by-box basis using the corresponding air calibration vector. That is, for each box i out of a total of n boxes for a given pixel, the projection data of the box can be divided by the corresponding air calibration correction value of the box, as described in the following expression 2:

[0087]

[0088] For example, when the MD calibration vector for the XL focal spot is used to correct projection data acquired with the L focal spot, the resulting spectral difference can be compensated by box-by-box air calibration vector normalization, as expressed below.

[0089] As stated in Equation 3:

[0090]

[0091] Therefore, for each energy box of the relevant pixel / detector, the ratio between the relevant air calibration correction value of the energy box's XL air calibration vector and the relevant air calibration correction value of the energy box's L air calibration vector, where the XL air calibration correction value is normalized by the sum of the XL air calibration correction values ​​of each energy box of the detector, and the L air calibration correction value is normalized by the sum of the L air calibration correction values ​​of each energy box of the detector, is then calculated. The projection data is then divided by the sum of the air calibration correction values ​​of the L air calibration vectors for each energy box of the relevant detector, and then multiplied by this ratio to obtain the normalized photon count at the energy box of the relevant detector. In this way, the projection data is compensated for the change in photon count caused by the difference in focal spot size in the MD. The MD calibration vector is then applied to the compensated projection data to generate one or more material decomposition sine maps.

[0092] At 612, method 600 includes reconstructing an image from the corrected projection data (e.g., a material decomposition sine curve), and at 614, displaying the reconstructed image on a display device. For example, the display device could be... Figure 2 The imaging system 200 and the display device 232.

[0093] Now for reference Figure 7 This illustrates the methods for determining which PCCT systems to apply (such as...). Figure 1 and Figure 2 A method 700 that uses a set of vectors of projection data acquired from a subject by PCCT systems 100 and 200. In various embodiments, method 700 may serve as described above. Figure 6 This is part of the execution of method 600.

[0094] At 702, method 700 includes receiving parameter settings for a scan performed on a subject from a scanning protocol used for scanning. The received parameter settings may include, for example, focal spot size parameters, aperture parameters, bowtie shape parameters, tube voltage parameters, tube current parameters, collimation parameters, and / or other parameters.

[0095] At 703, method 700 includes retrieving and applying the stacked calibration vector. The stacked calibration vector is independent of the focal spot size.

[0096] At 704, method 700 includes determining whether the size of the focal spot used for scanning is XL. If the focal spot size is determined to be XL, method 700 proceeds to 706. At 706, method 700 includes retrieving and applying, for the XL focal spot, at least the tube current calibration vector, air calibration vector, and MD calibration vector associated with the scanning parameters used in the scanning. Alternatively, if the focal spot size is determined not to be XL, method 700 proceeds to 708.

[0097] At 708, method 700 includes determining whether the size of the focal spot used for scanning is L. If the size of the focal spot is determined to be L, method 700 proceeds to 710.

[0098] At 710, method 700 includes retrieving and applying a tube current calibration vector and an air calibration vector associated with the combination of scan parameters used in the scan for the L focal spot. At 712, method 700 includes retrieving and applying an MD calibration vector associated with the scan parameters used in the scan, but for the XL focal spot. In other words, the MD calibration vector associated with the combination of scan parameters used in the scan for the L focal spot is not retrieved and applied, and the MD calibration vector associated with the scan parameters used in the scan for the XL focal spot is replaced with the MD calibration vector associated with the combination of scan parameters used in the scan for the L focal spot. Using the MD calibration vector associated with the scan parameters for the XL focal spot instead of the L focal spot actually used in the scan may reduce the image quality of the image reconstructed from the corrected projection data; however, projection data corrected using the MD calibration vector associated with the scan parameters for the XL focal spot can advantageously be normalized using the air calibration vector for the L focal spot to offset the reduction in image quality, as described above with reference to method 600.

[0099] Alternatively, if it is determined at 708 that the size of the focal spot is not L, then method 700 proceeds to 714. At 714, method 700 includes determining whether the size of the focal spot used for scanning is S. If it is determined that the size of the focal spot is S, then method 700 proceeds to 716.

[0100] At 716, method 700 includes retrieving and applying a tube current calibration vector and an air calibration vector associated with the scan parameters used in the scan for the S focal spot. At 718, method 700 includes retrieving and applying an MD calibration vector associated with the scan parameters used in the scan, but for the XL focal spot. In other words, the MD calibration vector associated with the combination of scan parameters used in the scan for the S focal spot is not retrieved and applied, and the MD calibration vector associated with the combination of scan parameters used in the scan for the XL focal spot is replaced with the MD calibration vector associated with the combination of scan parameters used in the scan for the S focal spot.

[0101] If it is determined at 714 that the focal spot size is not S, then the focal spot size can be inferred to be XS, thus method 700 proceeds to 720. At 720, method 700 includes retrieving and applying the tube current calibration vector and air calibration vector associated with the scan parameters used in the scan for the XS focal spot. At 722, method 700 includes retrieving and applying the MD calibration vector associated with the combination of scan parameters used in the scan, but for the XL focal spot, as in steps 712 and 718. In other words, the MD calibration vector associated with the combination of scan parameters used in the scan for the XS focal spot is not retrieved and applied, and the MD calibration vector associated with the combination of scan parameters used in the scan for the XL focal spot is replaced by the MD calibration vector associated with the combination of scan parameters used in the scan for the XS focal spot. Method 700 ends.

[0102] It should be understood that although method 700 is described as using an MD calibration vector associated with an XL focal spot to correct projection data acquired at L, S, and XS focal spot sizes, the MD calibration vector associated with any of the XL, L, S, and XS focal spots can be advantageously used to correct projection data at other focal spot sizes via method 700. For example, an MD calibration vector associated with an L focal spot can be used to correct projection data acquired at XL, S, and / or XS focal spot sizes; an MD calibration vector associated with an S focal spot can be used to correct projection data acquired at XS, L, and / or XL focal spot sizes; and so on.

[0103] Additionally or alternatively, method 700 may include retrieving and applying MD calibration vectors associated with more than one focal spot to correct projection data acquired at other focal spot sizes. For example, in one embodiment, projection data acquired using an L focal spot may be corrected using a stacked calibration vector and an MD calibration vector associated with an XL focal spot (e.g., steps 710-712), and projection data acquired using an XS focal spot may be corrected using a stacked calibration vector and an MD calibration vector associated with an S focal spot (e.g., modified steps 720-722). Such cases may rely on MD calibration vectors collected during the calibration phase for a combination of scan parameters at both XL and S focal spot sizes, which can improve the quality of the image reconstructed at a smaller focal spot size at the cost of longer calibration time and storing a larger amount of calibration data.

[0104] Figure 8 , Figure 9 and Figure 10 An exemplary image reconstructed from projection data that has been corrected as described herein is shown. References Figure 8The diagram illustrates a first reconstructed image 800 of the phantom, where image 800 is reconstructed from projection data acquired with an XS focal spot and corrected using an XS stacking calibration vector, an XS tube current calibration vector, an XS air calibration vector, and an XS MD calibration vector. In other words, during a previous calibration phase, calibration vectors are generated for the XS focal spot size to correct for variations in the detector response in the projection data. Therefore, image 800 has high image quality (e.g., at the desired level).

[0105] and Figure 8 compared to, Figure 9 A second reconstructed image 900 of the phantom is shown, which is reconstructed from projection data acquired with the same scanning parameters (including XS focal spot size) as image 800. However, in image 900, the projection data is corrected using XL stacking calibration vector, XL tube current calibration vector, XL air calibration vector, and XL MD calibration vector. Due to the use of XL calibration vectors for the XS focal spot size, image 900 has lower quality than image 800, where the ring artifact 902 is more visible than in image 800. Therefore, image 900 represents a situation where calibration vectors generated for different focal spot sizes are replaced by calibration vectors generated for the focal spot size used in the scan, but without performing [the necessary steps]. Figure 6 The air calibration procedure is used to adjust the X-ray count to normalize for focal spot size differences (e.g., the XL air calibration vector does not correspond to the focal spot used in the scan).

[0106] and Figure 9 compared to, Figure 10 A third reconstructed image 1000 of the phantom is shown, which was reconstructed from projection data acquired with the same scanning parameters (including the XS focal spot size) as image 800. However, in image 1000, the projection data is corrected using the XL stacking calibration vector and the XL MD calibration vector instead of the XS tube current calibration vector and the XS air calibration vector. The XS air calibration vector, corresponding to the XS focal spot used in the scan, is used as described above regarding... Figure 6 The projection data is normalized as described in the air calibration normalization. Because the projection data is normalized using an air calibration vector for the XS focal spot size, the quality of image 1000 is improved relative to image 900 and is similar to that of image 800. However, since the correction of the projection data in image 1000 does not depend on the stored MD, less memory is required in the PCCT system, and the time spent generating the calibration vector is reduced. Therefore, by using… Figure 10 The method adopted in this study improves the efficiency of PCCT systems without sacrificing image quality.

[0107] Therefore, as described herein, by reducing the total number of MD calibration vectors on which the PCCT system relies to compensate for variations in material density of portions of the anatomical structures of the subject scanned by the PCCT system, the total amount of calibration data (e.g., calibration vectors) stored in the PCCT system's memory and the corresponding time spent calibrating the PCCT system can be advantageously reduced. Instead of generating different sets of calibration vectors for each focal size, a single set of MD calibration vectors can be generated that can be applied to various focal size settings to correct projection data. To compensate for the spectral difference in photon counts resulting from using MD calibration vectors from a single set of MD calibration vectors (e.g., for a single focal size) instead of using MD calibration vectors generated based on different focal sizes to correct projection data acquired at different focal sizes, the corrected projection data can be normalized by multiplying by the ratio of the air calibration correction corresponding to the actual focal size used during scanning to the air calibration correction corresponding to the substituted focal size.

[0108] By using a single set of MD calibration vectors to correct projection data acquired at each focal spot size and using an air calibration vector to normalize the corrected projection data, the time spent performing calibration and the amount of MD calibration data collected can be significantly reduced without correspondingly degrading the quality of the reconstructed images. The reduced time spent performing calibration allows for greater availability of the PCCT system for patient scanning. Furthermore, by reducing the amount of computational and memory resources consumed by the PCCT system during the calibration phase, the operational efficiency of the PCCT system is improved, where the same or similar scan results can be obtained with less computation. The saved computational and memory resources can then be advantageously used for other imaging tasks and / or for other subjects. The technical advantage of using MD vectors of a single focal spot size, rather than MD vectors generated for specific focal spot sizes, to correct projection data acquired via the PCCT system is that it reduces the amount of calibration data stored in the PCCT system and the corresponding time spent generating calibration vectors, thereby improving the efficiency of PCCT system usage.

[0109] This disclosure also provides support for a method for a photon-counting computed tomography (PCCT) system, the method comprising: performing a scan using the PCCT system with a first focal spot of a first size; applying a material decomposition (MD) calibration vector to correct projection data acquired during the scan, the MD calibration vector being generated for a second focal spot of a second size, different from the first size; reconstructing an image from the corrected projection data; and displaying the image on a display device. In a first example of the method, the method further comprises: correcting and normalizing the projection data using a first air calibration vector generated for the first focal spot of the first size and a second air calibration vector generated for the second focal spot of the second size. In a second example of the method, optionally including the first example, correcting and normalizing the projection data using the first air calibration vector and the second air calibration vector further comprises, for each of a certain total number of energy boxes of each detector of the PCCT system, dividing the projection data of the energy box by the sum of the air calibration correction values ​​of the first air calibration vectors of each energy box of the detector. In a third example of the method, optionally including one or both of the first and second examples, correcting and normalizing the projection data using the first and second air calibration vectors also includes, for each energy box in a certain total number of energy boxes for each detector in the PCCT system, dividing the projection data of the energy box by the air calibration correction value of the first air calibration vector of the energy box. In a fourth example of the method, optionally including one or more or each of the first to third examples, correcting and normalizing the projection data using the air calibration vectors also includes, for each energy box of each detector in the PCCT system: calculating a ratio between the first air calibration correction value of the second air calibration vector of the energy box and the second air calibration correction value of the first air calibration vector of the energy box, the first air calibration correction value being normalized by the sum of the air calibration correction values ​​of the second air calibration vectors of each energy box of the detector; and dividing the corrected projection data of the energy box by the sum of the air calibration correction values ​​of the first air calibration vectors of each energy box of the relevant detector, and then multiplying the result by the ratio to obtain the normalized photon count at the energy box of the detector. In a fifth example of the method, one or more or each of the first to fourth examples may be optionally included, and the method further includes: applying a stacking calibration vector to correct the projection data acquired during scanning, wherein the generated stacking calibration vector is independent of the focal spot size. In a sixth example of the method, one or more or each of the first to fifth examples may be optionally included, wherein a first focal spot of a first size is a large (L) focal spot, and a second focal spot of a second size is an extra-large (XL) focal spot.In a seventh example of the method, one or more or each of the first to sixth examples may be included, wherein a first focal spot of a first size is an ultra-small (XS) focal spot, and a second focal spot of a second size is a small (S) focal spot. In an eighth example of the method, one or more or each of the first to seventh examples may be included, wherein, under a first condition that the first focal spot is an XL focal spot, a first MD calibration vector associated with the XL focal spot is applied to the projection data; and under a second condition that the first focal spot is an L focal spot, a second MD calibration vector associated with the L focal spot is not applied, and a first MD calibration vector associated with the XL focal spot is applied to the projection data. In a ninth example of the method, one or more or each of the first to eighth examples may be included, wherein, under a first condition that the first focal spot is an S focal spot, a third MD calibration vector associated with the S focal spot is applied to the projection data; and under a second condition that the first focal spot is an XS focal spot, a fourth MD calibration vector associated with the XS focal spot is not applied, and a third MD calibration vector associated with the S focal spot is applied to the projection data.

[0110] This disclosure also provides support for a photon-counting computed tomography (PCCT) system, the PCCT system comprising: an X-ray source that emits an X-ray beam toward a subject to be imaged; a photon-counting detector that receives the attenuated X-ray beam from the subject; a data acquisition system (DAS) operatively connected to the detector; and a computer including a processor and a non-transitory memory operatively connected to the DAS, wherein instructions are stored in the non-transitory memory, and when executed, these instructions cause the processor to: use the PCCT system to During the scan: photon counts are collected at each detector of the PCCT system; stacking calibration correction values, tube current calibration correction values, air calibration correction values, and material decomposition (MD) calibration correction values ​​are applied to the photon counts to obtain one or more material decomposition sinograms; an image is reconstructed based on the one or more material decomposition sinograms; and the image is output to a display device, wherein the air calibration correction values ​​and tube current calibration correction values ​​are based on a first focal spot size used during the scan, and the MD calibration correction value is based on a second focal spot size not used during the scan, the second focal spot size being different from the first focal spot size. In a first example of the system, the stacking calibration correction values, tube current calibration correction values, air calibration correction values, and MD calibration correction values ​​are retrieved from the stacking calibration vector, the first air calibration vector, the tube current calibration vector, and the MD calibration vector stored in non-transitory memory, respectively. In a second example of the system, optionally including the first example, at each detector, the stacking calibration correction values, tube current calibration correction values, air calibration correction values, and MD calibration correction values ​​are applied to generate a material decomposition sinogram. In a third example of the system, one or both of the first and second examples are optionally included, wherein the photon count at each energy box is normalized by the sum of air calibration correction values ​​across all energy boxes of the detector. In a fourth example of the system, one or more or each of the first to third examples are optionally included, wherein the photon count at each energy box is normalized by an air calibration correction value applied at the energy box.In a fifth example of the system, one or more or each of the first to fourth examples may be optionally included, with additional instructions stored in non-transitory memory that, when executed, cause the processor to: receive a second air calibration vector from the non-transitory memory, the second air calibration vector being generated for a second focal spot size; and at each energy box of each detector in the PCCT system: calculate a ratio between a first air calibration correction value of the energy box's second air calibration vector and a second air calibration correction value of the energy box's first air calibration vector, the first air calibration correction value being normalized by the sum of the air calibration correction values ​​of the second air calibration vectors of each energy box of the detector; and divide the corrected projection data of the energy box by the sum of the air calibration correction values ​​of the first air calibration vectors of each energy box of the relevant detector, and multiply the result by the ratio to obtain a normalized photon count at the energy box of the detector. In a sixth example of the system, one or more or each of the first to fifth examples is optionally included, wherein the first focal spot size is an extra-large (XL) focal spot, and the second focal spot size is one of a large (L) focal spot, a small (S) focal spot, and an extra-small (XS) focal spot. In a seventh example of the system, one or more or each of the first to sixth examples is optionally included, with additional instructions stored in non-transitory memory, which, when executed, cause the processor to: during the calibration phase of the PCCT system: generate a set of stacked calibration vectors, a set of tube current calibration vectors, a set of air calibration vectors, and a set of MD calibration vectors for a focal spot of the first size and store them in non-transitory memory; and generate a set of air calibration vectors and a set of tube current calibration vectors for a focal spot of a different size than the first size and store them in non-transitory memory, without generating a set of MD calibration vectors for the focal spot of the different size. In the eighth example of the system, one or more or each of the first to seventh examples may be optionally included, with additional instructions stored in non-transitory memory. These additional instructions, when executed, cause the processor to: during the calibration phase: generate a set of stacked calibration vectors, a set of tube current calibration vectors, a set of air calibration vectors, and a set of MD calibration vectors for a first focal spot of a first size and a second focal spot of a second size, and store them in non-transitory memory; and generate a set of air calibration vectors and a set of tube current calibration vectors for focal spots of a size different from both the first and second sizes, and store them in non-transitory memory, without generating a set of MD calibration vectors for focal spots of a size different from both the first and second sizes.

[0111] This disclosure also provides support for a method for calibrating a photon-counting computed tomography (PCCT) system, the method comprising: during a calibration phase of the PCCT system: setting the focal spot size scan parameters of the PCCT system to a first size; generating a set of stacked calibration vectors, a set of tube current calibration vectors, a set of air calibration vectors, and a set of MD calibration vectors; storing the set of stacked calibration vectors, the set of air calibration vectors, the set of tube current calibration vectors, and the set of MD calibration vectors in the memory of the PCCT system; setting the focal spot size scan parameters of the PCCT system to one or more different sizes; and for each of the one or more different sizes: generating a set of air calibration vectors and a set of tube current calibration vectors and storing them in the memory, without generating a set of MD calibration vectors.

[0112] When describing elements of various embodiments of this disclosure, the articles “a,” “an,” and “the” are intended to indicate the presence of one or more such elements. The terms “first,” “second,” etc., do not indicate any order, quantity, or importance, but are used to distinguish one element from another. The terms “comprising,” “including,” and “having” are intended to be inclusive and indicate that additional elements may exist in addition to the listed elements. As used herein, the terms “connected to,” “coupled to,” etc., indicate that an object (e.g., a material, element, structure, component, etc.) may be connected to or coupled to another object, regardless of whether the one object is directly connected to or coupled to the other object, or whether one or more intervening objects exist between the one object and the other object. Furthermore, it should be understood that references to “an embodiment” or “an embodiment” of this disclosure are not intended to be construed as excluding the existence of additional embodiments also incorporating the referenced features.

[0113] In addition to any modifications previously indicated, many other variations and alternative arrangements can be devised by those skilled in the art without departing from the spirit and scope of this specification, and the appended claims are intended to cover such modifications and arrangements. Therefore, although the information has been described in particular and in detail above in conjunction with what is now considered to be the most practical and preferred aspects, it will be apparent to those skilled in the art that many modifications can be made without departing from the principles and concepts set forth herein, including but not limited to changes in form, function, mode of operation, and purpose. Likewise, as used herein, in all respects, embodiments and implementations are intended to be illustrative only and should not be construed as limiting in any way.

Claims

1. A method (600) for a photon counting computed tomography (PCCT) system, the method (600) comprising: The PCCT system is used to perform a scan with a first focal spot of a first size (606); Material decomposition (MD) calibration vectors are applied to correct the projection data acquired during the scan (608), the material decomposition (MD) calibration vectors being generated for a second focal spot of a second size, which is different from the first size; Reconstruct the image from the corrected projection data (612); and The image (614) is displayed on a display device.

2. The method (600) according to claim 1, further comprising correcting and normalizing the projection data using a first air calibration vector generated for the first focal spot of the first size and a second air calibration vector generated for the second focal spot of the second size (610).

3. The method (600) of claim 2, wherein correcting and normalizing the projection data using the first air calibration vector and the second air calibration vector further comprises, for each of a certain total number of energy boxes of each detector of the PCCT system, dividing the projection data of the energy box by the sum of the air calibration correction values ​​of the first air calibration vector of each energy box of the detector.

4. The method (600) of claim 2, wherein using the first air calibration vector and the second air calibration vector to correct and normalize the projection data further comprises, for each of a certain total number of energy boxes for each detector of the PCCT system, dividing the projection data of the energy box by the air calibration correction value of the first air calibration vector of the energy box.

5. The method (600) of claim 2, wherein using the air calibration vector to correct and normalize the projection data further comprises: For each energy tank of each detector in the PCCT system: Calculate the ratio between the first air calibration correction value of the second air calibration vector of the energy box and the second air calibration correction value of the first air calibration vector of the energy box, wherein the first air calibration correction value is normalized by the sum of the air calibration correction values ​​of the second air calibration vector of each energy box of the detector, and the second air calibration correction value is normalized by the sum of the air calibration correction values ​​of the first air calibration vector of each energy box of the detector; as well as The corrected projection data of the energy box is divided by the sum of the air calibration correction values ​​of the first air calibration vector of each energy box of the associated detector, and the result is then multiplied by the ratio to obtain the normalized photon count at the energy box of the detector.

6. The method of claim 1 (600, 700) further includes applying a stacking calibration vector to correct the projection data acquired during the scan (703), wherein the generated stacking calibration vector is independent of the focal spot size.

7. The method (600, 700) according to claim 1, wherein the first focal spot of the first size is a large (L) focal spot, and the second focal spot of the second size is an extra-large (XL) focal spot.

8. The method (600, 700) according to claim 1, wherein the first focal spot of the first size is an ultra-small (XS) focal spot, and the second focal spot of the second size is a small (S) focal spot.

9. The method (600, 700) according to claim 6, wherein: Under the first condition that the first focal spot is an XL focal spot, the first MD calibration vector associated with the XL focal spot is applied to the projection data (706); as well as Under the second condition that the first focal spot is an L focal spot, the second MD calibration vector associated with the L focal spot is not applied, and the first MD calibration vector associated with the XL focal spot is applied to the projection data (712).

10. The method (600, 700) according to claim 6, wherein: Under the first condition that the first focal spot is an S focal spot, the third MD calibration vector associated with the S focal spot is applied to the projection data; as well as Under the second condition that the first focal spot is an XS focal spot, the fourth MD calibration vector associated with the XS focal spot is not applied, and the third MD calibration vector associated with the S focal spot is applied to the projection data.

11. A photon counting computed tomography (PCCT) system (100, 200), comprising: X-ray source (104), which emits an X-ray beam toward the subject to be imaged; A photon counting detector (108) receives the X-ray beam attenuated by the subject; A data acquisition system (DAS) operatively connected to the detector; and Computer (216), the computer including a processor and a non-transitory memory operatively connected to the DAS, wherein instructions are stored in the non-transitory memory, the instructions causing the processor, when executed, to: During the scan performed using the PCCT system (100, 200): Photon counts are collected at each detector in the PCCT system (100, 200); The stacking calibration correction value, tube current calibration correction value, air calibration correction value, and material decomposition (MD) calibration correction value are applied to the photon count to obtain one or more material decomposition sine curves; The image (1000) is reconstructed based on the sinusoidal plots of the one or more material decompositions, and The image (1000) is output to the display device (232); The air calibration correction value and the tube current calibration correction value are based on a first focal spot size used during the scan, and the MD calibration correction value is based on a second focal spot size not used during the scan, the second focal spot size being different from the first focal spot size.

12. The PCCT system (100, 200) according to claim 11, wherein the stacking calibration correction value, the tube current calibration correction value, the air calibration correction value and the MD calibration correction value are retrieved from the stacking calibration vector, the first air calibration vector, the tube current calibration vector and the MD calibration vector stored in the non-transitory memory, respectively.

13. The PCCT system (100, 200) according to claim 12, wherein at each detector (108), the stacking calibration correction value, the tube current calibration correction value, the air calibration correction value and the MD calibration correction value are applied to generate a material decomposition sine curve.

14. The PCCT system (100, 200) according to claim 13, wherein the photon count at each energy cell is normalized by the sum of air calibration correction values ​​across all energy cells of the detector (108).

15. The PCCT system (100, 200) of claim 13, wherein the photon count at each energy chamber is normalized by the air calibration correction value applied at the energy chamber.

Images