Extended-field imaging collimator for radiological imaging and methods using radiological imaging - Patents.com

Abstract

[Solution] A radiation imaging system includes a collimator configured to filter radiation emitted from a target object, the collimator including a plurality of apertures non-uniformly distributed on the collimator. A maximum acceptance angle of the plurality of apertures is 15° or less. The radiation imaging system further includes a detector that detects radiation that has passed through the collimator. The collimator is positioned at a distance from the detector such that a point on an upper surface of the detector facing the collimator is simultaneously illuminated by two or more of the plurality of apertures.

Description

[Technical field]

[0001] Priority This application claims priority to and the benefit of U.S. Provisional Patent Application No. 63 / 168,778, filed March 31, 2021, and U.S. Provisional Patent Application No. 63 / 071,540, filed August 28, 2020, which claims priority to U.S. Patent Application No. 17 / 399,768, filed August 11, 2021, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] In radiation imaging, such as molecular medicine imaging (also known as nuclear medicine imaging), images are generated that represent the distribution of a radiopharmaceutical for medical diagnosis. Prior to imaging, a subject, such as a patient, is injected with the radiopharmaceutical. The radiopharmaceutical emits radioactive photons that pass through the body and are detected by a photon detector. Based on the information from the received photons, the photon detector can then determine the distribution of the radiopharmaceutical within the patient. This distribution represents the patient's physiology, and thus the distribution image provides valuable clinical information for diagnosing various diseases and conditions in cardiology, oncology, neurology, etc.

[0003] The collimator and photon detector work together to produce an image. A collimator is a device that guides the photon path (guiding the photons to follow a certain path). Unlike X-rays and CT, where photons are emitted from a known source position, in radiation imaging, photons are emitted from an unknown position inside the subject's body. Without a collimator, photons from all directions would be recorded by the photon detector, which could make image reconstruction difficult. Therefore, a collimator is used to guide the photons into possible paths to reconstruct the image, similar to the lens in a photographic camera. Although existing radiation imaging systems are generally suitable for their intended purposes, they are not completely satisfactory in all respects. For example, existing collimators and detectors do not excel in both imaging resolution and signal sensitivity, and they often must be deployed by making a trade-off between one or the other. Therefore, improvements in radiation imaging systems are desirable. The prior art documents relevant to the invention of this application are as follows (including documents cited during the international phase after the international filing date and documents cited when the application entered the national phase in other countries). (Prior art document) (Patent Documents) (Patent Document 1) U.S. Patent Application Publication No. 2020 / 0146641 (Patent Document 2) U.S. Patent Application Publication No. 2020 / 0261034 (Patent Document 3) U.S. Patent Application Publication No. 2015 / 0216488 (Patent Document 4) U.S. Patent Application Publication No. 2007 / 0064876 Summary of the Invention [Means for solving the problem]

[0004] According to various embodiments, the present disclosure provides a radiation imaging system. The radiation imaging system includes a collimator configured to filter radiation emitted from a target object, the collimator including a plurality of apertures distributed non-uniformly on the collimator, the plurality of apertures having a maximum acceptance angle of 15° or less, and a detector for detecting radiation passing through the collimator, the collimator being spaced apart from the detector such that a point on an upper surface of the detector facing the collimator is simultaneously illuminated by two or more of the plurality of apertures. In some embodiments, a point on an upper surface of the detector is illuminated by a predetermined percentage of the plurality of apertures, the percentage being less than about 25%. In some embodiments, the number of the plurality of apertures is greater than one thousand. In some embodiments, the plurality of apertures differ in at least one of aperture size, aperture shape, acceptance angle, aperture length, and aperture pitch. In some embodiments, the distance between the collimator and the detector is about 0.5 to about 10 times (e.g., about 1.5 to about 10 times) the thickness of the collimator. In some embodiments, the plurality of openings form a predetermined aperture pattern including a repeating base pattern. In some embodiments, the repeating base pattern is a coded aperture pattern. In some embodiments, an illumination area of ​​one of the plurality of openings is substantially equal to an area of ​​the base pattern. In some embodiments, the collimator includes a first portion and a second portion, the first portion including through holes uniformly distributed on the first portion, and the second portion including through holes non-uniformly distributed on the second portion and corresponding to the plurality of openings, whereby the through holes of the first portion are blocked by the second portion. In some embodiments, a thickness of the first portion is about 2 to about 10 times that of the second portion.In some embodiments, the collimator is a first collimator, the detector is a first detector, and the system further includes a second collimator and a second detector coupled to the second collimator, and the second collimator is attached to the second detector.

[0005] According to various embodiments, the present disclosure provides a radiation imaging system, comprising: a first collimator configured to filter radiation emitted from a target object, the first collimator including a first plurality of apertures forming a first aperture pattern; a first detector associated with the first collimator for detecting radiation passing through the first collimator; a second collimator configured to filter radiation emitted from the target object, the second collimator including a second plurality of apertures forming a second aperture pattern; and a second detector associated with the second collimator for detecting radiation passing through the second collimator, the target object being disposed between the first collimator and the second collimator, and the first aperture pattern being different from the second aperture pattern. In some embodiments, the first collimator is in contact with the first detector, and the second collimator is spaced apart from the second detector. In some embodiments, a distance between the second collimator and the second detector is about 0.5 to about 7 times (e.g., about 1.5 to about 7 times) the thickness of the second collimator. In some embodiments, the first plurality of apertures are uniformly distributed on the first collimator, and the second plurality of apertures are non-uniformly distributed on the second collimator. In some embodiments, the second aperture pattern includes a repeating coding pattern. In some embodiments, the repeating coding pattern is one of a Uniform Redundant Array (URA) pattern, a Modified Uniform Redundant Array (MURA) pattern, and a Perfect Binary Array (PBA) pattern. In some embodiments, the facing directions of the first and second detectors have an offset angle.

[0006] According to various embodiments, the present disclosure provides a collimator configured to filter radiation emitted from a target object. The collimator includes a first portion including a first plurality of uniformly distributed through-holes and a second portion including a second plurality of non-uniformly distributed through-holes, the first and second portions being spaced apart and aligned such that photons emitted by the object are blocked by the second portion when passing through some of the first plurality of through-holes. In some embodiments, the second plurality of through-holes form a coding pattern. In some embodiments, the second portion is operable to slide relative to a surface of the first portion facing the second portion, thereby changing the alignment of the first and second plurality of through-holes.

[0007] According to various embodiments, the present disclosure provides a method of correcting misalignment in an imaging system, the method comprising the steps of providing a collimator and a detector having a digitized pixel grid, illuminating the collimator with a flood light source, recording a signal intensity at each pixel of the digitized pixel grid, the signal intensity being generated by illuminating the collimator, deriving a descriptor from the recorded signal intensities, generating a set of offsets introduced to the digitized pixel grid, determining from the set of offsets a selected offset that optimizes the descriptor, and reconstructing the digitized pixel grid based on an actual optimal offset derived from the selected offsets. In some embodiments, the selected offset comprises a pair of offset values ​​in two orthogonal directions. In some embodiments, the descriptor is one of a contrast value, a peak value, and a valley value of the recorded signal intensity. In some embodiments, the contrast value is determined by a ratio of a peak value and a valley value of a signal intensity line corresponding to the recorded signal intensity at the digitized pixel grid. In some embodiments, determining the selected offset comprises sweeping through the range of offsets and selecting a selected offset that maximizes or minimizes the descriptor, In some embodiments, reconstructing the digitized pixel grid comprises adjusting the selected offset by a fixed offset to generate the actual optimum offset, and reconstructing the digitized pixel grid by the actual optimum offset. [Brief description of the drawings]

[0008] The present disclosure is best understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: It should be noted that, in accordance with common practice in the art, the various components are not drawn to scale and are used for illustrative purposes only. Rather, the dimensions of the various components have been arbitrarily expanded or reduced for clarity of illustration. [Figure 1A] 1A, 1B, and 1C are schematic diagrams of an exemplary radiation imaging system in accordance with various aspects of the present disclosure. [Figure 1B-1C] 1A, 1B, and 1C are schematic diagrams of an exemplary radiation imaging system in accordance with various aspects of the present disclosure. [Figure 2A-2B] 2A, 2B, 2C, 2D, 2E, and 2F are partial perspective, cross-sectional, and plan views of a collimator according to some embodiments of the present disclosure. [Fig. 2C-2D] 2A, 2B, 2C, 2D, 2E, and 2F are partial perspective, cross-sectional, and plan views of a collimator according to some embodiments of the present disclosure. [Fig. 2E-2F] 2A, 2B, 2C, 2D, 2E, and 2F are partial perspective, cross-sectional, and plan views of a collimator according to some embodiments of the present disclosure. [Diagram 3] FIG. 3 is a partial cross-sectional view of a radiation imaging system according to some embodiments of the present disclosure. [Figure 4] FIG. 4 is a partial cross-sectional view of a radiation imaging system according to some alternative embodiments of the present disclosure. [Figure 5A-5B] 5A and 5B are partial plan views of a collimator having non-uniformly distributed openings, according to various embodiments of the present disclosure. [Figures 6A-6C] 6A, 6B, and 6C are plan views of collimators having a repeating base pattern according to some embodiments of the present disclosure. [Figure 7] FIG. 7 illustrates the decomposition of an original image into sub-images in a fast image reconstruction algorithm according to various aspects of the present disclosure. [Figure 8A-8C] 8A, 8B, 8C, 9A, 9B, 9C, 10A, and 10B show cross-sectional views of several embodiments of lightweight collimators according to some embodiments of the present disclosure. [Figure 9A-9C]8A, 8B, 8C, 9A, 9B, 9C, 10A, and 10B show cross-sectional views of several embodiments of lightweight collimators according to some embodiments of the present disclosure. [Figure 10A-10B] 8A, 8B, 8C, 9A, 9B, 9C, 10A, and 10B show cross-sectional views of several embodiments of lightweight collimators according to some embodiments of the present disclosure. [Figure 11A-11B] 11A, 11B, 12A, 12B, and 13 show plan and cross-sectional views of several embodiments of a radiological imaging system having alignment adjustments. [Figure 12A-12B] 11A, 11B, 12A, 12B, and 13 show plan and cross-sectional views of several embodiments of a radiological imaging system having alignment adjustments. [Figure 13] 11A, 11B, 12A, 12B, and 13 show plan and cross-sectional views of several embodiments of a radiological imaging system having alignment adjustments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The following disclosure provides many different embodiments or examples for implementing different features of the present disclosure. To simplify the present disclosure, specific examples of components and arrangements are described below. These are, of course, merely examples and are not intended to be limiting. Any changes and further modifications to the described apparatus, system, and method, and any further application of the principles of the present disclosure are naturally contemplated, as would normally occur to one skilled in the art in the relevant technical field of the present disclosure. For example, features, components, and / or steps described in connection with one embodiment can be combined with features, components, and / or steps described in connection with another embodiment of the present disclosure, even if the combination is not explicitly shown, to form yet another embodiment of the apparatus, system, or method according to the present disclosure. In addition, although reference numbers and / or letters may be repeated in various examples in the present disclosure, such repetition is for the purpose of brevity and clarity, and does not by itself dictate a relationship between the various embodiments and / or configurations described.

[0010] Furthermore, in the following disclosure, an element on, connected to, and / or coupled to another element may include an embodiment in which the element directly contacts the other element, and may also include an embodiment in which an additional element is interposed between the element and the other element, and the element does not directly contact the other element. Furthermore, spatial relative terms, such as "lower," "upper," "horizontal," "vertical," "above," "over," "below," "directly below," "upward," "downward," "top," "bottom," and the like, and derivatives thereof (e.g., "horizontally," "downward," "upward," and the like), are used in this disclosure to facilitate the relationship of one element to another element. Such spatial relative terms are intended to encompass different orientations of a device that includes the element. Furthermore, when a numerical value or numerical range is described with "about," "approximately," and the like, the term is intended to encompass a numerical value that is within a reasonable range that includes the stated numerical value, such as, for example, a numerical value within ±10% of the stated numerical value, or other numerical value that would be understood by one of ordinary skill in the art. For example, the term "about 5 cm" encompasses a dimensional range of 4.5 cm to 5.5 cm.

[0011] The present disclosure relates to collimators used in radiation imaging, and more particularly to collimators with non-uniformly distributed apertures used in nuclear medicine (molecular) imaging systems. The term "non-uniformly distributed apertures" refers to collimator apertures (through holes) that differ in at least one of the aperture profiles, including, but not limited to, aperture size, aperture shape (including cross-sectional shape and longitudinal shape), aperture length, aperture pitch, and aperture direction.

[0012] In radiation imaging, such as nuclear medicine (molecular) imaging systems, collimators and detectors work in conjunction to generate an image that represents the distribution of a radiopharmaceutical in the subject's body. Many nuclear medicine imaging systems, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) imaging systems, use one or more detectors to acquire imaging data, such as gamma ray or photon imaging data. Before acquiring an image, a radiopharmaceutical is usually orally administered or injected into a subject, such as a patient. The radiopharmaceutical undergoes nuclear decay and emits gamma photons with a specific speed and characteristic energy by direct or indirect annihilation. One or more detector units are placed around the subject and the emission is recorded or monitored. In many cases, for convenience of manufacturing and data processing, the detectors are configured in a planar shape, so that the data is acquired in a 2D matrix format, a configuration often called a projection. Based on the recorded information, including the location, energy, and number of detections of the detected events, an image that represents the distribution of the radiopharmaceutical can be reconstructed and the function of a particular part of the subject (e.g., a part of the patient's body) can be observed.

[0013] However, existing collimator and detector designs have various problems. For example, collimators with parallel holes are usually closely coupled (or attached) to the detector to reduce crosstalk between sensing pixels in the detector, resulting in only one possible photon path to any sensing pixel. A longer collimator aperture usually improves the imaging resolution of the imaging system but degrades the signal sensitivity. Therefore, the length of the collimator aperture must be determined by considering the trade-off between imaging resolution and signal sensitivity.

[0014] The present disclosure presents embodiments of collimator designs in which a collimator with non-uniformly distributed apertures is placed at a distance away from a detector, and certain areas of the detector are simultaneously illuminated by photons passing through multiple apertures. In other words, at least some of the apertures of the collimator have overlapping illumination areas on the detector. By moving the collimator away from the detector to utilize a larger aperture size (or wider acceptance angle), the effective length of the collimator aperture is increased, improving imaging resolution without sacrificing signal sensitivity. Also, in some embodiments of the present disclosure, the collimator apertures have a repeating pattern. In further embodiments, the pattern may be one of a uniformly redundant array (URA) pattern, a modified uniformly redundant array (MURA) pattern, a perfect binary array (PBA) pattern, a random pattern, a pseudorandom pattern, and other suitable patterns. The repeating pattern allows the radiation imaging system to employ simplified image reconstruction algorithms, reducing computational complexity and improving system performance. In various embodiments, the collimator may also employ a lightweight design by including thin plates that define apertures that are not completely filled with closed holes. This novel collimator design therefore improves the performance of radiological imaging systems in many aspects.

[0015] 1A-1C show an exemplary radiological imaging system 100 (specifically, a nuclear medicine imaging system) incorporating features of the present disclosure according to some embodiments. FIG. 1A is a side cross-sectional view of a portion of the system 100 in the XZ plane. FIG. 1B and FIG. 1C are axial views of the system 100 in the YZ plane in two embodiments. The imaging system 100 can be used to medically examine or treat a target subject, such as a patient. The imaging system 100 includes an integrated gantry 102 that further includes a rotation mechanism (e.g., a rotor oriented about a gantry central bore) configured to support and rotate one or more detectors 108 (two detectors 108 are shown in opposing positions) about an axial axis (e.g., the X direction as shown). In one embodiment illustrated in FIG. 1B, the two detectors 108 are operable to rotate along a circle 152 in opposing positions, where normal directions 154A and 154B to the top surfaces of the two detectors 108 respectively point toward the center of the circle 152 and in opposite directions. In an alternative embodiment shown in FIG. 1C, the two detectors 108 are similarly operable to rotate along the circle 152 in opposing positions, with the normal directions 154A and 154B to the top surface of each of the two detectors 108 pointing to the center of the circle 152. The difference between the two is that the normal directions 154A and 154B in FIG. 1C are not completely opposite to each other, but form a small angle Υ. The angle Υ may range from about 0.5° to about 20°, depending on the system performance needs, according to some embodiments. The details of the setup in FIG. 1B and FIG. 1C are described below. Each detector 108 operates in conjunction with a collimator 110. The collimator 110 is a device that guides the photon path. Unlike photons emitted from a known source position as in X-rays and CT, in molecular imaging, photons are emitted from an unknown position inside the subject's body. Without the collimator 110, photons from all directions would be recorded by the detector 108, which may make image reconstruction difficult. Therefore, the collimator 110 is used to direct the photon path to reconstruct the image.The imaging system 100 further includes a patient table 112 coupled to a table support system 114, which may be coupled directly to a floor or to the gantry 102 via a base. The patient table 112 is configured to slide relative to the table support system 114, facilitating movement of the patient 150 to and from an examination position substantially aligned with the axial axis. The control console 120 provides operation and control of the imaging system 100, such as in any manner known in the art. The control console 120 provides operation and control of the imaging system 100, such as in any manner known in the art. For example, the control console 120 can be used by an operator or technician to control mechanical movements such as rotation of the rotor 104, movement, rotation, or tilt of the detector 108 and collimator 110, and sliding of the patient table 112. The imaging system 100 further includes computer components (not shown), such as a data storage device, an image processor, an image storage device, and a display, for acquiring data and reconstructing nuclear medicine images. In some embodiments, one or more of the computer components may be partially or wholly located remotely (e.g., using cloud computing). In some embodiments, one or more of these components are located locally or remotely.

[0016] In some embodiments, the detector 108 is a semiconductor detector, such as made of cadmium telluride (CdTe), cadmium zinc telluride (CZT), or high purity germanium (HPGe). In some embodiments, the detector 108 is a scintillator detector, such as made of sodium iodide (NaI) or cesium iodide (CsI). In some other embodiments, the detector 108 may also be a scintillator coupled with a miniature photomultiplier tube (PMT), a silicon photomultiplier tube (SiPMT), or an avalanche photodiode. One or more radiopharmaceuticals administered orally or injected into the patient 150 undergo nuclear decay, emitting radiation (e.g., gamma photons) with a particular speed and characteristic energy by direct or indirect annihilation. The detector 108 is placed in proximity to the patient 150 to record or monitor the emission of radiation. Based on the recorded information, including the location, energy, and number of detections of the detected events, an image showing the distribution of the radiopharmaceutical can be reconstructed and the condition or function of the body part of the patient 150 can be observed.

[0017] The collimator 110 includes a number of walls (also known as septa) that define one or more openings (also called through holes). In various embodiments, the septa are made of a heavy metal such as lead or tungsten. The thickness of the septa is large enough to block most of the radiation, depending on the energy of the photons, such that the photons pass mainly through the small openings on the plate. A larger thickness is required to image higher energy gamma rays. The collimator 110 is disposed between the detector 108 and an imaging subject, such as a patient 150. The collimator openings determine the direction and angular range (acceptance angle) through which radiation can pass to reach a particular location on the detector. The collimator 110 may be a single-hole collimator, a multi-hole collimator, a coded aperture collimator, or any other suitable type of collimator, depending on the number and geometry of the openings.

[0018] The imaging system 100 may include other components necessary for an imaging gantry, such as connectors for coupling components together (e.g., coupling the detector 108 and collimator 110 together), motors for moving the components, photon blocking components, housing components for housing the other components, etc. For example, the coupling and shielding components 116 may couple the detector 108 and the collimator 110 such that they move (e.g., rotate) together and block radiation (photons) from reaching the detector 108 through a path other than the collimator 110. In another embodiment, the detector 108 and the collimator 110 may move independently relative to each other.

[0019] FIG. 2A shows a perspective view of an exemplary multi-hole (or multi-aperture) collimator 202, and FIG. 2B shows a cross-sectional view of the collimator 202 (along section line BB in FIG. 2A). The collimator 202 has a number of holes 208, each having a predetermined height H and a diameter (or width) W. The height H is also the thickness of the collimator 202. The holes 208 are also referred to as apertures or through-holes. The acceptance angle Θ of the aperture 208 is defined by the angle between two rays R0 and R1 that travel from an end point on the upper opening of the aperture 208 to an opposite end point on the lower opening. Only incident photons (or radiation) that propagate within the acceptance angle Θ can pass through the aperture 208 (not considering the negligible fraction of rays that may be transmitted through the septum 212). As mentioned above, the septum 212 is made of a radiation-absorbing heavy metal(s) or alloy, such as lead or tungsten. The septum 212 absorbs most of the radiation that does not leave the target direction (or propagate toward the target direction). Collimators for high-energy radiation have much thicker septums than collimators for low-energy radiation. The septums are typically designed so that the transmission of unwanted photons through the septum does not exceed 5%, and in some cases, 1%. It should be noted that the radiation or photons that are blocked or absorbed by the collimator do not require 100% blocking, since a very small percentage of photons (e.g., 5% or less) may still penetrate the thickness of the radiation-absorbing material. In other words, blocking (or other similar terms) means that the majority of the photons (e.g., 95% or more, or 99% or more) are absorbed by the radiation-absorbing material.

[0020] The collimator 202 provides position information of detected photons by limiting the acceptance angle Θ of the incident photons. Typically, the smaller the acceptance angle Θ, the higher the imaging resolution provided by the collimator 202. The collimator 202 can have a number of substantially identical elongated apertures 208 arranged side by side and parallel to each other. Elongated apertures entail small acceptance angles Θ, and therefore correspondingly high imaging resolution. The collimator 202 has a thickness H greater than 15 mm, e.g., 20 mm to 70 mm, and an aperture diameter (or width) W of about 1 mm to 5 mm, thereby providing a large aspect ratio (H / W) and a small acceptance angle Θ. The acceptance angle Θ of the parallel hole collimator 202 is limited to a small angle, e.g., less than 15°. If the acceptance angle Θ is too large (e.g., greater than 15°), the imaging resolution provided by a particular parallel hole collimator will be low and may be considered undesirable. Unless otherwise stated, the acceptance angle of the exemplary collimators of the present disclosure is less than 15°. In addition to the holes of FIG. 2B, which have a rectangular “xz” cross section, the above acceptance angle concept can be extended to holes of other shapes, such as those of convergent or divergent collimators, where the holes may be trapezoidal in shape and have a slope that represents the maximum angular difference of the light rays that can pass through the holes.

[0021] The collimator 202 shown in FIG. 2A is a parallel-hole collimator with uniformly distributed apertures. In the illustrated embodiment, the apertures 208 are substantially identical, and each aperture has the same height H and the same diameter or width W, except for variations caused by limitations in fabrication accuracy. Thus, each aperture has the same acceptance angle Θ, which is small (e.g., less than 15°, such as about 5° in a specific example). The aperture pitch P is also substantially the same on the collimator 202. That is, on the collimator 202, the aperture spacing S (thickness of the partition) is uniform. In this specification, apertures with uniform aperture shape, aperture size, aperture length, and aperture pitch are referred to as "uniformly distributed apertures."

[0022] 2C and 2D show plan views of two arrangements of parallel-hole collimators with uniformly distributed apertures (such as collimator 202), but other arrangements are possible and do not depart from the spirit and scope of the present disclosure. In FIG. 2C, the apertures 208 are arranged in orthogonal rows and columns to form an orthogonal two-dimensional grid arrangement. Each aperture 208 can be considered to be aligned with a square unit grid 240 in the grid network (represented by dashed lines in FIG. 2C). Two adjacent apertures 208 have the same pitch P, where P=W+S, W is the width (or diameter) of the aperture 208, and S is the minimum spacing (or partition thickness) between two adjacent apertures 208. In FIG. 2D, the apertures 208 are arranged in a staggered arrangement with adjacent rows in succession, and the two adjacent rows are offset from each other by a predetermined distance (or offset). This offset may be W in one embodiment, or other suitable value in alternative embodiments. Each opening 208 can be considered to be aligned with a hexagonal unit grid 240 in the grid network (represented by dashed lines in FIG. 2D). Two adjacent openings 208 have the same pitch P, where P=W+S, W is the width (or diameter) of the opening 208, and S is the minimum spacing (or partition thickness) between two adjacent openings 208. In some alternative embodiments (not shown), the openings 208 can be aligned with honeycomb-shaped unit grids 240 in the grid network. FIGS. 2E and 2F show variants of FIGS. 2C and 2D, respectively. One or more of the unit grids 240 are designed with no opening(s) through the grid, which corresponds to having openings filled (or closed) by partition material (represented by dashed circles in FIG. 2E and FIG. 2F). Such an arrangement of openings is considered to have a variable opening pitch or to have closed holes, and therefore does not fall under the category of a parallel-hole collimator with uniformly distributed openings. That is, the openings in Figures 2E and 2F are non-uniformly distributed.

[0023] 3 is a schematic cross-sectional view of the collimator 202 shown in FIG. 2B and the detector 204 that operate in conjunction to generate an image representative of the distribution of the radiopharmaceutical in a target object 206 (e.g., a patient). The collimator 202 is disposed between the target object 206 and the detector 204 and is configured to filter radiation by blocking certain photons and passing others. In the illustrated embodiment, coupling and shielding components between the collimator 202 and the detector 204 are omitted for simplicity. In this embodiment, the collimator 202 is a parallel-hole collimator in which the openings or holes therein have rectangular cross-sections and are arranged parallel to one another. In alternative embodiments, the openings or holes in the collimator 202 may have diverging cross-sections, converging cross-sections, or other cross-sectional shapes.

[0024] The detector 204 includes an array of sensing pixels 214, such as a rectangular array, a square array, or other suitable array of pixels 214. In operation, each sensing pixel 214 independently records or monitors the amount of radiation incident on it and generates a signal associated with the amount of radiation (e.g., a voltage or current). The sensing pixels 214 may be substantially the same size and shape (e.g., circular, rectangular, or square). In various embodiments, the sensing pixels 214 are approximately 1x1 mm in size. 2 ~Approx. 5x5mm 2The pitch P' of the detector pixel array may range from less than 1 mm to about 6 mm in various embodiments. In one example, a CZT or silicon photomultiplier (SiPM) based detector 204 may be fabricated to a size of 4 cm x 4 cm, where the detector 204 comprises a 16x16 detector pixel array with a unit pixel size of 2.5 mm x 2.5 mm. In some embodiments, the detector 204 includes appropriate electronic circuitry (e.g., ASIC) for collecting and processing signals generated from the detector pixels 214. In some other embodiments, the detector pixels 214 are not physically distinct from one another, as is the case in most PMT-based detector systems, but are simply the product of the digitization of a continuous detector surface. For example, the detector surface may be digitized as a grid network identical to the grid network of the collimator 202, with each unit grid in the surface corresponding to one opening of the collimator 202.

[0025] In FIG. 3, for simplicity, it is assumed that the pitch P′ of the sensing pixels is substantially equal to the aperture pitch P, and each sensing pixel 214 is directly below a corresponding aperture 208. Furthermore, the collimator 202 is closely coupled (or attached) to the detector 204 with a minimum spacing between them so that photons can only propagate along the aperture 208 to reach the corresponding sensing pixel 214 directly below it, although there may be minimal or negligible crosstalk (e.g., photons passing through a partition or photons passing through one aperture and impinging on a sensing pixel directly below another aperture). In this disclosure, the terms “closely coupled” and “attached” both refer to a spacing between the collimator and the detector that is less than half the thickness of the collimator, including cases where the collimator 202 and the detector 204 are in physical contact.

[0026] In a parallel-hole collimator, the imaging resolution decreases rapidly as the imaging source moves away from the collimator. The aperture size W and length (or effective length) H eFor a target object 206 located at a vertical distance Z above the upper surface 210 of the collimator, the imaging resolution R c is given by the following equation (1).

[0027]

number

[0028] Effective aperture length H e is H e =H - 2 / μ, where H is the aperture length and μ is the linear attenuation coefficient of the collimator material. For lead, μ=22.43 cm at 150 keV. -1 ,

[0029]

number

[0030]

number

[0031] As can be seen from equation (1), the imaging resolution R c increases linearly with distance Z, and at the same distance Z, H e If is greater than R c is smaller (i.e., the imaging resolution is better). In other words, the longer the aperture (i.e., the larger H), the better the imaging resolution. However, a longer aperture has an adverse effect on signal sensitivity in an inverse square relationship. If the collimator efficiency, G, is defined as the ratio of the amount of radiation (e.g., gamma rays) that passes through the collimator to the amount of radiation emitted from the source, G is expressed as He -2 is directly proportional to

[0032]

number

[0033] FIG. 4 shows an alternative collimator design, called a "spread field imaging (SFI) collimator," in which the aperture length is uniformly increased, resulting in higher imaging resolution without sacrificing signal sensitivity. Compared to the parallel hole collimator 202 shown in FIG. 3, the SFI collimator 203 shown in FIG. 4 has at least two distinct features. First, the spacing D between the SFI collimator 203 and the detector 204 is significantly larger compared to the configuration in FIG. 3. Second, the apertures 208 of the SFI collimator 203 are not all the same, but differ in at least one respect, as will be further described below.

[0034] By increasing the spacing D between the SFI collimator 203 and the detector 204, photons passing through one aperture 208 can impinge on an extended area on the detector 204 beyond the aperture 208. That is, radiation from the target object 206 passes through one aperture 208 and illuminates an extended area on the detector 204 that is substantially larger than the size of the aperture 208, and extends to an area directly below another adjacent aperture. In contrast, when the collimator 202 is closely coupled to the detector 204 as shown in FIG. 3, radiation from the target object 206 propagating through one aperture 208 illuminates only an area on the detector 204 that is substantially equal to the size of the aperture 208. As shown in FIG. 4, incident photons that are incident between the lines A1 and A2 that define the acceptance angle Θ pass through the aperture 208 to the detector 204 and "illuminate" the detector 204. The illuminating area of ​​each aperture 208 is the surface area of ​​the detector 204 bounded by the lines that define the acceptance angle of the aperture 208. For example, the illuminating area of ​​the central aperture 208 is defined by the surface area at the detector 204 between the lines A1 and A2 that intersect the detector 204. The size and shape of the illuminating area depends on the profile of the corresponding aperture and the spacing D between the collimator 202 and the detector 204. In some embodiments, the illuminating area of ​​an aperture 208 on the detector 204 is at least twice the size of the corresponding unit grid, which includes the corresponding aperture 208 and half of the partition surrounding the corresponding aperture 208 (note that the boundary of a unit grid is at the center of the partition between the unit grid and an adjacent unit grid). Furthermore, the illuminating area of ​​one aperture 208 may overlap with the illuminating area of ​​an adjacent aperture. For example, in various embodiments, more than 10%, 20%, or 50% of the illuminated area may overlap with other illuminated areas (considering only radiation passing through opening 208 and excluding radiation transmitted through a bulkhead surrounding opening 208). Figure 4 shows the overlap area between an illuminated area defined by lines A1 and A2 below opening 208 and another illuminated area defined by lines A1' and A2' below an adjacent opening.That is, a point (or sensing pixel 214) within the overlapping area of ​​the detector 204 can receive radiation from one or more apertures 208. Furthermore, in some embodiments, any one sensing pixel 214 in the detector 204 is simultaneously illuminated by no more than a predetermined percentage of the total number of apertures 208, which percentage is less than 25%. In some embodiments, this percentage may be less than 10%, less than 5%, or less than 2%. In some other embodiments, the illuminated area of ​​any aperture is no more than a predetermined percentage of the total illuminated area (through all apertures) of the concatenated detectors 204, which percentage may be less than 10%, less than 5%, or less than 2%. In contrast, the apertures in a coded aperture collimator or multi-pinhole collimator typically illuminate a larger percentage of the total illuminated area than the embodiments of the present disclosure. For example, the illuminated area of ​​one aperture in these collimators may be greater than 25% or greater than 40% of the total illuminated area of ​​all apertures in each collimator, thereby introducing a significant amount of crosstalk into the imaging system. By limiting the illumination area of ​​any one aperture to 10%, 5%, or even 2% or less of the total illumination area (through all apertures) in the coupled detector 204 (depending on system performance needs), crosstalk from different apertures is effectively reduced and good performance in imaging resolution and signal sensitivity can be achieved.

[0035] Shielding 244 (usually a heavy metal material) is provided along the perimeter area to cover (or seal) the space between the collimator 203 and the detector 204, preventing radiation from passing through the space to reach the detector 204. Gaps are shown in FIG. 4 between the shielding 244 and the SFI collimator 203, and between the shielding 244 and the detector 204, for illustrative purposes only. In various embodiments, the shielding 244 is deployed to provide a closed (or enclosed) area where only radiation passing through the collimator 203 can reach the detector 204.

[0036] In this embodiment, the apertures 208 of the SFI collimator 203 are not all the same. The apertures 208 may also differ in aperture shape, aperture size, or aperture length. Different aperture sizes may result in different acceptance angles Θ, but in this embodiment, the maximum acceptance angle Θ is less than 15°. As described above, an acceptance angle Θ less than 15° provides desirable imaging resolution. Also, for a given collimator thickness H, a smaller acceptance angle Θ results in a smaller aperture size, and therefore a smaller collimator dimension. An acceptance angle Θ less than 15° provides a good compromise between designing a compact collimator and manufacturing difficulties (e.g., mechanical tolerances). Furthermore, an acceptance angle Θ greater than 15° increases crosstalk from adjacent apertures, resulting in poor resolution and more complex post-imaging processing. In some applications, depending on the needs of system performance, the acceptance angle Θ can be less than about 10°, or even less than about 5°, to achieve higher imaging resolution and reduce crosstalk. In some embodiments, some of the apertures 208 of the SFI collimator 203 are closed and are substantially blocked from allowing photons to pass through. Thus, the aperture pitch also varies across the collimator 203. For example, in FIG. 5B, the pitch and partitions around 208' are relatively different from the other partitions. Apertures that have non-uniformity in at least one parameter of the aperture profile (e.g., aperture shape, aperture size, aperture length, aperture pitch, etc.) are referred to as "non-uniformly distributed apertures." FIG. 5A illustrates one embodiment of an SFI collimator 203 with different aperture shapes (e.g., circular and square), aperture sizes, and different aperture pitches (caused by blocked apertures represented by dashed circles). In FIG. 5A, each aperture 208 is aligned (e.g., center-to-center alignment) with a respective unit grid 240. FIG. 5B illustrates yet another embodiment of an SFI collimator 203, in which there is at least one aperture 208' that is offset from the unit grid 240. An SFI collimator 203 having such an array of apertures can be considered a parallel hole collimator with non-uniformly distributed apertures.5A and 5B show only a portion of the SFI collimator 203. In one embodiment, the total number of apertures 208 on the collimator 203 is greater than one thousand (1000), such as approximately two thousand (2000), five thousand (5000), ten thousand (10000), thirty thousand (30000), or even one hundred thousand (100000). Because each aperture only illuminates a small portion of the detector 204 surface area, and there is significant crosstalk or multiplexing between the apertures 208, a large number of apertures is beneficial for higher sensitivity, a better range of field of view (FOV), and efficient utilization of the detector 204 surface area.

[0037] In some embodiments, some or all of the openings 208 of the collimator 203 can be arranged in a repeating pattern, such as repeating patterns of size 3x3, 4x4, 3x5, 5x5, etc. The repeating pattern is referred to as a base pattern. In some embodiments, the base pattern has an odd number of rows and / or columns such that there is one central hole in the pattern, which may be advantageous in data processing. FIG. 6A shows a plan view (or a portion of a plan view) of one embodiment of the SFI collimator 203. At least a portion of the SFI collimator 203 includes a repeating base pattern of a 5x5 unit cell, and in one base pattern illustrated in FIG. 6B, white pixels represent open openings and gray pixels represent closed openings. The open openings are surrounded by partitions. The dotted lines in FIG. 6A represent the boundaries between the base patterns. In the illustrated embodiment, the openings are aligned with a square unit grid and can expand to the size of the entire unit grid. The base pattern can be a coded aperture pattern, such as a URA, MURA, PBA, random, or pseudorandom pattern. In some embodiments, the number of repeating patterns is greater than 2x2, for example, 3x3, 3x5, 5x5, etc. The basic pattern illustrated in FIG. 6A is a MURA 5 pattern, which is repeated 3 times in the X direction and 3 times in the Y direction to form a collimator (or a portion of a collimator) with a size of 15x15. In various embodiments, the basic pattern can be repeated any number of times in each of the X and Y directions as required by system requirements. FIG. 6C shows a top view of another embodiment of an SFI collimator 203 (or a portion thereof), in which the MURA 5 basic pattern is repeated 25 times in the X direction and 25 times in the Y direction to form a collimator (or a portion of a collimator) with a size of 125x125. In the illustrated embodiment, the shape and size of the openings are the same. In some embodiments, the openings may differ in shape or size, or both.

[0038] Returning to FIG. 4, the spacing D between the collimator 203 and the detector 204 can be selected such that the illuminated area of ​​each opening 208 on the detector 204 is equal to or less than the repeating basic pattern, and is at least twice as large as the spacing (pitch) of the openings. The size of the illuminated area of ​​the openings 208 on the detector 204 is substantially equal to the size of the basic pattern of the collimator 203, as illustrated by dotted lines B1 and B2 in FIG. 4. As will be described in more detail below, making the size of the illuminated area substantially equal to the basic pattern (or an integer multiple of the basic pattern) provides a fast image reconstruction algorithm that significantly improves system efficiency and accuracy. As a result, the spacing D between the collimator 203 and the detector 204 is optimized when it is greater than half the aperture length (collimator thickness) H. In various embodiments, D can be less than 10 times H, for example, between 1.5 and 10 times H, between 1 and 7 times H, between 1.5 and 7 times H, or other suitable range. If D is too small, such as less than 1.5 times H, there is no significant effect on improving resolution. On the other hand, if D is greater than 10 times H, crosstalk from adjacent apertures will be high, increasing complexity during image reconstruction and causing degradation of resolution. In some examples, D is approximately 1, 1.5, 2, 2.5, 3, or 3.5 times H.

[0039] Further, referring to FIG. 4, the area that can be seen through the aperture directly above a sensing pixel 214 of the detector 204 is equal to the imaging resolution R n and is given by the following equation (2):

[0040]

number

[0041] If a photon passes through one aperture and enters a detection region directly beneath another aperture, crosstalk or multiplexing is present. In contrast, for any sensing pixel on the surface of a detector equipped with a parallel-hole collimator (e.g., collimator 202 shown in FIG. 3), there is only one aperture through which a photon can pass (except for transmission through a septum, an event that parallel-hole collimator designs must suppress). This crosstalk effect typically reduces resolution, since there are multiple possible paths a detected photon can take at a given location on the detector, and for a source at a given point, there are multiple apertures through which radiation can pass to reach the detector. This is true even when all apertures are essentially identical, as in parallel-hole collimators.

[0042] On the other hand, for an SFI collimator 203 as shown in Figure 4, the apertures are not all the same. Specifically, the apertures within the visible region of a sensing pixel on the detector surface (where visible region means the collection of all apertures through which photons can pass to reach a sensing pixel on the detector surface) are not all the same. Thus, light sources that are close to each other cast different shadows on the detector due to different patterns of apertures below. These different shadows can cause the reconstruction algorithm to skew the original light source distribution, i.e., the object image, to look like the original light source distribution. n This helps to recover the image with high resolution approaching that of the detector. The detector surface is typically digitized as a network of grids, each grid representing the boundary of a sensing pixel. In some embodiments, the aperture patterns observed by adjacent sensing pixels are different.

[0043] Similar to single photon emission computed tomography (SPECT) imaging, images (also called projections) can be acquired from multiple angles by rotating a coupled pair of collimator (such as collimator 203) and detector (such as detector 204) around the target object. In this acquisition mode, the coupled collimator and detector move together without relative motion to each other. The forward projection p at a particular angle α can be expressed as:

[0044]

number

[0045]

number

[0046]

number

[0047] The algorithm in (5) requires a lot of time to multiply the number of angles by the number of voxels and pixels, and the storage space of the K matrix is ​​also large. A projection at one angle can be digitized from 64x64 up to 512x512 pixels, so typically 60 or 120 angles are required. Also, object images often have 128x128x128 or 256x256x256 voxels, so the matrix Kα(i,j) becomes very large.

[0048] The matrix K and algorithm can be simplified if the apertures are constructed by repeating a basic pattern and corresponding through holes in all repeating patterns have the same shape and dimensions (even if the holes are different in the basic pattern). Typically, for a repeating basic pattern with size XxY (X and Y can be different), there are only a number of patterns equal to multiples of the product of XxY that a point source can project onto the detector. Figure 6A or 6B is an example where the repeating pattern is a 5x5 (X=Y=5) MURA 5 pattern. Let m be the index (grid unit including open and closed holes) of the holes in the basic pattern, m=1,...25. In this example, we consider a plane located at a distance Z from the collimator 203 (see Figure 4), and a point source at any point on this plane projects a given pattern onto the detector 204. Here, the size of the projected pattern is much smaller than the size of the detector, so the pattern that a point source can project onto the detector is only XxY=25, and depending on the index of the aperture directly under this point source (but excluding the border area), psf m,Z The detector pixel size is denoted as (x,y) where x=-M,...M and y=-N,...N. Here, the size of the detector pixels is the same as the collimator pitch (hole spacing), or a fraction of it. Thus, the 25 patterns can be reduced to a basic 5x5 pattern by indexing by aperture. On the other hand, there are f patterns where the pixel (also called voxel) size is the same as the collimator pitch (hole spacing), and the pixels are aligned with the collimator pitch. ZThe image of an object at a distance Z expressed as f Z Here, in each sub-image, the pixels on the aperture with the same index as the aperture are kept the same, and the remaining pixels are set to 0. Let m be the pattern index, then the slice f of the object image at distance Z is Z is the sub-image f Z It can be expressed as (j,m).

[0049]

number

[0050]

number

[0051]

number

[0052] Equation (7) also gives the projection pattern psf if the object image is constructed from slices parallel to the collimator plane, denoted by a distance Z. m,zWe show that is the same for different angles α, which further simplifies the algorithm.

[0053] One way to run equation (5) independently for each angle is to exclude α from the summation and calculate the estimate f along the distance Z. Z (j) is added up to find the angle p' α This is a method to obtain the secondary projection from the object image f to p'. α Since the projection geometry for deriving p' is similar to that of parallel-hole collimation, image reconstruction algorithms for parallel-hole collimation, such as filtered back projection (FBP), algebraic reconstruction (ART), and OSEM, can be used to obtain p'. α f can be reconstructed from

[0054] One alternative is to interpolate fz to α,z where f α,z is constructed from slices parallel to the collimator plane (denoted by Z) at the above angle. Then, a forward projection, back projection step is performed before applying the interpolation,

[0055]

number

[0056] Returning to FIG. 4, with respect to the collimator 203, the height H of the opening (i.e., the thickness of the collimator 203) may be much larger than its opening width or diameter W. For example, the opening width W may be less than 2 mm, and the opening height H may be greater than 20 mm. However, for closed-type openings, the opening does not need to be completely filled, but rather filled to a sufficient thickness to block radiation from passing through the opening. For example, when the radioisotope Tc-99m is used as the partition material, a 3 mm thickness of lead can block more than 99% of the incident radiation of 140 keV gamma rays. Therefore, it is possible to manufacture the collimator with a light weight. Referring to FIG. 8A and FIG. 8B, the collimator 203 is a collimator having an open hole section and a closed hole section, and includes two sections. One section is a section similar to a parallel hole collimator having uniformly distributed through holes (also called the parallel hole section 262). The other section is a substantially thinner plate 264 (but thick enough to substantially block radiation) with non-uniformly distributed through-holes, with fewer through-holes (e.g., about half as many) than the through-holes in the parallel-hole section 262. The collimator 203 shown in FIGS. 8A and 8B is also referred to as a multi-section collimator. The through-holes in the thin plate 264 are aligned with the through-holes in the parallel-hole section 262 to together define the aperture pattern of the collimator 203, while the other through-holes in the parallel-hole section 262 are blocked by the opaque material of the thin plate 264. Thus, the through-holes in the thin plate 264 define the final opening locations of the collimator 203. The thin plate 264 is also referred to as a patterned aperture section 264. In some embodiments, all the through-holes in the parallel-hole section 262 are substantially identical, while the through-holes in the patterned aperture section 264 are different and may include closed apertures. Additionally, the patterned aperture section 264 may be movable in some embodiments, such as being operable to slide horizontally relative to a surface of the parallel hole section 262 .Thus, by moving the patterned aperture section 264 relative to the parallel hole section 262, multiple projections can be obtained at one angle.

[0057] The patterned aperture section 264 has a thickness H1 sufficient to block the target radiation. In various embodiments, the thickness H2 of the parallel-bore section 262 may be about 2 to about 10 times the thickness H1 of the coded aperture section 264. For example, an aperture length of 25 mm may be composed of a parallel-bore section 262 having a thickness of 20 mm and a patterned aperture section 264 having a thickness of 5 mm. In this way, the weight of the entire collimator can be significantly reduced compared to a fully filled closed aperture. The patterned aperture section 264 may be on the side of the parallel-bore section facing the detector (FIG. 8A) or on the side facing the target object (FIG. 8B), or in the middle of two sub-parallel-bore sections. (For example, sub-parallel-bore sections 262-a and 262-b in FIG. 8C, where H2-a+H2-b=H2). In fact, a closed aperture requires only a thin layer of heavy metal material to block radiation. This thin layer can also be divided into multiple segments as needed. Therefore, this new design starts by fabricating regular parallel through holes, and then a thin layer can be placed anywhere within the open opening to create a closed opening.

[0058] Although the apertures shown in Figures 8A-8C have the same cross section and aperture width, there are many different designs that allow different aperture profiles. For example, the lightweight design can also be applied to collimators with the same pitch of apertures, but with different aperture sizes and partitions. Some such examples are shown in Figures 9A-9C. In Figure 9A, collimator 203 has some apertures with diameter (or width) W and some apertures with smaller diameter (or width) W'. Instead of using partitions of constant width throughout the thickness of collimator 203 to create narrower aperture sidewalls, the partitions may have thicker portions only at the lateral ends. The thicknesses H1-a and H1-b at the lateral ends are sufficient to block the target radiation and maintain a smaller acceptance angle Θ, similar to the narrower aperture design. The above multi-section design concepts described with reference to Figures 8A-8C can also be applied to the exemplary collimator of Figure 9A. FIG. 9B shows a collimator 203 with one parallel hole section 262 and two patterned aperture sections 264-a / 264-b that define the aperture size. In some cases, only one of 264-a / 264-b is needed to adjust the acceptance angle Θ. The apertures may also have different shapes in both cross and vertical cross sections. In some embodiments, the apertures may have different orientations. In the patterned aperture sections 264-a / 264-b, the aperture size may be defined by a pattern of open and closed apertures as shown in FIG. 9C. With collective reference to FIGS. 8A-9C, one parallel hole section 262 is paired with different patterned aperture sections 264a and / or 264b to create different collimator configurations. Thus, the multi-section collimator according to the present disclosure provides a lightweight design and also provides a low-cost solution. In particular, it should be noted that Figures 8A-8C and 9A-9C show embodiments in which adjacent sections of the multi-section collimator are in physical contact with one another.In an alternative embodiment, adjacent sections are closely coupled to each other without physical contact (to prevent photons entering through one hole and exiting through another), as long as collimator performance is guaranteed.

[0059] Reference is now made to FIG. 10A. Unlike the exemplary collimator shown in FIGS. 8A-9C, in which the parallel hole section 262 and the patterned aperture section 264 are attached to each other, in the exemplary collimator shown in FIG. 10A, the parallel hole section 262 is spaced apart from the patterned aperture section 264. Furthermore, the patterned aperture section 264 may be closely coupled to the detector 204. In the illustrated embodiment, the sensing pixels 214 of the detector 204 have corresponding openings in the patterned aperture section 264 to receive illumination from the parallel holes in the parallel hole section 262 above. The openings in the patterned aperture section 264 are intentionally different in size, resulting in a different number of parallel holes in the parallel hole section 262 that can illuminate the pixel 214. For example, the sensing pixel 214a has a corresponding opening in the patterned aperture section 264 that is larger than the adjacent sensing pixel 214b. As a result, the sensing pixel 214a can be illuminated by multiple parallel holes in the parallel hole section 262, bounded by lines C1 and C2 intersecting the bottom surface of the parallel hole section 262. In contrast, the pixel 214b is illuminated only by a single parallel hole directly above it in the parallel hole section 262, bounded by lines D1 and D2 intersecting the bottom surface of the parallel hole section 262, because the corresponding opening of the patterned aperture section 264 is smaller. The number of parallel holes in the parallel hole section 262 that can illuminate the same sensing pixel 214 depends on the acceptance angle of the corresponding opening of the patterned aperture section 264. The number of parallel holes in the parallel hole section 262 that can illuminate the same sensing pixel 214 also depends on the distance H3 between the parallel hole section 262 and the patterned aperture section 264. In principle, the distance H3 is such that at least a portion of the pixels 214 of the detector 204 can be illuminated by two or more of the parallel holes 208. Note that in the alternative embodiment shown in FIG. 10B, it is the parallel hole section 262 that is closely coupled to the detector 204, while the patterned aperture section 264 is spaced apart from 262.

[0060] The design presented in this invention can be adapted to other variants with multiple elongated holes, such as collimators with convergent or divergent holes, modified by the two features presented above.

[0061] Returning to Figures 1A-1C, the radiation imaging system 100 can use at least two different sets of coupled collimators and detectors. In one such embodiment, one set has an SFI (for extended field imaging) collimator 203 (e.g., as shown in Figures 4, 6A-6C, 8A-8C, 9A-9C, or 10A-10B) spaced apart from the coupled detector, and the other set has a parallel-hole collimator 202 attached to the coupled detector (e.g., as shown in Figure 3). Typically, in this configuration, the aperture (and / or acceptance angle) of the parallel-hole collimator 202 is smaller than that of the SFI collimator 203. As shown in equation (1), the parallel-hole collimator has good resolution at close ranges (small Z) due to its small aperture, but the imaging resolution drops off rapidly as the target object moves away from the collimator. In contrast, the SFI collimator 203 provides superior imaging resolution when the target object moves away from the collimator. During multi-angle image acquisition, the two sets of coupled collimators and detector modules rotate around the target object to acquire images (projections). Naturally, a certain portion of the target object will be closer to one collimator at a given angle and away from the target object at the opposite angle, except for the portion close to the center of rotation (COR). By combining these two collimators, for positions closer to the parallel-hole collimator 202, a larger weight can be assigned to the update coefficients estimated from the projection of the parallel-hole collimator 202, and for positions farther from the parallel-hole collimator 202, a larger weight can be assigned to the update coefficients estimated from the projection of the SFI collimator 203. In other words, the weighting between the estimates of the two sets of coupled collimators and detector modules depends on the distance. Another advantage of this configuration is that a quick estimate of the object image can be obtained by reconstruction using only the parallel hole collimator, which can be used as an initial estimate for iterative reconstruction using projections from both collimators.The method of reconstructing the projections from the multi-angle parallel hole can be selected from Filtered Back Projection (FBP), Algebraic Reconstruction (ART), Ordered Subset Expectation Maximization (OSEM), etc. Then, to mitigate the low scan times due to the parallel hole collimator of the present invention having only half the acquisition angle compared to the conventional scan where both cameras have parallel hole collimators, a step of image reconstruction at a lower resolution (by applying low pass filtering (LPF) and / or reconstructing larger pixels) may be performed. In another embodiment, at least two sets of concatenated collimator and detector modules are used in the system, both of which include SFI collimators 203 (e.g., as shown in Figures 4, 6A-6C, 8A-8C, 9A-9C, or 10A-10B) located at a distance from the concatenated detector. However, the elongated through holes of the two collimators have different acceptance angles. For example, the maximum acceptance angle of one SFI collimator 203 is at least twice that of the other SFI collimator 203 (e.g., 10° for one and 5° for the other). In another embodiment, at least two sets of coupled collimator and detector modules are used in the system, where at least one detector is coupled to an SFI collimator 203 having a long narrow hole (small acceptance angle) and positioned away from the coupled detector (e.g., as shown in Figures 4, 6A-6C, 8A-8C, 9A-9C, or 10A-10B), and another detector is coupled to a multi-pinhole collimator including a coded aperture collimator, where the maximum acceptance angle is significantly larger than the SFI collimator 203, such as greater than 20°.

[0062] Referring to FIG. 1B, when two identical sets of coupled collimators and detectors in opposite positions are used in the radiographic imaging system 100, each set of coupled collimators and detectors only needs to rotate half a circle in equal sweeping steps. By combining data from the first and second semicircles, one circle of images is acquired. For example, in a 3° sweeping step, the first set sequentially acquires images at angles of 0°, 3°, 6°, ... 177°, and the second set sequentially acquires images at angles of 180°, 183°, 186°, ... 357°. When two different sets of coupled collimators and detectors are used in the radiographic imaging system 100, each set needs to rotate one circle because each set "observes" in a different way. To maintain the same operating time, each set sweeps one circle with twice the sweeping steps. For example, in a 6° sweep step, the first set sequentially acquires images at angles of 0°, 6°, 12°, ... 180°, 186° ... 354°, and the second set sequentially acquires images at angles of 180°, 186°, 192°, ... 354°, 0°, ... 174°. For each given angle swept by the system, two images are acquired, one image from the first set and another image from the second set. In other words, the two sets "observe" in different ways, but each set repeats by acquiring an image from the same angle as the other set acquired an image. In contrast, referring to FIG. 1C, the two different sets of coupled collimators and detectors are not in completely opposite positions, but are offset by an angle of half the angle of the sweep step. For example, in the case of a 6° sweep step, the directions 154A and 154B in which the two sets face form an angle Υ of about 3°. By offsetting the orientation of the two sets, system 100 is able to capture each image from a different angle.For example, with a sweep step of 6° and an offset of 3°, the first set would sequentially acquire images at angles of 0°, 6°, 12°, ... 180°, 186°, ... 354°, and the second set would sequentially acquire images at angles of 183°, 189°, 195°, ... 357°, 3°, ... 177°.

[0063] Reference is now made to Figures 11A and 11B. In a radiation imaging system, performance is optimized if the openings of the collimator are aligned with the detector's sensing pixels. Figure 11A shows a plan view in the XY plane of a portion of an exemplary SFI collimator 203 (basic pattern shown in Figure 6B) superimposed on a detector 204. Figure 11B shows a schematic cross-sectional view in the XZ plane of a portion of the collimator 203 and the detector 204 along the section line AA in Figure 11A. The detector 204 shown in Figures 10A and 10B includes an array of individual sensing pixels 214. The individual sensing pixels 214 may be based on a CZT or SiPM structure. Typically, the sensing pixels 214 are arranged in a grid network (also called a pixel grid) formed by rows and columns, such as rows i=1,2,3,... and columns j=1,2,3,... in the illustrated embodiment. Any sensing pixel 214 located at a position (x, y) in the XY plane can be represented by an index (i, j) in the grid network (i and j are integers) and has a corresponding aperture (or a coding pattern or a closed aperture in a partition) in the grid network (also called an aperture grid) of the collimator 203. Since the sensing pixels 214 in Figures 11A and 11B are individually and physically distinct from each other, alignment between the detector and the collimator can be easily achieved, i.e., by placing the sensing pixel 214 directly under the corresponding aperture of the collimator 203 such that the pixel grid and the aperture grid overlap.

[0064] Reference is now made to Figures 12A and 12B. As explained in relation to Figures 3 and 4, in some embodiments, the sensing pixels of the detector are not physically distinct from one another, as is the case in most PMT-based detector systems, but are simply the product of the digitization of a continuous detector surface. Figure 12A shows a plan view in the XY plane of a portion of an exemplary SFI collimator 203 (basic pattern shown in Figure 6B) superimposed on such a detector 204. Figure 12B shows a schematic cross-sectional view in the XZ plane of a portion of the collimator 203 and the detector 204 along the section line AA in Figure 12A. The detector surface is digitized as a pixel grid that is the same as the corresponding aperture grid of the collimator 203, i.e., the spacing between grid points is the same. That is, the unit grid of each pixel on the detector surface has the closest corresponding aperture (or closed-type aperture in the coding pattern or septum) in the collimator 203. The unit grid of each pixel on the detector surface is also referred to as a pixel of the detector. The pixels of the detector are arranged in a grid network formed by rows i=1,2,3,... and columns j=1,2,3,... and can be labeled as pixels (i,j) (i and j are integers). Ideally, in a perfectly aligned state, the pixel grid of the detector and the aperture grid of the collimator should overlap. However, as shown in Figs. 12A and 12B, there are many cases where misalignment occurs between the pixel grid and the aperture grid, such as during assembly of the device, resulting in misalignment in the X direction (denoted as Δx) and / or in the Y direction (denoted as Δy). Such misalignment leads to degradation of the system. For example, as shown in Fig. 12B, in a perfectly aligned state, an incident photon passing through the aperture of the collimator 203 that should be recorded as an event (e.g., count) occurring at pixel (3,1) is instead recorded as an event occurring at pixel (3,2), which reduces the contrast of the aperture pattern in the recorded signal.

[0065] Taking a photomultiplier tube (PMT) based detection system as an example (and detection systems using other structures as well), the PMT works by converting incident photons into photoelectrons at a photocathode. These electrons generate a number of secondary electrons from a series of charged cathodes, generating a measurable current pulse at the anode. Based on the measurable current pulses at adjacent PMTs, a position (x,y) on the continuous XY plane can be determined as the position where the incident photon event occurred. Based on the mapping between the continuous XY planes and the index (i,j), the event occurring at the position (x,y) is then digitized as the event occurring at pixel (i,j). However, as mentioned above, misalignment of the initial mapping between the continuous XY planes and the index (i,j) can lead to poor performance. Image contrast provides a way to adjust for misalignment due to offset. In order for the human eye and mind to recognize an image, contrast must be present in the acquired image array. Any part of the subject must cause the signal intensity recorded at different pixels to change. The simplest is the transmission contrast caused by partially or totally opaque structures such as collimator openings and closed openings (or septa). The amount of contrast present in the image determines the precision with which the misalignment is adjusted. FIG. 13 shows a setup to compensate for the misalignment by an algorithm without physically moving the collimator. In FIG. 13, a flood source 242, such as a Co-57 or Tc-99m rectangular flood source, provides a uniform illumination field above the collimator 203. The flood source 242 can be placed in close contact with the collimator 203, such as less than half the thickness of the collimator 203, including when the collimator 203 and the flood source 242 are in physical contact. When illuminated by the flood source 242, the image acquired by the detector 204 is an image of the flood source 242, including the shadow cast by the collimator 203. Because the illumination field is uniform, the contrast in the acquired image is based on the transparent and opaque features of the collimator 203.That is, pixels under a closed aperture (or partition), or a small aperture, will have a lower image signal strength compared to pixels under an open aperture or a large aperture.

[0066] Contrast can be defined as a metric of variation (e.g., ratio or absolute difference) between the peaks and valleys of a single intensity in an image. As an example, the signal intensity line 244 in FIG. 13 has V p and V v One way to maximize contrast is to p / V v ratio, or

[0067]

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[0068] Because the detector surface is digitized into a pixel grid, the misalignment can be compensated for by an algorithm, i.e., maximizing contrast, without physically moving the collimator. In one exemplary method, each signal emerging from the detector surface (e.g., a current pulse from the PMT in the case of a PMT-based detection system) is recorded as an event with coordinates (x,y). Thus, each entry in the data list (also called list mode) is an attribute vector that contains information about a single detected photon. For example, a sequence of events can be recorded as (x n ,y n ) (n=1,2,3,...), where each pair represents the X- and Y-coordinates of the nth detected event. Based on the mapping between the continuous XY plane and the discrete index (i,j), the total number of records for pixel (i,j), denoted as C(i,j), is

[0069]

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[0070]

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[0071] Next, to "offset" the pixel grid, we introduce an offset pair (δx, δy) into the digitization function and recalculate C(i,j). Thus, C(i,j) is

[0072]

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[0073]

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[0074] Alternatively, instead of sweeping δx and δy together, the method may first perform a sweep in the X direction only (by fixing δy) to obtain an optimized offset δx in the X direction, and then perform a sweep in the Y direction only (with the optimized δx) to obtain an optimized offset δy in the Y direction. Similarly, the method may first perform a sweep in the Y direction, and then a sweep in the X direction. Additionally, the method may use a sweep of V from the entire pixel grid to calculate the contrast. p and V v Instead of selecting C(i,j) two-dimensionally, the method may select a row (by fixing i in the set of C(i,j), such as signal intensity line 244 in FIG. 13), sweep (δx, δy) to get the maximum contrast in the row, and apply the selected offset pair (δx, δy) to the entire pixel grid. Similarly, the method may select a column (by fixing j in the set of C(i,j)), sweep (δx, δy) to get the maximum contrast in the column, and apply the selected offset pair (δx, δy) to the entire pixel grid.

[0075] It should be noted that different optimization methods may be required for different hole patterns. In some patterns, the maximum or minimum value of a recorded signal intensity descriptor (e.g., contrast, signal intensity at peak, signal intensity at valley, etc.) is used to maximize / minimize the maximum contrast, maximum peak value, or minimum valley value, etc. When a repeating hole pattern is used, corresponding pixels (corresponding to the same hole in the base pattern) can be added together to reduce randomness (i.e., quantum noise) in the pixel values. Also, in some embodiments, when a particular hole pattern is used, the selected offset pair (δx, δy) from the optimization routine is at a fixed offset away from the actual optimal offset, which may be determined by the hole pattern. For example, in some hole patterns, the maximum pixel value in the flood source image is located at the center of adjacent 2x2 open openings. In that case, the fixed offset is half the hole pitch, and the actual optimal offset used to align the pixels to the holes will be corrected by the fixed offset. That is, after determining the selected offset pair (δx, δy), a further step may optionally be performed of adding or subtracting a fixed offset to or from the selected offset pair (δx, δy) to obtain an actual optimum offset to use for misalignment adjustment.

[0076] Without intending to be limiting, one or more embodiments of the present disclosure provide numerous advantages for imaging with radiation at a target object, such as a patient, For example, a collimator design having a repeating coding pattern spaced apart from an associated detector provides superior imaging resolution without sacrificing signal sensitivity, thus improving system performance.

[0077] The above description outlines the features of some embodiments so that those skilled in the art can better understand the aspects of the present disclosure. Those skilled in the art will appreciate that the present disclosure can be readily used as a basis for designing or modifying other processes and structures for carrying out the same purpose and / or achieving the same advantages as the embodiments introduced herein. Those skilled in the art should also recognize that such equivalent structures do not depart from the spirit and scope of the present disclosure, and that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the present disclosure. It is therefore appropriate that the appended claims be broadly interpreted in a manner consistent with the present disclosure.

Claims

1. 1. A radiation imaging system, comprising: a collimator configured to filter radiation emitted from a target object, a plurality of openings non-uniformly distributed on the collimator; a maximum acceptance angle of the plurality of openings is 15° or less; the plurality of openings form a predetermined aperture pattern including a base pattern, the base pattern being repeated more than four times within the aperture pattern. The collimator; a detector for detecting radiation passing through the collimator; having the collimator is positioned at a distance from the detector such that a point on an upper surface of the detector facing the collimator is simultaneously illuminated by two or more of the plurality of apertures; During detection, the collimator remains stationary relative to the detector. Radiation imaging system.

2. 2. The radiation imaging system of claim 1, wherein a point on the top surface of the detector is illuminated by a predetermined percentage of the plurality of apertures, the percentage being less than 25%.

3. 2. The radiation imaging system of claim 1, wherein the number of the plurality of apertures is greater than one thousand.

4. 2. The radiation imaging system according to claim 1, wherein the plurality of apertures are different in at least one of aperture size, aperture shape, light acceptance angle, aperture length, and aperture pitch.

5. 2. The radiation imaging system of claim 1, wherein the distance between the collimator and the detector is 1.5 to 10 times the thickness of the collimator.

6. 2. The radiation imaging system of claim 1, wherein said basic pattern is a coded aperture pattern.

7. 2. The radiation imaging system of claim 1, wherein an illumination area of ​​one of the plurality of openings is within a range of + / -10% of an area of ​​the basic pattern.

8. 2. The radiation imaging system of claim 1, wherein the collimator includes a first portion and a second portion, the first portion including through holes uniformly distributed on the first portion, and the second portion including through holes non-uniformly distributed on the second portion and corresponding to the plurality of openings, whereby the through holes of the first portion are blocked by the second portion.

9. 9. The radiation imaging system of claim 8, wherein the thickness of the first portion is between 2 and 10 times that of the second portion.

10. 10. The radiation imaging system of claim 1, wherein the collimator is a first collimator and the detector is a first detector, the system further comprising: A second collimator; and a second detector coupled to the second collimator; having the second collimator is attached to the second detector; Radiation imaging system.

11. 1. A radiation imaging system, comprising: a first collimator configured to filter radiation emitted from a target object, the first collimator including a first plurality of apertures forming a first aperture pattern; a first detector associated with the first collimator to detect radiation passing through the first collimator; a second collimator configured to filter radiation emitted from the target object, a second plurality of openings forming a second opening pattern; the second plurality of openings being non-uniformly distributed on the second collimator; a maximum acceptance angle of the second plurality of openings is less than or equal to 15°; the second aperture pattern includes a basic pattern, the basic pattern being 3x3 or greater in size and being periodically repeated more than four times within the second aperture pattern; the second collimator; a second detector associated with the second collimator to detect radiation passing through the second collimator; and having the target object is disposed between the first collimator and the second collimator, and the first aperture pattern is different from the second aperture pattern; During detection, the second collimator remains stationary relative to the second detector. Radiation imaging system.

12. 12. The radiation imaging system of claim 11, wherein the first collimator is in contact with the first detector.

13. 13. The radiation imaging system according to claim 12, wherein the distance between the second collimator and the second detector is 1.5 to 7 times the thickness of the second collimator.

14. 12. The radiation imaging system of claim 11, wherein the first plurality of apertures are uniformly distributed on the first collimator.

15. 12. The radiation imaging system of claim 11, wherein the basic pattern comprises a coding pattern.

16. 16. The radiation imaging system of claim 15, wherein the coding pattern is one of a Uniformly Redundant Array (URA) pattern, a Modified Uniformly Redundant Array (MURA) pattern, and a Perfect Dual Array (PBA) pattern.

17. 16. The radiation imaging system of claim 15, wherein a normal direction to a top surface of the first detector and a normal direction to a top surface of the second detector form a non-zero angle.

18. 1. An apparatus for medical imaging, comprising: a plate having a top surface, a bottom surface, and a plurality of holes extending from the top surface to the bottom surface, the plurality of holes being arranged in a repeating pattern of a base pattern, the base pattern being a coding pattern; a detector configured to acquire an image generated from photons passing through the plurality of holes in the plate, the plate has a thickness, and a spacing between the plate and the detector is at least 1.5 times the thickness of the plate; During detection, the plate remains stationary relative to the detector. The detector and having The irradiation area of ​​one of the plurality of holes is within a range of + / - 10% of the area of ​​the basic pattern. Device.

19. 20. The apparatus of claim 18, wherein the base pattern comprises a grid of odd rows and odd columns.

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