# Patent

## Title
X-ray chopper wheel assembly and method

## Abstract

An x-ray chopper wheel assembly, and corresponding method, include a chopper wheel having a solid area configured to block x-ray radiation received at a source side of the chopper wheel from an x-ray source. The chopper wheel defines one or more openings configured to pass x-ray radiation from the source side of the chopper wheel to an output side of the chopper wheel. The assembly further includes a source-side scatter plate arranged relative to the chopper wheel with a source-side gap in a range of approximately 0.2 mm to approximately 2.0 mm between the source-side scatter plate and the source side of the chopper wheel. The assembly and method can be used to limit leakage of scattered x-rays from the assembly, such as to safe levels for operation, while being significantly lighter than existing confinement enclosures.

## Background
RELATED APPLICATIONS
This application is a continuation of U.S. application Ser. No. 16/935,787, filed Jul. 22, 2020, which is a continuation of U.S. application Ser. No. 15/946,425, filed Apr. 5, 2018, now U.S. Pat. No. 10,770,195, which claims the benefit of U.S. Provisional Application No. 62/482,064, filed on Apr. 5, 2017. The entire teachings of the above applications are incorporated herein by reference.


BACKGROUND
X-ray backscatter imaging has been used for detecting concealed contraband, such as drugs, explosives, and weapons, since the late 1980s. Unlike traditional transmission x-ray imaging, which creates images by detecting the x-rays penetrating through an object, backscatter imaging uses reflected or scattered x-rays to create the image.
SUMMARY
In example embodiment assemblies, a disk chopper wheel need not be enclosed in a full shielded housing. Instead, embodiments incorporate a novel, open-geometry disk chopper wheel that includes one or more scatter plates especially configured to limit x-ray leakage and greatly reduce the weight of the chopper disk assembly relative to prior art systems. Embodiments are designed with one or more scatter plates on a source side of the disk chopper wheel, and embodiments may also optionally include one or more scatter plates on an exit side of the disk chopper wheel. The scatter plates are designed to absorb x-rays that are scattered off the chopper wheel, either in the forward or backward directions. A shielded structure may also be added to enclose a fan beam entering the chopper wheel assembly in a region between an x-ray source (e.g., x-ray tube) and the disk assembly.
In one embodiment, an x-ray chopper wheel assembly includes a disk chopper wheel configured to rotate about a rotation axis thereof. The rotation axis is perpendicular to a rotation plane of the disk chopper wheel, and the disk chopper wheel has a solid cross-sectional area in the rotation plane and is configured to absorb x-ray radiation received from an x-ray source at a source side of the disk chopper wheel (an input side at which radiation from an x-ray source is initially incident). The disk chopper wheel also defines one or more radial slit openings configured to pass x-ray radiation from the source side of the disk chopper wheel to an output side of the disk chopper wheel
The x-ray chopper wheel assembly also includes a source-side scatter plate having a solid cross-sectional area in a plane parallel to the rotation plane of the disk chopper wheel. The source-side scatter plate is configured to absorb x-ray radiation and defines an open slot therein configured to pass x-ray radiation. The solid cross-sectional area of the source-side scatter plate is substantially smaller than the solid cross-sectional area of the disk chopper wheel. The assembly further includes a support structure configured to secure the source-side scatter plate substantially parallel to the rotation plane of the disk chopper wheel with a source-side gap between the source-side scatter plate and the source side of the disk chopper wheel.
The solid cross-sectional area of the source-side scatter plate in the plane parallel to the rotation plane of the disk chopper wheel may be less than 75%, less than 50%, less than 25%, or less than 10% of the cross-sectional area of the disk chopper wheel. The source-side gap may be in a range of approximately 0.5 mm to approximately 1.0 mm. The source-side scatter plate may be formed from tungsten or another high-Z material. The source-side scatter plate may have a thickness on the order of 1.0 mm. The cross-sectional area of the source-side scatter plate may be in a range of about 100% to about 5,000% larger or in a range of about 500% to about 10,000% larger than an open cross-sectional area of one of the one or more radial slit openings in the rotation plane of the disk chopper wheel.
The source-side scatter plate may have a plate width in a direction parallel to a radial direction of the disk chopper wheel, and this plate width may be in a range of about 10% to about 70% greater than a slit length of one of the one or more radial slit openings in the radial direction of the disk chopper wheel. The source-side scatter plate may be formed of pure or alloyed lead, tin, iron, or tungsten.
The assembly may further include an output-side scatter plate, which may have a solid cross-sectional area in a plane parallel to the rotation plane of the disk chopper wheel, with the output-side scatter plate configured to absorb x-ray radiation. The output-side scatter plate may define an open slot therein configured to pass x-ray radiation, and the solid cross-sectional area of the output-side scatter plate in the plane parallel to the rotation plane of the disk chopper wheel may be substantially smaller than the solid cross-sectional area of the disk.
The support structure may be further configured to secure the output-side scatter plate substantially parallel to the rotation plane of the disk chopper wheel with an output-side gap between the output-side scatter plate and the disk chopper wheel. The support structure may be further configured to secure the disk chopper wheel at the rotation axis thereof.
The support structure may include an inner portion configured to secure the disk chopper wheel at the rotation axis thereof, with one or more radial spokes extending from the inner portion and configured to secure the source-side scatter plate. The support structure may further include a source-side portion and an output-side portion, with the source-side and output-side portions configured to be connected together and to secure the disk chopper wheel therebetween. The support structure may be formed of aluminum. The support structure may be configured to be mounted within a handheld x-ray scanner or within a fixed-mount or mobile x-ray scanning system.
The x-ray chopper wheel assembly may further include a shield structure configured to enclose the x-ray radiation in a region of travel between the x-ray source and the source-side scatter plate.
The x-ray source may be configured to output x-rays having an end-point energy in a range of about 120 kiloelectron volts (keV) to about 450 keV.
The source-side scatter plate may be configured to output a fan beam of x-rays through the open slot therein, and the assembly may be configured to output a pencil beam of x-rays. The disk chopper wheel and source-side scatter plate may be arranged relative to each other to substantially confine x-ray radiation scattered therefrom. Substantial confinement may also be achieved based on an arrangement of the disk chopper wheel and source-side scatter plate with the optional output-side scatter plate.
The substantial confinement may limit leakage of scattered radiation to no more than 50% leakage of the radiation that is scattered or to a dose of no more than 5 milli-Rem per hour at a distance of 5 cm away from an outer surface of the assembly, whichever is greater. The substantial confinement may limit leakage of scattered radiation to no more than 10% of scattered radiation or to a dose of no more than 0.5 milli-Rem per hour at a distance of 5 cm away from the outer surface of the assembly, whichever is greater.



BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments.
FIG. 1A is a photograph or illustration of a handheld x-ray scanner incorporating an embodiment chopper wheel assembly.
FIG. 1B is an illustration of a fixed-mount x-ray scanning system configured to scan luggage on a conveyor belt, the system incorporating an embodiment chopper wheel assembly.
FIG. 2 is a perspective illustration of a disk chopper wheel and source-side scatter plate that may be used in an embodiment chopper wheel assembly.
FIG. 3 is a perspective illustration of a prior art chopper wheel with a full enclosing shield surrounding the wheel.
FIG. 4A is a perspective illustration of an embodiment chopper wheel assembly for x-ray scanning.
FIG. 4B is an exploded, perspective-view illustration of the embodiment chopper wheel assembly of FIG. 4A.
FIG. 5A is a cross-sectional view illustration of a disk chopper wheel, scatter plates, and a shield structure that form part of the assembly of FIGS. 4A-4B.
FIG. 5B is a magnified view of a portion of the cross-sectional illustration of FIG. 5A showing additional dimensional details and features of the embodiment assembly of FIGS. 4A-4B.
FIG. 6 is a cross-sectional view illustration of the full chopper wheel assembly illustrated in FIGS. 4A-4B.
FIG. 7 is a cross-sectional view illustration of the disk chopper wheel used in the embodiment chopper wheel assembly of FIGS. 4A-4B.
FIG. 8A is a cross-sectional view illustration of a source-side scatter plates used in the embodiment chopper wheel assembly of FIGS. 4A-4B.
FIG. 8B is a cross-sectional view illustration of an output-side scatter plate used in the embodiment chopper wheel assembly of FIGS. 4A-4B.
FIG. 9 is a magnified view of a portion of the cross-sectional illustration of FIG. 5A, further illustrating the action of the disk chopper wheel, source-side scatter plate, an output-side scatter plate, and shield assembly of the embodiment chopper wheel assembly of FIGS. 4A-4B, specifically the action of these components to substantially confine scattered x-ray radiation.
FIG. 10 is a graph showing calculated x-ray leakage for various gaps between the source-side scatter plate and disk chopper wheel of FIGS. 4A-4B as a function of scatter plate width.



DETAILED DESCRIPTION
A description of example embodiments follows.
An x-ray tube has been used to generate x-rays that are collimated into a fan beam by a slot in an attenuating plate. The fan beam is then typically “chopped” into a pencil beam by a rotating “chopper wheel” with slits therein. As the chopper wheel rotates, the wheel slits rotate across the fan beam, causing the x-ray pencil beam to scan over an object being imaged.
For backscatter imaging, the intensity of the x-rays scattered in the backward direction is then recorded by one or more large area backscatter detectors as a function of the position of the illuminating, scanning pencil x-ray beam. By moving the object through the plane in which the x-ray pencil beam scans, either on a conveyor or under the object's own power, a two-dimensional backscatter image of the object is produced. Chopper wheels usually include three basic types: a rotating disk, a rotating wheel, or a rotating hoop rotated through the fan beam.
Existing x-ray backscatter imaging systems that implement a rotating disk chopper wheel also include a chopper wheel enclosure that completely contains the disk chopper wheel in a shielded housing, as described in relation to FIG. 3, for example. The bearings that support the disk are often mounted onto the interior of the housing, with the drive motor that rotates the disk being mounted onto the outside of the housing.
The shielded housing typically includes an aluminum box, for example, that is lined internally with sufficient lead to absorb any x-rays that are incident directly upon it, as well as any x-rays that have scattered from the chopper disk. An entry slot is typically provided for the incident fan beam of x-rays emitted from the x-ray tube to enter the shielded housing. An exit slot is also provided in the housing to allow the sweeping pencil beam, created by the rotating disk chopper wheel, to exit the housing.
For a 120 kiloelectron Volt (keV) to 160 keV backscatter x-ray system designed for scanning baggage, for example, the shielding is typically between 1⁄8 inches thick and 1⁄4 inch thick. For a chopper disk that is 18 inches in diameter, for example, the lead shielding in such a disk housing can, therefore, weigh between about 40 pounds and 80 pounds. Moreover, for a system with a 4 inch diameter disk, the shielding in such a system can weigh between about 3 pounds and about 6 pounds. It would be advantageous to have a way to reduce these relatively high weights. A reduction in weight and cost would be advantageous for all types of rotating disk x-ray scanning systems, including forward and backward scattering systems and fixed-mount and mobile x-ray scanning systems. Moreover, reducing the weight of the chopper disk and related assembly would be especially advantageous to the feasibility of developing and deploying handheld x-ray scanning systems. However, any possible weight-reduced system should also include a way to maintain the degree of x-ray scattering confinement and related safety of use that existing shielded housings provide.
Disclosed herein are embodiment x-ray chopper wheel assemblies that can provide scanning x-ray pencil beams while shielding and attenuating scattered x-rays to safe levels without the weight of existing shielded housings. Embodiments can provide radiation safety levels comparable to existing, full-shielded disk housings such as those described in connection with FIG. 3, even with significantly reduced system weight relative to existing shielded disk housings. The reduced weight is advantageous for various types of x-ray scanning systems, and especially for handheld devices, such as the handheld system described in connection with FIG. 1A, for which system weight especially should be minimized for ease of use.
FIG. 1A is a photograph or illustration showing a handheld x-ray scanner 155 that incorporates an embodiment x-ray chopper wheel assembly. Various embodiment chopper wheel assemblies, such as those illustrated hereinafter in connection with FIGS. 1B, 4A, and 4B, for example, may be used to specifically facilitate operation of a handheld x-ray scanner, fixed-mount x-ray scanning system, such as that illustrated in FIG. 1B, or a mobile x-ray scanning system, such as a truck-mounted x-ray scanning system. Nonetheless, embodiments are particularly advantageous for handheld scanning systems such as the scanner 155, which can be carried and moved by a person to scan a vehicle, luggage, or other items flexibly to detect contraband, safety issues, etc.
As illustrated in FIG. 1A, a hand 153 of a person holds the scanner 155, and the scanner is directed at a vehicle wheel 151, an example target for x-ray scanning for contraband, and the scanner 155 is configured to produce a real-time image 155 of the vehicle wheel. An embodiment chopper wheel assembly, such as those illustrated in FIG. 1B or 4A-4B, for example, may be mounted within a handheld x-ray scanner, such as that illustrated in FIG. 1A. In addition, embodiments may be used advantageously by mounting them within a fixed-mount scanning system, such as that illustrated in FIG. 1B or a similar mobile x-ray scanning system that is mounted on a truck or other vehicle, for example.
FIG. 1B is an illustration of an embodiment fixed-mount x-ray scanning system used to scan luggage 11 that is moved along a conveyor belt 27. The scanning system includes an embodiment chopper wheel assembly 100 that is used in connection with an x-ray tube 14 and attenuating plate 17 to produce a pencil beam 23 that scans the luggage 11 with a vertical sweep as the luggage is moved along the conveyor belt 27. The transmitted pencil beam 23 is detected by a detector 25, which is connected via a cable 26 to an analyzer/monitor 13 that analyzes the detected signals from the detector 25 and displays them to show an image 12 of items within the luggage, such as contraband. In addition, the backscattered x-rays may be detected in backscatter detectors (not shown) positioned on the source-side of the system.
The chopper wheel assembly 100 of FIG. 1B particularly includes a disk chopper wheel 101 with radial slits 121. The disk chopper wheel is secured to a chopper wheel mount 105 by a rotation axis of the disk chopper wheel 101 to allow chopper wheel rotation 24. The assembly 100 also includes a source-side scatter plate 103 that defines an open slot 107 therein. The source-side scatter plate 103 is secured by a support structure 102 to be substantially parallel to the rotation plane of the disk chopper wheel 101 with a source-side gap between the source-side scatter plate and the source side of the disk chopper wheel. These details are further illustrated in relation to the alternative embodiment of FIGS. 4A-4B and the additional details of these embodiments as illustrated in FIGS. 5A-5B, 6, 7, 8A-8B, and 9.
The disk chopper wheel 101 is configured to rotate with a rotation 24 about a rotation axis that is perpendicular to a rotation plane of the disk chopper wheel. These details are further illustrated and described in connection with the specific embodiment shown in FIGS. 4A-4B and additional details thereof as illustrated in FIGS. 5A-5B, 6, 7, 8A-8B, and 9.
The disk chopper wheel 101 has a solid cross-sectional area in the rotation plane, as further illustrated in FIG. 7 in connection with the embodiment of FIGS. 4A-4B. The disk chopper wheel 101 is configured to absorb x-ray radiation received from an x-ray source (here, the x-ray tube 14) at a source side of the disk chopper wheel (the side of the chopper wheel closest to the x-ray tube source 14). However, when the radial slit openings 121 in the disk chopper wheel 101 intersect with the open slot 107 in the source-side scatter plate 103 along the direction of travel of the x-rays 15, then the pencil beam 23 may pass through the source-side scatter plate 103 and disk chopper wheel 101 to an output side of the disk chopper wheel. This is further described in connection with FIG. 9, for example. The radial slit openings in the disk chopper wheel 101 and in chopper wheels in other embodiment assemblies described herein may have chamfered edges and may also may be tapered, as described in International Application PCT/US2015/061952, filed on Nov. 20, 2015, and published with International Publication Number WO 2016/081881 A1, which is incorporated herein by reference in its entirety.
The source-side scatter plate 103 has a solid cross-sectional area in a plane that is parallel to the rotation plane of the disk chopper wheel, as further illustrated in connection with FIG. 5A and FIG. 8A, for example. The source-side scatter plate 103 is configured to absorb x-ray radiation at solid portions of the plate, and also pass x-ray radiation through the open slot 107 to produce the pencil beam 23.
Advantageously, in embodiments described herein, such as those in FIGS. 1B, 2, and 4A-4B, for example, the solid cross-sectional area of the source-side scatter plate is substantially smaller than the solid cross-sectional area of the disk chopper wheel. This is very different from prior art designs such as that shown in FIG. 3, wherein a chopper wheel is completely enclosed by a full enclosure to confine radiation. The substantially smaller source-side scatter plate having the features described herein results in similar confinement of scattered x-rays, as illustrated in connection with FIGS. 9-10, for example, and also provides for substantially reduced weight of a chopper wheel assembly, facilitating handheld x-ray scanning, as illustrated in FIG. 1A, in addition to any x-ray scanning application.
The source-side scatter plate 103 may be formed of tungsten, another material having a high Z (atomic number), or an alloy of one of these materials, etc. The source-side scatter plate may have a thickness in certain embodiments on the order of 1.0 mm, for example. Thickness of the scatter plate is further illustrated in connection with FIG. 5B, for example.
The cross-sectional area of the source-side scatter plate 103 may be in a range of about 100% to about 5,000% larger than an open cross-sectional area of one of the radial slit openings 121 in the disk chopper wheel, for example. An open cross-sectional area of one of the radial slit openings is further illustrated in connection with FIG. 7, for example. However, in yet other embodiments, the source-side scatter plate cross-sectional area may be in a range of, for example, 500% to 10,000% larger than the cross-sectional area of a radial slit opening in the chopper wheel. In general, the cross-sectional area of the source-side scatter plate should be sufficient to intersect the entire fan beam 16 with the exception of part of the fan beam incident at the open slot 107. In this manner, at any orientation of a radial slit 121, only a pencil beam 23 can emerge from the chopper wheel when a portion of a radial slit 121 is aligned with a portion of the open slot 107. The source-side scatter plate 103 may be formed of pure or alloyed lead, tin, iron, tungsten, or another high Z material, for example.
While the assembly 100 of FIG. 1B includes only a source-side scatter plate 103 for confinement of scattered x-rays, in other embodiments, such as that described in connection with FIGS. 4A-4B, for example, an output-side scatter plate may also be included. The output-side scatter plate, like the source-side scatter plate, can have a solid cross-sectional area in a plane parallel to the rotation plane of the disk chopper wheel that is, where the solid cross-sectional area of the output-side scatter plate is substantially smaller than the solid cross-sectional area of the disk chopper wheel. Furthermore, the output-side scatter plate may be configured to absorb, like the source-side scatter plate, x-ray radiation and may define an open slot therein, like the open slot 107, that is configured to pass x-ray radiation. Moreover, while the source-side scatter plate 103 is a single-layered plate, other source-side or output-side scatter plates within the scope of embodiments may include multi-layered plates. It should be noted that an output-side scatter plate may have any of the other characteristics described herein for the source-side scatter plate.
The support structure 102 that secures the source-side scatter plate 103 may be advantageously formed of aluminum or another lightweight material. This is because the support structure 102 need not be relied upon for x-ray shielding or scattering confinement. Instead, the source-side scatter plate 103 (and in other embodiments, the output-side scatter plate) perform this function. Again, the limited size of the source-side scatter plate relative to the disk chopper wheel, together with the ability of other components such as the chopper wheel mount 105 and support structure 102 to be formed of lightweight materials, enable dramatically lower weight for various types of x-ray scanning systems, such as the handheld scanner 155 of FIG. 1A. Embodiment support structures may be configured to support bearings on a disk chopper wheel that rotates. Support structures may also be configured to support a drive motor that rotates the chopper wheel.
Embodiment x-ray assemblies may be used in systems using a wide range of x-ray energies. An x-ray source such as the x-ray tube 14 may be configured to output x-rays having an end-point energy in a range of about 120 kiloelectron volts (keV) to about 450 keV, for example. Furthermore, in other embodiments, this energy range may be between about 120 keV and about 160 keV, for example. In particular, handheld scanning systems, such as that of the scanner 155 of FIG. 1A, may use x-rays on the lower end of this energy region, for example 120 keV.
In particular, in the embodiment of FIG. 1B, the source-side scatter plate is configured to output a fan beam 16 of x-rays through the open slot 107, and the assembly, including the scatter plate 103 and chopper wheel 101, may be configured to output the pencil beam 23.
FIG. 2 is a perspective-view illustration of an embodiment of the disk chopper wheel 101 of FIG. 1B, together with the source-side scatter plate 103. As illustrated in FIG. 2, the disk chopper wheel 101 may be rotated by the motor 234 or other mechanism in various embodiments. An x-ray tube 214 is used as a source of x-rays.
FIG. 3 is a perspective-view illustration of a prior art chopper wheel 218a that is fully enclosed by a chopper wheel enclosure 330a-b. X-rays enter the enclosure through an entry slot 319, are periodically blocked or passed by slits in the chopper wheel 218a, and pass as a pencil beam through the output exit slot 336 in the enclosure portion 330b. Each portion of the chopper wheel enclosure 330a, 330b must have certain thickness 338 to limit leakage of scattered x-rays from the enclosure.
A significant disadvantage of the arrangement of FIG. 3 is the weight that the enclosure has. As will be understood, the enclosure must be of sufficiently high atomic number and thickness to be able to limit x-ray exposure outside of the enclosure. This causes the enclosure to add significant weight to a scanning system and is a major disadvantage for any scanning system, especially a handheld or otherwise mobile scanning system that must be held or transported.
A significant advantage of the embodiments illustrated in FIGS. 1B, 2, and 4A-4B, for example, is that a source-side scatter plate (and optionally an output-side scatter plate) and the disk chopper wheel can perform the desired shielding to obtain the appropriate safety standards while being significantly lighter than the prior art embodiments in the prior art design illustrated in FIG. 3.
FIG. 4A is a perspective-view illustration of an embodiment x-ray chopper wheel assembly 400. The assembly 400 includes a disk chopper wheel 401 that is configured to rotate about a rotation axis 440. In the illustration of FIG. 4A, the rotation axis 440 coincides with the Z axis, as shown. The rotation axis 440 is perpendicular to a rotation plane of the disk chopper wheel 401. The rotation plane is parallel to the XY plane that is shown in FIG. 4A. The rotation plane is further illustrated in FIG. 5A. The disk chopper wheel 401 has a solid cross-sectional area in the rotation plane that is illustrated in FIG. 7. The wheel 401 is configured to absorb x-ray radiation traveling in a direction 444 from an x-ray source (not shown in FIG. 4A) that is received at a source side of the chopper wheel (the side where on x-rays are first incident, traveling along the direction 444). The disk chopper wheel 401 defines radial slits openings 421 around the wheel, and these radial slit openings are configured to pass x-ray radiation from the source side of the wheel to an output side of the disk chopper wheel. The source and output sides are further illustrated in FIG. 5B.
The assembly 400 further includes a source-side scatter plate 403 that has a solid cross-sectional area in a plane parallel to the rotation plane of the wheel. This cross-sectional area is illustrated in FIG. 8A. The source-side scatter plate 403 is configured to absorb x-ray radiation, and it defines an open slot therein that is configured to pass x-ray radiation. The open slot is further illustrated and described in connection with FIGS. 5A-5B and 8A, for example. Advantageously, the solid cross-sectional area of the source-side scatter plate is substantially smaller than the solid cross-sectional area of the disk chopper wheel, providing for operation of the assembly with significantly reduced weight and x-ray confinement similar to that of the prior art chopper wheel enclosure of FIG. 3.
The source-side scatter plate 403 is secured by a support structure 402a-b that secures the source-side scatter plate substantially parallel to the rotation plane of the disk chopper wheel with a source-side gap between the source-side scatter plate and the source side of the disk chopper wheel, as further illustrated in FIGS. 5A-5B, for example. While an output-side scatter plate is generally optional, the assembly 400 does include an output-side scatter plate 404 that is secured by the support structure 402a-b to be substantially parallel to the rotation plane of the disk chopper wheel, similar to the source-side scatter plate 403. The support structure maintains an output-side gap between the output side of the scatter plate and the disk chopper wheel, as illustrated in FIG. 5B. In alternative embodiments not illustrated, the source-side and output-side scatter plates may form a single solid piece, the two scatter plates of which are connected by a bridge over the top of the chopper wheel 401 in FIG. 4A. Further in alternative embodiments, such a bridge structure may also be formed of a high-Z material to enhance shielding.
The output-side scatter plate 404 has a solid cross-sectional area in a plane parallel to the rotation plane of the disk chopper wheel, as illustrated in FIG. 8B. The output-side scatter plate is configured to absorb x-ray radiation, yet it also defines an open slot therein (illustrated in FIG. 8B) that is configured to pass x-ray radiation that emanates through the source-side scatter plate 403 and slits 421 in the chopper wheel. Advantageously, the solid cross-sectional area of the output-side scatter plate 404, like that of the input source-side scatter plate, is substantially smaller than the solid cross-sectional area of the disk, further providing for a lightweight assembly.
In the embodiment assembly 400, the support structure 402a-b is further configured to secure the disk chopper wheel 401 at the rotation axis 440. Advantageously, therefore, the support structure 402a-b performs both the functions of securing the chopper wheel and the functions of securing the source-side and output-side scatter plates 403 and 404, respectively. Further, in the embodiment assembly 400, it will be noted that the support structure includes the two portions 402a and 402b on the source side and output side of the chopper wheel, respectively. This provides a particularly robust and stable configuration that performs many needed support functions. However, in other embodiments, such as in the assembly 100 illustrated in FIG. 1B, a support structure may be one-sided, and the chopper wheel and support structure may be secured and mounted separately, while still being secured with the source-side scatter plate being substantially parallel to the chopper wheel and having the appropriate gap between the source-side scatter plate and the source side of the chopper wheel.
Further in the embodiment assembly 400 in FIG. 4A, the support structure 402a-402b includes an inner portion 472 that is configured to secure the disk chopper wheel 401 at the rotation axis 440 thereof, and the support structure 402b further includes radial spokes 442 that extend outward from the inner portion 472 and are configured to secure both the source-side scatter plate 403 and output-side scatter plate 404 with the appropriate alignment and gap with respect to the chopper wheel. The support structure 402a-b does this by means of hardware 446 that secures the two sides of the support structure 402a and 402b together while simultaneously securing the chopper wheel 401, as further illustrated in the exploded-view drawing of the assembly in FIG. 4B. Accordingly, the source-side portion 402a and output side portion 402b of the support structure are configured to be connected together and to secure the disk chopper wheel between the two portions of the support structure.
The support structure 402a-b is formed of aluminum, advantageously, for lighter weight. In other embodiments, other materials may be used. Nonetheless, aluminum may be advantageously used because of cost, sufficient rigidity and strength, and because the source-side and output-side scatter plates provide the desired shielding, while the support structure need not be relied upon for x-ray shielding.
The assembly 400 further includes an optional shield structure 405 that is configured to enclose the x-ray radiation in a region of travel between the x-ray source (e.g., x-ray tube, not shown in FIG. 4A) and the source-side scatter plate 403. The shield structure 405 may be formed of a high-Z material, for example, such as tungsten, lead, iron, or another high-Z material having sufficient thickness to prevent incident or scattered x-rays from being emitted outside of the device. The particular function and features of the shield structure 405 are further illustrated in FIGS. 4B, 5A-5B, 6, and 9, for example.
FIG. 4B is an exploded, perspective-view illustration of the assembly 400 of FIG. 4A. As illustrated in greater detail in FIG. 4B, the hardware 446 includes securing features 492a at ends of the spokes of the source-side support structure 402a and securing features 492b at ends of the spokes of the support structure 402b on the output side.
As also illustrated in greater detail in FIG. 4B, the chopper wheel 401 includes bearings 484 on either side thereof, which are configured to fit into securing features 482a-482b within the support structure portions 402a and 402b, respectively, in order to secure the disk chopper wheel 401 at the rotation axis 440 thereof. The support structure portions 402a and 402b further include features for securing the source-side and output-side scatter plates 403 and 404, respectively. Also illustrated in greater detail in FIG. 4B are an open slot 454 defined by the source-side scatter plate 403, as well as an open slot 456 defined in the output-side scatter plate 404. These open slots, which allow for x-rays to pass through, are further described in connection with FIGS. 5A-5B, 6, 8A-8B, and 9.
FIG. 5A is a cross-sectional profile view of the chopper wheel 401, source side and output-side scatter plates 403 and 404, respectively, and the shield structure 405 of the embodiment assembly 400 of FIGS. 4A and 4B. As also illustrated in FIG. 5A, the chopper wheel rotates about the rotation axis 440, which is perpendicular to a rotation plane 580 of the chopper wheel 401. In this illustration, the rotation axis 440 coincides with the z-axis in the Cartesian coordinates shown. The rotation plane 580 of the chopper wheel is perpendicular to the rotation axis 440 and lies in a plane parallel to the XY plane in the Cartesian coordinates shown. The source-side scatter plate 403 is secured in a plane 581 that is parallel to the rotation plane 580 of the disk chopper wheel.
As illustrated in FIG. 5A further, the output-side scatter plate 404, like the source-side scatter plate 403, is secured to be substantially parallel to the rotation plane of the disk chopper wheel 401. An output-side gap between the output-side scatter plate and the disk chopper wheel is illustrated in greater detail in FIG. 5B.
FIG. 5B is a magnified view of a portion of the cross-sectional profile illustration shown in FIG. 5A. FIG. 5B particularly illustrates various dimensions of the embodiment assembly 400 of FIGS. 4A-4B. The source-side scatter plate 403 has a thickness 568 and a source-side gap 550 between the scatter plate 403 and the chopper wheel 401. The source-side scatter plate 403 further includes the source-side scatter plate having a source-side slot width 590 that allows x-rays to pass through to a source side 576 of the disk chopper wheel 401. When the slot 454 is blocked by a solid portion of the chopper wheel 401, x-ray radiation is blocked from passing through the chopper wheel. On the other hand, as the chopper wheel rotates, when a radial slit of the chopper wheel intersects with the source side slot 454 along a direction of the x-ray travel 444, x-rays 444 pass through the slot 454 and through the radial slit defined in the chopper wheel 401.
The output-side scatter plate 404 similarly has a thickness 570 and an output-side gap 552 between the scatter plate 404 and the output side 578 of the disk chopper wheel.
The source-side gap 550 may be in a range of approximately 0.5 mm to approximately 1.0 mm, for example. As this gap increases, leakage of scattered x-rays also increases, as illustrated in FIGS. 9-10, for example. Other example source side gaps may be in a range of approximately 0.2 mm to approximately 2.0 mm, approximately 0.5 mm to approximately 1.25 mm, approximately 0.5 mm to approximately 0.75 mm, approximately 0.02 inches to approximately 0.04 inches, or approximately 0.03 inches, for example. In the context of source-side gaps or output-side gaps, as used herein, “approximately” denotes a tolerance of +/−0.25 mm.
As used herein, the source-side scatter plate 403 may be considered to be “substantially parallel” to the rotation plane of the chopper wheel when the source-side scatter plate and rotation plane of the disk chopper wheel are sufficiently parallel such that the chopper wheel may freely rotate without contacting the scatter plate 403. In a similar manner, the output-side scatter plate 404 may be considered to be “substantially parallel” to the chopper wheel 401 when the chopper wheel may freely rotate without risk of contact with the scatter plate 404. Where there is some degree of slight angle between either of the scatter plates and the rotation plane of the chopper wheel, the gap 550 or gap 552 may be considered to be the average distance between the plate 403 and the source side 576 of the disk chopper wheel or the average distance between the scatter plate 404 and the output side 578 of the disk chopper wheel.
FIG. 5B also illustrates that the source-side scatter plate 403 has a source-side plate width 588, measured parallel to the y-axis in FIG. 5B. Similarly, the output-side scatter plate 404 has an output-side plate width 589, similarly measured. The plate widths 588 and 589, which are measured in a direction parallel to a radial direction of the disk chopper wheel along the vertical y-axis in FIG. 5B, may be in a range of about 10% to about 70% greater than a slit length of one of the radial slit openings in the radial direction of the disk chopper wheel. These radial slit lengths are further illustrated in FIG. 7, and the plate widths are further illustrated in FIGS. 8A and 8B, respectively.
In general, as the plate width increases, leakage of scattered x-rays decreases for a given gap. In general, greater scatter plate width relative to slit length of radial slits in the chopper wheel leads to greater confinement and less leakage of x-rays. The relationship is further illustrated in FIG. 10 for the embodiment of FIGS. 4A-4B, assuming that only the source-side scatter plate in FIGS. 4A-4B is used, since the source-side scatter plate has a much larger impact on reducing x-ray leakage. Nonetheless, substantial confinement can occur with a limited size of a source-side scatter plate, as described herein, leading to operation of an x-ray scanner meeting leakage standards comparable to those of the prior art design in FIG. 3. As used herein, “substantial confinement” of x-ray radiation denotes that the disk chopper wheel and source-side scatter plate are arranged relative to each other with gaps, plate width, etc. such that x-ray leakage of scattered radiation is limited to no more than 50% leakage of the radiation that is scattered by the wheel, or to an x-ray radiation dose of no more than 5 milli-Rem per hour at a distance of 5 cm away from an outer surface of the assembly, whichever is greater. The substantial confinement may further include limiting leakage of scattered radiation to no more than 10% of radiation that is scattered by the assembly, or to a radiation dose of no more than 0.5 milli-Rem per hour at a distance of 5 cm away from the outer surface of the assembly, such as from the outer surface of the support structure, whichever is greater. In some embodiments, radiation leakage is limited to that which would be achieved by a full shield enclosure such as the enclosure 330a-b illustrated in FIG. 3 having a thickness and material similar to those of a given embodiment source-side scatter plate. In general, X-ray leakage may be limited to that which is considered safe for a particular scanning environment or application by adjusting plate width and gap as desired. “Substantial confinement” as used herein may also be achieved with the aid of an output-side scatter plate, such as the plate 404 of the embodiment assembly 400. “Substantial confinement” as used herein may also be achieved with the aid of the optional shield structure 405 arranged relative to the disk chopper wheel and source-side scatter plate.
Plate width is preferably greater than the lengths of radial slits in the chopper wheel, and the scatter plates all preferably fully overlap in cross section with the radial slits in the scatter plate, in order to enhance shielding. Nonetheless, it is also preferable for scatter plate width to be a small as possible in order to minimize total assembly weight. Accordingly, in example embodiments, as described above, the plate widths 588 and 589 may be in a range of about 10% to about 70% greater than a slit length of one of the radial slit openings. Furthermore, plate widths may be in other example ranges, such as about 5% to about 100%, about 10% to about 80%, about 20% to about 70%, about 30% to about 60%, or about 40% to about 50% greater than the slit length, depending on the plate gaps 550 and 552 and the desired maximum radiation leakage. In the context of plate widths, “about” as used herein denotes a tolerance of +/−5%.
FIG. 6 is a cross-sectional, profile-view illustration of the full assembly 400 illustrated in FIGS. 4A-4B.
FIG. 7 is a cross-sectional profile-view of the disk chopper wheel 401 of the assembly 400 illustrated in FIGS. 4A-4B. The disk chopper wheel 401 has a diameter 783. Radial directions 774a and 774b are shown for the disk chopper wheel. In the illustration of FIG. 7, these radial directions happen to be aligned parallel to the y-axis and x-axis, respectively. A radial slit length 793 of the radial slit openings is measured along the radial direction. The width of the source-side scatter plate 403 may be only slightly greater than the radial slit length 793. For example, the plate width may be about 10% to about 70% greater than the radial slit length. The size of the source-side scatter plate relative to the slit length may vary depending on the leakage tolerance. Calculated leakage is illustrated in FIG. 10 for various source-side scatter plate gaps and scatter plate widths 588. Scatter plate width 588 is also referred to as “height” in FIG. 10. Also illustrated in FIG. 7 is an open cross-sectional area 787 of the radial slits.
The chopper wheel has a total area 786, which is the total cross-sectional area of solid portions of the chopper wheel, including solid portions of the inner hub 449 and of the outer disk 448 and excluding the open areas constituting the radial slits and excluding any other holes or openings introduced into the chopper wheel, such as holes in the inner hub 449 illustrated in FIG. 4A. Where wheel slits or other openings include chamfering, the cross-sectional area of the solid portions of the chopper wheel may be considered to be the cross-sectional area through the center of the wheel (in the rotation plane).
FIG. 8A is a cross-sectional profile view of the source-side scatter plate 403 of the embodiment assembly 400. The cross section is in the plane 581 illustrated in FIG. 5A, which is parallel to the rotation plane 580 of the disk chopper wheel. The solid cross-sectional area 886 may be substantially smaller than the solid cross-sectional area 786 of the chopper wheel, as illustrated in FIG. 7, and as previously described. In addition to the source-side plate width 588, the plate 403 has a source-side plate length 885.
FIG. 8B is a cross-sectional profile view of the output-side scatter plate 404, which has a total solid cross-sectional area 887. This area 887 may be substantially smaller than the total cross-sectional solid area 786 of the disk chopper wheel for further advantages beyond those described above in relation to the solid cross-sectional area of the source-side scatter plate 403. Furthermore, the output-side scatter plate may have any of the features described herein in relation to the source-side scatter plate. In the embodiment of FIGS. 4A-4B, the slot width 591 of the output-side plate 404 is greater than the slot width of the source-side scatter plate 403. However, in other embodiments, these relative slot widths may differ or be the same. In addition to the output-side plate width 589, the plate 404 has an output-side plate length 889.
As described in connection with FIG. 3, in existing chopper wheel assemblies, the chopper wheel is completely enclosed by a chopper wheel enclosure in order to provide adequate x-ray shielding and safety. Accordingly, existing assemblies result in the chopper wheel enclosure being at least somewhat larger in cross-sectional area than the chopper wheel. In contrast to the existing assembly in FIG. 3, the solid cross-sectional area of the source-side scatter plate of the embodiment assembly 400, which is illustrated in FIG. 8A in greater detail, advantageously may be substantially smaller than the solid cross-sectional area of the disk chopper wheel.
As used herein, a solid cross-sectional area of the source-side scatter plate is “substantially smaller” than the solid cross-sectional area of the disk chopper wheel of an embodiment assembly when either the source-side plate width 588 or the source-side plate length 885 of the source-side scatter plate is smaller than the diameter 783 of the disk chopper wheel. In various embodiments, both the width 588 and length 885 of the source-side scatter plate may be smaller than the diameter 783 of the disk chopper wheel.
In some embodiments, the solid cross-sectional area of the source-side scatter plate may be smaller than a corresponding full enclosure would need to be in an enclosure width or length to enclose the chopper wheel fully. Further, in various embodiments, the source-side scatter plate may be smaller in weight than a corresponding full-shield enclosure would need to be to provide a comparable level of x-ray shielding. In various example embodiments, the solid cross-sectional area of the source-side scatter plate may be less than 90%, less than 70%, less than 50%, less than 40%, less than 30%, less than 25%, less than 15%, or less than 10% of the cross-sectional area 786 of the disk chopper wheel. Nonetheless, it is preferable for the solid cross-sectional area of the source-side scatter plate to be less than 50%, less than 25%, or less than 10% of the cross-sectional area 786 of the disk chopper wheel in order to reduce assembly weight the most and obtain maximum benefits of embodiment assemblies over the existing assembly in FIG. 3. This example solid cross-sectional area 787 of shielding material of the source-side scatter plate, which is significantly reduced relative to the enclosure 330a-b illustrated in FIG. 3, is a major advantage of embodiments described herein for reduced weight and material usage. Similar dimensional characteristics may apply to the output-side scatter plate relative to the disk chopper wheel.
FIG. 9 is a cross-sectional profile view showing part of the illustration of FIG. 5B, further magnified to show x-ray confinement properties. The shield structure 405 substantially encloses x-ray radiation 444 in a region of travel 951 between the x-ray source (not shown in FIG. 9) and the source-side scatter plate 403. Any x-rays 444 that are not traveling straight toward the scatter plate 403, perpendicular to the scatter plate, may be safely absorbed by the shield structure 405. Furthermore, scattered x-rays 444′ that are scattered from the source-side scatter plate may also be safely absorbed by the shield structure 405. The shield structure 405 may be considered to substantially confine x-rays traveling in the region 951 when any leakage x-rays are reduced to the level that is safe for the assembly to operate.
FIG. 9 further illustrates the operation of the source-side scatter plate 403 to prevent x-ray leakage. When x-rays 444 traverse the slot 454 in the source-side scatter plate and strike a solid portion of the disk chopper wheel 401, the scattered x-rays 444′ are absorbed by the scatter plate, and very few scattered x-rays 444′ escape, assuming that the gap between the source-side scatter plate and chopper wheel is sufficiently small, as already described herein. A similar principle applies to the output-side scatter plate 404, which is used in the embodiment illustrated in FIGS. 4A-4B. In the case of the output-side scatter plate 404, most of the scattered x-rays that need to be absorbed are scattered from the edges of the radial slits in the disk chopper wheel 401.
FIG. 10 is a graph illustrating calculated x-ray leakage as a function of scatter plate height (plate “width,” as used herein) for various gap sizes between the source-side scatter plate and the source side of the chopper wheel illustrated in FIGS. 4A-4B and 5B. The combination of the disk chopper wheel, source-side scatter plate, and output-side scatter plate used in the assembly 400 of FIGS. 4A-4B are arranged relative to each other to substantially confine x-ray radiation scattered therefrom, as illustrated in FIG. 9.
FIG. 10 shows the calculated leakage that will occur, which can guide a setting of gap width between the source-side scatter plate and the source side of the disk chopper wheel and a determination of desired plate width to decrease x-ray leakage to an acceptable level. As will be noted, as scatter plate width increases, leakage of scattered x-rays decreases. Conversely, as gap increases, the number of scattered x-rays that are leaked outside the assembly increases. “Substantial confinement” of scattered x-rays, as used herein, is further described herein in connection with FIG. 5B. The substantial confinement may limit leakage of scattered radiation to no more than 50% leakage of the radiation that is scattered by the wheel, or to an x-ray radiation dose of no more than 5 milli-Rem per hour at a distance of 5 cm away from an outer surface of the assembly, whichever is greater. The substantial confinement may limit leakage of scattered radiation to no more than 10% of scattered radiation or to a dose of no more than 0.5 milli-Rem per hour at a distance of 5 cm away from the outer surface of the assembly, whichever is greater.
The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.
While example embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments encompassed by the appended claims.

## Claims

1. An x-ray chopper wheel assembly comprising:
a chopper wheel having a solid area configured to block x-ray radiation received at a source side of the chopper wheel from an x-ray source, the chopper wheel defining one or more openings configured to pass x-ray radiation from the source side of the chopper wheel to an output side of the chopper wheel; and
a source-side scatter plate arranged relative to the chopper wheel with a source-side gap in a range of approximately 0.2 mm to approximately 2.0 mm between the source-side scatter plate and the source side of the chopper wheel.

2. The x-ray chopper wheel assembly of claim 1, wherein the source-side gap is in a range of approximately 0.5 mm to approximately 1.25 mm.

3. The x-ray chopper wheel assembly of claim 2, wherein the source-side gap is in a range of approximately 0.5 mm to approximately 0.75 mm.

4. The x-ray chopper wheel assembly of claim 2, wherein the source-side gap is in a range of approximately 0.02 mm to approximately 0.04 mm.

5. A method of limiting x-ray leakage from an x-ray chopper wheel assembly, the method comprising:
configuring a chopper wheel of an x-ray chopper wheel assembly to have a solid area configured to block x-ray radiation received at a source side of the chopper wheel from an x-ray source;
configuring the chopper wheel to define one or more openings to pass x-ray radiation from the source side of the chopper wheel to an output side of the chopper wheel; and
arranging a source-side scatter plate of the chopper wheel assembly relative to the chopper wheel with a source-side gap in a range of approximately 0.2 mm to approximately 2.0 mm between the source-side scatter plate and the source side of the chopper wheel to limit leakage of scattered x-rays from the x-ray chopper wheel assembly.

6. A method of limiting x-ray leakage from an x-ray chopper wheel assembly, the method comprising:
configuring a disk chopper wheel of an x-ray chopper wheel assembly to receive, at a source side of the disk chopper wheel, x-ray radiation from an x-ray source; and
arranging a source-side scatter plate of the x-ray chopper wheel assembly relative to the disk chopper wheel to cause a substantial confinement of x-rays that are scattered from the disk chopper wheel.

7. The method of claim 6, wherein arranging a source-side scatter plate to cause the substantial confinement includes arranging the source-side scatter plate to limit leakage of scattered radiation to no more than 10% of scattered radiation or to a dose of no more than 0.5 milli-Rem per hour at a distance of 5 cm away from the outer surface of the assembly, whichever is greater.

8. The method of claim 6, wherein:
configuring the disk chopper wheel includes configuring to rotate about a rotation axis thereof, the rotation axis perpendicular to a rotation plane of the disk chopper wheel, the disk chopper wheel having a solid cross-sectional area in the rotation plane,
the method further including configuring the source-side scatter plate to have a solid cross-sectional area in a plane substantially parallel to the rotation plane of the disk chopper wheel, with the solid cross-sectional area of the source-side scatter plate being less than 50% of the cross-sectional area of the disk chopper wheel.

9. The method of claim 8, further including configuring the source-side scatter plate to have the solid cross-sectional area less than 25% of the cross-sectional area of the disk chopper wheel.

10. The method of claim 9, further including configuring the source-side scatter plate to have the solid cross-sectional area less than 10% of the cross-sectional area of the disk chopper wheel.

11. The method of claim 6, wherein arranging a source-side scatter plate of the x-ray chopper wheel assembly relative to the disk chopper wheel includes securing the source-side scatter plate in the plane substantially parallel to the rotation plane of the disk chopper wheel with a source-side gap between the source-side scatter plate and the source side of the disk chopper wheel, the source-side gap being in a range of a in a range of approximately 0.2 mm to approximately 2.0 mm.

12. The method of claim 6, further including configuring the source-side scatter plate to be comprised of pure or alloyed lead, tin, iron, tungsten, or another high-Z material.

13. The method of claim 6, further including configuring the source-side scatter plate to have a thickness on the order of 1.0 mm.

14. The method of claim 6, further comprising:
configuring the disk chopper wheel to define one or more radial slit openings configured to pass x-ray radiation from the source side of the disk chopper wheel to an output side of the disk chopper wheel; and
configuring a cross-sectional area of the source-side scatter plate to be in a range of about 100% to about 5,000% larger than an open cross-sectional area of one of the one or more radial slit openings in a rotation plane of the disk chopper wheel.

15. The method of claim 6, further comprising:
configuring the disk chopper wheel to define one or more radial slit openings configured to pass x-ray radiation from the source side of the disk chopper wheel to an output side of the disk chopper wheel; and
configuring the source-side scatter plate to have a plate width in a direction parallel to a radial direction of the disk chopper wheel, the plate width being in a range of about 10% to about 70% greater than a slit length of one of the one or more radial slit openings in the radial direction of the disk chopper wheel.

16. The method of claim 6, further comprising:
configuring the disk chopper wheel to rotate about a rotation axis thereof, with the rotation axis perpendicular to a rotation plane of the disk chopper wheel, and to have a solid cross-sectional area in the rotation plane; and
configuring an output-side scatter plate, to define an open slot therein configured to pass x-ray radiation, to absorb x-ray radiation over a solid cross-sectional area in a plane parallel to the rotation plane of the disk chopper wheel, and to have the solid cross-sectional area of the output-side scatter plate substantially smaller than the solid cross-sectional area of the disk.

17. The method of claim 6, further comprising:
configuring the source-side scatter plate to output a fan beam of x-rays through an open slot defined therein; and
configuring the disk chopper wheel with the arranged source-side scatter plate to output a pencil beam of x-rays.

18. A method of limiting x-ray leakage from an x-ray chopper wheel assembly, the method comprising:
receiving, at a source side of a disk chopper wheel of the x-ray chopper wheel assembly, x-ray radiation from an x-ray source; and
substantially confining x-rays that are scattered from the disk chopper wheel by using a source-side scatter plate arranged relative to the disk chopper wheel.

19. The method of claim 18, wherein substantially confining x-rays includes using the source-side scatter plate arranged with a source-side gap in a range of approximately 0.2 mm to approximately 2.0 mm between the source-side scatter plate and the source side of the chopper wheel.