X-ray source for 2D scanning beam imaging

Abstract

A two-dimensional X-ray scanner that includes a beam steerer for steering an electron beam to impinge upon a target; and a collimator further including an aperture adapted for travel in an aperture travel path for rotating the X-ray beam plane spanned by the electron beam impinging upon the target along a focal track for emitting a scanning X-ray beam.

Claims

1. A two-dimensional X-ray scanning system comprising: an X-ray scanner comprising: a beam focuser; a beam steerer for scanning an electron beam on a path along an X-ray production target as a function of time; and an aperture adapted for travel in an aperture travel path relative to the X-ray production target, wherein the X-ray scanner remains stationary with respect to the object of inspection; and a detector configured to detect X-rays passing through an object or scattered by the object of inspection and generate two-dimensional data indicative of the detected X-rays.

2. A The two-dimensional X-ray scanning system of claim 1, wherein the aperture is an intersection of a fixed slit and a moving slit.

3. The two-dimensional X-ray scanning system of claim 1, wherein the X-ray production target is a planar target block.

4. The two-dimensional X-ray scanning system of claim 1, wherein the X-ray production target is convex.

5. The two-dimensional X-ray scanning system of claim 4, wherein the X ray scanner is configured to have a predefined take-off angle and wherein, during operation, the electron beam is steered across the convex X-ray production target to maintain the pre-defined take-off angle with the travelling aperture.

6. A method for sweeping an X-ray beam across an object of inspection in two dimensions using a two-dimensional X-ray scanner wherein the X-ray scanner is configured to remain stationary with respect to the object of inspection, the method comprising: varying a direction of a beam of electrons relative to a target upon which the beam of electrons impinges; and coupling X-rays generated at the target via an aperture that moves along a prescribed path as a function of time.

7. The method in accordance with claim 6, wherein coupling X-rays generated at the target may include coupling the X-rays via an intersection of a fixed slit and a moving slit.

8. The method in accordance with claim 6, wherein the target is a planar target block.

9. The method in accordance with claim 6, wherein the target is convex.

10. The method in accordance with claim 9, wherein the two-dimensional X ray scanner is configured to have a predefined take-off angle and wherein, during operation, the electron beam is steered across the convex X-ray production target to maintain the pre-defined take-off angle with the travelling aperture.

11. A two-dimensional X-ray scanner comprising: a beam steerer for steering an electron beam to impinge upon a target and thereby emit an X-ray beam; and a collimator comprising an aperture adapted for travel in an aperture travel path in order to rotate the electron beam impinging upon the target while maintaining an angular alignment with the target, wherein the two-dimensional X-ray scanner is configured to remain stationary relative to an object under inspection.

12. The two-dimensional X-ray scanner in accordance with claim 11, wherein the target is a planar target block.

13. The two-dimensional X-ray scanner in accordance with claim 11, wherein the target is convex.

14. The two-dimensional X-ray scanner in accordance with claim 13, wherein the two-dimensional X ray scanner is configured to have a predefined take-off angle and wherein, during operation, the electron beam is steered across the convex X-ray production target to maintain the pre-defined take-off angle with the travelling aperture.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features and advantages of the present specification will be further appreciated, as they become better understood by reference to the detailed description when considered in connection with the accompanying drawings:

(2) FIG. 1A is a schematic depiction of an electronic beam scanner;

(3) FIG. 1B depicts another electronic beam scanner;

(4) FIG. 1C schematically depicts a hybrid electromagnetic/mechanical scanning of an X-ray beam with a collimator in a first position with a limited field of view, in accordance with an embodiment of the present specification;

(5) FIG. 1D schematically depicts a hybrid electromagnetic/mechanical scanning of an X-ray beam with a collimator in a second position with an increased size of the apparent focal spot, in accordance with another embodiment of the present specification;

(6) FIG. 2A depicts a planar cross-section of a hybrid electrical/mechanical scanner, in accordance with a wide-angle embodiment of the present specification;

(7) FIG. 2B shows a planar cross-section of a hybrid electrical/mechanical scanner, in accordance with the wide-angle embodiment of FIG. 2A with the electron beam striking the target at a different location;

(8) FIG. 2C shows a planar cross-section of a hybrid electrical/mechanical scanner, in accordance with a wide-angle embodiment of FIG. 2A with the electron beam striking the target at a different location;

(9) FIG. 3A is a perspective view of a two-dimensional scanning X-ray source cut away to show a convex target, in accordance with an embodiment of the present specification; and

(10) FIG. 3B is a perspective view of the X-ray source of FIG. 3A, with a chopper wheel cut away in order to show an X-ray beam window, in accordance with an embodiment of the present specification.

DETAILED DESCRIPTION

(11) In various embodiments, the present specification provides a method and apparatus for acquiring wide field-of-view backscatter X-ray images of stationary objects without rotating the source in an X-ray imaging system.

(12) The following definitions are provided to further describe various aspects of the present specification in some embodiments:

(13) The term beam angle refers to an instantaneous exit angle of a beam from a scanning device measured in relation to a center line of the angular beam span. (The beam angle, thus, varies from instant to instant as the beam is scanned.)

(14) The term snout is defined as an enclosure that is opaque to the radiation in question and comprises one or more defined openings through which radiation is allowed to emerge.

(15) The term snout length is defined as the normal distance between a target where X-rays are generated and an aperture within a snout from where the generated X-rays emerge from the snout. The snout length determines the system's collimation length (see below).

(16) The term collimation length is defined as the shortest distance between the focal spot on the X-ray production target and an aperture serving to collimate an emergent X-ray beam.

(17) The term take-off angle is defined as the angle between the direction of X-ray beam extraction through the aperture and the plane that is tangent to the target surface at the focal spot.

(18) The term scan head encompass any structure which contains an X-ray source for two-dimensional scanning, whether by moving the scan head or in accordance with teachings of the present specification.

(19) Where an element is described as being on, connected to, or coupled to another element, it may be directly on, connected or coupled to the other element, or, alternatively, one or more intervening elements may be present, unless otherwise specified.

(20) The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting. The singular forms a, an, and the, are intended to include the plural forms as well.

(21) The present specification is directed towards multiple embodiments. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the specification. Language used in this specification should not be interpreted as a general disavowal of any one specific embodiment or used to limit the claims beyond the meaning of the terms used therein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the specification. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present specification is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the specification have not been described in detail so as not to unnecessarily obscure the present specification.

(22) In the description and claims of the application, each of the words comprise include and have, and forms thereof, are not necessarily limited to members in a list with which the words may be associated.

(23) It should be noted herein that any feature or component described in association with a specific embodiment may be used and implemented with any other embodiment unless clearly indicated otherwise.

(24) An electromagnetic scanner is now described with reference to FIG. 1A. A scanning electron beam X-ray source, designated generally by numeral 100, comprises an electron gun 101, a beam focuser 102 (also referred to herein as a focus lens 102), a beam steerer 103 (also referred to herein as deflection module 103), and a beam controller 104 which scans a focused electron beam 105 along a focal path 115 on an X-ray production target 110. Beam focuser 102 and beam steerer 103, alone or together, may be referred to herein as a focus and deflection module, designated generally by numeral 106. Collimator 120, which is stationary with reference to the X-ray production target 110, contains an aperture 125, creating a scanning X-ray beam 130 that spans a beam plane 135. X-ray beam 130 may be referred to herein as X-ray pencil beam 130 without regard to the precise cross-section of the beam.

(25) Electrons 105 emerging from gun 101 are steered by focus lens 102 and deflection module 103, governed by beam controller 104, such that electron beam 105 is scanned on a focal path 115 along X-ray production target 110 (also referred to herein as target 110). X-rays emitted through aperture 125 during a scan of electron beam 105 lie within a beam plane defined as the unique plane containing the focal path 115 and the aperture 125. If focal path 115 is not a straight line and/or aperture 125 is not a simple aperture but formed by a collimator as taught in U.S. Pat. Nos. 9,117,564 and 9,257,208, both assigned to American Science and Engineering and incorporated herein by reference, then X-rays emitted through aperture 125 during a scan of electron beam 105 lie on a non-planar surface. For simplicity we will still refer to the surface as a beam plane.

(26) An inspection object 140 is placed in the path of the beam plane 135. As the scanning X-ray beam 130 traverses the beam plane 135, scattered and/or transmitted X-rays from a scan line 142 are recorded by X-ray detectors (not shown). The inspection object 140 may be imaged by moving it successively along an axis 144 transverse to beam plane 135 while collecting scan lines. This method and apparatus is further described in U.S. Pat. No. 4,045,672, assigned to Watanabe, which is incorporated herein by reference.

(27) Another electromagnetic scanner (EMS) 50 is described with reference to FIG. 1B. Electrons in an electron beam 501 are focused and steered by beam controller 505 so as to sweep over a target 508, which may optionally be water-cooled. Beam controller 505 applies electric and/or magnetic fields for confining and steering electron beam 501, and, in particular, beam controller 505 includes beam steering coil 519. The source of electrons typically is an electron gun 101 (shown in FIG. 1A) from which electrons in electron beam 501 are emitted. Impingement of electron beam 501 onto target 508 produces X-rays 511 into a snout 515 that has a single-exit aperture 517 at its apex. (The vacuum seal, or window (not shown) may be anywhere, and is typically close to target 508 to minimize the vacuum volume.) The emerging X-ray beam 520 is swept in angle as electron beam 501 is swept across target 508.

(28) As described with reference to FIG. 1A, the collimator 120 of the electromagnetic scanner (such as the one shown in FIG. 1A) remains stationary during the course of inspection of an object. FIGS. 1C and 1D illustrate electromagnetic scanner embodiments 160 wherein the collimator 120 is moved during the course of the inspection process. Referring to FIGS. 1C and 1D, the movement of collimator 120 creates a sweeping beam plane 137 and allows keeping the inspection object 140 stationary with reference to the scanning electron beam X-ray source 100 (shown in FIG. 1A). In accordance with this method, the extent of the beam plane's sweep angle, and thus the field of view, may be limited by the heel effect at one end, as shown in FIG. 1C, where the intensity of the beam 130 is degraded towards one extremum of its motion due to attenuation within the X-ray production target 110 itself. At the other extremum, spatial resolution may be lost due to the increasing size of the apparent focal spot, as would occur in FIG. 1D. A practical range for the beam plane's sweep angle is 30 to 40.

(29) FIG. 2A depicts a planar cross-section of a hybrid electrical/mechanical scanner, in accordance with a wide-angle embodiment of the present specification. In embodiments, the term wide-angle is used to denote an angle exceeding the aforementioned range of 30 to 40 by a factor ranging from two to three. In an embodiment, the angle may be 120 as depicted in FIGS. 2A, 2B and 2C. Focused, steered electron beam 205 impinges upon X-ray production target 210. Successive lines are generated by moving collimator 220 having an aperture 225 (wherein the beam plane moves with aperture 225), along aperture travel path or range 270 (also referred to herein as lateral travel 270) which extends from a first end or outer boundary 236 of the beam plane to the second end or outer boundary 237, as shown in FIG. 2A, whereby scanning X-ray beam 230 emerges from aperture 225. It should be appreciated that the beam plane is perpendicular to FIG. 2A and therefore its projection onto FIG. 2A is the X-ray beam 230. Since the beam emerges from the aperture, it must also move with the aperture.

(30) The beam plane is turned or rotated incrementally by moving aperture 225. The aperture travel range is designated by the extrema (or outer bounds) ranging from a first end 236 of the beam plane to the second end 237, while the nominal snout length is designated by numeral 280. While in FIGS. 1C and 1D the axis of rotation for the beam plane is the focal path 115 (shown in FIGS. 1A, 1B) on the X-ray production target 110, the wide angle embodiment depicted in FIG. 2A does not feature a simple rotational axis for the beam plane. Instead the beam plane is approximately tangent to the convex X-ray production target 210. The time needed for the aperture 225 to travel its path 270 constitutes the image frame acquisition time. Accordingly, frame rates fast enough for backscatter motion imaging become advantageously possible.

(31) Referring to FIGS. 1C and 1D, when using a flat (planar) X-ray production target 110, the angular range (which has an identical meaning, herein, to the term angular span, and corresponds to the range over which the beam plane rotates, i.e., the angular extent of motion of the beam plane) between the beam planes depicted in FIGS. 1C and 1D depends on the so-called heel effect, as in cone beam imaging with film or a flat panel detector. By virtue of the heel effect, the intensity of the beam 130 is degraded towards the extreme of its motion due to attenuation within the target 110 itself. Typically, 30 to 40 of angular range are used with the take-off angle starting at about 1. The other limit is due to the enlargement of the apparent focal spot and the associated loss in spatial resolution.

(32) Referring to FIGS. 1C and 1D, assuming a 12 (300 mm) snout length, a 500 mm long focal track will create an angular beam span of about 80 in the beam plane 137. Assuming a planar target with a 40 angular range for the take-off angle and thus the beam plane, this EMS would cover a 44 (1.31 m) wide and 84 (2.5 m) high image at 5 (1.5 m) from the collimator. With a 12 (300 mm) snout length (as defined above), the lateral travel path 154 (i.e. the vertical path of the electron beam's focal spot on the target) of the aperture needs to be 8.6 (218 mm). Therefore, for certain snout lengths, an angular beam span range of 40 to 80 degrees may be achieved by a have a track length of 150 mm to 600 mm, preferably 200 mm to 500 mm.

(33) In one embodiment of the present specification, aperture 225 is made to travel on an arc with the X-ray production target 210 at its center in order to maintain angular alignment. In an embodiment, the radius of the arc is approximately 12. In an embodiment, an X-ray transparent floater is used in an arc shaped mercury filled pipe to enable the aperture travel on an arc hydraulically, wherein the mercury blocks the X-rays and the floater forms the aperture.

(34) Since the position of electron beam 105/205 on target 110/210 can be easily controlled using an X-Y deflection module (similar to deflection module 103 shown in FIG. 1A), converting from a conventional, flat production target 110 (shown in FIGS. 1C and 1D) to a target 210 with a convex surface allows extending the angular range. While the simplest convex surface is cylindrical, other convex shapes may be employed within the scope of the present specification. As is known, the limiting heel angle is with reference to the tangential plane at the focal track, and a convex shape provides a range of tangential planes depending upon the positioning of the focal track.

(35) FIGS. 2A, 2B and 2C show planar cross-sections of a hybrid electrical/mechanical scanner, in accordance with other wide-angle embodiments of the present specification. Referring to FIGS. 2A, 2B and 2C, by using a conservative 30 take-off range 250 from a quarter-round target 210 creates a 120 angular range 260, as shown in FIGS. 2B and 2C, where FIG. 2B shows the steered electron beam 205 strike the target 210 at a first outer boundary 206 and FIG. 2C shows the steered electron beam 205 strike the target 210 at a second outer boundary or extrema 207. The aperture 225 would be near extremum 236 for the electron beam deflection shown in FIG. 2B and near extremum 237 for the electron beam deflection shown in FIG. 2C. The electron beam is steered so that a desired take-off angle is maintained. Accordingly, the focal track is moved with the aperture to maintain the desired take-off angle.

(36) Hence, in various embodiments, by moving the comparably small collimator and not the entire X-ray source, the field of view of an X-ray imaging system can be increased by a factor of 3 or more over that of a conventional, heel-effect-limited X-ray source. This would, however, necessitate a fairly large X-ray exit window and the moving aperture 225 would have to travel linearly 2{square root over (3)} times the snout length 280. For a 150-mm snout length the aperture 225 would have to travel linearly over a distance of approximately 520 mm to achieve a 120 angular range. If only a 90 angular range is needed, aperture 225 must travel twice the snout length 280. Accordingly, a curved travel path may be preferable.

(37) An embodiment of a two-dimensional scanner, designated generally by numeral 300, is shown in perspective in FIG. 3A. A scanning aperture (such as aperture 225 in FIG. 2A) is achieved by rotating slits 302 of chopper disk 304 across X-ray beam window 310, which is shown with chopper 304 removed in FIG. 3B. Slit 302 is an example of a moving slit. Electrons from source 301 scan a target block 303 (which may be planar, or convex, as shown), with Bremsstrahlung X-rays confined by snout 305 to emerge only at the aperture created where rotating slit 302 intersects with X-ray beam window 310. X-ray beam window 310 is an example of a fixed slit. In the embodiment shown in FIG. 3A, rotating slit 302 is aligned radially with respect to an axis of rotation (not shown) of chopper disk 304 as one example.

(38) FIG. 3B is another depiction of the X-ray source of FIG. 3A, cutaway to show convex target 303 and X-ray beam window 310. The breadth of X-ray window 310 defines the line of pivot points for the X-ray beam as the electron beam scans along the target and thus creates the fast scan lines. The breadth of X-ray window 310 depends upon the desired field of view, and in an embodiment, is approximately equal to the lateral travel path 270. In another embodiment, the breadth dimension of the X-ray window is within ten percent (10%) of the lateral travel path dimension. The rate of angular change of the beam plane caused by moving the aperture is much slower.

(39) Scanning with chopper disk 304 for rotating apertures/slits 302 across X-ray beam window 310 is one way to achieve the moving aperture 225 (shown in FIG. 2A), and is suitable when the system does not require a large beam angle. Other ways of implementing a moving aperture are within the scope of the present specification, and the following examples are provided without limitation: a rotating twisted slit collimator, variations of which are described in U.S. Pat. Nos. 4,745,631, 4,995,066, and 5,038,370, assigned to Philips Corp. and European Patent No. 1,772,874, assigned to Bundesanstalt fr Materialforschung and Prufung (BAM), all of which are incorporated herein by reference; translating an aperture like the twisted slit described in U.S. Pat. Nos. 9,117,564 and 9,257,208 assigned to American Science and Engineering, Inc. (both incorporated herein by reference), with an actuator linear motor; and a hoop with parallel slits rotating with respect to a common axis of rotation.

(40) Embodiments of a two-dimensional scanner, in accordance with the foregoing teachings, may advantageously provide fast two dimensional image acquisition, with imaging at a rate of multiple frames per second made possible for the first time. The field of view provided by systems enabled hereby may be multiple times the field of view of a stationary tube system in size. Thus, 120 azimuth is now possible, vs. current limits of 30-40.

(41) A stationary two-dimensional scanner in accordance with the foregoing teachings may be particularly useful in situations that require a scanner that is compact in the lateral direction, or where it is important to operate close to the target without risk of accidentally contacting the target, or where movement of the scan head could be problematic for the platform on which the scan head is mounted. Examples, provided without limiting intent, include: inspecting aircraft, where any accidental collision renders the aircraft legally non-airworthy until a certified mechanic can inspect the aircraft to verify that no damage has been done; inspecting suspected improvised explosive devices (IEDs), where any accidental contact could detonate the IED; inspection of IEDs or any other application using a robot mounted imaging system. Space on a robotic vehicle is typically very limited, and a shifting or even rotating scanner might change the center of balance of the entire assembly which can be a problem, particularly on uneven terrain; medical X-ray applications, where the scanner must operate in close proximity to the patient without touching the patient or interfering with medical personnel working on the patient.

(42) Eliminating the need to move the scanner is also helpful in cases where high precision of beam placement is needed. Examples, provided without limiting intent, include: imaging at a distance, where small movements could translate to large position errors of the beam; Non-Destructive Testing (NDT) applications which often require very high resolution; NDT and Explosive Ordnance Disposal (EOD) applications which might use the image data for precision measurements of the target. EOD systems might use the measurement results to help aim a disruptor, or for forensic work, in addition to simply detecting the presence of an IED; applications which sum data from multiple repeat frames to build up image statistics over a period of time (also likely for NDT or EOD applications).

(43) It should be noted that the formation and scanning of X-ray pencil beam may be employed for any manner of imaging, such as transmission, sidescatter, or backscatter imaging, for example, within the scope of the present specification.

(44) The above examples are merely illustrative of the many applications of the system and method of present specification. Although only a few embodiments of the present specification have been described herein, it should be understood that the present specification might be embodied in many other specific forms without departing from the spirit or scope of the specification. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the specification may be modified within the scope of the appended claims.