Composite material x-ray collimator and method of manufacturing thereof

A composite material pre-patient collimator for shaping an x-ray beam in a computed tomography (CT) system is disclosed. The pre-patient collimator includes a base comprised of a first material having a first material density and an insert mechanically coupled to the base and being comprised of a second material, the second material comprising a moldable material having a second material density greater than the first material density and that is sufficient to block high frequency electromagnetic energy. The base comprises a plurality of structural features by which the insert is molded to the base, with the moldable material of the insert forming a connection with the plurality of structural features to mechanically couple the base and the insert.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to computed tomography (CT) imaging and, more particularly, to a composite material pre-patient x-ray collimator for use as part of a CT imaging system and a method of manufacturing thereof.

Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis, which ultimately produces an image.

In operation, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. The x-ray source is typically in the form of an x-ray tube that emits x-rays at a focal point, with the x-rays being emitted along diverging linear paths in an x-ray beam. A pre-patient collimator is employed for shaping a cross-section of the x-ray beam and for directing the shaped beam through the patient and toward the detector array. The detector array typically includes a collimator for collimating x-ray beams, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.

In CT imaging systems, the pre-patient collimator used for shaping the x-ray beam has historically been constructed by machining a monolithic piece of tungsten. Forming the pre-patient collimator from tungsten was appropriate because of the material's radiation blocking ability and structural properties. However, it is recognized that tungsten is an expensive material and difficult to machine. Additionally, in newer CT imaging systems that implement larger patient coverage, faster rotation speed, and larger bore sizes, pre-patient collimators formed from tungsten become even less desirable. That is, in such newer CT imaging systems, the centripetal acceleration (i.e., G-load) increases dramatically on the pre-patient collimator due to the increasing radius from the center of rotation, faster rotation speed of the components in the gantry, and larger pre-patient collimator size needed to block the beam in large-coverage systems. The weight and forces imposed on the pre-patient collimator are of concern as it affects dynamic balance of the CT imaging system, as well as agility of motion for the collimator.

Lead has also been recognized as a possible material from which to construct a pre-patient collimator, as lead also exhibits ideal radiation blocking capabilities associated with its material density. Unfortunately, similar to the use of tungsten pre-patient collimators, the high density of lead means that a pre-patient collimator constructed of lead is affected by the G-load increase in newer CT imaging systems. Additionally, lead is recognized as being too soft to be useful as a monolithic material and is not compliant under the Restriction of Hazardous Substances Directive (RoHS).

Therefore, it would be desirable to design a pre-patient collimator that combines the blocking power of a high-density material with the structural support of a lower density substrate material, therefore cutting back on weight and cost of the collimator, while preserving robustness, radiation blocking ability, and RoHS compliance.

BRIEF DESCRIPTION OF THE INVENTION

The invention is a directed method and apparatus for providing a composite material pre-patient x-ray collimator for use as part of a CT imaging system.

According to one aspect of the invention, a pre-patient collimator for shaping an x-ray beam in a computed tomography (CT) system includes a base comprised of a first material, the first material having a first material density, and an insert mechanically coupled to the base and being comprised of a second material, the second material comprising a moldable material having a second material density greater than the first material density and that is sufficient to block high frequency electromagnetic energy. The base comprises a plurality of structural features by which the insert is molded to the base, with the moldable material of the insert forming a connection with the plurality of structural features to mechanically couple the base and the insert.

According to another aspect of the invention, a method of manufacturing a pre-patient collimator for use in a computed tomography (CT) system includes the steps of forming a base from a first material, the base being formed so as to have a plurality of geometrical features thereon and molding a second material onto the base to form an insert, the second material comprising a material having a material density greater than that of the first material and that is sufficient to block high frequency electromagnetic energy. The second material is injection molded onto the base such that the second material forms a mechanical bond with the plurality of geometrical features to secure the insert to the base.

According to yet another aspect of the invention, a computed tomography (CT) system includes a rotatable gantry having an opening to receive an object to be scanned, a high frequency electromagnetic energy projection source configured to project a high frequency electromagnetic energy beam toward the object, and a collimator positioned between the high frequency electromagnetic energy projection source and the object configured to shape the high frequency electromagnetic energy beam, the collimator comprising a pair of blades. The CT system also includes a detector array configured to detect high frequency electromagnetic energy passing through the object and generate a detector output responsive thereto, a data acquisition system (DAS) connected to the detector array and configured to receive the detector output, and an image reconstructor connected to the DAS and configured to reconstruct an image of the object from the detector output received by the DAS. Regarding the collimator, each blade of the collimator further includes a metallic base formed of a first material and comprising a plurality of geometrical features thereon formed therein and an insert mechanically coupled to the base that is formed of a radiation blocking material having a material density greater than a material density of the first material, with the insert being mechanically coupled to the metallic base by way of the plurality of geometrical features, such that the blade is free of adhesives and fasteners for coupling the metallic base and the insert.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The operating environment of the invention is described with respect to a sixty-four-slice computed tomography (CT) system. However, it will be appreciated by those skilled in the art that the invention is equally applicable for use with other multi-slice configurations. Moreover, the invention will be described with respect to the detection and conversion of x-rays. However, one skilled in the art will further appreciate that the invention is equally applicable for the detection and conversion of other high frequency electromagnetic energy. The invention will be described with respect to a “third generation” CT scanner, but is equally applicable with other CT systems.

Referring toFIG. 1, a computed tomography (CT) imaging system10is shown as including a gantry12representative of a “third generation” CT scanner. Gantry12has an x-ray source14that projects a beam of x-rays toward a detector assembly or collimator18on the opposite side of the gantry12. Referring now toFIG. 2, detector assembly18is formed by a plurality of detectors20and data acquisition systems (DAS)32. The plurality of detectors20sense the projected x-rays16that pass through a medical patient22, and DAS32converts the data to digital signals for subsequent processing. Each detector20produces an analog electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuated beam as it passes through the patient22. During a scan to acquire x-ray projection data, gantry12and the components mounted thereon rotate about a center of rotation24.

Rotation of gantry12and the operation of x-ray source14are governed by a control mechanism26of CT system10. Control mechanism26includes an x-ray controller28that provides power and timing signals to an x-ray source14and a gantry motor controller30that controls the rotational speed and position of gantry12. An image reconstructor34receives sampled and digitized x-ray data from DAS32and performs high speed reconstruction. The reconstructed image is applied as an input to a computer36which stores the image in a mass storage device38.

Computer36also receives commands and scanning parameters from an operator via console40that has some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus. An associated display42allows the operator to observe the reconstructed image and other data from computer36. The operator supplied commands and parameters are used by computer36to provide control signals and information to DAS32, x-ray controller28and gantry motor controller30. In addition, computer36operates a table motor controller44which controls a motorized table46to position patient22and gantry12. Particularly, table46moves patients22through a gantry opening48ofFIG. 1in whole or in part.

As shown inFIG. 2, CT system10also includes a pre-patient collimator50mounted on gantry12and positioned in proximity to x-ray source14. Collimator50is constructed to shape the cross-section of x-ray beam16into a shape that matches the shape of detector array18, such as a rectangular shape, for example. The collimator50thus ensures that a patient being scanned is not subjected to an unnecessary dose of x-rays.

Referring now toFIG. 3, an exploded view of the collimator50is provided according to one embodiment of the invention. As shown inFIG. 3, collimator50is formed of a pair of blades51that are adjustable relative to one another so as to vary the size of an aperture53formed therebetween for allowing the x-ray beam16(FIG. 2) to pass there through. Varying of the size/shape of aperture53thus determines the cross-section of x-ray beam16and provides for the control and modification of the beam to a desired size/shape, such as to match the shape of detector array18.

As shown inFIG. 3, each blade51is further formed of a base52and an insert54attached to the base52. According to embodiments of the invention, each blade51of collimator50is formed as a composite component, in that the base52and the insert54are constructed of different materials. More specifically, the base52and the insert54are formed of materials having different densities. Each blade51is formed from a lower density structural material and a high-density radiation blocking material to achieve a resulting composite part that serves to selectively block radiation while also minimizing a weight and cost of the overall collimator50.

The base(s)52of each blade51of collimator50is constructed to provide structural support to the collimator50and allow for securing to gantry12of the CT system10(FIG. 1), while also being designed to lower the overall weight and cost of the collimator50. As such, the base52is composed of a lower density structural material that is selected based on its ability to preserve the robustness of the collimator50without adding undue weight and cost to the collimator50. As used herein, the term “lower density structural material” refers to a material having a density that is not sufficient to block high frequency electromagnetic energy (e.g., x-ray radiation) from passing there through. According to embodiments of the invention, the base52may therefore be formed of aluminum, steel, or another similarly acceptable material, that can be machined to have desired structural characteristics, as set forth in detail below.

The insert54of collimator50is constructed to provide radiation blocking within the collimator50. As such, the insert54is composed of a high-density radiation blocking material. As used herein, the term “high-density radiation blocking material” refers to a material having a density that is sufficient to block high frequency electromagnetic energy (e.g., x-ray radiation) from passing there through. According to embodiments of the invention, the insert54may therefore be formed, in part, of tungsten or another similarly acceptable material, that serves to block and shape the beam of x-rays16emitted from x-ray source14(FIG. 1), for example. According to an exemplary embodiment, the insert54is formed of a moldable high-density radiation blocking material, such as tungsten impregnated plastic, so that the insert54can be secured to base52by way of mechanical bonding, as set forth in detail below.

According to embodiments of the invention, the composite material blades51of collimator50are constructed such that base52is mechanically bonded to insert54without the use of adhesives or mechanical fasteners (e.g., bolts, screws, etc.). According to an exemplary embodiment, insert54is formed of a moldable material (e.g., tungsten impregnated plastic) that is molded onto base52, such as by way of mechanical over-molding or injection molding, to form an inseparable blade51in collimator50. To facilitate the mechanical bonding of the base52and the insert54, the base52is constructed to include a plurality of geometrical or structural features thereon that “lock” the insert54to the base52during a molding of the insert54thereto.

Referring now toFIG. 4, a detailed view of base52is shown according to an exemplary embodiment of the invention. As shown inFIG. 4, base52includes a plurality of geometrical/structural features56,58,60thereon that provide for a mating of insert54thereto when insert54is applied/formed via a mechanical over-molding or injection molding process. The base52includes a series of undercuts56for receiving the insert54, with the undercuts56being formed at opposing ends and sides of an insert placement area62, for example. The base52also includes a series of holes58spaced apart in the insert placement area62, with the holes58having counter bores60on an exit surface64of the base52opposite from where insert54is placed. According to an exemplary embodiment, undercuts56and holes58with counter bores60are formed in base52by way of a machining operation, such as according to standard machining procedures of an aluminum/steel material. According to another embodiment, a notch65is cut into the back of the base to serve as a gate for the introducing high-density radiation blocking material of the insert in the molding process.

Referring now toFIG. 5, a detailed view of insert54is shown according to an exemplary embodiment of the invention. As shown inFIG. 5, insert54includes a main face66that is generally formed in insert placement area62of base52. According to an exemplary embodiment, face66is formed as a curved face that accommodates an asymmetric x-ray beam, optimizes placement from x-ray source14(FIG. 1) based on a radius of curvature of the face, and provides full x-ray beam blocking. Alternatively, it is recognized that face66could also be formed as a straight (i.e., non-curved) face, according to another embodiment of the invention. Included on insert54are lips or protrusions68formed at opposing ends of face66, with the lips/protrusions68being formed to mate with undercuts56formed on base52(FIG. 4). Also included on insert54are a series of anchors70that extend out from a back surface72of face66and down through holes58formed in base52. At an end of the anchors70distal from face66, circular flanges74are formed that mate with the counter bore60of holes58, to lock the anchor70to base52. As indicated above, insert54is formed by way of a mechanical over-molding or injection molding process, such that the lips/protrusions68of face66and the anchors70form a locking mechanical bond with the geometrical features56,58,60of the base52, i.e., the undercuts56and holes58with counter bores60formed on/in the base52.

According to an embodiment of the invention, each blade51of the collimator50can thus be manufactured by first forming an base52from a piece of aluminum or steel, for example, with the aluminum/steel being machined to form an base having a plurality of geometrical features formed thereon. As set forth above, the geometrical features may be in the form of a series of undercuts56and holes58having counter bores60formed therein. Upon machining of the base52, the insert54is molded to the base by way of an over-molding or injection molding process. In molding the insert54to the base52, a number of protrusions68are formed on the insert that mate with the undercuts56of the base. Additionally, a number of anchors70are formed on the insert54that mate with the holes58and counter bores60of the base52. The molding of the insert54to the base52forms a mechanical bond there between that secures the insert to the base, without the need for any adhesives and/or fasteners.

Referring now toFIG. 6, a package/baggage inspection system100is shown according to an embodiment of the invention, with the package/baggage inspection system100incorporating a pre-patient collimator50such as shown inFIG. 3. As shown inFIG. 6, package/baggage inspection system100includes a rotatable gantry102having an opening104therein through which packages or pieces of baggage may pass. The rotatable gantry102houses a high frequency electromagnetic energy source106as well as a detector assembly108having scintillator arrays comprised of scintillator cells similar to that shown inFIG. 6or7. A conveyor system110is also provided and includes a conveyor belt112supported by structure114to automatically and continuously pass packages or baggage pieces116through opening104to be scanned. Objects116are fed through opening104by conveyor belt112, imaging data is then acquired, and the conveyor belt112removes the packages116from opening104in a controlled and continuous manner. As a result, postal inspectors, baggage handlers, and other security personnel may non-invasively inspect the contents of packages116for explosives, knives, guns, contraband, etc.

Beneficially, the composite material pre-patient collimator50can be optimized for radiation blocking ability, rigidity, and weight. The reduced weight of the collimator reduces demand on motors and bearings of the CT system since it is lighter. Additionally, the structure of the composite material pre-patient collimator blade, and the mechanical bonding/locking provided thereby, eliminates the need for any separate fasteners or adhesives to be used, thereby also eliminating any leakage points that might be created by the use of such fasteners. Additionally, the molding of the blocking material insert to the base allows for flexibility in geometry and application, and reduces waste and cost by providing a near net shape of the molded insert, such that the resulting insert is more environmentally friendly than using adhesives, for example. Still further, the structure of the composite material pre-patient collimator provides for machining of the assembly that is easier and cheaper than machining a pure tungsten collimator assembly.

Therefore, according to one embodiment of the invention, a pre-patient collimator for shaping an x-ray beam in a computed tomography (CT) system includes a base comprised of a first material, the first material having a first material density, and an insert mechanically coupled to the base and being comprised of a second material, the second material comprising a moldable material having a second material density greater than the first material density and that is sufficient to block high frequency electromagnetic energy. The base comprises a plurality of structural features by which the insert is molded to the base, with the moldable material of the insert forming a connection with the plurality of structural features to mechanically couple the base and the insert.

According to another embodiment of the invention, a method of manufacturing a pre-patient collimator for use in a computed tomography (CT) system includes the steps of forming a base from a first material, the base being formed so as to have a plurality of geometrical features thereon and molding a second material onto the base to form an insert, the second material comprising a material having a material density greater than that of the first material and that is sufficient to block high frequency electromagnetic energy. The second material is injection molded onto the base such that the second material forms a mechanical bond with the plurality of geometrical features to secure the insert to the base.

According to yet another embodiment of the invention, a computed tomography (CT) system includes a rotatable gantry having an opening to receive an object to be scanned, a high frequency electromagnetic energy projection source configured to project a high frequency electromagnetic energy beam toward the object, and a collimator positioned between the high frequency electromagnetic energy projection source and the object configured to shape the high frequency electromagnetic energy beam, the collimator comprising a pair of blades. The CT system also includes a detector array configured to detect high frequency electromagnetic energy passing through the object and generate a detector output responsive thereto, a data acquisition system (DAS) connected to the detector array and configured to receive the detector output, and an image reconstructor connected to the DAS and configured to reconstruct an image of the object from the detector output received by the DAS. Regarding the collimator, each blade of the collimator further includes a metallic base formed of a first material and comprising a plurality of geometrical features thereon formed therein and an insert mechanically coupled to the base that is formed of a radiation blocking material having a material density greater than a material density of the first material, with the insert being mechanically coupled to the metallic base by way of the plurality of geometrical features, such that the blade is free of adhesives and fasteners for coupling the metallic base and the insert.