Patent Number: 061756156
Section: description

DETAILED DESCRIPTION OF THE INVENTION A radiation imager system 10, such as a computed tomography (CT) system, incorporating the device of the present invention is shown in schematic form in FIG. 1. CT system 10 comprises a radiation point source 20 and a radiation detector 30 and a collimator 50 disposed between radiation source 20, typically an x-ray source, and detector panel 40. Radiation detector 30 typically comprises a panel 40 having an array of photosensor pixels 42 (only a few of which are shown in phantom for purposes of illustration) coupled to a scintillator (not shown) that together convert incident radiation into electrical signals. The detector elements in conventional CT systems are arranged in a one-dimensional array. Advanced volumetric CT systems have detector elements arranged in two-dimensional array, as illustrated in FIG. 1. The radiation detector elements are coupled to a signal processing circuit 60 and thence to an image analysis and d i splay circuit 70. This FIG. 1 arrangement allows an object or subject 90 to be placed at a position between the radiation source and the radiation detector, for examination or inspection of the object or subject. Collimator 50 is positioned over radiation detector panel 40 to allow passage of radiation beams that emanate along a direct path from radiation source 20, through exam subject 90, and to radiation detector panel 40, while absorbing substantially all other beams of radiation that strike the collimator. The construction of embodiments of the present invention for collimator 50, as well as the details of the fabrication of these collimators, are discussed in detail below. FIG. 2 is a cross-sectional view of a representative portion of a first embodiment of the collimator of the present invention. FIG. 3 is a slightly larger cross-sectional view of collimator 100. Collimator 100 is preferably fabricated from a solid, monolithic block or slab of a radiation absorbent material, such as tungsten. A plurality of channels or passages 102 are formed in the slab, extending completely through the slab from a first surface 104 to a second surface 106. The channels 102 extending through collimator 100 are "focally aligned", meaning t hat each of the channels has a central longitudinal axis L aligned or collinear with a respective orientation angle of the radiation source, such that extensions of the longitudinal axes L converge at a point corresponding to the position of radiation point source 20 in the imager assembly, as show n by the converging lines in FIG. 2. In that way, the channels 102 permit radiation originating at the radiation point source to pass through the collimator 100 to impinge upon detector 40. At the same time, the channels are oriented such that scattered or stray radiation not originating at or traveling directly from the radiation point source will impinge upon a portion of the collimator 100, such as first surface 104, or a wall 108 of a channel, and be absorbed by the collimator material prior to the radiation reaching a detector element 42. As a result, substantially the only radiation reaching the detector 40 will be radiation emanating directly from the radiation source 20 which passes through the object or subject 90, and which continues through to the detector. The image obtained is therefore minimally degraded by detection of scattered radiation. The fabrication process for producing collimators in accordance with the FIG. 2 embodiment advantageously permits custom design or tailoring of the collimator for different imaging situations, or for use in imaging devices having different configurations. As noted previously, the collimator is preferably formed from a single monolithic slab of a high atomic number material (e.g., an atomic number of about 72 or greater) which can absorb radiation of the type intended to be employed in a particular radiation detector or imager. This slab may be of a thickness on the order of several millimeters (e.g., 2-10 mm), with the thickness depending upon the energy of the radiation to be used and the imaging precision required, for example. As seen in the flow diagram of FIG. 4, the fabrication process begins with the use of a CAD (computer aided design) program, which generates a drawing of a two-dimensional collimator based upon overall imager system parameters, including the distance at which the collimator 100 will be placed from the radiation point source 20 in the imaging device, the size and position or location of the detector elements 42 on detector 40, and the spacing distance, if any, between the collimator 100 and detector 40. The CAD program preferably generates digital data files referred to as stereo-lithographic (STL) files. The CAD drawing or STL files contain information which defines the position, size, and orientation of the channels 102 which will extend through collimator 100 once fabrication is completed. In general, the size, orientation and position of the channels is determined by the distance of the collimator 100 from the radiation point source 20 in a given imager system, the size and location of the individual detector elements 42 on the detector panel 40, and the distance, if any, between the collimator 100 and the detector panel 40. The exit opening 110 of each of the channels 100 typically is sized and shaped to correspond to the size of the detector element 42 disposed adjacent to that channel. Where the collimator is not disposed in intimate contact with the detector panel 40, the sizing of the exit opening typically is also designed to account for spacing between the collimator 100 from the detector panel so as to allow the radiation passing from the collimator to be incident over the surface area of the respective detector elements 42. Based on the size and shape of the exit openings 110, the channel will generally have tapered walls which extend along imaginary planes defined by the respective edges of the exit opening 110 and the radiation point source 20. The size and position of the entrance openings 112 to the channels of the collimator 100 are thus dictated by the tapering walls 108 (that is, the dimensions of the channel are greater at first surface 104 of the collimator than at second surface 106 of the collimator) of the channels at the point that the channels reach the first or front surface 104 of the collimator. The exit and entrance openings 110, 112, respectively, on a collimator 100 designed for use with a two dimensional array of detector elements are schematically illustrated in FIG. 5. This figure shows entrance openings 112 in solid lines and exit openings 110 in broken lines. The geometric complexity of the channels and the differences in geometry from channel to channel can be better appreciated in this view as well. The generated STL files are typically used for control of a machining device, such as an electro-deposition machining (EDM) device, to machine out the material from block 101 to create the geometrically complex channels 102 which extend through the finished collimator. The geometric complexity of the channels is a result of the fact that the entrance and exit openings of the channels, and angles of orientation of the channels relative to the front and rear surfaces 104, 106 (respectively) of the collimator may all vary as a function of their distance from a central axis extending from the front surface of the collimator through a center of the radiation source 20. The CAD program and STL files generated permit the precise machining of these highly complex channels. In addition, a significant advantage of using CAD/STL files is that collimators having different channel characteristics can readily be made by revising the drawings or files or creating new drawings or files based on the device parameters which may be different for different imaging devices or for different imaging conditions in the same imaging device. As a result, this focally-aligned 2D collimator design and fabrication process have a great deal of flexibility despite the complexity of machining the many different channel configurations, and of machining at compound angles relative to the surfaces of the collimator. Collimators can thus be fabricated which are optimized for varying end uses. Generally, high energy (approximately 320-450 KeV) industrial x-ray imagers will be larger and have greater slab thicknesses and wall thicknesses (thickness of the material separating adjacent channels) to enhance the ability of the collimator to block the undesired radiation from reaching the detector 40. Collimators optimized for use with somewhat lower x-ray energies, used in medical imaging (approximately 120 KeV), for example, may have one or more of the following characteristics so as to be adapted for use in a medical system: a smaller slab thickness, or a thinner wall thickness. Two-dimensional collimators 100 as described above serve to reduce or suppress detection of scatter radiation. Due to the fact that such collimators have a substantial thickness (as noted above), as compared with thin sheets having collimation openings therein (e.g., openings over one or more detector columns or rows) and due to the fact that the web 150 of the collimator remaining after the channels have been machined is also of relatively substantial thickness (e.g., about 2 mm to about 10 mm of a high atomic number material for high energy x-rays in an industrial CT system), if the collimator is installed in a stationary position in the imager system, it is necessary to conduct an oversampling of the source distribution (e.g., a 4.times. sampling) to ensure that the detector elements of pixels 42 obtain an accurate image of the entire object being imaged, and not one with discrete sections corresponding to the grid of channels. Optionally, the imager system can be designed such that the collimator 100 is mounted to a vibrating platform 300 (FIG. 3) that will move the collimator 100 relative to the detector panel 40 such that the exit openings of the channels move to expose the detector elements to non-scattered radiation that otherwise would have been blocked or absorbed by the web portion 150 of the collimator. The platform vibration would be set such that each detector pixel sees the collimator walls and the exit opening of the channel for the same amount of time to ensure evenness (that is, uniformity) of exposure. An alternative embodiment of the present invention is schematically illustrated in FIGS. 6, 7 and 8. This alternative embodiment approximates the performance of the focally aligned 2D collimator of FIG. 2 by performing a one-dimensional (1D) collimation in a first plane, immediately followed by a further 1D collimation in a second plane which is orthogonal to the first plane. The net effect of the two collimations approximates the effectiveness and performance of a 2D collimator, and is generally superior to the effectiveness of a 1D collimator. Collimator 200 comprises first collimation section 204, which is made up of a plurality of first plate sets 201 (a representative one of which is illustrated in FIG. 6) of collimator plates 202. Each of the first plate sets 204 define a focally aligned (as that term is used herein) passage 206 adapted to allow to pass therethrough incident radiation emanating from a radiation point source. The axis of the passage is defined in a plane between the radiation point source and an underlying row (or other configuration) of detectors. In a conventional 1D collimator, scattered x-ray photons are prevented from reaching the detector in the plane of collimation of the collimator, but scattered photons originating in the plane orthogonal to that are not suppressed from reaching the detector elements. In this embodiment, collimator 200 further comprises a second collimation section 212. Second collimation section comprises a plurality of second plate sets 203. Second plate sets comprise collimator plates 210 that are positioned to create a respective focally aligned passages 216 arranged to collimate in a plane orthogonal to the plane of collimation of the first collimation section. The structure of the second collimation section will be essentially identical to that of the first collimation section, with the possible exception that the plates may be arranged such that passages 216 are adjusted to account for the different distance or spacing from the point source 20. Otherwise, the second collimation section appears, in end view, essentially identical to the first collimation section illustrated in FIG. 7. Collimator plates comprise a material selected to provide a desired level of attenuation given design information on energy level of x-ray radiation in the system and the imaging geometry used. Commonly, materials such as tungsten, lead, and natural uranium are efficacious collimator materials for use in imaging systems of the present invention. As seen in the substantially schematic illustrations in FIGS. 7 and 8, the plates of each of the first and second collimation sections are joined in fixed relationship to each other by a plurality of brackets 220 which make up a frame 222. The first and second collimation sections are also preferably secured in position relative to each other by brackets which also make up part of frame 222. One example of frame 222 comprises a box-type structure of a material transparent to the x-ray radiation (e.g., plastic or the like) that is fabricated to provide brackets (or grooves) 220 that receive collimator plates. For the 2-D arrangement, each of first and second collimator sections 204, 212, comprise a respective frame 222. The frames are disposed orthogonal to one another to provide the desired 2-D collimator structure. The collimator sections are typically fastened to the detector assembly (e.g., with bolts, snaps, or the likes) such that the sections can be removed and repositioned, if necessary. The collimator 200 is structured such that radiation passes successively through first collimation section 204 and second collimation section 212, with the effect that radiation not emanating directly from the radiation point source is, in large part, absorbed by plates of either the first or second collimation section. Collimator 200 thus is often referred to as a pseudo-2D or hybrid-2D collimator. FIG. 8, which illustrates the orthogonal orientation of plates 202 of first collimation section 204 and plates 210 of second collimation section 212, shows that passages 206 and 216, in combination and in succession, approximate the channels 102 of the collimator 100 according to the first preferred embodiment. For the purposes of clarity, only the leading edges 220, 222 of plates 202, 210, respectively, are shown in the view of FIG. 8. The broken lines illustrate that plates 210 are disposed underneath plates 202 in this illustration. In simulations conducted using a model of the collimator 200 shown in FIGS. 6, 7 and 8, this embodiment of the collimator demonstrated performance comparable to a true 2D collimator under moderate scatter conditions, such as are experienced in medical x-ray imaging. For example, for a given workpiece and energy of x-rays, the amount of the scatter signal reaching the detector array is typically less than about 20% of the primary x-ray signal reaching the array, and generally is between about 5% to about 10% of the primary signal reaching the array. The amount of scatter (e.g., the scatter signal as a percent of primary signal, is commonly less is medical imaging than in industrial imaging, where the composition and the geometry of parts being imaged generally contribute to a higher amount of scatter of incident x-rays. In extreme scatter conditions, such as are experienced in industrial x-ray imaging, the performance of collimator 200 is degraded. Nonetheless, given the relatively more complex design and fabrication of a true 2D collimator, there are many applications where the pseudo-2D collimator 200 would provides a desirable combination of performance and production cost. While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.