Patent Number: 
Section: description

With reference to FIG. 1, a computed tomography (CT) imaging apparatus or CT scanner 10 includes a gantry 12. An x-ray source 14 and a source collimator 16 cooperate to produce a fan-shaped, cone-shaped, wedge-shaped, or otherwise-shaped x-ray beam directed into an examination region 18 which contains a subject (not shown) such as a patient arranged on a subject support 20. The subject support 20 is linearly movable in a Z-direction while the x-ray source 14 on a rotating gantry 22 rotates around the Z-axis. In an exemplary helical imaging mode, the rotating gantry 22 rotates simultaneously with linear advancement of the subject support 20 to produce a generally helical trajectory of the x-ray source 14 and collimator 16 about the examination region 18. However, other imaging modes can also be employed, such as a single- or multi-slice imaging mode in which the gantry 22 rotates as the subject support 20 remains stationary to produce a generally circular trajectory of the x-ray source 14 over which an axial image is acquired. After the axial image is acquired, the subject support optionally steps a pre-determined distance in the Z-direction and the axial image acquisition is repeated to acquire volumetric data in discrete steps along the Z-direction. A radiation detector 30 is arranged on the gantry 22 across from the x-ray source 14. In the exemplary CT scanner 12, the radiation detector 30 spans a selected angular range that preferably comports with a fan angle of the x-ray beam. The radiation detector 30 includes several rows of detectors along the Z-direction for acquiring imaging data along a portion of the Z-direction in each projection view. The radiation detector 30 is arranged on the gantry 22 opposite to the x-ray source 14 and rotates therewith so that the radiation detector 30 receives x-rays that traverse the examination region 14 as the gantry 22 rotates. A plurality of anti-scatter elements 32, such as spaced anti-scatter plates, are arranged on the radiation detector 30 and are oriented with respect to a spatial focal point 34 generally corresponding to an origin or convergence point of the x-ray beam. The spatial focal point 34 is typically on the anode of the x-ray source 14. The detector 30 is a focus-centered detector centered on the spatial focal point 34. Instead of the arrangement shown in FIG. 1, it is also contemplated to arrange the radiation detector on a stationary portion of the gantry encircling the rotating gantry such that the x-rays continuously impinge upon a continuously shifting portion of the radiation detector during source rotation. With continuing reference to FIG. 1, the gantry 22 and the subject support 20 cooperate to obtain selected projection views of the subject along a helical trajectory or other trajectory of the x-ray source 14 relative to the subject. The path of the x-ray source 14 preferably provides substantial angular coverage for each voxel of the imaged region of interest to reduce image artifacts. Projection data collected by the radiation detector 30 are communicated to a digital data memory 40 for storage. A reconstruction processor 42 reconstructs the acquired projection data, using filtered backprojection, an n-PI reconstruction method, or other reconstruction method, to generate a three-dimensional image representation of the subject or of a selected portion thereof which is stored in an image memory 44. The image representation is rendered or otherwise manipulated by a video processor 46 to produce a human-viewable image that is displayed on a graphical user interface (GUI) 48 or another display device, printing device, or the like for viewing by an operator. Preferably, the GUI 48 is additionally programmed to interface a human operator with the CT scanner 12 to allow the operator to initialize, execute, and control CT imaging sessions. The GUI 48 is optionally interfaced with a communication network such as a hospital or clinic information network via which image reconstructions are transmitted to medical personnel, a patient information database is accessed, or the like. With continuing reference to FIG. 1 and with further reference to FIG. 2, the anti-scatter elements 32 are arranged between first and second generally symmetrical, substantially planar, alignment plates or boards 601, 602. The alignment plates 601, 602 are preferably arranged in a large arc, generally parallel to one another. The alignment plates 601, 602 are thin metallic plates, preferably made of a corrosion-resistant metal, such as stainless steel, which are supported by corresponding rigid support elements 621, 622, respectively. Preferably, the support elements 621, 622 are components or cast portions of a detector support frame that mechanically supports, secures, and/or retains functional components of the radiation detector 30 including the anti-scatter elements 32. With continuing reference to FIG. 2 and with further reference to FIGS. 3 and 4, each alignment plate 601, 602, numbered generally as 60, includes a plurality of anti-scatter element alignment openings 70 formed therein. As shown in FIG. 4, the anti-scatter element module alignment openings 70 are arranged in pairs along radial lines 72 that converge at the spatial focal spot 34 which coincides with the x-ray source 14 or a convergence of the fan-shaped, cone-shaped, wedge-shaped, or otherwise-shaped x-ray beam produced by the cooperating x-ray source 14 and source collimator 16. In FIG. 4, a few exemplary radial lines 72 are shown to indicate the alignment of pairs of anti-scatter element alignment openings 70 with the spatial focal point 34. In the embodiment of the alignment plate 60 shown in FIG. 3, an additional opening 74 is arranged between each pair of anti-scatter element alignment openings 70. The extra opening 70 is preferably aligned along the radial line 72 of the pair of anti-scatter element alignment openings 70, and provides a pass-through for a fastener that secures the anti-scatter element or module 32 to the rigid support element 62. Further additional alignment openings 76 in the alignment plates 60 are optionally included to align the alignment plates 60 with the support elements 621, 622 or to align other elements of the radiation detector 30. With continuing reference to FIGS. 2-4 and with further reference to FIGS. 5A, 5B, and 5C, the anti-scatter elements or modules 32 each include a plurality of anti-scatter plates or vanes 80 arranged generally in conformity with the rays or planes 72 and separated by spacer plates 82 that are generally parallel to the anti-scatter plates 80 and define a selected spacing and convergence angle between anti-scatter plates 80. The non-scattered radiation is directed parallel to the anti-scatter plates 80 and pass therebetween, while scattered radiation angularly deviates from parallel with the anti-scatter plates 80 and is typically absorbed by the anti-scatter plates 80. Although the anti-scatter plates or vanes 80 are generally parallel to one another, those skilled in the art will recognize that precisely parallel plates do not exactly align with the spatial focal point 34. That is, precisely parallel planes do not contain any points in common, and hence cannot contain the spatial focal point 34 in common. Preferably, the generally parallel anti-scatter plates or vanes 80 are each aligned with a plane that intersects the spatial focal point 34. Such planes are close to, but not exactly, parallel over a length L of the anti-scatter plate 80 since L is short compared a distance between the anti-scatter module 32 and the spatial focal point 34. In a preferred embodiment for obtaining the preferred generally parallel arrangement of anti-scatter plates 80 in the module 32, the sides of the spacer plates 82 that contact the anti-scatter plates 80 are preferably slightly non-parallel. An angle of the non-parallel sides is selected to provide a slight tilt of the contacting anti-scatter plates 80 relative to one another to closely align each anti-scatter plate 80 with a plane that intersects the spatial focal point 34. The anti-scatter plates or vanes 80 are preferably formed of a material with a high atomic number that is highly absorbing for radiation produced by the x-ray source 14, such as tantalum, tungsten, lead, or the like. The spacer plates 82 are formed of a material that is substantially translucent to radiation produced by the x-ray source 14, and are suitably formed of a plastic material. In a preferred embodiment, the spacer plates 82 are substantially hollow molded plastic frames, rather than full molded plastic slabs, to further reduce radiation absorption in the spacer plates 82. The arrangement of generally parallel anti-scatter plates 80 and spacer plates 82 is secured at the sides by two end caps 841, 842. Each end cap 84 includes alignment pins or other alignment protrusions 86 that are aligned along the radial line or plane 72, as best seen in FIG. 5A. In a preferred embodiment, the protrusions 86 of one end cap 842, align with the protrusions 86 of the other end cap 842, as best seen in FIG. 5B, so that the two end caps 841, 842 are interchangeable. Optionally, an adhesive such as a pressure-sensitive adhesive is disposed between contacting surfaces of the anti-scatter plates 80 and the spacer plates 82 to provide additional structural support. With continuing reference to FIGS. 2-5C and with further reference to FIGS. 6 and 7, the alignment protrusions 86 of the anti-scatter modules 32 mate with the anti-scatter alignment openings 70 of the alignment plates 601, 602 to align the anti-scatter modules 32 with the spatial focal point 34. Because both the openings and the protrusions are defined with precision, the modules are precisely aligned upon insertion. No adjustment in the alignment is necessary. As best seen in FIGS. 2 and 6, the rigid support elements 621, 622 include recess troughs 90 aligned with the anti-scatter alignment openings 70 that provide space for the protrusions 86 to pass through the alignment openings 70. The recess troughs 90 do not provide precise alignment and hence need not be formed with close tolerances. In the preferred illustrated embodiment the two alignment plates 601, 602 cooperate in aligning the anti-scatter modules 32. However, it is also contemplated to employ only a single alignment plate 60. With reference to FIGS. 8 and 9, the anti-scatter elements 32 are aligned using the alignment protrusions 86 and fastened in the radiation detector 30 using threaded fasteners 100 that pass through the openings 74. It should be noted that in FIG. 8, the side that faces the x-ray tube 14 is facing down. Although in FIG. 8 the support elements 621, 622 are omitted to show the alignment openings 70, 74, the fasteners 100 preferably secure to the support elements 621, 622. Additional protrusions or pins 102 preferably extend from a backside of each anti-scatter module 32 to provide alignment for photodetector array modules 104 that align with the anti-scatter modules 32. The pins 102 of each anti-scatter module 32 precisely mate with precision openings 106 of a corresponding photodetector array module 104 to provide alignment of the photodetector array module 104 with its corresponding anti-scatter module 32. With continuing reference to FIG. 9 and with further reference to FIG. 10, each photodetector array module 104 includes a substrate 108 on which is disposed a photodetector array 110. A scintillator layer or array 112 is disposed on the photodetector array 110 to provide conversion of x-rays to light that is detectable by the photodetector array 110. The photodetector array 110 is preferably a monolithic array of silicon photodiodes, amorphous silicon, charge-coupled devices, or other semiconductor photodetectors that is divided into individual detector elements 114 by wafer-level photolithographic processing of the monolithic photodiode array, by a mask 116 of tungsten or other x-ray absorbing material, or by a combination of processing and masking. The alignment of the photodetector array module 104 to the anti-scatter module 32 arranges the detector elements 114 in the gaps between the anti-scatter plates 80 as shown in FIG. 10. The detector elements 114 view between the anti-scatter plates 80, i.e. view through the spacer plates 82 such that scattered radiation which angularly deviates from the unscattered radiation is substantially absorbed by the anti-scatter plates 80 and does not reach the detector elements 114. With continuing reference to FIGS. 1-10 and with further reference to FIG. 11, a preferred method 120 for assembling the radiation detector 30 is described. In a step 122, the alignment plates 601, 602 are aligned onto the corresponding support elements 621, 622 using at least some of the additional alignment openings 76, and are secured thereto, e.g. using fasteners that pass through selected openings 76. In a step 124, the anti-scatter elements or modules 32 are aligned with the anti-scatter alignment openings 70 by coupling the alignment projections 86 with the anti-scatter alignment openings 70 of the alignment plates 601, 602, and the anti-scatter modules 32 are secured to the support elements 621, 622 using the fasteners 100. In a step 126, each photodetector array module 104 is aligned to each corresponding anti-scatter module 32 using the mating alignment pins 102 and openings 106, and the photodetector array module 104 is secured to the anti-scatter module 32, the support elements 62, or another suitable support. It will be appreciated that if the photodetector array modules 104 are secured to corresponding anti-scatter modules 32, then the alignment steps 124, 126 are optionally reversed. That is, the step 126 of aligning the photodetector array modules 104 to the anti-scatter modules 32 can be performed first, with each photodetector array module 104 aligned and secured to a corresponding anti-scatter module 32, followed by alignment of the anti-scatter modules 32 with attached photodetector array modules 104 to the alignment plates 60 in the step 124. In a step 128, the assembled radiation detector 30 is aligned and mounted to the computed tomography scanner gantry 22. The aligned anti-scatter modules 32 of the radiation detector 30 cooperatively define a spatial focal spot 34, as best seen in FIGS. 4, 5A, and 7. The radiation detector 30 is aligned on the rotating gantry 22 such that the spatial focal spot 34 coincides with a spatial convergence of the rays of the x-ray cone-, wedge-, or otherwise-shaped beam produced by the cooperating x-ray source 14 and source collimator 16. Alternatively, in the step 128 the x-ray source 14 and the source collimator 16 are aligned with respect to the spatial focal spot 34 associated with the radiation detector 30. The assembly method 120 described with particular reference to FIG. 11 relies upon the alignment plates 601, 602 accurately and precisely defining the alignment of the anti-scatter modules 32 through the anti-scatter alignment openings 70. The support elements 621, 622 similarly are aligned with respect to the alignment plates 601, 602 using at least some of the additional alignment openings 76. The alignment openings 70, 76 are precisely and accurately positioned. Furthermore, for manufacturing purposes, the alignment plates 601, 602 are preferably mass-produced with close tolerances in the positioning and sizing of the alignment openings 70, 76. In a preferred embodiment, the alignment plates 601, 602 are interchangeable, so that a single part is mass-produced for manufacturing quantities of the radiation detector 30. With reference to FIG. 12, a preferred photolithographic method 150 for manufacturing the alignment plate 60 is described. The method 150 operates on a stock metal plate 152, which is preferably thin (e.g., about 0.025 cm thick) and cut to at least approximately correspond to the desired lateral dimensions of the alignment plate 60. The stock metal plate 152 is preferably a stainless steel plate which is advantageously strong and corrosion-resistant. However, an aluminum alloy or other material can also be used. In one suitable embodiment, the stock metal plate 152 is cut mechanically to define the shape of the alignment plate 60. In a preferred embodiment, however, the mechanical cutting of the stock metal plate is limited to defining a rectangular or other regular shape whose dimensions exceed the outer dimensions of the desired alignment plate 60. In this latter embodiment, the photolithographic method 150 described below precisely defines the outer dimensions of the alignment plate simultaneously with formation of the openings 70, 74, 76. A selected photoresist film is applied to both sides of the metal plate 152. The photoresist is preferably applied using evaporation, a spin-on photoresist application method, or other method that produces a uniform and well-controlled thickness of photoresist on both sides of the stock metal plate 152. The photoresist film is exposed to a selected light using a pattern mask in a step 156. As is known in the art, photoresist is a light-sensitive substance whose resistance to certain types of etching chemicals is altered by exposure to light. With positive photoresists, exposure to light weakens resistance to the chemical etching. With negative photoresists, exposure to light strengthens resistance to the chemical etching. Interposing the pattern mask between the light and the photoresist film during the exposure step 156 causes selective exposure of the photoresist film. For a positive photoresist, the pattern mask blocks exposure except in the areas to be etched, i.e. the openings 70, 74, 76. For a negative photoresist, the mask blocks exposure only in the areas to be etched, i.e. the openings 70, 74, 76. The pattern mask is preferably constructed from a computer-assisted drawing (CAD) design using known methods. The pattern mask can also be generated by photographic replication and optional reduction or enlargement of a precise and accurate manual drawing of the target light exposure pattern. The exposed photoresist is developed in a step 158. The developing step 158 includes optional annealing or other curing of the exposed photoresist to optimize etching characteristics of the light-exposed and unexposed regions, followed by chemical etching in a developer chemical that selectively removes the light-exposed regions of the photoresist film (for positive photoresist) or the regions of the photoresist film which were not exposed to light (for negative photoresist). The developing step 158 causes the photoresist to be patterned such that those areas of the metal plate 152 which are to be removed, i.e. the openings 70, 74, 76, are not covered by photoresist, while the remainder of the metal plate 152 remains covered. The metal plate 150 with the patterned photoresist is etched in a step 160 using an etchant that etches the metal plate 150 but leaves the developed photoresist substantially unaffected. Hence, the exposed regions of the patterned photoresist corresponding to the openings 70, 74, 76 are etched, while the photoresist-coated remainder of the metal plate 150 is left substantially unaffected. For the preferred embodiment in which the photolithography process 150 defines the outer dimensions of the desired alignment plate 60, the photoresist pattern preferably additionally includes a continuous contour exposed region through which the etchant can cut out the alignment plate 60 in a precise and accurate fashion. Similarly, the through-holes 74 for the fasteners 100 or other features of the alignment plate 60 are suitably incorporated into the photoresist pattern and hence formed in the metal plate 150 during the etching step 160. After the etching step 160, the developed photoresist 162 is removed in a step 162. Typically a solvent such as acetone or the like suitably removes the developed photoresist while leaving the metal substantially unaffected. It will be appreciated that a small amount of residual photoresist contamination will typically remain after the cleaning step 162. Since small amounts of residual contamination do not affect the functional use of the alignment plate 60, the photoresist removal step 162 preferably uses a solvent exposure which leaves small amounts of residue contamination remaining on one or more surfaces of the alignment plate 60. Such residual contamination can be detected, for example, using sensitive chemical surface analysis techniques such as Auger electron spectroscopy, x-ray photoemission spectroscopy (XPS), or the like. The photoresist application, exposure, developing, metal etching, and photoresist removal steps 154, 156, 158, 160, 162 are well-known in the photolithographic arts, and the skilled artisan can select an appropriate photoresist, metal etchant, and photoresist solvent, and corresponding appropriate photolithographic parameters such the photoresist thickness, exposure time, etching time, and the like to optimize the method 150 for selected types of stock metal plates, for available photolithography facilities, and so forth. In one suitable embodiment, although the photoresist is applied to both sides of the metal plate 152 in the step 154, the pattern-defining step 156 is applied to only one side of the metal plate 152. In this case the developed photoresist has openings only on the exposed side, and the etching step 160 etches the openings 72, 74, 76 from the exposed side. In another suitable embodiment, the pattern-defining step 156 is applied to both sides of the metal plate 152 so that the etching step 160 etches the openings 72, 74, 76 simultaneously from both sides of the metal plate 152. This embodiment beneficially reduces the etching time by about a factor of two. However, precise relative alignment of the exposed patterns on the two sides should be achieved using known pattern mask alignment techniques, so that during the etching step 160 the simultaneously etched openings from the two opposite sides line up and properly join. In actually constructed embodiments, the alignment plate 60 has an accuracy in hole placement that is better than 0.0025 cm across a 100 cm area. However, undercutting or other imperfections introduced during the etching step 160 may produce openings 72, 76 which are not optimally defined with respect to circularity and diameter. To improve circularity and diameter accuracy of the openings 72, 76, the openings 72, 76 are optionally mechanically reamed in a step 164 to more precisely define the shape and size of the openings. The starting stock metal plate has been found to have an optimal thickness of about 0.025 centimeters for stainless steel. Thicker plates result in reduced hole diameter accuracy, while thinner plates result in reduced mechanical strength of the alignment plate 60. In addition to high precision and accuracy in the placement of alignment openings, those skilled in the art will recognize substantial additional advantages in using photolithography to define the alignment openings and other structures of the alignment plates 60. One particular advantage is that the manufacturing cost of the alignment plate is generally independent of the number of alignment openings formed therein. Hence, the conventional arrangement of a restricted number of anti-scatter modules which each include a plurality of anti-scatter plates is not necessary. Rather, the anti-scatter plates 80 and spacer plates 82 can be directly installed without the module-defining end caps 84. With reference to FIG. 13, an anti-scatter element 32xe2x80x2 which omits the end caps 84 is described. Components of the anti-scatter module 32xe2x80x2 that generally correspond with elements of the anti-scatter module 32 are designated by corresponding primed reference numbers herein. Spacer plates 82xe2x80x2 are modified compared with the spacer plates 82 to include alignment nubs or pins 86xe2x80x2 that mate with alignment openings in alignment plates 601xe2x80x2, 602xe2x80x2, which are modified compared with the alignment plates 601, 602 by including a higher density of anti-scatter alignment openings corresponding to the alignment nubs or pins 86xe2x80x2 of the spacer plates 82xe2x80x2. The anti-scatter plates or vanes 80xe2x80x2 are substantially similar to the anti-scatter plates 80, and are held between contacting spacer plates 82xe2x80x2 frictionally or using an adhesive such as a pressure-sensitive adhesive. The support elements 621xe2x80x2, 622xe2x80x2 include recess troughs 90xe2x80x2 dimensioned to provide space for the nubs or pins 86xe2x80x2 that project through the anti-scatter plates 601xe2x80x2, 602xe2x80x2. With reference to FIG. 14, another anti-scatter element 32xe2x80x3 which omits the end caps 84 is described. Components of the anti-scatter module 32xe2x80x3 which generally correspond with elements of the anti-scatter module 32 and the anti-scatter module 32xe2x80x2 are designated by corresponding double-primed reference numbers herein. The anti-scatter plates or vanes 80xe2x80x3 are modified compared with the anti-scatter plates 80 and the anti-scatter plates 80xe2x80x2 to include alignment nubs, pins, or extensions 86xe2x80x3 that mate with alignment openings in alignment plates 601xe2x80x3, 602xe2x80x3, which are modified compared with the alignment plates 601, 602 by including a higher density of anti-scatter alignment openings corresponding to the alignment nubs, pins, or extensions 86xe2x80x3 of the anti-scatter plates 80xe2x80x3. The spacer plates 82xe2x80x3 are substantially similar to the spacer plates 82, and preferably do not include nubs or projections. The spacer plates 82xe2x80x3 are held between contacting anti-scatter plates 80xe2x80x3 frictionally or using an adhesive such as a pressure-sensitive adhesive. The support elements 621xe2x80x3, 622xe2x80x3 include recess troughs 90xe2x80x3 dimensioned to provide space for the nubs, pins, or extensions 86xe2x80x3 that project through the anti-scatter plates 601xe2x80x3, 602xe2x80x3. In the various anti-scatter elements 32, 32xe2x80x2, 32xe2x80x2, it is to be appreciated that the alignment protrusions, nubs, pins, or extensions 86, 86xe2x80x2, 86xe2x80x3 can be cylindrical extensions, slots, or the like. The extensions 86xe2x80x2, 86xe2x80x3 can be correspond to extensions of the spacer plate 82xe2x80x2 or the anti-scatter plate 80xe2x80x3, respectively, to a length greater than the separation of the alignment plates 60xe2x80x2, 60xe2x80x3, such that the extensions 86xe2x80x2, 86xe2x80x3 are planar tabs substantially spanning a length of a side of the spacer plate 82xe2x80x2 or the anti-scatter plate 80xe2x80x3. In this arrangement the alignment openings of the alignment plates 60xe2x80x2, 60xe2x80x3 corresponding to each spacer plate 82xe2x80x2 or anti-scatter plate 80xe2x80x3 are single long slots each receiving a planar tab. Although the radiation detector 30 has been described with reference to a computed tomography imaging scanner, it is readily modified for use in other imaging systems. For example, a gamma camera for nuclear medical imaging typically includes detector arrays substantially similar to the detector array 110 with scintillators suitable for converting radiation produced by an administered radiopharmaceutical to light detectable by the detector array. Gamma cameras further typically include radiation collimators that define radial directions or narrow viewing cones corresponding to each detector element. Those skilled in the art can readily adapt the alignment plates 60, 60xe2x80x2, 60xe2x80x3 to precisely and accurately align collimators on a gamma camera. In such an adaptation, since the collimators of a gamma camera preferably define precisely parallel projections, the spatial focal point 34 described herein is suitably located at mathematical infinity, corresponding to precisely parallel radial lines 72. Analogously, these techniques can be applied to conventional x-ray, digital x-ray, fluoroscopy, and the like. The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.