Patent Number: 052934179
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A radiation imager system 10, such as a medical 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, typically an x-ray source, and a radiation detector 30 comprising a plurality of radiation detector modules or panels 40 and a plurality of collimators 50 disposed between radiation source 20 and detector panels 40. Each detector panel comprises a plurality of detector elements (not shown) which produce an electrical signal in response to the incident radiation. The detector elements are typically arranged in a one- or two-dimensional array on each detector plate 40. The radiation detector elements are coupled to a signal processing circuit 60 and thence to an image analysis and display circuit 70. Detector plates 40 are mounted on a curved supporting surface 80 which is positioned at a substantially constant radius from radiation point source 20. This arrangement allows a subject 90 to be placed at a position between the radiation source and and the radiation detector for examination. Collimators 50 are positioned over radiation detector panels 40 to allow passage of radiation beams that emanate directly from radiation source 20, through exam subject 90, to radiation detector panels 40, while absorbing substantially all other beams of radiation that strike the collimator. The details of steps in the fabrication, and the resulting structure, of collimators 50 are set out below. In accordance with this invention, material is selectively removed from each of a plurality of collimator plates to form a plurality of passages in each plate. Two representative plates, 210a and 210b, are illustrated in FIGS. 2(a) and 2(b) respectively. Passages 215 extend between openings in opposite surfaces of each plate. Preferably the shape of the sidewalls (e.g., vertical or slanted) in each individual plate is substantially the same, and each plate has sidewalls shaped similarly to those in adjoining plates. In one embodiment of this invention, the collimator plates comprise relatively thin (i.e., having a thickness less than about 0.25 mm) sheets of radiation absorbent material. The radiation absorbent material is selected to exhibit good absorption characteristics for radiation having the wavelength distribution emitted by radiation source 20, and typically comprises a material having a relatively high atomic number, i.e. about 72 or greater. Examples of such material include tungsten, gold, and lead. Conventional photolithographic techniques are advantageously used to selectively remove material from collimator plates 210 to shape passages 215. For example, a mask 220a is formed on collimator plate 210a and a mask 220b is formed on collimator plate 210b, each mask having a selected pattern chosen to result in the formation of passages in the respective plates so that when the plates are assembled or stacked together the adjoining passages in the plates will form channels through the assembled collimator with respective axes having a respective selected orientation. If the photoresist used in the photolithographic processes does not adhere well to the radiation absorptive material, a transfer mask may be used in order to form a mask of a material that does adhere well to the material to be etched. The pattern of the mask is selected for each collimator plate and typically results in the passages being positioned in slightly different places on each respective plate. The desired positions of the passages on the plate are dependent on the location of the plate with respect to the underlying radiation detector elements in the assembled collimator device, the arrangement of detector elements in the detector array, and the path along which radiation emanating from the radiation point source passes to the detector element. After the mask is formed, the collimator plates are etched to form a plurality of passages 215 (portions of the collimator plates that are removed in the etching process are shown in dotted cross hatching in FIGS. 2(a) and 2(b)). Known etching processes are used to form the passages, such as wet etching of tungsten. Alternatively, masks can be formed on both sides of the collimator plate and the plate then etched simultaneously from both sides. To assist with alignment of the collimator plates, an alignment hole 217 may advantageously be formed in each collimator plate at the time passages 215 are formed. One or more alignment holes are positioned in the same respective positions on each collimator plate to be used as a reference point so that the plates can be properly positioned with respect to one another when they are stacked together to form the collimator. In an alternative embodiment of the present invention, collimator plates comprise collimator substrates 310 coated with radiation absorbent material 330, as illustrated in FIGS. 3(a) and 3(b) respectively. Substrate 310 comprises photosensitive material, i.e., a material that will react to exposure to light in a manner similar to photoresist. Such a material may lose its photosensitive characteristics once it has been exposed and processed. One example of this type of substrate material is the Corning, Inc. product known as Fotoform.RTM. glass. Collimator substrate 310 is selectively exposed through a mask to a light source so that the light exposes areas of the photosensitive substrate corresponding to a selected pattern for each collimator plate. For example, an optically opaque mask 312 is formed by conventional methods on a first surface 310a of collimator substrate 310. The pattern of openings in mask 312 corresponds to the pattern of detector elements in radiation detector panel 40 (FIG. 1). For example, mask 312 has a pattern mimicking the arrangement, i.e., rows and columns, and the cross-sectional shape of detector elements at the interface between radiation detector panel 40 and collimator 50 (FIG. 1). Alternatively, mask 312 need not be on the surface of the collimator substrate but can be positioned with respect to the substrate in accordance with known photolithographic techniques to provide the desired exposure of the photosensitive material in substrate 310. In any event, the pattern of the mask is selected to expose areas of photosensitive collimator substrate 310 of sufficient size and orientation so that upon completion of fabrication of collimator 50, the surface of each radiation detector element for receiving the radiation is exposed to radiation passing along the desired paths from the radiation source. Collimator substrate 310 is then etched using conventional techniques appropriate for the substrate photosensitive material to remove the exposed photosensitive material and thus create a plurality of passages 320 through the substrate, as illustrated in FIG. 3(a). Portions of the photosensitive material that are removed in the etching are shown in dotted cross hatching in the figure. Each of these passages extends between openings in opposite surfaces of the collimator plate. Preferably the sidewalls of the passages on each individual plate have substantially the same shape and orientation, and are of substantially the same shape and orientation as the passage sidewalls in other plates used in the assembled imager system. A radiation absorbent material layer 330 (FIG. 3(b)) is then applied on collimator substrate 310 so as to cover at least the surfaces of the substrate which will be exposed to radiation when assembled in an imager device. The radiation absorbent material applied on the far interior wall of the channel is shown in dotted cross hatching. For example, many types of radiation absorbent material can be applied through known vapor deposition techniques. Radiation absorbent material 330 is selected to absorb radiation of the energy level and wavelength emitted by radiation source 20 (FIG. 1). The radiation absorbent material typically has a relatively high atomic number, e.g., greater than about 72, and advantageously comprises tungsten, gold or lead when the radiation used in the imager device is x-ray. The thickness of the radiation absorbent material layer is selected to provide, when the collimator is assembled, efficient absorption of radiation. This selected thickness depends on the nature of the radiation and the energy level of the radiation when it strikes the collimator. For example, in a CT system using an x-ray point radiation source of about 100 KeV positioned approximately one meter from the detector array, the collimator plates would need to present a collective tungsten thickness in a range of between about 30 to 40 mils along the path of the radiation to be absorbed. After application of the radiation absorbent material, the cross-sectional area of the opening or the void space in the passage is substantially the same as the area for receiving radiation on the detector element which it adjoins so as to allow substantially all radiation rays emanating along direct paths from the radiation source to strike the detector element. The collimator plates are then stacked, i.e., assembled one over the other as shown in FIG. 4(a), to form a collimator body 455 and aligned so that respective passages in the collimator plates form a plurality of respective channels 420 through the collimator body. The collimator plates are advantageously aligned in the stacking process by positioning an alignment hole 417 about an alignment rod 430. Alternatively, optical alignment devices aimed through alignment holes 417 or alignment of the edges of the plates can be used to provide correct alignment of the passages when stacking the collimator plates. In the assembled collimator 50 of FIG. 1, shown in a detailed view in FIG. 4(a), each collimator plate 410 comprises a patterned sheet of radiation absorbent material or alternatively comprises a photosensitive material substrate coated with a radiation absorbent material. Each channel is defined by sidewalls 418 of the respective passages in each collimator plate. The sidewalls of each respective passage in adjoining collimator plates form a step-shaped boundary 422 of channel 420 in collimator body 455. As illustrated in FIG. 4(b), a longitudinal axis 424 of each channel is substantially equidistant from a pair of longitudinal tangent lines 423 passing along the portions of sidewalls 418 which extend furthermost into the channel. The orientation of the tangent lines towards a convergence point above the collimator (i.e., the radiation point source) is exaggerated for illustration purposes. The longitudinal axis for each channel will have a unique selected orientation angle, varying in magnitude and orientation (i.e., displacement in an x or y direction, or a combination of those directions, in the plane of the radiation detector array). For example, in the plane of the cross-sectional view presented in FIG. 4(a), axis 424' has a selected orientation angle .beta. and axis 424" has a selected orientation angle .differential., each of which are in the plane of the drawing but which differ in magnitude and in direction of displacement with respect to the radiation source. With a two-dimensional array of radiation detectors 42, the various selected orientation angles would also be displaced in a plane normal to the plane of the cross-sectional illustration of FIG. 4(a). The magnitudes of the selected orientation angles typically range between about 0.degree. and 10.degree.. In accordance with the present invention, each longitudinal axis of each respective channel in the collimator body is aligned with a respective selected orientation angle, which angle corresponds to the direct path between radiation point source 20 and radiation detector element 42 adjoining the channel (FIG. 4(a)). The radiation beams spread out from the point source so as to strike each radiation detector element disposed on a planar array at a slightly different angles respectively, the magnitude and orientation of which depend on the position of the detector in the array. The pattern of the passages in each collimator plate is selected so that when the plates are stacked together each of the channels formed has an axis oriented along a selected orientation angle that corresponds with the path of a radiation beam from the point source to the radiation detector in the assembled imager. The number of collimator plates used in the assembly of the collimator body is dependent on the energy level and wavelength of the radiation to be collimated and hence the overall thickness of radiation absorptive material necessary to absorb radiation striking the collimator. As illustrated in FIG. 4(a), in the assembled device, collimator body 455 is disposed to adjoin radiation detector panel 40. Radiation detector elements 42 are positioned in an array on detector panel 40 and each typically comprises a scintillator coupled to a photodetector. Collimator body 455 is positioned to allow incident radiation on a direct path between the radiation source and each one of the radiation detector elements 42 to pass through the channels in the collimator. Beams of radiation that are not aligned with such a direct path strike the collimator body and are absorbed. The collimator of the present invention is readily used with either a one-dimensional or a two-dimensional array of radiation detector elements. A plan view of a collimator fabricated in accordance with the present invention and showing a representative number of channels 420 appears in FIG. 5. The figure has been marked to show left, right, upper, and lower edges solely to provide a reference for ease of discussion, and the selection and positioning of such references is not meant to consitute any limitation on the structure or positioning of the device of the invention. Channel openings 425 in the surface of the collimator closest to the radiation source are shown in dark outline and channel openings 425' on the opposite surface of collimator body 455 are shown in phantom. In the two-dimensional array the center channel is in substantial vertical alignment with the radiation source, and the opening 425' of the channel on the side of the collimator body opposite the radiation source is aligned with the opening in the surface closest to the radiation source. As the radiation beams spread out as they emanate from the point source, each of the openings 425' has a slightly larger cross-sectional area than its respective opening 425 in the surface of the collimator closest to the radiation source. Openings 425' for channels on the left, right, top, or bottom are also slightly offset from being in vertical alignment with their respective openings 425. The direct path from the radiation source to a radiation detector in the upper left hand corner, for example, is offset both to the left and to the upper side of the array. The selected orientation angle of the axis of the channel is substantially aligned with this direct path, and the channel thus extends through the collimator body at this angle. The selected orientation angle for each channel is different from any other channel in the collimator. Such a structure, which would be extremely difficult and time consuming to construct with conventional collimator fabrication techniques, is readily produced in accordance with this invention. 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.