Patent Number: 052705498
Section: summary

FIELD OF INVENTION This invention relates to a ring collimator for a gamma ray or radioisotope camera and a method of making it. BACKGROUND OF INVENTION A typical gamma ray or scintillation camera such as described in U.S. Pat. Nos. 4,859,852, 4,584,478, 4,095,107, 4,228,515, 4,593,198, 4,782,233, 4,831,261, 4,837,439, and 5,021,667 uses a collimating device which acts as a lens to project onto a position detector a shadow of parallel, converging or diverging gamma rays from a radioisotope tracer in a patient's body. The position detector includes a scintillation crystal coupled to an array of photodetectors and associated position analysis electronics including, for example, a computer. Initially in planar cameras the collimators were made by stacking corrugated sheets alternately inverted to create a multiplicity of collimator holes in the collimation direction generally transverse to the stacking direction. An adhesive such as epoxy is used to attach the layers to each other. The collimator holes may have any desired cross-sectional shape, e.g., square, triangular, hexagonal, round. With the advent of circular or ring gamma ray cameras the same stacking technique was used, but the stack was made with a cylindrical jigging surface against one face of the collimator stack so that while the corrugated plates continued to be stacked one on top of the other, the two faces of the stack defined a curved cylindrical surface. Ring cameras normally use three or more arcuate collimator segments per collimating ring to obtain multiple views from which to reconstruct the image tomographically. These collimator rings may have parallel, converging, or diverging collimator holes and the directions of the holes may vary from segment to segment and/or layer to layer in the collimator, i.e., both in the plane of the cylindrical axis and circumferentially. While this construction technique does maintain good alignment of the collimator holes in the plane of the plates, the plates are often misaligned so that the planes of the various plates do not have the same desired parallelism, convergence, or divergence. This causes degradation in image resolution in the transaxial plane of the camera, which is particularly problematic in conventional high-resolution three dimensional cameras. This technique is also quite labor-intensive and expensive to implement because each of the collimator segments is independently fabricated and then must be mutually aligned with the others and assembled. Further, these arcuate segments are easily deformed to depart from their ideal curvature during extensive handling in fabrication of the arcuate segments, shipping, and final assembly of the segments into a cylindrical (annular) ring collimator. An even more important problem is the difficulty in precisely aligning during assembly each of the segments relative to the others in their orientation about and in the plane of the axis. Fabrication of collimator segments by this technique of stacking in the circumferential direction requires a corrugated plate for each hole about the circumference of the collimator ring and this is usually quite a large number. In the general case the collimator is much larger in circumference than in its axial extent. In another approach the curvature of the segments is achieved by stamping arcuate, often crescent-shaped corrugated sections and stacking them alternately inverted to create the collimator holes running in the direction between the crescent-shaped curved edges. These crescent-shaped plates are stacked in the axial direction, i.e., along the axis of the cylindrical collimator ring, to form a cylindrical arcuate collimator segment. This approach reduces the number of corrugated plates used if the collimator ring employs only a few segments, and thus reduces the labor and time in fabricating a particular collimator ring. This approach also improves the transaxial alignment of the collimator holes because all the corrugations for a segment plate are formed by the same forming operation. But there still remains the problem of misalignment of the plane of the plates with the desired parallelism, divergence or convergence. There also remains the problem of mutual alignment of the collimator segments at assembly so that each segment is precisely aligned relative to the other segments in their orientation about the collimator axis. And these segments too are susceptible to deformation of the curvature during handling in fabrication, shipping and assembly. SUMMARY OF INVENTION It is therefore an object of this invention to provide an improved ring collimator for a radioisotope camera and a method of making it, which is simpler, easier, less expensive and less labor-intensive. It is a further object of this invention to provide such an improved ring collimator for a radioisotope camera and method of making it in which the transaxial alignment of the collimator holes is extremely precise and independent of assembly accuracy. It is a further object of this invention to provide such an improved ring collimator for a radioisotope camera and method of making it which is much more rugged and resistant to deforming during fabrication, shipping and assembly. It is a further object of this invention to provide such an improved ring collimator for a radioisotope camera and method of making it in which the alignment of segments to one another about the axis of the collimator ring is fixed and precise, independent of the number of segments employed and independent of whether the orientation of the collimator holes is parallel, divergent or convergent. It is a further object of this invention to provide such an improved ring collimator for a radioisotope camera and method of making it in which the desired parallelism, divergence and convergence can be more easily maintained from plate to plate in each segment. It is a further object of this invention to provide such an improved ring collimator for a radioisotope camera and method of making it in which all the segments are simultaneously fabricated and assembled together in a unitary structure. It is a further object of this invention to provide such an improved ring collimator for a radioisotope camera and method of making it which eliminates a separate step of segment assembly. This invention results from the realization that a truly simple and effective yet inexpensive and rugged collimator for a radioisotope camera can be achieved by stacking in the axial direction a plurality of corrugated foils or plates made in the form of a closed annular radio-opaque plate which contains all of the segments. This invention features a method of making an annular cylindrical multi-hole collimator for a radioisotope camera including forming a closed annular radio-opaque plate having a plurality of corrugations extending from the inner to the outer radius of the plate and defining at least one collimator segment section and junction. A plurality of the plates are stacked cylindrically axially on one another with their peaks and valleys aligned to form an annular cylindrical multi-hole collimator with at least one segment, and each plate is bonded to the adjacent plate. In a preferred embodiment, a radio-opaque filler medium may be applied between the plates at the junctions between segment sections. The stacking may include arranging the plates on one another relative to alignment means. The step of arranging may include aligning indicia on the plates with guide means. The indicia may include at least one hole. Aligning the indicia may include registering a corrugation of a plate with a guide pin parallel to that corrugation. End caps may be secured at each end of the collimator stack. The invention also features an annular cylindrical multi-hole collimator for a radioisotope camera including a plurality of closed annular radio-opaque plates each having a plurality of corrugations extending from the inner to the outer radius of the plate defining at least one collimator segment section and junction, and means for bonding the plates together with their peaks and valleys aligned to form an annular cylindrical multi-hole collimator with at least one segment. In a preferred embodiment there is a radio-opaque filler medium disposed in the plates at the junctions between segment sections, and end caps may be used at each end of the collimator stack. The invention also features a method of making an annular cylindrical multi-hole collimator for a radioisotope camera including the steps of forming a closed annular radio-opaque plate having a plurality of corrugations extending from the inner to the outer radius of the plate defining at least one collimator section and junction, and forming a closed annular flat radio-opaque plate, and stacking cylindrically, axially, alternately a plurality of the flat and corrugated plates on one another to form an annular cylindrical multi-hole collimator with at least one segment, and bonding each of the plates to its adjacent plates. In a preferred embodiment there may be applied a radio-opaque filler medium between the plates and the junctions between segment sections, the stacking may include arranging the plates on one another relative to the alignment means, and the arranging may include aligning indicia on the plates with the guide means. The indicia may include at least one hole. Aligning the indicia may include registering a corrugation of a plate with a guide pin parallel to that corrugation. End caps may be secured at each end of the collimator stack. The invention also features an annular cylindrical multi-hole collimator for a radioisotope camera including a plurality of closed annular radio-opaque plates each having a plurality of corrugations extending from the inner to the outer radius of the plate defining at least one collimator segment section and junction. There are a plurality of closed annular flat radio-opaque plates; means for bonding the flat corrugated plates alternately together to form an annular cylindrical multihole collimator with at least one segment. There may be a radio-opaque filler medium between the plates and the junctions between segment sections and there may be end caps mounted at each end of the collimator stack.