Methods of Collimator Fabrication

1. Field of Invention

This invention relates in general to grid-like structures of the type suitable for use as collimators for shielding radiation receiving and imaging devices from the effects of distorting radiation, and more particularly to structures of the above type suitable for use with high energy, i.e. 100 to 1,000 KEV, radiation.

2. Summary of Prior Art

The use of such structures as collimators is well known as may for example be seen from the Anger camera case. This device is a special type of radiation receiver used by the medical profession to locate and judge the extent of diseased tissue within a patient's body by the creation of photograph-like images of radioactive concentrations therein. A radioactive material is injected into the patient's bloodstream or administered orally which tends to collect in the diseased tissue. Formation of an image of an object which is a radioactive source and which therefor is its own source of radiation, however, presents a situation nonanalogous to formation of an image of an object which is illuminated by common light, or even X-rays, from a separate source, as in conventional photography. In order to get a clear image of a radioactive concentration a selection must be made from the rays emanating from the concentration in all directions of those rays which will clearly produce the image. This selection may be made so as to produce an enlarged, a miniaturized, or a same-size image of the concentration, but in all cases nonselected rays must be kept from the receiver. A collimator of a radiation absorbing material such as lead has been found to perform the selection function well and is presently used with all such devices for this purpose.

The Anger camera has thus become a significant medical tool both for diagnostic purposes and as a means to facilitate surgery by decreasing exploratory time because the spatial location of the diseased area is precisely known and by assuring all diseased tissue is found because the precise extent of the diseased area is also known.

Presently the above-described units are used with radiation energy levels of about 150 KEV, and many types of collimators have been produced for this energy level which are operationally effective and relatively efficiently manufacturable. An example of one such collimator is a number of corrugated sheets of lead approximately 0.010 inch thick having flattened ridges, sealed together by epoxy cement in a ridge-to-ridge configuration. Units of this type are particularly useful in examinations using a scintillation camera. Various methods have been tried to increase the clarity of the images formed by collimators, but for one reason or another each was unsatisfactory.

For example, casting the collimator as a single unit using removable pins in the mold to provide the holes has been tried. This method while producing an operational device is impractical since due to high friction between the cast lead and the pins and the fact that some collimators are convergent or divergent (to allow enlarged or miniaturized image formation) relative to the radiation source each of the pins used to create the holes must be removed individually. This process is time consuming and costly, especially when one realizes that some such collimators have 1000 or more such holes.

A second exemplary attempt was to form corrugated lead sheets and assemble them between successive straight strips of foil or ridge to ridge as described above. This alternative sometimes failed due in this case to joint leakage i.e. the epoxied joints are permeable to radiation and since these joints are adjacent to each other in a straight line in this case too much distorting radiation reaches the receiver. Further, attempts to avoid this problem in this alternative by creating an overlap raised insurmontable technical assembly problems.

The present invention solves the above problem by taking advantage of the subtle fact that joint leakage is only a problem with respect to rays which are substantially non-parallel with the holes. Stated in slightly different terms, this means that the penetration of rays substantially parallel to those passing through the holes through the joints do not effect the image enough to cause concern. Thus, it was found that the successful operational characteristics of the single unit casting may be successfully approximated using modules adapted to fit together to form a grid-like pattern with a series of mortis-tenon type joints and that successful units are thereby possible at essentially all energy levels, the only limiting factor being the sophistication of the module fabrication method used. The details of two exemplary embodiments of this invention are set forth below.

It is thus an object of the present invention to provide a collimator suitable for use with essentially all energy levels or radiation which is modular in construction thereby avoiding the problems of single unit casting, yet which is easy to fabricate and assemble, and which has no passable path for distorting rays.

It is also an object of the present invention to provide a method of collimator manufacture which is efficient at production rates.

Further, it is an object of the present invention to provide a collimator which may be easily adapted to fit within any desired overall shape and which may be given any optimum hole shape chosen.

These, as well as other features, objects, and advantages of the present invention, will be more clearly understood by reference to the following detailed description of two exemplary embodiments of the present invention and to the drawings in which:

FIG. 1 is a plane view of an assembled collimator in accord with the present invention suitable for use with an Anger camera,

FIG. 2 is an enlarged perspective view of a portion of a cast collimator module in accord with the first exemplary embodiment of the present invention;

FIG. 3 is an enlarged cross sectional view of two modules in accord with the first exemplary embodiment of the present invention in assembled configuration;

FIG. 4 is a cross-sectional view of a portion of two modules in accord with the first exemplary embodiment of the present invention in assembled relation defining round holes;

FIG. 5 is a cross-sectional view of a portion of two modules in accord with the first exemplary embodiment of the present invention in assembled relation defining hexagonal holes; and

FIG. 6 is a cross-sectional view of a portion of two modules in accord with the second exemplary embodiment of the present invention in assembled configuration.

In providing a collimator as shown in FIG. 1 suitable for use with high energy radiation, 150 to 1000 KEV, the present invention specifically recognizes that a collimator cast as a unit in the configuration of FIG. 1 is the best known high energy collimator from an operational standpoint. It is also known from low energy work that modularization presents great economies in the efficiency and flexibility of production it allows. The present invention thus combines these divergent concepts in such a way as to optimize both operational and production efficiency.

FIG. 2 shows as a first exemplary embodiment a cast module for the above purpose. As used herein the term "cast" is specifically comtemplated to include die casting, permanent mold casting, powdered metal techniques, extruding, lead filled epoxies, and other similar fabrication methods. From the lower side 2 of the base portion 4 of this module a first plurality of columns indicated at 6 project at spaced intervals parallel to each other. Each of these columns is of substantially rectangular cross section and extends from the top 8 to the bottom 10 of base portion 4. Similarly, a second plurality of columns indicated at 12 project from the upper side 14 of the base portion 4 of this modular in the area directly opposite and channels 16 formed by the columns 6. The columns 12 are also parallel to each other, extend from the top 8 to the bottom 10 of base portion 10, and are of substantially rectangular cross-section. (Note: In the preferred case, columns 12 taper somewhat along their height dimension 18.) The following chart indicates what I have found to be the preferred dimensions for such a module for two given radiation ranges.

Given the above described modules then, the present invention contemplates that the outer edge 32 of the columns 12 of one module be inserted in and affixed within the channels 16 of a second module, as shown in FIG. 3 in a series of modified form mortis-tenon joints, and so on until the collimator grid structure generally indicated at 33 desired is complete. The affixation mentioned above is contemplated to be simple press-fitting, but also may be cemented especially in the case where the tolerences set for the slight taper of the columns 12 are too large to assure consistantly tight press-fitting.

FIGS. 4 and 5 indicate two alternative hole shapes of the many which a person skilled in the art might desire. The important pont is that the mortis-tenon relationship between the columns 12 and channels 16 must be maintained. Otherwise one is limited only by the practical feasibility of casting the desired indentations into the sides 34 of the columns 12, the portions of the upper side 14 of the base 4 between the columns 12, and the upper face 36 of the columns 6.

In providing a collimator as shown in FIG. 1 suitable for use with radiation generally in the range of 100 - 150 KEV, I have found, however, that a foil module may be preferable as such reduses the septa thickness thereby increasing efficiency and sensitivity, eases manufacture, and reduces cost. FIG. 6 shows a second exemplary embodiment of the present invention adapted for use in the foil context. The modules of this embodiment, generally indicated at 42, are composed of a foil of material on the order of 0.010 inch thick which is opague to the radiation to be used bent such that a series of ridges 43 and channels 45 are formed on either side of the foil sheet 44 and 46 respectfully. Each such channel 45 is contemplated to be slightly trapezoidal in shape, i.e. open end 48 being only slightly larger than closed end 50, the distance indicated at 52 being substantially equal to the outer measurement 54 of closed end 50. These modules are then contemplated to be assembled by inserting the ridges 43 on one side 44 of a first foil module into the open ends 48 of the corresponding channels 45 on side 46 of a second foil module and affixing them in place. It is clear that in so doing the ridges 43 on side 46 of the second module will also be inserted and affixed within the open ends 48 of the corresponding channels 45 on side 44 of the first module. Further, the trapezoidal shape of the channels assures a correct grid pattern as shown in FIG. 1 will result.

The collimator assembly in either case is then completed by locking the assembled grid structure 33 into a frame representatively shown at 40 filling the open areas 38 between the grid 33 and the frame 40 with lead or some other shielding material.

It should be understood that the embodiments and practices described and portrayed herein have been presented by way of disclosure, rather than limitation, and that various substitutions, modification, and combinations may be effected without departure from the spirit and scope of this invention in its broader aspects. For example, the columns of each module need not necessiarly be parallel to each other nor need they define channels which are perpendicularly orientated with respect to the top 8 and the bottom 10 or which are of substantially contant width and depth from the top 8 to the bottom 10. Also, the use of such collimators is specifically contemplated to extend beyond the above recited Anger camera example to scanners and other radiation receiving equipment, and in some contexts to radiation producers as well.