Patent Number: 039379698
Section: summary

Radiation cameras typically employ a radiation collimator of some type between the raditation sensitive tranducer and the radioactive object under investigation. The most widely used radiation camera is the Anger-type scintillation camera (U.S. Pat. No. 3,011,057) which is employed in hospitals to obtain an image of the distribution of a radiopharmaceutical introduced into the body of a human patient. The purpose of a radiation collimator is to provide substantially one gamma ray transmissive passageway between each elemental volume of the radioactive object and a corresponding elemental volume of the transducer. The most commonly used collimator is the multichannel collimator which comprises a number of collimating apertures separated from each other by a volume of radiation--opaque material--most commonly lead. It is well known that radiation collimator design involves basically the parameters of aperture size and shape, septal thickness, and aperture length. These are the parameters which determine the resolution and efficiency of the collimator for gamma rays of a particular energy. In general, the septal thickness, which is the thickness of the walls separating adjacent collimating apertures, is chosen in accordance with the energies of gamma rays to be collimated so that the collimator will blcok substantially all gamma rays which enter the collimator at an angle and location such that they would otherwise tranverse the wall between two apertures. Thus the septal thickness must be relatively large for high energy gamma emitting isotopes, but for low energy isotopes the septum or wall between apertures may be quite thin. Indeed, it is desirable to employ only the septal thickness actually required for the gamma ray energy involved in order to avoid unnecessary loss of efficiency. Initially, all multi-channel collimators were of the parallel channel type. The first parallel channel collimators were cast-lead collimators. Later, extruded lead collimators consisting of comb-shaped pieces assembled to provide square-channels were designed to achieve better imaging efficiency for low energy isotopes. As imaging with technetium- 99m having a 140 KEV gamma energy began to dominate imaging procedures and improvements in resolution of radiation cameras were made, radiation collimators with very thin septa (i.e., about 0.010 inches) were needed to avoid loss of efficiency in imaging. Faced with this need, some investigators turned to the general approach of using corrugated foil of a material which is relatively opaque to low energy gamma rays. The corrugation approach would at first seem to offer a simple solution to the problem of constructing a collimator with septa of about 0.010 inch in thickness by enabling a large number of corrugated strips of metal foil to be mounted together in rows to build up a structure of the desired size. It turns out, however, that the use of a multiplicity of corrugated strips creates rather severe tolerance problems because the corrugations must be extremely uniform from strip-to-strip or they won't match up at the surfaces which are to be mated and fastened together throughout the length of each strip. The larger the collimator and the smaller the corrugation, the greater the tolerance problem. In addition, since lead is the preferred collimator material from a cost standpoint, the corrugating of lead foil which has a very low tensile strength creates an additional problem, especially when relatively large volume production is needed. The problems involved in employing corrugated lead foil in the construction of a low energy radiation collimator are solved in accordance with the present invention by mounting the corrugated strips of foil between successive straight strips of foil, thereby eliminating the basic tolerance problem in forming the corrugated strips so they can be matched up and in handling the rather easily distorted strips after they have been formed. In accordance with another aspect of this invention, this solution also leads to an improved method of constructing a corrugated-strip collimator using adhesives such as epoxy rather than welding or similar techniques. In addition, in accordance with a further aspect of this invention, the problem of forming corrugated lead strips is solved by using a highly advantageous new method in which straight strips of lead foil are corrugated by using a pair of substantially meshed gear-like members. These initially corrugated strips either are employed as is or, to produce a collimator with superior uniformity, are subjected to further forming by pressing them between male and female forming dies. The general techniques of this invention are also extremely useful in manufacturing corrugated diverging or converging collimators in an inexpensive manner. A diverging collimator has the multiple collimator channels focused at a point some distance away and arranged to diverge in the transducer to object sense so that objects larger than the transducer can be imaged. A converging collimator has the focused channels arranged to converge in the transducer to object sense so that objects smaller than the transducer are imaged in a magnified way. The first commercially available diverging collimator was designed for medium energy gamma rays because such a design employs thick septa between channels and can be made by conventional lead casting or drilling techniques. Attempts to use casting techniques to produce a low energy diverging collimator failed because void areas in the thin septa could not be avoided. An approach involving assembling about 15,000 individual thin-walled lead tubes, each having an appropriate taper to produce divergent channels, was successful, but turned out to be extremely costly to manufacture. Adapting the corrugated collimator approach to produce diverging and converging collimators reduces the manufacturing cost dramatically and produces a collimator which is virtually identical in performance to the multiple tapered tube design. To produce a collimator with converging channels a pair of gear-like members having tapered teeth are employed to produce initial corrugated strips which have tapered corrugations generally pointing to a common focus. The initial corrugated strips are pressed between a set of male and female forming dies to produce highly regular, tapered, triangular corrugations which focus to a point. These corrugated strips are mounted between straight strips, using an alignment fixture, and the taper of each strip causes the assembled strips to focus to a common line. The final collimator structure has collimating apertures which focus to a short line segment such that no observable distortion due to imperfect focus is noticeable.