Focusing collimator and method for making it

A focusing collimator has a plurality of corrugated strips which are built up in a stack. Within each stacking plane of interest, each strip is shaped in cross-section as a part of a different radial sector of a common predetermined annulus.

BACKGROUND OF THE INVENTION 
The invention relates to collimators, and more particularly relates to 
focusing collimators which are built up by stacking up corrugated strips, 
one adjacent the next. 
One example of this type of collimator is a conventional cone beam 
collimator which is built up from a plurality of identical strips. Each 
strip is corrugated to create a plurality of channels which all aim at a 
single focal point. The strips are tapered in cross-section and in the 
ideal case all the focal points of the individual strips would coincide at 
a single focal point. 
In practice, conventional construction techniques used for cone beam 
collimators do not produce collimators which focus to a single point. This 
is because the strips are identical, and are stacked in a planar fashion. 
It is geometrically impossible to maintain a punctate focal point if 
identically tapered strips are stacked up on a flat plane. This causes 
geometric distortion in the planar image and subsequent loss of resolution 
in the back-projected (or tomographic) image. This degradation--stacking 
error--is not limited to cone beam collimators; it exists in all focusing 
collimators in which corrugated strips are stacked one adjacent the other. 
Additionally, a conventional collimator has channels of constant size in 
the stacking plane. This has the consequence that sensitivity drops off 
rapidly from the center of the collimator to the edge. 
It is therefore one object of the invention to provide a focusing 
collimator which has less stacking error than do known collimators. 
Another object is to provide such a collimator which is comparatively easy 
to manufacture, particularly using automated and automatable assembly 
techniques. 
Still a further object is to provide such a collimator which has a 
reasonably constant sensitivity over its entire surface. 
Yet another object is, in general, to improve on known focusing 
collimators, and particularly known collimators which converge or diverge 
in one or two or orthagonal directions, e.g. cone beam collimators. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is provided a focusing collimator 
which comprises a plurality of corrugated strips which are of different 
shapes in cross-section. The strips are stacked up in a stacking plane, 
and the cross-sections of the strips are defined by the intersections of 
each of the strips with the stacking plane. Within each stacking plane of 
interest, each strip is shaped in cross-section as a part of a different 
radial sector of a common predetermined annulus. Because this annulus is 
common to all the strips which make up the collimator, a punctate or 
substantially punctate focal point within the stacking plane of interest 
is inherently built into the device. 
In accordance with the invention, each strip can be individually sized (as 
by automatic equipment) and then stacked on a flat surface. Alternatively, 
a plurality of strips can all be stacked (as by manual assembly) on a 
mandrel having a curvature equalling the inner radius of curvature of the 
annulus and then machined flat, as by milling. 
Each of the strips may subtend a constant solid angle. In this case, the 
channels in the collimator also subtend a constant solid angle. This has 
the consequence that the channels of the collimator increase in size from 
center to edge. This results in a more uniform sensitivity across the 
entire surface of the collimator, instead of a steady reduction in 
sensitivity from center to edge that results from using channels of 
constant diameter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The preferred embodiment described below is a cone beam collimator. 
However, the invention is not limited to cone beam collimators; it is 
equally applicable to any collimator which is built up of a stack of 
corrugated strips which focus in the stacking plane, and where it is 
desired to minimize or eliminate stacking error. 
In FIGS. 1 and 1A, a strip generally indicated by reference numeral 2 is 
shown on end and from the side, respectively. (FIGS. 1 and 1A are merely 
illustrative, and are not to scale.) The strip 2 has a plurality of 
channels 4. The channels 4 are all aimed at a common focal point 6 to form 
a two-dimensional fan beam generally indicated by reference numeral 8. 
The channels 4 are separated from each other by corrugations 10. The 
corrugations 10 may be formed in a variety of ways, but the details of 
corrugations 10 are not part of the invention. 
While the term "strip" as used herein may denote a unitary element made for 
example of a single sheet of corrugated material, the term also 
encompasses a two-element structure such as a corrugated member backed 
with a flat sheet. As used herein, the term "strip" includes all 
structures which either by themselves or in cooperation with neighboring 
ones form channels which collimate, e.g., gamma radiation. In the 
preferred embodiment, the finished collimator is intended for use with a 
gamma ray scintillation camera so that the strips 2 are made of lead, but 
this not a part of the invention. The material used for the strips 2 
depends upon the type of radiation to be collimated. 
A disadvantage of known cone beam collimators for gamma radiation is 
illustrated in FIG. 2. In a conventional finished collimator, a plurality 
of finished strips 2A-2H are stacked adjacent each other in a stacking 
plane. (FIG. 2 is exaggerated for clarity and is not to scale; there would 
always be more than 8 strips in a collimator and their misalignment would 
never be so pronounced.) Each of the finished strips 2A-2H in the 
collimator is identical to all the others, and each has an individual 
focal point 6A-6H. As can be seen in FIG. 2, the focal points 6A-6H do not 
coincide in the stacking plane; they are spaced from each other. 
This is because during fabrication of a conventional collimator, the 
identical finished strips 2A-2H are stacked one adjacent the other on a 
flat surface represented by line 12 and are bonded together in a finished 
unit. As more strips are added, the cumulative error of focusing 
increases. This decreases resolution of a backprojected image. 
As is schematically shown in FIG. 3, individual semi-finished strips 
14A-14K in accordance with a preferred embodiment of the invention are all 
identically shaped and are all different radial sectors of a common 
predetermined annulus generally indicated by reference numeral annulus 16. 
In their side views, each of the strips 14A-14K is corrugated in the same 
way as is the strip 2 in FIG. 1A. A single focal point, namely the center 
16A of the annulus 16, is therefore built into the assembly. In accordance 
with invention, these individual semi-finished strips 14A-14K can be used 
to manufacture a cone beam collimator in one of two preferred ways. 
The first preferred method is illustrated in FIG. 4. In this case, the 
semi-finished strips 14A etc. are all stacked, as by hand, on a mandrel 
18. The mandrel 18 has a cylindrical cross-section and has the same radius 
of curvature (and the same center 16A) as the inner radius of curvature of 
the annulus 16. The semi-finished strips 14A etc. are bonded together (as 
by adhesive) so that they form a unitary structure. This is then milled or 
otherwise machined along the two parallel planes 20 and 22 to form a flat 
plate. 
Another alternative is illustrated in FIG. 5. Here, the semi-finished 
strips 14A, etc. are shaped, as by automatic machinery, in accordance with 
their intended positions before they are stacked and bonded together. For 
example, finished strip 24A is formed by cutting down one of the 
semi-finished strips 14 in the manner shown in FIG. 3A, strip 24I is 
formed by cutting down one of the semi-finished strips 14 in the manner 
shown in FIG. 3B, and so forth. Each individual strip to be included in 
the collimator is appropriately dimensioned, and stacked up on a flat 
plane 26. All the finished strips 24A etc. are bonded together (as by 
adhesive) to form the finished collimator. 
A single strip from a known cone beam collimator may differ but slightly, 
or even imperceptibly, from a single strip from a finished collimator in 
accordance with the invention. Thus, FIG. 1 can equally well be said to 
illustrate a known collimator strip and a strip for making a collimator in 
accordance with the invention. Further, two strips from a collimator in 
accordance with the invention may appear identical. This is because the 
geometrical relationships in accordance with the invention do not reside 
in the dimensions of any one particular strip, but rather in the overall 
relationship between the dimensions of all of the strips in the 
collimator, taken as a class. In accordance with the invention, there is a 
relationship between the cross-sectional shape of each strip and the 
position of that strip in the finished collimator. While this relationship 
may not be observable from a comparison of neighboring strips because of 
manufacturing tolerances, it is nonetheless true of all the strips, taken 
as a class. 
Because each of the illustrated finished strips is a radial sector, each is 
wider at its radially outward end than at its radially inward end. This 
means that, as shown in FIG. 1A, the channels in each finished strip are 
wider at one end than at the other. This relationship is preserved in the 
illustrated preferred embodiment, so that at the edges of the finished 
collimator, the channel openings are wider than they are at the center. In 
this preferred embodiment, each channel subtends a constant solid angle, 
(i.e. is conical) so that sensitivity remains constant over the entire 
surface of the collimator rather than dropping off from center to edge. 
It is not necessary that each of the semi-finished strips 14A etc. subtend 
the same solid angle. As long as all of them are radial sectors of a 
common predetermined annulus, they can subtend different angles and yet 
retain the punctate focus of the preferred embodiment discussed above. 
Thus, for example, the strip 14A can have half the angular width of strip 
14B. This can be selected when, for example, it is desired to have 
cylindrical channels in each strip rather than conical ones. 
Those skilled in the art will understand that changes can be made in the 
preferred embodiments here described, and that these embodiments can be 
used for other purposes. Such changes and uses are within the scope of the 
invention, which is limited only by the claims which follow.