An appartus is described for scanning relatively small near sources of penetrating radiation to obtain the energy distribution thereof, wherein a collimator assembly is interposed between a radiation source and a radiation detector. The collimator assembly has a plurality of plates aligned in parallel planes with respect to a common axis normal to their centers. All the plates have similar random distribution of apertures. All apertures on each plate are the same size. The size of respective plates, the size of the apertures of said respective plates, and the spacing between said respective plates vary precisely according to a predetermined ratio to produce radiation channels which converge to a focal point. The radiation incident to the radiation detector is maximized when the focal point and the radiation source are co-incident

Hoover, R. B.

Underwood, J. H.

English

NASA Technical Reports Server

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MAR 141975
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https://ntrs.nasa.gov/search.jsp?R=19750011544 2020-03-19T20:49:03+00:00Z
United States Patent
Hoover et al.
[in 3,869,615
[45] Mar. 4, 1975
[54] MULTIPLATE FOCUSING COLLIMATOR
[75] Inventors: Richard B. Hoover, Huntsville, Ala.;
James H. Underwood, Palos Verdes
Peninsula, Calif.
[73] Assignee: The United States of America as
represented by the Administrator of
the National Aeronautics and Space
Administration, Washington, D.C.
[ 2 2 ] Filed: June 28, 1973
[ 2 1 ] Appl. No.: 374,441
[52] U.S. Cl 250/508, 250/510, 250/505
[5 I ] Int. Cl G21f 5/00
[58] Field of Search 250/355, 508, 505, 513,
250/482, 510
[56] References Cited
UNITED STATES PATENTS
2,133,385 10/1938 Freeman 250/508
2.638.554 5/1953 Bartow et al 250/355
2.665,387 1/1954 Bartow 250/508
2.741.710 4/1956 Bartow et al 250/508
2.959,680 11/1960 Green 250/513
OTHER PUBLICATIONS
"Construction and Testing of Large Area X-Ray Col-
limators," Stewart et al., J. Phys., Vol. 41, No. 12,
( 1 9 7 1 ) , pp. 966-968.
Primary Examiner—James W. Lawrence
Assistant Examiner—B. C. Anderson
Attorney, Agent, or Firm—George J. Porter; L. D.
Wofford, Jr.; John R. Manning
[ 57 ] ABSTRACT
An appartus for scanning relatively small near sources
of penetrating radiation to obtain the energy distribu-
tion thereof, wherein a collimator assembly is inter-
posed between a radiation source and a radiation de-
tector. The collimator assembly has a plurality of
plates aligned in parallel planes with respect to a com-
mon axis normal to their centers. All the plates have
similar random distribution of apertures. All apertures
on each plate are the same size. The size of respective
plates, the size of the apertures of said respective
plates, and the spacing between said respective plates
vary precisely according to a predetermined ratio to
produce radiation channels which converge to a focal
point through corresponding ones of the similar ran-
dom distribution of apertures of the plates. The radia-
tion incident to the radiation detector is maximized
when the focal point and the radiation source are co-
incident.
10 Claims, 3 Drawing Figures
FOCAL
POINT
; I
.. i ' A l E N T t O H A R 4 IS;5 3.869,615
FOCAL
POINT
FIG.i
20
FIG.3
DETECTOR
48
47
RADIATION SOURCE ^/
X40
3,869,615
MULTIPLATE FOCUSING COLLIMATOR
ORIGIN OF THE INVENTION
The invention described herein was made by employ-
ees of the U.S. Government and may be manufactured
and used by or for the Government of the United States
of America for governmental purposes without the pay-
ment of any royalties thereon or therefore.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a focusing optical collimator
capable of high resolution scanning of relatively small
near sources of penetrating radiation. It has particular
use in the electromagnetic radiations, including high
energy X-rays and gamma rays, as well as with neutrons
and particles.
2. Description of the Prior Art
It is often desirable, particularly in the field of nu-
clear medicine, to determine the energy distribution of
a small near source of radiation by obtaining an image
thereof through the use of a collimator. Previously, this
has been accomplished by "focussing collimators" such
as that described in "A Focussing Collimator for Re-
search in Scanning," Journal of Nuclear Medicine 3, 10,
1962. These devices consist of a sheet of lead or suit-
able material containing an array of conical channels
oriented such that their lines which pass through their
centers all converge to a point. This device is then used
to scan a region believed to contain a radiation source.
When the device is oriented such that this 'focal point'
coincides with a source the signal received at the scin-
tillator is maximized. A major disadvantage with the
prior art is that leakage through the septa separating
adjacent channels produces a rather broad point spread
function and thereby degrades the resolution.
Applicants' copending patent application entitled
"Multiple Plate—Multiple Pinhole Collimator," Ser.
No. 51,317 filed June 30, 1970, now U.S. Pat. No.
3,749,911, discloses a collimator that transmits radia-
tion traveling parallel to its optical axis and eliminates
off-axis radiation. The collimator consists of a plurality
of similar sized plates, all of which are pierced with an
identical random array of similar sized apertures. The
plates are mounted perpendicular to a common or opti-
cal axis with like apertures on consecutive plates axially
aligned. The collimator, when interposed between a
near source of radiation and a penetrating radiation de-
tector, transmits radiation traveling parallel to its opti-
cal axis to produce an image that is an array of dots cor-
responding to the apertures in the plates and confined
to the boundaries of the source. To obtain an accurate
representation of the intensity distribution of the
source (i.e., fill in the image) the collimator may be
translated perpendicular to its optical axis. During this
translation the source and detector remain motionless.
SUMMARY OF THE INVENTION
The present invention relates to a novel collimator
assembly of the spaced plate variety which consists of
a sheet of lead or other suitable material wherein each
plate is pierced with an identical random orientation of
apertures. While the orientation of each array of aper-
tures is identical, the size of apertures and the area cov-
ered by each plate changes between successive plates,
and the spacing between adjacent plates is fixed so as
'°
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to produce radiation channels which converge to a sin-
gle focal point.
Accordingly, it is an object of the present invention
to provide an optical collimator which is capable of
high angular resolution in scanning relatively small
near sources of penetrating radiation and discerning
the energy distribution therein.
Another object of the invention is to provide a colli-
mator which may be used with electromagnetic radia-
tion particularly high energy X-rays, gamma rays and
with neutrons.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded pictorial view of the collimator
assembly according to the invention with parts broken
away to illustrate the radiation channels obtained with
the present invention;
FIG. 2 shows the manner of assembling the multi-
plate collimator of FIG. 1; and
FIG. 3 illustrates a radiation detector system using
the collimator assembly of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
The invention consists of a multiplicity of plates,
each of which is pierced with a plurality of apertures.
The apertures have the same relative orientation on
each plate. The apertures are all the same size the same
relative orientation on each plate but the aperture size
and spacings change from plate to-plate in a very pre-
cise manner, so as to produce radiation channels which
converge to a single point.
FIG. 1 shows an assembly 20 comprising four plates
21 through 24. Each of plates 21 through 24 has a simi-
lar array of randomly located apertures 25 through 28
respectively.
The size of each plate and the apertures contained
therein are related to the size of the other plates and
the apertures contained therein according to a prede-
termined ratio, determined by certain parameters of
the assembly. There is a constant linear rate of de-
crease of corresponding parameters of plates 21
through 24, with all parameters being focused to the
focal point 29 of the assembly.
Thus if plate 21 comprises a rectangular plate with
sides having lengths of al and bl, and defines random
distribution of similar apertures 25 comprising circles
having a diameter dl, plate 22 comprises a rectangular
plate with sides having lengths a2 and bl which are less
than al and bl, and defines a similarly configured ran-
dom distribution of circular apertures 26 of diameter
dl which is less than dl, in accordance with the follow-
ing ratios, all of which are based on well known princi-
ples of geometry including the law of similar triangles
which governs the proportional relationship of corre-
sponding parts of similar triangles:
(d\ld2) = (al/a2) = (b\lb2)
The same ratios are also valid between plates 21 and
23, plates 21 and 24 and, in general, between any of the
plates comprising the assembly. Thus
and
= (al/a4) = (M/M)
The ratio of the spacings between the focal point and
the plane of any two plates is equal to the ratio of the
aperture diameters of corresponding plates. Thus if SI
designates the spacing between plate 21 and the focal
point, S2 the spacing between plate 22 and the focal
3,869,615
point, and S3 the spacing between plate 23 and the
focal point, the following ratios are valid:
(Sl/rfl)=(S2/</2) = (S3/</3)
The result of fabricating the plates of the assembly to
conform to the ratios described above is that all corre-
sponding plate parameters converge to a common focal
point 29, which produces radiation channels as ex-
plained hereafter which converge to a single point.
The plates 21 through 24 may be produced to meet
the above-discussed ratios using conventional photo-
etching techniques. Thus'a master photographic plate
can be used in conjunction with a photographic en-
larger to focus the image of the apertures of the plates
to be photoetched. At different enlargements, aperture
size and rectangle area can then be maintained accord-
ing to the predetermined ratio, while maintaining the
identical orientation of the random array of apertures
on all plates. See R. B. Hoover, American Journal of
Physics, Vol. 9, pp. 861-876 (1969) for a discussion of
techniques for fabrication of plates containing a ran-
dom array of apertures of various shapes.
The plates may also be constructed such that the
thickness thereof is also governed by the predeter-
mined ratio. Although this is not required, it is useful
for high energy applications.
To form a complete collimator, the plates 21 through
24 are assembled in aligned relationship with respect to
the perpendicular axis 30, extending normal to their
centers as shown in FIG. 2. Rods 31 extend through
holes 32 defined in the corners of the plates, and nuts
33 are fitted to the threaded ends of the rods to hold
the assembly together. Spacers 34 through 36 are inter-
posed between adjacent plates 21 through 24 to pro-
vide the spacing required by the predetermined ratio
discussed heretofore. The final alignment of the aper-
tures of the plates is thus established by the engage-
ment of the edges of holes 32 with rods 31 and with the
use of spacers 34 through 36 constructed in accor-
dance with the predetermined ratios. Since the edges of
the holes 32 are formed photographically from a com-
mon master which insures a near-perfect identity of lo-
cation between individual pieces and since the rods 31
can be easily made nearly perfectly linear, the align-
ment of corresponding apertures of the plates within
the assembled collimator is accurate.
FIG. 3 illustrates an application to which the collima-
tor 40 according to the present invention is particularly
suited. This particular apparatus is designed to scan a
source of penetrating radiation 41 to discern its energy
distribution. Radiation source 41 may be gamma rays
and X-rays. The radiation emitted from radiation
source 41 passes through collimator 40 and is incident
to radiation detector 42. The detector has an output
which varies with the magnitude of the incident radia- ,,
tion and may comprise for example a scintillator. Since
the radiation channels defined by the apertures of the
collimator 40 interposed between source 41 and detec-
tor 42 converge to a single point, the focal point
thereof, orientation of the collimator 40 such that its
 6Q
focal point coincides with the radiation source 41 will
maximize the radiation received by the detector 42.
Conventional means can be employed to change the
position of the collimator 40 during scanning. For ex-
ample, a stepping motor 45 may be connected to a piv-
 65
oted collimator support 47 by means of link 48, so that
the operation of the motor 45 causes the collimator 40
to incrementally scan the source of radiation 41.
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What is claimed is: *
1. In an apparatus for scanning small near sources of*
penetrating radiation to obtain the energy distribution
thereof, wherein a collimator assembly is interposed
between a radiation source and a radiation detector,
the improvement comprising:
a collimator assembly having a plurality of similar
plates aligned in parallel planes, said plates having
a common axis through all their respective centers,
said axis being normal to said parallel planes; each
said plate having a plurality of apertures the same
size, the plurality of apertures on each said plate
having a similar random distribution; the relative
size of said plates and the spacing between said re-
spective plates being arranged to produce radiation
channels which converge to a focal point through
corresponding ones of the similar random distribu-
tion of apertures of the plates, the diameter of the
apertures and the length of the sides of said respec-
tive plates being directly proportional to the dis-
tance from said respective plates to the said focal .
point, the radiation incident on the radiation detec-
tor being maximized when the said focal point and
the radiation source coincide.
2. The apparatus as recited in claim 1 further com-
prising:
support means to maintain the plurality of plates in
aligned position, and spacing means interposed be-
tween successive plates to maintain the required
spacing therebetween to satisfy the predetermined
ratio.
3. The apparatus as recited in claim 1 wherein the
ratio of the diameters of the apertures of any two plates
is equal to the ratio of the corresponding sides, respec-
tively, of the same two plates.
4. The apparatus as recited in claim 1 wherein the
ratio of the distance from said focal point to a first plate
to the distance from said focal point to a second plate
is equal to the ratio of the length of a side of said first
plate to the length of the corresponding side of said sec-
ond plate.
5. The apparatus as recited in claim 4 further com-
prising:
support means to maintain the plurality of plates in
aligned position, and spacing means interposed be-
tween successive plates to maintain the required
spacing therebetween to satisfy the predetermined
ratio.
6. A collimator assembly for pentrating radiation
comprising:
a plurality of similar plates aligned in parallel planes,
said plates having a common axis through their
centers, normal to said parallel planes, each said
plate having a plurality of equal-sized apertures ar-
ranged in a similar random distribution; the relative
size of said plates and the spacing between said
plates being arranged to produce radiation chan-
nels which converge to a focal point through corre-
sponding apertures of said plates; the diameter of
the apertures and the length of the sides of said re-
spective plates being directly proportional to the
distance from said focal point to said respective
plates.
7. The collimator assembly recited in claim 6 further
comprising:
support means to maintain the plurality of plates in
aligned position, and spacing means interposed be-
3,869,615
tween successive plates to maintain the required
r-' spacing therebetween to satisfy the predetermined
ratio.
8. The collimator assembly as recited in claim 6
wherein the ratio of the diameters of the apertures of
any two plates is equal to the ratio of the corresponding
sides, respectively, of the same two plates.
9. The collimator assembly as recited in claim 6
wherein the ratio of the spacings between the focal
point and the plane of any two plates is equal to the 10
ratio of the aperture diameters of corresponding plates.
10. The collimator assembly recited in claim 9 fur-
ther comprising:
support means to maintain the plurality of plates in
aligned position, and spacing means interposed be-
tween successive plates to maintain the required
spacing therebetween to satisfy the predetermined
ratio.
* * * * *
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Multiplate focusing collimator

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