80
FOR:
a
t
"22"
_
4# fa
“a
a
40 cm
=
B=
——_—__—_—_+]
ko,
|+-_— 20em
Net
CITTTE
etal
0.25
0.!
= 10
(&,=450 key}
4
Re = 3.361
x
I
|
Ri,
= 2.299
F
=i,462
RA,
= 3.026
Fo
o= ft.ill
R,
= 3.272
~
|
1.027
a)
[
I
1
l
|
x
|
|
(2)
[
Il
t
I
]
wt
me
t
q
1
Fig. 60—Comparison of single, double, and quadrupole point source approximations to a uniform line source
f, thus obtained, for a choice of parameters typical of
many counting systems, is compared with the response
of one-, two-, and four-point sources having equal
total intensity and distributed symmetrically. It is
found that two sources result in a response which is
11% low, three sources (not illustrated)—d %, and four
sources—2.7 %. There is one big drawback. The point
source nearest the detector contributes 56% of the
total response for four sources, and the first two contribute 82%. They must, therefore, be located accurately at the designated places, but of course in many
phantom systemsthis is impractical, if not impossible.
On the other hand, one might envision a phantom
system in which a uniform distribution within any
selected organ, or indeed any arbitrary activity distribution, could be simulated by a set of equal point
sources located at uniquely determined points of a
three-dimensional grid or coordinate system. A computer program permitting a systematic investigation of
organ size, shape and composition based upon such a
concept has been reported by Snyderet al.
We are inclined to believe that a more useful approach is to determine the most probable response and
the statistical distribution about the mean resulting
from successively large numbers of points locai
random. One mighttry to do this empirically by p
a sufficient number of beads in a suitable enc]
shaking them up thoroughly, and observing tl
sultant count rate. The question of what constit
“sufficient number, and what is the distribution
tion would then be evaluated from a large num!
repeated trials. A more sophisticated approach
involve Monte Carlo simulation by computer. In
gation of the properties of an ensemble of sc
along these lines is continuing.
In conclusion, an alternate system for the safe
ing of small amounts of radioactivity into phat
has been described. We feel that the method has
improved flexibility (being capable of simulating
about anyarbitrarily chosen source configuration
safety. Further analysis of the random spatial «i
bution assumed by multiple sources, and the resu
detector response in realistic, three-dimensional «
dinates is necessary in order to realize its full pote:
REFERENCES
1. Grotenhuis, I. M. Properties and Uses of Radiating 4
spheres. Radioactive Pharmaceuticals, Ed. G. A. An+