136
M. BRUCER
absorption andscatter is to look at the shape
of the spectrum.
Figure 6 shows the pulse-height spectrum
from an iodine 131 point source. Superimposed over this is the pulse-height spectrum
from a patient’s neck in which a thyroid
gland was present. These two spectra are ar-
Acta Radiolégica
Interamericana
taining 100 ml of iodine 131 solution The
size and shape of these nine standards clo-
sely simulate most of the standards that are
In current use.
The effect of the size and shape of these
standards is shown in (Fig 8). When a
5 cc test-tube sample is used as the base line
7
SCALE
N@tiTl] CIFFERENTIAL PULSE MEIGHT SPECTAUM
tant
WE FULL
ru
+ ARTE ETHC SCALE © COUNTE ere
153
m
WORMALIZED #O1NT
ts
ETM ot8H
10 wt
Fuh
THE DIFFERENCE SPECTRUM
HIRE? OF OER!
rn
.
~ ee
ARY
GONE SPECTR, 4 MINUS IODINE POINT SOURCE
Fes dd. HEIGHT
x
.
.
“
.
ee
-
-
.
p
7
‘
oot
oo
-
TEST Tenet
Smt Using SAMPLE
FUEL
*
;
go
Fic. 6. Pulse-height speetrum of a patient's neck super-
Fic. 7. The 100 per cent total dose standards used in
the survey manikin kit.
spectrum.
for comparison, there is very little change in
major peaks. When the point-source spec-
trum is subtracted from the extended source
Spectrum, it is possible to illustrate the differences that have cccurred by drawing out
what can be called a first-order difference
spectrum In (Fig 6) this differente spectrum shows that there is a great excess of
low-energy radiation produced in the patient's
neck. There is some absorption of the 640
kev peak but, it should be remembered, the
differences that might occur in the 640 kev
peak disaspear on the graph because the two
spectra have been normalized at this point.
With the shape of this difference spectrum kept in mind, we can look at what happens when the size and shape of the 100
per cent total dose standards that are commonly used is varied. Figure 7 shows the
selection of 100 per cent total dose standards
uscd in the survey manikin kit. These range
from a very small capsule containing 0.4 ml
of iodine 131 sclution, to a paper cup con-
'
;
i
iI
<j
PAPER CUR
100 mi
ve FULL
ood
bitrarily normalized at the 640 peak, and
the spectrum is smoothed to show only the
|
.
_ atm
imposed over the pulse-height spectrum from an iodine
131 point source to give a first-order difference
|
BEAKER |
(OO m4
Stfash
o1a@
(3)
BEAKER
—iqpiwe 13) POINt $ouRcE |
PATENT NECK WiTK TRYROS
eto eae
_
aly
its difference spectrum and that of the large
and small beakers and the paper cup. However, when the test-tube sample is compa-
red with a commonly used gelatin capsule
(in two different kinds of phantoms) it is
seen that there is a tremendous change in
the difference spectra. It is interesting that
none of these spectra even closely resemble
the spectrum from a patient’s neck. Therefore, when one is using any of these standards in air, or even many of the very carefully constructed neck-shaped phantoms, one
is not comparing the same things, and the
[00 per cent dose standard is not truly a
standard. During the course of our investigation of these spectra, we had almost reached the conclusion that a phantom for the
patient's neck could not be designed. Recently, however, a method has been devised that
makes it appear that such a phantom can be
designed but that the specifications will have
to be exceedingly strict. A number of these
phantoms have been made for clinical trial
by the Thyrcid Uptake Calibration Committee and they now being sold commercially.
;