130
Acta Radioldgica
Interamericana
M. BRUCER
By 1950 the National Bureau of Standards
in the United States in cooperation with
many other national bureaus had been able
to calibrate a sample of iodine with a high
degree of precision. This intercalibration was
on the basis of purely physical techniques
and was a laboratory procedure. The com-
are arbitrarily defined as anything under 100
kev; the low energies are defined as those
up to 250 kev; the medium energies are those up to 500 kev; and the high energies are
the rest of the spectrum. The high-energy
peak shown in the spectrum consists of two
gamma photons that are not very important
BABBITT METAL SHIELDS
DIFFERENCE BETWEEN
“TOO THIN" AND
"TOO _——
.
;
.
No F (TH)
ENERGY SPECTRA
of
TODINE 131
“. on reain in glam
AND
10.541 ta.) sire
Na HT)
ENERGY SPECTRA
". FQOINE 131
ae:
— MOCK. IODINE
on resin in bobbirt
#*
e
=
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“ONERGIER
:
.
‘HIGH -
ENERGIES
LecceeeROD kee. BS a ker,8thChore
Fig. 1. Comparison of the sodium ivdide speetra of iodine 131 and mock-iodine.
mittee decided in 1954 that one of the reasons for the wide variation in results was
the lack of a similar intercalibration stan-
dard that met clinical requirements. It was
felt that a clinical intercalibration procedure,
which would be adaptable to all the various
techniques in use, was necessary. Such a cli-
of Totat Area
~
.
o
Ole
.
0,3
xeRay
LOW
ENERGIES | ENERGIES
100 kaw
MEGIUM
ENERGIES
250 kev
‘hy
500 kev
sce
ENERGIES
$5th Channel
Fig. 2. The results of interposing succesive layers of
babbit metal between raw mock-iodine and the detecting crystal.
in thyroid uptake measurements because they
contribute so little to the total number of
counts recorded by most instruments. The
medium-energy peaks are the most promi-
to simulate the gamma-ray spectrum from
nent in the iodine spectrum. The low energy
peak consists mostly of Compton scattered
radiation. The peaks in the range arbitrarily
called X-rays energies consist both of photons emitted by the iodine and scatter from
interactions in both the instruments and the
mock-iodine are compared. In this figure it
is seen that when radioactive iodine is “view-
trum matches this iodine spectrum remarkably well. The high-energy peak is slightly
higher but this will not be picked up by
any instruments except complex spectrometers, which are seldom used for thyroid ra-
nical intercalibration demanded the use of
a long-lived isotope. The short half life of
radioactive todine was one of the main deterrents to clinical intercalibration.
A jong-lived gamma emitter can be made
iodine 131. This material is now called mockiodine and consists of the proper mixture of
barium 133 and cesium 137. In Fig. 1 the
sodium iodide spectra of iodine 131 and
ed” by a sodium iodide crystal, it emits a
number of gamma photonsof different energies. An arbitrary division has been made on
the spectrum to divide the energies emitted
into four classifications, The X-ray energies
surroounding media. The mock-iodine spec-
dioiodine uptake measurements. The medium-
energy peaks show the major difference between 10dine and mock-iodine. There is too
much of the 280 kev radiation and too little of the 360 kev radiation in mock-iodine.