Vol. VII, No 3-4
RADIOIODINE UPTAKE MEASUREMENT
To illustrate in summary form the effects
of many of these variables in thyroid uptake
measurement Fig. 10 shows the combination
of all the factors. One kind of instrument was
used, the Mediac, to measure two different
manikins. One was a high-thyroid, low bodybackground manikin; and the other was a
low-thyroid, high body-background manikin.
In the measurement of thyroid uptake the
four standard formulas were used; the O for-
mula (without any filtration), the A formula (with an “A” filter only), the B formula
(with a thyroid eclipsing shield only), and
the AB formula. Three different kinds of
phantoms were used. One was the moot shallow possible kind of phantom; and one, the
standard phantom, was a phantom in which
the spectrum of emission from the source
matched the spectrum of emission from the
small errorr. It was found that the actual
amount of error was highly related to the
thyroid uptake, and therefore the error is
given in percentage points of uptake rather
than in per cent error. This 3 percentage
points of accuracy, however, is possible only
TUC-RANK ORDER IMPORTANCE
A
B
WITH
SPECTROMETER
CONTROL
WITHOUT
SPECTROMETER
CONTROL
The spectrum
The phantom
centimeters from the surface of the patient’s
neck to the surface of the detector). It can
be seen from this figure that when a very
shallow phantom is used, there is a tendency
for a very wide range of answers. When the
too-deep phantom is used in the hig-thyroid
type of patient, all the answers are far too
high. When a standard phantom is used,
80 %
20 &
The formula
The phantom
The formula
10 %
The standard
50 %
25 Ye
The standard
20 %
The filter area*
(for absolute values only}
2%
The collimation®
The distance®
* Te reasonahie
2 %
1%
The filter thickness? 1 %
pMCLPonsicow
| Goan”
PRA POM
patient’s neck. The measurements were made
at five different distances (from 20 to 60
14}
errs.
PHAN POM
PHANTOM,
|g aa
20
30,
400
$00
Cem
|
2c
20
3
4G
AD
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Hi-thyraid, lo-baadybackground patient
AND FORMULA HAVE BEEN STANDARDIZED
DISTANCE, PHANTOM, STANDARD, FILTERS
except where there is no filtration, usually
a reasonably good answeris given.
The results in the low-thyroid, high bodybackground patient illustrate the value of the
thyroid eclipsing shield or “B-filter’”. Even
where the phantom is improperly designed,
if a thyroid eclipsing shield is used, the result is within reason.
An important question is, how accurate
can a thyroid uptake measurement be under
ideal conditions? In running through a very
large series of measurements on many different instruments and many different manikins, it was found that with exceptionally
stable equipment it is possible to measure
the thyroid uptake within about 3 percentages point of accuracy. If the thyroid uptake
is close to 90 per cent, then this is a very
eo 30 «4630 oem 20
30
0
80 Sem
80
SOS
ES
Ee
Lo-Thyroid, Hi-Bodybackgronnd patient
Fic. 10. Summary of the effects of many variables in
thyroid uptake measurement.
under ideal conditions of artifical manikins
and easily controlled geometry. When: pa-
tients are used, this accuracy cannot be achieved. It would appear however, that with
any instruments and with any kind of pa-
tient it is possible to achieve well within 10
percentage points of accuracy and probably,
if care is taken, 5 percentage points of ac-