Net thyroid count =40,000 − 2000 =38,000
= 205
Percent thyroid uptake
Standard deviation in uptake [using Eq. (4.7)]
=0.427 ×0.006373
=0.0027
% standard deviation in uptake
=0.63%
Thus, the thyroid uptake =42.7 ±0.63%.
It should be noted that although all counts were taken for 2 min, count
rates (cpm) were not used in the calculations. One can do similar calcula-
tions using count rates and obtain the same results.
Chi-Square Test
The chi-square (c^2 ) test is a useful test for verifying if the variations in a
set of measurements are due to statistical randomness of the data or due
to variations in entities, such as equipment, patients, and the like, used in
the measurements. The latter variations may be systematic, such as a fixed
voltage drop throughout the measurement or random, such as fluctuations
in voltage supply to the equipment. If there are Nmeasurements made of
a parameter, then for Gaussian distribution of the data, which is true in
radioactive measurement, the c^2 is given by
(4.8)
where Xiis the value of the ith measurement and is the average of Nmea-
surements. The value of Nshould be 10 or larger. The c^2 values are given in
Table 4.1 for various probability (p) values as a function of degree of
freedom, which is equal to the number of measurements minus one,N–1.
X
c^2 =()−
∑
XX
X
i
iN=×
0 0027
0 427
100
.
.
=×0 427 0 000011514 0 000029103..+.
=×⎛
⎝
⎞
⎠
+⎛
⎝
⎞
⎠
38 000
89 000
302
89 000
205
38 000
22
,
,, ,=×=
38 000
89 000
100 42 7
,
,
.%
st=+40 000 2000,Chi-Square Test 39