Physics and Radiobiology of Nuclear Medicine

Problem 4.3
A radioactive sample and the background were counted each for 5 min and
found to give 8000 counts and 3000 counts, respectively. Calculate the net
count rate, its standard deviation, and percent standard deviation.

Answer

Net count rate = 1600 − 600 =1000 cpm

Using Eqs. (4.3) and (4.4)

≈21 cpm

Thus, the count rate of the sample is 1000 ±21 cpm

Problem 4.4
A thyroid patient is given a^131 I–NaI capsule to measure the 24-hr thyroid
uptake. The 2-min counts are: standard, 90,000; room background, 1000;
thyroid, 40,000; and thigh, 2000. Calculate the thyroid uptake and its percent
standard deviation.

Answer
Net standard count =90,000 − 1000 =89,000

= 302

ss=+90 000 1000,

%. standard deviation of the count rate = %

21

1000

×= 100 2 1

=

46 9

224

.

.

=

1

224

2200

.

=+

1

5

1600 600

=+

1600

5

600

5

sc= ⎛

⎝

⎜

⎞

⎠

⎟ +

⎛

⎝

⎜

⎞

⎠

⎟

1600

5

600

5

22

Background count rate==cpm

3000

5

600

Gross sample count rate==cpm

8000

5

1600

38 4. Statistics of Radiation Counting