The Pharmaceutics and Compounding Laboratory
Radiopharmaceuticals:
TC-99M Sulfur Colloid

Generation of Radioactive Pertechnetate Ion

The pertechnetate ion (99mTcO-4) is created in a commercial generator. The radioactivity is generated, and then decays over time. This isotope has a half-life of 6 hours. You can calculate the expected radioactivity for any time after generation using a physical decay chart.

PHYSICAL DECAY CHART
 
Hours
Fraction
Remaining
 
Hours
Fraction
Remaining
0
1/12
1/6
1/4
1/2
1
2
3
4
5
1.000
0.990
0.981
0.971
0.944
0.891
0.794
0.708
0.631
0.562
6
7
8
9
10
11
12
18
24
0.501
0.447
0.398
0.355
0.316
0.282
0.251
0.126
0.063

For example: The generation activity and time for a sample of (99mTcO-4) obtained from a generator was 25 µCi at 9:00 a.m. You take your sample at 2:30 p.m. and find the assayed activity is 13.2 µCi. The expected activity can be calculated as:

25 µCi × 0.562 × 0.944 = 13.3 µCi

Obviously, the assayed activity and the calculated expected activity should be similar.