E-2 APPENDIX E Biodex Medical Systems, Inc. © 2014
SAMPLE CALCULATION
The following is a calculation of the decayed activity of Tc-99m, with an initial activity of
43 mCi, after 53 minutes:
6. A
0
= 43 mCi.
7. For Tc-99m, T
1/2
= 6.007 h.
8. Elapsed t = 53 min x
!!!!"
!"!!"#
= 0.8833!hr!
9. A = A
0
x 2
(-t/T1/2)
A = 43 mCi x 2
(-0.8833/6.007)
A = 43 mCi x 2
(-0.147045)
A = 43 mCi x 0.903
A = 38.83 mCi
Alternate Decay Calculation Method
If you are using a calculator with the Y
x
function, perform the following:
1. Measure the current activity of the radioisotope (A
0
), which can be compared
to the decayed calculated activity.
2. Look up half life T
1/2
of the radioisotope in Appendix E. Note the time units.
3. T
1/2
= Half life of the isotope
t = Elapsed Time
A = Current Activity
A
0
= Activity at the time of calibration
n = elapsed time / half life
4. Calculate A = A
0
/ 2
n
5. Calculate the elapsed time (t) in the same units as the (T
1/2
) that will elapse between when
the calculated activity measurement was made and when you wish to know what the decayed
activity of your radioisotope will be.
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