15. Calculate the composite efficiencies D.E. * G.E. using one of the following equations:
S is Standard ROI counts
B is Background ROI counts
A is the activity determined in step 7 under Source Preparation for isotope Xx-123 with
units of microcuries.
Exponent of “2”:
This is a decay correction. For short lived isotopes such as Tc-99m, it is important. For long
lived isotopes such as I-125, it may not be important unless the source is saved for future
efficiency checks.
T
1/2
is the half-life of isotope Xx-123, in units of hours, days, etc.
T and t are respectively the time of source measurement in the dose calibrator and the
same of counting the source in the well detector. Both should have the same units as T1/2.
For example:
If T = 08:31 h and t = 15:49 h, then T - t = -7.300 h.
If T
1/2
= 3.261 days for Ga-67, then T - t should be converted to days = -0.3041 days. In this
case, the decay result would be 0.937.
16. Calculate D.E. from the composite (D.E. * G.E.) found in step 10 above and from the
calculated value of G.E. found earlier in the discussion on GEOMETRIC EFFICIENCY. The
following equation defines the calculation.
Note that the two terms (D.E. * G.E.) and G.E. are both calculated as percents in the earlier
expressions and the above result preserves D.E. as a percentage.
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