The validity of the geometric mean method for estimating internal activities in nuclear medicine

The activity of an internal radioactive source may be calculated by comparing the geometric mean of the regional counts in a pair of conjugate gamma camera images with the geometric mean of the counts from a small surface reference source of known activity. Allowance must be made for the effects of differential forward scatter in the detector count rates from the internal and reference sources. The magnitude of these can be predicted, and a suitable correction factor for a given collimator and energy acquisition window calculated by applying a simple count rate build-up model to the observed count rate-depth data for a point and plate source in a water tank. Using a high resolution collimator with 99mTc sources in three sets of phantom trials, the model shows that a scatter build-up correction factor of 1.20-1.22 is suitable for an internal 99mTc source in a typical adult chest or abdomen.