Performance of a well counter and a dose calibrator for quantitative positron emission tomography

Quantitative nuclear medicine techniques such as positron emission tomography (PET) require precise and accurate knowledge of both the amount of radioactivity administered to a patient and the radioactivity concentration in blood as a function of time. In addition, the uncertainties in such measurements must be known so that the accuracy and precision of quantitative in vivo metabolic measurements may be estimated. In order to characterize and minimize the measurement errors for PET, well counter and dose calibrator performances were studied over long and short time periods using positron-emitting isotopes. To ensure accurate quantitation with well counters, the use of correction factors for sample volume and high count rate effects is essential. Only small drifts in well counter sensitivity were observed during a 24-h period, but longer-term drifts and poor performance emphasized the need for continuing quality control procedures. Similar behavior was observed for dose calibrators. Dose calibrator, rather than well counter, data should be used for PET imaging instrument calibration. The methods presented have direct application to any quantitative nuclear medicine program.