Device-dependent activity estimation and decay correction of radionuclide mixtures with application to Tc-94m PET studies

Multi-instrument activity estimation and decay correction techniques were d eveloped for radionuclide mixtures, motivated by the desire for accurate qu antitation of Tc-94m positron emission tomography (PET) studies. Tc-94m and byproduct Tc isotopes were produced by proton irradiation of enriched Mo-9 4 and natural Mo targets, Mixture activities at the end of bombardment were determined with a calibrated high purity germanium detector. The activity fractions of the greatest mixture impurities relative to 100% for Tc-94m av eraged 10.0% (Tc-94g) and 3.3% (Tc-93) for enriched targets and 10.1% (Tc-9 4g), 11.0% (Tc-95), 255.8% (Tc-96m), and 7.2% (Tc-99m) for natural targets. These radioisotopes have different half-lives (e.g., 52.5 min for Tc-94m, 293 min for Tc-94g), positron branching ratios (e.g., 0.72 for Tc-94m, 0.11 for Tc-94g) and gamma ray emissions for themselves and their short-lived, excited Mo daughters. This complicates estimation of injected activity with a dose calibrator, in vivo activity with PET and blood sample activity wit h a gamma counter. Decay correction using only the Tc-94m half-life overest imates activity and is inadequate. For this reason analytic formulas for ac tivity estimation and decay correction of radionuclide mixtures were develo ped. Isotope-dependent sensitivity factors for a PET scanner, dose calibrat or, and gamma counter were determined using theoretical sensitivity models and fits of experimental decay curves to sums of exponentials with fixed de cay rates. For up to 8 h after the end of bombardment with activity from en riched and natural Mo targets, decay-corrected activities were within 3% of the mean for three PET studies of a uniform cylinder, within 3% of the mea n for six dose calibrator decay studies, and within 6% of the mean for four gamma counter decay studies. Activity estimation and decay correction for Tc-94m mixtures enable routine use of Tc-94m in quantitative PET, as illust rated by application to a canine Tc-94m sestamibi study. (C) 2001 American Association of Physicists in Medicine.