title: Device-dependent activity estimation and decay correction of radionuclide mixtures with application to Tc-94m PET studies.
abstract: Multi-instrument activity estimation and decay correction techniques were developed for radionuclide mixtures, motivated by the desire for accurate quantitation of Tc-94m positron emission tomography (PET) studies. Tc-94m and byproduct Tc isotopes were produced by proton irradiation of enriched Mo-94 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 averaged 10.0% (Tc-94g) and 3.3% (Tc-93) for enriched targets and 10.1% (Tc-94g), 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 with a gamma counter. Decay correction using only the Tc-94m half-life overestimates activity and is inadequate. For this reason analytic formulas for activity estimation and decay correction of radionuclide mixtures were developed. Isotope-dependent sensitivity factors for a PET scanner, dose calibrator, and gamma counter were determined using theoretical sensitivity models and fits of experimental decay curves to sums of exponentials with fixed decay rates. For up to 8 h after the end of bombardment with activity from enriched 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 mean 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 illustrated by application to a canine Tc-94m sestamibi study.
authors.affiliation: Nuclear Medicine Department, Clinical Center, National Institutes of Health, Bethesda, Maryland 20892-1180, USA. smith@nmdhst.cc.nih.gov
authors.forename: M F
authors.initials: MF
authors.lastname: Smith
authors.forename: M E
authors.initials: ME
authors.lastname: Daube-Witherspoon
authors.forename: P S
authors.initials: PS
authors.lastname: Plascjak
authors.forename: L P
authors.initials: LP
authors.lastname: Szajek
authors.forename: R E
authors.initials: RE
authors.lastname: Carson
authors.forename: J R
authors.initials: JR
authors.lastname: Everett
authors.forename: S L
authors.initials: SL
authors.lastname: Green
authors.forename: P R
authors.initials: PR
authors.lastname: Territo
authors.forename: R S
authors.initials: RS
authors.lastname: Balaban
authors.forename: S L
authors.initials: SL
authors.lastname: Bacharach
authors.forename: W C
authors.initials: WC
authors.lastname: Eckelman
mesh_terms: D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D004285:Dogs; D006321:Heart / Q000000981:diagnostic imaging; D006801:Humans; D019047:Phantoms, Imaging; D011829:Radiation Dosage; D019275:Radiopharmaceuticals; D012542:Scattering, Radiation; D013667:Technetium; D017256:Technetium Tc 99m Sestamibi; D014055:Tomography, Emission-Computed / Q000295:instrumentation* / Q000379:methods / Q000706:statistics & numerical data
chemical_list: D019275:Radiopharmaceuticals; D013667:Technetium; D017256:Technetium Tc 99m Sestamibi