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 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.
Smith
M F
MF
Nuclear Medicine Department, Clinical Center, National Institutes of Health, Bethesda, Maryland 20892-1180, USA. smith@nmdhst.cc.nih.gov
Daube-Witherspoon
M E
ME
Plascjak
P S
PS
Szajek
L P
LP
Carson
R E
RE
Everett
J R
JR
Green
S L
SL
Territo
P R
PR
Balaban
R S
RS
Bacharach
S L
SL
Eckelman
W C
WC
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
D019275:Radiopharmaceuticals; D013667:Technetium; D017256:Technetium Tc 99m Sestamibi