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

Mark F. Smith, Margaret E. Daube-Witherspoon, Paul S. Plascjak, Lawrence P. Szajek, Richard E. Carson, James R. Everett, Shielah L. Green, Paul R. Territo, Robert S. Balaban, Stephen L. Bacharach, William C. Eckelman

Research output: Contribution to journalArticle

15 Scopus citations

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.

Original languageEnglish (US)
Pages (from-to)36-45
Number of pages10
JournalMedical physics
Volume28
Issue number1
DOIs
StatePublished - Jan 1 2001
Externally publishedYes

Keywords

  • Decay correction
  • PET
  • Positron emission tomography
  • Radioactive mixture
  • Technetium-94m

ASJC Scopus subject areas

  • Biophysics
  • Radiology Nuclear Medicine and imaging

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    Smith, M. F., Daube-Witherspoon, M. E., Plascjak, P. S., Szajek, L. P., Carson, R. E., Everett, J. R., Green, S. L., Territo, P. R., Balaban, R. S., Bacharach, S. L., & Eckelman, W. C. (2001). Device-dependent activity estimation and decay correction of radionuclide mixtures with application to Tc-94m PET studies. Medical physics, 28(1), 36-45. https://doi.org/10.1118/1.1333411