JNM: Rb-82 PET has low radiation exposure

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Dose estimates for rubidium-82 (Rb-82) based on quantitative whole-body biokinetics in humans suggest a reasonably low radiation exposure and is at the lower end of effective dose among cardiac imaging procedures that involve ionizing radiation, according to a study published in the October issue of the Journal of Nuclear Medicine.

Rb-82 dosimetry using human in vivo biokinetic measurements were evaluated by Senthamizhchelvan Srinivasan, PhD, postdoctoral fellow in the radionuclide therapy and dosimetry research lab, in the Russell H. Morgan department of radiology and radiological science at the Johns Hopkins University School of Medicine in Baltimore, and colleagues.

Ten healthy volunteers underwent dynamic PET/CT (six contiguous table positions, each with separate Rb-82 infusion). Source organ volumes of interest were delineated on the CT images and transferred to the PET images to obtain time-integrated activity coefficients and radiation doses were estimated by Srinivasan and colleagues.

The highest mean absorbed organ doses (mGy/MBq) were observed for the kidneys (5.81), heart wall (3.86) and lungs (2.96). Mean effective doses were 1.11 and 1.26 Sv/MBq, respectively, according to Srinivasan and colleagues.

"Inaccuracy in quantifying activity in the lungs using a gamma-camera and the unavoidably crude separation of heart chamber from wall activity may explain the nearly two-fold lower organ dose estimates," wrote Srinivasan and colleagues.

Additionally, the dose estimate for testes was based on one subject. However, the ovary dose estimate was extrapolated from male-to-female comparisons, and dose estimates for some organs were not obtained by the researchers, which they expressed as a limitation.

Absorbed dose estimates for the brain and gallbladder wall included in the present study have not been previously reported, Srinivasan and colleagues wrote. "Absorbed dose estimates for gastrointestinal tract walls in our study were obtained by assuming total self-absorption of beta-emissions—an overly conservative assumption because Rb-82 has highly energetic beta-(positron) emissions, and not all energy emitted is completely absorbed inside the thin gastrointestinal tract walls," noted the authors.

The authors concluded that the current Rb-82 dosimetry suggests reasonably low radiation exposure. On the basis of this study, a clinical Rb-82 injection of 2 x 1,480 MBq (80 mCi) would result in a mean effective dose of 3.7 mSv which is slightly above the average annual natural background exposure in the U.S. (3.1 mSv).