Radiation dosimetry of 82Rb in humans under pharmacologic stress

Abstract

(82)Rb is used with PET for cardiac perfusion studies. Using human biokinetic measurements, in vivo, we recently reported on the resting-state dosimetry of this agent. The objective of this study was to obtain (82)Rb dose estimates under stress.

Methods: (82)Rb biokinetics were obtained in 10 healthy volunteers (5 male, 5 female; mean age ± SD, 33 ± 10 y; age range, 18-50 y) using whole-body PET/CT. The 76-s half-life of (82)Rb and the corresponding need for pharmacologic vasodilation require that all imaging be completed within 10 min. To accommodate these constraints, while acquiring the data needed for dosimetry we used the following protocol. First, a whole-body attenuation correction CT scan was obtained. Then, a series of 3 whole-body PET scans was acquired after a single infusion of 1.53 ± 0.12 GBq of (82)Rb at rest. Four minutes after the infusion of a 0.56 mg/kg dose of the vasodilator, dipyridamole, 3 serial whole-body PET scans were acquired after a single infusion of 1.50 ± 0.16 GBq of (82)Rb under stress. The time-integrated activity coefficient (TIAC) for stress was obtained by scaling the mean rest TIAC obtained from our previous rest study by the stress-to-rest TIAC ratio obtained from the rest-stress measurements described in this report.

Results: The highest mean organ-absorbed doses under stress were as follows: heart wall, 5.1, kidneys, 5.0, lungs, 2.8, and pancreas, 2.4 μGy/MBq (19, 19, 10.4, and 8.9 mrad/mCi, respectively). The mean effective doses under stress were 1.14 ± 0.10 and 1.28 ± 0.10 μSv/MBq using the tissue-weighting factors of the International Commission on Radiological Protection, publications 60 and 103, respectively.

Conclusion: Appreciable differences in source-organ biokinetics were observed for heart wall and kidneys during stress when compared with the previously reported rest study. The organ receiving the highest dose during stress was the heart wall. The mean effective dose calculated during stress was not significantly different from that obtained at rest.

FIGURE 1

Graphical representation of Equation 2. Shaded regions in A, B, C, and D show ${\int }_0^{\infty }[a_{R,RS}(r_S,t)],{\int }_0^T[a_{R,RS}(r_S,t)],{\int }_0^T[a_R(r_S,t)]$, and [ ${\stackrel{\sim }{a}}_{R,RS}^{\prime }(r_S)$], respectively, where T represents peak uptake time.

FIGURE 2

Comparison of mean TIAC (h) calculation at rest based on 3-time-point whole-body PET (current study) and multibed multiple-time-point PET (previous study (2)) for selected source organs.

FIGURE 3

Comparison of absorbed dose estimates for ⁸²Rb at stress (current study) and at rest (previous study (2)).