New effective dose estimates for Rubidium-82 based on dynamic PET/CT imaging in humans

Objectives Published radiation dose estimates for Rubidium-82 vary widely, and no comprehensive study in man has yet been conducted. With the increasing use of Rb-82 PET for myocardial perfusion imaging, improved dosimetry information is needed for accurate risk assessment. The purpose of this study was to measure Rb-82 internal organ and effective doses with PET/CT in humans.

Methods We recruited 26 cardiac patients and 4 normal subjects with no cardiac history. Dynamic 3D PET scans were acquired (GE Discovery RX/VCT) over 10 minutes following IV injection of 10 MBq/kg Rb-82. Images were reconstructed using FORE-OSEM and 8 mm Hann filter. Cardiac scans of the chest were acquired at rest for all 30 subjects, plus one additional scan of the Head, Neck, Abdomen, Pelvis, or Thighs. Mean Rb-82 residence times were determined in 22 source organs using volumes-of-interest (VOI) drawn on the fused PET/CT images. For large organs a small VOI was used to avoid partial volume losses. For small organs the average activity above 80% threshold was multiplied by the volume at 40% of the peak value within the organ VOI. Target organ doses and the effective dose estimate were calculated using OLINDA/EXM 1.0 according to ICRP 60, and recalculated according to ICRP 103.

Results A total of 283 organs were measured across the 60 scans, with at least 4 samples obtained in each source organ. The average ‘adult’ effective dose for Rubidium-82 was found to be 0.00074 mSv/MBq using ICRP 60. Using ICRP 103, the male and female effective doses were 0.00074 and 0.00092 mSv/MBq respectively. The highest absorbed organ dose was to the kidneys.

Conclusions These dose estimates for Rubidium-82 are the first to be measured directly with PET/CT in humans, and are 4 to 4.5 times lower than previous ICRP 60 values based on a theoretical blood flow model. The new values derived from human studies suggest a typical effective dose of 0.6 mSv per scan with 3D PET.

Research Support DRAXimage, Ontario Research Fund