Faculty Bibliography

Estimating the radiation dose received by the fetus from nuclear medicine procedures is important because of the greater sensitivity of rapidly developing fetal tissues to ionizing radiation. (18)F-fluoro-L-thymidine (FLT) uptake is related to cellular proliferation and is currently used to monitor tumor progression and response to therapy. This study was undertaken to estimate-on the basis of biodistribution data obtained by PET/CT in pregnant rhesus monkeys-radiation absorbed dose to a human fetus administered (18)F-FLT. METHODS: Three pregnant rhesus macaques (gestational age, 113 +/- 8 d) were administered (18)F-FLT and imaged for 2 h on a PET/CT scanner. Time-activity curves for maternal and fetal organs were generated in anatomic regions of interest identified via CT. Doses were estimated using OLINDA/EXM and the 6-mo-pregnant human model. RESULTS: The extrapolated whole-body maternal dose obtained, 11.4 microGy/MBq, is similar to the previously reported adult female dose of 15.6 microGy/MBq. The estimated total-body dose to a human fetus is 24 microGy/MBq. Significant long-term (18)F-FLT accumulation in fetal liver resulted in a fetal liver dose of 53 microGy/MBq. CONCLUSION: The fetal dose estimate in a 6-mo-pregnant human using (18)F-FLT is slightly greater than that reported for (18)F-FDG. (18)F-FLT trapping in the fetal liver should be considered in the risk-benefit analysis of (18)F-FLT PET examination in pregnant patients.

Environmental factors have long been thought to have a role in the etiology of idiopathic Parkinson's disease (PD). Since the discovery of the selective neurotoxicity of MPTP to dopamine cells, suspicion has focused on paraquat, a common herbicide with chemical structure similar to 1-methyl-4-phenylpyridinium (MPP+), the MPTP metabolite responsible for its neurotoxicity. Although in vitro evidence for paraquat neurotoxicity to dopamine cells is well established, its in vivo effects have been ambiguous because paraquat is di-cationic in plasma, which raises questions about its ability to cross the blood brain barrier. This study assessed the brain uptake of [(11)C]-paraquat in adult male rhesus macaques using quantitative PET imaging. Results showed minimal uptake of [(11)C]-paraquat in the macaque brain. The highest concentrations of paraquat were seen in the pineal gland and the lateral ventricles. Global brain concentrations including those in known dopamine areas were consistent with the blood volume in those structures. This acute exposure study found that paraquat is excluded from the brain by the blood brain barrier and thus does not readily support the causative role of paraquat exposure in idiopathic Parkinson's disease.