Yoshimune Ogata

Measurement of the Induced Radionuclides in Production of Radiopharmaceuticals for Positron Emission Tomography (PET)

The radioactive by-products contained in an entire series of target foil, [18O]H2O and synthesis apparatus were identified and quantified. From the perspective of waste management, 60Co induced in Havar foil should be taken into consideration. Because the exempt activity of 60Co in BSS is 0.1 MBq, the used Havar foil should be managed more than for 20 years. The radionuclides in the [18F]-FDG synthesis apparatus are negligible. Equivalent doses at skin and to tissues were estimated assuming a point source at a distance of 30 cm in air. The annual equivalent doses at skin and equivalent dose at deep tissues of such an operating staff will be 56 and 8.3 μSv, respectively, as two times the remove of the target foil and five hundreds times the synthesis of the [18F]-FDG. When proper radiation protection is provided, the exposure from the cyclotron management and the [18F]-FDG synthesis process will not cause meaningful radiological risk to the operating staff. The activity concentration of 3H, 180kBq-cm−3, detected in the target water, is 3,000 times the legal limit of effluent for 3H. The operators should take care of the treatment of the target water when they make a distillation for reuse and a disposal.

Measurement of Thermal Neutron Capture Cross Section and Resonance Integral of the Reaction 127I(n, γ)128I

The thermal neutron (2,200 m/s neutron) capture cross section (σ0) and the resonance integral (I 0) of the reaction 127I(n, γ)128I were obtained. The targets of the iodine were irradiated within and without a cadmium capsule at Rikkyo University Research Reactor. Characteristic features of neutron field were monitored by Co/Al and Au/Al wires. Gamma-rays from the irradiated targets were measured by a high purity Ge detector. The γ-ray spectra were taken successively and the decay of peak-area counts of the 443 keV γ-ray of 128I was followed. A half-life of 128I obtained was 24.72±0.03 min. The σ0 and I 0 of the reaction 127I(n, γ)128I were deduced from the analysis of obtained γ-ray spectra according to the Westcotts convention and were 6.40±0.29 barns and 162±8 barns, respectively. The previous measurements of the σ0 and I 0 of the reaction 127I(n, γ)128I were reviewed and differences between the present values and the previous results were discussed with special emphasis on the experimental results by ...

Evaluation of radiocesium concentrations in new leaves of wild plants two years after the Fukushima Dai-ichi Nuclear Power Plant accident

Radiocesium ((137)Cs) transfer to plants immediately after the Fukushima Dai-ichi Nuclear Power Plant accident was investigated by collecting newly emerged leaf and soil samples between May 2011 and November 2012 from 20 sites in the Fukushima prefecture. Radiocesium concentrations in leaf and soil samples were measured to calculate concentration ratios (CR). Woody plants exhibited high CR values because (137)Cs deposited on stems and/or leaves were transferred to newly emerging tissues. The CR values in 2012 declined as compared to that in 2011. Exchangeable (137)Cs rates in soil (extraction rate) samples were measured at five sites. These rates decreased at four sites in 2012 and depended on environmental conditions and soil type. Both CR values and extraction rates decreased in 2012. However, CR values reflected the changes in extraction rates and characteristics of each species. Amaranthaceae, Chenopodiaceae, and Polygonaceae, which had been identified as Cs accumulators, presented no clear (137)Cs accumulation ability. In 2012, the perennial plant Houttuynia cordata and deciduous trees Chengiopanax sciadophylloides and Acer crataegifolium displayed high CR values, indicating that these species are (137)Cs accumulators and may be considered as potential species for phytoremediation.

Feasibility of in vivo thyroid 131I monitoring with an imaging plate

A new in vivo thyroid 131I monitoring method was devised by using an imaging plate (IP). A thyroid image obtained with a realistic neck-thyroid phantom showed a unique shape characteristic of the thyroid gland. A 131I thyroid imaging allows visual confirmation of thyroid accumulation of 131I. The detection limit of the IP system of 290 Bq was about 1/100 of the screening level of 30 kBq in cases of public emergencies. The IP system is applicable for thyroid 131I monitoring.

Tritium separation by electrolysis using solid polymer electrolyte

Hydrogen isotope separation effect by electrolysis of water was theoretically investigated and was compared with experimental results. The separation mechanism was analyzed as the hydrogen isotope exchange reaction between water and diatomic hydride that consists of hydrogen and cathode material. The equilibrium constants of the isotope exchange reaction were calculated from reduced partition function ratio. Using the constants, the separation factor (SF) of the isotopes was calculated according to the two-phase distribution theory for isotopes. Experimentally, light or heavy water spiked with tritiated water was electrolyzed by a device with a solid polymer electrolyte, which equipped with SUS, Ni, or carbon cathode. Thus, the SFs were experimentally obtained. Calculated SFs were well agreed with the experimentally values for SUS and Ni cathodes, and that for carbon cathode was somewhat small then the experimental value.

Development of a PET/Cerenkov-light hybrid imaging system.

PURPOSE Cerenkov-light imaging is a new molecular imaging technology that detects visible photons from high-speed electrons using a high sensitivity optical camera. However, the merit of Cerenkov-light imaging remains unclear. If a PET/Cerenkov-light hybrid imaging system were developed, the merit of Cerenkov-light imaging would be clarified by directly comparing these two imaging modalities. METHODS The authors developed and tested a PET/Cerenkov-light hybrid imaging system that consists of a dual-head PET system, a reflection mirror located above the subject, and a high sensitivity charge coupled device (CCD) camera. The authors installed these systems inside a black box for imaging the Cerenkov-light. The dual-head PET system employed a 1.2×1.2×10 mm3 GSO arranged in a 33 × 33 matrix that was optically coupled to a position sensitive photomultiplier tube to form a GSO block detector. The authors arranged two GSO block detectors 10 cm apart and positioned the subject between them. The Cerenkov-light above the subject is reflected by the mirror and changes its direction to the side of the PET system and is imaged by the high sensitivity CCD camera. RESULTS The dual-head PET system had a spatial resolution of ∼1.2 mm FWHM and sensitivity of ∼0.31% at the center of the FOV. The Cerenkov-light imaging systems spatial resolution was ∼275 μm for a 22Na point source. Using the combined PET/Cerenkov-light hybrid imaging system, the authors successfully obtained fused images from simultaneously acquired images. The image distributions are sometimes different due to the light transmission and absorption in the body of the subject in the Cerenkov-light images. In simultaneous imaging of rat, the authors found that 18F-FDG accumulation was observed mainly in the Harderian gland on the PET image, while the distribution of Cerenkov-light was observed in the eyes. CONCLUSIONS The authors conclude that their developed PET/Cerenkov-light hybrid imaging system is useful to evaluate the merits and the limitations of Cerenkov-light imaging in molecular imaging research.

Distribution of thermal neutron flux around a PET cyclotron.

The number of positron emission tomography (PET) examinations has greatly increased world-wide. Since positron emission nuclides for the PET examinations have short half-lives, they are mainly produced using on-site cyclotrons. During the production of the nuclides, significant quantities of neutrons are generated from the cyclotrons. Neutrons have potential to activate the materials around the cyclotrons and cause exposure to the staff. To investigate quantities and distribution of the thermal neutrons, thermal neutron fluxes were measured around a PET cyclotron in a laboratory associating with a hospital. The cyclotron accelerates protons up to 18 MeV, and the mean particle current is 20 &mgr;A. The neutron fluxes were measured during both 18F production and 11C production. Gold foils and thermoluminescent dosimeter (TLD) badges were used to measure the neutron fluxes. The neutron fluxes in the target box averaged 9.3 × 106 cm−2 s−1 and 1.7 × 106 cm−2 s−1 during 18F and 11C production, respectively. Those in the cyclotron room averaged 4.1 × 105 cm−2 s−1 and 1.2 × 105 cm−2 s−1, respectively. Those outside the concrete wall shielding were estimated as being equal to or less than ∼3 cm−2 s−1, which corresponded to 0.1 &mgr;Sv h−1 in effective dose. The neutron fluxes outside the concrete shielding were confirmed to be quite low compared to the legal limit.

Tritium separation from heavy water by electrolysis with solid polymer electrolyte

A tritium separation from heavy water by electrolysis using a solid polymer electrode layer was specified. The cathode was made of stainless steel or nickel. The electrolysis was performed for 1 hour at 5, 10, 20, and 30 °C. Using a palladium catalyst, generated hydrogen and oxygen gases were recombined, which was collected with a cold trap. The activities of the samples were measured by a liquid scintillation counter. The apparent tritium separation factors of the heavy and light water at 20 °C were ∽2 and ∽12, respectively.

Monitoring of 131I in milk and rain water in Japan following the reactor accident at Chernobyl and estimates of human thyroidal dose equivalents.

Iodine-131 in milk and in rain water in Nagoya, Japan, (a location 8,000 km from Chernobyl) was monitored between May and July 1986. The 131I concentration in rain water ranged from 43.1 Bq L-1 on 4 May to 15 mBq L-1 on 12 July, and that in milk ranged from 21.8 Bq L-1 on 19 May to 11 mBq L-1 on 14 July. Iodine-131 concentrations in milk were estimated to be 4 to 6 times greater than those in rain water during the first few weeks after the accident. Both concentrations decreased with approximately the same effective half-life of 5.9 +/- 0.3 d for rain water and 5.0 +/- 0.2 d for milk. The 131I concentration in milk sold in markets varied from dairy to dairy and ranged from 0.07 to 0.2 times that in fresh milk. The maximum thyroidal dose equivalents estimated for an adult man and for a baby were far lower than the population annual dose equivalent to the thyroid from natural radiation.

Development of the modified sum-peak method and its application.

As the sum-peak method requires the total count rate as well as the peak count rates and the sum peak count rate, this meets difficulties when a sample contains other radionuclides than the one to be measured. To solve the problem, a new method using solely the peak and the sum peak count rates was developed. The method was theoretically and experimentally confirmed using (60)Co, (22)Na and (134)Cs. We demonstrate that the modified sum-peak method is quite simple and practical and is useful to measure multiple nuclides.