Effectiveness of an Imaging Plate System in Emergency 131I Thyroid Monitoring.

The effect of temperature and shielding on the lower detection limit of a thyroid I monitoring system was investigated in an anthropomorphic thyroid-neck phantom fitted with an imaging plate. The phantom was loaded with an I aqueous solution and monitored with the imaging plate for 10 min. After exposure, the plates were incubated with or without the shield at 0, 10, 20, 25, 30, or 40°C. The latent image was read out at 0 min to 7 d after exposure. The thyroid equivalent doses corresponding to the detection limit were calculated in six age categories, using the inhalation equivalent dose coefficients recommended by the International Commission on Radiological Protection. The detection limit was distributed between 0.13 and 4.2 kBq, and depended on the age of subjects, elapsed time, temperature, and shielding provision. The maximum detection limit of 4.2 kBq was below the emergency screening level of 30 kBq in Japan. The thyroid equivalent dose corresponding to the detection limit ranged from 0.17 to 46 mSv. From the maximum equivalent dose of 46 mSv, the effective dose was estimated as 1.8 mSv, lower than the annual effective dose limit of 20 mSv for radiation workers. At 2 d after exposure, the measured dose was below the annual effective dose limit of 1 mSv for the public, regardless of age, temperature, and shielding provision. The imaging plate system effectively monitors the I thyroid levels in emergency situations.

Determination of half value layers of X-ray equipment using computed radiography imaging plates.

A method for determining half value layers (HVLs) of inverter-type X-ray equipment using a computed radiography (CR) systems was developed. This method is similar to the traditional method, where the air kerma (K) is measured using an ionization based dosimeter and increasing aluminum (Al) absorber thickness, but utilized an imaging plate (IP) and the sensitivity index (S number) of the CR system as the dosimeter and the dosimeter reading, respectively. The IP and the S number were calibrated using an ionization chamber having traceability to the National Standard Ionization Chamber. A modified version of the S number definition equation K=a × S(-b) was used to translate the S number to K values for X-ray beams produced using tube voltages ranging from 50 to 120 kV and additional Al filtration up to 2.5mm. The coefficient 'a' varied depending on the beam quality, while the coefficient 'b' showed a constant value of 0.991. The HVLs in the range from 1.8 to 5.5mm Al that were obtained with this method were in good agreement with those obtained with the traditional method, as uncertainties were between -7 and 4%. This method can be used to determine the HVLs of inverter-type X-ray equipment within an acceptable accuracy.

Feasibility of in vivo measurement of 239 Pu distribution in lungs using an imaging plate.

An in vivo measurement system using an imaging plate (IP) system was developed, which displayed images reflecting (239)Pu distribution in the lung of a phantom. The detection limits of the IP system for 1-12h exposures were between 1670 and 245 Bq at a 1.6 cm chest wall thickness. The detection limit of the IP system for a 2.5h exposure was equal to that of a germanium detector for a 0.5h measurement. The IP system could be used as a new device for in vivo measurement of (239)Pu in the lung.

Radiation effects on communication performance of radio frequency identification tags.

Radioactive materials (sources) are managed by bookkeeping and stocktaking. The radiation protection section staffs should check the sources manually. Annual effective dose concerning stocktaking of them are estimated at some mSv concerning fingers. A radio frequency identification (RFID) tag's absorbed dose is estimated at some dozen Gy. RFID for stocktaking automatically was devised. Radiation effects on the communication performance of RFID tags were investigated by using response times and read ranges as indices. The RFID system was composed of a computer, a detector, and transponders (tag) consisting of an integrated circuit chip and an antenna. The tag is joined to the source for identification. The tags were irradiated at doses between 5 and 5,000 Gy by an x-ray irradiator. The response times and the read ranges were tracked from 40 to 23,200 min after irradiation. Relative read ranges fluctuated between 0.9 and 1.1 in the dose region less than 2,000 Gy, but fluctuated greatly in the dose region beyond 2,000 Gy. Malfunctioning tags appeared from 3,000 Gy, and all tags malfunctioned in the dose region over 4,500 Gy. The threshold dose leading to malfunction was determined to be 2,100 Gy. Time variation of relative read ranges was classified into four patterns. The pattern shifted from pattern 1 to 4 when the dose was increased. The relative read ranges lengthened in pattern 1. The relative read rages were approximately 1.0 in pattern 2. The read ranges tentatively shortened, then recovered in pattern 3. The tags malfunctioned in pattern 4. Once the tags malfunctioned, they never recovered their performance. Radiation enhances or deteriorates communication performance depending on dosage. Tags can spontaneously recover from radiation deterioration. The time variation of the read ranges can be illustrated by enhancement, deterioration, and recovery. The mechanism of four patterns is explained based on the variation of the frequency harmonization strength and activation voltage by irradiation. The annual effective dose of radiation protection section staffs can be reduced considerably.

(90)Y bremsstrahlung emission computed tomography using gamma cameras.

OBJECTIVE: This study demonstrates images obtained by (90)Y bremsstrahlung emission computed tomography (BECT), and characterizes the system performance of gamma cameras. METHODS: (90)Y BECT images of phantoms were acquired using a gamma camera equipped with a medium energy general purpose parallel-hole collimator. Three energy window widths of 50% (57-94 keV) centered at 75 keV, 30% (102-138 keV) at 120 keV, and 50% (139-232 keV) at 185 keV were set on a (90)Y bremsstrahlung spectrum. The images obtained with three energy windows were reconstructed using filtered back projection (FBP) and ordered subsets expectation maximization (OSEM) methods. The images of the sum window were obtained by fusing the images of the 75, 120, and 185 keV windows. RESULTS: The OSEM method improved the full width at half maximum by 20% and the standard deviation by 9% compared with the FBP method. BECT displayed (90)Y biodistribution and quantified (90)Y activity. BECT images obtained with OSEM method using the 120 keV window showed the highest resolution and lowest uncertainty. The sum window showed the highest sensitivity, while its resolution was 10% inferior to that of the 120 keV window. One whole-body image can be taken over 100 min using the sum window. An absorber to cover the body surface reduced background by 30%. CONCLUSIONS: (90)Y BECT imaging can be used for patient assessment without modifying current treatment procedures.

(99m)Tc thyroid imaging system using multiple imaging plates.

A system for taking static thyroid (99m)Tc images was devised by using multiple imaging plates (IPs) and a low-energy high resolution collimator. System spatial resolution of the IP systems and the gamma camera was determined by referring to standards set by the National Electrical Manufacturers Association. Sensitivity was represented by using lower detection limits (LDLs). The sensitivity and resolution of IP systems using 16 IP probes connecting two collimators and 9 IPs were determined by using a 20 ml thyroid phantom, and compared with the sensitivity of gamma cameras. The sensitivity of the IP systems increased in proportion to the number of IPs. The sensitivity and resolution of a probe using 6 IPs and a high resolution collimator were equivalent to or superior to the gamma camera for taking static thyroid (99m)Tc images. IP systems can be applied clinically as mobile static nuclear imaging devices. The performance of IP systems should be thoroughly investigated for combinations of various collimators and the number of IPs in order to verify their efficacy for imaging all organs.

Research on radiation protection in the application of new technologies for proton and heavy ion radiotherapy.

Particle radiotherapy using proton and heavy ion beams has shown improved clinical results and is a promising cancer therapy which is expected to gradually spread in Japan. There are, however, no special regulations for radiotherapy treatment facilities. They have been operated under the same safety regulations as for a research facility using a research accelerator. Significantly high-energy radiation is necessary for particle radiotherapy compared with conventional radiation therapy. The treatment facility, therefore, should have a large accelerator, which is installed in a room with a thick shield wall. Data on radiation protection for such high energy medical facilities is fragmentary and insufficient. In this study, we examined the necessity of other regulations for the safe operation of medical facilities for particle radiotherapy. First, we measured activation levels of the therapeutic devices and of patients. Next the safety level of the medical facility was evaluated from the viewpoint of radiation protection. We have confirmed the facilities can be safely operated by present regulations given in the Law Concerning Prevention from Radiation Hazards due to Radiation Isotopes, etc. or the Law for Health Protection and Medical Care.

Development of 241Am lung monitoring system using an imaging plate.

A new 241Am lung monitoring system without shielding was devised by using an imaging plate system. The Lawrence Livermore National Laboratory's realistic torso phantom containing a 241Am lung was covered by imaging plates sealed in lightproof bags. The imaging plate system displayed 241Am lung images characteristic of the lung shape of the torso phantom. The imaging plate system's lower detection limits of 14 Bq for 60 min exposure and 6 Bq for 300 min were the same levels as those of the phoswich detectors and the germanium detectors placed in shielded rooms. The imaging plate system for 60 min exposure detected about 2% of the annual limit of 740 Bq for 241Am inhalation. A lung monitoring system using imaging plates is applicable for 241Am lung monitoring.

Development of dosimetry using detectors of diagnostic digital radiography systems.

Dosimetry using an imaging plate (IP) of computed radiography (CR) systems was developed for quality control of output of the x-ray equipment. Sensitivity index, or the S number, of the CR systems was used for estimating exposure dose under the routine condition: exposure dose from 1.0 to 1.0 x 10(2) microC kg(-1), tube voltages from 50 to 120 kV, and added filtration from 0 to 4.0 mm Al. The IP was calibrated by using a 6 cc ionization chamber having traceability to the National Standard Ionization Chamber. The uncertainty concerning the fading effect was suppressed less than 1.9% by reading the latent image 4 min+/-5 s after irradiation at the room temperature 25.9+/-1.0 degrees C. The S number decreased linearly on the logarithmic graph regardless of the beam quality as exposure dose increased. The relationship between the exposure dose (E) and the S number was fitted by the equation E=a' X S(-b). The coefficient a' decreased when the added filtration and the tube voltage were increased. The coefficient b was 0.977+/-0.007 in all beam qualities. The dosimetry using the IP and the equation can estimate the exposure dose in a range from 9.0 x 10(-2) to 5.0 microC kg(-1) within an uncertainty of +/-5% required by the Japanese Industry Standard. This dose range partially included the doses under routine condition. The doses between 1.0 and 1.0 x 10(2) microC kg(-1) under the routine condition can be shifted to the 5% region by using an absorber. The IP dosimetry is applicable to the quality control of the CR systems.

Electromagnetic malfunction of semiconductor-type electronic personal dosimeters caused by access control systems for radiation facilities.

High frequency electromagnetic fields in the 120 kHz band emitted from card readers for access control systems in radiation control areas cause abnormally high and erroneous indicated dose readings on semiconductor-type electronic personal dosimeters (SEPDs). All SEPDs malfunctioned but recovered their normal performance by resetting after the exposure ceased. The minimum distances required to prevent electromagnetic interference varied from 5.0 to 38.0 cm. The electric and magnetic immunity levels ranged from 35.1 to 267.6 V m(-1) and from 1.0 to 16.6 A m(-1), respectively. Electromagnetic immunity levels of SEPDs should be strengthened from the standpoint of radiation protection.

Radioactive byproducts in [18O]H2O used to produce 18F for [18F]FDG synthesis.

Potential radioactive byproducts in [(18)O]H(2)O irradiated with 9.6 MeV protons to produce (18)F were analyzed theoretically and experimentally. Twenty two nuclear reaction cross sections included in the National Nuclear Data Center's (NNDC) data base were selected from the possible nuclear reactions between 9.6 MeV protons and a silver havar target. Ten radionuclides: (52)Mn, (55)Fe, (55)Co, (56)Co, (57)Co, (58)Co, (59)Ni, (95)Tc, (96)Tc and (109)Cd were detected experimentally in [(18)O]H(2)O by using high purity germanium semiconductor detectors. The activities of the 10 radionuclides were distributed between 4B q and 1.2k Bq. These activities were less than the reference values given in the International Basic Safety Standards. The radionuclides derived from nuclear reactions between a silver target body and 9.6 MeV protons at a beam current 25 microA for 60 min irradiation would be exempt from restrictions for radioactive waste. The purified [(18)F]FDG prepared from (18)F produced by irradiating a silver havlar target with 9.6 MeV protons was not contaminated by the radionuclides.

Abnormal responses of electronic pocket dosimeters caused by high frequency electromagnetic fields emitted from digital cellular telephones.

High frequency electromagnetic fields emitted from digital cellular telephones (cell phones) occasionally cause abnormally high and erroneous indicated dose readings on electronic pocket dosimeters (EPDs). Electric field strength distribution around a cell phone transmitting 1.5 GHz band with a maximum power of 0.8 W was analyzed by using an isotropic probe with tri-axial dipole antennas. Five types of EPDs were exposed to the fields for 50 s under configurations relative to the cell phone. The electric field distribution expanded around the phone's antenna and had a maximum electric field strength of 36.5 +/- 0.3 V m(-1). The cell phone gave rise to erroneous indicated dose readings on four out of five EPDs. The maximum value of erroneous indicated dosage for 50 s reached 1,283 microSv, which was about 2.6% of the annual effective dose limit of 50 mSv. The electromagnetic susceptibility of the EPDs was higher in the sections where the semiconductor detectors or electric circuit boards were located. The distance required to prevent electromagnetic interference differed for each EPD and ranged from 2.0 to 21.0 cm from the cell phone. The electric and magnetic field immunity levels of the EPDs varied from 9.2 V m(-1) to greater than 37.6 V m(-1), and from 0.03 A m(-1) to greater than 0.51 A m(-1). The EPDs displayed erroneous dose readings during exposure but recovered their normal performance after the cell phone ceased transmitting. The electromagnetic immunity levels of the EPDs were either equal to or greater than the IEC-standard. The immunity levels should be enhanced greater than the IEC-standard from the standpoint of radiation protection. The simplest and most reliable measure to prevent potential malfunction is to prohibit the radiation workers from carrying cell phones to their workplace.

Effects of high-frequency electromagnetic fields emitted from card readers of access control systems on electronic pocket dosimeters.

High-frequency electromagnetic fields in the 120 kHz band emitted from card readers for access control systems caused abnormally high doses on electronic pocket dosimeters (EPDs). All EPDs recovered their normal performance by resetting after the exposure ceased. The electric and magnetic immunity levels of the EPDs were estimated by using the distances needed to prevent electromagnetic interference.

Tritium in [18O]water containing [18F]fluoride for [18F]FDG synthesis.

The presence of tritium in enriched [18O]water irradiated with 9.6 MeV protons used to produce [18F]fluoride by the 18O(p, n)18F reaction was inferred from the cross sections and threshold energies of the 18O(p, t)16O reaction, and the existence of tritium was confirmed experimentally. Tritium was also detected in both [18O]water recovered for recycling and waste acetonitrile solutions. The purified [18F]FDG was not contaminated with 3H. The amount of 3H discharged into the air was far less than the International Basic Safety Standard Level.

Characterization of an in vivo thyroid 131I monitoring system using an imaging plate.

The effects of neck diameter, thyroid volume, and prethyroid tissue thickness on a count-activity conversion coefficient and the detection limit of a thyroid 131I monitoring system with an imaging plate (IP) were estimated by using an anthropomorphic thyroid-neck phantom. The conversion coefficient and detection limit of the IP system was approximately constant for normal Japanese adults regardless of their neck diameters, thyroid volumes, and prethyroid tissue thicknesses. The IP system is a new option for thyroid 131I monitoring.

In situ calibration of a low-level, in-line, flow liquid scintillation analyzer.

A new method to calibrate a flow liquid scintillation detector in an in-line waste water monitor was developed and its operation was verified. The liquid scintillation detector for detecting low energy beta emitters was one of three detectors of the monitor that aimed to analyze radionuclide concentrations in the waste water before draining it into sewer. To calibrate the liquid scintillation detector, a 20-cm3 calibration vial containing a reference solution was connected to the water sampling channel with a three-way valve. Ten measurements needed to define a quench correction curve required only 200 cm3 solution. The counting efficiencies of the liquid scintillation detector for 3H and for 14C under the average quenching level of the waste waters were estimated at 27% and 58%, respectively. The calibration using the vial took into account the sampling and mixing system, which would not be the case with sealed quenched standards. Although active solution was used, the method was simple, generated little active waste, had low possibility of contamination, and reduced costs and labor for the calibration.

In situ calibration and evaluation of the accuracy of a waste water monitoring system.

A new method to estimate the reliability of a waste water monitoring system was developed and substantiated. To estimate the reliability means (1) to verify the representativeness of the sample fed into the monitoring system, and (2) to calibrate the detectors of the monitoring system. To verify the representativeness, test water spiked with 32P was prepared in a storage tank, and the monitoring system was operated. For all of the experiments, the radioactivity concentrations of the water fed into the monitoring system agreed well with those of the water in the storage tank at 725 +/- 45 s after the start of the sampling pump. Therefore, the representativeness was verified. Calibration was carried out with active water in the storage tank and in a specially designed calibration tank. The efficiency of a plastic scintillation detector (PSD), one of the detectors used in the monitoring system, for 32P was determined to be 4.91 +/- 0.10%. Using the calibration tank significantly reduced the cost and labor for calibration. In addition, calibration with natural potassium (40K) proved to be effective for periodic checks of the detector.