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We developed a Gd2Si2O7 (GPS) scintillator-based alpha imaging detector and demonstrated its effectiveness by evaluating the actual Pu particle and 222Rn progeny. The GPS scintillator plate was prepared by a sintering method. The outer dimensions of the GPS scintillator plate were $5\times 5$ cm, and the scintillator layer was approximately $50~\mu \text{m}$ on a 3-mm-thick high-transparency glass. The plate was optically coupled to a position-sensitive photomultiplier tube (Hamamatsu H8500, Hamamatsu, Japan) with silicone grease. The developed imaging detector exhibited good uniformity; Pu particle activities were accurately evaluated at 14 different positions and the difference in activity was within ±6%. The radon-222 progeny counts were reduced by 65.3% by applying an energy window. Although the Pu/222Rn progeny activity ratio was 1/51, the Pu particle was successfully identified among the 222Rn progeny within the 5-min measurement time. The imaging detector has an excellent ability for detecting Pu among the 222Rn progeny. Thus, this detector is useful for alpha contamination monitoring in high-radon-background environments.

Recently, we found that the decay times between $\alpha $ particles and $\gamma $ photons were different for the recently developed scintillators. We also noticed that $\alpha - \beta $ ratios were different for these scintillators. We expect that there might be a relation between these two properties. Thus, we measured the $\alpha - \beta $ ratios and the decay time differences between $\alpha $ particles and $\gamma $ photons for the newly developed scintillators and investigated the relationship between them. We measured the $\alpha - \beta $ ratios and decay times for five scintillators: (LaGd)2Si2O7 (LaGPS), Gd2Si2O7 (GPS), ceramic Gd3Al2Ga3O12(GGAG), ceramic (YGd)3(GaAl)5O12:Ce (YGAG), and single-crystal Gd3Al2Ga3O12 (GAGG). After the measurements, we estimated the correlation between the $\alpha - \beta $ ratios and the decay time differences between $\alpha $ particles and $\gamma $ photons. Among these scintillators, as the $\alpha - \beta $ ratios increased, decay time differences decreased with the correlation coefficient ( $R^{2}$ ) of more than 0.8. We conclude that there was a negative correlation between $\alpha - \beta $ ratios and the decay time differences between $\alpha $ particles and $\gamma $ photons for the measured newly developed scintillators.

Because the sensitivity of positron emission tomography (PET) system increases in proportion to the inverse of the diameter of the detector ring, brain PET systems are being planned and developed. However, count losses due to the dead-time of the detector blocks increases as the ring diameter decreases. Since the count losses decrease the noise equivalent count rate (NECR) of the system, the NECR of brain PET system may be lower than that of a whole-body PET system in clinical dose range. In this paper, we compared the NECRs of brain PET systems with that of a whole-body PET system using a Monte Carlo simulation. We simulated the PET system for a 27-cm-diameter detector ring, a 54-cm-diameter detector ring, and an 89-cm-diameter detector ring and evaluated the NECRs. In the clinical dose range, the NECR of the small-diameter brain PET system was lower than those of the whole-body PET system unless the electronics was advanced. We conclude that the small-diameter brain PET system with conventional electronics has not so much advantage except for the lower cost and the higher spatial resolution. However, advanced electronics can reduce degradation of NECR, so the small-diameter brain PET system with advanced electronics will be useful for clinical studies.

The purpose of this paper is to identify effective methods to reduce the interference between PET and MRI. First, the author theoretically calculated the magnetic and electrical fields from the radiofrequency (RF) coil as a function of the distance from it. Then with a 0.3-T MRI and a silicon photomultiplier (Si-PM) block detector, the author measured the RF-induced noise signal amplitudes as a function of the distance from the coil. Although both the magnetic and electrical fields decreased dramatically as the distance from the coil increased, the magnetic field decreased more rapidly than the electrical field. From experiments, the induced noise signal decreased rapidly as the distance from the coil increased. With these results, the most effective method to reduce the interference between PET and MRI is taking the distance between the coil and the Si-PM arrays. Using the Cu shield around the detector will not be very effective in PET/MRI systems since it is located in the near field where the magnetic field in RF is dominant.

The luminescence imaging of water during irradiation of beta particles with energy lower than the Cerenkov-light threshold has widely been considered impossible because such particles do not emit Cerenkov light. Contrary to this consensus, we found that luminescence imaging of water is in fact possible with such relatively low-energy beta particles. Beta particles from a 500-kBq 45Ca beta source (maximum energy: 257 keV) were irradiated to pure water, and the source’s luminescence images were acquired with a high-sensitivity, cooled charge-coupled device camera. We also conducted image acquisition for an acrylic plate and a plastic scintillator. The water’s luminescence image during beta particle irradiation became visualized after a 1200-s acquisition time. The luminescence intensity with the beta irradiation of water was 0.54 photons/MeV, and the luminescence intensity with that of the acrylic plate was 3.3 photons/MeV. Consequently, we have shown that luminescence imaging of water using beta particles with energy lower than the Cerenkov-light threshold is a promising new method for beta particle detection and distribution measurements.

Eu:SrI2 bulk single crystals with 1.5-in diameter were grown using the Bridgman–Stockbarger technique and special-shaped crucibles. We will report on the growth of the 1.5-in size Eu:SrI2 single crystal and the preliminary test of the same sample cut in size of ${10 \times 10 \times 10}$ mm3 with polishing. An energy resolution of 3.2% full width at half maximum (FWHM) was obtained for 137Cs. After cutting and polishing the grown crystal to the size of ${3 \times 3 \times 3}$ mm3, ${8 \times 8}$ matrix Eu:SrI2 arrays were fabricated. We made a test module consisting of Eu:SrI2 arrays, which was optically coupled with position sensitive photomultiplier tube (PSPMT) and multi-pixel photon counter (MPPC) array. The position and energy performance of the test module were evaluated using 137Cs and 57Co radioactive sources. As a result, we were able to resolve all pixels clearly. A good energy resolution of 6.7% ± 0.7% (FWHM) and 8.2% ± 2.5% (FWHM) for 662 keV was obtained using PSPMT and MPPC arrays, respectively. In the same way, energy resolution of 12.7% ± 1.3% and 14.7% ± 3.4% (FWHM) for 122 keV was obtained using PSPMT and MPPC-array, respectively. These energy resolutions considerably degraded compared with an energy resolution of the single piece of Eu:SrI2 sample owing to the thinness of the Teflon reflector. This conclusion can be considered as a strong motivation for future research on the choice of reflector materials and structure of the Eu:SrI2 array.

The intrinsic spatial resolution of the detector is an important aspect to reconstruct high quality nuclear medicine images. In order to realize a high spatial resolution, small pixelated Cerium doped Gd3Al2Ga3O12 (Ce:GAGG) scintillators–known for their high light output, high density and relatively fast decay time, were coupled to a digital silicon photomultiplier (dSiPM) for read out. This detector module consists of a $ {24 \times 24}$ Ce:GAGG scintillator block of 0.4 mm $ {\times }0.4$ mm $ {\times }5$ mm crystal elements and a 2 mm thick light guide made of acrylic resin. The digital silicon photomultiplier (dSiPM), DPC-3200-22-44, was used to acquire 2-D position histogram, energy resolution and coincidence timing resolution. The 2-dimensional position histogram of the Ce:GAGG block detector irradiated with 22Na gamma photons showed that most pixels were clearly resolved with an average peak-to-valley (P/V) ratio of ~3.4. The average energy resolution of the Ce:GAGG block detector was 17.8%. At trigger scheme 1 ( $ {1}^{\mathrm {st}}$ photon trigger), the coincidence timing resolution was 342 ± 7 ps FHWM when acquired in coincidence with a 2 mm $ {\times }2$ mm $ {\times }6$ mm LSO crystal (after skew correction). This study demonstrates that a pixelated Ce:GAGG block coupled with dSiPM provides good position performance and time resolution. We conclude that the highly pixelated Ce:GAGG detector module may be a good candidate for implementing ultrahigh resolution nuclear medicine systems and applications requiring good timing resolution.

Optical fiber is a promising material for integrated positron emission tomography (PET) and magnetic resonance imaging (MRI) PET/MRI systems. Because its material is plastic, it has no interference between MRI. However, it is unclear whether this material can also be used for a single photon emission tomography (SPECT)/MRI system. For this purpose, we developed an optical fiber-based block detector for a SPECT/MRI system and tested its performance by combining $1.2 \times 1.2 \times 6~\hbox{mm} {{\rm Y}_2}{\rm SiO}_5$ (YSO) pixels into a $15 \times 15$ block and was coupled it to an optical fiber image guide that used was 0.5-mm in diameter with 80-cm long double clad fibers. The image guide had $22 \times 22~\hbox{mm}$ rectangular input and an equal size output. The input of the optical fiber-based image guide was bent at 90 degrees, and the output was optically coupled to a 1-in square high quantum efficiency position sensitive photomultiplier tube (HQE-PSPMT). The parallel hole, 7-mm-thick collimator made of tungsten plastic was mounted on a YSO block. The diameter of the collimator holes was 0.8 mm which was positioned one-to-one coupled to the YSO pixels. We evaluated the intrinsic and system performances. We resolved most of the YSO pixels in a two-dimensional histogram for Co-57 gamma photons (122-keV) with an average peak-to-value ratio of 1.5. The energy resolution was 38% full-width at half-maximum (FWHM). The system resolution was 1.7-mm FWHM, 1.5 mm from the collimator surface, and the sensitivity was 0.06%. Images of a Co-57 point source could be successfully obtained inside 0.3 T MRI without serious interference. We conclude that the developed optical fiber-based YSO block detector is promising for SPECT/MRI systems.

For small animal positron emission tomography (PET) research using high radioactivity, such as dynamic studies, the resulting high random coincidence rate of the system degrades image quality. The random coincidence rate is increased not only by the gamma photons from inside the axial-field-of-view (axial-FOV) of the PET system but also by those from outside the axial-FOV. For brain imaging in small animal studies, significant interference is observed from gamma photons emitted from the body. Single gamma photons from the body enter the axial-FOV and increase the random and scatter coincidences. Shielding against the gamma photons from outside the axial-FOV would improve the image quality. For this purpose, we developed a body shield for a small animal PET system, the microPET Primate 4-ring system, and evaluated its performance. The body shield is made of 9-mm-thick lead and it surrounds most of a rat’s body. We evaluated the effectiveness of the body shield using a head phantom and a body phantom with a radioactivity concentration ratio of 1:2 and a maximum total activity of approximately 250 MBq. The random coincidence rate was dramatically decreased to ~1/10, and the noise equivalent count rate (NECR) was increased ~6 times with an activity of 7 MBq in the head phantom. The true count rate was increased to ~35% due to the decrease in system deadtime. The average scatter fraction was decreased to 1/2.5 with the body shield. Count rate measurements of rat were also conducted with an injection activity of approximately 25 MBq of [C-11]N,N-dimethyl-2-(2-amino-4-cyanophenylthio) benzylamine ([C-11]DASB) and approximately 70 and 310 MBq of 2-deoxy-2-(F-18)fluoro-D-glucose ([F-18]FDG). Using the body shield, [F-18]FDG images of rats were improved by increasing the amount of radioactivity injected. The body shield designed for small animal PET systems is a promising tool for improving image quality and quantitation accuracy in small animal molecular imaging research.

Ce doped Gd3Al2Ga3O12 (Ce:GAGG) is a newly developed single-crystal scintillator which has a large light output and longer emission light wavelength. The longer wavelength of the scintillation photons will produce a larger signal when coupled to typical silicon photomultiplier (Si-PM) as the quantum efficiency of semiconductor based photodetector is generally higher for light with longer wavelength. A block detector with higher spatial resolution may thus be realized by combining Ce:GAGG with Si-PM arrays. To achieve the highest possible spatial resolution for PET and SPECT detectors, we developed an ultrahigh resolution block detector using 0.4 mm × 0.4 mm × 5 mm Ce:GAGG pixels assembled to form a 24 × 24 matrix that is coupled to an Si-PM array and evaluated the performance. All Ce:GAGG pixels were separated in the 2-dimensional position histograms for Cs-137 (662 keV) gamma photons with an average peak-to-valley (P/V) ratio of 2.4. The energy resolution was 21.6% FWHM for Cs-137 (662 keV) and 23.8% for Co-57 (122 keV) gamma photons. Since Ce:GAGG does not contain naturally occurring radioisotope (Lu), beta-gamma true coincidences can be avoided and randoms are reduced when used for PET detectors. Furthermore, this property, together with its high light output and good intrinsic energy resolution, make the scintillator suited for SPECT detectors. An ultrahigh resolution PET/SPECT hybrid system might be an interesting application using Ce:GAGG/Si-PM block detectors.

A new single-crystal Cerium doped Gd3Al2Ga3 O123Al2Ga3 O12 (GAGG) scintillation crystal with high luminosity, high density and relatively fast decay time has successfully been grown. We report on the first performance results of the new GAGG scintillation crystal read out with silicon photomultipliers (SiPM) from Hamamatsu (MPPC) and FBK. The best energy resolution (511 keV peak of Ge-68) of 7.9% was attained with GAGG coupled to MPPC and 9.0% with the FBK SiPM after correcting for non-linearity. On the other hand, the best coincidence resolving time (FWHM) of polished 3 × 3 × 5 mm3 and 3 × 3 × 20mm3crystals were 464 ±12 ps and 577 ±22 ps for GAGG crystals compared to 179 ±8 ps and 214 ±6 ps for LYSO crystals respectively with MPPCs. The rise time of GAGG was measured to be 200 ps (75%) and 6 ns (25%) while the decay time was 140 ns (92%), 500 ns (7.7%) 6000 ns (0.3%).

Brain functional studies using PET have advantages over fMRI in some areas such as auditory research in part because PET systems produce no acoustic noise during acquisition. However commercially available PET systems are designed for whole body studies and are not optimized for brain functional studies. We developed a low cost, small, wearable brain PET system named PET-Hat dedicated for brain imaging. It employs double counter-balanced systems for mechanical supports of the detector ring while allowing the subject some freedom of motion. The motion enables subject to be measured in the sitting position and move relatively freely with the PET during acquisition. The detector consists of a Gd2 SiO5 (GSO) block, a tapered light guide and a flat panel photomultiplier tube (FP-PMT). Two types of GSO are used for depth-of-interaction (DOI) separation allowing the use of a small ring diameter without resolution degradation. The tapered light guide allows the use of larger GSO blocks with fewer FP-PMTs. Sixteen detector blocks are arranged in a 280 mm diameter ring. Transaxial and axial field-of-view (FOV) are 20 cm and 4.8 cm, respectively. Energy resolution of the block detectors was ~15% full width at half maximum (FWHM) and timing resolution was ~4.6 ns FWHM. Transaxial resolution and axial resolution at the center of the FOV were ~4.0 mm FWHM and ~3.5 mm FWHM, respectively. Sensitivity was 0.7% at the center of the axial FOV. Scatter fraction was ~0.6. Hoffman brain phantom images were successfully obtained. We conclude that the PET-Hat is a promising, low cost, small, wearable brain PET system for brain functional studies.

In order to detect the breast cancer at the early stage with high efficiency, we intend to develop positron emission mammography (PEM), using Pr:LuAG single crystal as a scintillator of this scanner. PEM has the same mechanism of PET, where two 511 keV annihilation gamma-rays are detected by two gamma cameras at opposite side, and reconstruction of the data shows the position of the cancer with malignancy grades. After cutting and polishing processes for each crystal to a physical dimensions of 2.1 × 2.1 × 15 mm3, Pr:LuAG array covered with BaSO4 reflector was used as a sensor head. The one camera unit consisted of 20 × 64 pixels optically coupled with three H8500-03 multi anode PMTs. Our PEM required four cameras at each side. Finally, eight cameras were installed in both sides of the prototype instrument and the spatial resolution was evaluated using the 18F in the breast phantom.

A multi-slice depth-of-interaction (DOI) MR-compatible PET system for imaging small animals such as mice or rats was developed and tested It employs DOI block detectors made of mixed lutetium silicates (MLSs), optical fibers, and multi-channel position sensitive photo-multiplier tubes (PSPMTs). The size of a single MLS was 2.5 mm times 3.5 mm times 3.5 mm and eight MLSs were combined to form a DOI block detector using the Anger principle and connected to four optical fibers with light guides. Thirty-two sets of block detectors were arranged in a 51 mm diameter circular ring. The scintillation light signals from the DOI detectors are read from the side of the detector ring. The configuration makes the diameter of the detector ring small and easy to use in the small imaging area of the MRI. Position histograms showed good separation for all block detectors. The timing resolution was 6 ns FWHM. The measured transaxial spatial resolution was 2.8 mm FWHM at the center of the image and the axial resolution was 3 mm FWHM. Sensitivity was around 0.23% at the center of the axial field of view (FOV). Simultaneous imaging of PET and MRI was also tested using a 0.15 T permanent magnet system with a 30-mm FOV. The detector part of the PET system was set inside the gradient coil of the MRI system and the RF coil was positioned inside the FOV of the PET system. No difference was found in the PET images among outside MRI, inside MRI without gradient and RF, and inside MRI with the gradient and the RF sequences. Also there is no noticeable change of the MRI images with and without the detector ring of the PET system. We also successfully obtained simultaneously measured rat brain with the PET and MRI. These results indicate that the developed MR-compatible PET system can be used for simultaneous measurements with an MRI system.

This paper investigates issues in preparing corpora for developing speech-to-speech translation (S2ST). It is impractical to create a broad-coverage parallel corpus only from dialog speech. An alternative approach is to have bilingual experts write conversational-style texts in the target domain, with translations. There is, however, a risk of losing fidelity to the actual utterances. This paper focuses on balancing a tradeoff between these two kinds of corpora through the analysis of two newly developed corpora in the travel domain: a bilingual parallel corpus with 420 K utterances and a collection of in-domain dialogs using actual S2ST systems. We found that the first corpus is effective for covering utterances in the second corpus if complimented with a small number of utterances taken from monolingual dialogs. We also found that characteristics of in-domain utterances become closer to those of the first corpus when more restrictive conditions and instructions to speakers are given. These results suggest the possibility of a bootstrap-style of development of corpora and S2ST systems, where an initial S2ST system is developed with parallel texts, and is then gradually improved with in-domain utterances collected by the system as restrictions are relaxed

This paper addresses the challenging problem of automatically evaluating output from machine translation (MT) systems that are subsystems of speech-to-speech MT (SSMT) systems. Conventional automatic MT evaluation methods include BLEU, which MT researchers have frequently used. However, BLEU has two drawbacks in SSMT evaluation. First, BLEU assesses errors lightly at the beginning of translations and heavily in the middle, even though its assessments should be independent of position. Second, BLEU lacks tolerance in accepting colloquial sentences with small errors, although such errors do not prevent us from continuing an SSMT-mediated conversation. In this paper, the authors report a new evaluation method called “g Rader based on Edit Distances (RED)” that automatically grades each MT output by using a decision tree (DT). The DT is learned from training data that are encoded by using multiple edit distances, that is, normal edit distance (ED) defined by insertion, deletion, and replacement, as well as its extensions. The use of multiple edit distances allows more tolerance than either ED or BLEU. Each evaluated MT output is assigned a grade by using the DT. RED and BLEU were compared for the task of evaluating MT systems of varying quality on ATR's Basic Travel Expression Corpus (BTEC). Experimental results show that RED significantly outperforms BLEU.

In this paper, we describe the ATR multilingual speech-to-speech translation (S2ST) system, which is mainly focused on translation between English and Asian languages (Japanese and Chinese). There are three main modules of our S2ST system: large-vocabulary continuous speech recognition, machine text-to-text (T2T) translation, and text-to-speech synthesis. All of them are multilingual and are designed using state-of-the-art technologies developed at ATR. A corpus-based statistical machine learning framework forms the basis of our system design. We use a parallel multilingual database consisting of over 600 000 sentences that cover a broad range of travel-related conversations. Recent evaluation of the overall system showed that speech-to-speech translation quality is high, being at the level of a person having a Test of English for International Communication (TOEIC) score of 750 out of the perfect score of 990.

We propose and test a one-dimensional (1-D) sharing block detector using dual photo-multiplier tubes (PMT). The detector consists of scintillator blocks and dual PMTs. The dual PMTs are optically coupled on the scintillator blocks and overlap in one dimensional. With this configuration, the scintillator size of the block detector can be reduced to half that of a conventional block detector in one direction, and isotropic spatial resolution can be obtained in transaxial and axial directions using commercially available dual PMTs. First, we measure the position and energy response for minimum configuration; one GSO block and two dual PMTs. The size of a single GSO is 2.9 mm/spl times/2.9 mm/spl times/20 mm. The scintillators are arranged in an 8/spl times/8 matrix with multi-layer optical film inserted partly between scintillators to obtain the resolved position response. We obtain good position and energy responses with this configuration. Next, we fabricate a 1-D sharing block detector with five GSO blocks and six dual PMTs. With this configuration, we also obtain reasonable position and energy responses. These results indicate that the proposed 1-D sharing block detector is a promising solution for developing high-resolution and low cost PET systems.

We proposed and tested the a block detector for a multislice, depth-of-interaction (DOI) MR-compatible positron emission tomography (PET). The detector consists of a block detector, four optical fibers and photo-multiplier tubes (PMTs). The block detector is a lutetium oxyorthosilicate (LSO) DOI detector comprising eight LSOs arranged in a 2/spl times/2/spl times/2 matrix based on the concept proposed by Murayama et al. The size of a single scintillator of a prototype block detector was 2/spl times/2/spl times/2 mm. The scintillation photons are collected from the side of the scintillator block detector and are transferred to four channels of the position-sensitive PMT (PSPMT) using four optical fibers several meters long. The outputs of the PSPMT signals are calculated to determine the position of gamma interaction in the block detector. Results show that although the light loss from using the fiber was around 90%, there were sufficient transferred scintillation photons to obtain the photo-peak and to calculate the position of gamma interaction in the block detector with reasonable separation. We also modified the block detector to improve the performance of the detector. The size of a single scintillator of the modified block detector was enlarged to be 2.5 mm (transaxial) /spl times/ 3.5 mm (axial) /spl times/ 3.5 mm (depth) to improve the sensitivity. Good position responses as well as time resolution of 5.6-ns full-width at half-maximum (FWHM) were obtained for the modified block detector. With these results, we conclude that a two-rings, two-layer DOI-MR-compatible PET can be realized using the proposed block detector.

We developed and tested a new underground radon detector using an optical fiber. Previous underground radon detector used a photo-multiplier tube (PMT) of small diameter behind the chamber thus the diameter of the underground radon detector was determined by the size of the PMT. The larger diameter of the detector needed considerable labor for drilling a hole in under ground soil. The new underground radon detector consists of a small chamber, an optical fiber, and a PMT. The small chamber is a scintillation detector using a ZnS(Ag) film. The optical fiber transfers the scintillation light produced in the chamber to the PMT that positioned out of the soil. In these configurations, the size of the detector could not be determined by the size of the PMT. The diameter of the optical fiber used was 5mm and the outside diameter of the detector that buried into the soil could be reduced to be 12mm. Although the light loss due to the optical fiber was about 90%, the signal level of scintillation was much larger than the noise level of the PMT and electronics. We measured performance of the underground radon detector. The energy response had a clear distribution due to alpha particles by radon and its decay products. Both the rise time and fall time of the detector was approximately 2 hours. Sensitivity was approximately 0.01 counts/h/Bq/m/sup 3/, one third of the previous underground radon detector. These results indicate the developed radon detector can be used for continuous measurements of radon concentration in underground soil with easy handling.

A new input function monitoring system has been developed using the GSO detector assembly for both PET and SPELT quantitative studies. Due to the rapid rise time of the pulse (about 10nsec), the coincidence time window can be reduced < 1nsec, reducing contribution of randoms associated with the high background activity surrounding the detector. Large light output improved the energy resolution of approximately 11% for 511keV photons, and 20% at 140 keV, as compared with the BGO detector, enabling the use of this system also in SPELT studies. The paired assembly of crystals provided the absolute sensitivity of approximately 7% for PET and 75% for PET tracers. Multiple arrangement of the paired detectors provided possibility of correcting for the transit time of radioactivity through the catheter tube. This study demonstrated that the present system can be of use in both clinical and small animal studies using SPECT and PET tracers.

A new method for a four-stage depth of interaction (DOI) detector is proposed. The four-stage DOI detector is constructed with two kinds of Gd/sub 2/SiO/sub 5/:Ce (GSO) crystals doped with different amounts of Ce, 0.5 mol% and 1.5 mol%. The amount of Ce in GSO determines the scintillation decay time constant, and it is 60 ns for the 0.5 mol% GSO and 35 ns for 1.5 mol% GSO. This difference led to the introduction of pulse-shape discrimination, which would distinguish between two kinds of event data from respective GSOs and sort them into two groups. By independently applying Anger-type position arithmetic to the data of each group, two two-dimensional (2-D) histograms are obtained. The crystal of interaction can be identified on these histograms in which only 0.5 mol% or 1.5 mol% GSO crystal elements are expressed. To evaluate this method, we constructed the four-stage DOI detector by alternately stacking 1.5 mol% GSO crystal stages and 0.5 mol% stages. The result of a scanning measurement with a /sup 137/Cs gamma-ray beam proved that the DOI detector had enough accuracy in crystal identification.

We have developed a phoswich detector composed of a plastic scintillator and a BGO, for a continuous blood sampling system. The beta particle (positron) from the tube is detected by the plastic scintillator and emits 511 keV gamma photons. The BGO scintillator that is optically coupled to the plastic scintillator detects one of the gamma photons. Since the decay time of the plastic scintillator and the BGO are very different, it is possible to discriminate true (beta+gamma) events from background gamma events. First, the pulse height and pulse shape of plastic scintillator for beta particles and BGO for 511 keV gamma photons were measured and compared to estimate the possibility of the proposed method. Then proposed phoswich detector was fabricated and tested. Absolute sensitivity for Ga-68 (maximum energy of 1.90 MeV) and F-18 (maximum energy of 633 keV) positrons were measured and compared with conventional beta detector in similar size. The absolute sensitivity of the developed detector was O.15 counts/Bq for Ga-68 positrons; at the center of the detector that was approximately 5 times higher sensitivity than the conventional beta detector. The absolute sensitivity of the developed detector for F-18 positrons was 0.017 counts/Bq while that of the conventional beta detector was zero. The count rate of the developed detector was linear up to 10 kcps. The background count rate was small. These results indicate that the developed detector is useful not only for higher energy positrons such as 0-15 but also lower energy positrons such as F-18 or C-11.

A real-time radon monitoring system that can simultaneously measure radon concentrations in multiple sites was developed and tested. The system consists of maximum of four radon detectors, optical fiber cables and a data acquisition personal computer. The radon detector uses a plastic scintillation counter that collects radon daughters in the chamber electrostatically. The applied voltage on the photocathode for the photomultiplier tube (PMT) acts as an electrode for radon daughters. The thickness of the plastic scintillator was thin, 50 /spl mu/m, so as to minimize the background counts due to the environmental gamma rays or beta particles. The energy discriminated signals from the radon detectors are fed to the data acquisition personal computer via optical fiber cables. The system made it possible to measure the radon concentrations in multiple sites simultaneously.

Dynamic images of positron distribution in artery was measured from the skin surface of human subject using beta camera for noninvasive measurement of the input function. After estimating the possibility of the method by some phantom studies, the beta camera was attached from the skin surface on the artery of wrist during the PET measurement using the O-15 water injection method. After injection of O-15 water, dynamic images of the artery were measured. Regions of interest (ROIs) were set on the artery and on the adjacent area of the artery. From the ROIs analysis, a time activity curve similar to that measured by the invasive technique was noninvasively obtained.

Tapered fiber optic plate (tapered fiber) is a fiber optic plate which input size and output size is different. One can magnify the light photon image by using the tapered fiber with low light loss. The tapered fiber may be a useful component for a high resolution positron emission tomograph (PET) block detector. Thus, investigations on resolution improvement were performed by using the tapered fiber on a PET block detector. The block detector consists of a scintillator block, a tapered fiber and a position sensitive photomultiplier tube (PSPMT). The light photon image of the scintillator block is magnified by tapered fiber which is optically coupled to the PSPMT. Light loss by using the tapered fiber was 27%. With the taper fiber, scintillator blocks consist of 11/spl times/11 matrix of 1.67 mm/spl times/1.67 mm/spl times/10 mm BGOs as well as 8/spl times/8 matrix of 2.0 mm/spl times/2.0 mm/spl times/10 mm were resolved in two dimensional distribution. It is concluded that tapered fiber is one solution for high resolution PET block detectors.

The authors have developed a GSO depth of interaction detector. Three GSO scintillators with different concentration of Ce were chosen and stacked along depth of interaction direction and optically coupled to a photomultiplier tube (PMT) or a position sensitive photomultiplier tube (PSPMT). In the beginning, pulse shape, light output and zero-cross time were measured so as to select a proper combination of GSO scintillators with different concentrations of Ce. Selecting three GSO scintillators with different concentration of Ce, a single stacked GSO/PMT depth of interaction detector was fabricated for estimating the possibility of this method. Using the zero-cross time analysis, three peaks correspond to decay times of the three GSO scintillator were obtained. Finally, a GSO/PSPMT depth of interaction block detector was fabricated and tested. The block detector consists of 7/spl times/7 matrix of GSO scintillators which have three layers of different concentrations of Ce optically coupled to a PSPMT. The pulse shape analysis was used for determination of depth of interaction direction while the Anger logic was used for transaxial and axial directions. Good separations were obtained in depth of interaction direction as well as transaxial and axial direction. These results indicate that the GSO block detector may become one solution for developing a positron emission tomograph (PET) with depth of interaction detection capability.

We have developed and tested a high resolution beta camera for a direct measurement of positron distribution on the brain surface of animals. The beta camera consists of a thin CaF/sub 2/(Eu) scintillator, a tapered fiber optic plate (tapered fiber) and a position sensitive photomultiplier tube (PSPMT). The tapered fiber is the key component of the camera. We have developed two different sizes of beta cameras. One is 20 mm diameter field of view camera for imaging the brain surface of cats. The other is 10 mm diameter camera for that of rats. Spatial resolutions of the beta camera for cats and rats were 0.8 mm FWHM and 0.5 mm FWHM, respectively. We demonstrated that developed beta cameras may overcome the limitation of the spatial resolution of positron emission tomography (PET).

Factors which affect count-rate characteristics for a positron emission tomograph (PET) were investigated using a model based on the data acquisition system for a direct slice using Headtome IV. Object size and geometrical design were found to affect the count-rate capability. Low efficiency or low packing fraction of the scintillator degraded the count rate. Thinner slice thickness, larger ring diameter, and shorter length of the slice shield seriously degraded the count rate, and the true-to-single ratio was inversely proportional to each of them. The electronics had relatively small affect compared with the affect of geometrical design. Predominant electronics factors were the deadtimes of the group encoder and the coincidence circuit, while the effect of time window was insignificant. It is concluded that to obtain a better count rate, higher efficiency, larger slice thickness, smaller detector ring diameter, and longer slice shield are desirable as well as minimization of the deadtime of the system when designing a PET. >

The use of the public data network for facsimile communication has remarkable advantages with regard to high speed and reliable transmission and, in addition, it is well applicable to provide value added facilities for future service extension. The Research and Development Laboratories of Kokusai Denshin Denwa (KDD) Company, Ltd. have developed an experimental facsimile communication system on the packet switched data network (PSDN). The system is composed of packet switching exchanges (PSE), high speed digital facsimile terminal equipment, a facsimile communication processor (FCP), and facsimile packet assembly/ disassembly equipment (FAX-PAD). The facsimile equipment in the system adopts a two-dimensional modified READ code and has the interface protocol to the PSDN. The FCP provides the store-and-forward facilities to provide such services as delayed delivery, multiaddress delivery, and mailbox service. The FAX-PAD realizes the interworking between facsimile equipments on the public switched telephone network (PSTN) and those on PSDN. After various experiments including international field tests, it was confirmed that the system is useful for future advanced facsimile service.

A whole-body positron emission tomograph called Headtome-IV has been developed, and its physical performance has been investigated. An in-plane spatial resolution of 4.5 mm was realized with stationary sampling at the center of the field of view. The axial slice thicknesses were 9.5 and 9.0 mm for direct and cross planes, respectively. By moving the gantry framework axially, transaxial images of 14 or 21 slices are obtained quasi-simultaneously. The real-time large-scale cache memory system allowed real-time corrections for deadtime and radionuclide decay and real-time weighted integration for the purpose of a rapid calculation of rate-constant images. >

Headtome IV is a PET (positron emission tomograph) which consists of 96 detector units per ring. Each detector unit consists of eight 3-mm-wide bismuth germanate (BGO) crystals, a pair of light guides, and a dual module photomultiplier tube and uses pulse-height-ratio logic to determine the crystal of interaction. In-plane resolution of 4.5 mm full width at half maximum (FWHM) was obtained without any sampling motion such as wobble. The authors investigated three in-plane resolution characteristics of the Headtome IV, i.e. resolution with distance from the center, variation of the in-plane resolution and the effect of gain imbalance. >

The relationship between the ring diameter of the transmission line source in a positron emission tomograph and the signal-to-noise ratio of the transmission data is estimated. It is confirmed that a smaller diameter improves the signal-to-noise ratio of both transmission and emission images. This signal-to-noise ratio improvement is clearly shown as an improved count density of true events. It is shown that the count density is inversely proportional to the diameter. >