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ABSTRACT: We found that Bi<sub>4</sub>Ge<sub>3</sub> O<sub>12</sub> (BGO) scintillator can be elements of a four-layer depth of interaction (DOI) detector and it was proved with a 12×12×4 array of BGO crystals in dimensions of 2.9 mm×2.9 mm×7.5 mm coupled to a 256-channel flat panel position sensitive photomultiplier tube. Appropriate reflector insertion in the array makes all crystal identification possible on one position histogram. Despite the large refractivity and small light output of BGO, the four-layer BGO detector showed no significant variation in the full energy peaks among all crystal elements. When no optical grease was used in the construction of the BGO DOI-block and irradiated with gamma-rays from <sup>137</sup>Cs, a top layer crystal has 80% of light output relative to the bottom layer. The obtained two-dimensional position histogram by the irradiation was clear enough to allow identification of the crystals of interaction. Profiles of the histogram show peak-to-valley ratio of 1.9:1 for the top layer crystals and larger ratio for other layer crystals in the experiment.
IEEE Transactions on Nuclear Science 03/2006; · 1.45 Impact Factor
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ABSTRACT: Previously, we proposed a new depth of interaction (DOI) encoding method and proved that it worked successfully with four-layered Gd<sub>2</sub>SiO<sub>5</sub> crystals for a small animal positron emission tomography (PET) detector. We are now planning to develop a small animal PET scanner, jPET-RD (for rodents with DOI detectors), which has both high resolution and high sensitivity by the use of a DOI detector with a 32×32×4 crystal array. The scintillator for the detector will be Lu<sub>2(1-x)</sub>Y<sub>2x</sub>SiO<sub>5</sub> (LYSO). In this work, we evaluated performance of a DOI detector composed of four layers of a 12×12 LYSO (Lu: 98%, Y: 2%) crystal array by irradiating 511 keV gamma rays uniformly. The new encoding method was used for crystal identification. The size of each crystal was 1.46 mm×1.46 mm×4.5 mm. The crystal block was coupled to a 256-channel flat panel position sensitive photomultiplier tube, which has 16×16 multi anodes at intervals of 3.04 mm. As we expected, all crystals are expressed on a single two-dimensional position histogram without overlapping. Energy resolution of all events is 21.8% and time resolution of all events is 0.69 ns in FWHM. When layers are counted from the top, the energy resolutions of the first, second, third, and fourth layer events are 11.6%, 12.3%, 13.3%, and 19.1% and the time resolutions are 0.60ns, 0.59ns, 0.60ns, and 0.66ns, respectively.
IEEE Transactions on Nuclear Science 03/2006; · 1.45 Impact Factor
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ABSTRACT: jPET-RD is designed to achieve high sensitivity as well as high spatial resolution by the use of four-layer depth of interaction (DOI) information of the detector. We have previously proposed the DOI encoding method that enables four layers DOI identification using only single kind crystal elements. The basic idea was tested by using Gd<sub>2</sub>SiO<sub>5</sub>, and the first prototype detector was developed using Lu<sub>2(1-x)</sub>Y<sub>2x</sub>SiO<sub>5</sub> (LYSO). In this work, we prepared a pair of jPET-RD prototype detectors composed of four layers of a 32 (transaxial)×8 (axial) LYSO (Lu: 98%, Y: 2%) crystal block and a 256-channel flat panel position sensitive photomultiplier tube (256ch FP-PMT). The size of each crystal element is 1.46 mm×1.46 mm×4.5 mm. The crystal block (46.5 mm×11.6 mm×18.0 mm) is placed on the central area of a 256ch FP-PMT (49 mm×49 mm useful area) and coupled with silicone rubber. First, we evaluated performance of the prototype DOI detector by uniform gamma ray irradiation. Then response functions of the prototype DOI detector were measured with collimated single gamma rays and finally coincidence responses are estimated with a pair of prototype DOI detectors in the experimental setup which simulates jPET-RD system. In the performance evaluation, the energy resolution of all events was 14.7% and the time resolution was found to be 0.66 ns. The response functions were 1.56 mm FWHM and 4.51 mm FWHM in average in transaxial and depth direction, respectively. The FWHMs of coincidence responses were 5.4 mm (non-DOI) and 3.7 mm (averaged DOI). It is confirmed that the spatial resolution is improved by using DOI information.
Nuclear Science Symposium Conference Record, 2005 IEEE; 11/2005
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ABSTRACT: The final design of the 4-layer DOI detector module for jPET-D4 system was determined and we produced 120 of the DOI crystal blocks. The jPET-D4 detector module consists of four layers of 16 by 16 Gd<sub>2 </sub>SiO<sub>5</sub> (GSO) crystals and a 256 channel flat panel position sensitive photomultiplier tube (256ch FP-PMT). Two kinds of GSO crystals that show different scintillation decay time constants are used in the upper and lower two layers in order to distinguish these layers using pulse shape discrimination. Proper reflector insertion in the crystal block enables to identify crystals of detection in each two layers. For mass production of the DOI crystal block composed of 1024 crystal elements with the proper reflector arrangement, we used appropriate tools which promote simple construction as well as uniform configuration. The construction took only 3 hours per one crystal block. The produced 120 DOI crystal blocks were estimated their characteristics; pulse shape discrimination, energy resolution and full energy peak. The results show that misidentification in each GSO layer is less than 5% on pulse shape discrimination, the average of energy resolutions for the central four crystals of the 1st (farthest from PMT), 2nd, 3rd and 4th-layer are 15.7 plusmn 1.0%, 15.8 plusmn 0.6%, 17.7 plusmn 1.2% and 17.3 plusmn 1.4%, respectively and full energy peak variation among four layers is less than 5% in average
Nuclear Science Symposium Conference Record, 2005 IEEE; 11/2005
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ABSTRACT: A 256 channel flat panel position sensitive photomultiplier tube (256ch FP-PMT) was evaluated for application to a depth of interaction (DOI) PET detector composed of small crystal elements. The 256ch FP-PMT has 52 × 52 mm opening area with 89% useful area. Its 256 anodes are placed at a 3.04 mm interval. To estimate the FP-PMT performance, light spread functions of anodes at central and peripheral regions of a prototype 256ch FP-PMT were measured with a Gd<sub>2</sub>SiO<sub>5</sub> (GSO) crystal sized 1.42 × 1.42 × 4.5 mm. The crystal cross section area is about one quarter of one anode area of the FP-PMT. It was found that light spread functions of peripheral anodes were similar to the functions of central anodes and also the functions measured with a 2.9 × 2.9 × 7.5 mm GSO crystal which has almost the same dimensions of cross-section as the anode area. Transit time fluctuation among the 256ch FP-PMT anodes was estimated by measuring time resolutions of a LuYSiO<sub>5</sub> (LYSO) crystal at some central and peripheral positions. The LYSO crystal has the same dimensions as the smaller GSO crystal. BaF<sub>2</sub> was used for a reference detector. The obtained distributions show about the same time resolution between the central and peripheral positions and the full width at half maximum is (366 ± 15) ps. The transit time fluctuation is ± 106 ps. Two-dimensional position histograms of a 32 × 32 GSO crystal array and two layers of 9 × 9 GSO crystal arrays are obtained by uniform gamma-ray irradiation. The crystal dimensions are 1.42 × 1.42 × 4.5 mm so that the 32 × 32 array covers all the useful area and indicates variation of crystal identification performance over the useful area. The 9 × 9 × 2 array is coupled to peripheral region. The resultant histograms confirm that a 256ch FP-PMT has enough capability for identifying crystals of this size even on the periphery and in DOI arrangement.
IEEE Transactions on Nuclear Science 03/2005; · 1.45 Impact Factor
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ABSTRACT: A new method to acquire four-layer depth of interaction (DOI) information is proposed for the next generation positron emission tomography scanner (jPET-D4) that realizes high resolution and high sensitivity. The detector module of the jPET-D4 is a 16×16×4 Gd<sub>2</sub>SiO<sub>5</sub>: Ce (GSO) multicrystal array coupled with a 256 ch flat panel position sensitive photomultiplier tube (256 ch FP-PMT) having large opening area. The first challenge to encode DOI information was carried out with 8×8 array of units consisted of 2×2×4 crystal elements. The unit is developed for four-layer DOI encoding in previous report. Its crystal identification performance is evaluated by uniform gamma ray irradiation. The measured scintillation events are mapped on a two-dimensional (2-D) position histogram according to the relative ratio of the multianode output of the FP-PMT. However, peaks corresponding to the crystal elements of one unit form a colony in the resultant 2-D position histogram and there is large space between adjacent colonies. In the new method, the reflector arrangement which makes proper light sharing in the multicrystal array decreases such wasted space. Consequently, peak-to-valley on the 2-D position histogram was improved to 3.3:1 from 1.8:1. We found energy performance was also enhanced by the new method.
IEEE Transactions on Nuclear Science 03/2005; · 1.45 Impact Factor
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ABSTRACT: Previously we proposed a new depth of interaction (DOI) encoding method and proved that it worked successfully with four-layered Gd<sub>2</sub>SiO<sub>5</sub> crystals for a small animal positron emission tomography (PET) detector. We are now planning a small animal PET scanner, jPET-RD (for Rodents with DOI detectors), which has both high resolution and high sensitivity. Scintillator for the scanner will be Lu<sub>2(1-x)</sub>Y<sub>2x</sub>SiO<sub>5</sub> (LYSO). In this work, we evaluated the DOI detector composed of four layers of 12×12 LYSO (Lu: 98%, Y: 2%) crystal array by irradiating 511 keV uniform gamma rays. For crystal identification, the new encoding method was used. The size of each crystal is 1.44 mm×1.44 mm×4.5 mm. The crystal block was coupled to a 256-channel flat panel position sensitive photomultiplier tube, which has 16×16 multi anodes at intervals of 3.04 mm. In this measurement, all crystals are expressed on a single two-dimensional position histogram without overlapping as we expected. Energy resolution of all events is 21.8 % and time resolution of all events is 0.69 ns in FWHM. The energy resolutions of the first, second, third, and fourth are 11.6%, 12.3%, 13.3%, and 12.5% and the time resolutions are 0.60 ns, 0.59 ns, 0.60 ns, and 0.66 ns, respectively.
Nuclear Science Symposium Conference Record, 2004 IEEE; 11/2004
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ABSTRACT: We found that Bi<sub>4</sub>Ge<sub>3</sub>O<sub>12</sub> (BGO) scintillator can be a feasible alternative for a 4-layer depth of interaction (DOI) detector and was proved with a 12times12times4 array of BGO crystals in dimensions of 2.9 mmtimes2.9 mmtimes7.5 mm coupled to a 256 channel flat panel position sensitive photomultiplier tube. Appropriate reflector insertion in the array makes all crystal identification possible on one position histogram. Despite the large refractivity and small light output of BGO, the 4-layer BGO detector showed no significant variation in the full energy peaks and energy resolutions among crystal elements. When no optical grease was used in the construction of the BGO DOI-block and irradiated with gamma-rays from <sup>137</sup>Cs, 0.71 full energy peak of a top layer crystal relative to a bottom layer crystal and about 20 % energy resolutions for each layer crystal was achieved. The obtained two-dimensional position histogram from the irradiation was clear enough to allow identification of the crystals of interaction. Profiles of the histogram show peak-to-valley ratio of 1.9:1 for the top layer crystals and larger ratio for other layer crystals
Nuclear Science Symposium Conference Record, 2004 IEEE; 11/2004
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ABSTRACT: A 256ch flat panel position sensitive photomultiplier tube (FP-PMT) is a promising device for a PET detector because of its large opening area, 52 mm × 52 mm, and small dead space. The useful area of the 256ch FP-PMT is 89% to the opening area. The 256ch FP-PMT contains 16 × 16 matrix anodes with 3.04 mm intervals between them so that the 256ch FP-PMT affords optical coupling with a 16 × 16 array of scintillation crystals having 3 mm × 3 mm bottom area. Its 14.7 mm thickness will also ensure a compact volume and less weight for the PET apparatus. Using a prototype 256ch FP-PMT, we measured light spread function of a central anode and crystal identification ability with 2.9 mm × 2.9 mm × 7.5 mm Gd<sub>2</sub>SiO<sub>5</sub> (GSO) crystals. The full width at half maximum of the light spread function was found to be 4.6 mm. As regards positioning performance, the resultant positioning image map assures its capability for crystal identification in a 16 × 16 array of the GSO crystals. For the use of a 256ch FP-PMT having a large useful area, we made a new proposal for easier construction of an array with many crystal elements. The way utilizing multilayer polymer mirrors, a reflector, properly processed by laser can be applied to other various shaped detectors.
IEEE Transactions on Nuclear Science 03/2004; · 1.45 Impact Factor
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ABSTRACT: We are now planning to develop a positron emission tomograph dedicated to small animals such as rats and mice which meets the demand for higher sensitivity. We propose a new depth of interaction (DOI) detector arrangement to obtain DOI information by using a four-layer detector with all the same crystal elements. In this DOI detector, we control the behavior of scintillation photons by inserting the reflectors between crystal elements so that the DOI information of four layers can be extracted from one two-dimensional (2D) position histogram made by Anger-type calculation. As a preliminary experiment, we measured crystal identification performance of the DOI detector which consists of four layers of a 16 × 16 crystal array using Gd<sub>2</sub>SiO<sub>5</sub> crystals with Ce concentration of 0.5 mol %. Each crystal is 1.42 mm × 1.42 mm × 4.5 mm. A crystal block is optically coupled to a 256-channel flat panel position sensitive photomultiplier tube whose opening area is 52.0 mm × 52.0 mm. We obtained sufficient positioning performance for this four-layer DOI detector on the 2D position histogram. We concluded it would be a promising device to realize a small animal positron emission tomography scanner with high sensitivity and high resolution.
Nuclear Science Symposium Conference Record, 2003 IEEE; 11/2003
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ABSTRACT: As part of the next generation PET project, uniformity of full energy pulse height for all crystal elements was improved in the depth of interaction (DOI) detector constructed of three-dimensional crystal arrays. In our previous report, we found that the DOI detector constructed of four stages of a 2 × 2 Gd<sub>2</sub>SiO<sub>5</sub>:Ce (GSO) crystal array provides good crystal identification performance but poor uniformity of the energy pulse height distribution. The upper stage crystal elements which stay further from the photocathode of a PMT have a tendency to show lower energy pulse height. For example, the ratio of the full energy peak of the top stage crystal to the bottom stage one was about 0.3. We designed a new DOI detector improved in the uniformity. By optimizing crystal surface finishes, reflector configurations, and optical coupling between crystal elements, we got comparable energy pulse height from the upper stage crystals to the bottom stage crystals. Despite this change of detector conditions, good separation between each area corresponding to crystal elements is maintained on two-dimensional histograms obtained by Anger-type position calculation. The uniform full energy pulse height of every stage crystal allows a narrower dynamic range of the electrical circuits, and may give a great advantage in getting an accurate scatter correction. It also improves energy and timing resolution.
IEEE Transactions on Nuclear Science 11/2003; · 1.45 Impact Factor
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ABSTRACT: As part of the next generation PET development project, performances of one-dimensional GSO crystal arrays were measured and analyzed for the development of the depth-of-interaction (DOI) detector constructed of three-dimensional GSO crystal arrays. We considered two structures of one-dimensional crystal arrays, one is a "straight line type" which has the crystals of the array stacked in series, and the other is a "U shaped type" which is a crystal block of stacked crystals bent at the center. By scanning collimated gamma rays along the crystals, we measured the pulse height distributions from one end or both ends of the crystal array for different conditions of the surface of each crystal (rough or chemical etching) and arrangement of the reflector. The analysis of these data showed how these conditions affected the performances of the DOI detector. We also measured two-dimensional pulse height distributions for the "U shaped type". The analysis of the data on the combination of crystal surface treatment and reflector arrangement provided a method to optimize performances of the DOI detector.
Nuclear Science Symposium Conference Record, 2002 IEEE; 12/2002
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ABSTRACT: As part of the next generation PET project, uniformity of full energy pulse height for all crystal elements was improved for the development of the depth of interaction (DOI) detector constructed of three-dimensional crystal arrays. In our previous report, we found a DOI detector constructed of four stages of a 2 by 2 GSO crystal array provides poor uniformity of the energy pulse height distribution. The upper stage crystal elements which are separated from the photocathode of a PMT have a tendency to have a lower energy pulse height. For example, the ratio of the full energy peak of the top stage crystal to the bottom stage was about 0.3. We designed a new DOI detector to overcome this problem. By optimizing reflector arrangement, condition of crystal surface, and optical coupling between crystal elements, we got an energy pulse height in the upper stage which was almost the same height as for the bottom stage crystals. There was also good separation between each area corresponding to crystal elements on two-dimensional histograms calculated by an Anger-type position calculation. The uniform full energy pulse height of every stage crystal simplifies the electrical circuits, and may give a great advantage in getting an accurate scatter correction. It also improves energy and timing resolution.
Nuclear Science Symposium Conference Record, 2002 IEEE; 12/2002
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ABSTRACT: 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</sub>SiO<sub>5</sub>: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</sup>Cs gamma-ray beam proved that the DOI detector had enough accuracy in crystal identification.
IEEE Transactions on Nuclear Science 07/2002; · 1.45 Impact Factor
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ABSTRACT: A new method for a 4-stage depth of interaction (DOI) detector was proposed. The 4-stage DOI detector is constructed with two kinds of Gd<sub>2</sub>SiO<sub>5</sub>:Ce (GSO) crystals doped with different amount 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 introduce pulse shape discrimination, which would distinguish between two kinds 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 2-dimensional 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 4-stage DOI detector by alternately stacking 1.5 mol% GSO crystal stages and 0.5 mol% stages. The result of <sup>137</sup>Cs gamma ray beam scanning measurement proved that the DOI detector had enough accuracy in crystal identification.
Nuclear Science Symposium Conference Record, 2001 IEEE; 12/2001
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K. Takahashi,
N. Inadama,
H. Murayama,
T. Yamaya,
E. Yoshida,
F. Nishikido,
K. Shibuya,
Chih Fung Lam,
I. Oda,
K. Kitamura, H. Kawai
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ABSTRACT: Positron emission tomography (PET) and optical imaging are two major techniques for molecular imaging. The PET scanner which enables simultaneous detection of optical imaging will be a powerful tool in molecular imaging. Here, we propose a new detector that detects both signals: annihilation photons for PET and photons for optical imaging. It is based on a four layered depth-of-interaction (DOI) PET detector. Since the detector must be set at a close distance to an object for optical imaging, the DOI information becomes important to obtain uniform spatial resolution in PET imaging over the field-of-view. The proposed PET/optical imaging detector consists of a scintillation crystal block and a position sensitive photo multiplier tube (PS-PMT). While the bottom of the crystal block is optically coupled to the PS-PMT, four side surfaces of the crystal block are covered with reflectors. A dichroic mirror is placed on the top of the crystal block. The dichroic mirror allows fluorescence photons (longer wavelength than 600 nm) to pass through while it reflects scintillation photons (wavelength around 450 nm) generated within the crystal block. Owing to the dichroic mirror, both scintillation photons originating inside the crystal block and fluorescence photons coming from outside the block can be detected by the same PS-PMT. To evaluate of the DOI-PET/ optical detector, we first measured the influence of the dichroic mirror. The results show that the dichroic mirror maintains DOI-PET detector performance without significant loss of fluorescence photons for optical imaging.
Nuclear Science Symposium Conference Record, 2007. NSS '07. IEEE;
