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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. For the positron emission tomography (PET), we newly designed a four-layer depth of interaction (DOI) detector, which is constructed of all the same crystal elements. In this DOI detector, appropriate insertion of the reflector controls 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 (2-D) 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 Gd2 SiO5 crystals with Ce concentration of 0.5 mol %. Each crystal is in dimensions of 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 2-D position histogram. We concluded that it would be a promising device to realize a small animal PET scanner with high sensitivity and high resolution. [ABSTRACT FROM AUTHOR]

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 3.04mm intervals between them so that the 256ch FP-PMT affords optical coupling with a 16 × 16 array of scintillation crystals having 3 mm pact 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 Gd2SiO5 (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. [ABSTRACT FROM AUTHOR]

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 x 2 Gd[sub2]SiO[sub5]: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. [ABSTRACT FROM AUTHOR]