Kenji Ishimatsu

Performance Study of Whole-Body, Multislice Positron Computed Tomograph POSITOLOGICA-II

A 3-detector ring, 5-slice whole-body positron CT has been developed and is being tested. The PCT, POSITOLOGICA-II, has a total of 480 BGOs (160/ring), and employs continuous rotation scan (0.5rps). By using a 15mm wide BGO, a 9.2mm FWHM spatial resolution for reconstructed image is obtained at the center of FOV. Measured phantom diameter dependence of the true count rate shows good agreement with theoretically anticipated characteristics, including maximum sensitivity at around 30cm dia. Sensitivities for 20cm dia. phantom are 28 and 38 kcps/¿Ci/ml for in-plane and cross-plane, respectively, including scattered coincidences. Relative system detection efficiency measured with a line source at FOV center is 96% at 15ns time window (90% at 12ns), basing on 100% efficiency in 20-24ns.

Evaluation of Slice Shield Collimators for Multi-Layer Positron Emission Computed Tomographs

Evaluation of slice shield collimators is carried out using theoretical event rate formulas. This approach permits calculation of system performance in terms of sensitivity, scatter fraction, and high-counting rate characteristics. Electronics counting efficiency is also taken into account. A rational approach to optimal design is presented along with design examples. Effectiveness of collimator replacement is suggested. A system using sub-collimator with relatively small detector ring radius is proposed. It achieves a level of performance similar to that of a conventional collimator system with large ring radius.

Advanced coded-aperture imaging system for nuclear medicine

An advanced coded imaging system is described, and some results of phantom experiments are presented. The advanced method uses a pair of coherent codes (+ 1 and -1 codes) and has many advantages compared with conventional ones. One of the greatest advantages is that there are no sidelobes in the focal plane and only a few in other planes. Therefore, when an object can be regarded as two-dimensional, it is perfectly reconstructed with high detecting efficiency, and this is successfully simulated by a thyroid phantom with 99mTc. Moreover, this system has an ability to reconstruct tomograms, which is also shown by using ring phantoms piled on one another with some cold spots in their shells. From these experimental results it may be concluded that the new system is useful for practical applications, for example, to nuclear medicine.

Coded-aperture imaging system for reconstructing tomograms of human myocardium

To increase the detection efficiency and improve the spatial resolution, a coded-aperture imaging method is applied to nuclear medicine. The aperture consists of nine pinholes arranged in a square grid. Three kinds of coding are sequentially used to record the same number of projections including parallax and overlap. The overlapped images are partially separated, and good tomograms of a ring phantom and a human myocardium are reconstructed using a modified backprojection algorithm with variable damping factor.

Production system for 18F-2-deoxy-2-fluoro-D-glucose--a trial for automatic production.

We have developed a production system for 18F-2-deoxy-2-fluoro-D-glucose (18F-2FDG), which assures reliable production with easy handling and reduces radiation exposures to the operator. Chemical procedures in this system are the same as manual method developed in NIRS. This system has 2 operation modes; one is remote controlled manual operation mode and the other is microcomputer controlled automatic operation mode. In remote controlled mode, we tested this system 5 times and 18F-2FDG synthesized was supplied for clinical use once. The mean radiochemical yield of 18F-2FDG from the target gas recovery with decay time correction was 8%, that is the same as in the manual synthesis. It took about 2 hours from end of bombardment (EOB) to end of synthesis (EOS). Since this time is shorter than in manual synthesis, the available activity at EOS is increased.

Nuclear battery with silicon p-n junctions

シリコンp-n接合素子および147Pm線源 (有効放射能0.1～0.2Ci) をそれぞれ5枚交互に重ねあわせて原子力電池を構成した。最大出力1.2μW, エネノレギー変換効率0.3%を得た。単色電子ビームを使用した実験から, 素子の最大出力は入力の1.2～1.3乗で増加することがわかった。147Pm線源に対して最大出力を与えるシリコンp-n接合素子の接合部深さは2～4μであった。さらに, 147Pmを線源にした場合, 素子の放射線損傷による最大出力の低下は5%/6, 200時間であった。