No abstract available

[en] In a scintillation camera detector, a pair of outputs from a number of photomultipliers aligned in the X-direction, for example, are applied to associated intermediate taps of two delaylines through two associated groups of attenuators adjusted so that the delay times and attenuations of the delay line outputs correspond proportionally to the positional coordinates, in the X and -X directions, at which the scintillators of the photomultipliers are located. Both outputs of the delay lines are formed, expanded and then fed to ratio circuits for obtaining respective ratio signals to a Z signal. Both ratio signals are fed to a subtraction circuit where a difference signal is obtained for determining the position of a luminous point. Since the contributions of the photomultipliers remote from the luminous point are separated in time by insertion of the delay lines, the resolution of the camera can be improved. (Ohno, Y.)

[en] In recent years there has been growing concern over the biological effects of low-dose X-rays, but few studies have addressed this issue. Our laboratory had observed flies (Drosophila melanogaster) irradiated with low-dose X-rays tend to emerge earlier than normal flies. This observation led us to quantitatively examine the effects of low-dose X-irradiation on development in the fly. Following exposure of prepupal (day 5) flies to 0.5 Gy X-rays, the time to emergence was slightly shorter than in the sham controls. This tendency was increased when the X-ray exposure came during the pupal stage (day 7). In these flies, the time to eclosion decreased significantly, by an average of 30 h sooner than sham controls. A further experiment examined whether such radiation effects could be observed in the unexposed F1 generation of exposed individuals. Greater radiation effects on early F1 emergence were seen when the time between exposure and mating was 3 days, indicating an effect on early spermatid development. Early F1 emergence was also observed after exposure of female flies to X-rays during late previtellogeny. Furthermore, rapid emergence could be induced in the F1 embryos of unexposed parents by transferring the polar cytoplasm (precursor cells of the germ cell line) from F1 embryos of exposed flies. These results show that radiation-induced effects can be transmitted to the next generation through the germ cell line

[en] Both single photon emission computed tomography (SPECT) and positron emission tomography (PET) technologies have been dramatically improved during the recent one decade. They play an important role in providing the functional information of organ tissues, especially the heart and brain. SPECT system has entered the era of quantitative measurement associated with the improvement of scattering correction, which is approaching the performance of PET. For PET, sophisticated technologies, such as high resolution and sensitivity, have been developed. PET system providing three-dimensional data acquisition is promising. This review focuses on the current status of both SPECT and PET, with special emphasis on design and performance in both systems, their sensitivity and resolution, correction for the quantitative measurement in SPECT, and three-dimensional PET. (N.K.) 53 refs

[en] In order to improve image quality in positron emission tomography (PET), a PET device needs to achieve full utilization of emitting photons, and it must be intimately linked to any 3D image reconstruction algorithm. Both the analytical and the iterative 3D reconstruction algorithms suitable for 3D PET systems are classified and evaluated from a computational point of view. For the analytical algorithms, the computation time is analyzed briefly and the required performances of a computer for 3D PET systems are provided. It is suggested that successful utilization of their algorithms depends not only on the performances of a computer, but also on the size of the image space due to the practical constraints of detected photon counts. Some guidelines are provided for the choice of an appropriate reconstruction algorithm in a given situation. (author)

[en] Cone-beam single photon emission computed tomography (SPECT) offers the potential for a large increase in sensitivity as compared with parallel hole or fan-beam collimation. Three-dimensional image reconstruction was approximately accomplished by backprojecting filtered projections using a two-dimensional fan-beam algorithm. The cone-beam projection data were formed from mathematical phantoms as analytically derived line integrals of the density. In order to reduce the processing time, the filtered projections were backprojected into each plane parallel to the circle on which the focal point moved. Discrepancy of source position and degradation of resolution were investigated by computer simulation in three-dimensional image space. The results obtained suggest that, the nearer to the central plane or the axis of rotation, the less image degradation is performed. By introducing a parameter of angular difference between the focal point and a fixed point in the image space during rotation, degradation of the reconstructed image can be estimated for any cone-beam SPECT system. (author)

[en] In order to improve image quality in positron emission tomography (PET), a PET device needs to achieve full utilization of emitting photons, and it must be intimately linked to any 3D image reconstruction algorithm. Both the analytical and the iterative 3D reconstruction algorithms suitable for 3D PET systems are classified and evaluated from a computational point of view. For the analytical algorithms, the computation time is analyzed briefly and the required performances of a computer for 3D PET systems are provided. It is suggested that successful utilization of their algorithms depends not only on the performances of a computer, but also on the size of the image space to be reconstructed due to the practical constraints of detected photon counts. Some guidelines are provided for the choice of an appropriate reconstruction algorithm in a given situation. (author)

[en] In order to improve image quality in positron emission tomography (PET), a PET device needs to achieve efficient utilization of emitting photons, and it must be intimately linked to any 3 D image reconstruction algorithm. A lot of 3 D image reconstruction algorithms are available due to the redundancy of the projection data. The condition to proceed 3 D image reconstruction proposed by Orlov is interpreted with the central slice theorem of the 3 D Radon transform in order to provide a subset of the projection directions. The 3 D reconstruction algorithm proposed by Orlov is analyzed and evaluated as a part of the filtered backprojection (FILBK) algorithms to develop the extended Orlov's equation. It is suggested that the Orlov's expression can be expanded to more general algorithms with introduction of weighted summation among possible projection directions and their selected direction area, all of which are satisfied with the Orlov's condition. Such a generalization is promising to characterize different 3 D image reconstruction algorithms in a given situation. (author)

[en] This paper proposes scintillation cameras which can handle input pulse rates beyond 1 Mcps with negligible loss of spatial resolution and energy resolution at low pulse rates. The cameras are based on the ''variable sampling-time technique'', in which phototube current pulses are first shortened in width and then selectively intergrated by gated intergrators followed by sample-and-hold circuits. The period of integration before sampling is not fixed but shortened by the arrival of the following pulse to keep a high pulse-pair time resolution. An approximate expression for the count rate characteristics has been derived, and its validity was confirmed by Monte Carlo simulaton. The result indicates that the maximum observed count rate will reach 420-520 kcps with energy window fraction of 0.5 for typical design parameters. (author)

[en] Several iterative reconstruction methods appropriate for emission tomography are compared in regard to the rate of convergence and image quality, by using the noise-free and noisy projection sets generated from a mathematical phantom using a computer. Two modified versions of the simultaneous iterative reconstruction technique (SIRT) method are introduced as the additive SIRT (ASIRT) and the multiplicative SIRT (MSIRT). The MSIRT method is analogous to the iterative space reconstruction algorithm (ISRA). Simulation studies proved that the MSIRT method has faster convergence and better image quality than the ISRA method. It is suggested that, though the expectation maximization (EM) method gives good estimates for the same noisy projection set among all the methods reported here, the fast methods such as the filtered iterative reconstruction algorithm (FIRA) and the conjugate gradient method in weighted version (CONGRW) are good candidates for practical image reconstruction. (author)