This paper proposes a continuously rotating positron ECT system, in which the detectors are arranged on a circular ring with "non-uniform" spacing so as to provide much finer sampling interval in the projections than the conventional circular ring system. The address signals on the coincidence events are read out through a rotary photo-coupler, and the detector system can be rotated at a desired speed. Suitable detector arrangements are searched which provide sufficiently fine sampling interval with reasonable sampling density uniformity.Assuming about 50 detectors are arranged on a circle, the sampling characteristics of various detector arrangements have been examined. An iterative search method has also been applied to find a reasonable arrangement. The best arrangement has been obtained by the iterative method.It has an effective sampling interval of the order of (1/200)R, where R is the radius of the detector array, and the sampling density uniformity after 5-point smoothing is -7.8 X, +18 % in the range of + R/2.

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.

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