1. A single-photon emission computerized tomography imaging apparatus comprising: first ring-shaped means including thereon a plurality of first detector elements and surrounding an object under examination to which radioisotope has been administered; second ring-shaped means including thereon a plurality of second detector elements and concentrically surrounding the first ring-shaped means, wherein a gamma ray irradiated from the administered radioisotope through the object firstly scatters in one of the first detector elements, producing a first detector signal and thereafter the scattered gamma ray disappears in one of the second detector elements, producing a second detector signal; first analyzing means including a plurality of first single channel analyzers coupled to the corresponding first detector elements for analyzing whether an energy level of the first detector signal is involved in a first predetermined level width of the first single channel analyzers and producing a first analyzing signal; second analyzing means including a plurality of second single channel analyzers coupled to the corresponding second detector elements for analyzing whether an energy level of the second detector signal is involved in a second predetermined level width of the second single channel analyzers and producing a second analyzing signal; image reconstruction means for reconstructing a computerized tomographic image signal by processing the first and second analyzing signals, said first and second analyzing signals containing both first data on the positional information of the first and second detector elements upon which said gamma ray and scattered gamma ray are incident, and second data on the energy information derived from the first and second analyzing means; and means for displaying a computerized tomographic image by receiving the reconstructed tomographic image signal.