1. An image reconstruction method, comprising the steps of: constructing a system model that has a plurality of concentric rings each segmented into a plurality of sectors; constructing a system matrix associated with one view, by using some of all the sectors constituting the system model, the some of all sectors being located on respective projection lines concerning the one view; creating a system matrix associated with another view, by transforming the system matrix associated with the one view; reconstructing an image through iterative reconstruction by using the system matrix and projection data; and transforming the image into an image composed of rectangular pixels.