1. An image reconstruction method for structuring planar images into three-dimension images, being adopted to perform an image reconstruction operation with respect to a plurality of measured values, respectively corresponding to a plurality of lines of response (LOR) obtained from the detecting of an object-to-be-imaged, comprising steps of: (a) utilizing a plurality of voxels, each having a first radioactivity value, to construct an image domain with reference to the object-to-be-imaged; (b) projecting the plural voxels with respect to each LOR in respective for converting the same to a projection domain while comparing the result of the projection respectively with the measured values to obtain a plurality of calibrated values corresponding to the plural LORs in respective; (c) obtaining a weighting for each voxel based on an operation performed upon the first radioactivity value of each voxel with respect to a corresponding LOR, wherein the weighting of a voxel with respect to a LOR is the ratio of the first radioactivity value of the voxel and the totality of the first radioactivity value of all voxels passing by the LOR; (d) performing a back-projection operation basing on the weighting of each voxel of each LOR so as to perform a calculation upon the calibrated values of each LOR with respect to the weightings for obtaining a second radioactivity value for each voxel of each LOR; (e) converting the second radioactivity value of each voxel into an updated first radioactivity value by projecting the same back to the image domain; and (f) repeating the step (b) to step (f) by an iterative manner.