Abstract: A method and apparatus are provided for reconstructing 3D image. The method may include the steps of: detecting a plurality of line of response (LORs) emitted from an object; transforming the plurality of LORs into first sinogram data; back-projecting the first sinogram data with a plurality of projection angles to produce image data for the object; and projecting the produced image data with the plurality of projection angles to transform the image data into second sinogram data. The back-projecting may include filling pixels of image plane for each of the plurality of projection angles with the first sinogram data and rotating a coordinate axis of the image plane with corresponding projection angle to produce the image data. The projecting may include rotating the image data with corresponding projection angle in an opposite direction before projecting the image data with the plurality of projection angles. The projecting and the back-projecting may use symmetry properties in coordinate space.