A method and apparatus for reconstructing an image by generating a scatter-corrected sinogram are provided. In the reconstructed image in which initial scattering is not corrected, an emission position of a photon is determined. A graphics processing unit (GPU) calculates migration of the photon using a Monte-Carlo simulation. A 3-dimensional (3D) line of response (LoR) may be calculated through a position in which the photon reaches a detector. The calculated LoR may be converted into a 3D sinogram form and stored in a prompt sinogram and a scatter-corrected sinogram. A final scatter-corrected sinogram may be generated by adjusting a scale of the scatter-corrected sinogram.
