Provided is a method for projection and back-projection for method and apparatus for image processing. The method for projection and back-projection includes the steps of: projecting the center of a detector cell included in a detection unit onto a common axis； mapping the boundaries of pixels in a pixel grid onto the common axis； projecting the center of the detector cell included in the detection unit onto the set common axis； and determining a pixel value on the basis of the step for mapping the boundaries of pixels onto the set common axis.
