1. A medical image photographing apparatus comprising: an X-ray generator configured to irradiate an X-ray toward an object that is located in a three-dimensional (3D) virtual grid space which includes a plurality of voxels; an X-ray detector comprising a plurality of detecting elements and configured to detect the X-ray that has propagated through the object; and an image processor configured to process projection image data corresponding to the detected X-ray based on a volume of a first region within the plurality of voxels, through which the X-ray has propagated.