1. A method for iteratively reconstructing a volumetric image of an overall volume from radioactive emissions, the method comprising: a) obtaining radioactive emissions from the overall volume, the overall volume comprising at least a part of a body organ or other body portion; b) using the radioactive emissions to reconstruct an initial volumetric image of said overall volume, said initial volumetric image containing an initial location and initial shape of said at least a part of a body organ or other body portion and an initial estimation of number of photons emitted from said at least a part of a body organ or other body portion; and c) reconstructing a further volumetric image from said initial volumetric image by an iterative process using object implantation for refining reconstruction, wherein said object implantation includes: providing a model of at least a portion of the overall volume, said model including a general location and shape of said at least apart of a body organ or other body portion and an expected number of photons emitted from said at least a part of a body organ or other body portion; replacing, at said general location, at least a portion of said initial volumetric image with said general shape of said at least a part of a body organ or other body portion, based on said model; determining an improved estimation of a number of photons emitted from said at least a portion of said initial volumetric image, based on said expected number of photons, wherein said improved estimation is an increase in number of photons over said initial estimation; and replacing said initial estimation of number of photons with said improved estimation, wherein said object implantation is used one or more times during said iterative process, each time for providing a better starting point for performing a next iteration of said iterative process, whereby said improved estimation is used to redistribute photon counts in an iteration.