1. A method for positive emission tomography (PET) target image segmentation, the method comprising: receiving, by a computer system, captured and digitized image data of a selected target; determining, by the computer system, an initial volume estimate of the selected target based on the image data of the selected target; determining, by the computer system, an initial concentration ratio based on an initial source background ratio and the initial volume estimate of the selected target employing a concentration ratio table; determining, by the computer system, a desired threshold from the initial concentration ratio and the initial volume estimate employing a threshold table; and determining, by the computer system, a final volume estimate of the selected target based on the determined desired threshold.