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