Gating with anatomically varying durations

An apparatus for reconstructing a volumetric image of an overall volume from radioactive emissions, the apparatus comprising:

• at least one SPECT or PET detecting unit configured for obtaining radioactive emissions from the overall volume, said overall volume encompassing at least part of a body organ or other body portion;
• an image reconstruction module configured for reconstructing 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 reconstructing a further volumetric image from said initial volumetric image by an iterative process using an object implantation module,
• said object implantation module configured for providing a model of at least a portion of the overall volume, said model including a general location and shape of said at least a part 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,
• wherein said image reconstruction module is further configured for reconstructing the further volumetric image by an iterative process, comprising
• 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.