1. A method for quantitative single photon emission computed tomography (SPECT) with multiple photopeaks, the method comprising: detecting, with a SPECT detector, emissions from a patient, the emissions being at different photopeak ranges of an isotope with respective multiple photopeaks; forward projecting, in reconstruction from the emissions, for a first of the different photopeak ranges; forward projecting, in the reconstruction from the emissions, for a second of the different photopeak ranges; backward projecting in the reconstruction form the emissions, the backward projecting combining feedback from the first and second photopeak ranges; and generating an image as a function of the reconstruction.