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 from the emissions, the backward projecting combining feedback from the first and second photopeak ranges, and wherein backward projecting comprises combining conjugate gradients from the multiple photopeaks; and generating an image as a function of the reconstruction.