1. A method for reconstructing a medical image of a subject, comprising: providing current scan data, selected from the group consisting of PET (Positron Emission Tomography) scan data and SPECT (Single-Photon Emission Computed Tomography) scan data, to a processor, said current scan data resulting from a current scan introduction of a tracer into the subject, said subject also having a tracer introduced into the subject prior to said current scan introduction, and the previously-introduced tracer producing residual activity in said current scan data; in said processor, automatically detecting, from said current scan data, a time of said current scan introduction; in said processor, automatically estimating said residual activity dependent on the detected time of said current scan introduction; in said processor, executing an iterative reconstruction algorithm to reconstruct an image of the subject from said current scan data and, in the iterative execution of said reconstruction algorithm, separating said residual activity in said current scan data from non-residual activity in said current scan data to thereby correct the reconstructed image of the subject from contributions made by said residual activity; and making the reconstructed image available in electronic form at an output of said processor.