1. A method of correcting for residual activity due to an earlier tracer in a later PET or SPECT scan image at reconstruction, thereby generating a residual-corrected later image, the method comprising the steps of: estimating the residual activity by detecting the time of an introduction of a tracer for the later PET or SPECT scan; and separating the residual activity from the true counts during iterative reconstruction of the PET or SPECT scan image.