A diagnostic imaging device includes detector elements (16) for detecting y-rays indicative of nuclear decay events. Pairs of concurrently detected γ-rays define lines of response (LORs) which are collected, time stamped, and compiled in list-mode. In tissue perfusion studies, it is beneficial to use the data that concurrently maximizes contrast and signal-to-noise ratio in the reconstructed images. Using the list-mode data, events in an adjustable temporal window (33) are reconstructed and the reconstructed images are analyzed to determine a figure of merit based on contrast and signal-to-noise properties of the image. By iteratively adjusting the temporal window, extending its start point (36) backwards in time, and repeating the reconstructing, analyzing, and adjusting steps, an image with an optimal figure of merit is obtained.