Transverse sections representing the magnitude of clearance rates or the mean transit time of radionuclide flow through known volumes of interest are computed from angular projections. First the integrated counts for all time intervals at each projection bin for each angle are computed. These modified projection sums are convolved with a filter and backprojected to form an integrated event image. A second image is formed by backprojected the sum over time of the projected values for each angle weighted time. The ratio of the second image to the first image gives a mean clearance time image with high signal to noise and accuracy after thresholding to remove noise in zero-flow regions. The algorithm is verified by computer simulations and has a major benefit in the savings of computational time by a factor of 10 for anticipated clinical studies of brain and kidney blood flow and lung ventilation and perfusion.