1. A method for estimating kinetic parameters from image data, the method comprising using a processor to execute instructions for: providing a model of kinetic contributions from first and second physiological regions; grouping voxels of the image data into first and second groups; determining an average value of the factors associated with the first group, the factors corresponding to blood-flow time activity curves; incorporating the average value of the factors associated with the first group into the model of kinetic contributions from the first and second physiological regions; and estimating the kinetic parameters based on the model of kinetic contributions.