1. A method for estimating kinetic parameters from image data, the method comprising: using a processor for executing 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 into the model; estimating the kinetic parameters based on the model; determining a first seed voxel for which a sum of distances between the first seed voxel and other voxels of the image data is greatest; and selecting, from ungrouped voxels, a first predefined number of ungrouped voxels that are located nearest to the first seed voxel; and wherein grouping voxels of the image data into the first group comprises defining a first group that includes the selected ungrouped voxels and the first seed voxel.