1. A method using a priori data for a non-invasive input function definition to a pharmacokinetic modeling procedure, comprising the acts of: receiving a medical image; providing the a priori data that describes at least one attribute of the received medical image; with the provided a priori data and for the received medical image, computing a reference input time-of-activity curve (TAC) having at least one major parameter; and defining the input function to the pharmacokinetic modeling procedure using the computed reference input, wherein a region-of-interest (ROI) definition module receives the computed reference input TAC; wherein the defining act comprises the acts of: identifying best-matching voxels of the received medical image that best match the computed TAC; region-growing using the identified best-matching voxels to automatically define at least one ROI.