1. A method using a priori data for a non-invasive input function definition to a pharmacokinetic modeling procedure, comprising the steps of: receiving a medical image ( 101 ); providing a priori data that describes at least one attribute of the received medical image ( 303. 1 - 303. 2 ); with the provided a priori data ( 303. 1 - 303. 2 ) and for the received medical image, computing a reference input time-of-activity curve (TAC) ( 301 ) having at least one major parameter; and defining the input function to the pharmacokinetic modeling procedure using the computed reference input TAC ( 302 ).