1. A method of estimating a parameter of physiological significance, the method comprising: (a) providing one or more images of a tissue in a subject to whom a dose of a contrast agent (CA) has been administered; and (b) determining, using a computer equipped with image processing software, the concentration or relative concentration of the agent in a region or regions of interest in the tissue, thereby generating concentration data; (c) describing the time-based behavior of concentrations of CA within the tissue using a pharmacokinetic model that is based on a set of pharmacokinetic model parameters; and (d) fitting, using computer code, the pharmacokinetic model to the concentration data, varying one or more parameters, wherein the best fit estimates a parameter of physiological significance.