1. A method for quantifying contrast agent behavior of an administrated contrast agent within a region of interest, comprising: obtaining a time sequence of measurements, I(t), of contrast agent concentration starting before the contrast agent enters the region of interest and a time sequence of measurements, J(t), of the contrast agent concentration as the contrast agent perfuses the region of interest; deconvolving I(t) from J(t) to obtain an estimate of the tissue impulse response function for the region of interest, R′(t), where R′(t) is constrained to take into account at least one of physiologic and physical considerations and is chosen such that I(t) R′(t) is the maximum likelihood estimate of J(t), where denotes convolution; and determining the behaviour of the contrast agent in the region of interest based on R′(t).