1. A method for determining quantitative blood perfusion indices, comprising: administering a bolus containing an optical contrast agent and an MRI contrast agent; determining an arterial input function from optical contrast agent concentration samples after the bolus was administered; determining a tissue function from MRI contrast agent samples in a tissue volume of interest, wherein the contrast optical agent concentration samples and the MRI contrast agent concentration samples are correlated in time; determining at least one quantitative perfusion index from the arterial input function and the tissue function.