1. A method for use in assessing collateral blood flow to a tissue region of interest, comprising: obtaining at least a first pulsatility measurement and a second pulsatility measurement indicative of the volume of blood flow to a tissue region of interest, wherein one of said first and second pulsatility measurements is obtained with a plurality of arteries supplying blood to the tissue region of interest, and wherein the other of said first and second pulsatility measurements is obtained with at least one of said plurality of arteries substantially occluded; and, employing said first pulsatility measurement and said second pulsatility measurement to obtain a value employable to assess collateral blood flow to the tissue region of interest.