1. A method for enabling an ultrasound system to identify a boundary between a tissue area and a blood-filled area, both lying within a region of interest ROI) to which a contrast agent has been administered, said method comprising the steps of: a) transmitting a first ultrasound beam at a first power level into said ROI, processing first echo return signals and storing first digital values in accord therewith; b) transmitting a second ultrasound beam at a second power level into said ROI, processing second echo return signals and storing second digital values in accord therewith; and c) deriving phase information from coherent time segments of said first digital values and second digital values to determine a boundary location between said tissue area and said blood-filled area.