The present invention proposes an ultrasound imaging system and a method of calculating a perfusion parameter ratio. The proposed approach comprises: acquiring a time intensity curve of a contrast agent for each region unit of a plurality of region units of an imaging region of a subject, each region unit comprising one or more points of the imaging region selecting, among the plurality of region units, a first, second and third sets of region units respectively corresponding to normal tissue, vessel and background noise, based on the time-intensity curves calculating a perfusion parameter for each of one or more region units of the first set of region units and a perfusion parameter for a region of interest in the imaging region based on the calculated time-intensity curves calculating a reference perfusion parameter based on the perfusion parameters of the one or more region units and calculating a ratio value for the region of interest as a ratio of the perfusion parameter of the region of interest and the reference perfusion parameter. Since the reference perfusion parameter is calculated based on the one or more region units which are automatically selected, the approach requires no manual manipulation and the calculated perfusion parameter ratios are independent of operator experience.
