1. An ultrasonic diagnostic imaging system for the evaluation of plaque by contrast enhanced ultrasound, the system comprising: an ultrasound imaging probe having an array transducer configured to acquire a sequence of ultrasound images of a plaque region in a carotid artery during contrast agent delivery; a time intensity curve calculator configured to form a plurality of time intensity curves for the plaque region, wherein the plaque region comprises a plurality of pixels and wherein the plurality of time intensity curves includes a time intensity curve for each pixel in the plurality of pixels of the plaque region; a comparator configured to determine, for each pixel in the plurality of pixels in the plaque region, whether contrast is present in a given pixel based on the time intensity curve for the given pixel; and a display configured to display the degree of perfusion in the plaque region.