1. A method for constructing an image of a region within a vasculature indicative of blood flow within the region, the method comprising the steps of: emitting an ultrasonic waveform from within a lumen of the vasculature, the ultrasonic waveform thereafter propagating through the region within the vasculature; sensing echoes of the emitted ultrasonic waveform arising from reflections of the ultrasonic waveform from tissue and blood within the region; converting the sensed echoes into an echo waveform; repeating the emitting, sensing and converting steps a plurality of times to obtain a set of echo waveforms arising from the sensed echoes from the region; combining the set of echo waveforms to form a modified echo waveform indicating areas within the region occupied by relatively dynamic features, including flowing blood, the portions of the set of echo waveforms arising from relatively static features in the region, including tissue, being substantially attenuated in the modified echo waveform; and processing the modified echo waveform to provide a first image of the region of the vasculature, the first image primarily representing relatively dynamic features in the region.