An apparatus and method are described for imaging blood flow from within a vasculature. An ultrasound catheter probe carrying an ultrasound transducer array is inserted within a blood vessel. The transducer array emits ultrasound excitation signals and receives ultrasound echo waveforms reflected from blood and tissue in a region of the vasculature. A series of the echo waveforms resulting from a series of excitation signals are combined in a manner such that the echo signals from static features in the region, such as tissue and plaque, are significantly attenuated. The combined signal primarily represents the relatively dynamic features in the region (i.e. the blood flow). A blood flow image is constructed from the combined signal. The blood flow image is colorized and combined with an image of the relatively static features in the region. Thereafter, the combined image is displayed on a video display.