1. An ultrasound imaging catheter, comprising: a catheter body having proximal and distal ends; an acoustic window portion coupled to the distal end of the catheter body, the acoustic window portion having an interior surface; a phased array ultrasound transducer housed within the acoustic window portion and configured so that a gap exists between the phased array ultrasound transducer and the interior surface of the acoustic window; an acoustic coupling fluid filling the gap between the transducer array and the interior surface of the acoustic window; and a drive motor positioned near the proximal end of the catheter body and mechanically configured and coupled to the phased array ultrasound transducer so as to rotate the phased array ultrasound transducer about its long axis.