Ultrasonic apparatus and method for high resolution intravascular imaging to assist in treating endovascular lesions and to monitor the results of interventional treatment. An ultrasonic transducer is carried by the distal end of a catheter adapted for insertion into a vessel, and either the transducer or another element is rotated and/or translated relative to the catheter to image different portions of the vessel, an inflatable balloon carried on the distal extremity of the catheter covering at least part of a longitudinal section through which the ultrasonic transducer is swept during imaging without substantially attenuating the ultrasonic waves emitted from the transducer.