A single transducer element that is capable of oscillation at a plurality of natural resonant frequencies may be used in an ultrasonic imaging catheter assembly including a catheter body configured to be inserted and guided through the vascular system of a living being, a lumen and a rotatable imaging core adapted to pass through the lumen, the imaging core including a flexible drive-shaft. Because the transducer element is capable of oscillation at a plurality of natural resonant frequencies, a user can switch from one frequency to another in order to improve the depth of field or resolution without having to switch out the catheter or imaging core.