An ablation apparatus comprises an ultrasonic transducer which includes a piezoelectric element having a cylindrical shape; a plurality of external electrodes disposed on the outer surface of the piezoelectric element; and at least one internal electrode disposed on the inner surface of the piezoelectric element. The at least one internal electrode provides corresponding internal electrode portions that are disposed opposite the external electrodes with respect to the piezoelectric element, the external electrodes and the at least one internal electrode to be energized to apply an electric field across the piezoelectric element. The ultrasonic ablation zones of the external electrodes are distributed in a staggered configuration so as to span one or more open arc segments around the longitudinal axis, and the ultrasound ablation zones of all external electrodes projected longitudinally onto any lateral plane which is perpendicular to the longitudinal axis span a substantially closed loop around the longitudinal axis.