Ablation apparatus such as a catheter carrying an ultrasonic ablation device including an ablation transducer and a balloon reflector structure for directing ultrasonic energy from the ablation transducer into a ring-like ablation region is provided with an imaging ultrasonic transducer mounted on an imaging probe which may be inserted into or through a passageway extending through the catheter and into or through the ablation device to image the ablation region or neighboring regions. Alternatively, the imaging transducer may be mounted within the balloon reflector structure or distal to this structure.