An ultrasonic transceiver apparatus for intracorporeal use, comprises an undamped ultrasonic transceiver for placing in a confined intracorporeal space, the transceiver vibrating at an instantaneous frequency and being excited to cause vibration at said instantaneous frequency to produce an ultrasonic ablation beam for ablating surrounding tissues and being further excited to cause vibration by primary echo signals returning from surrounding tissues—from separate excitations during quiet periods between the ablation—to monitor ablation progress. A signal processor isolates the primary echo signals from ringing, secondary echoes and extraneous noise also received from the transceiver and uses presence or absence of the characteristic frequency, or of a body characteristic frequency such as pulse or breathing, as an isolation criterion.
