A dual use ultrasonic transducer device for combined sensing and power transmission, comprises a first piezoelectric transducer sized for placement in a body lumen； a power unit enabling an ultrasonic power beam for tissue ablation in a tissue ablation region； and a sensing unit enabling an ultrasonic sensing beam for sensing at said tissue ablation region. In one example, a single piezoelectric surface is electrically connected to a mounting； and the mounting provides a first region of the piezoelectric surface with a first relatively high level of damping and a second region of said piezoelectric surface with a second relatively low level of damping, thereby to enable sensing from said first region and power transmission from said second region. Changes in efficiency of the transducer or the treatment during use may be inferred from changes in the impulse response or impedance or changes in the temperature of liquids that have flowed past the transducer.
