A medical system includes a shaft, an inflatable balloon, a radial array of ultrasound transducers and a processor. The shaft is configured for insertion into a body of a patient. The inflatable balloon is coupled to a distal end of the shaft and configured to perform a treatment to surrounding anatomy. The ultrasound transducers are distributed circumferentially around the distal end of the shaft inside the balloon, and configured to transmit ultrasound waves at respective radial directions and receive respective ultrasound reflections. The processor is configured to estimate and output to a user, based on the ultrasound reflections received from the ultrasound transducers, an extent of mechanical contact between the balloon and the surrounding anatomy.