A medical device includes a capacitive micromachined ultrasonic transducer (CMUT) array configured to emit ultrasound to target tissue, and at least one thermoelectric cooler mechanically coupled with the CMUT array and configured to cool non-target tissue heated by the ultrasound. The medical device may be implemented in a catheter together with a solid thermal conductor coupled to the thermoelectric cooler and extending along the catheter, to conduct heat away from the thermoelectric cooler. A catheter or catheter sleeve includes a tubular wall for insertion into a body channel, and at least one thermoelectric cooler coupled to the tubular wall for cooling the body channel wall. A catheter sleeve includes tubular casing for insertion into a body channel and capable of encasing a catheter, and at least one sensor coupled to the tubular casing for sensing one or more properties of the body channel wall, such as temperature and pressure.