Apparatus is provided for use with skin of a subject, the apparatus including a handheld unit and an ultrasound transducer disposed in the handheld unit and configured to transmit ultrasound energy into the skin. An outwardly-facing surface of the handheld unit includes a transducer-skin interface that is configured (a) to contact the skin when the handheld unit is positioned against the skin and (b) to slide against the skin. A sensor is disposed in the handheld unit and is configured to detect a distance between the transducer-skin interface to a bone under the skin of the subject when the transducer-skin interface is positioned against the skin. Control circuitry is configured to drive the ultrasound transducer to apply the ultrasound energy into the skin at different respective power levels according to the detected distance.