A medical ultrasound device is disclosed. The device comprises an elongated body having a proximal end, a distal end (10) and a distal end region (1). One or more ultrasound transducers (4) for generating acoustic radiation are positioned in the distal end region, inside the elongated body. A transmission element (5) which is substantially transparent to acoustic radiation is positioned in the radiation path of the acoustic radiation, and a controller unit is operatively connected to the ultrasound transducer. The controller unit detects the acoustic path length through the transmission element and determines the temperature at the distal end from the detected acoustic path length. In an embodiment, the medical device is an ultrasound RF ablation catheter.