Implantable systems and methods (e.g., using an implantable medical device) for measuring distance including a transmit/receive acoustic transducer implantable at a first location for transmitting and receiving acoustic signals, an echo acoustic transducer implantable at a second location for receiving the acoustic signal from the transmit/receive acoustic transducer and in response thereto transmitting a return echo signal to be received by the transmit/receive acoustic sensor, and a controller to control transmission of the acoustic signal from the transmit/receive acoustic transducer at a transmit time and determine a receive time corresponding to the time the transmit/receive acoustic transducer receives the return echo signal. The distance between the transmit/receive acoustic transducer and the echo acoustic transducer is determined as a function of the transmit time and the receive time.
