A measuring device, comprising, (a) an ultrasonic acoustic transceiver unit capable of sending and receiving acoustic signals into the body of a patient; (b) frequency modulating circuitry which drives said transceiver unit with a signal whose frequency varies with time; and (c) processing circuitry which extracts an indication of a distance, from at least one signal detected by said transceiver unit, said signal being a reflection of a transmission of said transceiver unit driven by said time varying frequency signal. Optionally, the indication is a distance between walls of a bladder.