Data is communicated from an implanted section to an external location. The implanted section includes a transducer such as a piezoelectric element with an ultrasound reflecting surface that moves in response to an applied driving signal. A sensor generates an output signal that depends on a sensed parameter, and a control circuit drives the transducer based on the sensors output. The transducers response to the driving signal is repeatable such that the value of the output signal can be determined by measuring the variations in the velocity of the surface using externally applied Doppler ultrasound, and computing the value of the sensors output from the measured variations in velocity.
