A system for measuring the size of a gastric lumen includes a gastric banding device, an antenna positioned outside of the patient's body, and a receiver in operative communication with the antenna. The banding device comprises two orthogonal ultrasonic transceiver modules, a microprocessor, and a transmitter in electrical communication with the microprocessor. The microprocessor is in operative communication with each of the two transceiver modules. The transmitter transmits signals to a receiver positioned outside of a patient's body. Another system for measuring the size of a gastric lumen includes a gastric banding device having a first coiled conductor. The system further comprises a circuit external to the banding device, the circuit comprising a second coiled conductor. The circuit further comprises a tunable frequency generator and a spectrum analyzer, the circuit being tunable to allow a resonant frequency.