As part of the present invention, a motion detector for detecting motion inside of a body may include a first oscillator to produce an electrical scanning signal and a second oscillator to produce an electrical Doppler signal. A vibrating element may vibrate in response to the scanning and Doppler signals. A vibration transducer may receive vibrations reflected from a structure inside the body and may convert the reflected vibrations into an electrical signal. Doppler shifts between the transmitted and received vibrations may be used to estimate the velocity of the structure inside the body.