Abstract: A closed-loop single-crystal ultrasonic sonomicrometer capable of identifying the myocardial muscle/blood interface and continuously tracking this interface throughout the cardiac cycle using a unique piezoelectric transducer. During steady-state, a reference volume SV.sub.m is adjusted so that it remains within the myocardium while half of an interface volume SV.sub.i is in the myocardium and one-half is in the ventricular chamber. Every 100 ns, a 0.5 .mu.s duration ultrasonic burst of 10 MHz pulses is transmitted from the piezoelectric transducer. Myocardial layers and red blood cells in the ventricular chamber reflect the ultrasonic energy back to the piezoelectric transducer. The echoes are then amplified and applied to a Doppler decoder, which then mixes them with a local oscillator signal having a 2 kHz offset relative to the transmitted signal. When the offset signal is mixed with returning ultrasonic echoes, the resulting Doppler signals are shifted in the frequency domain by 2 kHz.