A bi-directional real-time blood flowmeter using an implantable CMUT array

This paper describes the development of a bidirectional pulsed-wave ultrasound Doppler blood flow measuring system using a miniature CMUT array suitable for minimally invasive diagnosis. On this flowmeter, the CMUT devices, integrated on a silicon probe smaller in diameter than a human' hair, were used both for ultrasound transmission and reception. The Doppler shift was recognized by comparing the demodulated echo ultrasound with the incident ultrasound. The demodulated ultrasound signal was sampled by a pulse delayed in time domain from the transmitted burst which corresponds to detecting the signal at a specific distance. Two sets of Doppler signals were connected to a phase quadrature demodulator for detection of the flow direction (either forward or reverse with respect to the CMUT array). With this interface circuit, both the rate and direction of blood flow were displayed in real time about 5-10 ms per signal segment. The resolution of the flowmeter was 2 mm/second per 10 Hz of Doppler frequency shift.