The initial Doppler blood flow measurement using an Implantable CMUT array

This paper describes the initial Doppler blood flow velocity measurement using an implantable capacitive micromachined ultrasonic transducer (CMUT) array. This CMUT array was integrated on a probe-shaped silicon substrate of a typical shank cross-section of 40 mu m (thickness) x 60 mu m (width) and a length of 4-10 millimeters. Smaller in diameter than a human's hair, this CMUT probe can be implanted inside the tissue with a minimal tissue disruption and is potentially useful for monitoring blood flow deep inside the tissue. Pulsed-wave approach was used in this blood flow velocity measurement experiment using ultrasound with a central frequency of 2.2MHz. The time domain method was used for data analysis in order to achieve a higher blood flow rate resolution using a broadband ultrasound signal. The pulse Doppler changes on successive echoes scattered from the moving red blood cells were counted as a progressive time shift. Using correlation procedure, the time difference or distance the red blood cells travel between the two transmitted pulses can be determined. From the correlation curve as a function of period numbers and the time lag between two successive echoes, the blood flow velocity was derived with a resolution of 1.5mm/s.