Design for an ultrasound-based instrument for measurement of tissue blood flow

Quantitative measurement of tissue blood flow is of importance for many reasons: detection of capillary flow disturbances, evaluating indications for amputation, investigating vasospastic conditions, and evaluating vasoactive drugs or the effect of sympathectomy. Methods of investigating tissue blood flows include skin thermometry, thermal conductance or clearance, transcutaneous PO2, laser doppler flux, and photoplethysmographic techniques. A major reason that ultrasound has not previously been applied to this problem is that at the very low blood velocities in the capillaries (0.1 to 1 mm/sec) the Doppler shift is small (on the order of 1 Hz/MHz of carrier signal). Advantages of ultrasound are a noninvasive procedure and penetration depths greater than that possible with optical techniques. This paper discusses the development of a Doppler ultrasound instrument which allows spectral resolution of the velocities in tissue blood flow. Briefly, a low phase noise oscillator at 7.5 Mhz is used with two transducers to obtain the shifted signal. After mixing with the fundamental, the resulting low frequency signal is fed into a high resolution spectrum analyzer for analysis.