A high-frequency continuous-wave Doppler ultrasound system for the detection of blood flow in the microcirculation

Basic ultrasound physics and several clinical and experimental observations suggest that high-frequency Doppler ultrasound (HFD) operating in the frequency range 20-100 MHz holds the promise of detecting blood flow in the microcirculation. This article describes a directional, continuous-wave (CW), 1- to 200-MHz Doppler ultrasound system. The system electronics have a dynamic range of 100 dB, a noise floor of 10 nV and a directional isolation of 50 dB. The development of a 40-MHz Doppler transducer composed of two, 81-micron-thick, lithium niobate crystals that have been air-backed and transmission-line tuned for maximum sensitivity is described. This device is used to test the CW Doppler system using string and capillary phantoms and in vivo tissue. We show that HFD can detect and measure velocities on the order of the blood velocities found in the capillaries (1 mm/s) and arterioles (5 mm/s) with suitable velocity (50-500 microns/s) and temporal (20-250 ms) resolutions. In vivo measurements demonstrate that HFD is sensitive to the detection of blood flow in small vessels.