A high-frequency pulsed-wave Doppler ultrasound system for the detection and imaging of blood flow in the microcirculation.

Previous work with a 40-MHz continuous-wave Doppler ultrasound system has demonstrated the potential of high-frequency Doppler ultrasound (HFD), operating in the frequency range 20-200 MHz, to detect blood flow in the microcirculation. This paper describes a directional, pulsed-wave high-frequency Doppler ultrasound (PW HFD) system that was designed and constructed further to investigate this potential. The PW HFD system electronics have a dynamic range of > 80 dB, a noise floor of 250 nV, a directional isolation of 45 dB and operate over the frequency range 1-200 MHz. The system is tested using a focused PVDF transducer that is tuned for maximum sensitivity at 50 MHz, has a -6 dB lateral beamwidth of 70 microns and -6 dB depth-of-focus of 0.90 mm. This permits the practical use of sample volumes with dimensions 70 microns laterally by 90-900 microns axially. The PW HFD system can operate in duplex mode by either sharing the Doppler transducer or using a second PVDF transducer for imaging. Tests with string and capillary flow phantoms demonstrate that the PW HFD system is capable of detecting velocities on the order of the blood velocities found in the capillaries (0.5 mm/s) and arterioles (5 mm/s) with suitable velocity (30-300 microns/s) and temporal (15-100 ms) resolutions. In vivo measurements demonstrate that PW HFD can detect and measure blood velocities of less than 5 mm/s in arterioles and venules with diameters as small as 20 microns and 35 microns, respectively, using a sample volume of only 70 microns laterally by 150 microns axially. Preliminary experiments with high-frequency colour Doppler (HFCD) and high-frequency power Doppler (HFPD) imaging are also presented.