Optimization of real-time high frequency ultrasound for blood flow imaging in the microcirculation

The first real-time high frequency ultrasound system capable of visualizing and measuring blood flow parameters in the superficial microvasculature has been developed. The system uses a recently reported method, called swept-scan or swept-Doppler, to rapidly acquire 2-D frames for color flow frame rates approaching 10 fps. To optimize the system, a new high frequency flow phantom was constructed. RF data from the phantom was processed to optimize wall filter order, cutoff and attenuation for a 25 MHz, f/2 transducer for both M-mode and swept-mode. These parameters were then used in processing measurements of flow velocity and volume flow rate from within the phantom at a typical operating PRF of 500 Hz (scan speed of 1 mm/sec). For swept-scanning at 500 Hz, the mean and maximum flow velocities were found to be linear over a range of mean axial velocities ranging from 0.3 to 3.0 mm/sec (0.88 to 8.8 mm/sec angle corrected), though the mean velocities were over estimated due to the wall filter cutoff. The measured flow rates were consistent with the known flow rates, after considering the finite size of the resolution cell. Optimized color flow settings were then applied to in vivo imaging of the microcirculatory flow within the nail bed region of a human finger.