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

The first high frequency ultrasound system able to image blood flow in the microcirculation in real-time has been developed. 2-D color flow frames are rapidly acquired using a recently reported method[l] to achieve frame rates approaching 10 fps. A new flow phantom was constructed in order to tune the wall filter order, cutoff and attenuation for a 25 MHz, f/2 transducer. RF data were acquired in both M-mode and swept-mode, and processed in order to tune the wall filter. These filters were then used in making controlled measurements of flow velocity and volume flow rate for a typical PRF of 500 Hz (1 min/sec scan speed). Over the input of mean axial velocities ranging from 0.3 to 3.0 mm/sec (0.88 to 8.8 mm/sec angle corrected), the measured mean and maximum flow velocities were linear, with slight over-estimation of mean velocities due to the wall filter cutoff. Without correction for finite beam size, the volume flow rates were over-estimated by a factor of 2. The color flow settings were then applied to image microcirculatory flow within the nail bed of a human finger, where they were tested and optimized for a variety of vessel sizes and flow velocities.