High-frequency 3-D color-flow imaging of the microcirculation

Abstract

High-frequency (> 20 MHz) ultrasound (US) flow imaging has the potential to be an important tool for assessing microvascular blood flow in superficial tissues noninvasively. In this paper, we describe the development and evaluation of a 3-D US flow imaging system capable of operating at center frequencies in the 20- to 50-MHz range. Flow images are made for tissue volumes of sizes up to 10 mm laterally and 5 mm in depth, permitting a range of scientific and clinical applications. To acquire data sets in a reasonable time, the 2-D sections were derived from data collected with a transducer that was scanning continuously in a direction perpendicular to the beam axis. Due to spectral broadening effects induced by scanning tissue, significant tradeoffs must be made between frame rate, lateral resolution and the minimum detectable blood velocity. 3-D flow images were reconstructed with flow data acquired from a series of adjacent planes. The system was evaluated at a center frequency of 50 MHz, using two PVDF transducers with lateral resolutions of 43 microm and 65 microm and axial resolutions of 66 microm to 72 microm, respectively. Velocity ranges were from below 1 mm/s to 25 mm/s. In vivo validation experiments using the mouse ear demonstrated the ability to follow branching patterns of closely spaced microvessels from 30 microm to 100 microm in diameter. Experiments conducted on mouse tumors successfully imaged microvessel morphology in the tumor microcirculation.