Echo-Lagrangian particle tracking: an ultrasound-based method for extracting path-dependent flow quantities

Abstract

Eulerian, ultrasound-based velocimetry has become a popular tool for evaluating non-optically accessible flows, and has demonstrated great potential for medical flows. In contrast, the current study presents a Lagrangian method of extracting path-dependent dynamics from time-resolved ultrasound images referred to here as echo-Lagrangian particle tracking (echoLPT). Ultrasound system parameters specific to Lagrangian tracking are detailed for recording pulsatile flow through an idealized stenosis model. Furthermore, seeding materials and image processing procedures are discussed in order to improve signal-to-noise ratio and minimize particle image ambiguity. The pathlines that result from echoLPT reveal mixing downstream of the stenosis, and yield time-resolved, path-dependent information. As a means to demonstrate the value of echoLPT, particle residence time (PRT) in the post-stenotic region is calculated. PRT is the length of time a fluid parcel remains within a region of interest, and is used to highlight the effects of pulsatility. For the pulsatile flows tested, PRT is shown to increase with the frequency of pulsation as fluid is swept into the recirculation region, while PRT is decreased with increasing mean Reynolds number and amplitude ratio.