The measurement of multi-component temporal blood velocity and shear stress distributions in the cardiovascular system is important in hemodynamic evaluation of patients with various cardiovascular diseases since changes in local flow patterns may reflect development and progression of pathology. The current technique for obtaining multiple blood flow components, MRI phase velocity imaging, is cumbersome and limited by its poor temporal resolution and high cost.

We have recently developed an ultrasound based velocimetry technique, termed echo particle image velocimetry (Echo PIV), to measure velocity profiles, multiple velocity vectors and local shear rate in arteries and opaque fluid flows by identifying and tracking flow tracers (ultrasound contrast microbubbles) within these flow fields. The original system was implemented on images obtained from a commercial echocardiography scanner. Although promising, this system was limited in spatial resolution and measurable velocity range.

In this work, the key requirements for accurate multi-component blood flow velocimetry for peripheral vascular imaging were first investigated. Then a custom designed Echo PIV system was developed from this analysis, with increased spatial resolution and measurable velocity range. Using this system as an example, standard rules for characterizing Echo PIV performance were proposed to shed light on optimizing Echo PIV techniques for general blood flow applications.

Then this system was employed for initial measurements on tube flows, rotating flows, jet flows, and in vitro carotid artery and abdominal aortic aneurysm (AAA) models to acquire the local velocity distributions in these flow fields and shear rate distributions. The experimental results verified the accuracy of this technique and indicate the promise of the custom designed Echo PIV system in capturing complex flow fields non-invasively. However, the performance of this system was still limited by its 2D nature and the inadequacy in high frame rate imaging in a large field of view.

Since ultrasound transducer characteristics are a vital part of the Echo PIV system, additional efforts were devoted to the design and characterization of novel broad-bandwidth ultrasonic transducers. Simulation of conventional piezoelectric transducers for Echo PIV applications and development of novel techniques involving capacitive micromachined ultrasound transducers (cMUTs) were explored. A coupled non-linear model incorporating array transducer and contrast agents was developed to study the nonlinear interaction between high frequency ultrasound pressure, soft tissue and microbubbles. The simulation results could provide solutions for hardware selection for ultrasound systems as well as guidance for improving imaging sensitivity.