Ultrasound Imaging Velocimetry using Polyvinyl Alcohol Shelled Microbubbles

Abstract: Current research within the field of ultrasound contrast agents (UCAs) aims at developing capsules which are not only acoustically active, but also have a chemically modifiable surface. This enables use in new areas, including targeted drug delivery and theranostics. For such purposes, air-filled microbubbles (MBs) with a polyvinyl alcohol (PVA) shell are being studied. Ultrasound imaging velocimetry (UIV) is a technique used to evaluate various types of liquid flows by tracking patterns caused by UCAs across ultrasound images, and has shown great potential for flow measurements in terms of accuracy. The aim of this thesis was to implement a basic UIV program in Matlab to investigate the flow behavior of air-filled PVA MBs being pumped through a phantom, mimicking a blood vessel. The images were acquired using the programmable Verasonics research system by plane wave imaging with coherent compounding, and UIV was implemented as a post-processing technique. Three parameters were varied to study how the UIV performance and flow behavior of the MBs were affected: the concentration of MBs, the flow velocity, and the transducer voltage. The resulting velocity vector fields showed that it is possible to track PVA MBs using the implemented UIV program, and that the concentration 5·106 MBs/ml gave the best results out of the five concentrations tested. The generated velocity vector fields indicated a turbulent and pulsatile flow behavior, which was in line with the predicted flow behavior, although there was a disparity between the measured average flow velocity of the MBs and the predicted flow velocity. It was also observed that the MBs were increasingly pushed in the axial direction with increasing voltage, as according to theory. Even though a more advanced UIV algorithm could improve the accuracy of the velocity measurements, the results show possible use of air-filled PVA MBs in combination with UIV.