| Summary: | <p>The utility of microbubble agents in a variety of diagnostic and therapeutic ultrasound techniques has been widely demonstrated, most notably in Contrast Enhanced Ultrasound (CEUS) imaging. Unfortunately, the underlying mechanisms of their response to ultrasound excitation are poorly understood, restricting the development of promising techniques, such as quantitative perfusion imaging. A significant reason for this is that current microbubble characterisation techniques suffer from one or more of the following limitations: i) large experimental uncertainties, ii) physical restrictions on microbubble response and iii) failure to provide large data sets suitable for statistical analysis. This thesis presents a new technique to overcome these limitations. A co-axial microfluidic device is used to hydrodynamically confine microbubbles through the focal region of a laser and ultrasound field. The magnitude of light scattered by isolated microbubbles during ultrasound excitation is converted to radius using Mie Scattering theory. This technique is capable of obtaining large samples (>10<sup>3</sup>/min) of microbubbles to be efficiently characterised.</p> <p>The response of a commercial contrast agent, SonoVue®, is first investigated for a range of ultrasound exposure parameters; frequency (2 MHz – 4.5 MHz), peak negative pressure (6 kPa - 400 kPa) and pulse length (3 cycles – 8 cycles). Second the device is used to investigate the effect of composition and fabrication on microbubble response to similar ultrasound conditions. The results demonstrate a very large variability in microbubble response independent of initial size, indicating a significant lack of uniformity of coating properties. This is further supported by quantitative fluorescence imaging and quasi-static pressure chamber measurements.</p> <p>The implications of the findings for CEUS imaging and the development of microbubble contrast agents are discussed, as well as the limitations and suggested improvements of the characterisation technique.</p>
|
|---|