Yuan Liu

Graduation Semester and Year




Document Type


Degree Name

Doctor of Philosophy in Biomedical Engineering



First Advisor

Baohong Yuan


Ultrasound-modulated fluorescence (UMF) imaging has been proposed as a novel imaging modality by combining ultrasound and optical imaging techniques for early cancer detection. In UMF, a focused ultrasound beam is used to modulate the diffused fluorescence photons in the acoustic focal region, and by specifically analyzing the modulated photons, one can isolate and quantify the fluorescence properties within the ultrasonic focal area. Therefore, UMF is able to provide fluorescence contrast while maintaining ultrasound resolution in tissue. The major challenge of UMF is to extract the weakly modulated fluorescence signal from a bright and unmodulated background, i.e. the low modulation efficiency. This work is focused on investigating and developing novel UMF contrast agents and imaging systems, to improve the modulation efficiency of UMF for biological applications.This work can be categorized into two major parts: the contrast agent and the imaging system. In the contrast agent part, firstly four different fluorescent probes, ranging from 5 nm to 1 &mum in diameter, were used to study the size effect of fluorescent probes on UMF modulation efficiency. Next, two novel microbubble-based UMF contrast agents (single fluorophore labeled microbubbles and donor-acceptor labeled microbubbles) were developed to further improve the modulation efficiency. These designs take advantage of the microbubbles' oscillations in size in response to ultrasound to modulate the inter-fluorophore distance and the quenching efficiency. As a result, the fluorescence emissions were modulated, presented as UMF signal. In the imaging technique part, a novel optical system consists of a confocal microscopic system and a gated and intensified charge-coupled device (ICCD) camera system was developed first in order to characterize the contrast agents. The high-speed oscillations of microbubbles in 3-dimensions were characterized, and their modulation efficiencies were evaluated and optimized. After that, those contrast agents were utilized for UMF imaging in water and scattering mediums using a sensitive ultrasound combined optical imaging system. Results showed that the modulation efficiency was improved by approximately a factor of two when the size of the fluorescent particles was increased from 5 nm to 1 &mum. However, this improvement was still not sufficient for UMF imaging in biomedical applications. Excitingly, the microbubble-based contrast agents were successfully developed and demonstrated UMF signal with high modulation efficiency. The dynamics of the microbubbles under various ultrasound pressures were clearly observed along both horizontal plane (x-y plane) and vertical direction (z direction) using the developed optical imaging system. It was shown that the UMF strength were highly dependent on the microbubbles' oscillation amplitude and the initial surface fluorophore-quenching status. A UMF modulation efficiency of ~40% was detected corresponding to a size change of ~33% from individual microbubbles of both types, thought the donor-acceptor labeling scheme presented more complex quenching mechanisms compared to the single-fluorophore labeling scheme. In the end, UMF signals from a 500-&mum tube filled with both microbubble-based contrast agents were detected in water and a scattering medium using the UMF imaging system. These results indicate that fluorescent microbubbles can be used as promising UMF contrast agents. When combined with the developed UMF system, they can potentially be used for fluorescence-based molecular imaging in future.


Biomedical Engineering and Bioengineering | Engineering


Degree granted by The University of Texas at Arlington