Date of Award

Summer 8-15-2022

Author's School

McKelvey School of Engineering

Author's Department

Biomedical Engineering

Degree Name

Doctor of Philosophy (PhD)

Degree Type



Capable of structural, functional, molecular, and metabolic imaging with high spatial resolution in vivo, photoacoustic microscopy (PAM) is an emerging tool in biomedical research. Further, recent advances in multi-parametric acquisition and analysis make PAM uniquely capable of simultaneously mapping the total concentration of hemoglobin (CHb), oxygen saturation of hemoglobin (sO2), and blood flow speed. However, there are multiple major limitations, including, the large footprint, low imaging speed, limited imaging contrast, and insufficient axial resolution, which impedance its applications in basic and translational research. In this dissertation, I presented potential solutions to overcome the limitations above.In the first part of this dissertation, considering that the large footprint and relatively low scanning speed of existing bench-top multi-parametric PAM systems have limited its clinical translation, I have developed a handheld PAM system that enables real-time acquisition and display of multi-parametric functional images of the human skin microvasculature. I tested the performance of this system by imaging the human finger cuticle in vivo. My results show that multi-parametric PAM images can be acquired at 3-µm resolution and 10-Hz frame rate over a field of view of 200×200 µm2. Taking advantage of the high spatiotemporal resolution, the traverse of single red blood cells through individual cuticle capillaries can be visualized, from which the blood flow speed can be quantified. Furthermore, the multi-parametric measurement enables comprehensive quantification of the oxygen saturation and release in individual capillaries. I demonstrated the initial utility of this new technique by studying the human microvascular reactivity to blood pressure cuffing and monitored the response of the oxygen extraction fraction (OEF) and oxygen metabolic rate (MRO2) to different cuff pressures. In Chapter 2, I have developed an ultra-high-speed multi-parametric PAM system, which achieves a 112-fold improvement in imaging speed over traditional multi-parametric PAM, enables simultaneous acquisition of ~500 densely sampled B-scans by superposing the rapid laser scanning across the line-shaped focus of a cylindrically focused ultrasonic transducer over the conventional mechanical scan of the optical-acoustic dual foci. A novel optical-acoustic combiner is designed and implemented to accommodate the short working distance of the transducer, enabling convenient confocal alignment of the dual foci in the reflection mode. This new system enables continuous monitoring of microvascular hemoglobin concentration, blood oxygenation, and flow over a 4.5×3 mm2 area in the awake mouse brain with high spatial and temporal resolution (6.9 µm and 0.3 Hz, respectively). Simultaneous visualization of the interaction between neurons and vascular function (blood perfusion, flow, and especially oxygenation) is challenging due to the limitations of existing microscopy modalities. To date, no single microscopy is able to visualize neurons and vasculature concurrently. In Chapter 3, Yifeng and I have developed a novel integration of TPM and multi-parametric PAM, which enables simultaneous imaging of microvasculature function and fluorescence contrasts in the mouse brain. I designed a customized parabolic mirror to realize TPM imaging with high numerical aperture and PAM imaging with good sensitivity. With integrated two-photon and multi-parametric photoacoustic microscopy, I demonstrated simultaneous imaging of the interplay between neural activity and microvascular function during whisker stimulation with a frame rate up to ~20 Hz. Lastly, I explored the application of the micro-ring resonator (MRR), a novel polymer-based nanophotonic device, in multi-parametric PAM. Taking advantage of the high sensitivity and large acceptance angle of the MRR, I have developed an isotropic-resolution multi-parametric PAM using label-free tracking of red blood cells. With the high temporal resolution of the new PAM system, I demonstrated real-time monitoring of the microvascular changes in the structure, oxygenation, and blood flow during ischemic stroke. Moreover, with the cellular resolution of the new PAM system along all three spatial dimensions, I demonstrated the structural and functional changes of the 3D microvasculature in response to the focal ischemia challenge.


English (en)


Song Hu

Committee Members

Quing Zhu, Chao Zhou, Jin-Moo Lee, Manu Goyal,

Available for download on Friday, November 01, 2024