Date of Award
Doctor of Philosophy (PhD)
Photoacoustic tomography (PAT) combines rich optical contrast and high ultrasonic resolution in optically scattering tissue at depths. Taking advantage of its 100% sensitivity to optical absorption, PAT has been widely applied to structural, functional and molecular imaging, with both endogenous and exogenous contrasts, at superior depths than pure optical methods. This dissertation explores novel absorption contrast mechanisms of PAT based on optical/thermal patterns, endogenous cellular chromophores, nanoparticles, small-molecule dyes and genetically-encoded proteins. With these novel contrasts, the proof-of-concept applications of PAT have been extended to include homogenous flow measurements, targeted angiogenesis imaging and therapy, label-free white blood cell imaging, 3D-whole-organ cell nuclei imaging with a subcellular resolution, and in vivo neural activity imaging with voltage/calcium-sensitive indicators.
Specifically, Chapter 1 introduces photoacoustic microscopy (PAM) and photoacoustic computed tomography (PACT) systems and discuss the motivation of the dissertation.
Chapter 2 describes two photoacoustic (PA) flow measurement methods with optical and thermal patterns, which are applicable to homogenous flowing medium. In the first method, a Doppler frequency shift in PA signals of the flow was detected and used to calculate flow speeds. In the second method, unique features in an externally imposed thermal pattern of the flow, captured by repeated B-scans along the flow direction with a PAM system, revealed different flow speeds.
Chapter 3 explores the unique PA contrast of macrophages, an important type of white blood cells. Macrophages were imaged by PAM without any label, and their measured PA spectrum was distinctive from the hemoglobin spectrum, so they can be potentially differentiated from red blood cells in the blood stream. Next, with a microtomy-assisted PAM system, cell nuclei distribution in whole organs, including mouse brain and mouse lung, were imaged with subcellular resolution.
Chapter 4 introduces a type of target copper nanoparticles, which are less expensive and more biocompatible than its counterpart gold nanoparticles. The PA signals of neovasculature in the mouse flank were enhanced by the ___3-targeted copper nanoparticles. Moreover, the work shows the first example of a systemically targeted antiangiogenic drug delivery with a photoacoustic contrast nanoparticle in vivo.
Chapter 5 demonstrates the voltage imaging capability of PA. A voltage sensitive dye with sufficient signal change was discovered and used as a PA voltage indicator for the first time. The mechanism was characterized through both PA imaging and spectroscopic methods. Its use was explored in a mouse epilepsy model and cortical electrical stimulation model in vivo. Finally, the deep imaging potential of PA was realized by imaging the voltage response of cells under 4.5 mm thick slice of rat brain tissue using a PACT system.
Chapter 6 proves the neural calcium imaging capability of PA with a genetically encoded calcium indicator. In a fly model, I ambiguously demonstrated for the first time that PA can be used to imaging neural activities in the fly brain without the interference signals from hemoglobin. In the a live-mouse-brain-slice model, I successfully demonstrated the deep imaging capability of PA for calcium imaging by imaging through a 2-mm-thick scattering medium with a PACT system.
Lihong V. Wang
Mark A. Anastasio, Gregory Lanza, Jung-Tsung Shen, Jin-Moo Lee