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Advancing Photoacoustic Imaging Technology with Compact Source, Fluorescence Co-registration, Spectral Encoding, Matrix Detection and FRET
Date of Award
Doctor of Philosophy (PhD)
Photoacoustic tomography, which detects non-radiative decay, is an emerging biomedical imaging modality that can provide 3D ultrasonically scalable images of biological tissue ranging from organelles to organs. Pure optical imaging modalities (e.g., optical coherence tomography and diffuse optical tomography) encounter a fundamental limitation of either penetration or spatial resolution at depths beyond one optical transport mean free path (~1 mm) due to strong light scattering by biological tissue. Photoacoustic imaging, however, provides a high ultrasonic spatial resolution for deep imaging by utilizing ultrasonic detection of the photoacoustic waves generated by absorbed diffuse light. By exploiting the rich optical absorption contrasts of biomolecules, photoacoustic imaging has been used to image both biological structure (e.g., internal organs and sentinel lymph nodes) and function (e.g., tumor hypoxia and brain oxygenation). The ability of photoacoustic imaging (photoacoustic microscopy and photoacoustic computed tomography) systems, to render three-dimensional volumetric images relies on illuminating light-absorbing chromophores using a pulsed laser system and recording the photoacoustic time-of-flight signals on a two-dimensional surface facing the photoacoustic source. My doctoral research focuses on hardware advances in both exciting and detecting photoacoustic signals. Förster resonance energy transfer (FRET) imaging in deep biological tissue using photoacoustic techniques is also explored.
This first part of my dissertation presents some novel photoacoustic excitation and detection technologies implemented in photoacoustic imaging. Fiber lasers have been proposed as a fast and compact alternative to current excitation sources for photoacoustic imaging, especially in the clinical environment. Its intrinsic optical-fiber-based amplification and output make the system easy to maintain. We developed a 1064 nm photoacoustic microscope based on a fiber laser system, which features a pulse repetition rate of 50 kHz. We demonstrated detection of circulating melanoma cells in blood. Photoacoustic and fluorescence imaging provide complementary optical absorption and fluorescence contrasts, respectively. We developed a dual modality imaging system that combines photoacoustic and fluorescence microscopy. The two sub-systems are naturally integrated by sharing the same laser source, objective lens and image scanner. We reported in vivo imaging of hemoglobin oxygen saturation and oxygen partial pressure in single blood vessels. Spectral (multi-wavelength) photoacoustic imaging must possess high wavelength-switching speed when applied in dynamic functional imaging. We implemented a digital-mirror-device (DMD)-based spectral-encoding photoacoustic imaging system. As a wavelength multiplexing element, DMD features a fast frame rate and pixelated manipulation flexibility. Compared with internal wavelength tuning of a narrow-band laser, external wavelength tuning based on a digital mirror device improves the data acquisition speed of spectral photoacoustic microscopy. Compared with external wavelength scanning of a wide-band laser with the same pulse energy, spectral encoding improves the signal-to-noise ratio. A two-dimensional (2D) array transducer can acquire three-dimensional (3D) photoacoustic imaging without mechanical scanning; therefore, by using a small number of laser firings, higher imaging frame rates can be achieved. We presented an integrated photoacoustic and ultrasonic 3D volumetric imaging system based on a modified commercial ultrasound imaging system (iU22, Philips Healthcare) with a 2D array transducer (X7-2, Philips Healthcare). The imaging system enables rendering of co-registered 3D ultrasound and photoacoustic images. In vivo 3D photoacoustic mapping of the sentinel lymph node using methylene blue dye was demonstrated in a rat model.
The second part of my dissertation focuses on photoacoustic Förster resonance energy transfer (FRET) imaging. FRET provides fluorescence signals sensitive to intra- and inter-molecular distances in the 1-10 nm range. Widely applied in the optical imaging environment, FRET enables visualization of physicochemical processes in molecular interactions and conformations. We reported photoacoustic imaging of FRET, based on non-radiative decay that produces heat and subsequent acoustic waves. The experimental results show that photoacoustic imaging, through its ability to three-dimensionally image tissue with scalable resolution, provides a beneficial biomedical tool to broaden the in vivo application of the FRET technique
Lihong V Wang
Jin-Moo Lee, James Miller, Jung-Tsung Shen