Towards clinical photoacoustic imaging: developing next-generation endoscopy systems and exploring new contrast agents

Photoacoustic imaging holds great clinical promise because it achieves high-resolution tomographic imaging at depths. Moreover, its strong spectroscopic imaging capability provides a wealth of molecular and functional information based on. Still, despite recent advances, existing photoacoustic systems cannot be readily applied in the clinical environment. This dissertation aims to push the frontier of clinical photoacoustic imaging from both technological and applicational perspectives. The first part of this dissertation describes the development of photoacoustic endoscopy (PAE) systems for imaging human Barrett\u27s esophagus and studying preterm birth. We have developed optical resolution-PAE, which significantly improved lateral resolutions, laparoscopic-PAE, which can guide minimally-invasive surgeries, and catheter-based-PAE, which opens up new opportunities to image the human esophagus. For each system, we tested and optimized the imaging performance in phantom and animal experiments, and then validated them in humans. The second part of the dissertation describes advanced photoacoustic imaging aided by contrast agents. Specifically, gold nanoparticles were used to quantify biological diffusion photoacoustically. In addition, ion nanosensors were applied for continuously monitoring therapeutic lithium concentration in deep tissue in vivo

DMD-based spatially Fourier-encoded photoacoustic microscopy

We present spatially Fourier-encoded photoacoustic microscopy using a digital micromirror device (DMD). The spatial fluence distribution of laser pulses is Fourier-encoded by the DMD, and a series of such encoded photoacoustic (PA) measurements enables decoding of the spatial distribution of optical absorption. By imaging a chromium target, we demonstrated the throughput and Fellgett advantages, which increased the PA signal-to-noise ratio (SNR) compared to raster scanning. The system was used to image two biological targets, a monolayer of red blood cells, and melanoma cells. The enhanced SNR benefited PA images by increasing the image’s contrast-to-noise ratio and target identifiability

Photoacoustic elastography

Elastography can noninvasively map the elasticity distribution in biological tissue, which can potentially be used to reveal disease conditions. In this Letter, we have demonstrated photoacoustic elastography by using a linear-array photoacoustic computed tomography system. The feasibility of photoacoustic elastography was first demonstrated by imaging the strains of single-layer and bilayer gelatin phantoms with various stiffness values. The measured strains agreed well with theoretical values, with an average error of less than 5.2%. Next, in vivo photoacoustic elastography was demonstrated on a mouse leg, where the fat and muscle distribution was mapped based on the elasticity contrast. We confirmed the photoacoustic elastography results by ultrasound elastography performed simultaneously

Photoacoustic microscopy with an enhanced axial resolution of 5.8 μm

The axial resolution of photoacoustic microscopy (PAM) can be enhanced by reducing the speed of sound within the imaging region of interest. This principle was demonstrated on a previously-reported PAM system, which utilized a 125 MHz ultrasonic transducer for signal detection and the Wiener deconvolution for signal processing. With sound slowed by silicone oil immersion, we have achieved a finest axial resolution of 5.8 μm for PAM, as validated by phantom experiments. The axial resolution was also enhanced in vivo when mouse ears injected with silicone oil were imaged. After injection of silicone oil, the blood vessels were resolved more clearly. When tissue-compatible low-speed liquids become available, this approach may find applications in PAM as well as in other imaging modalities, such as photoacoustic computed tomography and ultrasound imaging

Intracellular temperature mapping with fluorescence-assisted photoacoustic-thermometry

Measuring intracellular temperature is critical to understanding many cellular functions but still remains challenging. Here, we present a technique–fluorescence-assisted photoacoustic thermometry (FAPT)–for intracellular temperature mapping applications. To demonstrate FAPT, we monitored the intracellular temperature distribution of HeLa cells with sub-degree (0.7 °C) temperature resolution and sub-micron (0.23 μm) spatial resolution at a sampling rate of 1 kHz. Compared to traditional fluorescence-based methods, FAPT features the unique capability of transforming a regular fluorescence probe into a concentration- and excitation-independent temperature sensor, bringing a large collection of commercially available generic fluorescent probes into the realm of intracellular temperature sensing

Spatially Fourier-encoded photoacoustic microscopy using a digital micromirror device

We have developed spatially Fourier-encoded photoacoustic (PA) microscopy using a digital micromirror device. The spatial intensity distribution of laser pulses is Fourier-encoded, and a series of such encoded PA measurements allows one to decode the spatial distribution of optical absorption. The throughput and Fellgett advantages were demonstrated by imaging a chromium target. By using 63 spatial elements, the signal-to-noise ratio in the recovered PA signal was enhanced by ∼4×. The system was used to image two biological targets, a monolayer of red blood cells and melanoma cells

Slow-sound photoacoustic microscopy

We propose to enhance the axial resolution of photoacoustic microscopy (PAM) by reducing the speed of sound within the imaging region of interest. With silicone oil immersion, we have achieved a finest axial resolution of 5.8 μm for PAM, as validated by phantom experiments. The axial resolution was also enhanced in vivo when mouse ears injected with silicone oil were imaged. When tissue-compatible low-speed liquid becomes available, this approach may find broad applications in PAM as well as in other imaging modalities, such as photoacoustic computed tomography and ultrasound imaging

Optical sectioning by wide-field photobleaching imprinting microscopy

We present a generic wide-field optical sectioning scheme, photobleaching imprinting microscopy (PIM), for depth-resolved cross-sectional fluorescence imaging. Wide-field PIM works by extracting a nonlinear component that depends on the excitation fluence as a result of photobleaching-induced fluorescence decay. Since no specific fluorescent dyes or illumination modules are required, wide-field PIM is easy to implement on a standard microscope. Moreover, wide-field PIM is superior to deconvolution microscopy in removing background fluorescence, yielding a six-fold improvement in image contrast

Photoacoustic tomography of vascular compliance in humans

Characterization of blood vessel elastic properties can help in detecting thrombosis and preventing life-threatening conditions such as acute myocardial infarction or stroke. Vascular elastic photoacoustic tomography (VE-PAT) is proposed to measure blood vessel compliance in humans. Implemented on a linear-array-based photoacoustic computed tomography system, VE-PAT can quantify blood vessel compliance changes due to simulated thrombosis and occlusion. The feasibility of the VE-PAT system was first demonstrated by measuring the strains under uniaxial loading in perfused blood vessel phantoms and quantifying their compliance changes due to the simulated thrombosis. The VE-PAT system detected a decrease in the compliances of blood vessel phantoms with simulated thrombosis, which was validated by a standard compression test. The VE-PAT system was then applied to assess blood vessel compliance in a human subject. Experimental results showed a decrease in compliance when an occlusion occurred downstream from the measurement point in the blood vessels, demonstrating VE-PAT’s potential for clinical thrombosis detection