Sihua Yang

Functional imaging of cerebrovascular activities in small animals using high-resolution photoacoustic tomography.

Photoacoustic imaging (PAI) is a noninvasive, nonionizing modality based on the differences in light absorption of various biological tissues. PAI utilizes the endogenous contrast characteristics of traditional optical imaging, while benefiting from high spatial resolution of the ultrasound imaging. A PAI system was developed to reconstruct the two-dimensional cross section image and to visualize the cerebrovascular activities of mouse in vivo. The spatial resolution of the PAI system was determined to be 0.110 mm by a two-point-source phantom with the Rayleigh criterion. The potential applications of the system were clearly demonstrated by successfully mapping a traumatic lesion in the mouse brain cerebral cortex, by its ability to monitor physiological changes in the brain due to carotid ligation and drug stimulation, and two-dimensional sliced images of a traumatic mouse brain at different depths were also provided. Our experimental results indicate that PAI has the potential for studying of traumatic brain injury and physiological functions of the brain.

Noninvasive photoacoustic imaging of the developing vasculature during early tumor growth

In this study, we monitor the progress of vasculature in early tumor growth using photoacoustic imaging over a 20 day period after subcutaneous inoculation of breast cancer tumor cells in a mouse. With 532 nm laser pulses employed as an irradiation source, the photoacoustic images were obtained through the photoacoustic signals received by a hydrophone in orthogonal mode. The morphological characteristics of vasculature in tumor region are clearly resolved in the photoacoustic images, and the change in structure as well as the increase in density can be identified. Moreover, the average photoacoustic signal strength of vasculature in tumor region, which is highly correlated with the total hemoglobin concentration of blood, is enhanced during early tumor growth. These results indicate the feasibility of detecting early stage tumor and monitoring the progress of anti-angiogenic therapy by photoacoustic imaging.

Fast full-view photoacoustic imaging by combined scanning with a linear transducer array

We present a fast full-view photoacoustic imaging system for visualizing tissue structures using a linear transducer array with combined scan. In this system, a 128-element linear transducer array was used to detect photoacoustic signals by combined scanning of electronic scan and mechanical scan. An improved limited-field filtered back projection algorithm with directivity factors was applied to reconstruct the optical absorption distribution. The experiments of phantoms and in vivo blood vessels in a rat brain were performed with this system. And a clear view of the curve boundaries of objects and the network of blood vessels of rats brain were acquired. The experimental results demonstrate the multi-element photoacoustic imaging system has the ability of imaging complicated structures of objects.

Photoacoustic molecular imaging with antibody-functionalized single-walled carbon nanotubes for early diagnosis of tumor

Single-walled carbon nanotubes (SWNT) in a poly(ethylene)ghycol solution are a biocompatible transporters with strong optical absorption in the near-infrared region, in which the biological tissue is almost transparent with very low absorbance. Here, antibody-functionalized SWNTs for tumor early detection with photoacoustic molecular imaging in vivo are reported. To lay the groundwork for this goal and insure system stability, images were collected in tissue simulating phantoms to determine appropriate detectable concentrations of SWNTs. Preliminary in vitro and in vivo results showed that a high contrast and a high efficient targeting of integrin alpha(v)beta(3) positive U87 human glioblastoma tumours in mice could be achieved. The nontoxicity of functionalized SWNTs has also been demonstrated in our experiment; this feature ensures that SWNTs can be used for clinical applications. This study suggests that photoacoustic molecular imaging with antibody-functionalized SWNTs has the potential to be an effective early tumor diagnosis method.

Real-time optoacoustic monitoring of vascular damage during photodynamic therapy treatment of tumor

The optoacoustic technique is a noninvasive imaging method with high spatial resolution. It potentially can be used to monitor anatomical and physiological changes. Photodynamic therapy (PDT)-induced vascular damage is one of the important mechanisms of tumor destruction, and real-time monitoring of vascular changes can have therapeutic significance. A unique optoacoustic system is developed for neovascular imaging during tumor phototherapy. In this system, a single-pulse laser beam is used as the light source for both PDT and for concurrently generating ultrasound signals for optoacoustic imaging. To demonstrate its feasibility, this system is used to observe vascular changes during PDT treatment of chicken chorioallantoic membrane (CAM) tumors. The photosensitizer used in this study is protoporphyrin IX (PpIX) and the laser wavelength is 532 nm. Neovascularization in tumor angiogenesis is visualized by a series of optoacoustic images at different stages of tumor growth. Damage of the vascular structures by PDT is imaged before, during, and after treatment. Rapid, real-time determination of the size of targeted tumor blood vessels is achieved, using the time difference of positive and negative ultrasound peaks during the PDT treatment. The vascular effects of different PDT doses are also studied. The experimental results show that a pulsed laser can be conveniently used to hybridize PDT treatment and optoacoustic imaging and that this integrated system is capable of quantitatively monitoring the structural change of blood vessels during PDT. This method could be potentially used to guide PDT and other phototherapies using vascular changes during treatment to optimize treatment protocols, by choosing appropriate types and doses of photosensitizers and doses of light.

Characterization of Lipid-Rich Aortic Plaques by Intravascular Photoacoustic Tomography Ex Vivo and In Vivo Validation in a Rabbit Atherosclerosis Model With Histologic Correlation

BACKGROUND Histologic studies have demonstrated that lipid content and its spatial distribution is related to plaque vulnerability. However, in vivo imaging is still limited. Photoacoustic imaging may provide novel in vivo insights into these lipid-rich plaques. OBJECTIVES This study sought to examine whether intravascular photoacoustic tomography (IVPAT) allows localization and quantification of lipid content in atherosclerotic plaques. METHODS Rabbits fed with a high-fat/high-cholesterol diet served as the atherosclerotic model. Catheter-based IVPAT was used to evaluate pixel-based lipid relative concentration (LRC) of the vessel wall. The aorta of 4 groups of rabbits (n = 12) were examined ex vivo with IVPAT after 0, 5, 10, and 15 weeks of a high-fat diet, respectively. Six rabbits underwent 3-dimensional (3D) IVPAT after 20 weeks of the high-fat diet. Three rabbits were examined in vivo using IVPAT without interruption of blood flow. Concentration-based lipid map and quantitative index were calculated. For subsequent histologic correlation, all specimens were evaluated with Oil Red O staining. RESULTS Cross-sectional LRC maps allowed visualization of concentration and depth information of lipid content in the atherosclerotic plaques. Lipid accumulation within plaque, assessed by the maximum LRC, mean LRC, and high lipid content area correlated to duration of a high-fat diet. Three-dimensional LRC maps enabled overall evaluation of focal plaques in an intact explanted aorta including spatial and structural features. In vivo-obtained LRC maps accurately showed the structure of lipid core with high contrast. Ex vivo and in vivo IVPAT results were highly consistent with histological results. CONCLUSIONS In an animal model, IVPAT allowed characterization of spatial and quantitative features of lipid-rich plaques.

Thermoacoustic molecular tomography with magnetic nanoparticle contrast agents for targeted tumor detection.

PURPOSE The primary feasibility steps of demonstrating the ability of microwave-induced thermoacoustic (TA) in phantoms have been previously reported. However, none were shown to target a diseased site in living subjects in thermoacoustic tomography (TAT) field so far. To determine the expressions of oncogenic surface molecules, it is quite necessary to image tumor lesions and acquire pathogenic status on them via TAT. METHODS Compared to biological tissues, iron oxide nanoparticles have a much higher microwave absorbance. Fe3O4/polyaniline (PANI) nanoparticles were prepared via polymerization of aniline in the Fe304 superparamagnetic fluids. Then Fe3O4/PANI was conjugated to folic acid (FA), which can bind specifically to the surface of the folate receptor used as a tumor marker. FA-Fe3O4/PANI targeted tumor was irradiated by pulsed microwave at 6 GHz for thermoacoustic detection and imaging. RESULTS The effect of the Fe3O4/PANI superparamagnetic nanoparticles for enhancing TAT images was successfully investigated in ex vivo human blood and in vivo mouse tail. Intravenous administration of the targeted nanoparticles to mice bearing tumors showed fivefold greater thermoacoustic signal and much longer elimination time than that of mice injected with nontargeted nanoparticles in the tumor. The specific targeting ability of FA-Fe3O4/PANI to tumor was also verified on fluorescence microscopy. CONCLUSIONS Fabricated iron oxide nanoparticles conjugated with tumor ligands for targeted TAT tumor detection at the molecular level was reported for the first time. The results indicate that thermoacoustic molecular imaging with functionalized iron oxide nanoparticles may contribute to targeted and functional early cancer imaging. Also, the modified iron oxide nanoparticles combined with suitable tumor markers may also be used as novel nanomaterials for targeted and guided cancer thermal therapy.

Noninvasive monitoring of traumatic brain injury and post-traumatic rehabilitation with laser-induced photoacoustic imaging

A photoacoustic imaging system was used for noninvasive monitoring of traumatic mouse brain in vivo with high-quality reconstructed images. Traumatic lesions accompanying with hemorrhage in the mouse cortical surface were accurately mapped, and foreign bodies of two small copper wires inserted in the mouse brain were also detected. Furthermore, the time course of morphological changes of cerebral blood during rehabilitation process of a mouse brain with traumatic brain injury was obtained using a series of photoacoustic images. Experimental results demonstrate that photoacoustic technique holds the potential for clinical applications in brain trauma and cerebrovascular disease detection.

Intracellular label-free gold nanorods imaging with photoacoustic microscopy

Noninvasive photoacoustic microscopy was developed to image intracellular gold nanorods with high optical-absorption contrast. The endocytosed gold nanorods in MCF7 cells can be detected and imaged with the home-made photoacoustic microscope. Cell nucleus and gold nanorods in cytoplasm were clearly identified after hematoxylin and eosin (H&E) staining with dual-wavelength excitation. The intracellular gold nanorods were successfully monitored, and that the time-dependent uptake and distribution of the gold nanorods in the cells were clearly shown. The result demonstrated an application of photoacoustic microscopy for complements to imaging of nonfluorescent nanoparticles, which will arm the in vivo microscopic imaging method to the nano-bio research.

Optical-resolution photoacoustic microscopy based on two-dimensional scanning galvanometer

An optical-resolution photoacoustic microscopy system was designed and fabricated by integration of a two-dimensional scanning galvanometer, an objective lens, an unfocused ultrasound transducer, and a sample stage. The lateral resolution of the system was measured to be ∼500 nm. Ex vivo erythrocytes were used to test the imaging capability of the system, and a single erythrocyte was mapped with high contrast. Furthermore, invivo blood vessels of a mouse ear were clearly shown, and the injured blood vessels were also monitored. The experimental results demonstrate that galvanometer-based photoacoustic microscopy holds clinical potential in detecting lesion of erythrocyte and blood vessel.