Noninvasive Submillimeter-Precision Brain Stimulation by Optically-Driven Focused Ultrasound

High precision neuromodulation is a powerful tool to decipher neurocircuits and treat neurological diseases. Current non-invasive neuromodulation methods offer limited millimeter-level precision. Here, we report an optically-driven focused ultrasound (OFUS) for non-invasive brain stimulation with submillimeter precision. OFUS is generated by a soft optoacoustic pad (SOAP) fabricated through embedding candle soot nanoparticles in a curved polydimethylsiloxane film. SOAP generates a transcranial ultrasound focus at 15 MHz with a lateral resolution of 83 micrometers, which is two orders of magnitude smaller than that of conventional transcranial focused ultrasound (tFUS). Effective OFUS neurostimulation in vitro with a single ultrasound cycle is shown. Submillimeter transcranial stimulation of mouse motor cortex in vivo is demonstrated. An acoustic energy of 0.02 J/cm^2, two orders of magnitude less than that of tFUS, is sufficient for successful OFUS neurostimulation. By delivering a submillimeter focus non-invasively, OFUS opens a new way for neuroscience studies and disease treatments.Comment: 36 pages, 5 main figures, 13 supplementary figure

Multi-contrast Photoacoustic Microscopy

Photoacoustic microscopy is a hybrid imaging modality with high spatial resolution, moderate imaging depth, excellent imaging contrast and functional imaging capability. Taking full advantage of this powerful weapon, we have investigated different anatomical, functional, flow dynamic and metabolic parameter measurements using photoacoustic microscopy. Specifically, Evans-blue dye was used to enhance photoacoustic microscopy of capillaries; label-free transverse and axial blood flow was measured based on bandwidth broadening and time shift of the photoacoustic signals; metabolic rate of oxygen was quantified in vivo from all the five parameters measured by photoacoustic microcopy; whole cross-sectional imaging of small intestine was achieved on a double-illumination photoacoustic microscopy with extended depth of focus and imaging depth; hemodynamic imaging was performed on a MEMS-mirror enhanced photoacoustic microscopy with a cross-sectional imaging rate of 400 Hz. As a maturing imaging technique, PAM is expected to find new applications in both fundamental life science and clinical practice

Review of photoacoustic imaging plus X

Photoacoustic imaging (PAI) is a novel modality in biomedical imaging technology that combines the rich optical contrast with the deep penetration of ultrasound. To date, PAI technology has found applications in various biomedical fields. In this review, we present an overview of the emerging research frontiers on PAI plus other advanced technologies, named as PAI plus X, which includes but not limited to PAI plus treatment, PAI plus new circuits design, PAI plus accurate positioning system, PAI plus fast scanning systems, PAI plus novel ultrasound sensors, PAI plus advanced laser sources, PAI plus deep learning, and PAI plus other imaging modalities. We will discuss each technology's current state, technical advantages, and prospects for application, reported mostly in recent three years. Lastly, we discuss and summarize the challenges and potential future work in PAI plus X area

Multi-scale volumetric dynamic optoacoustic and laser ultrasound (OPLUS) imaging enabled by semi-transparent optical guidance

Major biological discoveries have been made by interrogating living organisms with light. However, the limited penetration of unscattered photons within biological tissues severely limits the depth range covered by optical methods. Deep-tissue imaging has been achieved by combining light and ultrasound. Optoacoustic imaging uniquely exploits optical generation of ultrasound to render high-resolution images at depths unattainable with optical microscopy. Recently, laser ultrasound has further been suggested as a means of generating broadband acoustic waves for high-resolution pulse-echo ultrasound imaging. Herein, we propose an approach to simultaneously interrogate biological tissues with light and ultrasound based on layer-by-layer coating of silica optical fibers with a controlled degree of transparency. We exploit the time separation between optoacoustic signals and ultrasound echoes collected with a custom-made spherical array transducer for simultaneous three-dimensional optoacoustic and laser ultrasound (OPLUS) imaging with a single laser pulse. OPLUS is shown to enable large-scale comprehensive anatomical characterization of tissues along with functional multi-spectral imaging of spectrally-distinctive chromophores and assessment of cardiac dynamics at ultrafast rates only limited by the pulse repetition frequency of the laser. The suggested approach provides a flexible and scalable means for developing a new generation of systems synergistically combining the powerful capabilities of optoacoustics and ultrasound imaging in biology and medicine.Comment: 21 pages, 4 figure

High-Speed Photoacoustic Microscopy In Vivo

The overarching goal of this research is to develop a novel photoacoustic microscopy: PAM) technology capable of high-speed, high-resolution 3D imaging in vivo. PAM combines the advantages of optical absorption contrast and ultrasonic resolution for deep imaging beyond the quasi-ballistic regime. Its high sensitivity to optical absorption enables the imaging of important physiological parameters, such as hemoglobin concentration and oxygen saturation, which closely correlate with angiogenesis and hypermetabolism--two hallmarks of cancer. To translate PAM to the clinic, both high imaging speed and high spatial resolution are desired. With high spatial resolution, PAM can detect small structural and functional changes early; whereas, high-speed image acquisition helps reduce motion artifacts, patient discomfort, cost, and potentially the risks associated with minimally invasive procedures such as endoscopy and intravascular imaging. To achieve high imaging speed, we have constructed a PAM system using a linear ultrasound array and a kHz-repetition-rate tunable laser. The system has achieved a 249-Hz B-scan rate and a 0.5-Hz 3D imaging rate: over ~6 mm × 10 mm × 3 mm), over 200 times faster than existing mechanical scanning PAM using a single ultrasonic transducer. In addition, high-speed optical-resolution photoacoustic microscopy: OR-PAM) technology has been developed, in which the spatial resolution in one or two dimension(s) is defined by the diffraction-limited optical focus. Using section illumination, the elevational resolution of the system has been improved from ~300 micron to ~28 micron, resulting in a significant improvement in the 3D image quality. Furthermore, multiple optical foci with a microlens array have been used to provide finer than 10-micron lateral resolution--enabling the system to image capillary-level microvessels in vivo--while offering a speed potentially 20 times faster than previously existing single-focus OR-PAM. Finally, potential biomedical applications of the developed technology have been demonstrated through in vivo imaging of murine sentinel lymph nodes, microcirculation dynamics, and human pulsatile dynamics. In the future, this high-speed PAM technology may be adapted for clinical imaging of diabetes-induced vascular complications or tumor angiogenesis, or miniaturized for gastrointestinal or intravascular applications

Development of High-speed Photoacoustic Imaging technology and Its Applications in Biomedical Research

Photoacoustic (PA) tomography (PAT) is a novel imaging modality that combines the fine lateral resolution from optical imaging and the deep penetration from ultrasonic imaging, and provides rich optical-absorption–based images. PAT has been widely used in extracting structural and functional information from both ex vivo tissue samples to in vivo animals and humans with different length scales by imaging various endogenous and exogenous contrasts at the ultraviolet to infrared spectrum. For example, hemoglobin in red blood cells is of particular interest in PAT since it is one of the dominant absorbers in tissue at the visible wavelength.The main focus of this dissertation is to develop high-speed PA microscopy (PAM) technologies. Novel optical scanning, ultrasonic detection, and laser source techniques are introduced in this dissertation to advance the performance of PAM systems. These upgrades open up new avenues for PAM to be applicable to address important biomedical challenges and enable fundamental physiological studies.First, we investigated the feasibility of applying high-speed PAM to the detection and imaging of circulating tumor cells (CTCs) in melanoma models, which can provide valuable information about a tumor’s metastasis potentials. We probed the melanoma CTCs at the near-infrared wavelength of 1064 nm, where the melanosomes absorb more strongly than hemoglobin. Our high-speed PA flow cytography system successfully imaged melanoma CTCs in travelling trunk vessels. We also developed a concurrent laser therapy device, hardware-triggered by the CTC signal, to photothermally lyse the CTC on the spot in an effort to inhibit metastasis.Next, we addressed the detection sensitivity issue in the previous study. We employed the stimulated Raman scattering (SRS) effect to construct a high-repetition-rate Raman laser at 658 nm, where the contrast between a melanoma CTC and the blood background is near the highest. Our upgraded PA flow cytography successfully captured sequential images of CTCs in mouse melanoma xenograft model, with a significantly improved contrast-to-noise ratio compared to our previous results. This technology is readily translatable to the clinics to extract the information of a tumor’s metastasis risks.We extended the Raman laser technology to the field of brain functional studies. We developed a MEMS (micro-electromechanical systems) scanner for fast optical scanning, and incorporated it to a dual-wavelength functional PAM (fPAM) for high-speed imaging of cerebral hemodynamics in mouse. This fPAM system successfully imaged transient changes in blood oxygenation at cerebral micro-vessels in response to brief somatic stimulations. This fPAM technology is a powerful tool for neurological studies.Finally, we explored some approaches of reducing the size the PAM imaging head in an effort to translate our work to the field of wearable biometric monitors. To miniaturize the ultrasonic detection device, we fabricated a thin-film optically transparent piezoelectric detector for detecting PA waves. This technology could enable longitudinal studies on free-moving animals through a wearable version of PAM

Photoacoustic microscopy

Photoacoustic microscopy (PAM) is a hybrid in vivo imaging technique that acoustically detects optical contrast via the photoacoustic effect. Unlike pure optical microscopic techniques, PAM takes advantage of the weak acoustic scattering in tissue and thus breaks through the optical diffusion limit (∼1 mm in soft tissue). With its excellent scalability, PAM can provide high-resolution images at desired maximum imaging depths up to a few millimeters. Compared with backscattering-based confocal microscopy and optical coherence tomography, PAM provides absorption contrast instead of scattering contrast. Furthermore, PAM can image more molecules, endogenous or exogenous, at their absorbing wavelengths than fluorescence-based methods, such as wide-field, confocal, and multi-photon microscopy. Most importantly, PAM can simultaneously image anatomical, functional, molecular, flow dynamic and metabolic contrasts in vivo. Focusing on state-of-the-art developments in PAM, this Review discusses the key features of PAM implementations and their applications in biomedical studies

Optical-Resolution Photoacoustic Microscopy

Optical microscopy, providing valuable biomedical insights at the cellular and organelle levels, has been widely recognized as an enabling technology. Mainstream optical microscopy technologies, including single-/multi-photon fluorescence microscopy and OCT, have demonstrated extraordinary sensitivities to fluorescence and optical scattering contrasts, respectively. However, the optical absorption contrast of biological tissues, which encodes essential physiological/pathological information, has not yet been fully assessable. The emergence of biomedical photoacoustics has led to a new branch of optical microscopy--OR-PAM. As a valuable complement to existing optical microscopy technologies, OR-PAM detects optical absorption contrasts with exquisite sensitivity: i.e., 100%). Combining OR-PAM with fluorescence microscopy or optical-scattering-based OCT: or both) provides comprehensive optical properties of biological tissues. Moreover, OR-PAM encodes optical absorption into acoustic waves, in contrast to the pure optical processes in fluorescence microscopy and OCT, and thus provides background-free detection. The acoustic detection in OR-PAM mitigates the impacts of optical scattering on signal degradation and naturally eliminates possible interferences: i.e., crosstalks) between excitation and detection, which is a common problem in fluorescence microscopy due to the overlap between the excitation and fluorescence spectra and imperfect extinction of the filter. Unique for high-resolution imaging of optical absorption, OR-PAM has demonstrated broad biomedical applications in fields such as neurology, ophthalmology, vascular biology, and dermatology. My doctoral research focuses on developments and biomedical applications of OR-PAM. The first part of my dissertation discusses the development of three generations of OR-PAM towards high-resolution, high-sensitivity, high-speed, and wide FOV in vivo imaging. In this section, I provide a comprehensive description of OR-PAM, including the principle, system design, system configuration, experimental procedures, laser safety, functional imaging scheme, and example biomedical applications at a variety of in vivo anatomical sites: i.e., skins, eyes and brains). The second part of my dissertation focuses on the application of OR-PAM in vascular biology, with an emphasis on neovascularization. In this section, I demonstrate longitudinal OR-PAM monitoring of the morphological: i.e., vessel diameter, length, tortuosity and volume) and functional: i.e., sO2) changes of angiogenic microenvironment at the capillary level, in both a non-disease TetON-HIF-1 transgenic mouse model and a cancer xenograft model in mouse ear. The last part of my dissertation focuses on the application of OR-PAM in neurology, with an emphasis on cortical stimulation, Alzheimer\u27s disease, and ischemic stroke. In this section, I use label-free OR-PAM for both acute monitoring of microvascular responses to direct electrical stimulations of the mouse somatosensory cortex through a cranial opening and longitudinal monitoring of the morphological and functional changes of cortical vasculature in a transient middle cerebral artery occlusion mouse model. I also explore the potential of OR-PAM for transcranial monitoring of amyloid plaque growth in an AD mouse model

Photoacoustic Imaging of the Eye

Photoacoustic imaging (PAI) is a novel, hybrid, non-ionizing, and non-invasive imaging technology with high-resolution, high sensitivity, high-contrast, and high depth of penetration. Hence, it has particularly useful applications in eye investigations. It can provide both anatomic and functional ocular characterizations. Many eye diseases, including macular degeneration and diabetic retinopathy, involve abnormalities in the vasculature, and thus the ability of PAI to affectively visualize the vasculature can be incredibly helpful to evaluate normal and disease states of the eye. In future research, PAI of the eye can be dramatically improved in terms of its resolution, use of contrast agents for molecular imaging, safety evaluations to develop a clinically approved system, and integration with existing fundus imaging modalities. Multimodality ocular imaging platforms have also been successfully developed by a combination of photoacoustic microscopy (PAM) with other optical imaging such as optical coherence tomography (OCT), scanning laser ophthalmoscopy (SLO), and fluorescence microscopy (FM). The multimodal images can accurately be acquired from a single imaging system and co-registered on the same image plane, enabling improved evaluation of eye disease states. In this book chapter, the potential application of photoacoustic imaging of the eye in both research and clinical diagnosis are comprehensively discussed as a powerful medical screening technique for visualization of various ocular diseases