Photoacoustic flow measurements with gold nanoparticles

Surface-Functionalized Modified Copper Sulfide Nanoparticles Enhance Checkpoint Blockade Tumor Immunotherapy by Photothermal Therapy and Antigen Capturing

Nanomaterial-based tumor photothermal therapy (PTT) has attracted increasing attention and been a promising method for cancer treatment because of its low level of adverse effects and noninvasiveness. However, thermotherapy alone still cannot control tumor metastasis and recurrence. Here, we developed surface-functionalized modified copper sulfide nanoparticles (CuS NPs). CuS NPs can not only be used as photothermal mediators for tumor hyperthermia but can adsorb tumor antigens released during hyperthermia as an antigen-capturing agent to induce antitumor immune response. We selected maleimide polyethylene glycol-modified CuS NPs (CuS NPs-PEG-Mal) with stronger antigen adsorption capacity, in combination with an immune checkpoint blocker (anti-PD-L1) to evaluate the effect of hyperthermia, improving immunotherapy in a 4T1 breast cancer tumor model. The results showed that hyperthermia based on CuS NPs-PEG-Mal distinctly increased the levels of inflammatory cytokines in the serum, leading to a tumor immunogenic microenvironment. In cooperation with anti-PD-L1, PTT mediated by CuS NPs-PEG-Mal enhanced the number of tumor-infiltrating CD8⁺ T cells and inhibited the growth in primary and distant tumor sites of the 4T1 tumor model. The therapeutic strategies provide a simple and effective treatment option for metastatic and recurrent tumors.

Photoacoustic tomography: principles and advances

Photoacoustic tomography (PAT) is an emerging imaging modality that shows great potential for preclinical research and clinical practice. As a hybrid technique, PAT is based on the acoustic detection of optical absorption from either endogenous chromophores, such as oxy-hemoglobin and deoxy-hemoglobin, or exogenous contrast agents, such as organic dyes and nanoparticles. Because ultrasound scatters much less than light in tissue, PAT generates high-resolution images in both the optical ballistic and diffusive regimes. Over the past decade, the photoacoustic technique has been evolving rapidly, leading to a variety of exciting discoveries and applications. This review covers the basic principles of PAT and its different implementations. Strengths of PAT are highlighted, along with the most recent imaging results.

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.

Photoacoustic imaging

Photoacoustic imaging, which is based on the photoacoustic effect, has developed extensively over the last decade. Possessing many attractive characteristics such as the use of nonionizing electromagnetic waves, good resolution and contrast, portable instrumention, and the ability to partially quantitate the signal, photoacoustic techniques have been applied to the imaging of cancer, wound healing, disorders in the brain, and gene expression, among others. As a promising structural, functional, and molecular imaging modality for a wide range of biomedical applications, photoacoustic imaging can be categorized into two types of systems: photoacoustic tomography (PAT), which is the focus of this article, and photoacoustic microscopy (PAM). We first briefly describe the endogenous (e.g., hemoglobin and melanin) and the exogenous (e.g., indocyanine green [ICG], various gold nanoparticles, single-walled carbon nanotubes [SWNTs], quantum dots [QDs], and fluorescent proteins) contrast agents for photoacoustic imaging. Next, we discuss in detail the applications of nontargeted photoacoustic imaging. Recently, molecular photoacoustic (MPA) imaging has gained significant interest, and a few proof-of-principle studies have been reported. We summarize the current state of the art of MPA imaging, including the imaging of gene expression and the combination of photoacoustic imaging with other imaging modalities. Last, we point out obstacles facing photoacoustic imaging. Although photoacoustic imaging will likely continue to be a highly vibrant research field for years to come, the key question of whether MPA imaging could provide significant advantages over nontargeted photoacoustic imaging remains to be answered in the future.

Nanorod-based flow estimation using a high-frame-rate photoacoustic imaging system

A quantitative flow measurement method that utilizes a sequence of photoacoustic images is described. The method is based on the use of gold nanorods as a contrast agent for photoacoustic imaging. The peak optical absorption wavelength of a gold nanorod depends on its aspect ratio, which can be altered by laser irradiation (we establish a wash-in flow estimation method of this process). The concentration of nanorods with a particular aspect ratio inside a region of interest is affected by both laser-induced shape changes and replenishment of nanorods at a rate determined by the flow velocity. In this study, the concentration is monitored using a custom-designed, high-frame-rate photoacoustic imaging system. This imaging system consists of fiber bundles for wide area laser irradiation, a laser ultrasonic transducer array, and an ultrasound front-end subsystem that allows acoustic data to be acquired simultaneously from 64 transducer elements. Currently, the frame rate of this system is limited by the pulse-repetition frequency of the laser (i.e., 15 Hz). With this system, experimental results from a chicken breast tissue show that flow velocities from 0.125 to 2 mms can be measured with an average error of 31.3%.