Nonionizing photoacoustic cystography with near-infrared absorbing gold nanostructures as optical-opaque tracers

Surfactant-Stripped Semiconducting Polymer Micelles for Tumor Theranostics and Deep Tissue Imaging in the NIR-II Window

Photoacoustic imaging (PA) in the second near infrared (NIR-II) window presents key advantages for deep tissue imaging owing to reduced light scattering and low background signal from biological structures. Here, a thiadiazoloquinoxaline-based semiconducting polymer (SP) with strong absorption in the NIR-II region is reported. After encapsulation of SP in Pluronic F127 (F127) followed by removal of excess surfactant, a dual functional polymer system named surfactant-stripped semiconductor polymeric micelles (SSS-micelles) are generated with water solubility, storage stability, and high photothermal conversion efficiency, permitting tumor theranostics in a mouse model. SSS-micelles have a wideband absorption in the NIR-II window, allowing for the PA imaging at both 1064 and 1300 nm wavelengths. The PA signal of the SSS-micelles can be detected through 6.5 cm of chicken breast tissue in vitro. In mice or rats, SSS-micelles can be visualized in bladder and intestine overlaid 5 cm (signal to noise ratio, SNR ≈ 17 dB) and 5.8 cm (SNR over 10 dB) chicken breast tissue, respectively. This work demonstrates the SSS-micelles as a nanoplatform for deep tissue theranostics.

Systematic imaging in medicine: a comprehensive review

Systematic imaging can be broadly defined as the systematic identification and characterization of biological processes at multiple scales and levels. In contrast to "classical" diagnostic imaging, systematic imaging emphasizes on detecting the overall abnormalities including molecular, functional, and structural alterations occurring during disease course in a systematic manner, rather than just one aspect in a partial manner. Concomitant efforts including improvement of imaging instruments, development of novel imaging agents, and advancement of artificial intelligence are warranted for achievement of systematic imaging. It is undeniable that scientists and radiologists will play a predominant role in directing this burgeoning field. This article introduces several recent developments in imaging modalities and nanoparticles-based imaging agents, and discusses how systematic imaging can be achieved. In the near future, systematic imaging which combines multiple imaging modalities with multimodal imaging agents will pave a new avenue for comprehensive characterization of diseases, successful achievement of image-guided therapy, precise evaluation of therapeutic effects, and rapid development of novel pharmaceuticals, with the final goal of improving human health-related outcomes.

Deep tissue photoacoustic imaging of nickel(II) dithiolene-containing polymeric nanoparticles in the second near-infrared window

Photoacoustic imaging is gaining great attention in the medical world due to its significant potential for clinical translation. Light excitation in the second near-infrared (NIR-II) window (1000-1350 nm) has resolution and penetration depth suitable for several clinical applications. However, the significant challenge exists for clinical translation because of the absence of notable intrinsic chromophores in this clinically significant optical range to generate diagnostic images. Methods: We present newly developed a biocompatible nickel dithiolene-based polymeric nanoparticle (NiPNP), which have a strong and sharp absorption peak at 1064 nm, as a photoacoustic contrast agent to boost specific absorbance in the NIR-II window for in vivo deep tissue imaging. Results: We confirm the enhanced PA signal by NiPNP's strong light absorption in the NIR-II window (287% higher than that of NIR-I) and deep tissue imaging capability (~5.1 cm) through in vitro experiment. We have successfully acquired diagnostic-quality in vivo photoacoustic images in deep tissue (~3.4 cm) of sentinel lymph nodes, gastrointestinal tracts, and bladders of live rats by using clinically viable imaging system. Conclusions: Our results prove that with strong absorption in the NIR-II window and with deeper imaging depth, the clinical translation of photoacoustic imaging with NiPNP is feasible for preclinical studies and thus would facilitate further clinical investigations.

Review on practical photoacoustic microscopy

Photoacoustic imaging (PAI) has many interesting advantages, such as deep imaging depth, high image resolution, and high contrast to intrinsic and extrinsic chromophores, enabling morphological, functional, and molecular imaging of living subjects. Photoacoustic microscopy (PAM) is one form of the PAI inheriting its characteristics and is useful in both preclinical and clinical research. Over the years, PAM systems have been evolved in several forms and each form has its relative advantages and disadvantages. Thus, to maximize the benefits of PAM for a specific application, it is important to configure the PAM system optimally by targeting a specific application. In this review, we provide practical methods for implementing a PAM system to improve the resolution, signal-to-noise ratio (SNR), and imaging speed. In addition, we review the preclinical and the clinical applications of PAM and discuss the current challenges and the scope for future developments.

Current concepts in nanostructured contrast media development for in vivo photoacoustic imaging

To overcome the endogenous photoacoustic contrast arising from endogenous species, specific contrast agents need to be developed, allowing PAI to successfully identify targeted contrast in the range of wavelength in which the interference from the biomatrix is minimized.

Bi2Se3 nanoplates for contrast-enhanced photoacoustic imaging at 1064 nm

Photoacoustic (PA) imaging is a high-resolution biomedical imaging modality, which can be used to visualize biological tissues located beyond the limited penetration depth of existing optical imaging techniques. An optical wavelength of 1064 nm is of great interest in PA imaging due to low intrinsic absorption at this wavelength. Reduced absorption implies an increased depth of imaging, which enables several new clinical applications such as bladder imaging, gastrointestinal (GI) imaging, and sentinel lymph node (SLN) imaging. In addition, a 1064 nm Nd:YAG laser system enables a high power, cost-effective, and compact laser-based PA imaging system. However, at this wavelength, due to low intrinsic contrast, high absorption exogenous PA contrast agents are necessary for imaging. To this end, we present new Bi2Se3 nanoplates as PA contrast agents at 1064 nm wavelength for PA imaging. We successfully synthesized Bi2Se3 nanoplates and they exhibited relatively strong PA signals at 1064 nm. We confirmed the increased imaging depth of penetration by imaging the Bi2Se3-containing tube located 4.6 cm deep in biological tissues. We present in vivo PA imaging of the bladder, GI tract, and SLN in mice using a Bi2Se3 contrast agent establishing the clinical feasibility of these agents with a clinical photoacoustic/ultrasound imaging system. Our results confirm that Bi2Se3 nanoplates are promising PA contrast agents at 1064 nm that offer a high optical absorbance in the second NIR region providing a high contrast imaging and increased depth of penetration.

A novel bacteriochlorin-gold nanoparticle construct for photoacoustic imaging

Herein we report the synthesis of new gold nanoparticle-bacteriochlorin conjugate (Au-NP-PS) as a highly efficient photoacoustic (PA) imaging agent. The conjugate was obtained by reacting a water soluble thio-bacteriochlorin (derived from bacteriochlorophyll-[Formula: see text] with Au-NP. The resulting NPs were characterized by UV-vis, TEM (transmission electron microscopy)/HRTEM (high resolution TEM) and diffraction pattern. The results obtained from in vitro studies suggest that due to conjugation between the PS and Au-NPs, the photoacoustic (PA) signals from the Au-NP-PS were greatly enhanced by a factor of 2.4 compared to that from PS alone.

Delay-multiply-and-sum-based synthetic aperture focusing in photoacoustic microscopy

We propose an improved version of a synthetic aperture focusing technique (SAFT) based on a delay-multiply-and-sum algorithm for acoustic-resolution photoacoustic microscopy (AR-PAM). In this method, the photoacoustic (PA) signals from multiple scan-lines are combinatorially coupled, multiplied, and then summed. This process can be considered a correlation operation of the PA signals in each scan-line, so the spatial coherent information between the PA signals can be efficiently extracted. By applying this method in conventional AR-PAM, lateral resolution and signal-to-noise ratio in out-of-focus regions are much improved compared with those estimated from the previously developed SAFT, respectively, thereby achieving the extension of the imaging focal region. Our phantom and in vivo imaging experiments prove the validity of our proposed method.

Real-time Near-infrared Virtual Intraoperative Surgical Photoacoustic Microscopy

We developed a near infrared (NIR) virtual intraoperative surgical photoacoustic microscopy (NIR-VISPAM) system that combines a conventional surgical microscope and an NIR light photoacoustic microscopy (PAM) system. NIR-VISPAM can simultaneously visualize PA B-scan images at a maximum display rate of 45 Hz and display enlarged microscopic images on a surgeon's view plane through the ocular lenses of the surgical microscope as augmented reality. The use of the invisible NIR light eliminated the disturbance to the surgeon's vision caused by the visible PAM excitation laser in a previous report. Further, the maximum permissible laser pulse energy at this wavelength is approximately 5 times more than that at the visible spectral range. The use of a needle-type ultrasound transducer without any water bath for acoustic coupling can enhance convenience in an intraoperative environment. We successfully guided needle and injected carbon particles in biological tissues ex vivo and in melanoma-bearing mice in vivo.

In vivo photoacoustic and fluorescence cystography using clinically relevant dual modal indocyanine green

Conventional X-ray-based cystography uses radio-opaque materials, but this method uses harmful ionizing radiation and is not sensitive. In this study, we demonstrate nonionizing and noninvasive photoacoustic (PA) and fluorescence (FL) cystography using clinically relevant indocyanine green (ICG) in vivo. After transurethral injection of ICG into rats through a catheter, their bladders were photoacoustically and fluorescently visualized. A deeply positioned bladder below the skin surface (i.e., ~1.5-5 mm) was clearly visible in the PA and FL image using a laser pulse energy of less than 2 mJ/cm2 (1/15 of the safety limit). Then, the in vivo imaging results were validated through in situ studies. Our results suggest that dual modal cystography can provide a nonionizing and noninvasive imaging tool for bladder mapping.

Multiplane spectroscopic whole-body photoacoustic imaging of small animals in vivo

We have successfully developed a multiscale acoustic-resolution photoacoustic tomography system in a single imaging platform. By switching between ultrasound transducers (center frequencies 5 and 40 MHz) and optical condensers, we have photoacoustically imaged microvasculatures of small animals in vivo at different scales. Further, we have extended the field of view of our imaging system to entire bodies of small animals. At different imaging planes, we have noninvasively imaged the major blood vessels (e.g., descending aorta, intercostal vessels, cephalic vessels, brachial vessels, femoral vessels, popliteal vessels, lateral marginal vessels, cranial mesenteric vessels, mammalian vessels, carotid artery, jugular vein, subclavian vessels, iliac vessels, and caudal vessels) as well as intact internal organs (e.g., spleen, liver, kidney, intestine, cecum, and spinal cord) of the animals in vivo. The spectroscopic whole-body photoacoustic imaging clearly reveals the spectral responses of the internal structures. Similar to other existing preclinical whole-body imaging systems, this whole-body photoacoustic tomography can be a useful tool for small-animal research.