Photoacoustic Imaging: Contrast Agents and Their Biomedical Applications

Abstract

Photoacoustic (PA) imaging as a fast‐developing imaging technique has great potential in biomedical and clinical applications. It is a noninvasive imaging modality that depends on the light‐absorption coefficient of the imaged tissue and the injected PA‐imaging contrast agents. Furthermore, PA imaging provides superb contrast, super spatial resolution, and high penetrability and sensitivity to tissue functional characteristics by detecting the acoustic wave to construct PA images. In recent years, a series of PA‐imaging contrast agents are developed to improve the PA‐imaging performance in biomedical applications. Here, recent progress of PA contrast agents and their biomedical applications are outlined. PA contrast agents are classified according to their components and function, and gold nanocrystals, gold‐nanocrystal assembly, transition‐metal chalcogenides/MXene‐based nanomaterials, carbon‐based nanomaterials, other inorganic imaging agents, small organic molecules, semiconducting polymer nanoparticles, and nonlinear PA‐imaging contrast agents are discussed. The applications of PA contrast agents as biosensors (in the sensing of metal ions, pH, enzymes, temperature, hypoxia, reactive oxygen species, and reactive nitrogen species) and in bioimaging (lymph nodes, vasculature, tumors, and brain tissue) are discussed in detail. Finally, an outlook on the future research and investigation of PA‐imaging contrast agents and their significance in biomedical research is presented.

Indocyanine Green–Coated Gold Nanoclusters for Photoacoustic Imaging and Photothermal Therapy

“…PA imaging has become increasingly popular due to its deep signal penetration (ࣘ5-6 cm), high resolution (ࣙ5 µm), and decreasing instrument costs. [12][13][14][15][16] Furthermore, because the molecular contrast agents for PA imaging are optimized for non-radiative emission, many of these same materials can be utilized for PTT, allowing dual clinical functionality. [9] PTT has a potential advantage over photodynamic therapy, in that it does not require the presence of oxygen, the concentration of which may be limited in large solid tumors.…”

“…For this, we selected another material that has been proven dually efficacious in both PTT and PA imaging: gold nanoparticles (AuNPs). [5,13,16,24,25] Several groups have reported physiological studies of particles combining ICG and gold, but these particles have several key drawbacks. For example, they incorporated significant quantities of inert binding materials (serum albumin, [26,27] silica, [28][29][30][31] poly(styrene-alt-maleic anhydride), [32] polyethyleneimine, [32,33] chitosan, [27,34] and poly(lactic-co-glycolic acid [27] ), thereby reducing the per-particle loading of functionally active ICG and Au.…”

The Yin and Yang of PDT and PTT

“…Another aspect of using nanoparticles for either PDT or PTT is the ability to acquire additional contrast‐imaging information to guide treatment. For example, nanoparticles with high optical absorption can serve as contrast agents for photoacoustic tomography that can be used both to improve tumor localization and for dosimetry.…”

Polymer‐based activatable optical probes for tumor fluorescence and photoacoustic imaging

“…Due to negligible phonons scattering in living organisms, PAI provides deep tissue penetration (~6 cm) and high spatiotemporal resolution (~5 μm; Zhen, Feng, Xie, Zheng, & Pu, ). Recently, many materials have been used as PA imaging agents, including inorganic nanomaterials, carbon‐based nanomaterials, 2D materials, small organic molecules, and semiconducting polymer nanoparticles (Fu, Zhu, Song, Yang, & Chen, ). Compared to other imaging modalities such as magnetic resonance imaging, positron emission tomography, and single photon emission computed tomography, the optical imaging techniques are inexpensive, fast and nonradiative (X. Li, Kim, Yoon, & Chen, ).…”