Abstract
Photoacoustic imaging (PAI) and thermoacoustic imaging (TAI) are two emerging biomedical imaging techniques that both utilize ultrasonic signals as an information carrier. Unique advantages of PAI and TAI are their abilities to provide high resolution functional information such as hemoglobin and blood oxygenation and tissue dielectric properties relevant to physiology and pathology. These two methods, however, may have a limited detection depth and lack of endogenous contrast. An exogenous contrast agent is often needed to effectively resolve these problems. Such agents are able to greatly enhance the imaging contrast and potentially break through the imaging depth limit. Furthermore, a receptor-targeted contrast agent could trace the molecular and cellular biological processes in tissues. Thus, photoacoustic and thermoacoustic molecular imaging can be outstanding tools for early diagnosis, precise lesion localization, and molecular typing of various diseases. The agents also could be used for therapy in conjugation with drugs or in photothermal therapy, where it functions as an enhancer for the integration of diagnosis and therapy. In this article, we present a detailed review about various exogenous contrast agents for photoacoustic and thermoacoustic molecular imaging. In addition, challenges and future directions of photoacoustic and thermoacoustic molecular imaging in the field of translational medicine are also discussed.
Highlights
Photoacoustic imaging (PAI) is a burgeoning biomedical structural and functional imaging technique which utilizes ultrasonic signals as an information carrier that captures information about optical absorption property relevant physiology and pathology inside tissues
thermoacoustic imaging (TAI) offers higher spatial resolution than microwave imaging and receives much deeper imaging than most optical imaging techniques [4]. Since it is based on a different absorption mechanism, TAI can capture information about dielectric properties of the relevant physiology and pathology inside of tissues
Et al, explored perfluorocarbon-based nanoparticles by incorporating two different NIR fluorescent dyes, PPCy-C8 and cypate-C18, as PA and fluorescence imaging agents. They demonstrated the contrast agent’s ability of lymph node mapping with both modalities [21]. Besides these surface-modified dyes, there are many other dyes developed for PAI, such as Indocyanine-green-embedded PEBBLEs [16], ICG-polyethylene glycol (PEG) [17], ICG-enhanced carbon nanotubes [18], and ICG encapsulated in virus-mimicking nanoconstructs [19]
Summary
Photoacoustic imaging (PAI) is a burgeoning biomedical structural and functional imaging technique which utilizes ultrasonic signals as an information carrier that captures information about optical absorption property relevant physiology and pathology inside tissues. Unlike PAI, a TAI excitation source involves far-infrared light or microwaves It provides longer imaging depth due to a different electromagnetic radiation. TAI offers higher spatial resolution than microwave imaging and receives much deeper imaging than most optical imaging techniques [4] Since it is based on a different absorption mechanism, TAI can capture information about dielectric properties (such as the distributional difference of some polar molecules and ions) of the relevant physiology and pathology inside of tissues. Known as exogenous contrast agents, can be used to enhance images in medical scans of the tissues or organs Besides improve resolutions, they can potentially break through the imaging depth limit. A targeted contrast agent can be used for therapy in conjugation with drugs or photothermal therapy, and the double virtues of imaging and targeted therapy will be possessed
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