Abstract
For combining optical and ultrasonic imaging methodologies, photoacoustic imaging (PAI) is the most important and successful hybrid technique, which has greatly contributed to biomedical research and applications. Its theoretical background is based on the photoacoustic effect, whereby a modulated or pulsed light is emitted into tissue, which selectively absorbs the optical energy of the light at optical wavelengths. This energy produces a fast thermal expansion in the illuminated tissue, generating pressure waves (or photoacoustic waves) that can be detected by ultrasonic transducers. Research has shown that optical absorption spectroscopy offers high optical sensitivity and contrast for ingredient determination, for example, while ultrasound has demonstrated good spatial resolution in biomedical imaging. Photoacoustic imaging combines these advantages, i.e., high contrast through optical absorption and high spatial resolution due to the low scattering of ultrasound in tissue. In this review, we focus on advances made in PAI in the last five years and present categories and key devices used in PAI techniques. In particular, we highlight the continuously increasing imaging depth achieved by PAI, particularly when using exogenous reagents. Finally, we discuss the potential of combining PAI with other imaging techniques.
Highlights
Together with exogenous contrast agents [24], such as bio-compatible nanoparticles, chemical dyes and report genes, researchers have greatly expanded the photoacoustic imaging (PAI) application range, while improving imaging depth and quality in animal and human tissues alike. Depending on their imaging resolution and depth, PAI methods are approximately divided into PA microscopy (PAM), PA macroscopy (PAMac) and PA computed tomography (PAT or PACT)
Details of the exogenous contrast agents for PAI enhancement can be found in recent review papers [24,60], here we focus on the progress made in brain imaging using contrast agents over the past five years
The results demonstrated that functional PAI of the fluorescence quenching Voltage-sensitive dye (VSD) mechanism proved to be a promising approach to recording the brain activities in a chemoconvulsant rat model, at a sub-mm spatial resolution, with an intact scalp
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Ultrasonography and other related ultrasonic techniques are safe methods that are widely used in medical imaging, offering a deep imaging depth, high spatial resolution and a low device cost. Optical imaging techniques can provide molecular information about tissues and organs based on their optical properties, with a limited spatial accuracy and imaging depth. Diffuse optical tomography (DOT) has made great advances in terms of its imaging of organs and the blood circulation in peripherals [1], but in deep tissue, at the depth of several centimeters, its spatial resolution is only 5–10 mm. In order to provide both high contrast and high spatial resolution in deep tissue imaging, some form of hybrid imaging technique is most likely to succeed, one that combines acoustics and optics. By combining PAI with ultrasonography techniques, it is possible to gather both structural and functional information [6], while combining PAI with DOT allows a quantitative detection of biological chromophores in tissues [7]
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