Photoacoustic or optoacoustic imaging is an emerging imaging modality that utilizes a hybrid approach comprising optical illumination and subsequent detection of the ultrasound waves released due to thermoelastic expansion 7.
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Clinical applications
Photoacoustic imaging has limited clinical utility and is not widely available. In principle, as the light source can be integrated into an ultrasound linear array transducer, photoacoustic imaging could become an adjunct to routine sonography.
The most promising aspect of photoacoustic imaging in clinical imaging is its ability to selectively detect naturally occurring chromophores with distinct optical absorption spectra, such as hemoglobin, lipids, collagen and melanin 1,7.
Exogenous photoacoustic contrast agents can also be used, including dyes and nanostructures of various compositions 7.
Endogenous chromophores
Photoacoustic imaging using endogenous chromophores can differentiate deoxyhemoglobin and hemoglobin, directly visualizing ischemia/hypoxia. Similarly, tissue vascularity has been successfully performed (e.g. imaging of breast cancers, skin tumors, psoriatic skin lesions, wound healing, and inflammation of the small joints) 2-5.
Superficial, pigmented lesions (e.g. melanoma) can also be visualized directly.
Photoacoustic imaging has also been proposed as a tool to image disease activity in Crohn disease 6.
Exogenous chromophores
A number of dyes are approved for clinical use by the FDA including indocyanine green, Evans blue and methylene blue 1,7.
Although nanoparticles of various compositions (e.g. metallic, organic, inorganic semiconductor, and carbon) are potentially useful as targets of photoacoustic imaging, their potential toxicity has limited their study 7.
Technique
Physical principles
Photoacoustic imaging exploits thermoelastic expansion; the ability of certain compounds to absorb photons of specific wavelengths, converting this energy into heat and structural expansion. This expansion, in turn, results in pressure waves propagating away from the tissue. An ultrasound transducer can then detect these waves 7.
Typically, very short burst (nanosecond), narrow bandpass laser or LED illumination is used to excite biological tissues.
The frequency needed for photoacoustic imaging depends on the target, with different requirements for various endogenous chromophores and exogenous contrast agents 7.
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