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Photoacoustic or optoacoustic imaging (PAI) is an emerging imaging modality that utilizes a hybrid approach by using optical illumination of endogenous materials or administered fluorescent tracers, and consequent detection of the ultrasound waves released due to thermoelastic expansion.
During photoacoustic imaging, a typically nanosecond, narrow bandpass laser or LED illumination is used to excite biological tissues. Molecules absorb the optical energy and convert it to heat, which in turn results in thermoelectric expansion of the investigated tissue. This expansion generates acoustic waves that can be detected by US transducers. Clinical use of optical imaging has been hampered by the limited penetration of light in biological tissues. Photoacoustic imaging partially overcomes this limitation by using the emitted US waves for detection, increasing the maximum depth of the imaging to several centimeters.
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, melanin, and water 1.
Current use and potential future applications
Label-free photoacoustic imaging is a non-invasive method similar to the conventional US, and can be readily utilized in a variety of tasks. It can differentiate deoxyhemoglobin and hemoglobin, thereby directly visualizing ischemia/hypoxia. Superficial, pigmented lesions (e.g. melanoma) can also be visualized directly. Label-free photoacoustic imaging of tissue vascularity has been successfully employed in e.g. imaging of breast cancers, skin tumors, psoriatic skin lesions, wound healing, and inflammation of the small joints 2-5. PAI has been also proposed as a tool to image disease activity in Crohn disease 6. With the light source easily integrated into a linear array, US transducer photoacoustic imaging can be readily utilized as an adjunct in sonography.
The clinically approved fluorophores indocyanine green, and methylene blue can also be depicted with photoacoustic imaging as can most other fluorescent tracers 1. Selective labeling with these chromophores (e.g. labeled peptides or antibodies) can open up new perspectives in clinical molecular imaging.