Light scattering in biological tissue prevents optical microscopy from reaching depths beyond 1.5 mm. This drastically restricts the study of deep brain regions located beneath the cortex, even in small rodents [1].

PA imaging has been developed to image optically-absorbing structures (such as blood vessels) deep inside biological tissue [2]. This technique relies on the ultrasound waves emitted upon the absorption of pulsed illumination. As opposed to light, these pressure waves are not disturbed when propagating through soft tissue. The acoustic field can then be detected at the tissue surface in a non-invasive way. Optically-absorbing structures can thus be reconstructed with acoustic resolution [3].

Conventional piezoelectric-based detectors are nonetheless limited to low acoustic frequencies, and two-dimensional probes (for three-dimensional imaging) are highly complex to fabricate and use.

As an alternative, optical sensors of ultrasound have been developed to overcome these issues [4]. The LightEcho system from the French company Deepcolor Imaging is using a polymer-based Fabry-Perot sensor, which allows to image a volume of 15x15x10 mm3 with a resolution up to 50 µm. This system can image blood vessels [5], as well as exogenous chromophores [6].

References:

[1] V. Ntziachristos, “Going deeper than microscopy: the optical imaging frontier in biology,” Nature Methods, vol. 7, no. 8, pp. 603–614, Aug. 2010, doi: 10.1038/nmeth.1483.

[2] P. Beard, “Biomedical photoacoustic imaging,” Interface Focus, vol. 1, no. 4, pp. 602–631, Aug. 2011, doi: 10.1098/rsfs.2011.0028.

[3] A. Rosenthal, V. Ntziachristos, and D. Razansky, “Acoustic Inversion in Optoacoustic Tomography: A Review,” Current Medical Imaging Reviews, vol. 9, no. 4, pp. 318–336, Jan. 2014, doi: 10.2174/15734056113096660006.

[4] G. Wissmeyer, M. A. Pleitez, A. Rosenthal, and V. Ntziachristos, “Looking at sound: optoacoustics with all-optical ultrasound detection,” Light: Science & Applications, vol. 7, no. 1, p. 53, Aug. 2018, doi: 10.1038/s41377-018-0036-7.

[5] O. Ogunlade et al., “In vivo three-dimensional photoacoustic imaging of the renal vasculature in preclinical rodent models,” American Journal of Physiology-Renal Physiology, vol. 314, no. 6, pp. F1145–F1153, Jun. 2018, doi: 10.1152/ajprenal.00337.2017.

[6] O. Ogunlade et al., “In vivo photoacoustic imaging of a nonfluorescent E2 crimson genetic reporter in mammalian tissues,” JBO, vol. 25, no. 4, p. 046004, Apr. 2020, doi: 10.1117/1.JBO.25.4.046004.