Control of scattered coherent light and photoacoustic imaging : toward light focusing in deep tissue and enhanced, sub-acoustic resolution photoacoustic imaging

Abstract : Light focusing is a crucial requirement for high resolution optical imaging. In biological tissue though, refractive index inhomogeneities scatter light, preventing any focusing beyond one millimeter. Wavefront shaping techniques have been recently developed to partially compensate for light scattering after propagation through a scattering medium. These techniques require a measurement of the light intensity at the target point. These techniques hold much promise for performing wavefront correction in order to focus light deep inside scattering media. This would require a non-invasive measure of the light intensity at depth. In this PhD study, we propose to use the photoacoustic effect for such task. An optically absorbing structure under pulsed illumination indeed generates ultrasonic waves, whose amplitude is proportional to the absorbed light intensity. These ultrasounds mostly propagate in a ballistic way, and can therefore be detected with an external transducer. We have shown that photoacoustic imaging could be used to measure the transmission matrix of a scattering sample, enabling to focus light on absorbing structures as well as to retrieve mesoscopic properties of the medium. We have shown that the broadband spectral content of the photoacoustic signals can be harnessed to improve the focusing performances. Finally, we demonstrated that coherent illumination could be used to remove fundamentals artefacts, as well as to break the acoustic resolution limit of conventional deep tissue photoacoustic imaging.
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Thomas Chaigne. Control of scattered coherent light and photoacoustic imaging : toward light focusing in deep tissue and enhanced, sub-acoustic resolution photoacoustic imaging. Optics [physics.optics]. Université Pierre et Marie Curie - Paris VI, 2016. English. ⟨NNT : 2016PA066162⟩. ⟨tel-01402738⟩

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