BS4A.3.pdf Biomedical Optics 2014 © OSA 2014 Enhanced Photoacoustic Imaging with Speckle Illumination Jérôme Gâteau, Thomas Chaigne, Ori Katz, Sylvain Gigan and Emmanuel Bossy Institut Langevin, ESPCI ParisTech, CNRS UMR 7587, INSERM ERL U979, 1 rue Jussieu, 75005 Paris, France Author e-mail address: emmanuel.bossy@espci.fr Abstract: We investigate experimentally the use of speckle illumination for photoacoustic imaging. In particular, we demonstrate that otherwise invisible features are revealed through high-frequency signals fluctuations from different speckle realizations. OCIS codes: (110.5125) Photoacoustics; (120.6150) Speckle imaging; (030.6140) Speckle; 1. Introduction Photoacoustic imaging is considered to be mostly speckle-free for uniform illumination and large density of optical absorbers contained in structures with smooth boundaries [1]. The lack of acoustic speckle artifacts in photoacoustics has been attributed to the strong initial phase and amplitude correlation among the ultrasound waves generated by the individual absorbing molecules or particles after quasi-instantaneous optical excitation. Two related effects of this correlation are the directivity of the ultrasonic emission for elongated structures and the prominent boundary build-up for large structures. This results in visibility issues when imaging homogeneous structures using a photoacoustic system with uniform illumination, because of the limited bandwidth and/or the limited view of practical implementations [2]. In this work, we propose here to exploit optical speckle patterns naturally present in coherent illumination as a source of structured illumination, to break the homogeneity of absorbing structures. Speckle illumination has proved very useful in optical microscopy for optical sectioning [3], and was recently also exploited as a structured illumination source to surpass the optical diffraction limit [4]. As photoacoustic techniques aim at imaging beyond the depth achieved by optical microscopy [5], only random a-priori unknown structured illumination provided by the passage of coherent light through a scattering medium can be considered. In the present study, unlike in conventional photoacoustic imaging where a locally uniform illumination driven by diffusion is usually considered, we investigate the operation of a high frequency and limited- view photoacoustic system with optical speckle illumination of homogeneously absorbing structures. The final images are obtained by computing the local variations of the signal under the different speckle illumination [9]. 2. Material and methods Fig.1: (a) Schematic of the experimental setup. The laser input beam (diameter  6 mm) impinged on the diffuser surface, which was placed at a distance L of the imaging plane. A coordinate system is introduced for the imaging plane. Its origin is the middle point of the array. (b) A typical optical speckle pattern illuminating the sample (transverse grain size of 6.3 μm ± 0.8 μm). The experimental set-up is illustrated in Fig. 1(a). Optical excitation was performed with a Q-Switched Nd:YAG oscillator laser (Brillant, Quantel) delivering 4 ns duration pulses at λlaser =532 nm with a 10 Hz repetition rate, and a coherence length on the order of 1 mm. To generate the varying speckle illumination, the laser beam was passed through a ground glass diffuser (220 Grit, Thorlabs), mounted on a rotation mount. The rotating diffuser acted here as a random inhomogeneous dynamic medium producing temporally varying speckle patterns with no appreciable ballistic transmitted component. The main difference is the scattering- induced path-length differences. Path-length differences larger than the laser coherence length may reduce speckle contrast, and were avoided here by employing a thin scattering layer. The diffuser was positioned at a distance L=15 mm to 150 mm from the imaged plane of the sample. Varying this distance allowed to control the optical speckle grain size on the target plane [6] ([Fig.