Development of an acousto-optics imaging system
Dominique Ettori, Eric Tinet, Jean-Michel Tualle
Former PhD student: Kinia Barjean (march 2016)
In order to perform optical imaging within thick tissues, a promising method is acousto-optics imaging, which is an emerging technique in the field of biomedical optics that combines the contrast allowed by diffuse optical tomography with the resolution of ultrasound (US) imaging. The US wave modulates both the refractive index and the scattering particles position. Under a monochromatic light source this induces a modulation at the US frequency of the speckle pattern resulting from multiply scattered light. A main challenge with this technology is to record this very weak modulation, bearing in mind that the speckle grains do not oscillate in phase so that their modulations do not add coherently. Among many potential solutions to perform such measurements, the OMA team developed, in collaboration with Institut d’Electronique Fondamentale (Orsay) an ASIC (application specific intergrated circuit) which presents many advantages for this application: it is a low-cost and compact technology, and it allows accessing to temporal correlations of the speckle pattern, what should give of a new kind of contrast relevant for medical diagnosis.
We therefore started a collaboration with Institut Langevin, with the team of François Ramaz, which is specialized on acousto-optics imaging. This work was supported by the ANR project ICLM. The LPL developed an autonomous prototype (fig. 1) for an easy implementation on the setup at Institut Langevin. This system allows a lock-in measurement of the speckle beat at the US frequency, and therefore filters the so-called tagged photons, which interacted with the US beam. We thus have a spatial selection in the diffusion light determined by the shape of the US beam.
Fig. 1: Prototype for acousto-optics imaging
The tagged signal comes from the whole US beam, and as we used a continuous US beam in this experiment we a priori don’t have any axial resolution. We however developed a new protocol , Fourier-Tranform Acousto-Optic Imaging (FT-AOI), in order to get such axial resolution. This protocol is especially suited to our application and allows high performances in term of signal to noise ratio. We have shown that, by introducing a periodic phase shift on the US beam and using the relevant demodulation scheme, it was possible to get the Fourier transform of the axial acousto-optic profile. An axial profile (‘z-scan’) can then be reconstructed through a simple inverse Fourier transform. The figure 2 presents an acouto-optic image obtained with this method  using a mechanical scanning of the transverse direction. The two absorbing inclusions, inserted within a 2 cm thick scattering gel, are clearly resolved, illustratingthe potential spatial resolution of the method.
Fig. 2: Acousto-optic image of two absorbing inclusions inserted within a scattering gel