Photo-acoustic tomography based on laser optical feedback imaging of surface displacements.

We present how a laser optical feedback imaging (LOFI) setup can be used for the optical detection of ultrasound in photo-acoustic tomography (PAT). A PAT image is reconstructed by an inversion algorithm using surface displacement measurements made at several locations with our LOFI setup and following the optical irradiation with a pulsed Nd:YAG laser of a sample with absorbing inclusions. The width of the reconstructed inclusions and the signal-to-noise ratio (SNR) of the reconstructed images are first studied on the numerical model of a sample with three absorbing inclusions (i.e., with three acoustic punctual sources). Finally, an experimental PAT image of a phantom composed of two polyamide tubes with an internal diameter of 800 µm filled with red ink and submerged at -3.5 mm depth in a tank filled with water is reconstructed. Experimentally, the water surface displacement measurements have been made with our LOFI vibrometer, which provides an amplitude sensitivity of 1 nm (for a single-shot measurement) in a detection bandwidth of roughly 1 MHz adapted to the detection of the polyamide tubes. Under our experimental conditions, the surface energy densities of the LOFI focalized beam for the detection and of the pulsed Nd:YAG laser used for the irradiation, are compatible with the maximum permissive exposure for future biomedical measurements. The SNR and the resolution of the reconstructed PAT images are in good agreement with the theoretical predictions.