Photoacoustic imaging techniques consist in the calculation of the initial pressure distribution from measurements on surfaces of various shapes. The pressure distribution at the initial time can be caused by the laser‐induced expansion of optical heterogeneities in liquid, liquid‐like, or solid media for instance. Postulated mechanical properties and theoretical formulas are used to perform the backpropagation of the measured time traces, leading to the imaging of the initial pressure distribution.
Among reconstruction algorithms, those based on time‐reversal considerations are used because they give robust results even if inhomogeneities are present within the medium. The existing algorithms require the backpropagation of the compressional or the shear waves separately. However in laser ultrasonics, when the measurements of the lasergenerated displacements are performed at a surface of a solid plate, compressional and shear acoustic pulses can overlap, depending on the thickness of the plate. To overpass this limitation, a reconstruction algorithm that takes into account simultaneously both compressional and shear waves, even in overlapping cases, is of great interest. The purpose of this work is to propose a reconstruction algorithm for the imaging of optoacoustic sources distributed in the volume of a solid. It is based on the time‐reversal and the Huygens’ principles, and simulates, in a solid medium, the backpropagation of the measured acoustic waves, both compressional and shear waves. It allows analyzing the initial in‐depth and lateral distributions of the optoacoustic source.
To illustrate the method, we propose the following model experiment. We focus a laser beam to a circular spot of diameter 100 µm on the surface of a plate. The laser beam penetrates over a depth of about 220 µm depending on the optical properties of the material. The absorption of the laser beam leads to a heating of the material that locally expands in the illuminated area owing to thermal dilatation. This process therefore defines a volume expansion source. The normal displacements caused by the acoustic waves generated by this volume optoacoustic source are measured at a surface of the plate. By applying the reconstruction algorithm we have developed, good characterization of the indepth and lateral distributions of a volume optoacoustic source in a solid medium are obtained from recorded data. This work suggests important potentialities for the imaging of optical heterogeneities in microelectronic industry for instance, or for biomedical imaging of calcified tissues, where shear waves are expected to be generated.