Direct imaging of turbid media using long-time back-scattered photons, a numerical study

Abstract

Direct imaging is a convenient way to obtain information on the interior of a semi-transparent turbid material by non-invasive probing using laser beams. The major difficulty is linked to scattering which scrambles the directional information coming from the laser beam. It is found in this paper that the long-term multiple-scattered reflected photons may provide structural information on the inside of a material, which offers an interesting alternative to using information only from un-scattered or least-scattered photons as obtained from current direct imaging set-ups for thin media.Based on some observations on a non-homogeneous three layered 1-D slab irradiated by a laser pulse, a direct probing methodology making use of the long-term back-scattered photons is illustrated to recover inclusions positions in a turbid 2-D medium. First, the numerical model is presented. Second, an extended parametrical study is conducted on 1-D homogeneous and non-homogeneous slabs with different laser pulse durations. It is found that the reflected asymptotic logarithmic slope carries information about the presence of the inclusion and that short laser pulses are not necessary since only the decaying parts of the remanent optical signature is important. Longer laser pulses allow a higher level of energy injection and signal to noise ratio. Third, those observations are used for the probing of a 2-D non-homogeneous phantom.

Boundary Effects on the Efficiency of Direct Infra-Red Optical Tomography

“…The aim of this study is to investigate the response of direct imaging with respect to reflective boundary conditions. The study is purely numerical and the algorithm is detailed in [39,18,19]. The first section presents the model problem.…”

“…The first section presents the model problem. In the second section, direct imaging is explored in a way similar to [18,19,40] but with the focus put on the change in the contrast of the image when reflection occurs at the boundary.…”

“…The source terms are treated implicitly and the Euler method is used for time integration. More details may be found in [39,18].…”

“…This leads to a sharper log-slope. Now, it is important to recall the manner by which the image is formed [18]. The imaging contrast is obtained by backprojecting the log-slope of the reflected intensity along the half-line of sight following the laser beam direction.…”

“…Other areas are now very sensitive to this probing protocol making use of the diffuse signal as in the food industry [13], oceanography and fisheries [14,15]. More recently, it has been emphasized how the ALS could be used to reveal the presence of an inclusion inside a medium [16,17] and numerical studies have paved the way to a potential imaging tool for direct reconstruction using the ALS diffuse signal [18,19]. This kind of tool for direct imaging, in the best case, is limited to structural information and the optical properties (which are a sign of inclusion identification) remain unreachable.…”