Study Of Absorbing Inclusions In Turbid Media Slabs Using Time Resolved And CW Light Trasmission

H. O. DI ROCCO; D. I. IRIARTE; JUAN ANTONIO POMARICO; H. F. RANEA SANDOVAL; M. V. WAKS SERRA

Lima

Congreso; RIAO-OPTILAS 2010; 2010

PUCP

In the last decade, the use of red and near-infrared radiation has been proposed as a tool for a possible noninvasive imaging modality to detect pathological processes leading to changes in optical properties of tissue. Until now, a major obstacle in the achievement of such a modality is the strong scattering of light by biological tissues that dramatically deteriorates image quality and may smudge images structures inside the tissue, such as tumors. Despite its relative lower spatial resolution when compared to X- Rays, the interest in this novel noninvasive tool relies on its capability to optically characterize the inhomogeneities, so complementing the techniques used for localization. These optical parameters are the absorption coefficient , the scattering coefficient, the anisotropy factor g and the reduced scattering coefficient.The problem of scattered light admits three approaches, namely time resolved, steady state (or continuous wave: CW), and frequency-domain optical techniques. Experimentally, we investigate the transmitted light profiles when scattering and absorbing inhomogeneities are located at different depths inside slabs of turbid media. In particular, we present the case of cylindrical inclusions submerged in rectangular tanks of liquid phantoms. This inclusion is cylinder (diameter 14 mm and 160mm of longitude) of a mixture of water, milk (3:1), Indian ink dilution in distilled water (1:1000) and agarose (2%). The bulk of the phantom is the same liquid mixture water: milk (3:1) and the half concentration of Indian ink diluted (1:1000) that the cylindrical inclusion. We analyze the transilluminance light profiles when the phantom is scanned using a NIR laser. The study of the spatial profiles obtained with CW light shows an apparently contradictory effect when the absorption coefficient of the inclusion is higher than that of the bulk. In this case, the intensity profiles present a peak of higher intensity when the laser, the detector and the axis of the cylinder are all collinear, as it would be expected for a less absorbing inclusion. To address this problem, and provided that time resolved techniques can give more information than CW, we present an experiment simultaneously in Time Resolved (TR) and CW that shows this effect with one  and two inclusions of different diameter. The analysis of the distributions of times of flight of the photons (DTOF) considers not only the zero moment of the light pulses (equivalent to CW measurements) but also higher order moments. In particular, the first moment of this distribution provide important information about the position and the optical properties of the inclusions because the variations in the absorption coefficient change the shape of the DTOF giving a narrower pulse when the absorption increases. Moreover, the experiments were compared with a theoretical model for cylindrical inclusions and with Monte Carlo (MC) simulations using Graphics Processing Units based  on Compute Unified Device Architecture (CUDA) that reduce  the times of calculations