Propagation Of Polarized Light In Turbid Media And Its Application For Imaging

Interest in research on the propagation of polarized light in turbid medium has increased recently because the polarization effects in scattered light can yield useful information about biological material, especially in noninvasive optical diagnosis and imaging through body tissues. In this thesis, the properties of the state of polarization of scattered light from turbid media were investigated both in experimental method and Monte Carlo simulation, then the obtained results were practically applied in imaging.A Monte Carlo algorithm was presented to model the polarized light propagation in turbid media. The Stokes-Mueller formalism included polarization effect into the standard Wang-Jacques code was employed. The algorithm based on Single-scattering event according to Mie theory and keeps track of the polarization state of individual photons in multiple scattering media. It should be noted that our simulation fully considered the Fresnel matrices that describe reflection from the surface of the medium. We considered the effect of different incident polarization states and tissue optical parameters. The simulation well reproduced the 16, two-dimensional elements of the diffuse backscattering Mueller matrices and 4 Stokes vector. The results showed that: (1) Stokes Vectors vary as incident polarization state changes, while Mueller matrices remain the same. (2) Both Stokes Vectors and Mueller matrices are influenced by tissue optical parameters. The regime where we can get clear signals contracts as the scattering coefficient increases. From these findings we can concluded that Stokes Vectors are influenced by incident light and tissue characters, while Mueller matrices are determined only by tissue characters.By varying the incident polarization and the analyzer for the experimental measurements, the diffuse backscattering Mueller matrix elements were obtained. A depolarization value concept was introduced to describe the optical properties of backscattering media and the polarization properties of diffusely scattered light were studied as a function of medium concentration and detection angle. Furthermore, the interactions of polarized light with glucose dissolved in turbid medium were investigated. The experimental results showed that polarization preservation on multiple scattering affected by the glucose concentration, as well as turbid of the medium. Finally the accuracy of this measurement was discussed too.Based on the above investigation, we used the depolarization value to discriminate and image the hidden object in turbid media. An Intralipid with a given concentration was used as a tissue phantom in which a black rubber hiddenas an absorptive object. It is experimentally demonstrated that depolarization value of the backscattered light measured with and without the hidden object are different. Therefore, we can determine whether the object is present or absent in a turbid medium from the differences. To validate our method, the results were compared with those by the total intensity discrimination. The resolution and contrast of polarized discriminating was superior to the intensity approach under the same conditions of the turbidity of the medium and depth of object located. Furthermore, by scanning the sample transversely, the location of the hidden object was restored with the shape of the depolarization value profile. In addition, it also showed that the effectiveness of this approach depends largely on the properties of the surface to be imaged rather than the hidden object's geometry. This work suggests new possibilities for discriminating or imaging of objects inside a turbid media when intensity techniques are inappropriate.