Determining Optical Properties Of Turbid Media Using Incomplete PN Approximation To Radiance

The technique of interstitially determining the optical properties of tissue has a wide application in the interstitial photodynamic therapy,laser-induced interstitial thermotherapy and dianosis of cancer aiming at the tissue located deep within the body.The continuous wave(CW)radiance technique which can detect the photons from different directions at a selected point has become an important means of mini-mally invasive detection of tissue physiology and morphology information.However,the current CW radiance technique is difficult to accurately recover the optical prop-erties(absorption coefficient,?a,reduced scattering coefficient,?s')and the total he-moglobin concentration of the tissue with a low reduced albedo a'.Besides,the ra-diance technique has not been successfully applied to measure the full optical proper-ties(?a,scattering coefficient ?s,anisotropy factor g)of the tissue.This thesis is de-voted to develop an analytical method for recovering the optical properties based on a high-precision model of the photon transfer.The incomplete P3 approximation(P3in)has been developed firstly,which is the asymptotic part of the solution for the radiance by P3 approximation.The proposed P3in provides the simpler formulation than the P3,and the more accurate model than the diffusion approximation(DA)for describing the light propagation.Based on the P3in,an analytical method for recovering the optical properties directly from the radi-ance measurements has been developed.The simulation and experimental results show that,for the case of low a',the P3in based analytical method can greatly im-prove the accuracy of the recovered optical properties,when compared with the P3 based fitting method and the DA based analytical method.The solution of radiative transfer equation(RTE)by the P5 approximation for the case of an isotropic CW point source in an infinite turbid medium has be derived,which could be applied for describing light propagation in the near source regimes.Compared with the traditional P5 approximation expressed in a recursive form,the developed P5 approximation has a direct expression and the physical meaning of each item is clear,and thus the incomplete P5 approximation(P5in)for radiance can been further proposed.Based on the developed P5in,an analytical expression for directly calculating the full optical properties(?a,?s,g)using the radiance data measured in the near-source regimes has been constructed.The simulation and experimental re-sults show that the radiance technique has been first applied to simultaneously deter-mine ?a,?s and g of turbid media successfully.Based on the P5in and using the point spectral radiance measurement data,a fit-ting recovery method for obtaining the absorption related physiological parameters(hemoglobin concentration and oxygen saturation)and the scattering related morpho-logical parameters(scattering amplitude and scattering power)independently has been proposed.The results show that,the independent fitting method developed in this pa-per can avoid the crosstalk between the physiological parameters and the morpholog-ical parameters,and thus can improve the accuracy of the recovered physiological pa-rameters and morphological parameters,when compared with the traditional direct fitting algorithm based on the P3 approximation.This thesis has proposed the incomplete PN approximation model and based on such model has developed analytical methods for recovering optical properties and the independent fitting method for recovering physiological parameters and the morpho-logical parameters,which extends the application range of the optical properties and physiological parameters for the CW radiance technique in the minimally invasive detection of biological tissue.The results have proved that both the recovery accuracy and the application range have been optimized and improved by using the developed methods.