Performance Of The Models And Algorithms In The Image Reconstruction Of Diffuse Optical Tomography

Diffuse optical tomography(DOT)using near-infrared light is promising in many biomedical applications such as breast cancer detection and functional brain imaging since the detection using near-infrared light is harmless to biological tissues.Also the detection of DOT is convenient and anti-jamming to electromagnetic environments,and the price of the DOT system is low.Neither the detection nor the theory of DOT is complicated.However,many biomedical applications of DOT(including the optical detection of early breast cancer)are not applied to clinic yet.The primary difficult resides in the image reconstruction in practical applications,i.e.,either the quality of the reconstructed image is too bad to be used in medical diagnosis,or the computational time for the reconstruction is too long to be acceptable.The present study focuses on the performance(accuracy and computational efficiency)of the models and algorithms used in the image reconstruction of DOT.The goal is to develop efficient forward models and reconstruction algorithms in order to obtain the images whose quality are high enough to be used in medical diagnosis.The present thesis consists of seven chapters.A brief overview of the fundamental of DOT and various reconstruction methods at present is given in Chapter 1.The content of Chapter 2 is included to present a method to improve the computational efficiency of the Monte Carlo(MC)simulation of photon transport in turbid media.In Chapter 3,the deviations of two versions of the traditional diffusion model(compared with the MC simulation)are studied.Knowing the details of these deviations helps us to improve the accuracy of the diffusion models(as the forward models used in the image reconstruction of DOT)without debasing the computational efficiency of the models.And this is very important to the improvement of the quality of the reconstructed images(since the inverse problem of DOT is badly ill-posed).An introduction of the finite element method (FEM)to solve the diffusion equation is also included in Chapter 3.And the validity and efficiency of the zero boundary condition are discussed.The deviation of the diffusion model is relatively larger in the region near the surface of the medium than in the deep region.This surface deviation is studied in Chapter 4.A modification of the diffusion approximation is presented.This modification can be applied in both the pure diffusion model and the hybrid models(MC diffusion hybrid model and transport diffusion hybrid model)and significantly reduces the surface deviation without debasing the computational efficiency of the models.Therefore,the modified models are more suitable than the traditional modelsto be the forward models in DOT image reconstructions.This is the primary contribution of the present thesis.(It can improve the quality of the reconstructed images of DOT).The temporal autocorrelation of the diffuse light is also studied in this thesis.A straightforward method to calculate the correlation function in the MC simulation is present in Chapter 5.This method is fast and accurate.Also in Chapter 5 the accuracy of the correlation diffusion model is studied by comparing it with the MC simulation. Numerical experiments show that the diffusion model is really accurate to calculate the normalized temporal autocorrelation of the diffuse field(though the diffusion model is not so accurate to calculate the diffuse intensity),i.e.,the correlation diffusion model is almost exact.Therefore,the correlation diffusion model is an ideal forward model in the reconstruction of the dynamic properties(such as the Brownian coefficient)of the medium.The content of Chapter 6 is about the reconstruction algorithms and the quality of the reconstructed images in DOT.A two-step reconstruction algorithm based on the FEM and NCG(Nonlinear Conjugated Gradient,a nonlinear optimization method)is presented to improve the quality of the reconstructed images.The limitation of the reconstructed images by the forward model in the reconstructions is also studied.The limitation is that almost all the time the details of the absorption and scattering distribution of the medium are not correctly reconstructed due to the-low accuracy of the forward model and the ill-posedness of the inverse problem of the DOT imaging.Only the strong contrast across a large region and the significant characters of the reconstructed image are reliable and useful.The study in Chapter 6 reveals that the accuracyof the forward model used in the reconstruction of DOT is extremely important to the quality of the reconstructed image.In the last chapter,Chapter 7,we first give a brief description of the experiment work for this thesis,including an optical mammography system,a simple experiment,and a DOT/FDOT experiment system which we are building for research.And at last,we list the research tasks to be taken after the research of this thesis.