Active Microwave Imaging For Early Breast Cancer Detection

Breast cancer is the most common cancer found in women.Early prevention and diagnosis are key factors that can dramatically reduce the death rate of breast-cancer patients.Microwave imaging technology,a new non-invasive breast cancer detection method,has drawn more and more attention due to its superiority of low risk,high sensitivity and high contrast.One of the most prominent methods is the active microwave imaging method.It mainly involves two patterns,microwave tomography(MT)and confocal microwave imaging(CMI).By using Microwave Tomography,the electromagnetic fields outside the scattering object can be observed and then the distribution of electromagnetic characteristic parameters of the imaging region can be reconstructed.Finally,the position,shape and size distribution of the target can be achieved.In CMI,the energy distribution of the target region is obtained by focusing algorithm,and the position and size of the target are determined by distinguishing the strong and weak energy distribution.MT is similar to the X-ray computed tomography in modern medical use,and its physical interpretation is clearer and more valuable in clinical detection.This paper focuses on the research of MT,which belongs to the electromagnetic inverse scattering.The optimization iteration is usually applied in dealing with inverse scattering problems,which results in a large amount of CPU time spent on the forward solver calculation.The inverse scattering problem can be processed from the single-frequency domain,multi-frequency domain,and time domain.The time-domain method is more accurate in high-resolution imaging for breast cancer detection.Current research on breast cancer microwave imaging in the time domain generally applies the finite difference time domain(FDTD)method for iterative calculation.However,due to the constraint of Courant–Friedrich–Levy stability condition,the time step should be small enough to well match the small fine cells.In order to get a high resolution,the time step must be small enough,which results in an overlong time in CPU operation time.Therefore,increasing the efficiency of MT became the discussion and concerning focus.The research of this dissertation focuses on the unconditionally stable FDTD algorithm,the alternating-direction implicit finite difference time domain(ADI-FDTD)method as well as its application in microwave imaging of early breast cancer.The innovative contributions of this dissertation mainly include the following aspects:(1)Firstly,the EM scattering problems of one-dimensional layered lossy breast model are analyzed by using the ADI-FDTD method.The numerical results verify the feasibility and effectiveness of the ADI-FDTD method in MT for early breast cancer detection.(2)Then,in order to simulate a more real breast,a two-dimensional(2-D)semicircle breast model is applied in the ADI-FDTD calculation.The numerical results verify the feasibility and effectiveness of the ADI-FDTD method in MT for early breast cancer detection.(3)A fast nonlinear electromagnetic reconstruction algorithm for layered lossy media by using the ADI-FDTD method is proposed for time domain breast cancer MT.This algorithm is based on an adjoint method,and the nonlinear iterations apply the ADI-FDTD method to calculate the forward and adjoint field and adopt the Polak,Ribiere,Polyar conjugate-gradient optimization scheme.By comparing the simulation results based on the ADI-FDTD method and the FDTD method,the validity and efficiency of the proposed algorithm have been proved.In addition,the simulation results suggest that the ADI-FDTD method can be more efficient if higher resolution is required,thus further enhancing the clinical applicability of MT.(4)In this paper,the inverse scattering algorithm in(2)is extended to a 2-D case,and a 2-D time-domain inverse scattering algorithm based on ADI-FDTD method is proposed.An effective nonlinear electromagnetic reconstruction algorithm for 2-D semicircle breast model by using the ADI-FDTD method is proposed in this paper.By comparing the simulation results based on the ADI-FDTD method and the FDTD method,the validity and efficiency of the proposed algorithm have been proved.Meanwhile,the anti-noise performances of the algorithm are firstly analyzed in consideration of the existence of the noise during measurements.The simulating experimental results show that this ADI-FDTD based inverse scattering algorithm has characteristic of high accuracy and good performance on anti-noise.(5)In this paper,another type of active microwave imaging---the CMI is introduced and analyzed.The traditional back projection(BP)algorithm has been used to simulate the 2-D semicircle infinite cylindrical breast model.At first,it did not take into account that the propagation of electromagnetic waves transmits refraction through different dielectric layers,resulting in an error in imaging.In this way,the time delay is corrected by an approximate solution,so that the image can be correctly imaged.(6)Then,in order to improve the quality of imaging,the RCB algorithm is used to simulate and analyze the 2-D breast model,the size,number,and the number of tumors has been studied as well.The experimental results confirm the feasibility and efficiency of the CMI in the early breast cancer detection and prove the superiority of the RCB algorithm.