Multi-scale Thermoacoustic Imaging:Technology And Reconstruction Algorithm

Thermoacoustic(TA)imaging is a new and hybrid imaging modality which shows great potential because of high imaging contrast and spatial resolution.TA effect can be decomposed into two independent physical processes.The first process is the interaction between electromagnetic(EM)waves and biological tissue,and the second process is the generation and propagation of acoustic waves.Based on these two physical processes,this thesis focus on the study of the first process using multi-scale EM excitation and the second process by studying image reconstruction algorithm.The main content of the thesis is summarized as follows.Multi-scale excitation: In this thesis,the physical mechanism of microwave-induced and VHF-induced thermoacoustic techniques are discussed in depth.In order to solve the problem of limited contrast in microwave thermoacoustic imaging due to water molecules,VHF thermoacoustic imaging is applied to suppress imaging contrast derived from water molecules.In addition,this thesis firstly studied the imaging depth of VHF thermoacoustics.We adopted numerical EM simulation to evaluate the absorption of biological tissue under VHF excitation,and the results showed a rapid decay of electric field energy deposition in the tissue.This conclusion is consistent with our biological tissue imaging experiments.Image reconstruction algorithm: The thesis addresses two major issues in thermoacoustic reconstruction(1)biological acoustic heterogeneous effect;(2)high computation burden of compressive sensing(CS)-based reconstruction.The contributions of the thesis are summarized below:1.Proposed a multi-physical K-means clustering reconstruction method to address acoustic heterogeneous effect in human breast imaging.The proposed method is validated by imaging anatomically realistic numerical breast phantoms and biological tissues.Moreover,proposed a microwave-based dual imaging modality by combining the conventional microwave imaging(MWI)and thermoacoustic imaging(TAI).By applying the proposed dual-imaging modality,medical images with ultrasonic resolution and quantitative dielectric information of breast tissue can be obtained.2.Proposed a hierarchical dictionary compressive sensing thermoacoustic imaging method.Compared with the conventional CS-based method,the proposed method is able to decrease computation burden significantly without reducing image quality.3.Proposed an efficient dictionary construction method by simplifying the forward solution of the wave equation,which enables a fast calculation of the thermoacoustic dictionary through a time-shifting operation.The proposed method is validated by imaging numerical,biological phantoms and in-vivo mouse experiments using CS-based reconstruction.In summary,the thesis addressed some issues in physical mechanisms and image reconstruction raised in thermoacoustic imaging.All the methods proposed in this thesis have been verified by both numerical simulations and biological experiments.It is worth noting that this thesis is not only a scientific report or the collection of the author's work,but also the author's insights and unique understanding of thermoacoustic imaging.