热声成像技术是一种新型的，具有多物理特性的混合成像模式。由于热声图像兼具高对比度和高分辨率，热声成像被认为是具有广泛前景的生物医学影像方法。从热声成像的物理机理来分析，其包含两个独立的物理过程。第一个是生物组织与电磁波的相互作用，第二个是声波的产生与传播。基于上述分析，本文将分别从激励多尺度对第一个物理过程和图像重构算法对第二个物理过程进行研究。 本文的主要研究内容可以概括为以下两个方面。 激励多尺度方面：本文系统地研究了微波热声和VHF热声的成像机理。尤其针对水分子影响微波热声图像对比度的问题，使用了VHF热声技术来降低水分子对图像对比度的影响。同时，本文率先研究了VHF热声成像深度的问题。通过电磁仿真和生物组织成像实验研究了生物组织对VHF电磁能量的吸收及电场能量分布的问题。 图像重构算法方面：本文主要研究了(1)生物组织声学非均匀性降低重构质量的问题和(2)压缩感知热声成像算法计算量大的问题。 论文创新点归纳如下： 1.针对人体乳腺热声成像中声学非均匀特性降低图像质量的问题，提出了一种基于声学和电磁学对比度的多物理场K-means重构算法。该方法以乳腺癌检测为应用背景，通过数值仿真和生物组织成像实验验证了该方法能够较好地改善声学非均匀性对图像重构的影响。进一步地，针对多物理场K-means聚类成像方法中可能出现微波辐射和组织吸收不均匀导致边界信息失真的问题，提出了微波-热声双模成像方法。微波-热声双模方法的提出，改善了微波成像分辨率低，微波热声无定量信息和声学非均匀性影响热声图像重构的问题。 2.针对压缩感知热声成像算法高分辨率重构计算量大的问题，提出了多层字典热声压缩感知重构方法。与传统压缩感知热声重构方法相比，所提方法在保证图像质量的情况下，显著降低了高分辨率重构所需的时间。 3.针对压缩感知热声成像算法中字典矩阵建模时间长、内存占用大的问题，提出了一种基于快速时移和稀疏矩阵的快速热声字典建立方法，减少了字典建立时间，降低内存占用量。该方法通过数值仿真和微波热声成像实验以及光声活体成像实验说明了其有效性。 本文对热声成像技术中的成像机理和成像算法中的一些问题进行了研究。针对热声重构中声学非均匀特性和压缩感知重构中计算量大的问题提出了解决方法且所提方法均通过数值仿真和成像实验进行了验证。 关键词：微波热声成像，VHF热声成像，热声成像物理机理，图像重构算法

 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. Keywords: Microwave-induced thermoacoustic imaging, VHF-induced thermoacoustic imaging, Thermoacoustic imaging contrast mechanism, Image reconstruction algorithm