Research On Near-Infrared Light And Microwave-Induced Thermoacoustic Imaging Techniques For Functional Imaging Of Biological Tissues

Sensory,cognitive,motor,metabolic,and other biological tissue functions play a crucial role in maintaining the normal physiological functions of living organisms,coping with internal and external tasks and stimuli,and regulating emotional and psychological states.As an important tool in modern medical diagnosis and treatment,medical imaging technology provides a powerful means for obtaining and analyzing internal structural and functional information of biological tissues,helping researchers to better understand the mechanisms of biological tissue function,and providing strong support and impetus for the diagnosis and treatment of diseases and functional abnormalities in living organisms.On the other hand,the brain is a core component of the central nervous system,responsible for regulating and controlling aspects such as thinking,sensation,emotion,and behavior,and it is also the control center of the nervous system.Therefore,the performance of brain function occupies an important position in the life activities and behaviors of organisms.The introduction of medical imaging techniques enables people to perform anatomical and functional imaging of the brain.Anatomical imaging is used to display brain tissue structures,such as the skull,cortex,blood vessels,and nerves,and can be used for the diagnosis and treatment of diseases such as brain tumors or brain trauma.Brain functional imaging can record and display the activity(activation)or network connections of brain functional areas through imaging techniques,demonstrating the metabolic activity,blood oxygen levels,or hemodynamic responses of the brain when receiving specific tasks and stimuli or in a certain state.Existing brain imaging methods,such as optical molecular imaging,X-ray imaging,computed tomography,positron emission tomography,and functional magnetic resonance imaging,are relatively mature and have been widely used in scientific research and clinical diagnosis and treatment.However,most of these imaging techniques have limitations such as invasiveness,bulky equipment,high imaging cost,and unfriendliness for specific patient populations.In this dissertation,several novel brain functional imaging techniques are deeply studied,and their imaging principles,system design and key technologies are described in detail,and several experimental projects have been completed.In this dissertation,the principle of interaction between light and biological tissue is firstly introduced,and key tissue optical parameters such as absorption coefficient and scattering coefficient are introduced to describe the physical process of the interaction between light and biological tissue.Then,the mathematical model of light propagation in biological tissue and the hypothetical simplified model used in this dissertation are introduced.In the introduction of diffuse optical tomography,the detailed solution methods and steps of the forward and inverse problems are given,as well as the image reconstruction process adopted in this dissertation.Subsequently,the autonomous design and construction process of the diffuse optical tomography system are described in detail.In the development of the weak light signal detection and amplification unit for imaging depth,the design principles,methods,specific design steps,parameter specifications,schematic design,and layout design are extensively presented.As a result,the system achieves a depth of 35 mm in imaging a human brain phantom,reduces the overall cost by approximately 40%,and significantly improves integration and portability.This dissertation develops a complete method for accurately acquiring head models using a handheld 3D laser scanner and combines it with modeling software and in-house algorithms for source-detector registration and model mesh segmentation.This method successfully integrates the 3D scanning device into the diffuse optical tomography reconstruction process,fundamentally addressing the issue of mismatch between theoretical and actual models,significantly reducing positioning errors,simplifying the imaging procedure,and reducing imaging time by approximately one hour.In this dissertation,a 32x32-channel hollow cylindrical interface and a 40x40-channel flexible wearable brain-computer interface are developed,with the minimum S-D spacing of 19 mm and 15 mm,respectively.In the subsequent cylindrical phantom experiments and real human brain skulls,good experimental results have been achieved.The spatial resolution is greater than 12 mm,the temporal resolution is greater than or equal to 50.8 Hz,and the imaging depth is about 35 mm.It has the ability to image human brain functions.The forearm vascular occlusion test in the human body is a typical method for measuring whether a system can respond to changes in the hemodynamics of biological tissues.This dissertation proposes a dual-modal imaging system that combines nearinfrared spectroscopy(NIRS)and microwave-induced thermoacoustic tomography(TAT)for functional imaging of biological tissues.NIRS detects changes in the concentration of hemoglobin in biological tissues to obtain indicators such as blood oxygen saturation and hemodynamic parameters,thereby reflecting the level of tissue functional activity.TAT can provide high-resolution anatomical information of the tissue.The fusion of the two imaging modes can leverage their respective advantages,compensate for their shortcomings,and the imaging results can serve as a reference for each other.This dissertation first constructs a complete dual-modal imaging system and overcomes the two main challenges that affect the success of the experiment.Secondly,validation experiments are designed to demonstrate the effectiveness and performance level of the system.Finally,consistent conclusions are obtained in the forearm vascular occlusion test of five subjects,and the experiment is found to be repeatable.The introduction of TAT provides prior information about the anatomical structure of biological tissues for NIRS,aiding in the localization of blood vessels and muscles in the region and indirectly improving the spatial resolution of functional NIRS imaging to the millimeter-level equivalent of TAT,with an imaging depth of approximately 17.5 mm.The conclusion proves that the dual-modal imaging system can provide low-cost,non-invasive,in vivo anatomical and functional images of biological tissues,and the imaging results can be cross-validated.The spatial resolution of the dual-modal system is determined by the spatial resolution of the TAT.The experimental study of thermoacoustic imaging using metal wires holds significant importance in calibrating the spatial resolution of both thermoacoustic imaging and the aforementioned NIRS-TAT dual-modal imaging systems.This dissertation presents a simulation system for thermoacoustic imaging based on a fast ionization device,providing a reasonable explanation of the physical mechanism of metal wire thermoacoustic imaging.It states that the source of the thermoacoustic signal is the ohmic loss caused by the induced electromotive force generated on the coil.The magnitude of this thermal loss is quantitatively determined through simulation calculations.This study derives a theoretical conclusion that the amplitude of the thermoacoustic signal is determined by the rate of change of thermal loss power over time.Through analysis of the induced electromotive force signal,the key factors determining the spatial resolution of the microwave thermosonic imaging system are explored,specifically that the spatial resolution is determined by the pulse width of the induced electromotive force signal generated on the coil.This study further investigates the influence of parameters such as the size and position of the metal wire coil on calibrating the spatial resolution of the thermoacoustic imaging system,and provides standardized setting recommendations for the metal wire to guide actual experiments.