基于连续波的频域磁声耦合成像方法正问题研究

磁声耦合成像常用的时域信号检测方法易受到电磁干扰,限制成像质量。提出频域磁声成像方法,通过锁相放大实现微弱声信号的检测,提高信号检测精度。本研究建立了频域磁声耦合正问题数学模型,基于正弦波激励,对简单电导率边界模型进行了仿真研究和实验验证。仿真研究表明信号频域幅值相位反映了介质声源分布。实验验证了仿真结果,实现了声源幅值锁相检测和空间定位,检测精度达到10-7Pa,实现了激励电流低于1m A下的声检测。本研究提出的频域磁声耦合成像方法,有利于提高微弱磁声信号检测精度,在低频激励下实现高分辨率成像,对实现磁声耦合成像的介质内部电导率研究具有积极意义。     

注入电流式磁声成像的电导率模型构建和实验研究

基于磁声耦合效应的电导率成像是一种新型的生物组织功能成像技术,本课题旨在对注入电流式磁声成像方法进行仿真和实验研究.以金属圆环作为成像目标,将其置于稳恒磁场中,并对其加载微秒级正弦脉冲激励,对磁声耦合作用下产生的声振动进行检测和分析.借助有限元方法对磁声正问题进行仿真,同时建立注入电流式磁声耦合成像实验装置,对电导率模...     

基于ANSYS的磁声耦合效应声源的分布建模与仿真

感应式磁声成像(MAT-MI)是一种近年来提出的新型生物电阻抗成像方法,在理论与实验方面仍有很多问题值得研究和探讨,本研究设计仿真实验分析磁声耦合效应产生声源的分布及特点.结合电磁场理论分析MAT-MI声源与电导率分布、激励磁场、静态磁场和感应电场的关系,基于有限元方法构建了三维霍姆赫兹线圈和双层导体立方体的仿真模型,...     

基于声学不均匀特性的磁感应磁声成像声压解析

目的 依据人体声学参数,探索磁声耦合成像声信号在声学非均匀媒介中的传播对检测结果的影响.方法 参考人体组织声学特性建立仿真模型,借助有限元分析工具进行电磁场仿真,采用时域有限差分方法求解磁声信号在仿真模型中的传播,并在自由空间采集声压信号.最后,对比研究磁声信号在声学均匀模型和声学非均匀模型中的传播过程.结果 在两个声学模型中声源分布相同,声学非均匀模型中采集的声压信号峰值数量多于声源边界,且声压峰值间的时间间隔与声源边界之间的距离不匹配.结论 初步揭示了声学非均匀特性对磁感应磁声成像的影响,为声学非均匀媒介的重建算法研究奠定了一定基础.     

基于相关系数的磁声信号特征分析

为了研究注入电流式磁声信号所包含的特征,需研究采集到的磁声信号与输入激励以及声换能器系统响应函数之间的关系。本研究通过实验,采用不同电压幅值大小的单周期正弦脉冲信号作为激励源,通过对接收到的磁声信号进行分析,使用 matlab 进行计算得到了磁声信号与上述两者的相关系数,然后对获取的数值进行数理统计,结果表明：随着激励电压幅值的增加,磁声信号与激励源信号以及与声传感器响应的相关系数增大,并趋于稳定;磁声信号与输入激励源以及声传感器的相关系数的峰值特点不同,且声传感器对平行铜线之间的距离比较敏感。该研究为后续实验中选择激励源信号以及声传感器特性提供参考,同时对磁声信号的特征分析和实验信号图像的精确性重建提供了一定依据。     

编码激励磁声信号处理方法研究

基于磁声耦合效应的生物组织电特性检测成像方法,在神经电活动监测和肿瘤等疾病早期诊断方面具有重要研究意义。常用的单脉冲激励和接收模式的信噪比低,限制了磁声成像质量;信号平均处理方法则限制了成像速率。本研究提出了一种脉冲编码激励的磁声成像信号处理方法,采用编码序列激励模式和脉冲压缩的检测处理方法以提高信噪比,缩短信号处理时间。本研究通过仿真计算和磁声信号的实验测量,采用13位巴克(Barker)编码和16位格雷(Golay)编码,对磁声信号的编码激励和脉冲压缩处理方法进行了研究。结果表明,对金属丝模型,在未进行波形叠加平均的条件下,编码激励可明显提高磁声信号信噪比,如13位Barker脉冲编码和16位Golay编码处理方法,可分别提高磁声信号信噪比约20.96 dB和20.62 dB。同时处理时间明显缩短:在相同信噪比提升的情况下,13位Barker编码和16位Golay编码处理方法的整体采集处理时间约缩短为单脉冲激励平均处理方法的3.62%和4.73%。本研究提出的脉冲编码处理方法,对提高磁声信号信噪比、改善成像质量、提高整体成像速率具有重要意义。     

基于磁声耦合效应的电导率图像重建研究

磁声耦合成像技术利用电磁声三物理场耦合作用的原理,通过声检测方式重建电导率存在明显变化的组织轮廓,实现恶性肿瘤的早期检测。本文建立双层同心球模型,模拟被正常组织包裹的病变组织,利用声场中的时间反转法以及电磁场方程,求解组织中的电导率分布,研究了由检测声压计算电导率分布的重建算法,并进行了仿真分析。仿真结果显示重建的电导率分布图像在电导率边界处清晰可见,进而验证了应用磁声耦合成像对电导率边界进行功能成像的可行性,为恶性肿瘤的早期检测提供了一种新的功能成像方法。     

基于声换能器特性的磁感应磁声成像正问题分析

目的在考虑声换能器特性的基础上,对磁感应磁声成像的正问题进行研究,并探讨其声源的产生、传播及接收机制。方法参考CT的三维Phantom仿真模型建立磁感应磁声成像正问题的模型,并结合电磁场理论利用有限元软件comsol仿真分析模型内部电导率分布与磁场和磁感应电场的关系,得到模型内部的磁感应电流分布。然后再深入研究声偶极子模型,并检测分析声换能器的特性后,给出相应的仿真声信号检测结果。结果磁感应电流密度在中心位置处为0,在电导率边界处变化较大,声换能器的检测声场分布和声偶极子传播的指向性会极大的影响声换能器接收到的磁声信号的值。结论为磁感应磁声成像实验研究及由声信号重建物体内部的电导率分布提供理论基础。     

频域磁声耦合成像逆问题初步研究

对频域磁声耦合成像方法逆问题进行初步研究,为实现基于频域处理方法的磁声电导率成像打下基础。应用复平面矢量方法,基于声源和频域幅值相位特性,研究频域磁声成像逆问题数学模型和重建方法。对频域磁声信号随声源空间位置和幅度变化特性进行仿真,对简单模型不同声源分布的频域逆问题进行仿真重建。建立多频率下的声源幅值和相位矢量方程组,利用非线性方程组的优化算法求解声源信息。利用铜丝仿体,对矢量求解方法和逆问题声源重建进行实验验证。频域幅值和相位满足复平面声源矢量叠加理论。利用矢量方法,初步实现两个声源样本的声源幅度和位置的重建,成像分辨率小于1 cm,实验的声源重建位置误差小于±0.4 mm。通过对频域磁声成像逆问题的重建研究,为后续进行复杂模型和真实组织的频域磁声重建研究打下基础,对实现高分辨率成像具有重要研究意义。     

电流注入式磁声耦合成像技术仿真研究

目的:电流注入式磁声耦合成像技术是一种新型的生物组织电特性工程成像方法,具有高分辨率和高对比度的优点。探讨电流注入式磁声耦合成像的工作原理,为成像系统实验平台的设计提供理论参考,为真实生物组织电导率的成像奠定基础。方法:结合电场、磁场、超声耦合理论,运用有限元建模分析软件COMSOL Multiphysics建立电导率仿体模型,并进行电磁场分析,获得电导率仿体的电流密度分布。利用数学工具Matlab获得磁声成像声场分布,进而重建仿体模型声源。结果:从电磁场分析结果获知电导率的分布反映了仿体模型的结构形状,电流场为一无旋场,重建的声源图像精确地反映了仿体模型电导率的分布,与仿体模型的层析结构形状、尺寸高度一致。结论:电流注入式磁声成像为组织电阻抗分布的重建提供了一种新的方法,该进一步研究磁声耦合成像的理论与实验打下了基础。     

Magneto-acoustic imaging by continuous-wave excitation

The electrical characteristics of tissue yield valuable information for early diagnosis of pathological changes. Magneto-acoustic imaging is a functional approach for imaging of electrical conductivity. This study proposes a continuous-wave magneto-acoustic imaging method. A kHz-range continuous signal with an amplitude range of several volts is used to excite the magneto-acoustic signal and improve the signal-to-noise ratio. The magneto-acoustic signal amplitude and phase are measured to locate the acoustic source via lock-in technology. An optimisation algorithm incorporating nonlinear equations is used to reconstruct the magneto-acoustic source distribution based on the measured amplitude and phase at various frequencies. Validation simulations and experiments were performed in pork samples. The experimental and simulation results agreed well. While the excitation current was reduced to 10 mA, the acoustic signal magnitude increased up to 10^?7 Pa. Experimental reconstruction of the pork tissue showed that the image resolution reached mm levels when the excitation signal was in the kHz range. The signal-to-noise ratio of the detected magneto-acoustic signal was improved by more than 25 dB at 5 kHz when compared to classical 1 MHz pulse excitation. The results reported here will aid further research into magneto-acoustic generation mechanisms and internal tissue conductivity imaging.     

感应式磁声成像中的磁热声效应研究

探究感应式磁声成像中的磁热声效应, 分析磁声声源与热声声源分布及幅值特征。建立磁声及热声声源表达式, 数值仿真多层电导率仿真模型的感应电流分布、磁声+热声声源分布及声压信号, 对两种声源分布及重建结果进行比较。为验证仿真结果, 建立磁声成像实验平台, 采用导电橡胶圈仿体分别开展磁声和热声圆周扫描实验, 并对磁声和热声信号幅值和声源重建结果进行比较分析。仿真结果表明, 磁声声源分布集中在电导率边界处, 而热声声源分布展宽, 并且在边界和内部均有分布;磁声声源的最大值是热声声源的15倍。实验结果表明, 热声声压信号峰-峰值是混合信号的1/4。热声信号重建的声源图像在边界处更模糊, 磁声信号的声源重建图像边界清晰但存在伪影。仿真和实验结果均证明, 磁声成像中同时存在热声效应。从热声声源的分布及热声信号与磁声信号的幅值关系中可见, 热声效应在一定程度上会对磁声成像结果产生影响。     

电流注入式磁声成像中样本电导率边界的提取

目的借助步进电机对电流注入式磁声成像进行实验研究,同时对电导率边界重建算法进行探讨与比较。方法实验研究中,选用正弦脉冲信号作为实验研究的激励信号。选用铜丝环作为实验对象,通过步进电机旋转探头沿圆周运动扫描样本采集声信号,用声信号重建样本边界信息。在电导率边界重建算法设计中,利用了“最值一两侧单调”方法检测信号的峰值,使用“跟踪峰值确定边界走向”的方法查找峰值边界。结果实验结果表明,利用步进电机采集到的超声信号及新的电导率边界重建算法,在声信号数据不太理想的情况下寻找到良好的边界,算法适应性较好。所寻找到的边界中包含95％（190／200）以上角度中的边界信息。结论找到了铜丝环中铜丝中心线对应的峰值边界,同时找到了铜丝环外边界的上限和内边界的下限。     

Electrophysiological evidence for the efficiency of spoken word processing

The Cohort model (Marslen-Wilson, Spoken Word Recognition, MIT Press, Cambridge, MA, 1987, pp. 71-103) proposes that spoken words are frequently recognized at the first point in the acoustic signal where a unique lexical representation is specified. This proposal was tested in two experiments. In experiment 1 participants made speeded lexical decisions to spoken words and pseudowords. In experiment 2 participants passively listened without making overt responses. In both experiments the recognition points for words (the point past which no other lexical item was consistent with the acoustic signal) and deviation points for pseudowords (the point past which no real word is compatible with the acoustic signal) were manipulated. An ERP negativity in the region of the N400 component and RT occurred sooner for items with early than late recognition/deviation points when measures were time-locked to stimulus onset. In experiment 1, when time-locking was to recognition/deviation points, early and late words produced N400s and RTs with indistinguishable latencies, while late pseudowords produced faster RTs and earlier N400s than early pseudowords. Experiment 2 replicated the N400 effects for words, but only produced a trend in the same direction as experiment 1 for pseudowords.     

经颅磁声电刺激对前额叶皮层神经集群钙信号的影响EI北大核心CSCD

经颅磁声电刺激(TMAES)作为一种新型的脑神经调控和研究手段,利用超声和磁场耦合产生的感应电流调节不同脑区的神经电活动。钙离子作为神经信号第二特使,在神经信号传递中起着关键作用。为了探究经颅磁声电刺激对前额叶皮层放电活动的影响,将15只小鼠分为对照组、超声组和磁声组,磁声组接受超声强度为2.6 W/cm^(2)和磁感应强度为0.3 T的刺激,超声组仅接受相同强度的超声刺激,对照组无超声和磁场,持续刺激一周。通过光纤光度检测技术实时记录小鼠刺激过程中前额叶皮层钙离子浓度,并进行新物体识别实验,对比各组行为学差异和钙信号的时频分布。实验结果显示,经颅磁声组在刺激后10 s内钙瞬变信号均值(4.84±0.11)%大于超声组(4.40±0.10)%和对照组(4.22±0.08)%,且钙瞬变信号波形变缓,提示钙离子代谢变快;磁声组主要能量频带分布在0~20 Hz,超声组为0~12 Hz,对照组为0~8 Hz;磁声组认知指数为0.71,超声组为0.63,对照组为0.58,表明超声刺激和磁声电刺激均能提高小鼠的认知能力,但磁声组效果优于超声组。以上结果显示,经颅磁声电刺激能够改变前额叶皮层神经集群的钙稳态,实现对前额叶神经集群放电活动的调控,对认知功能具有促进效果。研究结果为进一步探索经颅磁声电刺激的深层神经作用机制提供了数据支撑和参考。     

Fourier-domain biophotoacoustic subsurface depth selective amplitude and phase imaging of turbid phantoms and biological tissue

A novel photothermoacoustic imaging modality utilizing a frequency-swept (chirped) intensity-modulated laser source and coherent frequency domain signal processing ("biophotoacoustics") was introduced for noninvasive imaging of biological tissues. The developed frequency-domain imaging system takes advantage of linear frequency modulation waveforms to relate depth of tissue chromophores to the frequency spectrum of the detected acoustic response and of a narrow signal detection bandwidth to improve signal-to-noise ratio (SNR). Application of frequency-domain photothermoacoustic (FD-PTA) imaging was demonstrated using turbid phantoms and ex-vivo specimens of chicken breast with embedded absorbing inclusions simulating tumors.     

Combining ultrasound-based elasticity estimation and FE models to predict 3D target displacement

During minimally invasive surgical procedures (e.g., needle insertion during interventional radiological procedures), needle–tissue interactions and physiological processes cause tissue deformation. Target displacement is caused by soft-tissue deformation, which results in misplacement of the surgical tool (needle). This study presents a technique to predict target displacement in three-dimensions (3D) by combining soft-tissue elasticity estimation using an ultrasound-based acoustic radiation force impulse (ARFI) technique and finite element (FE) models. Three different phantoms with targets are manufactured, and subjected to varying loading and boundary conditions. Ultrasound images are acquired using a 3D probe during loading and unloading of each phantom, and subsequently target displacement is calculated. 3D FE models of the phantoms are developed, and they are used to predict target displacement. The maximum absolute error in target displacement between the experiments and FE analyses is found to be 1.39 mm. This error is less than the smallest tumor diameter (2.0–3.0 mm) which can be detected in breast tissue. This study shows that the combination of soft-tissue elasticity estimation using the ARFI technique and 3D FE models can accurately predict target displacement, and could be used to develop patient-specific plans for surgical interventions.     

Magnetoacoustic tomography with magnetic induction (MAT-MI) for breast tumor imaging: numerical modeling and simulation

Magnetoacoustic tomography with magnetic induction (MAT-MI) was recently introduced as a noninvasive electrical conductivity imaging approach with high spatial resolution close to ultrasound imaging. In this study, we test the feasibility of the MAT-MI method for breast tumor imaging using numerical modeling and computer simulation. Using the finite element method, we have built three-dimensional numerical breast models with varieties of embedded tumors for this simulation study. In order to obtain an accurate and stable forward solution that does not have numerical errors caused by singular MAT-MI acoustic sources at conductivity boundaries, we first derive an integral forward method for calculating MAT-MI acoustic sources over the entire imaging volume. An inverse algorithm for reconstructing the MAT-MI acoustic source is also derived with spherical measurement aperture, which simulates a practical setup for breast imaging. With the numerical breast models, we have conducted computer simulations under different imaging parameter setups and all the results suggest that breast tumors that have large conductivity in contrast to the surrounding tissue as reported in the literature may be readily detected in the reconstructed MAT-MI images. In addition, our simulations also suggest that the sensitivity of imaging breast tumors using the presented MAT-MI setup depends more on the tumor location and the conductivity contrast between the tumor and its surrounding tissue than on the tumor size.