Developing microwave-induced thermoacoustic tomography:System,application,and reconstruction

A microwave-induced thermoacoustic imaging(MITAT)system is a non-destructive physical medical imaging method that combines the advantages of the high contrast of microwave imaging and the high resolution of ultrasound imaging.It uses the microwave as the excitation source and ultrasound as the information carrier.When different kinds of biological tissue absorb electromagnetic energy,it results in localized temperature rises.The thermal expansion will induce ultrasonic signals(i.e.,thermoacoustic signals),known as the thermoacoustic effect.The microwave absorption image of the sample can be reconstructed by algorithm processing.The MITAT contrast depends on different dielectric parameters of different kinds of tissue.We introduce the developed system and its application.In addition,the challenges and prospects of MITAT for further development are discussed.

Prospects of microwave-induced thermoacoustic imaging

Microwave-induced thermoacoustic imaging(MTAI)has advantages including the large imaging depth,high imaging resolution,high imaging contrast,and fast imaging speed.The thermoacoustic(TA)group of South China Normal University has dedicated to developing TA imaging for more than a decade and has made many breakthroughs.This review introduces these breakthroughs from two aspects including the improvement in techniques and the exploration of applications.On the technological level,there are ultrashort microwave pulse(USMP)-inducedTA imaging that can improve the imaging resolution,nonlinear thermoacoustic imaging(NTAI)that can improve the imaging contrast,polarized microwave-inducedthermoacoustic imaging(P-MTAI)that can obtain cellular-level alignment information,and more convenient and accurate handheld and multimodal probes.On the application side,the optimization and expansion have been carried out,mainly concentrating on breast and myocardial imaging.Finally,several current research directions are introduced,including the application of P-MTAI on joint imaging and research on whole-body imaging of small animals.

Metal reflector-enhanced thermoacoustic imaging as a guidance for puncture biopsy

Puncture biopsy is an important clinical technique to obtain diseased tissue for pathological diagnosis,where imaging guidance is critical.In this paper,we describe a metal reflector-enhanced microwave-induced thermoacoustic imaging(TAI)approach capable of guiding puncture biopsy for detection of breast cancer and joint diseases.Numerical experimentations simulating puncture guidance in breast cancer and knee gout models werefirst conducted using(CST STUDIO SUITE)(CST)software,and then ex-vivo experiments were performed followed by qualitative observations and semi-quantitative analysis.The results of both the simulations and ex-vivo experiments showed that our reflector-enhanced TAI could image the puncture needle in high resolution with a large depth of>12 cm.

Thermoacoustic tomography of in vivo rat brain

We present for the¯rst time in vivo imaging of rat brain using microwave-induced thermoacoustic tomography(TAT).The in vivo imaging of rat brain was realized through an unconventional delivery of microwave energy from the front of rat brain(while the transducer was scanned along coronal plane of the animal brain),which maximized the microwave penetration into the brain.In addition,we found that the imaging contrast was highly dependent on the direction of the electric¯eld polarization(EFP)and that more tissue structures/compositions could be revealed when both X-and Y-EFPs were used for TAT.The in vivo TAT images of rat brain obtained were compared with the 3.0 T MRI images and histological photographs,and numerous important brain anatomical structures were identi¯ed.An example of our TAT approach for imaging a foreign object embedded in a rat brain was also demonstrated.This study suggests that TAT has a great potential to be used in neuroscience studies and in noninvasive imaging of brain disorders.

Deep learning for image reconstruction in thermoacoustic tomography

Microwave-induced thermoacoustic tomography(TAT)is a rapidly-developing noninvasive imaging technique that integrates the advantages of microwave imaging and ultrasound imaging.While an image reconstruction algorithm is critical for the TAT,current reconstruction methods often creates significant artifacts and are computationally costly.In this work,we propose a deep learning-based end-to-end image reconstruction method to achieve the direct reconstruction from the sinogram data to the initial pressure density image.We design a new network architecture TAT-Net to transfer the sinogram domain to the image domain with high accuracy.For the scenarios where realistic training data are scarce or unavailable,we use the finite element method(FEM)to generate synthetic data where the domain gap between the synthetic and realistic data is resolved through the signal processing method.The TAT-Net trained with synthetic data is evaluated through both simulations and phantom experiments and achieves competitive performance in artifact removal and robustness.Compared with other state-of-the-art reconstruction methods,the TAT-Net method can reduce the root mean square error to 0.0143,and increase the structure similarity and peak signal-to-noise ratio to 0.988 and 38.64,respectively.The results obtained indicate that the TAT-Net has great potential applications in improving image reconstruction quality and fast quantitative reconstruction.

Analysis of influencing factors of excitation parameters for magnetoacoustic tomography with current injection

Magneto-acoustic tomography with current injection(MAT-CI) is a type of hybrid imaging;under the excitation of the static magnetic field, the thermoacoustic effect and the Lorentz force effect will exist at the same time. Therefore,the detected signal is a mixed signal generated by the simultaneous action of the two effects, but the influence of excitation parameters on the two effects is different. In this paper, for objects with different conductivity, the proportion of thermoacoustic signal(TA) and magneto-acoustic signal(MA) in the mixed signal is quantitatively analyzed in terms of three aspects: the magnetic induction intensity, pulse excitation and injection current polarity. Experimental and simulation analyses show that the intensity ratio of MA to TA is not affected when the conductivity varies from 0.1 S/m to 1.5 S/m and other conditions remain unchanged. When the amplitude of the pulse excitation and the strength of the magnetic induction are different, the growth rates of MA and TA are different, which has a significant impact on the proportion of the two signals in the mixed signal. At the same time, due to the Lorentz force effect, MA is affected by the polarity of the injected current and the direction of the static magnetic field. The combination of the static magnetic field and the injected current can not only distinguish the two signals in the mixed signal, but also effectively enhance the intensity of the mixed signal and improve the quality of the reconstructed image.

Application of time reversal mirror technique in microwave-induced thermo-acoustic tomography system

Microwave-induced thermo-acoustic tomography (MITAT) is a promising technique with great potential in biomedical imaging. It has both the high contrast of the microwave imaging and the high resolution of the ultrasound imaging. In this paper, the proportional relationship between the absorbed microwave energy distribution and the induced ultrasound source distribution is derived. Further, the time reversal mirror (TRM) technique based on the pseudo-spectral time domain (PSTD) method is applied to MITAT system. The simulation results show that high contrast and resolution can be achieved by the TRM technique based on PSTD method even for the received signals with very low signal-to-noise ratio (SNR) and the model parameter with random fluctuation.

微波热声成像的技术进展

微波热声成像是一种新的医学功能成像方式,本文从原理、硬件系统结构、重构算法以及实际成像结果等几个方面对其进行讨论,主要介绍了WangLV小组近几年的研究成果以及国外的发展,并讨论了其应用前景。

一种新型医用成像技术——微波激励热声CT

介绍了一种基于崭新学术思想的医用成像技术——微波激励热声 CT,这一技术架起了超声 CT和微波 CT功能和技术上优热互补的桥梁。在介绍它的成像原理的基础上 ,进而介绍了刚刚起步进行研究的两个实验装置和相应的成像实验 。

用于脑功能研究的生物光子学技术研究进展

脑成像技术是生物医学工程学研究的热点。现有传统脑成像手段,如X射线成像(X-ray)、磁共振成像(MRI)等已发展成熟,广泛应用于科学研究和临床诊疗。然而这些成像技术大多具有侵入性、设备体积庞大、成像成本高昂等局限性,且难以适用于特殊人群,如新生儿。该文介绍了几种可用于脑成像的微观光学分子成像模态,这些方法大多无创、成本较低且性能优异,具有广阔的发展前景。在给出了它们的成像原理、系统组成以及关键技术后,总结了已有的研究成果和现阶段研究进展。最后,通过列举和比较这几种成像模态的优缺点讨论了脑成像技术未来的发展方向。

生物组织微波热声成像高速数据采集系统的设计

在生物组织微波热声成像的研究中,对拥有极宽带宽的超声波进行数据采集是一个技术难点。本文设计了一个高速数据采集系统,可采集500MHz以下的超声波信号,经实际使用,效果令人满意。

一种新型的医学成像技术—热声成像

本文介绍了一种新型的医学成像技术———基于光声效应的热声断层成像。本文的介绍主要基于Kruger等人的研究[1],并包括相关的一些技术和原理 ,如分辨率和重建原理的数学推导及其应用前景的讨论。

注入式磁声成像中热声效应的初探

探究磁声成像中热声效应的影响。通过加载和不加载静磁场条件下,改变激励源的相位和幅值以及换能器的接收方向来验证铜、铝、石墨和凝胶四种不同特性样本被激发的磁声信号和热声效应信号,并进行分析。磁声成像中热声信号与激励电压的相位无关,其传播方向在各个方向都有,其幅值与激励的能量成正比。相对于其他样本而言,凝胶仿体中的热声效应的影响最大。在生物组织中进行磁声成像时,热声效应的影响不可忽略。

声聚焦层析增强的三维微波热声成像

本文提出了一种提高微波热声断层成像层析能力的方法和装置。基于热声成像原理和声聚焦理论,搭建了由超短脉冲微波源、384阵元环形探测器、声聚焦透镜、384-64通道采集切换系统、精密扫描位移平台构成的微波热声三维成像系统,并实现了模拟样品的断层成像。实验结果表明该系统能够实现亚毫米级分辨率的热声成像,通过声聚焦方法成倍地提高了其层析分辨率。这对推动微波热声CT技术走向临床具有重要的意义。

微波激励热声成像中微波源参数选择的理论分析

分析了微波激励热声成像中激励源参数对成像质量和对比度的影响,探讨使用理论和计算方法来选择微波激励源参数。首先从理论上分析微波激励源参数对成像质量的影响和微波源参数的选择标准,然后从微波热声成像的自身特点、微波对于人体的安全标准以及工程应用的可行性的角度出发,对一些关键参数进行理论计算得出参数选择的参考范围,同时指出进行参数选择时的关键影响因素。

Two-Dimensional Photoacoustic Imaging of Blood Vessel Networks within Biological Tissues

Photoacoustic tomography （PAT） is a powerful imaging merits and most compelling features of light and sound, technique for medical diagnosis because it combines the We describe a PAT experimental system constructed in our laboratory which consists of a Q-switched Nd：YAG pulse laser operating at 532nm with a 8-ns pulse width to generate the photoacoustic signals from a biological sample. Two-dimensional photoacoustic imaging of blood vessel networks lcm below the tissue surface is achieved, We also successfully demonstrate that the system is capable of imaging the blood vessels over the ex vivo rat brain with skull and skin intact.

基于多场耦合的生物组织微波热声信号分析

我们根据微波照射生物组织时电磁场、热场、声波场三者之间的耦合关系,对微波激励热声成像正问题展开研究。从实验样本热学和声学的空间不均匀性角度出发,应用有限元方法求解热声波传播方程。由于入射微波对不同组织的穿透深度不同,不同组织对微波能量的吸收率不同,导致样本内部初始热声源的位置不同,相应地影响到声压传播情况。仿真结果表明,不同检测点声压信号中的信息量是不同的,检测点的声压信号富含样本中组织结构和电声学特性的信息量,从而能够从检测点的信号反映出关于样本结构和组成的信息。

MICROW AVE-INDUCED THERMOA COUSTIC IMAGING FOR EARLY BREAST CA NCER DETECTION

Microwave induced thermoacoustic tomography(TAT)is a noninvasive,nonionizing modality based on the inherent differences in microwave absorpt ion of malignant breast tissues and normal adipose dominated breast tissues.In this paper,a TAT system based on multielement acquisition system was built to receive signals.Slices from different layers in the sample were composed into a three-dimensional(3D)volume.Based on the 3D volume,inherent differences in microw ave absorption bet ween different biological tisues can be converted into structure information.Our experimental results of some minicked and human tumors indicate that TAT may potentially be used to detect early-stage breast cancers with high contrast.

基于微波热声层析成像定量重建生物组织电导率的改进方法

目的生物电磁学参数中的电导率与组织的功能性信息直接相关,精准重建生物组织电导率在医学成像技术和医学诊断领域中有着重要意义。本文改进定量微波热声层析成像(microwave-induced thermoacoustic tomography,MTAT)算法,使组织电导率的重建精度提高。方法本文在利用有限元离散法求解热声波动方程和亥姆霍兹方程的基础之上,提出了一种基于正则化牛顿迭代法(regularized Newton iteration method,RNIM)定量重建组织电导率的改进方法。结果通过数值模拟实验和含不同浓度NaCl溶液的仿体实验,验证了算法改进的有效性。组织仿体实验结果表明,目标在不同位置、不同大小、不同对比度情况下,相比于定量微波热声层析成像采用拟合(fitting)的方法,采用正则化牛顿法定量重建的仿体电导率相对误差明显降低,重建目标精度提高。在仿体实验中采用RNIM方法重建相同浓度的单目标在不同位置的电导率变化幅度更小,以及重建多目标电导率的相对比值与实际更接近,实验结果验证了改进方法的稳定性。结论研究结果表明优化算法能更加准确地定量重建组织仿体的电导率,这对于肿瘤的定位和分期的早期筛查及精准诊疗,预防疾病恶化具有重要意义。

脉冲磁激励的磁声效应和热声效应的仿真研究

我们根据电磁场辐照低电导率样本时电磁场和声波场之间的耦合关系,对脉冲磁激励的磁声效应和热声效应展开研究。本文基于电磁场理论和声波传播理论知识,推导了脉冲磁激励的声压波动方程;构建二维载流线圈和低电导率圆形薄片模型以模拟成像物理环境,利用有限元方法分析瞬态电磁场;并且根据电磁场仿真结果,进行磁声效应和热声效应所产生的声场仿真计算。仿真结果表明,微秒级高斯脉冲磁场激励下,低电导率样本中热声效应比磁声效应大,且起主要作用;微秒级高斯脉冲磁场和0.2 T静磁场激励下,低电导率样本中热声效应和磁声效应共存。该研究为感应式磁声成像和磁热声成像的进一步应用奠定了基础。