多种光声成像方法研究

阐述了光声成像的基本原理,综述了现有的几种光声成像方法、原理及成果。介绍了国内外多个研究小组在光声成像领域所取得的一些成果,主要包括:用超声探测器记录光声信号的滤波反投影算法光声成像;超声探测器结合声透镜的傅里叶方法;用探测光做载波调制光声信号,将光声信号转化为光信号,经光学系统解调,将样品像显示在CCD摄像机上的实时光声成像等。对比了几种光声成像方法,并提出了一些光声成像的新思路。     

光声层析成像的信号处理

报道了采用滤波反投影的光声层析成像的信号处理方法，为了还原空间位置的光声信号，声探测器接收到的光声信号和探测器的脉冲响应在频域进行逆卷积处理；由于光声信号相对于触发时刻的延迟时间就是光声源到探测器的走时，重建时根据光声信号的延迟时间以及声速的距离，把速度势信号反投影到与探测器等距离的圆弧上。通过多个角度的反投影，能够重建出光声源的图像，但是由于反投影时光声信号投影到没有光声源的地方产生伪迹信号，模糊了光声源图像的边界，降低了图像的分辨率和对比度。因此在反投影之前采用CT成像中的R-T空间滤波函数与光声速度势信号进行卷积处理，然后再进行反投影成像；这种方法降低了由反投影带来的伪迹。应用这些处理方法，获得了埋藏深度为12mm的四个光吸收体的二维光声层析成像。     

Single‐Layer MoS2 Nanosheets with Amplified Photoacoustic Effect for Highly Sensitive Photoacoustic Imaging of Orthotopic Brain Tumors

Photoacoustic (PA) imaging, as a fast growing technology that combines the high contrast of light and large penetration depth of ultrasound, has demonstrated great potential for molecular imaging of cancer. However, PA molecular imaging of orthotopic brain tumors is still challenging, partially due to the limited options and insufficient sensitivity of available PA molecular probes. Here, the direct formation of single‐layer (S‐MoS₂), few‐layer (F‐MoS₂), and multi‐layer (M‐MoS₂) nanosheets by the albumin‐assisted exfoliation without further surface modifications is reported. It is demonstrated that the PA effect of the MoS₂ nanosheets is highly dependent on their layered nanostructures. Decreasing the number of nanosheet layers from M‐MoS₂ to S‐MoS₂ can both significantly enhance the near‐infrared light absorption and improve the elastic properties of the nanomaterial, resulting in greatly amplified PA effect. The in vitro experiments demonstrate that the prepared S‐MoS₂ with excellent biocompatibility can be efficiently internalized into U87 glioma cells, producing strong PA signals for highly sensitive detection of brain tumor cells, with a detection limit of ≈100 cells. Intravenous administration of S‐MoS₂ to both U87 subcutaneous and orthotopic tumor‐bearing mice shows highly efficient tumor retention and significantly enhanced PA contrast. Tumor tissue ≈1.5 mm below the skull can still be clearly visualized in vivo. Previous studies suggest that the fabricated S‐MoS₂ with amplified PA effect have high potential to serve as an efficient nanoplatform for sensitive PA molecular imaging and hold promising prospect for translational medicine.     

长焦区光声成像系统中的信号补偿

信号衰减问题在许多领域都备受关注,在光声成像系统中各种原因引起的信号衰减同样在很大程度上影响了系统的成像质量以及成像深度。从组织光学、声传输与接收等方面讨论了长焦区光声成像系统中引起信号衰减的因素及其特点,并在此基础上提出相应的补偿策略。补偿策略考虑到光声形成与传输过程中的光衰减与声衰减两方面因素对光声信号的影响,实现了从光与声两方面对检测信号综合补偿。信号补偿实验分别从模拟样品补偿成像与离体甲状腺成像展开。实验结果表明,补偿策略能够较好地对光声信号进行补偿,从而提高了系统的成像深度及成像质量。     

Improved reconstruction for MR spectroscopic imaging

Sensitivity limitations of in vivo magnetic resonance spectroscopic imaging (MRSI) require that the extent of spatial-frequency (k-space) sampling be limited, thereby reducing spatial resolution and increasing the effects of Gibbs ringing that is associated with the use of Fourier transform reconstruction. Additional problems occur in the spectral dimension, where quantitation of individual spectral components is made more difficult by the typically low signal-to-noise ratios, variable lineshapes, and baseline distortions, particularly in areas of significant magnetic field inhomogeneity. Given the potential of in vivo MRSI measurements for a number of clinical and biomedical research applications, there is considerable interest in improving the quality of the metabolite image reconstructions. In this report, a reconstruction method is described that makes use of parametric modeling and MRI-derived tissue distribution functions to enhance the MRSI spatial reconstruction. Additional preprocessing steps are also proposed to avoid difficulties associated with image regions containing spectra of inadequate quality, which are commonly present in the in vivo MRSI data     

Regionally Optimized Reconstruction for Partially Parallel Imaging in MRI Applications

Based on the conventional SENSE and GRAPPA, a regionally optimized reconstruction method is developed for reduced noise and artifact level in partially parallel imaging. In this regionally optimized reconstruction, the field-of-view (FOV) is divided into a number of small regions. Over every small region, the noise amplification and data fitting error can be balanced and minimized locally by taking advantage of spatial correlation of neighboring pixels in reconstruction. The full FOV image can be obtained by ldquoregion-by-regionrdquo reconstruction. Compared with the conventional SENSE, this method gives better performance in the regions where there are pixels with high SENSE g-factors. Compared with GRAPPA, it is better in the regions where all the pixels have low SENSE g-factors. In this work, we applied the regionally optimized reconstruction in four important imaging experiments: brain, spine, breast, and cardiac. It was demonstrated in these experiments that the overall image quality using this regionally optimized reconstruction is better than that using the conventional SENSE or GRAPPA.     

基于频率调制和空间编码的单PMT荧光分子断层成像

针对荧光分子断层成像数据采集方式存在的问题,提出了一种基于频率调制和空间编码的成像方法,旨在改进数据采集方案,缩短数据采集时间。在该方法中,激发光束被分成若干个子束,用作多点激发光源。这些子光束首先被调制成不同的频率,然后同时入射到目标表面的不同点上。在检测端,目标的出射光首先通过空间编码掩模,然后被引导至单光电倍增管。根据压缩感知理论,改变掩模的模式,进行稀疏重构恢复,最终得到目标表面荧光信号的分布。为了验证本文所提方法的可行性,设计了相应的仿真模拟实验,实验结果表明该方法可以较好地恢复原始图像,证明该方法的可行性。     

激光能量密度对激光诱导炽光技术测试碳烟粒径的影响

激光诱导炽光（Laser Induced Incandescence,LII）技术,由于其具有高的时间和空间分辨率,被认为是一种适合于测试碳烟浓度的技术,也可以用来测试碳烟主小球粒径。在LII 测试过程中,不同激光能量密度对测试精度影响很大,高激光能量密度具有较高的信噪比,但是对于碳烟粒径测试,为了避免升华,应尽量使用低的激光能量密度。具体在利用激光诱导炽光技术推断碳烟粒径的实验中所用的低能量密度为何值,需要结合理论模型分析和实验进行确定。首先基于LII测试过程的数学模型进行了理论分析,然后在乙烯层流扩散火焰的相同位置上,测试得到了不同激光能量密度下的碳烟主小球粒径,分析确定了测试所需的最佳激光能量密度。为进一步测试碳烟的主小球粒径奠定了基础。     

参考臂差分压缩关联成像方案研究

关联成像可以在不包含物体的参考光路中获得物体的像,提供了一种运用常规手段难以获得清晰图像的方法,现已成为量子光学领域的前沿和热点。然而,高分辨率成像和短成像时间一直是关联成像研究的热门。本文提出一种基于压缩感知的参考臂差分关联成像方案(简称,参考臂差分压缩关联成像),只对参考臂测量值做差分运算,而不对物体做差分运算。在方案中,参考臂多次测量的赝热光源强度的随机分布差分被选为压缩感知的测量矩阵,物臂多次测量的关联信息被选为测量值,利用压缩感知重建方法,高质量地恢复出物体信息。理论研究和数值仿真结果表明,该方案可获得文献[19]差分压缩关联成像方案一样的结果,即无论在成像分辨率还是在成像时间等关键性能上都明显优于常规关联成像方案。      

Optimized sampling distribution based on nonparametric learning for improved compressive sensing performance

In this work, an optimized nonparametric learning approach for obtaining the data-guided sampling distribution is proposed, where a probability density function (pdf) is learned in a nonparametric manner based on past measurements from similar types of signals. This learned sampling distribution is then used to better optimize the sampling process based on the underlying signal characteristics. A realization of this stochastic learning approach for compressive sensing of imaging data is introduced via a stochastic Monte Carlo optimization strategy to learn a nonparametric sampling distribution based on visual saliency. Experiments were performed using different types of signals such as fluorescence microscopy images and laser range measurements. Results show that the proposed optimized sampling method which is based on nonparametric stochastic learning outperforms significantly the previously proposed approach. The proposed method is achieves higher reconstruction signal to noise ratios at the same compression rates across all tested types of signals.     

基于激光超声的钛合金裂纹合成孔径组合成像研究

针对钛合金内部疲劳裂纹激光超声成像问题,提出了一种基于合成孔径聚焦技术的多模式组合成像方法。采用扫描激光源法,通过提取不同探测位置的时域模式回波信号,根据B-scan图中反射纵波与反射横波的幅值分布,将裂纹回波时域特征进行分区重组,对其进行反演聚焦完成了对内部裂纹的识别及图像重构。使用COMSOL Multiphysics对钛合金板材内部裂纹成像过程进行数值模拟并通过MATLAB图像算法验证了其可行性。与单一模式波成像相比,采用反射纵波与反射横波组合的成像方法有效减少了伪像,实现了对内部裂纹的定位及图像重构,重构裂纹位置X轴与Y轴相对误差均在3%以内。     

Self‐Assembly of Semiconducting Polymer Amphiphiles for In Vivo Photoacoustic Imaging

Despite the advantages of semiconducting polymer nanoparticles (SPNs) over other inorganic nanoparticles for photoacoustic (PA) imaging, their synthetic method is generally limited to nanoprecipitation, which is likely to cause the issue of nanoparticle dissociation. The synthesis of near‐infrared (NIR) absorbing semiconducting polymer amphiphiles (SPAs) that can spontaneously self‐assemble into homogeneous nanoparticles for in vivo PA imaging is reported. As compared with their counterpart nanoparticles (SPN1) prepared through nanoprecipitation, SPAs generally have higher fluorescence quantum yields but similar size and PA brightness, making them superior over SPN1. Optical and simulation studies reveal that the poly(ethylene glycol) (PEG) grafting density plays a critical role in determining the packing of SP segments inside the core of nanoparticles, consequently affecting the optical properties. The small size and structurally stable nanostructure, in conjunction with a dense PEG shell, allow SPAs to passively target tumors of living mice after systemic administration, permitting both PA and fluorescence imaging of the tumors at signals that are ≈1.5‐fold higher than that of liver. This study thus not only provides the first generation of amphiphilic optically active polymers for PA imaging, but also highlights the molecular guidelines for the development of organic NIR imaging nanomaterials.     

Regularized signal reconstruction for level-crossing sampling using Slepian functions

In this paper, we propose a method for efficient signal reconstruction from non-uniformly spaced samples collected using level-crossing sampling. Level-crossing (LC) sampling captures samples whenever the signal crosses predetermined quantization levels. Thus the LC sampling is a signal-dependent, non-uniform sampling method. Without restriction on the distribution of the sampling times, the signal reconstruction from non-uniform samples becomes ill-posed. Finite-support and nearly band-limited signals are well approximated in a low-dimensional subspace with prolate spheroidal wave functions (PSWF) also known as Slepian functions. These functions have finite support in time and maximum energy concentration within a given bandwidth and as such are very appropriate to obtain a projection of those signals. However, depending on the LC quantization levels, whenever the LC samples are highly non-uniformly spaced obtaining the projection coefficients requires a Tikhonov regularized Slepian reconstruction. The performance of the proposed method is illustrated using smooth, bursty and chirp signals. Our reconstruction results compare favorably with reconstruction from LC-sampled signals using compressive sampling techniques.     

The Application of Compressed Sensing for Photo-Acoustic Tomography

Photo-acoustic (PA) imaging has been developed for different purposes, but recently, the modality has gained interest with applications to small animal imaging. As a technique it is sensitive to endogenous optical contrast present in tissues and, contrary to diffuse optical imaging, it promises to bring high resolution imaging for in vivo studies at midrange depths (3–10 mm). Because of the limited amount of radiation tissues can be exposed to, existing reconstruction algorithms for circular tomography require a great number of measurements and averaging, implying long acquisition times. Time-resolved PA imaging is therefore possible only at the cost of complex and expensive electronics. This paper suggests a new reconstruction strategy using the compressed sensing formalism which states that a small number of linear projections of a compressible image contain enough information for reconstruction. By directly sampling the image to recover in a sparse representation, it is possible to dramatically reduce the number of measurements needed for a given quality of reconstruction.      

运动目标强度关联成像及其在遥感探测中的应用

开展了强度关联成像领域中对于运动目标成像的研究,为了克服目标与系统之间的相对运动引起的图像退化问题,提出了基于参考臂探测器记录的光场强度分布平移补偿进而提高运动目标强度关联成像分辨率的方案。理论和实验结果表明:通过对参考臂探测器记录的光场强度分布进行平移补偿,利用强度关联成像技术仍然可以获取运动目标的高分辨率图像。针对强度关联线性重构和稀疏约束非线性重构算法,分别讨论了遥感探测应用过程中探测噪声对运动目标强度关联成像重构质量的影响。此外,比较了所述技术与现有去运动模糊技术的优缺点。     

基于水体退化函数补偿方法的水下傅里叶单像素成像

水下光学成像的探测环境相对复杂。前向散射、后向散射和吸收极大地降低了水下光学成像的成像质量。单像素成像因其较高的抗噪声性而被认为是一种适合于水下光学成像的技术。对于水下单像素成像系统,目前存在的问题是需要采用结构光进行照明,当散斑在水下传播时,前向散射会使预先生成的散斑发生畸变。因此,重建结果的分辨率降低,重建结果模糊不清。为了减小前向散射对单像素成像系统的影响,对傅里叶单像素成像的重建过程进行改进。在空间谱域傅里叶单像素成像系统的水下退化函数进行估计,然后根据估计的退化函数实现目标空间谱反演。最后利用傅里叶变换对反演后的目标空间谱进行变换,最终获得目标的强度图像。理论分析和实验结果证明了该方法的有效性。利用该方法,能够有效地减小前向散射对成像质量的影响,提高了水下傅里叶单像元成像的重建结果质量。     

Anatomically constrained electrical impedance tomography forthree-dimensional anisotropic bodies

As shown previously for two-dimensional geometries, anisotropy effects should not be ignored in electrical impedance tomography (EIT) and structural information is important for the reconstruction of anisotropic conductivities. Here, we describe the static reconstruction of an anisotropic conductivity distribution for the more realistic three-dimensional (3-D) case. Boundaries between different conductivity regions are anatomically constrained using magnetic resonance imaging (MRI) data. The values of the conductivities are then determined using gradient-type-algorithms in a nonlinear-indirect approach. At each iteration, the forward problem is solved by the finite element method. The approach is used to reconstruct the 3-D conductivity profile of a canine torso. Both computational performance and simulated reconstruction results are presented together with a detailed study on the sensitivity of the prediction error with respect to different parameters. In particular, the use of an intracavity catheter to better extract interior conductivities is demonstrated     

基于Landweber迭代算法的欠采样恢复数字预失真技术

传统宽带数字预失真(DPD)为了更好地矫正功率放大器(PA)非线性特性,通常要求反馈通道带宽达到发送信号带宽的5倍,相应地要求更高采样率的模数转换器(ADC),这将导致数字预失真系统面临着硬件成本和能耗问题。针对这一问题,该文提出一种基于Landweber迭代算法的欠采样恢复(USR)数字预失真(Landweber-USR DPD)技术。这种以内外循环的方式进行处理,可将反馈通道带宽从理论要求的5倍降低至2倍,以良好的质量从欠采样的功放输出信号中恢复全频带的输出信号,使还原出的数据更接近真实的功放输出信号,以实现更好的预失真效果。实验选用基于单管氮化镓(GaN)器件的宽带F类功率放大器,在1.8 GHz工作频点下用5 MHz的长期演进(LTE)信号激励,反馈ADC速率分别设置为全采样速率(40 Msps)和欠采样速率(10 Msps)。实验结果充分证明了Landweber迭代算法恢复功放数据的可靠性以及Landweber-USR DPD技术的有效性,为宽带通信系统中数字预失真技术的工程实现提供了有效降低ADC采样率的思路和方法。     

金纳米棒的制备及其在光声成像中的应用

该文制备了巯基聚乙二醇（PEG）修饰的金纳米棒,该纳米材料在近红外区具有良好的光吸收特性,具备作为优良光声造影剂的潜质。该文通过透射电子显微镜（TEM）、紫外 可见（UV VIS）吸收光谱等测量手段对金纳米棒进行了形貌、结构、基本光学性能及光声成像效果等表征。实验结果表明,随着材料浓度的增加,体外光声信号的响应近似呈线性增长;经由PEG修饰,金纳米棒的生物相容性得到提高;PEG修饰后的金纳米棒对小鼠大脑皮层血管的成像效果得到提升。结果表明,PEG修饰的金纳米棒材料,在光声成像造影领域具有巨大的应用前景。     

Tomographic fluorescence imaging in tissue phantoms: a novel reconstruction algorithm and imaging geometry

A novel image reconstruction algorithm has been developed and demonstrated for fluorescence-enhanced frequency-domain photon migration (FDPM) tomography from measurements of area illumination with modulated excitation light and area collection of emitted fluorescence light using a gain modulated image-intensified charge-coupled device (ICCD) camera. The image reconstruction problem was formulated as a nonlinear least-squares-type simple bounds constrained optimization problem based upon the penalty/modified barrier function (PMBF) method and the coupled diffusion equations. The simple bounds constraints are included in the objective function of the PMBF method and the gradient-based truncated Newton method with trust region is used to minimize the function for the large-scale problem (39919 unknowns, 2973 measurements). Three-dimensional (3-D) images of fluorescence absorption coefficients were reconstructed using the algorithm from experimental reflectance measurements under conditions of perfect and imperfect distribution of fluorophore within a single target. To our knowledge, this is the first time that targets have been reconstructed in three-dimensions from reflectance measurements with a clinically relevant phantom.