一种基于复数主成分分析的磁共振波谱量化预处理方法

该文提出了一种应用复数主成分分析方法(CPCA)来获取波谱基木波型,自动校正频率和衰减系数偏移的方法。该方法首先对频率进行校正,然后拓宽CPCA的应用范围,把它使用到衰减系数偏移校正领域。文中的实现结果证明:该方法较好地实现了频率和衰减系数的校正,在同实域PCA方法性能比较上,前者的计算速度和估计精度明显占优。     

一种基于全局最小乘方法的磁共振波谱目标 频率选择的量化方法

磁共振波谱中,感兴趣成分的单独量化不但符合许多实际应用的要求,而且同时也提高了量化的速度和精度.为此本文提出了一种基于频率选择的波谱信号量化方法,该方法通过全局最小乘方法来估计波谱信号的非线性参数,又结合前向和后向的APES方法来估计其线性参数.文中的模拟和活体数据实现表明,本文方法在计算速度和参数估计的稳健性上要明显优于经典的HSVD和HTLS方法,同时其综合性能比Stoica等提出的频率选择方法SELF_-SVD还好.     

基于方向图乘积的阵列天线口径反演

介绍了一种利用近场测量系统进行阵列天线口径反演的新方法。该方法将近场测量系统采集的近场数据变换到平面波谱（PWS）,在平面波谱上进行天线单元因子补偿后运用口径场反演算法,重构出被测阵列天线口径面上各天线单元馈电端口的幅相分布。随后使用计算机模拟的方法在理想近场数据上加入单元通道误差,将乘积反演方法与传统插值反演方法在模拟结果上进行对比,随后分析了单元不一致给反演结果带来的影响,证明了该方法具有较高的反演精度,有效提高了阵列天线口径校正的效率。     

分析矩形光波导损耗的简易方法

利用有效折射率和修正的有效折射率方法对矩形光波导的损耗作了简单有效的分析，并且计算出了纵向传播系数和衰减系数的大小。     

一种改进的EPI伪影校正算法

EPI是功能磁共振成像(fMRI)广泛采用的一种扫描技术。成像物体的磁敏感性、化学位移、静磁场不均匀性以及系统硬件性能的不完善等因素导致EPI图像在相位编码方向存明显的ghost伪影,严重破坏了图像的质量。研究基于图像的后处理方法校正EPI伪影具有明显的实际临床应用价值。首先分析了现行相位校正算法和相位恢复算法的不足之处,然后提出了一种改进的EPI伪影校正算法。改进的算法充分利用两种方法的优点,并克服了各自的缺点,较好地实现了伪影校正。通过对实际的phantom图像处理,阐明了改进算法的稳定性及可靠性。     

数字波束形成的误差分析及校正方法

分析了数字波束形成过程中存在的主要误差，给出了各项误差的表达式，提出了误差校正的方法，说明了误差校正的重要性。最后给出了校正前后的试验结果，证明了校正方法的可行性。     

海水衰减系数的多角度后向散射测量技术研究

提出了一种基于后向散射式激光雷达方程的海水衰减系数现场快速测量方法.该方法通过接收角度的调整,得到不同距离、不同散射角度下的水体散射光信号,并根据由后向散射式激光雷达方程推导出的衰减系数公式计算测量水域的衰减系数.实验结果表明,采用多角度后向散射测量的方法得到的水体衰减系数的测量误差可控制在8%以内,且由实验结果的误差分析可知,随着系统与计算方法的改进,系统的测量误差可得到进一步减小.     

快速校正天波雷达电离层相位污染的改进方法

时变的电离层会对天波超视距雷达( OTHR)回波信号相位进行调制,产生相位污染,导致回波谱展宽。最大似然估计( MLE)法具有比相位梯度法更佳的污染校正效果,但计算量非常大。通过引入投影近似子空间跟踪法,提出了一种改进的MLE方法。改进方法采用递归手段估计最大特征值对应的特征向量,避免了特征值分解过程,能够显著降低计算量,污染校正效果与MLE法相当。理论分析与仿真对比表明改进方法普适性强,计算量只有MLE法的万分之一,更适合工程实现。     

大气散射模型下衰减系数对图像去雾的影响

本文针对当前在大气散射模型下的去雾算法存在图像质量较差问题,从模型原理上分析衰减系数的变化对于去雾后图像质量的影响.通过大气光成像的物理建模和数学分析,逐步推导出衰减系数,分析了其物理意义.结合颜色衰减先验去雾算法,并改变衰减系数,得到不同的效果图,根据图像的评价参数,对比分析衰减系数会带来的影响.实验显示,衰减系数的变化能在一定程度上影响图像的色彩、对比度,进而影响去雾的实际效果,可以通过适当的改变衰减系数优化图像去雾的效果.     

中国人血液的组织光学参数

本文介绍了血液组织光学特性的测量原理与方法，并测量获得了不同血型的中国人血液的组织光学参数，包括血液吸收系数、散射系数、全衰减系数、平均散射余弦、有效衰减系数和穿透深度等。本研究对激光照射血液疗法以及其它激光照射人体组织的诊断与治疗，具有重要的实用或参考价值。     

Inhomogeneity and multiple dimension considerations in magnetic resonance imaging with time-varying gradients

The inhomogeneity of the main magnetic field is a significant factor limiting the performance and increasing the cost of commercial magnetic resonance imaging (MRI) and spectroscopy machines. This is particularly true where shielding is employed to limit fringing fields. In this paper, we investigate the performance of a recently introduced MRI technique using time-varying gradients in the presence of such inhomogeneity. It is shown that the time-varying gradient imaging system can accommodate considerable inhomogeneities (1000 ppm in typical imaging application). The problem of selection of the gradient frequencies for simultaneous multiple dimension imaging in the presence of inhomogeneity is formulated and solved.     

Magnetic Fields as an Investigation Technique and Manipulation Tool for Phthalocyanine Molecular Aggregates

A polarized absorption spectroscopy (PAS) study of molecular aggregate solutions of phthalocyanine molecules in high magnetic fields is presented. We show that a magnetic field can induce macroscopic ordering of molecular aggregates (MAs), in a clean, non‐contact method, which is well suited for MA manipulation. We also demonstrate that magnetic‐field‐induced alignment in combination with PAS can be used to determine the MA size and the precise position of the absorption lines in concentrated solutions, as well as probe interaggregate interactions.     

Constrained reconstruction applied to 2-D chemical shift imaging

The method of constrained reconstruction, previously applied to magnetic resonance imaging (MRI), is extended to magnetic resonance spectroscopy. This method assumes a model for the MR signal. The model parameters are estimated directly from the phase encoded data. This process obviates the need for the fast Fourier transform (FFT) (which often exhibits limited resolution and ringing artifact). The technique is tested on simulated data, phantom data, and data acquired from human liver in vivo. In each case, constrained reconstruction offers spatial resolution superior to that obtained with the FFT     

一种鲁棒的人脑组织核磁共振图像分割算法研究

自动的人脑核磁共振(MR)图像分割是许多医学图像应用的关键问题.该文提出了一种有效的自动脑核磁共振图像的分割方法框架体系,脑MR分割框架体系由3个处理步骤构成.首先,采用基于水平集的方法将MR图像中非脑组织剔除,从脑图像中提取大脑组织结构.然后,对MR脑结构图像进行灰度不均匀性校正.最后,该算法采用最大后验分类器可以将人脑组织分为脑白质、脑灰质、脑髓液.在实验中对大量的MR脑图像数据应用该分割算法.实验结果充分证明该方法的有效性.这种分割算法适用于人脑核磁图像分析的各种实际临床应用.     

Accounting for signal loss due to dephasing in the correction of distortions in gradient-echo EPI via nonrigid registration

Gradient-echo (GE) echo planar imaging (EPI) is susceptible to both geometric distortions and signal loss. This paper presents a retrospective correction approach based on nonrigid image registration. A new physics-based intensity correction factor derived to compensate for intravoxel dephasing in GE EPI images is incorporated into a previously reported nonrigid registration algorithm. Intravoxel dephasing causes signal loss and thus intensity attenuation in the images. The new rephasing factor we introduce, which changes the intensity of a voxel in images during the registration, is used to improve the accuracy of the intensity-based nonrigid registration method and mitigate the intensity attenuation effect. Simulation-based experiments are first used to evaluate the method. A magnetic resonance (MR) simulator and a real field map are used to generate a realistic GE EPI image. The geometric distortion computed from the field map is used as the ground truth to which the estimated nonrigid deformation is compared. We then apply the algorithm to a set of real human brain images. The results show that, after registration, alignment between EPI and multi-shot, spin-echo images, which have relatively long acquisition times but negligible distortion, is improved and that signal loss caused by dephasing can be recovered.     

A magnetic resonance spectrometer for FIR frequencies; Measurement on LiErF4

A magnetic resonance spectrometer is described with an HCN laser and an optically pumped FIR laser as radiation sources. The latter enables us to measure magnetic resonances at many frequencies throughout the FIR range. High magnetic fields up to 35 T are generated by means of a capacitor discharge with 10 ms pulse duration. The results of our measurments on LiErF₄ are in good agreement with resonance conditions calculated from parameters found by Hansen et al[10]. Different aspects of FIR magnetic resonance spectroscopy are discussed.     

Surface-constrained volumetric brain registration using harmonic mappings

In order to compare anatomical and functional brain imaging data across subjects, the images must first be registered to a common coordinate system in which anatomical features are aligned. Intensity-based volume registration methods can align subcortical structures well, but the variability in sulcal folding patterns typically results in misalignment of the cortical surface. Conversely, surface-based registration using sulcal features can produce excellent cortical alignment but the mapping between brains is restricted to the cortical surface. Here we describe a method for volumetric registration that also produces an accurate one-to-one point correspondence between cortical surfaces. This is achieved by first parameterizing and aligning the cortical surfaces using sulcal landmarks. We then use a constrained harmonic mapping to extend this surface correspondence to the entire cortical volume. Finally, this mapping is refined using an intensity-based warp. We demonstrate the utility of the method by applying it to T1-weighted magnetic resonance images (MRIs). We evaluate the performance of our proposed method relative to existing methods that use only intensity information; for this comparison we compute the intersubject alignment of expert-labeled subcortical structures after registration.     

MRI of moving subjects using multislice snapshot images with volume reconstruction (SVR): application to fetal, neonatal, and adult brain studies

Motion degrades magnetic resonance (MR) images and prevents acquisition of self-consistent and high-quality volume images. A novel methodology, Snapshot magnetic resonance imaging (MRI) with Volume Reconstruction (SVR) has been developed for imaging moving subjects at high resolution and high signal-to-noise ratio (SNR). The method combines registered 2-D slices from sequential dynamic single-shot scans. The SVR approach requires that the anatomy in question is not changing shape or size and is moving at a rate that allows snapshot images to be acquired. After imaging the target volume repeatedly to guarantee sufficient sampling every where, a robust slice-to-volume registration method has been implemented that achieves alignment of each slice within 0.3 mm in the examples tested. Multilevel scattered interpolation has been used to obtain high-fidelity reconstruction with root-mean-square (rms) error that is less than the noise level in the images. The SVR method has been performed successfully for brain studies on subjects that cannot stay still, and in some cases were moving substantially during scanning. For example, awake neonates, deliberately moved adults and, especially, on fetuses, for which no conventional high-resolution 3-D method is currently available. Fine structure of the in-utero fetal brain is clearly revealed for the first time and substantial SNR improvement is realized by having many individually acquired slices contribute to each voxel in the reconstructed image.     

基于加权Schatten p范数最小化的磁共振图像重构方法研究

本文提出了一种基于加权Schatten p范数最小化（Weighted Schatten p-Norm Minimization,WSNM）的磁共振图像重构算法,该方法利用磁共振图像的非局部自相似性,并结合Schatten p范数和不同秩元素重要性的加权因子,实现磁共振图像重构过程的低秩约束.此外,采用交替方向乘子算法（Alternating Direction Method of Multipliers,ADMM）来求解基于WSNM磁共振图像重构的非凸最小化问题.实验结果表明,相比于最近的磁共振重构算法,基于WSNM的磁共振图像重构方法具有更好的重建效果,可获得更高的峰值信噪比（Peak Signal to Noise Ratio,PSNR）和更好的结构相似性（Structural Similarity,SSIM）.     

Band diagram for low-k/Cu interconnects: The starting point for understanding back-end-of-line (BEOL) electrical reliability

The starting point for describing the electrostatic operation of any semiconductor device begins with a band diagram illustrating changes in the semiconductor Fermi level and the alignment of the valence and conduction bands with other interfacing semiconductors, insulating dielectrics and metal contacts. Such diagrams are essential for understanding the behavior and reliability of any semiconductor device. For metal interconnects, the band alignment between the metal conductor and the insulating intermetal and interlayer dielectric (ILD) is equally important. However, relatively few investigations have been made. In this regard, we have investigated the band alignment at the most common interfaces present in traditional single and dual damascene low-k/Cu interconnect structures. We specifically report combined X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy (REELS) measurements of the Schottky barrier present at the ILD and dielectric Cu capping layer (CCL) interfaces with the Ta(N) via/trench Cu diffusion barrier. We also report similar measurements of the valence and conduction band offsets present at the interface between a-SiN(C):H dielectric CCLs and low-k a-SiOC:H ILDs (porous and non-porous). The combined results point to metal interfaces with the CCL having the lowest interfacial barrier for electron transport. As trap and defect states in low-k dielectrics are also important to understanding low-k/Cu interconnect reliability, we additionally present combined electron paramagnetic resonance (EPR) and electrically detected magnetic resonance (EDMR) measurements to determine the chemical identity and energy level of some electrically active trap/defect states in low-k dielectrics. Combined with the photoemission derived band diagrams, the EPR/EDMR measurements point to mid-gap carbon and silicon dangling bond defects in the low-k ILD and CCL, respectively, playing a role in electronic transport in these materials. We show that in many cases the combined band and defect state diagrams can explain and predict some of the observed reliability issues reported for low-k/Cu interconnects.