用于同时多层MRI的选择性射频脉冲的优化设计

近年来,为提高磁共振成像（MRI）信号信噪比（SNR）、缩短成像时间,同时多层成像技术受到了极大的关注.为了实现同时多层的选择性激发,现有的多层成像序列大多使用组合射频（RF）脉冲,该脉冲可包含多个独立的幅值相同相位不同的简单脉冲,由于其采用简单的线性叠加方法,该类脉冲射频功率随脉冲数量呈现平方增长,因而应用受限.针对这一问题,基于自旋动力学和优化控制原理,本文提出了一种针对同时多层MRI的选择性射频脉冲的数值优化方法,该方法充分运用射频脉冲的调控机制,获得优化脉冲,并配合层选梯度,可实现任意层厚、层间距、层数的同时高效选择性激发.最后,通过数字模体的同时多层模拟成像实验验证了优化脉冲的有效性.     

实空间高分辨象模拟计算中的动态收敛判据

给出实空间多片层(RSMS)法的动态收敛判据(RSMSD法),利用与晶体结构有关的截断条件,动态地确定了每两片层间传播因子的展开级数,完全克服了RSMS法固有的发散问题。RSMSD法的计算结果与快速傅里叶变换多片层(FFTMS)法的相应结果符合很好,这说明动态收敛判据是正确、有效的。 关键词：     

用于测量J偶合常数的同时多层选择性恒时J分解谱的方法

标量偶合是核磁共振（NMR）波谱的一个重要参数.其中氢-氢偶合能提供关于分子结构的有用信息.但是,在复杂的偶合网络中解析出氢-氢偶合常数（J_H-H）较为困难.本文提出了一种基于空间编码选择性恒时演化的测量J_H-H的方法,利用一次实验就能解析分子中所有氢核的偶合网络,并测量J_H-H.该方法被称为同时多层选择性恒时J分解谱（SMS-SECTJRES）.它结合空间编码梯度和选择性恒时演化,并利用平面回波谱成像（EPSI）采样模块,从不同的空间位置提取出对应不同氢核偶合网络的J分解谱,促进了NMR技术在分子结构解析中的进一步应用.     

HRTEM image contrast of short range order in Ni4Mo

The high resolution transmission electron microscope (HRTEM) imaging of short range order (SRO) in Ni4Mo was investigated by means of multi-slice image simulations. The HRTEM images of Ni4Mo exhibit locally bright dot patterns corresponding to the [001] projections of the N2M2-type (chalcopyrite-like) structure. The multi-slice simulations revealed that the N2M2 patterns are rationalized as the projection patterns of the SRO structure which consists of subunit cell clusters of D1a, D022 and Pt2Mo structures. The N2M2-type image contrast appears when both the fundamental fcc lattice reflections and the 1 1/2 0 diffuse scattering of SRO contribute enough to imaging. This suggests that a good coincidence in intensity distribution between the Fourier power spectra of HRTEM images and the electron diffraction patterns is one of the conditions for the image contrast of SRO to be interpreted in terms of the projection contrast.     

NMR velocimetry with 13-interval stimulated echo multi-slice imaging in natural porous media under low flow rates

Characterization and quantification of root water uptake processes play a key role in understanding and managing the effects of global climate change on agricultural production and ecosystem dynamics. Part of this understanding is related to the flow of water towards plant roots in soils. In this study we demonstrate for the first time, to our knowledge, that fluid flow in the voids of the pore space of a model soil system (natural sand) can be detected and mapped to an NMR image for mean flows as low as 0.06 mm/s even under the influence of internal magnetic field gradients. To accomplish this we combined multi-slice imaging with a 13-interval pulse sequence to the NMR pulse sequence 13-interval stimulated echo multi-slice imaging (13-interval STEMSI). The result is a largely reduced influence of the internal magnetic field gradients, leading to an improved signal-to-noise ratio which in turn enables one to acquire velocity maps where conventional stimulated echo methods fail.     

Combined simulated annealing and Shinnar-Le Roux pulse design of slice-multiplexed RF pulses for multi-slice imaging

Slice-multiplexed RF pulses have recently been introduced for simultaneous multi-slice imaging. Their novel aspect is that each slice is given a different linear phase profile, and hence a different slice-rephasing requirement, by the pulse. During readout, extra slice gradients are applied such that when one slice is rephased, the others are dephased to prevent aliasing. In this paper, an improved method of designing slice-multiplexed RF pulses is presented: component pulses which are optimized with simulated annealing for a specific rephasing are combined using Shinnar-Le Roux methods. In this way, non-linearities at higher flip angles are taken into account and more slices can be excited. Bloch simulations show the phase and amplitude profile of component pulses are faithfully preserved in the multiplexed pulse. Three- and four-slice multiplex pulses are demonstrated in gradient- and spin-echo in-vivo imaging.     

基于多尺度计算方法的活体磁共振谱自动相位矫正和代谢物定量分析(英文)

活体多片磁共振谱成像(MRSI)产生大量的波谱数据,因此需要使用自动的谱数据分析方法来获得不同组织代谢产物的定量分布图.然而,活体波谱通常产生严重的谱和基线变形,使得基于曲线拟合的谱定量数据分析方法失效.该文应用多尺度分析(Multiscale)方法自动确定兴趣代谢物在频率空间的谱峰特征(位置和线宽),然后通过叠代运算对该代谢物对应的谱峰进行独立的自动相位矫正和线型拟合.大脑波谱成像的实验结果表明,该方法可以方便、有效的获得代谢产物在大脑的分布,特别适宜于多片磁共振谱成像的代谢产物定量分析.     

An Accurate Image Simulation Method for High-Order Laue Zone Effects

A completely different formulation for simulation of the high order Laue zone （HOLZ） diffractions is derived. It refers to the new method, i.e. the Taylor series （TS） method. To check the validity and accuracy of the TS method, we take polyvinglidene fluoride （PVDF） crystal as an example to calculate the exit wavefunction by the conventional multi-slice （CMS） method and the TS method. The calculated results show that the TS method is much more accurate than the CMS method and is independent of the slice thicknesses. Moreover, the pure first order Lane zone wavefunction by the TS method can reflect the major potential distribution of the first reciprocal plane.     

Study of the influence of higher—order Laue zones on the exit wavefunction

Based on an analytical expression of electron dynamical diffraction derived recently,the exit wavefunctions of nonperiodic samples have been calculated.The results have shown that the amplitude and phase obtained by the analytical expression were in good agreement with those obtained using the multi-slice method.Besides,the influence of the higher-order Laue zone on the wavefunction for different sample thickness was studied in detail and the results showed that the exit wavefunction is dominated by the zero-order Laue zone.When the sample is very thin,the influence of the higher-order Laue zone should be considered.     

Moses: multiple oversampled slabs EPI sequence

A new technique is proposed which combines the advantages of phase encoded and multi-slice echo planar imaging (EPI) methods. Its principle is to interleave multiple phase encoded EPI slabs. This approach can provide a larger spatial coverage than multi-slice EPI for the same signal to noise ratio and total imaging time and a shorter minimum imaging time than 3D EPI for the same coverage and repetition time. Other advantages include availability of the steady state image contrasts and potentially lower acoustic noise and RF specific absorption rate compared to the standard multi-slice EPI. A full discussion of its potential as well as in vivo results at 1.5 and 3 Tesla are presented in this paper.     

高分辨电子显微象实空间象模拟方法的改进

在深入研究高分辨实空间象模拟方法的基础上,本文导出了一个通用的收敛判据。这个判据确定了实空间象模拟计算中,在x-y平面上的取样间距δ和在电子束入射方向上片层厚度δ的依赖关系,从而有效地克服了实空间法中存在的发散问题。此外,还给出了精确计算传播因子的新公式,并在动力学衍射计算中获得了和传统的FFT多片层法符合很好的结果。 关键词：     

手持数码相机获取场景三维点云的实现

利用计算机视觉进行三维点云获取是一种实用方便的三维信息提取方法,在工业检测和三维建模等领域应用广泛.鉴于传统固定相机方法中存在着灵活性差,适用范围小的问题,提出了一种手持单相机的三维点云获取方法.实验结果表明,该方法在室外场景点云获取应用中较传统方法在灵活性方面具有非常明显的优势.     

基于同时多层激发和并行成像的心脏磁共振电影成像

磁共振成像（MRI）无创无害、对比度多、可以任意剖面成像的特点特别适合用于心脏成像,却因扫描时间长限制了其在临床上的应用.为了解决心脏磁共振电影成像屏气扫描时间过长的问题,该文提出了一种基于同时多层激发的多倍加速心脏磁共振电影成像及其影像重建的方法,该方法将相位调制多层激发（CAIPIRINHA）技术与并行加速（PPA）技术相结合,运用到分段采集心脏电影成像序列中,实现了在相位编码方向和选层方向的四倍加速,并使用改进的SENSE/GRAPPA算法对图像进行重建.分别在水模以及人体上进行了实验,将加速序列图像与不加速序列图像进行对比,结果验证了重建算法的有效性,表明该方法可以在保障图像质量以及准确测量心脏功能的前提下成倍节省扫描时间.     

双棱镜干涉实验中调节方法的改进

分析了双棱镜干涉实验中传统调节方法存在的缺陷,改进了调节方法,使调节方法、步骤清晰,消除了传统调节方法中存在的某些盲目性,使学生能很快调节出干涉条纹,提高了实验效率。     

Large-range displacement measurement using sinusoidal phase-modulating laser diode interferometer

A signal processing method of realizing a large-range displacement measurement in a sinusoidal phasemodulating laser diode interferometer is proposed. The method of obtaining the dynamic value of the effective sinusoidal phase-modulating depth is detailed, and the residual amplitude modulation is also taken into account.Numerical simulations and experiments are carried out to compare this method with the traditional one.We prove that, with this method, the sinusoidal phase-modulating laser diode interferometer can realize a centimeter-level displacement measurement range with high precision, which is much better than the traditional method.     

Dynamic shim updating on the human brain

Dynamic alteration of shim settings during a multi-slice imaging experiment can improve static magnetic-field homogeneity over extended volumes. In this report, a pre-emphasized dynamic shim updating (DSU) system capable of rapidly updating all non-degenerate zeroth through second-order shims is presented and applied to high-field multi-slice imaging studies on the human brain. DSU is utilized in both non-oblique and oblique slicing geometries while updating in-plane and through-slice shims. Image-based magnetic-field maps are used to quantify homogeneity improvements and comparisons are made on a slice-specific basis between static global shimming and increasing orders of shim inclusion utilized DSU. The influence of oblique slicing geometry on DSU-utilized global homogeneity is also quantified computationally. Finally, the effect of DSU on susceptibility artifact reduction in single-shot axial-sliced EPI is analyzed using experimental acquisitions.     

光学材料光学均匀性的波长调谐两步绝对测量法

光学均匀性是光学材料的重要指标,目前高精度的测量方法一般采用绝对测量法,而该方法步骤繁琐,容易受环境影响。根据波长移相干涉仪的移相特点,提出了测量光学材料光学均匀性的波长调谐两步绝对测量法。该方法在波长移相干涉仪中通过平行平板放入测量和空腔测量两个步骤得到平行平板的光学均匀性。在模拟仿真验证方法的正确性后,进行了实验研究,并与传统的绝对测量法的测量结果进行比较。结果表明,波长调谐两步绝对测量法可用于测量平行平板的光学均匀性,且测量步骤简单,精度较高。     

Laser beam cleanup using improved model-based wavefront sensorless adaptive optics

An improved model-based wavefront sensorless adaptive optics algorithm is proposed for laser beam cleanup.Deformable mirror(DM) eigenmodes are used to replace traditional Lukosz modes in order to avoid DM fitting errors. The traditional method is based on a sophisticated calibration process and solving linear equations. In our method, coefficients of DM eigenmodes are estimated by adding bidirectional modal biases into the system and then solving parabolic equations. The calibration process is omitted in our method, which makes it more feasible.From simulation and experimental results, the corrective accuracy of the improved method is higher than the traditional one.     

基于虚拟线圈和卷积神经网络的多层同时激发图像重建

本文提出一种基于虚拟共轭线圈（Virtual Coil Concept,VCC）技术和k空间插值鲁棒人工神经网络（Robust Artificial-neural-networks for k-space Interpolation,RAKI）的图像重建方法,用于磁共振多层同时激发成像（Simultaneous Multi-Slice imaging,SMS）,该方法能够有效提升重建图像的质量,被命名为VIRGINIA（VIRtual conjuGate coIls Neural-networks InterpolAtion）.为了得到更高质量的SMS图像,本文提出的VIRGINIA方法利用磁共振线圈数据的复数共轭对称性质扩展了SMS所获取的多通道数据,并将扩展后的数据用于RAKI网络的训练,利用训练后的网络实现高质量的SMS图像重建.本文将VIRGINIA方法和其他SMS图像重建方法（RAKI和Slice-GRAPPA方法）进行了对比,并采用结构相似指数（Structural Similarity Index,SSIM）、峰值信噪比（Peak Signal-to-Noise Ratio,PSNR）和均方根误差（Root Mean Square Error,RMSE）对不同方法的重建图像进行了量化对比分析.结果显示,在相同的SMS加速倍数下,使用VIRGINIA方法进行重建的图像质量均好于RAKI方法,且远好于传统Slice-GRAPPA方法.     

谈谈光学检测的指导思想

指导思想是指人们在做一件事之前,考虑要采取什么样的技术路线,使用怎样的工具和方法等等。人类经过长期的经验积累和理论研究,发展出许许多多测量方法和测试仪器、测量工具。计算机的出现和普及,使光学测试领域也发生了革命性的变化。集光一机一电一算于一体的新的测试仪器和经典的仪器设备之间的取舍,往往是决策者不能回避的选择。世界上的事情是复杂的,有的事情,从这个角度看应该这样,而从另一个角度看也许应该那样。但是,不管用什么手段,又经济又能解决实际问题就是好手段。以制造一个4m镜面为例,谈谈个人的测试思路。