基于优化后流动敏感黑血序列的豆纹动脉3 T磁共振成像

豆纹动脉是大脑内部的重要动脉,其阻塞往往会导致腔隙性脑梗死.现在在临床上主要利用数字减影血管造影(Digital Subtraction Angiography,DSA)技术实现豆纹动脉成像,然而DSA的有创性是其重要的限制因素.有研究表明,在高场磁共振系统(7 T)下,时间飞跃法(Time-Of-Flight,TOF)已经能够得到较好的豆纹动脉影像,但是在临床使用的1.5 T或3 T磁共振系统下,由于豆纹动脉的管腔直径非常小(大约为0.3~0.7 mm)、血流速度比较慢,对其成像仍然是个挑战.该文主要研究了在3 T磁共振系统下使用流动敏感黑血(Flow-Sensitive Black-Blood,FSBB)序列对豆纹动脉进行成像的方法,并对该成像序列中流动敏感梯度的设计进行了优化,使其在扫描时间和图像分辨率、对比度、信噪比等方面都能够基本满足临床使用的要求.     

浑浊介质中图像对比度与成像方式的关系

浑浊介质中图像对比度的物理增强方法一直是研究热点, 目前学者们提出的距离选通成像、偏振差分成像和偏振距离选通成像均能提高图像的对比度, 但提高效果与成像距离的关系尚不明确. 本文分别利用以上三种成像方式及普通强度成像对处于不同浓度浑浊介质中的目标进行成像, 研究了图像强度和对比度随成像距离的变化情况. 结果表明: 从滤除的散射光强来看, 偏振距离选通成像最优, 而偏振差分成像在成像距离较远时优于距离选通成像; 三种成像方式滤除的散射光强值趋于稳定的阈值距离各不相同; 对比度改变相同量时, 偏振距离选通成像对应成像距离的变化量最大, 偏振差分成像次之, 强度成像最小, 且均与散射系数成反比. 本文对浑浊介质成像效果及机理的分析, 对进一步提高浑浊介质中目标的分辨及识别具有重要意义.     

基于神经网络拟合的腹部化学交换饱和转移成像

磁共振成像（Magnetic Resonance Imaging，MRI）化学交换饱和转移（Chemical Exchange Saturation Transfer，CEST）技术在临床诊断中展现了巨大的潜力，但在腹部成像中受到主磁场偏移量大的挑战，而且利用传统的非对称性分析法得到的酰胺质子转移（Amide Proton Transfer，APT）成像对比度受到核奥氏增强（Nuclear Overhauser Enhancement，NOE）效应的干扰.本文提出了一种基于神经网络拟合的CEST后处理方法，对每个像素采集得到的Z谱特征进行识别，不需要额外序列扫描即可得到背景参考Z谱与主磁场偏移量，用以校正和获得理想的Z谱，并进一步分离得到源自APT效应与NOE效应的信号.鸡蛋清和健康志愿者腹部成像结果显示，本文提出的基于神经网络的CEST后处理方法效果较好.     

Markov环境下“X”态基于CHSH不等式的量子非局域关联检验

量子非局域关联是量子理论最基础的特征之一.“X”态作为实验中常见的一种量子态,因其在演化过程中仍保持“X”形的稳定性,而被广泛应用于开放量子系统的研究中.利用Clauser-Horne-Shimony-Holt (CHSH)不等式,在Markov环境这种典型的开放量子系统下,研究了两种通过局部变换操作所关联的“X”态在振幅阻尼环境和相位阻尼环境中量子非局域检验结果随时间的演化情况.研究结果表明,在相位阻尼环境中,随着演化时间的增加,两种“X”态具有相同的CHSH不等式检验结果.在振幅阻尼环境中,利用局部变换操作得到的“X”态,可获得较长的成功进行量子非局域关联检验的演化时间.最后,详细给出了两种类型“X”态在相位阻尼环境和振幅阻尼环境中成功进行量子非局域关联检验的保真度范围.     

用TPPI方法消除2D COSY和MQSY实验中与辐射阻尼有关的谐波峰

在二维相关(COSY)和多量子(MQSY)实验中,TPPI(Time-proportional-phase-in-crement)方法被用于消除与辐射阻尼相关的谐波峰.二维实验中,^c/2混合脉冲常常非线性地改变磁化强度的进动相位.这一非线性变化和辐射阻尼一起导致依赖于化学位移偏置的谐波.然而,如果混合脉冲的相位正比于演化时间t₁,谐波消失,但水峰保持不变.     

基于深度递归级联卷积神经网络的并行磁共振成像方法

快速磁共振成像是磁共振研究领域重要的课题之一.随着大数据和深度学习的兴起,神经网络成为快速磁共振技术的重要方法.然而网络性能表现和网络参数量之间较难取得平衡,且对于多通道数据重建的并行成像问题,相关研究较少.本文构建了一种深度递归级联卷积神经网络结构,用于处理并行成像问题.这种网络结构在减少网络参数量的同时,能够尽可能地提高网络的表达能力,提高网络重建的精确度.实验结果表明,相较于传统并行成像方法,通过训练好的神经网络对欠采样磁共振数据进行重建,可以得到更准确的重建结果,且重建时间大大缩短.     

钆掺杂的类普鲁士蓝空心配位聚合物作为多模式成像探针及药物载体的研究

本文通过溶剂热法制备了具有多模式成像能力的钆掺杂的类普鲁士蓝空心配位聚合物,并包覆二氧化硅层以进一步应用. 磁共振成像实验表明纳米粒子表现出相当好的双模式磁共振成像能力. 此外,在各种波长的激光束下纳米粒子也会发出多色荧光. 由于其空心多孔结构,该聚合物具有1166 mg/g的高载药(阿霉素)能力和83.29%的药物封装效率,这使其成为潜在的药物载体平台. 特别在二氧化硅包覆之后,生物相容性也都得到了增强.     

能见距离测试条件及其对测试结果的影响

气象学的能见距离是在光电成像系统分析和设计中常用的大气性能评价参量。在能见距离的实际测试中,往往由于对测试条件的忽略或限制,使测量结果偏离能见距离。根据辐射度学原理,从理论上研究景物对比度在大气传输中的衰减,得到了在两种可能的条件下所测得的目标最大探测距离与能见距离的关系。利用这一关系可避免或减小测量偏差对成像系统分析设计的影响。     

功能磁共振成像技术及其在脑科学研究中的应用

功能磁共振成像(functional magnetic resonance imaging,fMRI)是用磁共振成像的方法研究人脑和神经系统的功能,它是磁共振成像的一种应用和深入发展.磁共振成像(magnetic resonance imaging,MRI)是核磁共振成像的简称,它是基于核磁共振(nuclear magnetic resonance,NMR)这一物理现象发展起来的.1946年物理学家首先发现核磁共振现象,直到70年代初,它一直沿着高分辨核磁共振波谱学的方向发展.1972年达马迪安(R.Damadian)提出磁共振成像的设想,并指出可以用磁共振成像仪扫描人体检查疾病.1973年劳特伯(P.Lauterbur)在<自然>杂志上发表了用试管样品得到的磁共振截面像,显示了磁共振成像的可能性.从此开始了磁共振成像的发展时期.1980年在实验室中获得了足够清晰的有医学诊断意义的人的头部磁共振图像.磁共振成像仪逐渐形成产业,开始进入医院,主要用于观测人体内部解剖学结构,确定肿瘤和其他疾病的位置.1990年对动物的实验表明,有可能用磁共振成像研究大脑功能.1991年发表了第一幅有意义的人的大脑功能的图像,显示出视觉刺激在大脑的反应,开始了脑功能磁共振成像的研究.至今刚刚过了几年的时间,这一研究领域已经得到了迅速发展.     

钆掺杂的类普鲁士蓝空心配位聚合物作为多模式成像探针及药物载体的研究（英文）

本文通过溶剂热法制备了具有多模式成像能力的钆掺杂的类普鲁士蓝空心配位聚合物,并包覆二氧化硅层以进一步应用.磁共振成像实验表明纳米粒子表现出相当好的双模式磁共振成像能力.此外,在各种波长的激光束下纳米粒子也会发出多色荧光.由于其空心多孔结构,该聚合物具有1166 mg/g的高载药(阿霉素)能力和83.29%的药物封装效率,这使其成为潜在的药物载体平台.特别在二氧化硅包覆之后,生物相容性也都得到了增强.     

高场强核磁共振下测量水的自旋晶格弛豫时间

在反转恢复脉冲序列中增加双极性梯度场脉冲以压制辐射阻尼效应,从而使之能够在高场强核磁共振波谱仪(Bruker AV600)中较为准确测量水的自旋-晶格时间.这一方法应用于商品化成像对比剂Gd-DTPA的弛豫率测试,得到的结果和文献相似,证实了该方法的可靠性.进一步应用于新合成成像对比剂TEMDP-EMFs性能的评价.     

On linear modeling of interface damping in vibrating structures

Dissipation of mechanical vibration energy at contact interfaces in a structure, commonly referred to as interface damping, is an important source of vibration damping in built-up structures and its modeling is the focus of the present study. The approach taken uses interface forces which are linearly dependent on the relative vibration displacements at the contact interfaces.The main objective is to demonstrate a straightforward technique for simulation of interface damping in built-up structures using FE modeling and simple, distributed, damping forces localized to interfaces where the damping occurs.As an illustration of the resulting damping the dissipated power is used for evaluation purposes. This is calculated from surface integrals over the contact interfaces and allows for explicit assessment of the effect of simulated interface forces for different cases and frequencies. The resulting loss factor at resonance is explicitly evaluated and, using linear simulations, it is demonstrated that high damping levels may arise even though the displacement differences between contacting surfaces at damped interfaces may be very small.     

Complex resonance and complex-frequency spectroscopy

The response of a medium to radiation with a complex frequency corresponding to an exponential time decrease in the radiation amplitude has been analyzed. The effect of complex resonance has been demonstrated when the real and imaginary parts of the complex radiation frequency approach the real and imaginary parts of the complex frequency of damping natural oscillations of medium oscillators. In resonance itself, the absolute value of the refractive index is divergent and radiation is completely reflected from the medium boundary. It has been shown that scanning of not only the real part, but also the imaginary part of the probe radiation frequency expands the capabilities of spectroscopy and makes it possible to, e.g., distinguish resonances even with coinciding (real) frequencies and close (of the same order of magnitude) widths.     

Interplay between nonlinearity,scan speed,damping, and electronics in frequency modulation atomic-force microscopy

Numerical simulations of the frequency modulation atomic force microscope, including the whole dynamical regulation by the electronics, show that the cantilever dynamics is conditionally stable and that there is a direct link between the frequency shift and the conservative tip-sample interaction. However, a soft coupling between the electronics and the nonlinearity of the interaction may significantly affect the damping. A resonance between the scan speed and the response time of the system can provide a simple explanation for the spatial shift and contrast inversion between topographical and damping images, and for the extreme sensitivity of the damping to a tip change.     

Solvent Magnetization Artifacts in High-Field NMR Studies of Macromolecular Hydration

With the use of high magnetic fields and improved quality factor ratings of the probeheads in modern NMR spectrometers, radiation damping becomes more and more important. In addition, the demagnetizing field effect from protonated solvents gains significance with the increase of the magnetic field strength. During a typical NMR pulse sequence the magnetic fields caused by these effects become time-dependent, which makes the system nonlinear and may, for example, measurably influence the precession frequencies of all nuclei in the sample. Since radiation damping can affect signals that are several kilohertz away from the solvent resonance, the amplitude, phase, and frequency of the desired signals can be disturbed so as to give rise to spectral artifacts. In particular when difference methods are used to obtain the final spectrum, the data sets may be severely deteriorated by such artifacts. This paper investigates effects from the demagnetizing field and from radiation damping with a selection of pulse sequences in use for studies of macromolecular hydration, and strategies are described for the detection and elimination of the ensuing artifacts.     

Cystic lymphangioma in the adult: an unusual axillary mass

Magnetic resonance imaging (MRI) has rapidly become an effective technique for the diagnosis and localization of most musculoskeletal diseases. Because MRI lacks ionizing radiation and has superior soft tissue contrast as compared with computed tomography without the need for intravenous contrast material, MRI is emerging as the study of choice for soft tissue masses. This report describes the use of MRI in the evaluation of an unusual axillary soft tissue mass in an adult, a cystic lymphangioma.     

An alternative tuning approach to enhance NMR signals

By using spin-noise type measurement we show that the resonance frequency of the reception circuit of classical NMR spectrometers does not match the Larmor frequency even if, in emission, the electronic circuit is perfectly tuned at the Larmor frequency and matches the amplifier impedance. We also show that this spin-noise method can be used to ensure a match between the Larmor frequency and the reception circuit resonance frequency. In these conditions, (i) the radiation damping field is in perfect quadrature to the magnetization and (ii) the NMR signal level and potentially the signal-to-noise ratio, are enhanced. This choice induces a change of the probe resonance frequency by several hundreds of kHz for 500 or 700 MHz spectrometer. We show that the resulting mismatch condition for emission can be removed by adding other tuning and matching degrees of freedom located on the excitation line (or by symmetry on the reception line) decoupled to the probe resonance circuit by the crossed diodes.     

Characterization and Suppression Techniques for Degree of Radiation Damping in Inversion Recovery Measurements

Radiation damping is a phenomenon in which transverse nuclear magnetization couples with the current in a coil used in nuclear magnetic resonance experiments. This results in an additional magnetic field that increases the relaxation pathway for the magnetization, which then relaxes back to equilibrium more quickly. Radiation damping has been shown to affect longitudinal relaxation time (T₁) measurement in inversion recovery experiments. In this work, we demonstrate that the extent of radiation damping depends upon the T₁ of the sample. Radiation damping difference spectroscopy is used to characterize the severity of radiation damping, while gradient inversion recovery is used for radiation damping suppression in T₁ measurements. At field strength of 9.4 T, for the radiation damping characteristic time (T_rd) of 50 ms, these investigations show non-negligible radiation damping effects for T₁ values greater than T_rd, with severe distortions for T₁ longer than about 150 ms, showing reasonable agreement with the predicted T_rd. We also report a discrepancy between published expressions for the characteristic radiation damping time.     

Contrast-enhanced dynamic MRI protocol with improved spatial and time resolution for in vivo microimaging of the mouse with a 1.5-T body scanner and a superconducting surface coil

Magnetic resonance imaging (MRI) is well suited for small animal model investigations to study various human pathologies. However, the assessment of microscopic information requires a high-spatial resolution (HSR) leading to a critical problem of signal-to-noise ratio limitations in standard whole-body imager. As contrast mechanisms are field dependent, working at high field do not allow to derive MRI criteria that may apply to clinical settings done in standard whole-body systems. In this work, a contrast-enhanced dynamic MRI protocol with improved spatial and time resolution was used to perform in vivo tumor model imaging on the mouse at 1.5 T. The needed sensitivity is provided by the use of a 12-mm superconducting surface coil operating at 77 K. High quality in vivo images were obtained and revealed well-defined internal structures of the tumor. A 3-D HSR sequence with voxels of 59x59x300 microm3 encoded within 6.9 min and a 2-D sequence with subsecond acquisition time and isotropic in-plane resolution of 234 microm were used to analyze the contrast enhancement kinetics in tumoral structures at long and short time scales. This work is a first step to better characterize and differentiate the dynamic behavior of tumoral heterogeneities.     

On surface plasmon damping in metallic nanoparticles

Two possible mechanisms of damping of surface plasmon (SP) oscillations in metallic nanoparticles (MNPs), not connected with the electron–phonon interaction, are investigated theoretically: (a) radiation damping of SPs and (b) resonant coupling of SP oscillations with electronic transitions in the matrix. For the mechanism (a) it is shown that the radiation damping rate is proportional to the number of electrons in a MNP and therefore this channel of energy outflow from the MNP becomes essential for relatively large particles. The strong frequency and size dependence of the radiation damping rate obtained allows us to separate the contributions of radiative processes and the electron–phonon interaction to the energy leakage. The investigation of the mechanism (b) shows that the rate of energy leakage of SP oscillations from a MNP does not depend on particle size and is fully determined by the optical characteristics of the matrix. It is demonstrated that for very small MNPs of -–3 5nm size, where the strong three-dimensional size quantization effect suppresses the electron–phonon interaction, the resonance coupling in certain cases provides an effective energy outflow. PACS 78.67.Bf