基于分子间多量子相干的矢量场磁共振成像模拟

对基于分子间多量子相干的矢量场成像进行了系统的模拟研究. 模拟结果表明：这类成像能够对几十微米量级的异质结构进行成像,并较好地抑制来自均匀介质的信号. 即使不存在梯度磁场情况下,通过相位循环该方法亦可实现对异质结构的成像. 这将有助于开拓基于分子间多量子相干成像的应用领域,深化对其成像机制的理解.     

MR IMAGING WITH INTERMOLECULAR MULTIPLE-QUANTUM COHERENCES (iMQCs):Fundamental Questions and Potentials

Pursuit of new contrast in imaging has been the driving force behind many innovative applications of physics in medical MRI. Even though the use of intramolecular multiple quantum coherence in NMR spectroscopy has a long history, intermolecular multiple quantum coherence (iMQC) among spins on different molecules, such as proton proton iMQC in water, was considered impossible for many years.     

Simultaneous acquisition of multiple orders of intermolecular multiple-quantum coherence images in vivo

Until recently, NMR imaging with intermolecular multiple-quantum coherences (iMQCs) has been based on the acquisition of a single echo. In vivo studies of iMQC image contrast would greatly benefit from a method that could acquire several orders of quantum coherence during the same acquisition. This would enable comparison of the image contrast for various orders and eliminate image coregistration problems between scans. It has previously been demonstrated that multiple orders of iMQC images can be simultaneously acquired of a simple phantom. Here, we examine the technique and its effect on biological tissue, both in vivo and in vitro. First, we establish the effectiveness of the iMQC sequence in vivo using earthworms as specimens. We then further show that the multi-CRAZED sequence enhances detection of next generation (nanoparticle) contrast agents on excised tumor tissue.     

Inhomogeneity-free heteronuclear iMQC

Intermolecular dipolar interactions between proton and carbon spins can be used to indirectly detect carbon spectra with high sensitivity. In this communication, we present a modified sequence that, in addition to the high sensitivity of heteronuclear intermolecular multiple quantum coherence (iMQC) experiments, retains the line narrowing capability characteristic of homonuclear zero-quantum coherences. We demonstrate that this sequence can be used to obtain high resolution (13)C spectra in the presence of magnetic field inhomogeneities, both for thermal and hyperpolarized samples, and discuss applications to water-hyperpolarized carbon imaging.     

Magnetic resonance microscopic imaging based on high-order intermolecular multiple-quantum coherences

Most imaging studies using intermolecular multiple-quantum coherences (iMQCs) have focused on the two-spin dipolar interactions--zero and double quantum coherences. Here, we report the results of various experimental studies to assess the feasibility of magnetic resonance microscopy with high-order iMQCs in model systems at 7 and 14 T. Experimental results demonstrated that the iMQC microscopic images with high coherence orders are readily observable at high field and have unique contrast depending on the sample microstructure and coherence order.     

Simultaneous acquisition of multiple orders of intermolecular multiple-quantum coherence images

Recent studies have demonstrated the ability to detect images based on intermolecular multiple-quantum coherences (iMQCs) that correspond to flipping of two or more separated spins simultaneously, as opposed to conventional magnetic resonance where only one spin is flipped at a time. Until now, iMQC imaging has only acquired one coherence signal per pulse sequence. Here we report a new sequence that successfully detects five orders of coherence (2, 1, 0, −1, and −2-quantum coherence images) in one pulse sequence, with each signal having its full intensity. The simultaneous acquisition highlights substantial contrast differences between conventional and iMQC images, and between the different types of iMQC images.     

High-resolution absorptive intermolecular multiple-quantum coherence NMR spectroscopy under inhomogeneous fields

Intermolecular multiple-quantum coherence (iMQC) is capable of improving NMR spectral resolution using a 2D shearing manipulation method. A pulse sequence termed CT-iDH, which combines intermolecular double-quantum filter (iDQF) with a modified constant-time (CT) scheme, is designed to achieve fast acquisition of high-resolution intermolecular zero-quantum coherences (iZQCs) and intermolecular double-quantum coherences (iDQCs) spectra without strong coupling artifacts. Furthermore, double-absorption lineshapes are first realized in 2D intermolecular multi-quantum coherences (iMQCs) spectra under inhomogeneous fields through a combination of iZQC and iDQC signals to double the resolution without loss of sensitivity. Theoretically the spectral linewidth can be further reduced by half compared to original iMQC high-resolution spectra. Several experiments were performed to test the feasibility of the new method and the improvements are evaluated quantitatively. The study suggests potential applications for in vivo spectroscopy.     

Intermolecular double-quantum coherence imaging without coherence selection gradients and its application in in vivo MRI

In the COSY Revamped with Asymmetric Z-gradient Echo Detection (CRAZED) experiments, magnetization is modulated by the distant dipolar field (DDF) generated by coherence selection gradient (CSG) commonly in sinusoidal wave-form and results in detectable intermolecular multiple-quantum coherence (iMQC) signal. IMQCs have some attractive features, but their intrinsic weak signal intensity prevents their widespread applications. In this paper, a new phase cycling scheme was applied to obtain intermolecular double-quantum coherence (iDQC) signal. It is found that DDF can arise from nonspherical sample geometry or background inhomogeneous field in the absence of CSGs, which is more efficient than that created from CSGs. The experimental results show that the resulting DDF can refocus the ± iDQC signals simultaneously and thus enhance the signal intensity to about two folds of that from the conventional CRAZED sequence. Theoretical prediction and experiments give coincident results.     

高阶多量子跃迁的一维直接检测——Raman磁共振方法

研究了异核AX₃自旋体系的初始态为最高阶量子相干态时的Raman磁共振谱.研究表明,唯有制备高阶多量子相干的初始态时才有可能对高阶多量子跃迁进行有效检测.积算符理论的精确性使得理论上准确地预言了Raman磁共振谱中各阶多量子跃迁频率、谱线强度,使得各阶多量子跃迁频率相互交错时仍有可能解析谱图.在Raman磁共振实验中采用了正负频偏交替变化采样的方法,对单量子信号进行压制,使其更有效地观察高阶多量子跃迁.     

ONE-DIMENSIONAL DETECTION OF HIGHER-ORDER MULTIPLE QUANTUM TRANSITIONS BY RAMAN MAGNETIC RESONANCE

研究了异核AX     

Functional magnetic resonance imaging with intermolecular multiple-quantum coherences

For the first time, we demonstrate here functional magnetic resonance imaging (fMRI) using intermolecular multiple-quantum coherences (iMQCs). iMQCs are normally not observed in liquid-state NMR because dipolar interactions between spins average to zero. If the magnetic isotropy of the sample is broken through the use of magnetic field gradients, dipolar couplings can reappear, and hence iMQCs can be observed. Conventional (BOLD) fMRI measures susceptibility variations averaged over each voxel. In the experiment performed here, the sensitivity of iMQCs to frequency variations over mesoscopic and well-defined distances is exploited. We show that iMQC contrast is qualitatively and quantitatively different from BOLD contrast in a visual stimulation task. While the number of activated pixels is smaller in iMQC contrast, the intensity change in some pixels exceeds that of BOLD contrast severalfold.     

Enhancement of magnetization transfer effects by inter-molecular multiple quantum filtered NMR

There are a number of methods that give MRI contrasts based on changes of the water M(z) magnetization as a result of magnetization transfer to macromolecules. In the present work we report that a combination of these methods with inter-molecular multiple quantum coherences (iMQC) gives enhanced effects. For the magnetization transfer contrast (MTC) method an effect of (M(z)/M(0)) becomes (M(z)/M(0))(l) where l is the rank of the tensors constituting the iMQC. A similar trend was found upon combining iMQC with the Goldman-Shen experiment. It is pointed out that the method is general for all magnetization transfer methods, including the nuclear Overhauser effect.     

Parallel frequency-domain optical coherence tomography scatter-mode imaging of the hamster cheek pouch using a thermal light source

We use a parallel frequency-domain optical coherence tomography (FDOCT) system to generate a scatter-mode image of the hamster cheek pouch epithelium. To our knowledge, this is the first optical coherence tomography (OCT) image of a biological sample obtained using a thermal light source in the frequency domain. The system employs an imaging spectrometer to acquire depth-resolved profiles from adjacent spatial points without the need for any scanning. To enable this imaging modality, we have considered that signals originating from multiple depths combine in a different manner in FDOCT compared to time-domain optical coherence tomography (TDOCT). Because a multicomponent FDOCT signal is a coherent sum, it is necessary to limit the number of modes that contribute to the detected signal. Conversely, multicomponent TDOCT signals can be represented as incoherent sums, where increasing the number of modes improves the signal.     

从离散Wigner函数的角度探讨量子相干性度量

量子相干性是量子信息处理的基本要素,在量子计算中扮演着重要的角色.为了便于讨论量子相干性在量子计算中的作用,本文从离散Wigner函数角度对量子相干性进行了探讨.首先对奇素数维量子系统的离散Wigner函数进行了分析,分离出表征相干性的部分,提出了一种可能的基于离散Wigner函数的量子相干性度量方法,并对其进行了量子相干性度量规范的分析;同时也比较了该度量与l_1范数相干性度量之间的关系.重要的是,这种度量方法能够明确给出量子相干性程度与衡量量子态量子计算加速能力的负性和之间不等式关系,由此可以解析地解释量子相干性仅是量子计算加速的必要条件.     

Assessment of the magnetic field distribution in yellow and red bone marrow by the MAGSUS technique

The MAGSUS imaging technique has been shown to provide insights into the distribution of the macroscopic and microscopic static magnetic field without requiring further data processing. Applications of the MAGSUS technique on bone marrow are reported in more detail in this article. Effects of the superposition of water and lipid signals and of considerable transverse relaxation on MAGSUS imaging are demonstrated, and adapted imaging parameters are presented. Examples of applications on marrow with different physiological and pathological compositions and different locations are shown. Appropriate adjustments for a reliable estimation of the trabecular density in peripheral yellow marrow and for an assessment of the field distribution in hemopoietic red marrow are reported. Osteoporotic peripheral marrow with reduced amount of trabecular structures and alterations due to osteodystrophia deformans can be simply revealed by this method. An estimation of the trabecular density can also be performed by MAGSUS in vertebral bodies of hematologically unaffected persons, but the interindividually differing amount of paramagnetic depositions in the marrow (e.g., hemosiderin) must be taken into account.     

非对称自旋-轨道耦合系统的多体量子相干含时演化

本文以具有非对称性自旋相互作用的三体自旋系统为研究对象,重点研究了三体量子相干含时演化规律.采用精确量子对角化和基于量子主方程的数值模拟方法,讨论了三体量子系统中多种量子相干组分及其退相干.研究发现,量子相干组分的含时演化与整个系统的初态量子特性紧密相关.当初态为可分离纯态时,在较短时间内,非对称相互作用有利于增加多体量子相干度.这些量子相干度因受噪声影响而逐渐衰减.当初态为类Werner态时,量子相干度的分布满足加和性,即三体量子相干度等于所有两体量子相干度之和.自旋之间非对称相互作用和环境噪声都会引起三体量子相干度大于所有两体量子相干度之和.这些结论有助于多体量子资源的制备.     

Novel methods for characterizing a decoupler channel using "undetectable" quantum coherences.

Exact solutions for the effect of time-independent RF pulses on any initial configuration of an IS J-coupled system demonstrate that on-resonance CW decoupling yields signals whose frequency depends on RF field strength and homogeneity. These signals are enhanced starting with "undetectable" antiphase and multiple quantum coherences, which can also produce centerband intensity to mimic the signal from decoupled Sx. Conversely, these coherences can be generated from Sx using a low-power pulse, B1 = J/2, of length (2J)-1, dubbed a "90J pulse" since it is the selective equivalent of {(2J)-1-90[I]}. Utilizing 90J pulses, new characterization-of-decoupler (COD) pulse sequences can determine the performance of an insensitive I-spin channel by observing large signals from either antiphase or multiple quantum coherences with the S-spin channel, allowing, in minutes rather than hours: (i) frequency calibration to an accuracy of 0.1 Hz; (ii) measurement of RF amplitudes over a 500-fold variation; and (iii) mapping of RF homogeneity along the sample axis with a single 1D B1 spectrum. These 90J coherence transfer pulses are of potential general use for selective spectroscopy.     

Diffraction-like phenomena in a periodic magnetization distribution at 1.5 T using the distant dipolar field (DDF)

In the CRAZED experiment (COSY revamped by asymmetric Z-gradient echo detection, Warren et al.), a spatially anisotropic magnetization distribution is created by application of a magnetic field gradient (strength G, duration tau) which in turn generates a response called the distant dipolar field (DDF). The DDF is a source of intermolecular multiple-quantum coherences (iMQC) which contain information on the distance d=pi/(gammaGtau) between pairs of dipolar-coupled spins. Diffraction-like phenomena may result for periodically structured samples. In this study, we report the observation of diffraction owing to the DDF at 1.5 T using a clinical whole-body tomograph. Based on the semi-classical treatment of the problem by Robyr and Bowtell, diffraction conditions were obtained for a CRAZED-type pulse sequence that selects iMQC of order N. The predicted distinct difference in N=2 and N not equal2 coherences, i.e., a dominant continuous course as a function of tau (N=2) and prominent diffraction peaks otherwise, could be verified in CRAZED experiments in a periodically structured sample selecting coherence orders N=2 and N=3. The diffractive signal component contains information on the geometric structure of the sample. Applications of this technique may permit the detection of changes in composition and geometry of periodic structures.     

混沌光场光子统计分布及二阶相干度的分析与测量

利用通信波段双通道单光子探测器,采用Hanbury Brown-Twiss关联测量方案,理论分析并实验测量了光反馈半导体激光器产生的混沌光场的光子统计分布及不同混沌状态光场的二阶相干度.通过对混沌光场二阶相干度g~((2))(τ)的理论分析,得出随着延迟时间和相干时间的变化,其与相干光、热光及单光子态的二阶相干度可明显区分并呈现出不同分布.同时实验上产生了频谱宽度6.7 GHz的混沌光场,测量了不同光子数分布的结果,并用高斯随机分布、泊松分布、玻色-爱因斯坦分布对光子数分布进行理论拟合,发现随着入射平均光子数的增加,光子数分布从玻色-爱因斯坦分布过渡到泊松分布,但整个过程都与高斯随机分布符合较好,且光场的二阶相干度g~((2))(0)由2降至1.通过改变偏置电流(I=1.0Ith-2.0Ith)和反馈强度(0—10%),实验上研究了混沌光场由低频起伏到相干塌陷的过程中不同状态宏观动力学特性与二阶相干度的对应关系.结果表明:混沌光场在此过程中始终呈现出明显的聚束效应,并在频谱宽度最大时达到最强;同时给出了光子计数测量中聚束效应减弱的物理原因.实验表明该系统及方法能很好地揭示不同状态混沌光场的光子统计特性.     

Triple-quantum-filtered imaging of sodium in presence of B(0) inhomogeneities

Triple quantum filtered sodium MRI techniques have been recently demonstrated in vivo. These techniques have been previously advocated as a means to separate the sodium NMR signal from different physiological compartments based on the differences between their relaxation rates. Among the different triple quantum coherence transfer filters, the three-pulse coherence transfer filter has been demonstrated to be better suited for human imaging than the traditional four-pulse implementation. While the three-pulse structure has distinct advantages in terms of RF efficiency, the lack of a refocusing pulse in the filter introduces an increased dependence on the main magnetic field inhomogeneities, which can sometimes lead to significant signal loss. In this paper, we characterize these dependencies and introduce a method for their compensation through the acquisition of a B(0) map and the use of a modified phase cycling scheme.