Region and volume dependencies in spectral line width assessed by 1H 2D MR chemical shift imaging in the monkey brain at 7 T

High magnetic fields increase the sensitivity and spectral dispersion in magnetic resonance spectroscopy (MRS). In contrast, spectral peaks are broadened in vivo at higher field strength due to stronger susceptibility-induced effects. Strategies to minimize the spectral line width are therefore of critical importance. In the present study, ¹H 2D chemical shift imaging at short echo times was performed in the macaque monkey brain at 7 T. Large brain coverage was obtained at high spatial resolution with voxel sizes down to 50 μl being able to quantify up to nine metabolites in vivo with good reliability. Measured line widths of metabolites decreased from 14.2 to 7.6 Hz with voxel volumes of 3.14 ml to 50 μl (at increased spatial resolution). The line width distribution of the metabolites (7.6±1.6 Hz, ranging from 5.5 to 10 Hz) was considerably smaller compared to that of water (10.6±2.4 Hz) and was also smaller than reported in ¹H MRS at 7 T in the human brain. Our study showed that even in well-shimmed areas assumed to have minimal macroscopic susceptibility variations, spectral line widths are tissue-specific exhibiting considerable regional variation. Therefore, an overall improvement of a gross spectral line width — directly correlated with improved spectral quality — can only be achieved when voxel volumes are significantly reduced. Our line width optimization was sufficient to permit clear glutamate (Glu)–glutamine separation, yielding distinct Glu maps for brain areas including regions of greatly different Glu concentration (e.g., ventricles vs. surrounding tissue).     

Improved resolution using symmetrically shifted pulses.

An approach to Hadamard phase encode the two halves of the F(1) dimension of a gHSQC experiment is presented. The phase encoding is achieved by excitation sculpting of the F(1) dimension using symmetrically shifted pulses. This approach (IMPRESS-improved resolution using symmetrically shifted pulses) increases the resolution of the F(1) dimension by exploiting spectral folding, but the folding is coded in the fashion of a Hadamard H(2) matrix. Editing of the IMPRESS spectra during processing sorts out spectral crowding which is a typical consequence of F(1) spectral folding. It is shown that for the same total experiment time, the IMPRESS-gHSQC experiment provides narrower peaks along the F(1) dimension compared to the normal gHSQC experiment. As a consequence of decreased linewidth, the peak height (sensitivity) is also increased.     

红外扫描和频振动光谱系统的激光线宽与光谱分辨率

本文报道了一个简化的利用可见光和红外光带宽来计算和频光谱分辨率的公式. 公式显示和频振动光谱的Voigt线宽可以通过振动模式的均匀线宽(洛伦兹线宽)、非均匀线宽(高斯线宽)、红外光与可见光的高斯线宽计算获得. 利用本实验室新搭建的频率分辨及偏振分辨的皮秒和频光谱系统验证了该公式的准确性. 实验结果显示,本激光系统获取的红外光的高斯线宽为1.5 cm^-1. 本激光系统的光谱分辨率约为4.6 cm^-1,结果与胆固醇单层膜光谱获取的光谱分辨率(3.5~5 cm^-1)基本一致.     

Super-Resolved Raman Spectroscopy

In this paper we present a super-resolved Raman spectroscopy configuration where, due to the proposed resolution improvement, more chemical species can be simultaneously detected. The idea includes usage of the basic property of Raman in which shifting the excitation wavelength will shift the absolute wavelength of the Raman spectral lines. Thus, encoding the spectra prior to analysis by the spectrometer allows obtaining spectral super-resolved sensing via time multiplexing super-resolution concepts applied over the spectral domain.     

High-resolution NMR spectra in inhomogeneous and unstable fields via the three-pulse method

In modern solution nuclear magnetic resonance (NMR), the spectral resolution is mainly dependent on the spatial homogeneity and temporal stability of the magnetic field. The spectral linewidths are usually proportional to the overall field homogeneity and the stability experienced by the sample. Many high-resolution NMR methods have been developed, but few are applicable in inhomogeneous and unstable fields. In this paper, a high-resolution three-pulse method based on intermolecular zero-quantum coherences (iZQCs) is proposed. Since this method is insensitive to field inhomogeneity and instability, spectral information such as the chemical shift can be retained in the resulting spectra. In comparison with the CPMG-HOMOGENIZED method, the new method provides almost pure solvent–solute iZQC signals.     

The determination of the true profile of XPS line by regularization method: I. Mathematical algorithm and numerical simulations

The resolution of XPS spectra is limited mainly by instrumental parameters like the spectral line width of exciting X-ray source and the finite energy resolution of the electron analyzer. A new algorithm of an inverse ill-posed problem has been proposed in which low resolution experimental XPS data can be enhanced by removing the instrumental functions. The regularization method with a special fast iteration algorithm is applied for determining the true profile line of a complex chemical compound if we know excitation and instrumental functions. This paper is structured as follows: (I) a mathematical algorithm is described and numerical simulation results are presented. (II) These algorithms have been applied to the poorly resolved C 1s spectrum of a poly-methyl methacrylate (PMMA) film studied by the conventional technique with monochromatic and polychromatic radiation. The resulting enhancement allows previously unresolved structure to be observed without any assumptions concerning number, position, shape of peaks and their ratio.     

可调空间分辨USEDCARS技术研究

相干反斯托克斯拉曼散射（CARS）技术是一种非常重要的燃烧诊断技术,该技术具有非常强的抗干扰能力和非常高的测量精度。但空间分辨力不足会使CARS技术产生很强的空间平均效应,引起成CARS光谱畸变,进而造成CARS光谱分析困难,无法通过CARS光谱反演燃烧场参数。针对非稳腔空间增强探测CARS（USEDCARS）技术存在的空间分辨不足以及空间分辨力不易改变的特点,分析了影响USEDCARS技术测量空间分辨力的各种因素,采用一组轴棱锥对USEDCARS系统中的泵浦激光进行环状光束整形,并通过调节轴棱锥之间的距离获得了不同直径的环状光束,在此基础上,建立了空间分辨可调USEDCARS诊断系统。开展了空间分辨力分析实验,获得了CARS信号强度随空间位置的分布数据,以CARS信号总强度95%包含的空间区域代表CARS的纵向空间分辨力,以此计算得到了CARS系统空间分辨力为1.7～6.5 mm连续可调。其中,高分辨力情况,达到了现有BOXCARS技术的空间分辨力。利用所建立的空间分辨可调USEDCARS诊断系统测量了酒精/空气预混火焰温度参数,获得了不同空间分辨条件下的CARS光谱。空间分辨力为1.7 mm时,获得了高质量CARS光谱,通过光谱拟合给出了所测火焰的温度信息。分辨力分别为4.9和6.5 mm时获得了较强的CARS信号,但存在光谱畸变。结果显示,空间分辨力对CARS信号的强度和空间平均效应有很大地影响,提高测量的空间分辨力可以有效消除空间平均效应,获得准确的CARS光谱,增强光谱拟合精度,同时空间分辨可调的特性使该系统能够更好地适应不同实验条件下的诊断工作。     

Spatial Resolution Enhancement in BOTDR by Spectrum Separation Method

BOTDR (Brillouin optical-fiber time domain reflectometry) is a promising technique for measuring the strain distribution along an optical fiber which can be used for diagnosing civil structures. However, there is a trade-off between the spatial and the frequency resolutions, and its practical spatial resolution is limited to about 1 m; to date the application scope has been restricted. In order to enhance the spatial resolution without decreasing the measurement accuracy, we intentionally introduce two kinds of strain within the spatial resolution length determined by the optical pulse width, which results in Brillouin gain spectrum separation. In this paper, we describe the theoretical analysis of the spectrum separation method together with experimental verification, where a spatial resolution of 0.5 m is demonstrated. We further propose a new parabolic data fitting method especially effective in the spectrum separation method.     

Entropy minimization and spectral dissimilarity curve resolution technique applied to nuclear magnetic resonance data sets

The use of entropy minimization and spectral dissimilarity is applied to three nuclear magnetic resonance (NMR) data sets. The data sets contain 2, 2, and 3 observables each. It was found that without any a priori information the sets of pure component spectra underlying the NMR spectroscopic observations could be extracted. These successful spectral resolutions suggest that a combined entropy minimization and spectral dissimilarity approach can be further developed for even larger NMR data sets containing a larger number of observables. Brief comparison to DECRA and PMF curve resolution results is also presented.     

On the voxel size and magnetic field strength dependence of spectral resolution in magnetic resonance spectroscopy

While the inherent low sensitivity of in vivo MR spectroscopy motivated a trend towards higher magnetic fields, B(0), it has since become apparent that this increase does not seem to translate into the anticipated improvement in spectral resolution. This is attributed to the decrease of the transverse relaxation time, T(2)*, in vivo due to macro- and mesoscopic tissue susceptibility. Using spectral contrast-to-noise ratio (SCNR) arguments, we show that if in biological systems the linewidth (on the frequency scale) increases linearly with the field, the spectral resolution (in parts per million) improves approximately as the fifth-root of B(0) for chemically shifted lines and decreases as about B(0)(4/5) (in hertz) for a structure of J-coupled multiplets. It is also shown that for any given B(0) there is a unique voxel size that is optimal in spectral resolution, linking the spectral and spatial resolutions. Since in practical applications the spatial resolution may be dictated by the target anatomy, nomograms to determine the B(0) required to achieve the desired spectral resolution at that voxel size are presented. More generally, the scaling of the nomograms to determine the achievable spectral and spatial resolutions at any given field is described.     

大孔径空间外差干涉光谱成像技术多谱段成像仿真

在大孔径空间外差干涉光谱成像技术（LASHIS）的基础上提出了一种多谱段成像方案.其采用LASHIS的外差探测原理,一方面,可通过较少的采样点数实现很高的光谱分辨率,保留了LASHIS的高光谱分辨率、高稳定性和高探测灵敏度的特点;另一方面,利用光栅的多级衍射性质,实现同一系统的多谱段同时探测,拓宽了光谱探测范围.首先,阐述了LASHIS多谱段成像方案的基本原理;然后,分析了多谱段探测与谱段解混方式;最后,对该方案进行了计算机仿真模拟,通过ZEMAX光线追迹的干涉图结果与理论计算结果相符合,验证了方案的正确性.基于LASHIS的多谱段成像方案所具有的高光谱分辨率、高探测灵敏度以及可实现同一系统的多谱段同时探测特点,尤其适合温室气体等高稳定性、高探测灵敏度的多谱段高光谱探测应用.     

Line shape considerations in ultrafast 2D NMR

We have recently proposed and demonstrated an approach that enables the acquisition of 2D nuclear magnetic resonance (NMR) spectra within a single scan. The approach is based on spatially encoding the spins' evolution along the indirect domain with the aid of a magnetic field gradient, and subsequently decoding this information numerous times over the course of the signal acquisition while spins are subject to a train of gradient echoes. The present paper discusses further considerations pertaining the 2D line shapes arising from this new way of collecting NMR data. Specific issues that are hereby addressed include (i) the effects introduced by fast relaxation onto the spatial encoding process, particularly the line widths and line shapes that will then arise in the frequency domain; (ii) approaches capable of correcting for the mixed-phase kernels resulting in these fast-relaxation cases, corresponding in essence to spatially encoded analogs of the TPPI and hypercomplex time-domain acquisition procedures; (iii) the enveloping characteristics imposed by the use of discrete excitation pulses on the attainable spectral widths along the indirect domain; and (iv) an analysis of the signal-to-noise characteristics of the methodology, with experimental corroborations of theoretical predictions and an illustration of the method's capabilities to analyze protein solutions in the mM-range concentration.     

空间调制干涉型阿达玛变换光谱成像仪

简要叙述了色散型阿达玛变换光谱成像仪的原理及仪器构成,指出其存在空间信息与光谱信息的错位和光谱分辨率受阿达玛编码模板码元宽度制约的缺陷.提出空间调制干涉型阿达玛变换光谱成像技术原理及仪器,利用横向剪切干涉仪获得所有阿达玛编码光信息在不同光程差处的干涉信号,对干涉谱进行傅哩叶变换和阿达玛变换得到目标的光谱.理论分析表明,干涉图的调制度不受阿达玛模板形状、大小等因素的影响,光谱分辨率与阿达玛模板的尺寸无关,不但避免了色散型阿达玛变换光谱成像仪中空间信息和光谱信息的错位,而且高能量通过率、高空间分辨率和高光谱分辨率成像容易同时实现.     

基于多分辨率融合距离的低质量星系光谱红移测量方法

星系光谱红移测量是大规模天体光谱巡天项目中的一个重要研究内容,其目的是从在光谱中测量出对应星系由于多普勒效应引起的红移。随着银河系外巡天项目的开展,观测目标距离(红移)越来越远,其星等越来越暗,光谱的质量也随之越来越差,如何能够有效准确地从这些低质量的光谱测量出红移是河外巡天面临的一个重要问题。基于此问题,充分考虑到低质量星系光谱的特点及数据特征,新定义了一种针对低质量巡天光谱数据的多分辨率融合距离,以此为基础提出一种针对低质量星系光谱的红移测量方法。该方法充分结合不同分辨率下光谱的特征,计算距离时首先将模板光谱和待测光谱同时降到多个相同分辨率下,该分辨率下所有波长采样点都计算一个偏差进而得到一个距离,然后将多个分辨率下得到的距离通过加权得到一个融合距离。基于多分辨率融合距离提出的星系红移测量方法,能够有效的解决低质量星系光谱的红移无法准确测量的问题。研究了不同信噪比下红移测量的精度,在信噪比大于5之后,该方法测量准确率可以达到90%以上。大量实验表明,提出的方法在星系光谱质量较低的情况可以非常准确地从中测量出红移,测量误差和红移大小无关,可以很好地应用于大规模巡天数据的星系光谱红移测量中。     

不同分辨率近红外光谱对汽车差速器油品牌鉴别的研究

研究了不同分辨近红外光谱对汽车差速器油品牌鉴别的效果与影响。分别采集了五种不同品牌汽车差速器油在4, 8, 16和32 cm-1分辨率下的透射光谱。去除采集波段范围两端噪声明显部分后采用10 522.28～4 443.425 cm-1范围光谱进行分析。不同分辨率光谱的主成分分析(principal component analysis, PCA)结果表明不同品牌差速器油可以被鉴别。同时基于不同分辨率光谱全谱所建立的偏最小二乘判别分析(partial least squares-discriminant analysis, PLS-DA)以及支持向量机(support vector machine, SVM)模型所获得的判别正确率相近且均高于90%。其中以8 cm-1分辨率下PLS-DA与SVM模型的判别正确率最高。上述研究结果表明分辨率对判别分析结果的影响较小。进一步对不同分辨率下的近红外光谱采用连续投影算法(successive projections algorithm,SPA)选择特征波数,发现不同分辨率下得到的特征波数均不相同,表明分辨率影响特征波数的选择。以不同分辨率特征波数建立的PLS-DA以及SVM模型取得了较好的判别分析效果,与以不同分辨率下全谱建立的判别分析模型效果相当。研究结果表明,分辨率对差速器油品牌鉴别影响较小,而对特征波数的选择影响较大,在实际应用中应考虑到分辨率对特征波数选择的影响。总体而言,不同分辨率的近红外光谱全谱与对应的特征波数都能有效的实现对汽车差速器油品牌的鉴别。     

非对称银纳米双环表面等离激元光谱特性

贵金属纳米材料在入射光激发下能够产生表面等离激元,即金属表面自由电子产生集体振荡。当其振荡频率与入射光频率相同时,发生表面等离激元共振,形成一种特殊的电磁场模式和光谱特性。利用该电磁场模式和光谱特性, 能够调节金属纳米材料的光谱学行为,例如通过改变金属纳米结构的大小、形状以及周围介质介电常数等参数, 在微纳尺度上实现光谱学信号的有效调控。目前,除了具有一定对称性的贵金属纳米材料被大量研究和应用外,非对称纳米结构的表面等离激元光谱特性也受到广泛关注。研究表明,在可见-近红外波段光谱范围内设计表面等离激元光电传感器件的关键问题在于,如何有效地调节其消光谱的共振波长、半峰宽以及峰值强度等主要特征参数。提出一种基于银纳米双环组成的非对称结构,利用时域有限差分方法,在可见-近红外波段内,通过分别改变银纳米双环的尺寸、间距及入射光偏振方向等参数,计算了该纳米结构在不同条件下的消光谱。结果表明,在0.4～3 μm的消光谱内,入射光能够激发产生两个独立的表面等离激元共振峰。通过研究峰值波长处的电场分布图发现,上述共振峰分别对应两种不同的电磁场模式。结果还表明,消光谱内两个独立的共振峰可以通过改变该双环结构的不同参数,被分别地进行调节。其中,可以通过改变该双环结构的半径来有效调节短波长峰的共振波长和半峰宽,同时保持长波长峰的共振波长和半峰宽基本不变。此外,通过改变两环间距或入射光偏振方向,可以分别以不同趋势来调节两个共振峰的峰值强度。在提出的非对称银纳米双环的消光谱中,获得了能够被分别调节的两个表面等离激元共振峰,研究结果能够为可见-近红外波段内基于银纳米材料光电传感器件的开发设计提供理论基础。     

用光学多通道分析仪系统作高分辨光谱测量的技术

通常的光多通道分析仪(OMA)存在着同时谱宽与分辨率的矛盾.本文所述方法,在不减小谱宽的前提下,可将该分析仪光谱分辨率提高1～2量级,使之可适用于高分辨率谱研究.     

观测平台转向时稀疏贝叶斯学习方位估计

针对观测平台转向时固定安装于其上的声呐线列阵指向快速变化导致的空间谱谱峰变宽问题,提出一种稀疏贝叶斯学习方位估计方法。该方法利用大地坐标系下不同接收快拍中的空域稀疏信号具有相同先验分布的特性,将转向过程中多个阵列指向分别对应的接收快拍信息联合处理,以求得目标方位。仿真分析与海试数据处理显示出,所提方法可以获得谱峰较尖锐的空间谱,具有较高的测向精度和角度分辨力,此外,对由左右舷模糊引起的伪峰有较强的抑制效果。所得结果表明,所提方法可以有效解决谱峰变宽问题,提升了平台转向时的方位估计性能,同时有效地利用阵列指向的变化提高了线阵抗左右舷模糊能力。      

Absorption spectra of unequal width bilayer graphene nanoribbons in a spatially modulated electric field

The optical absorption spectra of unequal width bilayer graphene nanoribbons can be effectively tuned by a spatially modulated electric field. The absorption spectra exhibit many prominent peaks’ structure owing to the one-dimensional subbands. The number, spectral intensity, and frequency of the absorption peaks depend sensitively on the magnitude, period and phase of the modulated electric potential. The relative displacement between the top and bottom nanoribbons also has strong influence on the spectra. For unequal width bilayer graphene nanoribbons without the interlayer hoppings, there exists an optical selection rule originating from the spatial symmetry of the electron wave functions. Most importantly, such a selection rule can be disrupted by the presence of the interlayer atomic interactions or a spatially modulated electric field. These theoretical predictions can be validated by absorption spectroscopy experiments.