MRI measurements of the dependence on T1 of the echo amplitudes using a multiple spin-echo scheme

Analytical calculations using the Bloch formalism were performed to assess the dependence on T₁ of the echo amplitudes for the Phase-Alternating Phase-Shift (PHAPS) multiple spin-echo protocol. Measurements in a 0.5 T MR imaging unit were performed to ratify the analytical results. especially for low T₂ values, the echo amplitudes were erroneous, with an increasing contribution from stimulated echo components with increasing T₁. Apart from affecting T₂ estimates, stimulated echoes generated a non-monoexponential signal decay of the echo trains. The results confirmed previous simulation studies as regards the dependence on T₁ of T₂ estimates from PHAPS.     

Identification of patients with hereditary haemochromatosis by magnetic resonance imaging and spectroscopic relaxation time measurements

A total of 4302 healthy blood donors were screened for elevated serum ferritin and transferrin saturation. Fifteen had increased serum ferritin at a follow-up examination. Five relatives of these donors also entered the study. Eleven patients had elevated liver iron concentrations, while five had normal liver iron concentrations. The R₂ relaxation rate in the liver was first measured with a conventional multi-spin-echo imaging sequence, and then by a volume-selective spectroscopic multi-spin-echo sequence, in order to achieve a minimum echo time of 4 msec. No correlation was found between the relaxation rate R₂ and the liver iron concentration, when R₂ was calculated from the imaging data. Multi-exponential transverse relaxation could be resolved when the spectroscopic sequence was used. A strong correlation between the initial slope of the relaxation curve and the liver iron concentration was found (r = 0.90, p < 0.001). Signal intensity ratios between liver and muscle were calculated from the first three echoes in the multi-echo imaging sequence, and from a gradient echo sequence. A strong correlation between the logarithm of the signal intensity ratios and the liver iron concentration was found. Although both spectroscopic T₂ relaxation time measurements and signal intensity ratios could be used to quantify liver iron concentration, the gradient echo imaging seemed to be the best choice. Gradient echo imaging could be performed during a single breath hold, so motion artifacts could be avoided. The accuracy of liver iron concentration estimates from signal intensity ratios in the gradient echo images was about 35%.     

铜化合物ESR g张量和A张量的相关性及其与结构的关系

对于某些含有特定配位原子的钢化合物,找出了钢的ESR g_‖和A_‖之间的线性相关性;总结了相关性与钢化合物的分子结构间的联系;认为相关性与分子对称性及分子轨道性有关,分子对称性的提高和钢-配体间成键分子轨道共价性的降低引起g_‖的增大和A_‖绝对值的减小,对某些钢化合物,g_‖与β₁²,A_‖与β₁²之间存在线性关系.总结出由实测的g_‖和A_‖估算分子轨道系数β₁²值的方法.     

Infrared fluorescence from exciting the v1 and v8 modes of CF2Cl2 with a TEA CO2 laser

Infrared fluorescence from CF₂Cl₂ molecules excited with a TEA CO₂ laser below dissociation threshold, is time resolved with a HgCdTe detector without spectral resolution in the range 1100-700 cm^-1. The signal fits well to a sum of three exponentials, which are interpreted as a bulk vibration-vibration (V-V) energy transfer between the fraction of highly excited and non-excited molecules in the irradiated volume, vibro-translation (V-T) deactivation and diffusional heat and mass transport to the surrounding. The measured V-V and V-T rates increase with vibrational excitation and the V-T deactivation is independent on the observed modes (v₁/v₆ or v₈) if v₈ or v₁ are excited respectively.     

Mo-SiO2-C3H6催化反应体系的ESR动态表征

文章考察了Mo-SiO₂催化剂体系在丙烯歧化反应过程中的动态ESR波谱,发现在催化歧化反应过程中有两种Mo⁵⁺顺磁中心,一种为扰动八面体配位,g_||=1.89,g_⊥=1.94;另一种为扰动四棱锥配位,g_||=1.86,g_⊥=1.95。测得同位素^95,97Mo⁵⁺的各向异性超精细耦合常数A_||=90.3×10^-4cm^-1,A_⊥=44.8×10^-4cm^-1;观测到反应产生的积炭信号,g≈2.0O₂;氧阴离子自由基信号g₁=2.018,g₂=2.011,g₃=2.005。用LCAO-MO理论对上述Mo⁵⁺的ESR波谱进行计算,求得分子轨道系数,发现并总结出△g_||/△g_⊥与△g成直线关系,并建议用△g_||/△g_⊥=4(△E(B₂→E))/(△E(B₂→B₁)(β₁/ε)²之比值作为衡量C_4v扰动程度的尺度。     

Selection of pulse sequences producing maximum tissue contrast in magnetic resonance imaging

The importance of spin density [N(H)] and spin-lattice (T₁) and spin-spin (T₂) relaxation in the characterization of tissue by nuclear magnetic resonance (NMR) is clearly recognized. This work considers which optimized pulse sequences provide the best tissue discrimination between a given pair of tissues. The effects of tissue spin density and machine-imposed minimum rephasing echo times (TE_MIN) for achieving maximum signal tissue contrast are discussed. A long TE_MIN sacrifices T₁-dependent contrast in saturation recovery (SR) and inversion recovery (IR) pulse sequences so that spin-echo (SE) becomes the optimum sequence to provide tissue contrast, due to T₂ relaxation. Pulse sequences providing superior performance may be selected based on spin density and T₁ and T₂ ratios for a given pair of tissues. Selection of the preferred pulse sequence and interpulse delay times to produce maximum tissue contrast is strongly dependent on knowledge of tissue spin densities as well as T₁ and T₂ characteristics. As the spin density ratio increases, IR replaces SR as the preferred sequence and SE replaces IR and SR as the pulse sequence providing superior contrast. To select the optimal pulse sequence and interpulse delay times, an accurate knowledge of tissue spin density, T₁ and T₂ must be known for each tissue.     

The adsorption of oxygen on Pd1/W(110)

The behavior of oxygen on Pd₁/W(110) has been investigated from 25 to 200 K by thermal desorption, UPS, XPS, and work function measurements. At 25 K only dioxygen species are present. A weakly bound O₂layer, containing O₂/Pd = 0.31 or 4.4 × 10¹⁴ O₂ molecules/cm² is desorbed at 35 K, leaving a coverage of O₂/Pd = 0.35 or 5 × 10¹⁴ O₂ molecules/cm². Heating to 200 K results in desorption of molecular O₂ as well as conversion to O, with O/Pd = 0.3. The molecular states, except the very weakly bound one, have high dipole moments with electron transfer to O₂, and thus correspond to Superoxide and peroxide species. These have UPS spectra quite different from physisorbed O₂. At 90 K adsorption is still mostly molecular with a sticking coefficient s near unity. At 200 K, adsorption is atomic with an initial s₀ = 0.8. This must be contrasted with Cu₁/W(110) where s₀ is unity even at 300 K. The difference can be explained by the much better size match of Pd and W, than Cu and W which makes it easier for Cu to take up momentum of impinging O₂ molecules.

The behavior of oxygen on Pd₁/W(110) is very similar to that on bulk Pd(111), suggesting that for oxygen the former surface resembles bulk Pd. This is not so for CO adsorption which is much weaker on Pd₁/W(110) than on bulk Pd. The reasons for this difference are not presently understood.     

In vivo relaxation of N-acetyl-aspartate, creatine plus phosphocreatine, and choline containing compounds during the course of brain infarction: a proton MRS study.

Localized water suppressed proton spectroscopy has opened up a new field of pathophysiological studies of severe brain ischemia. The signals obtained with the pulse sequences used so far are both T₁ and T₂ weighted. In order to evaluate the extent to which changes in metabolite signals during the course of infarction can be explained by changes in T₁ and T₂ relaxation times, eight patients with acute stroke were studied. STEAM sequences with varying echo delay times and repetition times were used to measure T₁ and T₂ of N-acetyl-aspartate (NAA), creatine plus phosphocreatine (Cr+PCr) and choline containing compounds (CHO) in a 27-ml voxel located in the affected area of the brain. Ten healthy volunteers served as controls. We found no difference in T₁ or T₂ of the metabolites between the patients and the normal controls. The T₂ of CHO was longer than that of NAA and Cr+PCr. Our results indicate that spectra obtained in brain infarcts and normal tissue with the same acquisition parameters are directly comparable with respect to relative signal intensities as well as signals scaled with internal and external standards.     

高价钼络合物化学——(Ⅱ) ESR和磁化率研究五价钼半胱氨酸体系的酸解反应

本文利用ESR和磁化率为手段研究了二钼半胱氨酯系列化合物,Na₂Mo₂(μ₂-X)(μ₂-Y)-O₂(cyst)₂(X=Y=O,(Ⅰ);X=O,Y=S,(Ⅱ);X=Y=S,(Ⅲ)),在不同浓度的盐酸溶液中离解的机制,并讨论了不同桥原子对该系列化合物化学稳定性的影响。     

Homonuclear correlation experiments for quadrupolar nuclei, spinning away from the magic angle

We present a set of homonuclear correlation experiments for half-integer quadrupolar spins in solids. In all these exchange-type experiments, the dipolar interaction is retained during the mixing time by spinning the sample at angles other than the “regular magic angle” (54.7°). The second-order quadrupolar interaction is averaged by different strategies for the different experiments. The multiple-quantum off magic angle spinning (MQOMAS) exchange experiment is essentially a regular MQMAS experiment where the quadrupolar interaction is averaged by combining magic angle spinning with a multiple- to single-quantum correlation scheme. The sample is spun at the magic angle at all times except during the mixing time which is added to establish homonuclear correlation. In the multiple-quantum P₄ magic angle spinning (MQP₄MAS) exchange experiment, the sample is spun at one of the angles at which the fourth-order Legendre polynomial vanishes (P₄ magic angle), the remaining second-order quadrupolar interaction now governed by a second-rank tensor is refocussed by the multiple to single-quantum correlation scheme. In the dynamic angle spinning (DAS) exchange experiment, the second-order quadrupolar interaction is averaged by correlating the evolution from two complementary angles. These experiments are demonstrated and compared, in view of their specific advantages and disadvantages, for ²³Na in the model compound Na₂SO₃.     

Developing of a fast cryodetector read-out for mass spectrometry

We report on the development of a SQUID read-out for a fast cryogenic detector with superconducting phase transition thermometer. To avoid the usual transformer matching and modulation–demodulation technique we are using a second SQUID to amplify the signal of a first stage input SQUID. A bandwidth of 2 MHz and a slew rate of 2×10⁵ Φ₀ s⁻¹ were achieved. The system is developed for the detection of large protein molecules in a time of flight mass spectrometer.     

Proton spectroscopy of human stroke: Assessment of transverse relaxation times and partial volume effects in single volume STEAM MRS

Proton T₂ relaxation times were measured in 13 stroke patients and 13 aged-matched normal subjects at 2.1 T. Spectra were acquired from an 8-cc volume using the STEAM sequence with echo times (TE) of 30.4 ms and 270.0 ms and repetition time of 2.8 s. Transverse relaxation times were estimated using two-point calculations. Percentage volume of infarct in the STEAM voxel was measured on spin-echo MRI encompassing the infarct and correlated with the peak amplitude of N-acetylated compounds (NA). T₂ values of NA, creatine, and choline resonances showed no significant difference between patients and controls. T₂ for lactate in patients was 780 ± 257 ms, respectively (mean ± SE, n = 7). In stroke patients, high inverse correlation was found between the absolute NA signal and partial volume of normal brain contributing to each spectrum (p < .001, r = 0.97). Together with unchanged T₂, this suggests that NAA largely disappears from infarcted tissue within 24 hr postinfarct.     

超声导波的频散补偿与模式分离算法研究

超声导波已被广泛应用于无损检测与评价,针对超声导波应用中普遍存在的模式混叠与频散问题,本文在采用频谱相位与信号时延函数建模导波频散信号传递系统的基础上,提出了一种单一模式的频散补偿算法。进一步地,通过将选择性频散补偿技术与导波模式分离相结合,本文算法克服了传统补偿算法所不能解决的多模式导波频散问题。板中的Lamb波A0,A1和S0混合模式仿真分析表明,本算法不仅可用于单一导波模式信号合成与频散补偿,而且可在实现多模式混合导波信号选择性频散补偿基础上,解决多模式导波信号中单一模式提取与分离。本文研究有助于超声导波多模式频散信号的分析与处理.      

相干瞬态微波波谱学

微波瞬态实验是研究气体分子转动弛豫过程的新的、有效的实验方法。本文将介绍相干瞬态微波波谱学中的瞬态吸收、瞬态发射和多脉冲序列实验,以及如何利用这些实验来测量弛豫时间T₁和T₂。     

CuTe,Cu2和Cu2Te的结构与势能函数

在Cu和Te的RECP(Relativistic Effective Core Potential)近似下,运用B3LYP方法,在LANL2DZ基组水平上对CuTe,Cu₂和Cu₂Te分子体系的结构进行优化计算.结果表明,CuTe和Cu₂分子的基电子状态分别为²Π和¹∑_g⁺,Cu₂Te分子的基态为单重态的C_2V构型,其电子状态为¹A₁.同时还计算了Cu₂Te分子基态的离解能、力常数和振动频率.采用最小二乘法拟合出CuTe和Cu₂分子Murrell-Sorbie势能函数参数.在此基础上,运用多体展式理论方法导出Cu₂Te分子基态势能函数的解析表达式,其势能面准确复现了平衡态的结构特征.     

D-T2二维核磁共振脉冲序列及反演方法改进设计

面对日益复杂的勘探对象, D-T₂二维核磁共振技术在实际应用中面临无法兼顾扩散系数测量范围和横向弛豫分辨率的困境. 脉冲序列作为D-T₂二维核磁共振数据采集的核心技术, 其性能优劣直接影响应用效果, 在综合对比PFG, STE-PFG, BP-PFG、改良式CPMG, 扩散编程, 多回波间隔CPMG脉冲序列性能的基础上, 有效融合脉冲梯度场、恒定梯度场D-T₂脉冲序列的优点, 本文提出一种基于脉冲梯度场的双变量、两窗口D-T₂脉冲序列改进设计. 针对两个窗口的D-T₂二维核磁共振数据反演, 为突破现有反演方法无法兼顾反演精度和解谱效率的瓶颈, 本着第二个窗口回波信号为主、第一个窗口回波信号为辅的原则, 本文提出一种同时使用两个窗口数据参与解谱的联合TSVD反演方法. 气水、油水、稠油、油气水模型不同信噪比条件下的数值模拟结果表明, 本文提供的D-T₂改进脉冲序列达到了平衡扩散系数测量范围和横向弛豫分辨率的设计要求, 本文提供的联合TSVD反演方法也有效平衡了反演精度要求和解谱效率. 文中的D-T₂改进脉冲序列及联合TSVD反演方法在复杂油气藏流体识别和产能预测中具有广泛的应用前景, 可为促进国内D-T₂二维核磁共振岩心分析技术的发展提供有利条件.     

Minimization of diffusive attenuation in T2-weighted NMR images of porous solids using turboSPI

This communication presents the results of T₂-weighted nuclear magnetic resonance imaging (MRI) of a water saturated porous volcanic rock using a fast single point imaging technique (turboSPI). Imaging porous materials with heterogeneously distributed mineral products, and air voids, using conventional imaging methods, which introduce T₂-weighting by increasing the time between the excitation and refocusing pulses, often results in high diffusive signal losses and susceptibility distortion. T₂-weighted images acquired of a water saturated porous rock using turboSPI with an effective echo time of 8.1 ms (actual inter-echo time of 0.9 ms) exhibit significantly decreased diffusive attenuation, compared to analogous images obtained with an inter-echo time of 8.1 ms.     

基于多弛豫信号补偿的快速磁共振T1ρ散布成像

定量磁共振成像（MRI）可量化组织特性,是科学研究和临床研究的重要工具.旋转坐标系下的自旋-晶格弛豫时间（T_1ρ）能反映水与大分子之间的低频交互作用,在3 T及以上的高场环境下,T_1ρ受水和不稳定质子之间化学交换的影响较大,通过测量弛豫率随自旋锁定场强度的变化而得到其分布情况（T_1ρ散布）,可用于分析和量化质子的交换过程,因此T_1ρ散布是一种重要的定量MRI技术.然而,获得不同自旋锁定场强下T_1ρ加权图像的时间过长,限制了其应用范围.针对这一问题,本研究提出一种基于多弛豫信号补偿策略的快速T_1ρ散布成像方法.该方法将不同锁定频率下的T_1ρ加权图像补偿到同一信号强度水平,并结合低秩与稀疏建立重建模型.实验结果表明,该方法在加速倍数高达7倍时仍获得了较好的重建结果.     

Nuclear quadrupole resonance echoes from hexamethylenetetramine

We investigated the echo phenomenon of nuclear quadrupole resonance (NQR) from hexamethylenetetramine (HMT). We detected the pure NQR echo signal of HMT with a short pulse interval. The intensity of the echo signal increased as the pulse interval time was decreased. We observed that a clean echo signal was generated even when the pulse interval was shorter than the decay time constant T₂^*. Since the short interval time gives a strong echo, our result insists that shorter interval time is preferred for the NQR detection.     

激光冷却OH分子的理论研究

采用高精度的多参考组态相互作用方法计算OH分子基态和第一激发态的势能曲线.为获得更精确的计算结果,在计算过程中考虑Davidson修正、标量相对论效应、核价相关效应和自旋轨道耦合效应.基于计算的Λ-S和?态的势能曲线,对一维径向薛定谔方程进行数值求解,得到各个电子态的光谱数据,与已报道的实验值和理论值相符合.获得OH分子的永久偶极矩、跃迁偶极矩、振动能级、Franck-Condon因子及辐射寿命,结果表明,A~2Σ~+→X~2Π跃迁具有高度对角化的Franck-Condon因子(0.9053)和短的辐射寿命(5.8363×10~(-7)s),符合激光冷却分子的条件.制定了激光冷却OH分子的具体方案,计算得到激光冷却跃迁A~2Σ~+→X~2Π所需的三束激光波长,主光束波长为307.1532 nm,两束重抽运激光波长为344.9163和349.7659 nm.计算结果为超冷OH分子的实验制备提供重要的理论依据.