Proton dynamics in antiferroelectric CsH3(SeO3)2 single crystal investigated by 1H NMR measurements

¹H nuclear magnetic resonance (NMR) measurements have been performed to study the proton dynamics associated with the antiferroelectric transition of a hydrogen-bonded single crystal of CsH₃(SeO₃)₂. Herein, ¹H NMR spectrum, shift, linewidth, and spin-lattice relaxation rate 1/T₁ are measured in the temperature range of 80–296 K with the c-axis parallel to a magnetic field of ~4.85 T. The spectrum exhibits a composite structure with two narrow peaks at 296 K; at a low temperature, this structure evolves into a single broad shape with three humps. This complex shape and evolution are deconvoluted into five or six components based on the number of inequivalent and disordered hydrogen sites. By estimating the chemical shift and linewidth for each proton site, we identify all peaks. The spin-lattice relaxation recovery exhibits a double-exponential behavior with two relaxation times, short T_1S and extremely large T_1L. Both T_1S and T_1L follow Arrhenius behavior. From the respective 1/T₁(T), the activation energies for proton motion are measured to be small: 1.16 ± 0.1 and 0.83 ± 0.06 kJ/mol for T_1S and T_1L, respectively. While the static NMR data, chemical shift and linewidth, show no evidence for the transition, the dynamic data 1/T_1L highlights a clear increase across T_N = 145 K, which is possibly a signature of the transition.     

Magnetic properties of Heisenberg triangular antiferromagnet V15 cluster studied by NMR

The magnetic properties of an s?=?1/2 Heisenberg triangular antiferromagnet V15 have been studied by proton nuclear magnetic resonance (NMR) at very low temperature down to 100 mK using a He³-He⁴ dilution refrigerator. In total spin S _T?=?3/2 ground state above 2.7 Tesla, proton spin-lattice relaxation rate (1/T₁) shows thermally activated behavior as a function of temperature. On the other hand, a temperature independent behavior of 1/T₁ at very low temperatures is observed in frustrated S _T?=?1/2 ground state below 2.7 Tesla. Possible origins for the peculiar behavior of 1/T₁ will be discussed in terms of magnetic fluctuations due to spin frustration.     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Three-dimensional T1, T2 and proton density mapping with inversion recovery balanced SSFP

By combining a balanced steady-state free precession (bSSFP) readout with an initial inversion pulse, all three contrast parameters, T₁, T₂ and proton density (M₀), may be rapidly calculated from the signal progression in time. However, here it is shown that this technique is quite sensitive to variation in the applied transmit RF (B₁) field, leading to pronounced errors in calculated values. Two-dimensional (2D) acquisitions are taxed to accurately quantify the relaxation, as the short RF pulses required by SSFP's rapid TR contain a broad spectrum of excitation angles. A 3D excitation using a large diameter excitation coil was able to correctly quantify the parameters. While the extreme B₁ sensitivity was previously problematic and has precluded use of IR-bSSFP for relaxometry, in this work these obstacles were significantly reduced, allowing the rapid quantification of T₁, T₂ and M₀. The results may further be used to simulate image contrast from common sequences, such as a T₁-weighted or fluid-attenuated inversion recovery (FLAIR) examination.     

In vivo NMR imaging and T1 measurements of water protons in the human brain

Nuclear magnetic resonance (NMR) proton density images of the human brain have been made by the FONAR method. Spin-lattice relaxation times, T₁, of water hydrogen protons have been determined at random positions within frontal and temporal regions of the human brain. The primary purpose of this ongoing research is to accumulate a large data base of normal T₁ values for water protons in normal human brain tissue. Our experience to data includes 31 measurements on 18 volunteer subjects, and the mean value ± standard deviation is 215 ± 42 msec. In addition, two metastatic lesions of the brain were studied and found to have T₁ values longer than those for normal brain tissue.     

Automated Detection and Characterization of Multiple Sclerosis Lesions in Brain MR Images

In the present study an automatic algorithm for detection and contouring of multiple sclerosis (MS) lesions in brain magnetic resonance (MR) images is introduced. This algorithm automatically detects MS lesions in axial proton density, T₂-weighted, gadolinium enhanced, and fast fluid attenuated inversion recovery (FLAIR) brain MR images. Automated detection consists of three main stages: (1) detection and contouring of all hyperintense signal regions within the image; (2) partial elimination of false positive segments (defined herein as artifacts) by size, shape index, and anatomical location; (3) the use of an artificial neural paradigm (Back-Propagation) for final removal of artifacts by differentiating them from true MS lesions. The algorithm was applied to 45 images acquired from 14 MS patients. The algorithm’s sensitivity was 0.87 and the specificity 0.96. In 34 images, 100% of the lesions were detected. The algorithm potentially may serve as a useful preprocessing tool for quantitative MS monitoring via magnetic resonance imaging.     

In Vivo Relaxation Time Measurements in the Human Placenta Using Echo Planar Imaging at 0.5 T

This paper presents the first in vivo measurements of the nuclear magnetic resonance relaxation times T₁ and T₂ at 0.5 T in the human placenta from 20 weeks gestational age until term, in both normal and compromised pregnancies. T₁ measurements were performed by using both an inversion recovery sequence and the Look-Locher echo planar imaging (EPI) sequence on a total of 41 women with normal pregnancies and 11 women with compromised pregnancies. T₂ measurements were performed by using a spin-echo EPI sequence on 36 women with normal pregnancies and 14 women with compromised pregnancies. In normal pregnancies, both the T₁ values measured with the inversion recovery sequence and the T₂ values were found to decrease with gestational age, the linear regression results gave T₁=−9.1t+1538 r²=0.23 p=0.03 T₂=−4.0t+338 r²=0.47 p=4 10⁻⁶ where t is the gestational age in weeks, and T₁ and T₂ are the relaxation times in milliseconds. T₁ values measured very rapidly with the Look-Locher EPI sequence, but, therefore, with a much lower signal-to-noise ratio, showed no significant trends.The T₁ values measured in the abnormal group were significantly lower than those measured in the normal group. Four out of eight patients with compromised pregnancies had placental T₁ values lying outside the 90% confidence limits for the normal population based about the regression line, significantly more than expected by chance (p = 0.005). Ten out of fourteen of the T₂ measurements in the abnormal group were below the regression line established for the normal group, with 4 lying below the 90% confidence interval, although these trends were only just significant (p = 0.06 and p = 0.03).     

Proton resonance imaging and relaxation in raw and cooked hen eggs

In-vivo proton magnetic resonance imaging and relaxation time measurements were carried out on raw and cooked eggs. The relaxation time measurements revealed several interesting results. While there was very little change in the T₁ values between the raw and cooked egg white, the T₂ values followed the decreasing trend expected of water becoming bound. However, both the CH₂ and CH₃ moieties of the yolk lipids registered a statistically significant increase in their relaxation times. The T₂ of the CH₂ group, in particular, increased by 113% in the cooked egg. No striking changes, however, were observed between the proton images of the raw and cooked eggs.     

Proton activity and spontaneous strain of Cs3H(SeO4)2 in the phase transition at 369 K

We have investigated the origin of the change in proton activity in the phase transition at T_II-III (=369 K) in Cs₃H(SeO₄)₂ from the viewpoint of its ferroelasticity by using ¹H NMR and X-ray measurements. It is found that the second moment of the ¹H NMR absorption line rapidly decreases at T_II-III with increasing temperature. From this result, we conclude that the hopping motion of a proton, which is the precursor motion in the superprotonic phase, becomes more active above T_II-III. This result is consistent with the fact that the electrical conductivity in phase II is larger than that in phase III. Furthermore, it is also found that the spontaneous strain decreases abruptly at T_II-III. From these results, it is deduced that the decrease in the spontaneous strain at T_II-III causes the increase in the proton activity at T_II-III. In addition, it is deduced that the increase in proton activity and the decrease in the spontaneous strain at T_II-III are closely related with the appearance of the superprotonic phase transition at T_I-II (=456 K).     

Calculation of Irreducible Water Saturation (S wirr) from NMR Logs in Tight Gas Sands

It is difficult to calculate irreducible water saturation (S _wirr) from nuclear magnetic resonance (NMR) logs in tight gas sands due to the effect of diffusion relaxation on the NMR T ₂ spectrum at present. By combining with classical Timur and Schlumberger—Doll Research (SDR) models, a novel model of calculating S _wirr is derived. The advantage of this novel model is that S _wirr can be calculated without a T ₂ cutoff, and all input information can be acquired from NMR logs accurately. With the calibration of 36 core samples, which were drilled from Xujiahe Formation in Bao-jie region of Triassic, Sichuan basin, southwest China, the values of these statistic model parameters are defined. Field examples of tight gas sands show that the proposed model is reliable. The S _wirr calculated with the proposed model match well with core analyzed results both in tight gas formations and water-saturated layers, the absolute error is in the range of ±4%. The calculated results by using 20.75 ms as the T ₂ cutoff are accurate in water-saturated layers but are overestimated in gas-bearing intervals. Defining 33 ms as the T ₂ cutoff is unusable both in gas-bearing and water layers.     

NMR-Based Liquid Explosives Detector

Liquid explosives pose a threat to security on airplanes and other public places, since they can easily be concealed as benign liquids. A detector, able to quickly identify liquids, would increase the possibility to detect such threats and speed up security checks. As a step towards a long-term goal to develop a liquid explosive detector, we have constructed an experimental setup based on a low-cost 1.1 T permanent magnet with huge static magnetic field gradient of 4.8 T/m, which allows us to measure proton relaxation times T ₁ and T ₂ and the self-diffusion coefficient D in liquid samples in a thin slice excited by radio-frequency pulses. We have developed a simple model in order to explain diffusion-enhanced non-exponential magnetization recovery in inversion recovery T ₁ experiment in this setup. Measuring a wide variety of liquid samples, we have demonstrated that it is possible to discriminate between the liquids based solely on these parameters. We discuss further improvements to the detection method, among those the choice of magnetic field, based on the fast field-cycling measurements.     

Temperature dependence of the lattice parameters in the S1G and S2G phases of HxBPA

The temperature dependence of the monoclinic lattice constants in the two smectic phases, S¹_G and S²_G, of HxBPA, has been obtained from X-ray diffraction data, in the temperature range 300 K < T < 240 K. The variation of b (unique axis) is consistent with the chain ordering in the S²_G phase indicated by Raman and NMR measurements.     

Spin echo SPI methods for quantitative analysis of fluids in porous media

Fluid density imaging is highly desirable in a wide variety of porous media measurements. The SPRITE class of MRI methods has proven to be robust and general in their ability to generate density images in porous media, however the short encoding times required, with correspondingly high magnetic field gradient strengths and filter widths, and low flip angle RF pulses, yield sub-optimal S/N images, especially at low static field strength. This paper explores two implementations of pure phase encode spin echo 1D imaging, with application to a proposed new petroleum reservoir core analysis measurement.In the first implementation of the pulse sequence, we modify the spin echo single point imaging (SE-SPI) technique to acquire the k-space origin data point, with a near zero evolution time, from the free induction decay (FID) following a 90° excitation pulse. Subsequent k-space data points are acquired by separately phase encoding individual echoes in a multi-echo acquisition. T₂ attenuation of the echo train yields an image convolution which causes blurring. The T₂ blur effect is moderate for porous media with T₂ lifetime distributions longer than 5 ms. As a robust, high S/N, and fast 1D imaging method, this method will be highly complementary to SPRITE techniques for the quantitative analysis of fluid content in porous media.In the second implementation of the SE-SPI pulse sequence, modification of the basic measurement permits fast determination of spatially resolved T₂ distributions in porous media through separately phase encoding each echo in a multi-echo CPMG pulse train. An individual T₂ weighted image may be acquired from each echo. The echo time (TE) of each T₂ weighted image may be reduced to 500 μs or less. These profiles can be fit to extract a T₂ distribution from each pixel employing a variety of standard inverse Laplace transform methods. Fluid content 1D images are produced as an essential by product of determining the spatially resolved T₂ distribution. These 1D images do not suffer from a T₂ related blurring.The above SE-SPI measurements are combined to generate 1D images of the local saturation and T₂ distribution as a function of saturation, upon centrifugation of petroleum reservoir core samples. The logarithm mean T₂ is observed to shift linearly with water saturation. This new reservoir core analysis measurement may provide a valuable calibration of the Coates equation for irreducible water saturation, which has been widely implemented in NMR well logging measurements.     

Laser induced collisional energy transfer processes in a rubidium-potassium mixture

Two laser induced collisional energy transfer processes in alkali vapours are investigated. For process K(4²S_1/2)+ Rb(5²P_3/2)+?ω_T→K(6²S_1/2)+Rb(5²S_1/2), a population inversion between the final state K(6²S_1/2) and lower states, leading to a superradiant emission has been observed. Comparison between experiment and theory shows a qualitative agreement in the limit of low atomic densities and laser power. However the transfer profile width is about two times larger than the calculated one for both processes.     

Proton NMR relaxation study of the CsHSO4 solid acid system

At 141 °C the solid acid CsHSO₄ is known to undergo transition to a superprotonic phase that is characterized by dramatic (several-order-of-magnitude) increases in hydrogen ion conductivity. Proton NMR spin-spin relaxation time T₂ measurements reported here for CsHSO₄ also reveal substantial increases (factors of 20-30) in the vicinity of the transition temperature. In the temperature range just below the transition (70-136 °C), T₂ increases by a factor of order 10 relative to the rigid-lattice regime, suggesting motional narrowing of the NMR resonance line. In the regime of motional narrowing, the activation energy barrier to diffusion is 0.40 eV, as determined from the present T₂ results. NMR spin-lattice relaxation T₁ measurements also show behavior consistent with transition to a regime of rapid hydrogen motion. In particular, proton T₁'s decrease with temperature (from 80 to 120 °C), and then drop sharply near the transition temperature. Above the transition temperature, T₁ exhibits a minimum in which the correlation time is found to be ∼2 ns.     

基于差分进化算法的核磁共振T2谱多指数反演

提出了一种基于差分进化(DE)算法的核磁共振弛豫信号多指数反演新方法. 将核磁共振T₂谱反演问题转化为带非负约束的非线性优化问题,不需要预先给定横向弛豫时间T_{2分布,直接利用差分进化算法进行反演计算. 在测量信号低信噪比的情况下,计算机模拟和实验数据反演都表明了该方法在分析处理NMR弛豫信号中的有效性. 关键词： 核磁共振 多指数反演 差分进化 岩心分析}     

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.     

NMR Study of proton transport in crystalline antimonic acid

Pulsed ¹H NMR measurements are reported for polycrystalline antimonic acid (Sb₂O₅·nH₂O) in the temperature range 145 K<T<330 K at 60 MHz. Motional narrowing occurs at T>220 K and the relaxation behaviour is compatible with fast proton transport with a distribution of activation energies.     

A New Approach of Two-Dimensional the NMR Relaxation Measurement in Flowing Fluid

Driven-equilibrium fast saturation recovery (DEFSR), as a new method for two-dimensional (2-D) nuclear magnetic resonance (NMR) relaxation measurement based on pulse sequence in flowing fluid, is proposed. The two-dimensional functional relationship between the ratio of transverse relaxation time to longitudinal relaxation time of fluid (T ₁/T ₂) and T ₁ distribution is obtained by means of DEFSR with only two one-dimensional measurements. The rapid measurement of relaxation characteristics for flowing fluid is achieved. A set of the down-hole NMR fluid analysis system is independently designed and developed for the fluid measurement. The accuracy and practicability of DEFSR are demonstrated.