MSE-MRI sequence optimisation for measurement of bi- and tri-exponential T2 relaxation in a phantom and fruit

The transverse relaxation signal from vegetal cells can be described by multi-exponential behaviour, reflecting different water compartments. This multi-exponential relaxation is rarely measured by conventional MRI imaging protocols; mono-exponential relaxation times are measured instead, thus limiting information about of the microstructure and water status in vegetal cells. In this study, an optimised multiple spin echo (MSE) MRI sequence was evaluated for assessment of multi-exponential transverse relaxation in fruit tissues. The sequence was designed for the acquisition of a maximum of 512 echoes. Non-selective refocusing RF pulses were used in combination with balanced crusher gradients for elimination of spurious echoes. The study was performed on a bi-compartmental phantom with known T₂ values and on apple and tomato fruit. T₂ decays measured in the phantom and fruit were analysed using bi- and tri-exponential fits, respectively. The MRI results were compared with low field non-spatially resolved NMR measurements performed on the same samples.     

Changes in Porcine Muscle Water Characteristics during Growth—An in Vitro Low-Field NMR Relaxation Study

This study investigates the effects of developmental stage and muscle type on the mobility and distribution of water within skeletal muscles, using low-field ¹H-NMR transverse relaxation measurements in vitro on four different porcine muscles (M. longissimus dorsi, M. semitendinosus, M. biceps femoris, M. vastus intermedius) from a total of 48 pigs slaughtered at various weight classes between 25 kg and 150 kg. Principal component analysis (PCA) revealed effects of both slaughter weight and muscle type on the transverse relaxation decay. Independent of developmental stage and muscle type, distributed exponential analysis of the NMR T₂ relaxation data imparted the existence of three distinct water populations, T_2b, T₂₁, and T₂₂, with relaxation times of approximately 1–10, 45–120, and 200–500 ms, respectively. The most profound change during muscle growth was a shift toward faster relaxation in the intermediate time constant, T₂₁. It decreased by approx. 24% in all four muscle types during the period from 25 to 150 kg live weight. Determination of dry matter, fat, and protein content in the muscles showed that the changes in relaxation time of the intermediate time constant, T₂₁, during growth should be ascribed mainly to a change in protein content, as the protein content explained 77% of the variation in the T₂₁ time constant. Partial least squares (PLS) regression revealed validated correlations in the region of 0.58 to 0.77 between NMR transverse relaxation data and muscle development for all the four muscle types, which indicates that NMR relaxation measurements may be used in the prediction of muscle developmental stage.     

Multi-component T1 relaxation and magnetisation transfer in peripheral nerve

We report here a study of longitudinal relaxation (T₁) and magnetisation transfer (MT) in peripheral nerve. Amphibian sciatic nerve was maintained in vitro and studied at a magnetic field strength of 3 T. A CPMG pulse sequence was modified to include either a saturation pulse to measure T₁ relaxation or an off-resonance RF irradiation pulse to measure MT. The resulting transverse relaxation (T₂) spectra yielded four components corresponding to three nerve compartments, taken to result from myelinic, axonal, and inter-axonal water, and a fourth corresponding to the buffer solution water in which the nerve sample was bathed. Each nerve component was analysed for T₁ relaxation and MT. All three nerve T₂ components exhibited unique T₁ relaxation and MT characteristics, providing further support for the assignment of the components to unique physical compartments of water. Numerical investigation of T_1sat measurements of each of the three nerve T₂ components indicates that while the two shorter-lived exhibit similar steady-state magnetisation transfer ratios (MTRs), their respective MT properties are quite different. Simulations demonstrate that mobile water exchange between these two components is not necessary to explain their similar steady-state MTR. In the context of the assignment of these two components to signal from myelinic and axonal water, this is to say that these two microanatomical regions of nerve may exhibit similar steady-state MTR characteristics despite possessing widely different MT exchange rates. Therefore, interpreting changes in MTR solely to reflect a change in degree of myelination could lead to erroneous conclusions.     

A Comparative Study of Water T1 and T2 Nmr Relaxation Times in Healthy and Pathological Blood Fluid

Water protons T₁ and T₂ relaxation times in samples of whole blood, obtained from healthy people and from patients affected by Macrocytic Anemia on one side and Lymphatic and Myeloid Leukemia on the other, have been measured with the FT NMR technique at 80 Mhz and at 25 °C. No significant difference with respect to the value of the spin lattice relaxation time parameter measured for the healthy control group is experimentally evident in the case of the Macrocytic Anaemia while the spin spin relaxation time increases in magnitude. On the reverse both the leukemic cases present a significant (p < 0.001) increase in the relaxation times with respect to the control group. The experimental relaxation data belonging to the anaemic case show a linear correlation with the red cells volume while that obtained for the two leukaemic cases appear linearly correlated with the total white cell numbers. From the relaxation data an estimate of the amount of water tightly bound to the white cells membrane can be determined which results roughly thirty times lower than that bound to the red cells membrane. In this work is also presented a step by step outline of the water relaxation behavior which starts with the pure water and ends with the water in the whole blood supported by relaxation experiments done on the isolated blood main components.     

Emulation of petroleum well-logging D-T2 correlations on a standard benchtop spectrometer

An experimental protocol is described that allows two-dimensional (2D) nuclear magnetic resonance (NMR) correlations of apparent diffusion coefficient D_app and effective transverse relaxation time T_2,eff to be acquired on a bench-top spectrometer using pulsed field gradients (PFG) in such a manner as to emulate D_app − T_2,eff correlations acquired using a well-logging tool with a fixed field gradient (FFG). This technique allows laboratory-scale NMR measurements of liquid-saturated cored rock to be compared directly to logging data obtained from the well by virtue of providing a comparable acquisition protocol and data format, and hence consistent data processing. This direct comparison supports the interpretation of the well-logging data, including a quantitative determination of the oil/brine saturation. The D − T₂ pulse sequence described here uses two spin echoes (2SE) with a variable echo time to encode for diffusion. The diffusion and relaxation contributions to the signal decay are then deconvolved using a 2D numerical inversion. This measurement allows shorter relaxation time components to be probed than in conventional diffusion measurements. A brief discussion of the numerical inversion algorithms available for inverting these non-rectangular data is included. The PFG-2SE sequence described is well suited to laboratory-scale studies of porous media and short T₂ samples in general.     

14N pulsed nuclear quadrupole resonance. 3. Effect of a pulse train. Optimal conditions for data averaging

The calculations developed in this paper aim at determining the optimal conditions of a NQR experiment when a transition is monitored by means of a pulse train with pulses of identical duration and signal acquisition after each pulse; coherences are assumed to vanish by effective transverse relaxation prior to every new pulse. These calculations demonstrate that, as in NMR, a steady state is effectively reached for any value of the recycle time. However, by contrast with NMR, it is shown that, for optimal data averaging under steady state conditions, the recycle time T can be kept as low as possible (the only limitation is the acquisition time). Nutation curves (signal amplitude versus pulse length) calculated in the steady state case are shown to depend strongly on the ratio T/T ₁ (T ₁: longitudinal relaxation time). The signal growth as a function of T/T ₁under averaging of the first transients has been evaluated as well as the number of pulses necessary for reaching a steady state.     

Effect of oxygen on the NMR relaxation properties of crude oils

Nuclear magnetic resonance relaxation measurements of bulk fluids provide a sensitive probe of the dynamics of molecular motion. Dissolved oxygen can interfere with this technique as its paramagnetic nature leads to a reduction of the paramagnetic relaxation times of the fluids. We studied this effect for the relaxation properties of crude oils that are in general characterized by a distribution of relaxation times. The samples were stock tank oils that have been exposed to air. We comparedT ₁ andT ₂ relaxation time distributions and their correlation functions of the initial (oxygenated) samples with those from the deoxygenated samples. Oxygen was removed from the oils with a freeze-thaw technique. As expected, the effect of oxygen is most apparent in oils with long relaxation times. In these oils the effect of oxygen can be described by an additional relaxation rate 1/T _1,2 ^ox to the transverse and longitudinal relaxation rates that is sample dependent but does not vary within the relaxation time distribution of the oil. Values of 1/T _1,2 ^ox for different crude oils were found to be in the range of 2.5 to 8.3 s. For crude oils that have components with relaxation times less than 100 ms, no significant oxygen effect is observed.     

Combining PFG and CPMG NMR measurements for separate characterization of oil and water simultaneously present in a heterogeneous system

When analyzingT ₂ relaxation time curves from an ordinary Carr-Purcell-Meiboom-Gill (CPMG) experiment in a multicomponent system, where internal magnetic field gradients broaden the line widths significantly, there is very little direct information regarding the mobility of the components and on the type of environment experienced by each component. Compared to a standard CPMG experiment, a combination of pulsed field gradient (PFG) methods with the CPMG experiment will increase the amount of information that is obtainable from the nuclear magnetic resonance (NMR) experiment on a system of components differing significantly in molecular mobility. We propose a method for achieving separate measurements of theT ₂ attenuation of two components simultaneously present within a sample, and we believe it to be generally valid for any system in which the components differ significantly in molecular mobility. The two components could be oil and water in porous rock, or fat and water in a biological tissue, where a separation of theT ₂ attenuations for the two components will add insight to the study of the systems. In order to verify the method we made use of a sample containing a mixture of oil and water in two separate bulk phases, and compared the results with PFG-CPMG experiments performed on samples containing oil or water only, respectively. The method was applied to systems containing glass spheres immersed in water and oil, and it was possible to obtain information about the physical environment of the components which otherwise is not easily obtainable. The method presented here is therefore presumably applicable to whole rock cores or tissue samples.     

NMR study of tomato pericarp tissue by spin-spin relaxation and water self-diffusion

Pericarp tissues of tomato varieties Quest and Cameron were studied by low-field nuclear magnetic resonance (NMR) at a controlled temperature of 20°C. The spin-spin relaxation times and the water diffusion coefficients were measured with Carr-Parcell-Meiboom-Gill and pulsed field gradient multi-spin-echo (PFGMSE) NMR sequences. Four relaxing components were extracted from the spin-spin relaxation. The components withT ₂=11 ms,T ₂=65 ms,T ₂=430 ms andT ₂=1500 ms were related to the nonexchangeable protons and water proton in each cell compartment (i.e., cell wall-extracellular space, cytoplasm and vacuole, respectively). In contrast to the relative intensities, theT ₂ values appeared insensitive to variety and harvest period. The difference in relative intensity was related to the size of the pericarp cell. The water self-diffusion coefficients for each cell compartment were determined simultaneously with the PFGMSE sequence. The water self-diffusion coefficients for the vacuole and cytoplasm were not affected by the harvest date or variety. However, the water self-diffusion in the cell wall-extracellular space was significantly different between the two varieties.     

In vivo NMR T2 relaxation of experimental brain tumors in the cat: A multiparameter tissue characterization

Experimental gliomas (F98) were inoculated in cat brain for the systematic study of their in vivo T₂ relaxation time behavior. With a CPMG multi-echo imaging sequence, a train of 16 echoes was evaluated to obtain the transverse relaxation time and the magnetization M(0) at time t = 0. The magnetization decay curves were analyzed for biexponentiality. All tissues showed monoexponential T₂, only that of the ventricular fluid and part of the vital tumor tissue were biexponential. Based on these NMR relaxation parameters the tissues were characterized, their correct assignment being assured by comparison with histological slices. T₂ of normal grey and white matter was 74 ± 6 and 72 ± 6 msec, respectively. These two tissue types were distinguished through M(0) which for white matter was only 0.88 of the intensity of grey matter in full agreement with water content, determined from tissue specimens. At the time of maximal tumor growth and edema spread a tissue differentiation was possible in NMR relaxation parameter images. Separation of the three tissue groups of normal tissue, tumor and edema was based on T₂ with T_2(normal) < T_2(tumor) < T_2(edema). Using M(0) as a second parameter the differentiation was supported, in particular between white matter and tumor or edema. Animals were studied at 1–4 wk after tumor implantation to study tumor development. The magnetization M(0) of both tumor and peritumoral edema went through a maximum between the second and third week of tumor growth. T₂ of edema was maximal at the same time with 133 ± 4 msec, while the relaxation time of tumor continued to increase during the whole growth period, reaching values of 114 ± 12 msec at the fourth week. Thus, a complete characterization of pathological tissues with NMR relaxometry must include a detailed study of the developmental changes of these tissues to assure correct experimental conditions for the goal of optimal contrast between normal and pathological regions in the NMR images.     

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.     

Mechanical detection of nuclear spin relaxation in a micron-size crystal

A room temperature nuclear magnetic resonance force microscope (MRFM), fitted in a 1 tesla electromagnet, has been used to measure the nuclear spin relaxation of ¹H in a micron-size (70 ng) crystal of ammonium sulfate. NMR sequences, combining both pulsed and continuous wave radio-frequency fields, have allowed us to measure mechanically T₂ and T₁, the transverse and longitudinal spin relaxation times. Because two spin species with different T₁ values are measured in our 7 μm thick crystal, magnetic resonance imaging of their spatial distribution inside the sample section have been performed. To understand quantitatively the measured signal, we carefully study the influence of spin-lattice relaxation and non-adiabaticity of the continuous-wave sequence on the intensity and time dependence of the detected signal. Received 23 February 2000     

Relaxation-Time and Diffusion NMR Microscopy of Single Neurons

Relaxation-time and diffusion-weighted NMR micrographs have been obtained for single neurons isolated from Aplysia californica. These images allow the nucleus and cytoplasm to be clearly differentiated, in contrast to proton spin-density images, which appear relatively homogenous. Images of the spatial distribution of T₁ and T₂ relaxivities and the diffusion coefficient (D), as well as average values for T₁, T₂, and D in the cytoplasm and nucleus, were calculated from sets of appropriately weighted images. In all cases, water in the nucleus had relaxation and diffusion properties markedly differing from those of cytoplasmic water, which in turn had properties which were distinct from those of free water. Additionally, the cytoplasmic T₂ was observed to triple following cell death, which is attributed to cytoplasmic dilution as water enters the cell. The work presented represents the first effort at a consistent exploration of the spatial distribution of NMR characteristics of water within intact single cells. These studies have implications both for modeling the NMR characteristics of water in neuronal tissues based on an understanding of the characteristics of water in different cell compartments and for understanding water/macromolecule interactions within cells. NMR microscopy studies such as these may help form a foundation for understanding and interpreting NMR characteristics measured from large assemblies of cells, i.e., spectroscopy and imaging of living tissues.     

Influence of magnetic impurities on proton spin relaxation of water in clay

The¹H nuclear magnetic spin relaxation of water in slurry of kaolin clay was investigated in the presence of magnetite (black iron oxide, Fe₃O₄) at 0.2 T and room temperature. The water spectra at high magnetite contents showed two different resonances, presumably from surface-associated water and free interstitial water. The difference in observed resonance frequencies increased as much as 200 ppm with increasing magnetite content. The apparent nuclear magnetic resonance intensity decreased biexponentially as a function of magnetite added. The observedT ₂^* values at low magnetite contents were in accordance with the predicted values from the resonance intensities and the estimated magnetic susceptibilities. TheT ₁ relaxation was multiexponential in character, so a uniform penalty program was used for the analysis of distribution. At 0.2 T for¹H, kaolin slurry containing less than 5.5 ppm magnetite did not differ significantly from magnetite-free clay in the longitudinal relaxation rates of water. However, higher concentrations of magnetite produced features in theT ₁ distribution significantly different from those of magnetite-free clay. TheT ₂ could be approximated by monoexponential relaxation, probably because the fast-decaying components relaxed before they could be recorded. The apparent transverse relaxation ratesR ₂ increased linearly as a function of magnetite content. On the basis of the comparison of spin-echo and Carr-Purcell-Meiboom-Gill data, an empirical relation was derived to describe the signal loss due to diffusion. It can be expressed by a power function of magnetite amount, which is multiplied by the sum of volume-dependent and volume-independent terms.     

NMR and quantum chemistry study of mesoscopic structuring in 3-methylpyridine/water/NaBr mixture in the vicinity of the phase separation boundary

²³Na and ⁸¹Br NMR spin-lattice relaxation times and signal half widths (Δ_1/2) have been measured in 3-methylpyridine (3MP)/H₂O/NaBr mixture along T?=?294 and 301 K isotherms gradually increasing the mass fractions of salt (X) up to the phase separation boundary. The extreme narrowing condition and thus 1/T ₁?=?1/T ₂?=?πΔ _1/2 was found to be valid in all cases. Discontinuous changes in slope of 1/T _1,2?=?f(X) were detected, and then corresponding points on the phase diagram (X, T) were attributed to the borderline between the molecular-ionic solution and the area of enhanced mesoscopic structuring. A very strong relaxation effect was observed for ⁸¹Br nuclei reaching relaxation rates of 14,000 s^–1. ²³Na and ⁸¹Br NMR relaxation data together with calculations of quantum chemistry model of electrical field gradient tensor evidence the migration of 3-methylpyridinium at increasing X from the anions hydration shells towards cations. An interchange from migration to steady distribution regimes is observed for anions and vice versa for cations at the borderline of the structured phase.     

Estimation of Saturation Exponent from Nuclear Magnetic Resonance (NMR) Logs in Low Permeability Reservoirs

The resistivity experimental measurements of 36 core samples, which were drilled from low permeability reservoirs of southwest China, illustrate that the saturation exponents are not agminate, but vary from 1.627 to 3.48; this leads to a challenge for water saturation estimation in low permeability formations. Based on the analysis of resistivity experiments, laboratory nuclear magnetic resonance (NMR) measurements for all 36 core samples, and mercury injection measurements for 20 of them, it was observed that the saturation exponent is proportional to the proportion of small pore components and inversely proportional to the logarithmic mean of NMR T ₂ spectrum (T _2lm). For rocks with high proportion of small pore components and low T _2lm, there will be high saturation exponents, and vice versa. The proportion of small pore components is characterized by three different kinds of irreducible water saturations, which are estimated by defining 30, 40 and 50 ms as T ₂ cutoffs separately. By integrating these three different kinds of irreducible water saturations and using T _2lm, a technique of calculating the saturation exponent from NMR logs is proposed and the corresponding model is established. The credibility of this technique is confirmed by comparing the predicted saturation exponents with the results from the core analysis. For more than 85 % of core samples, the absolute errors between the predicted saturation exponents from NMR logs and the experimental results are lower than 0.25. Once this technique is extended to field application, the accuracy of water saturation estimation in low permeability reservoirs will be improved significantly.     

NMR relaxometry study of gelatin based low-calorie soft candies

ABSTRACT

Soft candies are popular confectionery products. The most significant concern on the consumption of these products is the high amount of sugar and thus the high calories. The use of low-calorie sweeteners is a desirable trend in confectionery research. In this study, gelatin-based soft candies were formulated by using different sweeteners and their characterisation was performed using high and low field nuclear magnetic resonance (NMR) relaxation experiments. To complement the information obtained by NMR experiments, moisture content, water activity, texture analysis and differential scanning calorimeter experiments were also conducted. T₁ and T₂ relaxation times were measured at both low and high fields and also temperature-dependent measurements were conducted at the high field system. Candies were formulated by substitution of sucrose with maltitol, isomalt and stevia at 30%, 50% and 70% ratios. Significant difference was observed on relaxation times. T₁ values were best described by a mono-exponential model, whereas for T₂ relaxation times a bi-exponential model gave better results at both fields.     

Multi-components of T2 relaxation in ex vivo cartilage and tendon

The multi-components of T₂ relaxation in cartilage and tendon were investigated by microscopic MRI (μMRI) at 13 and 26 μm transverse resolutions. Two imaging protocols were used to quantify T₂ relaxation in the specimens, a 5-point sampling and a 60-point sampling. Both multi-exponential and non-negative-least-square (NNLS) fitting methods were used to analyze the μMRI signal. When the imaging voxel size was 6.76 × 10⁻⁴ mm³ and within the limit of practical signal-to-noise ratio (SNR) in microscopic imaging experiments, we found that (1) canine tendon has multiple T₂ components; (2) bovine nasal cartilage has a single T₂ component; and (3) canine articular cartilage has a single T₂ component. The T₂ profiles from both 5-point and 60-point methods were found to be consistent in articular cartilage. In addition, the depletion of the glycosaminoglycan component in cartilage by the trypsin digestion method was found to result in a 9.81–20.52% increase in T₂ relaxation in articular cartilage, depending upon the angle at which the tissue specimen was oriented in the magnetic field.     

DIRECT MEASUREMENT OF TRANSVERSE RELAXATION TIME OF INTERMOLECULAR MULTIPLE QUANTUM COHERENCES IN NMR

A one-dimensional NMR method is presented for measuring the transverse relaxation time, T_2,n, of intermolecular multiple quantum coherences (IMQCs) of coherence order n in highly polarized spin systems. The pulse sequence proposed in this paper effectively suppresses the effects of radiation damping, molecular diffusion, inhomogeneity of magnetic field, and variations of dipolar correlation distance, all of which may affect quantitation of T_2,n. This pulse sequence can be used to measure not only IMQC transverse relaxation time T_2,n(n>1) quickly and directly, but also the conventional transverse relaxation time. Experimental results demonstrate that the quantitative relationship between T_2,n(n≥1) and T₂ is T_2,n≈T₂/n. These results will be helpful for understanding the fundamental properties and mechanisms of IMQCs.