Comparison of multiple sclerosis clinical subgroups using navigated spin echo diffusion-weighted imaging.

The apparent diffusion coefficient (ADC) of tissue provides an indication of the size, shape, and orientation of the water spaces in tissue. Thus, pathologic differences between lesions in multiple sclerosis (MS) patients with different clinical courses may be reflected by changes in ADC measurements in lesions and white matter. Twelve healthy subjects and 35 MS patients with a relapsing-remitting (n = 10), benign (n = 8), secondary progressive (n = 8) and primary progressive (n = 9) clinical course were studied. T2-weighted and post-gadolinium T1-weighted images were obtained using a 1.5 T Signa Echospeed magnetic resonance imaging (MRI) system. Diffusion-weighted imaging was implemented using a pulsed gradient spin echo (PGSE) sequence with diffusion gradients applied in turn along three orthogonal directions in order to obtain the average apparent diffusion coefficient (ADCav). Navigator echo correction and cardiac gating were used to reduce motion artifact. ADC maps were derived using a two point calculation based on the Stejskal-Tanner formula. Diffusion anisotropy was estimated using the van Gelderen formula to calculate an anisotropy index. MS lesions had a higher ADC and reduced anisotropy compared with normal appearing white matter. Highest ADC values were found in gadolinium enhancing lesions and non-enhancing hypointense lesions on T1-weighted imaging. MS white matter had a slightly higher ADC and lower anisotropy than white matter of healthy subjects. Lesion and white matter ADC values did not differ between patients with different clinical courses of MS. There was no correlation between lesion ADC and disability. Diffusion-weighted imaging with measurement of ADC using the PGSE method provides quantitative information on acute edematous MS lesions and chronic lesions associated with demyelination and axonal loss but does not distinguish between clinical subtypes of MS.     

Determination of water self-diffusion coefficient in complex food products by low field 1H PFG-NMR: comparison between the standard spin-echo sequence and the T1-weighted spin-echo sequence

In 1990, Van Den Enden et al. proposed a method for the determination of water droplet size distributions in emulsions using a pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) T1-weighted stimulated-echo technique. This paper describes both the T1-weighted spin-echo sequence, an improved method based on this earlier work, and, the standard PFG spin-echo sequence. These two methods were compared for water self-diffusion coefficient measurement in the fatty protein concentrate sample used as a 'cheese model.' The transversal and longitudinal relaxation parameters T1 and T2 were determined according to the temperature and investigated for each sample; fat-free protein concentrate sample, pure anhydrous milk fat, and fatty protein concentrate sample. The water self-diffusion in fat-free protein concentrate samples followed a linear behavior. Consequently, the water self-diffusion coefficient could be easily characterized for fat-free protein concentrate samples. However, it seemed more complicated to obtain accurate water self-diffusion in fatty protein concentrate samples since the diffusion-attenuation data were fitted by a bi-exponential function. This paper demonstrates that the implementation of the T1-weighted spin-echo sequence, using the different T1 properties of water and fat phases, allows the accurate determination of water self-diffusion coefficient in a food product. To minimize the contribution of the 1H nuclei in the fat phase on the NMR echo signal, the fat protons were selectively eliminated by an additional 180 degrees pulse. This new method reduces the standard errors of diffusion data obtained with a basic spin-echo technique, by a factor of 10. The effectiveness of the use of the T1-weighted spin-echo sequence to perform accurate water self-diffusion coefficients measurement in fatty products is thus demonstrated.     

In vivo visualization of displacement-distribution-derived parameters in q-space imaging

OBJECTIVE: This study aimed to explore the potential of in vivo q-space imaging in the differentiation between different cerebral water components. MATERIALS AND METHODS: Diffusion-weighted imaging was performed in six directions with 32 equally spaced q values and a maximum b value of 6600 s/mm(2). The shape of the signal-attenuation curve and the displacement propagator were examined and compared with a normal distribution using the kurtosis parameter. Maps displaying kurtosis, fast and slow components of the apparent diffusion coefficients, fractional anisotropy and directional diffusion were calculated. The displacement propagator was further described by the full width at half and at tenth maximum and by the probability density of zero displacement P(0). Three healthy volunteers and three patients with previously diagnosed multiple sclerosis (MS) were examined. RESULTS: Simulations indicated that the kurtosis of a signal-attenuation curve can determine if more than one water component is present and that care must be taken to select an appropriate threshold. It was possible to distinguish MS plaques in both signal and diffusional kurtosis maps, and in one patient, plaques of different degree of demyelinization showed different behavior. DISCUSSION: Our results indicate that in vivo q-space analysis is a potential tool for the assessment of different cerebral water components, and it might extend the diagnostic interpretation of data from diffusion magnetic resonance imaging.     

In vivo determination of multiexponential T2 relaxation in the brain of patients with multiple sclerosis

In vivo measurement of T2 relaxation times in multiple sclerosis (MS) lesions by magnetic resonance imaging (MRI) is potentially useful for the evaluation of the disease activity. Seven patients with definite MS were investigated over a period of three years (19 examinations), using a whole-body MRI scanner operating at 0.15 T with a specially designed high-power radio-frequency head coil. A modified CPMG sequence with a 180 degree pulse interval of TE = 6 msec and 128 echoes was used for the T2 relaxation measurement of the areas of increased signal (AIS) and white matter (WM). A biexponential T2 analysis of each pixel of the spin-echo images was computed. The T2 relaxation processes were found to be a monoexponential function in WM. The T2 relaxation times of apparently normal white matter in MS patients was significantly longer than in control subjects. The T2 relaxation curves of the AIS were found in most cases to fit a biexponential function characterized by a short and a long T2. T2 long relaxation times of AIS were spread out over a wide range (150-560 msec). The study of T2 long histograms shows that some AIS can be divided into two or three parts depending on the T2 long values. Each of these parts may correspond to a pathological process such as edema, demyelination and gliosis. Evolution of T2 relaxation times over a period of time cannot as yet be correlated with modifications in the clinical state.     

In vivo evaluation of the reproducibility of T1 and T2 measured in the brain of patients with multiple sclerosis.

The precision (reproducibility) of relaxation times derived from magnetic resonance images of patients with multiple sclerosis (MS) were investigated. Measurements of 10 MS patients were performed at 1.5 T on two occasions within 1 wk. T1 and T2 was measured using a partial saturation inversion recovery sequence (6 points) and a Carr-Purcell-Meiboom-Gill phase alternating-phase shift multiple spin-echo sequence with 32 echoes. Regions of interest (ROI) were placed both in apparently normal white matter and plaques. The precision (+/- 1.96 SD) and the confidence intervals for T1 and T2 for white matter and plaques were calculated. The precision of T1 for white matter and plaques was respectively +/- 94 msec and +/- 208 msec. The precision of T2 for white matter and plaques was respectively +/- 18 msec and +/- 26 msec. For all measurements the coefficient of variation was about 9%. Judging from our own study and others as well, a precision better than 10% for T1 and T2 would seem unrealistic at present.     

Insights into brain microstructure from the T2 distribution

T2 weighting is particularly sensitive, but notoriously unspecific, to a wide range of brain pathologies. However, careful measurement and analysis of the T2 decay curve from brain tissue promise to provide much improved pathological specificity. In vivo T2 measurement requires accurate 180 pulses and appropriate manipulation of stimulated echoes; the most common approach is to acquire multiple echoes from a single slice. The T2 distribution, a plot of component amplitude as a function of T2, can be estimated using an algorithm capable of fitting a multi-exponential T2 decay with no a priori assumptions about the number of exponential components. T2 distributions from normal brain show peaks from myelin water, intra/extracellular water and cerebral spinal fluid; they can be used to provide estimates of total water content (total area under the T2 distribution) and myelin water fraction (MWF, fractional area under the myelin water peak), a measure believed to be related to myelin content. Experiments on bovine brain suggest that magnetization exchange between water pools plays a minor role in the T2 distribution. Different white matter structures have different MWFs. In normal white matter (NWM), MWF is not correlated with the magnetization transfer ratio (MTR) or the diffusion tensor fractional anisotropy (FA); hence it provides unique information about brain microstructure. Normal-appearing white matter (NAWM) in multiple sclerosis (MS) brain possesses a higher water content and lower MWF than controls, consistent with histopathological findings. Multiple sclerosis lesions demonstrate great heterogeneity in MWF, presumably due to varying myelin contents of these focal regions of pathology. Subjects with schizophrenia were found to have significantly reduced MWF in the minor forceps and genu of the corpus callosum when compared to controls, suggesting that reduced frontal lobe myelination plays a role in schizophrenia. In normal controls, frontal lobe myelination was positively correlated with both age and education; this result was not observed in subjects with schizophrenia. A strong correlation between MWF and the optical density from the luxol fast blue histological stain for myelin was observed in formalin-fixed brain, supporting the use of the MWF as an in vivo myelin marker.     

Self-diffusion of water and benzene molecules adsorbed in synthetic opal samples

The self-diffusion of adsorbed molecules of water and benzene in porous synthetic opals has been studied by pulsed field gradient nuclear magnetic resonance. The study indicates two “phases” of water molecules differing by the self-diffusion coefficients, indicating two types of porosities existing in the synthetic opals. The smallest self-diffusion coefficient characterizes water in ultramicropores found on the surface of nanospheres. The form of the diffusion decay depends on temperature in the region of high temperatures as a result of exchange between pores of different sizes. The ultramicropores are inaccessible for the largest benzene molecules. Water adsorption and self-diffusion data confirm that annealing of the opal samples at 1293 K caused the collapse of ultramicropores.     

Structural and dynamic properties of polyoxyethylene sorbitan monooleate micelle in water dispersion studied by pulsed field gradient NMR

The diffusion phenomenon of a nonionic surfactant, polyoxyethylene sorbitan monooleate (POE-SMO), micelle in aqueous solution was investigated by pulsed field gradient nuclear magnetic resonance (PFG NMR) with a high gradient strength of 17.4 T/m at the diffusion timet _d varied from 3 to 300 ms. This high gradient strength allowed us to measure the slow self-diffusion coefficient of POE-SMO micelle, and the short diffusion time below 10 ms showed the restricted diffusion of the micelle. At the shortt _d the self-diffusion of the micelle was restricted and the restricted sizes were 1.8, 1.5, and 0.8 μm for the POE-SMO concentration of 100, 200 and 300 mM, respectively, and 0.6 μm for the POE-SMO only. The possible reason of this restriction was assumed to be the formation of a spatial network or a micellar clustering. Furthermore, a proton exchange between water molecule and surfactant OH group on the micelle surface was proposed. With respect to this proposal, the residence time of the proton at the micelle surface and the thickness of the surface were investigated from proton self-diffusion coefficients by PFG NMR.     

Self-diffusion measurements by a mobile single-sided NMR sensor with improved magnetic field gradient

A simple and fast method of measuring self-diffusion coefficients of protonated systems with a mobile single-sided NMR sensor is discussed. The NMR sensor uses a magnet geometry that generates a highly flat sensitive volume where a strong and highly uniform static magnetic field gradient is defined. Self-diffusion coefficients were measured by Hahn- and stimulated echoes detected in the presence of the uniform magnetic field gradient of the static field. To improve the sensitivity of these experiments, a Carr-Purcell-Meiboom-Gill pulse sequence was applied after the main diffusion-encoding period. By adding the echo train the experimental time was strongly shortened, allowing the measurement of complete diffusion curves in less than 1min. This method has been tested by measuring the self-diffusion coefficients D of various organic solvents and poly(dimethylsiloxane) samples with different molar masses. Diffusion coefficients were also measured for n-hexane absorbed at saturation in natural rubber with different cross-link densities. The results show a dependence on the concentration that is in good agreement with the theoretical prediction. Moreover, the stimulated-echo sequence was successfully used to measure the diffusion coefficient as a function of the evolution time in systems with restricted diffusion. This type of experiment proves the pore geometry and gives access to the surface-to-volume ratio. It was applied to measure the diffusion of water in sandstones and sheep Achilles tendon. Thanks to the strong static gradient G(0), all diffusion coefficients could be measured without having to account for relaxation during the pulse sequence.     

In situ observation of diffusion and reaction dynamics in gel microreactors by chemically resolved NMR microscopy

The enzymatically catalyzed esterification reaction of propionic acid+1-butanol ⇆ propionic-acid-butyl-ester+water inside an immobilizing hydrogel environment has been investigated by means of spectroscopically resolved nuclear magnetic resonance (NMR) imaging. The alginate hydrogel was in the shape of a 3–4 mm diameter bead, with both the gel-forming water and the solvent (cyclohexane) being fully deuterated to simplify the identification of small amounts of reactants. In the absence of enzymes, the self-diffusion process of the separate reactants (propionic acid and butanol) proved to be severely slowed down compared to pure Fickian self-diffusion, and the concentration buildup inside the gel bead was shown to depend strongly on the properties of the reactants. Two-dimensional, non-chemically resolved images revealed that the diffusion process is not radially symmetric as expected, thus complicating the modelling of the diffusion and reaction kinetics. The reaction itself has been observed with chemical resolution in a time series of up to 40 h, clearly demonstrating the reduction of 1-butanol and production of water inside the gel bead.     

On the problem of field-gradient NMR measurements of intracrystalline diffusion in small crystallites--water in NaA zeolites as an example

Necessary conditions for measuring intracrystalline diffusion in small crystal size systems via field-gradient NMR are discussed. As an illustrative case self-diffusion coefficients of water adsorbed in NaA zeolites (average crystal diameter about 1 μm) have been measured by ¹H-NMR stimulated echoes in static magnetic field gradients of up to 180 T/m in the temperature range of 254–344 K. Obtaining intracrystalline diffusion coefficients necessitates a sufficiently high spatial resolution only provided by such large field gradients.     

水在不均匀纳米通道内自扩散性质的分子动力学研究

本文用分子动力学(MD)方法模拟了受限水在不均匀润湿性微通道中的自扩散性质.通过考察不均匀润湿通道内水的自扩散行为,发现在微通道中水的扩散性质表现出明显的尺度效应,随着通道高度的增加水的扩散增强.更重要的是,由于通道高度的不同,通道内的不均匀润湿段对水的扩散与均匀通道相比有不同的影响.当通道高度为0.8 nm时,不均匀润湿通道内水的扩散增强;当通道高度为1.0 nm时,不均匀润湿通道内水的扩散减弱;而当通道高度达到1.2 nm时,不均匀润湿通道内水的扩散基本相同.     

Water diffusivity in model biological membranes

Water self-diffusion was studied in model biological membranes (lipid bilayers of dioleoylphosphatidylcholine (DOPC) and dimyristoylphosphatidylcholine) by nuclear magnetic resonance with pulsed field gradient. The results for DOPC-water bilayers for three different orientations of the angle ? between the direction of the field gradient and the normal to the bilayer (?=57.4, 90, 0°) were presented. The differences in the diffusion decay shapes and values of the diffusion coefficients, obtained at different bilayer orientations and demonstrating highly anisotropic diffusion of the interbilayer water was discussed. This study also has shown some features of water diffusion in model lipid bilayers at the concentrations corresponding to the presence of bound and quasi-free water. The dependence of the water and lipid diffusion on the water content was considered from the point of view of the bilayers hydration and types of interbilayer water.     

Asymmetric self-diffusion with orientation-dependence of water molecule in finite timescale

Self-diffusion of water has been investigated by molecular dynamics simulations. It was found that the preference of the direction in self-diffusion of water is orientation dependent in a finite time. For a time of ~100 ps, there are more possibilities for water molecules moving along the initial dipole orientation than in the opposite direction. This reveals that self-diffusion of water molecules is asymmetric in a finite time. We tested four water models and found that they all show similar asymmetric diffusion, indicating that asymmetric diffusion of water is intrinsic behavior rather than induced by the water model. These results are important for understanding and application of asymmetric diffusion in research fields such as biological water and confined water in small dimensions.     

Ionic and molecular Self-Diffusion in Ion-Exchange materials for fuel energetics studied by pulsed field gradient NMR

Pulsed field gradient nuclear magnetic resonance technique was applied to investigate the self-diffusion mechanism of water, alcohol molecules and Li⁻ counterions in sulfocation exchangers with different structures of the polymeric matrix. It was shown that in the homogeneous perfluorinated sulfocation exchange membranes the ionic and water translation motions are controled by the hydrogen bond network forming in ionogenic channels at the high water content. At the low solvent content, the self-diffusion coefficients of methanol and ethanol are higher than the water self-diffusion coefficients. The influence of non-ion-exchange sorbed electrolyte on Li⁺ self-diffusion coefficients was observed in the heterogeneous sulfocation exchanger KU-23.     

Enhanced sensitivity to molecular diffusion with intermolecular double-quantum coherences: implications and potential applications

Apparent molecular self-diffusion rates for (1)H intermolecular double-quantum coherences (iDQCs) were measured in solvents covering a wide range of intrinsic diffusion coefficients at 1.5, 9.4 and 14T, and water iDQC diffusion-weighted images were obtained at 1.5T in human brains and at 9.4T in rat brains. Conventional single quantum coherence (SQC) measurements were also made in the same samples. Experimental results indicate that iDQCs are approximately twice as sensitive to diffusion as SQC. A general theoretical expression was derived, and a model was proposed to explain the phenomenon. Potential applications in DWI and brain fMRI were also discussed.     

Dynamic properties of water in silicalite-1 powder

Self-diffusion of D₂O in partially filled silicalite-1 crystals was studied at 25°C by ²H nuclear magnetic resonance (NMR) with bipolar field gradient pulses and longitudinal Eddy-current-delay. For the first time, reliable experimental diffusion data for this system were obtained. Analysis of NMR diffusion decays revealed the presence of a continuous distribution of apparent self-diffusion coefficients (SDCs) of water, ranging from 10⁻⁷ to ∼10⁻¹⁰ m²/s, which include values much higher and lower than that of bulk water (∼10⁻⁹ m²/s) in liquid phase. The observed distribution of SDC changes with variation of the diffusion time in the range of 10–200 ms. A two-site Kärger exchange model was successfully fitted to the data. Finally, the water distribution and exchange in silicalite-1 pores were described by taking into account (a) a gas-like phase in the zeolite pores, a gas-like phase in mesopores and an intercrystalline gas-like phase and (b) intercrystalline liquid droplets with intermediate exchange rate with the other phases. The other phases experience fast exchange on the NMR diffusion time scale. Diffusion coefficients and mean residence times of water in some of these states were estimated.     

A study of rotationally invariant and symmetric indices of diffusion anisotropy.

This study investigated the properties of a class of rotationally invariant and symmetric (relative to the principal diffusivities) indices of the anisotropy of water self-diffusion, namely fractional anisotropy (FA), relative anisotropy (RA), and volume ratio (VR), with particular emphasis to their measurement in brain tissues. A simplified theoretical analysis predicted significant differences in the sensitivities of the anisotropy indices (AI) over the distribution of the principal diffusivities. Computer simulations were used to investigate the effects on AI image quality of three magnetic resonance (MR) diffusion tensor imaging (DTI) acquisition schemes, one being novel: the schemes were simulated on cerebral model fibres varying in shape and spatial orientation. The theoretical predictions and the results of the simulations were corroborated by experimentally determined spatial maps of the AI in a normal feline brain in vivo. We found that FA mapped diffusion anisotropy with the greatest detail and SNR whereas VR provided the strongest contrast between low- and high-anisotropy areas at the expense of increased noise contamination and decreased resolution in anisotropic regions. RA proved intermediate in quality. By sampling the space of the effective diffusion ellipsoid more densely and uniformly and requiring the same total imaging time as the published schemes, the novel DTI scheme achieved greater rotational invariance than the published schemes, with improved noise characteristics, resulting in improved image quality of the AI examined. Our findings suggest that significant improvements in diffusion anisotropy mapping are possible and provide criteria for the selection of the most appropriate AI for a particular application.     

Water self-diffusion in Chlorella sp. studied by pulse field gradient NMR

The water self-diffusion behavior in chlorella water suspension was investigated by pulsed field gradient NMR technique. Three types of water was determined, which differs according to the self-diffusion coefficients; bulk water, extracellular and intracellular water. Intracellular and extracellular water self-diffusion were restricted, and the sizes of restriction regions were 3.4 microm and 17 microm, respectively. The water molecular exchange process between these three diffusion regions was investigated. The residence time and exchange rate constant for chlorella cells were obtained. The cell wall permeability determined from the rate constant as 3 x 10(-6) m/s agreed with the permeability 10(-6) m/s obtained from time dependence of intracellular water self-diffusion coefficient. The structural cluster model of chlorella cell is estimated to describe the extracellular water self-diffusion in chlorella water suspension.     

Novel MR image contrast mechanisms in epilepsy

A range of magnetic resonance (MR) parameters are introduced, which can give rise to image contrast by using suitable pulse sequences, and that can be measured quantitatively. Their relationship to tissue pathology is given as far as possible. Techniques for their measurement, and results from multiple sclerosis, stroke, and epilepsy are given. The parameters are proton density, T₁, T₂, transverse magnetisation decay, which gives estimates of extracellular water and myelin concentrations, magnetisation transfer ratio and T_1sat, and diffusion (including trace and anisotropy measured from the tensor matrix).