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PubMed2002

Study of pediatric brain development using magnetic resonance imaging of anisotropic diffusion.

The properties of water diffusion in human brain tissue can be characterized by diffusion tensors computed from diffusion weighted magnetic resonance images. Since these properties are strongly determined by the structural and geometrical characteristics of the tissue, the maturation process of white matter and gray matter tissue can be expected to be reflected in these images and derived tensor quantities. The purpose of this work was therefore to study the development of pediatric brain in terms of changes occurring in the observed diffusion behavior. Echo planar diffusion tensor imaging was performed on 22 (10 females and 12 males) full term newborn and infant patients, diagnosed in retrospect as neurologically healthy. The subjects were subdivided in three age categories. A number of quantities based on the diffusion images were calculated for each tissue type and age category, and the ability of these quantities to provide sensitive and consistent information about the tissue differences and evolution was evaluated. The results clearly illustrate that the rotationally invariant quantities (e.g., the highest diffusivity, anisotropy ratio and volume ratio) are superior to the rotationally variant ones (e.g., ADCs measured along the three axes of the magnet) often used in the clinic. On the basis of the anisotropy ratio and the volume ratio indices, a correlation between the white matter maturation and the evolution of the diffusion anisotropy could be established. The same quantities did not exhibit any age dependence for the gray matter tissues.

Anisotropy↗

PubMed1998

A theoretical study of the effect of experimental noise on the measurement of anisotropy in diffusion imaging.

Diffusion tensor imaging (DTI) is a modality known to be highly sensitive to the detrimental effects of experimental noise. Here, using Monte Carlo simulations, we compare and contrast how noise complicates the measurement of diffusion anisotropy in diffusion tensor and conventional diffusion-weighted imaging (DWI). As the signal-to-noise ratio (SNR) decreases below a value of approximately 20, the eigenvalues (lambda(i)) of the diffusion tensor D are found to diverge rapidly from their true values, with the result that the measured anisotropy can be significantly in error and isotropic structures falsely assigned a high level of anisotropy. The effect of noise on the rotationally variant indices, calculated from a conventional diffusion-weighted imaging experiment, is found to be much less insidious, because the apparent diffusion coefficients (ADCs) diverge only slowly as the signal-to-noise decreases. Thus, although rotationally variant indices almost always underestimate the true diffusion anisotropy, they show only a small susceptibility to experimental noise and hence, are preferred to their rotationally invariant counterparts when the signal-to-noise ratio is small.

Anisotropy↗

PubMed1998

Properties of a peptide containing DL-lactide/glycolide copolymer nanospheres prepared by novel emulsion solvent diffusion methods.

Novel emulsion solvent diffusion methods in water or oil were developed to prepare the peptide (TRH and elcatonin)-loaded PLGA nanospheres, via the coprecipitation of drug and polymer in the emulsion droplets induced by the diffusion of solvent. The PLGA nanospheres prepared by the emulsion solvent diffusion method in water were unimodally dispersed particles with an average diameter of about 250 nm, whereas those prepared by the emulsion solvent diffusion method in oil and the phase separation method exhibited bimodal distribution with average diameter of about 700 and 800 nm, respectively. The content and recovery of the peptide and nanospheres prepared by the emulsion solvent diffusion method in oil were superior to those of nanospheres prepared using other methods. The drug release properties from nanospheres depended strongly on their preparation processes. The nanospheres prepared by the emulsion solvent diffusion method in oil, released the drug continuously over 14 days, the rate of which was determined by the diffusion of drug in the rigid matrix structure of the nanosphere. The drug release behavior of nanospheres prepared by the phase separation method exhibited a triphasic release pattern with an initial burst and an induction period followed by the diffusion of drug through the walls of the reservoir type nanospheres, which were produced via the phase separation of polymer deposited around the emulsion droplets of the drug.

Biocompatible Materials↗

PubMed2003

De-airing of a cardiothoracic wound cavity model with carbon dioxide: theory and comparison of a gas diffuser with conventional tubes.

OBJECTIVES: To compare the efficiency of a new gas diffuser with conventional tubes for carbon dioxide (CO(2)) de-airing of a cardiothoracic wound cavity model, and to analyze how insufflation flow, outflow velocity, and diffusion affect de-airing. DESIGN: Technical study in vitro. SETTING: A nonventilated room at a University Hospital. INTERVENTIONS: De-airing by CO(2) insufflation via 3 methods was studied in a symmetric cardiothoracic wound model. MEASUREMENTS AND MAIN RESULTS: The studied insufflation devices were 2 open-ended tubes with an inner diameter of 2.5 mm and (1/4)-in (6.35 mm), respectively, and a gas diffuser (ie, a 2.5-mm tube with a diffuser at the end). CO(2) flows of 2.5, 5, 7.5, and 10 L/min were used. De-airing was assessed by measurement of remaining air content in a set of systematically distributed measuring points in the model. Three-, 2-, and 1-way analysis of variance all revealed significant interaction of device, flow, and depth on air content (p < 0.001). With tubes, the mean air content was 18% to 96% at the studied flows. With the gas diffuser, the mean air content in the cavity was below 0.2% at flows of 5 to 10 L/min. There was an exponential relation between calculated outflow velocity and air content. At a flow of 2.5 L/min, diffusion attenuated de-airing. CONCLUSION: These data imply that de-airing of a cardiothoracic wound by CO(2) insufflation depends on flow and outflow velocity. To compensate for diffusion with ambient air, the CO(2) flow should be >/= 5 L/min, and the outflow velocity should be about 0.1 m/s or less to avoid turbulence in the wound. This is only attainable with a gas diffuser.

Adult↗

PubMed2005

Determinants of the diffusion of computed tomography and magnetic resonance imaging.

OBJECTIVES: The aim of this study is to explain factors influential to the diffusion of computed tomography (CTs) and magnetic resonance imaging (MRIs). METHODS: Variables were identified from a review of the literature on the diffusion of health technologies. A formal process was applied to build a conceptual model of the mechanism that drives technology diffusion. Variables for the analysis were classified as predisposing, enabling, or reinforcing factors, in keeping with a model commonly used to explain the diffusion of health behaviors. Multiple regression analysis was conducted using year 2000 OECD data. RESULTS: The results of this study showed that total health expenditure per capita (p < .01, both CTs and MRIs) and flexible payment methods to hospitals (p < .05, both CTs and MRIs) were significantly associated with the diffusion of CTs and MRIs (adjusted R2 = 0.477, 0.656, respectively). CONCLUSIONS: This study presents a systematically developed model of the mechanism governing technology diffusion. Important findings from the study show that purchasing power, represented by total health expenditure per capita and economic incentives to hospitals in the form of flexible payment methods, were positively correlated with diffusion. Another important achievement of our model is that it accounts for all thirty OECD member countries without excluding any as outliers. This study shows that variation across countries in the diffusion of medical technology can be explained well by a logical model with multiple variables, the results of which hold profound implications for health policy regarding the adoption of innovations.

Diffusion of Innovation↗

PubMed2000

Direct in situ measurements of labile inorganic and organically bound metal species in synthetic solutions and natural waters using diffusive gradients in thin films.

The emerging technique of DGT (diffusive gradients in thin films) is shown to be capable of performing new speciation measurements in situ in natural waters. In DGT, metals are bound to a resin layer after passing through a well-defined diffusion layer. Cd was measured in solutions containing glycine, EDTA, and fulvic (FA) and humic acids (HA) by atomic absorption spectroscopy (AAS), anodic stripping voltammetry (ASV), and DGT. DGT measured similar labile fractions to ASV, with detailed differences being consistent with a thicker diffusion layer allowing more dissociation of labile complexes and a slower diffusion of FA and HA complexes through the gel. When single measurements are made in complex solutions with DGT, precise quantification is impossible due to uncertainties concerning the distribution of species with different diffusion coefficients. A new procedure was proposed based on the advantage of DGT of being able to control the pore size of the diffusive gel layer. Small (inorganic) species diffuse freely through all gels but larger FA and HA (organic) complexes diffuse less freely in more constrained gels. When measurements were made on known solutions of Cu and FA or HA, it was possible to quantify the inorganic and organic species separately. They agreed well with predictions made using the WHAM speciation code. Multiple DGT units were also deployed in situ in a stream with high dissolved organic carbon (14.6 mg/L). The systematic differences between the devices with different gel compositions enabled determination, for the first time, of the in situ concentrations of both labile inorganic and organic species in natural water. A single DGT device with a constrained gel can be used to quantify inorganic species directly, providing absolute accuracy is not required. This ability of DGT to measure well-defined fractions of metals in situ using a simple device gives it considerable potential as a regulatory tool. A direct speciation measurement may be preferable to modeling approaches which require diverse input data that are difficult to determine.

Benzopyrans↗

PubMed1994

Annexin IV reduces the rate of lateral lipid diffusion and changes the fluid phase structure of the lipid bilayer when it binds to negatively charged membranes in the presence of calcium.

Bovine annexin IV (endonexin) was bound to supported planar bilayers composed of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) in the first monolayer facing the substrate, and varying mole fractions of POPC, 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG) and small amounts of the fluorescent lipid analogs NBD-PC or NBD-PG in the second monolayer facing the large aqueous compartment. Lateral diffusion coefficients and mobile fractions of these phospholipids were measured by fluorescence recovery after photobleaching (FRAP) as a function of protein concentration and lipid composition in the presence of 2 mM CaCl2 or 1 mM EDTA. In the absence of annexin IV, the lateral diffusion coefficients depended only little on the POPC:POPG ratios and were approximately 3.0 microns2/s for NBD-PG (no Ca2+), 2.5 microns2/s for NBD-PG (2 mM Ca2+), and 1.6 microns2/s for NBD-PC (with or without 2 mM Ca2+). In the presence of 2 mM Ca2+ these diffusion coefficients decreased as a function of the added annexin concentration. A transition from a state with "rapid" lipid diffusion to a state with "slow" lipid diffusion occurred at about 80 nM annexin IV and was independent of the POPC:POPG ratio. In addition to reducing the lipid lateral diffusion coefficients, annexin IV also gave rise to two-component lateral diffusion of the lipids in these mixed bilayers. The split of the single diffusion coefficient of NBD-PG into two components occurred at most POPC:POPG ratios upon binding of annexin IV, but required higher annexin concentrations at mole fractions of POPC between 66 and 82 mol % than at high mole fractions of POPG or 90 mol % POPC.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1980

Lateral diffusion of M-13 coat protein in mixtures of phosphatidylcholine and cholesterol.

The translational diffusion of fluorescent-labeled M-13 phage coat protein (FITC-M-13), an integral membrane protein, has been measured in mixtures of phosphatidylcholines and cholesterol, using a pattern photobleaching technique. At temperatures below the chain-melting transition temperature of dimyristoylphosphatidylcholine (DMPC) (23.8 degrees C), the lateral diffusion coefficient of M-13 shows a marked increase when the cholesterol concentration is increased above 20 mol %. A similar marked increase in the lateral diffusion coefficient of a fluorescent phospholipid is also observed. At temperatures above the chain-melting transition temperature of DMPC, a minimum is observed in the lateral diffusion coefficient of FITC-M-13 for cholesterol concentrations in the vicinity of 25 mol %. This minimum in the diffusion coefficient of FITC-M-13 is also observed at 25 mol % cholesterol in egg phosphatidylcholine at 15 degrees C. No such minimum is observed for the lateral diffusion coefficient of the fluorescent lipid. The lateral diffusion coefficient of FITC-M-13 is large (greater than 10(-9) cm2/s) at all cholesterol concentrations for temperatures above the chain-melting transition temperature of the phosphatidylcholine. Several other proteins contain hydrophobic regions similar to that of the M-13 coat protein. We anticipate that a number of proteins of this type will show similar diffusional behavior, in particular exhibiting rapid diffusion throughout a wide range of lipid composition.

Cholesterol↗

PubMedDate not supplied

PFG-NMR measurements of the self-diffusion coefficients of water in equilibrium poly(HEMA-co-THFMA) hydrogels.

The self-diffusion coefficients for water in a series of copolymers of 2-hydroxyethyl methacrylate, HEMA, and tetrahydrofurfuryl methacrylate, THFMA, swollen with water to their equilibrium states have been studied at 310 K using PFG-NMR. The self-diffusion coefficients calculated from the Stejskal-Tanner equation, D(obs), for all of the hydrated polymers were found to be dependent on the NMR storage time, as a result of spin exchange between the proton reservoirs of the water and the polymers, reaching an equilibrium plateau value at long storage times. The true values of the diffusion coefficients were calculated from the values of D(obs) in the plateau regions by applying a correction for the fraction of water protons present, obtained from the equilibrium water contents of the gels. The true self-diffusion coefficient for water in polyHEMA obtained at 310 K by this method was 5.5 x 10(-10) m(2)s-1. For the copolymers containing 20% HEMA or more a single value of the self-diffusion coefficient was found, which was somewhat larger than the corresponding values obtained for the macroscopic diffusion coefficient from sorption measurements. For polyTHFMA and copolymers containing less than 20% HEMA, the PFG-NMR stimulated echo attenuation decay curves and the log-attenuation plots were characteristic of the presence of two diffusing water species. The self-diffusion coefficients of water in the equilibrium-hydrated copolymers were found to be dependent on the copolymer composition, decreasing with increasing THFMA content.

Diffusion↗

PubMedDate not supplied

Measurement of normal and anomalous diffusion of dyes within protein structures fabricated via multiphoton excited cross-linking.

We demonstrate microscale spatial and chemical control of diffusion within protein matrixes created through the use of nonlinear multiphoton excited photochemistry. The mobility of fluorescent dyes of different mass and composition within controlled cross-linked environments has been measured using two-photon excited fluorescence recovery after photobleaching (FRAP). The diffusion times for several rhodamine and sulforhodamine dyes within these fabricated structures were found to be approximately 3-4 orders of magnitude slower than in free solution. The precise diffusion times can be tuned by varying the laser exposure during the fabrication of the matrix, and the diffusion can be correlated with the mesh size determined by TEM and Flory-Rehner analysis. We find that the hydrophobic Texas Red dyes (sulforhodamines) exhibit diffusion that is highly anomalous, indicative of a strong interaction with the hydrophobic cross-linked protein matrix. These results suggests the use of these cross-linked protein matrixes as ideal model systems in which to systematically study anomalous diffusion. Finally, the diffusion can be tuned within a multilayered protein matrix, and this in conjunction with slow diffusion also suggests the use of these structures in controlled release applications.

Animals↗

PubMed2007

A study of water diffusion into a high-amylose starch blend: the effect of moisture content and temperature.

The effect of moisture content and temperature on water diffusion into a modified high amylose (< or = 90%) maize thermoplastic starch blend was investigated. Gravimetric and magnetic resonance imaging (MRI) studies were conducted to elucidate the diffusion mechanism and diffusion coefficients for this system. The diffusion coefficient data demonstrated that the rate of water diffusion into this blend was significantly dependent upon temperature and moisture content. Water diffusion was faster at higher temperatures and generally for samples stored at higher relative humidity environments. It was revealed from the gravimetric data that water diffusion into this starch blend was Fickian; however, further analysis of the MRI images found that the water diffusion mechanism was exponentially dependent on the concentration. This result was determined by comparing experimental water concentration profiles to a theoretical model calculated using the implicit Crank-Nicolson finite difference method.

Amylose↗

PubMed2001

Morphology of three lyotropic liquid crystalline biological NMR media studied by translational diffusion anisotropy.

The morphologies of three dilute liquid crystalline phases, which are widely used for biological NMR spectroscopy, are investigated by the study of tracer self-diffusion. The aqueous liquid crystalline media investigated include the common phospholipid bicelle medium, a phase consisting of a mixture of pentaethyleneglycol mono dodecyl ether and hexanol, and a medium containing cetylpyridinium bromide and hexanol. Threonine and water were used as tracer molecules for probing the aqueous environment, and tetramethylsilane (TMS) was for probing the lipophilic environment. Pulsed field gradient NMR was used to measure tracer self-diffusion rates in three orthogonal directions. Although results for the water-soluble tracers in bicelle media do not contradict the widely accepted disk-shaped bicelle model, the high TMS diffusion rate observed in the bilayer plane requires extensive transient edge-to-edge contacts of such disks. This morphology is essentially that of a heavily perforated lamellar bilayer phase and explains why this medium remains liquid crystalline well below the Onsager limit for disk-shaped nematogens. Below 25 degrees C, a bicelle mixture consisting of dimyristoyl phosphatidyl choline and dihexanoyl phosphatidyl choline remains isotropic, but tracer diffusion obstruction indicates that the particles are significantly oblate. The diffusion anisotropy in the penta(ethyleneglycol) mono dodecyl ether liquid crystals confirms the previously proposed alpha-lamellar phase. However, weak inhibition of aqueous-phase self-diffusion in the z direction points to the presence of bridge- or caplike obstructions, and the bilayers appear slightly permeable to water. If the previously proposed concentric cylinder superstructure of bilayers applies, the diffusion data indicate that the most outer cylinder must have a diameter greater than 50 microm. The tracer self-diffusion data for the cetylpyridinium bromide/hexanol medium is only compatible with a planar alpha-lamellar phase, with its local director orthogonal to the magnetic field, and a very large domain size over which the director remains parallel.

Anisotropy↗

PubMed2002

Residue-specific real-time NMR diffusion experiments define the association states of proteins during folding.

Characterizing the association states of proteins during folding is critical for understanding the nature of protein-folding intermediates and protein-folding pathways, protein aggregation, and disease-related aggregation. To study the association states of unfolded, folded, and intermediate species during protein folding, we have introduced a novel residue-specific real-time NMR diffusion experiment. This experiment, a combination of NMR real-time folding experiments and 3D heteronuclear pulsed field gradient NMR diffusion experiments (LED-HSQC), measures hydrodynamic properties, or molecular sizes, of kinetic species directly during the folding process. Application of the residue-specific real-time NMR diffusion experiments to characterize the folding of the collagen triple helix motif shows that this experiment can be used to determine the association states of unfolded, folded, and kinetic intermediates with transient lifetimes simultaneously. The ratio of the apparent translational diffusion coefficients of the unfolded to the folded form of the triple helix is 0.59, which correlates very well with a theoretical ratio for monomer to linear trimer. The apparent diffusion coefficients of the kinetic intermediates formed during triple helix folding indicate the formation of trimer-like associates which is consistent with previously published kinetic and relaxation data. The residue-specific time dependence of apparent diffusion coefficients of monomer and trimer peaks also illustrates the ability to use diffusion data to probe the directionality of triple helix formation. NMR diffusion experiments provide a new strategy for the investigation of protein-folding mechanisms, both to understand the role of kinetic intermediates and to determine the time-dependent aggregation processes in human diseases.

Alanine↗

PubMed2005

Diffusion measurements for molecular capsules: pulse sequences effect on water signal decay.

Diffusion NMR and, more recently, diffusion ordered spectroscopy (DOSY) are gaining popularity as efficient tools for the characterization of supramolecular systems in solution. Here, using diffusion NMR of hydrogen-bond molecular capsules, we demonstrate that the use of different diffusion sequences may have a dramatic effect on exchanging peaks. In fact, we found that the signal decay of the water peak in [(1a)(6)(H(2)O)(8)] is monoexponential in the pulsed gradient spin-echo (PGSE) and stimulated echo (PGSTE) sequences and biexponential in the longitudinal eddy current delay (LED) and the bipolar longitudinal eddy current delay (BPLED) sequences, routinely used in modern DOSY experiments. By performing these diffusion measurements on molecular capsules, in which water is not part of the molecular capsules, we demonstrate that this phenomenon is observed only for water molecules that exchange between two sites that differ considerably in their diffusion coefficients. Degeneration of the LED or the BPLED sequences into PGSTE-type sequences by shortening the te period resulted in the disappearance of the extra slow diffusing component. The origin, as well as the implications of the different results obtained from conventional diffusion sequences, such as the PGSE and PGSTE as compared with the LED and BPLED sequences generally used in DOSY experiments, are briefly discussed.

Diffusion↗

PubMed2005

An NMR determination of CO diffusion on platinum electrocatalysts.

We report the first direct measurement of CO diffusion on nanoparticle Pt electrocatalysts at the solid/liquid interface, carried out using 13C nuclear magnetic resonance (NMR) with a spin-labeling pulse sequence. Diffusion parameters were measured in the temperature range of 253-293 K for CO adsorbed on commercial Pt-black under saturation coverage. 2H NMR of the same system indicates that the electrolyte remains in the liquid state at temperatures where the CO diffusion experiments were performed. The CO diffusion parameters follow typical Arrhenius behavior with an activation energy of 6.0 +/- 0.4 kcal/mol and a pre-exponential factor of (1.1 +/- 0.6) x 10-8 cm2/s. Exchange between different CO populations, driven by a chemical potential gradient, is suggested to be the main mechanism for CO diffusion. The presence of the electrolyte medium considerably slows down the diffusion of CO as compared to that seen on surfaces of bulk metals under UHV conditions. This work opens up a new approach to the study of surface diffusion of adsorbed molecules on nanoparticle electrode catalysts, including the possibility of correlating diffusion parameters to catalytic activity in real world applications of broad general interest.

Carbon Monoxide↗

PubMed2006

Rotational and translational diffusion of peptide-coated CdSe/CdS/ZnS nanorods studied by fluorescence correlation spectroscopy.

CdSe/CdS/ZnS nanorods (NRs) of three aspect ratios were coated with phytochelatin-related peptides and studied using fluorescence correlation spectroscopy (FCS). Theoretical predictions of the NRs' rotational diffusion contribution to the correlation curves were experimentally confirmed. We monitored rotational and translational diffusion of NRs and extracted hydrodynamic radii from the extracted diffusion constants. Translational and rotational diffusion constants (D(trans) and D(rot)) for NRs were in good agreement with Tirado and Garcia de la Torre's as well as with Broersma's theories when accounting for the ligand dimensions. NRs fall in the size range where rotational diffusion can be monitored with higher sensitivity than translational diffusion due to a steeper length dependence, D(rot) approximately L(-)(3) versus D(trans) approximately L(-)(1). By titrating peptide-coated NRs with bovine serum albumin, we monitored (nonspecific) binding through rotational diffusion and showed that D(rot) is an advantageous observable for monitoring binding. Monitoring rotational diffusion of bioconjugated NRs using FCS might prove to be useful for observing binding and conformational dynamics in biological systems.

Amino Acid Sequence↗

PubMed2004

Effects of casein and fat content on water self-diffusion coefficients in casein systems: a pulsed field gradient nuclear magnetic resonance study.

The water self-diffusion coefficients in casein matrixes were measured using a pulsed field gradient spin-echo nuclear magnetic resonance technique (PFG-SE NMR). The dependence of the water self-diffusion coefficient on the casein concentration and the aqueous phase composition is reported in both a rehydrated native phosphocaseinate dispersion and a concentrated casein retentate. A model has been proposed to explain the different behavior of the water self-diffusion coefficient in the two casein systems. This model demonstrates that the water self-diffusion cannot be simply explained by the water content only. So, taking into account the specific effect of each constituent of the aqueous dispersing phase, the water self-diffusion reduction induced by the casein micelle can be modeled. The effect of fat on the water self-diffusion coefficients was investigated. Anhydrous milk fat-reconstituted retentate samples were used in order to estimate the obstruction effect of fat globules in the modeling process. The dependence of the self-diffusion coefficient of water on the fat and casein content is reported. A general model included the effect of the aqueous phase composition, and the obstruction effects of casein micelles and fat globules were proposed. This model was validated for water self-diffusion coefficients in industrial fatty retentates.

Caseins↗

PubMed2005

NMR relaxation and water self-diffusion studies in whey protein solutions and gels.

The changes in water proton transverse relaxation behavior induced by aggregation of whey proteins are explained in terms of the simple molecular processes of diffusion and chemical exchange. The water self-diffusion coefficient was measured in whey protein solutions and gels by the pulsed field gradient NMR method. As expected, water self-diffusion was reduced with increased protein concentrations. Whatever the concentration, the water molecules were free to diffuse over distances varying from 15 to 47 mum. Water diffusion was constant over these distances, demonstrating that no restrictions were found to explain the water hindrance. The modification in protein structure by gelation induced a decrease in water diffusion. The effects of protein concentration on water diffusion are discussed and modeled. Two approaches were compared, the obstruction effect induced by a spherical particle and the cell model, which considered two water compartments with specific self-diffusion coefficients.

Diffusion↗