SEARCH · PubMed Health

Results for “Diffusion”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,207 records · Page 67Linked to original sources

PubMed2006

Microstructural development of human brain assessed in utero by diffusion tensor imaging.

BACKGROUND: Diffusion-weighted MR imaging (DWI) has been shown to be a great tool to assess white matter development in normal infants. Comparison of cerebral diffusion properties between preterm infants and fetuses of corresponding ages should assist in determining the impact of premature ex utero life on brain maturation. OBJECTIVE: To assess in utero maturation-dependent microstructural changes of fetal cerebral white matter using diffusion tensor MR imaging. MATERIALS AND METHODS: An echoplanar sequence with diffusion gradient (b=700 s/mm(2)) applied in six non-colinear directions was performed between 31 and 37(+3) weeks of gestation in 24 fetuses without cerebral abnormality on T1- and T2-weighted images. Apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were measured in the white matter. RESULTS: Mean ADC values were 1.8 microm(2)/ms in the centrum semiovale, 1.2 microm(2)/ms in the splenium of the corpus callosum and 1.1 microm(2)/ms in the pyramidal tract. The paired Wilcoxon rank test showed significant differences in ADC between these three white matter regions. Mean FA values were 1.1%, 3.8% and 4.7%, respectively, in the centrum semiovale, corpus callosum and pyramidal tract. A significant age-related decrease in ADC and an increase in FA towards term were demonstrated in the pyramidal tract and corpus callosum. CONCLUSION: Diffusion tensor imaging in utero can provide a quantitative assessment of the microstructural development of fetal white matter. Anisotropic parameters of the diffusion tensor should improve with technical advances.

Anisotropy↗

PubMed2002

Two-photon excitation and photolysis by pulsed laser illumination modelled by spatially non-uniform reactions with simultaneous diffusion.

Two-photon absorption in the focus of a pulsed laser has the potential for localized photolysis of caged compounds, generating high concentrations of neurotransmitters, hormones and messengers. The concentrations of cage, intermediates and products in the femtolitre focal volume depend on reaction rates and diffusional exchange with the external volume. This problem of reaction with diffusion was analysed with analytical and numerical methods to determine simple relations between parameters useful in the design and interpretation of experiments. The diffraction-limited laser spot is approximated well by a sphere, radius A, in diffusional exchange with either an infinite uniform medium, representing extracellular photolysis, or within a non-permeable sphere, a "cell" of radius B, representing intracellular photolysis. Photolysis is modelled as sequential irreversible reactions, with either the excitation step alone, rate constant k(e), or with a subsequent "dark" reaction, rate constant k(p). For extracellular photolysis, steady-state depletion of a cage averaged in a spherical spot increases hyperbolically with k(e) with half-maximum depletion at k(e) = K0.5 = 2.5 D/A2, where D is the diffusion coefficient. With measured parameters for spot size A = 0.3 microm and diffusion D = 800 microm2/s, K0.5 = 22,200 s(-1). The optimal exposure for localized photolysis is the characteristic diffusion time tau = A2/D, 113 micros in this example, and is the time taken to reach 57% of steady state in the diffusion-limited case. In the two-step model, with excitation and "dark" reaction steps, rate constants both exceeding K0.5 are necessary to generate 50% of maximal product concentration in the illuminated volume. High concentrations of photolysis products depend particularly on a high excitation rate constant (k(e) > K0.5), and localization of the products requires fast dark reactions (k(p) > K0.5). If products diffuse faster than the cage, their steady-state concentrations are decreased, and concentration transients may occur. For localized intracellular photolysis, the duration of exposure that generates product concentration at the cell boundary, B, less than 10% of the spot concentration should be shorter than 0.043(B/A)3tau, and is determined by diffusion.

Biophysics↗

PubMed2001

How to perform diffusion-weighted imaging.

Diffusion-weighted imaging (DWI) has become an invaluable tool in the management of patients with stroke. DWI relies on detecting the random diffusion of water molecules. In normal tissues this movement may be restricted by the presence of cellular structures, which provide a barrier to free movement. This occurs in myelinated white matter, where movement is restricted more across than along fibres. This directional dependence is termed anisotropic restricted diffusion. The diffusion of water molecules can be made the dominant contrast mechanism within an image by applying large magnetic field gradients. The pulsed gradient spin echo (PGSE) sequence provides sensitivity to diffusion with gradient pulses either side of the 180 degrees refocusing pulse. This sequence is generally heavily T2 weighted. In order to detect normal anisotropic properties within the different components of the medullary core, association, commissural and projection fibres, DWI must be performed with sensitisation in at least three directions. PGSE sequences have been used to obtain the diffusion coefficient (D*), a measure of mobility at the molecular level within tissue. In acute infarction D* is increased; in brain death it is decreased. Diffusion contrast needs to be optimised in relation to the highly T2-dependent nature of PGSE sequences. This also requires a more detailed knowledge of how D* changes in disease, but information on nonischaemic neurological conditions is still very limited.

Adult↗

PubMed2006

Environmental monitoring of occupational exposure to N,N-dimethylformamide: comparison between active and diffusive sampling.

OBJECTIVES: The objective of this study is to optimize the evaluation of the exposure to N,N-dimethylformamide (DMF) in synthetic leather factories by diffusive samplers. The DMF exposure was monitored in synthetic leather factories by two sampler types: active and diffusive. METHODS: Air measurements were carried out using two different personal air samplers, a diffusive and an active one. The diffusive sampling method, TK200 with charcoal filters, was examined in comparison with pumping through NIOSH silica gel tubes workplace air as with the currently available "gold standard". The evaluation was carried out, in two different years but in the same season, for all the duration of the shift, i.e. 8 h on workers employed in five different factories in the district of Florence and Prato (Italy). RESULTS: The statistical and graphical analysis of data show a good correlation between active and passive samplers (r = 0.96, P < 0.001, n = 91), a good linear regression (DMF(diffusive )= 0.95 DMF(active) + 0.15, R (2) = 0.92), a not statistically significant difference between data (tested by paired t test and non-parametric Wilcoxon test). Moreover, all these results are confirmed for data lower and higher than TLV-TWA, in particular we found a significant Pearson correlation (r = 0.92, P < 0.001, n = 83; r = 0.92, P < 0.05, n = 8, respectively) and a significant linear regression (DMF(diffusive )= 0.88 DMF(active) + 0.73, R (2 )= 0.86; DMF(diffusive )= 0.90 DMF(active) + 3.76, R (2 )= 0.85). Besides, the analysis of graphical representations confirmed the previous evidences. Finally, we can not find a significant difference between different types of job. CONCLUSIONS: Due to the good agreement between the two groups of data, the TK200 samplers can be considered as a simpler approach than the pump for screening worker exposures to DMF.

Air Pollutants, Occupational↗

PubMed2004

Diffusivity of ions in agarose gels and intervertebral disc: effect of porosity.

The effect of tissue porosity on ion (sodium, potassium, and chloride) diffusivity in agarose gels and porcine intervertebral disc tissues was investigated using an electrical conductivity method. An empirical, constitutive model for diffusivity (D) of solutes in porous fibrous media was proposed: D/Do = exp[-alpha(r(s)/k(1/2))beta] where r(s) is the Stokes radius of a solute, kappa is the Darcy permeability of the porous medium, Do is the diffusivity in free solution, alpha and beta are two positive parameters whose values depend on material structure. It is found that alpha = 1.25 +/- 0.138, beta = 0.681 +/- 0.059 (95% confidence interval, R2 = 0.92, n = 72) for agarose gels and alpha = 1.29 +/- 0.171 and beta = 0.372 +/- 0.088 (95% confidence interval, R2 = 0.88, n = 86) for porcine annulus fibrosus. The functional relationship between solute diffusivity and tissue deformation was derived. Comparisons of our model prediction with experimental data on diffusion coefficients of macromolecules (proteins, dextrans, polymer beads) in agarose gels in the literature were made. Our results were also compared to the data on ion diffusivity in charged gels and in cartilaginous tissues reported in the literature. There was a good agreement between our model prediction and the data in the literature. The present study provides additional information on solute diffusivity in uncharged gels and charged tissues, and is important for understanding nutritional transport in avascular cartilaginous tissues under different mechanical loading conditions.

Animals↗

PubMed2005

Modeling of diffusion with partitioning in stratum corneum using a finite element model.

Partitioning and diffusion of chemicals in skin is of interest to researchers in areas such as transdermal penetration and drug disposition, either for risk assessment or transdermal delivery. In this study a finite element method is used to model diffusion in the skin's outermost layer, the stratum corneum (SC). The SC is considered to be a finite two-dimensional composite having different diffusivity values in each medium as well as a partition coefficient at the interfaces between media. A commercial finite element package with thermal analysis capabilities is selected due to the flexibility of this software to handle irregular geometries. Partitioning is accommodated through a change of variables technique. This technique is validated by comparison of model results with analytical solutions of steady-state flux, transient concentration profiles, and time lag for diffusion in laminates. Two applications are presented. Diffusion is solved in a two-dimensional "brick and mortar" geometry that is a simplification of human stratum corneum, with a partition coefficient between corneocyte and lipid. Results are compared to the diffusion in multiple laminates to examine effects of the partition coefficient. The second application is the modeling of diffusion with partitioning through an irregular geometry which is obtained from a micrograph of hairless mouse stratum corneum.

Administration, Cutaneous↗

PubMed2005

Cytotoxic edema is responsible for raised intracranial pressure in fulminant hepatic failure: in vivo demonstration using diffusion-weighted MRI in human subjects.

It is not clear whether cerebral edema in fulminant hepatic failure is predominantly vasogenic or cytotoxic, though cytotoxic edema due to astrocyte swelling is more likely. Diffusion-weighted magnetic resonance imaging can differentiate vasogenic from cytotoxic edema. We performed diffusion-weighted imaging in patients with fulminant hepatic failure to clarify the issue by measuring apparent diffusion coefficient, which quantifies movement of water molecule across cell membrane. Seven patients with fulminant hepatic failure underwent conventional and diffusion-weighted magnetic resonance imaging. Apparent diffusion coefficient was measured in four cortical areas and 12 deep white and gray matter regions in both cerebral hemispheres. Thirteen healthy subjects served as controls. The apparent diffusion coefficient values in patients and controls were compared using Wilcoxon signed rank test. Two patients who survived underwent repeat imaging using same protocol. Patients with FHF had significantly lower apparent diffusion coefficient in all cortical and deep white and gray matter regions of interest compared to controls (p < 0.001), suggesting cytotoxic cell swelling. In two survivors with repeat imaging, one showed complete resolution while the changes persisted in the other, suggesting ischemic injury. Cerebral edema in fulminant hepatic failure is predominantly due to cytotoxic edema.

Adolescent↗

PubMed1987

A method for the determination of diffusion coefficients for small molecules in aqueous solution.

A method is described for determining the diffusion coefficients of small solutes in limited volumes (approximately equal to 4-9 ml) of fluid. Diffusion is measured in a three-chamber diffusion cell across a central unstirred compartment. Compartments are separated by nitrocellulose membranes. The instantaneous concentration gradient and the instantaneous flux of solute into the dilute end compartment are derived from changes in the concentration of solute in the two stirred end compartments through time. The diffusion coefficient is calculated from the slope of the least-squares regression line relating the magnitude of the instantaneous solute flux to that of the instantaneous concentration gradient. The apparatus is calibrated with a solute of known diffusivity (KCl). Diffusion coefficients thus determined in water at 25 degrees C for CaCl2 (7.54 X 10(-6) cm2.s-1), Na2-ATP (7.01 X 10(-6) cm2.s-1), 2-deoxyglucose (5.31 X 10(-6) cm2.s-1), and D-Na-lactate (5.62 X 10(-6) cm2.s-1) differed by an average of 3.7% from literature values. The method described results in accurate estimates of diffusion coefficients by a simple and relatively rapid procedure.

Calcium Chloride↗

PubMed1987

The kinetics of the aerobic oxidation of ferrocytochrome c by cytochrome c oxidase in solvents of increased viscosity are partially diffusion controlled.

The steady-state spectrophotometrically determined initial velocity kinetics of the aerobic oxidation of ferrocytochrome c by cytochrome c oxidase were examined for effects of diffusion control in solvents of increased viscosity. Both glycerol/water and sucrose/water proved unsatisfactory as viscosogens due to weak competitive inhibition (Ki values of 2.6 M and 1.6 M, respectively). However, polyethylene glycol (PEG) was satisfactory as a viscosogen. The measured diffusion coefficient of ferrocytochrome c in PEG/water was shown to follow closely the Stokes-Einstein equation. In PEG/water mixtures at high ionic strength the minimum association rate constant (kmin = Vmax/(Km[EO]) is partly diffusion controlled with contributions from diffusion control and chemical activation control being about equal at 5 mPa X s, a viscosity that may be physiologically relevant. This finding can be interpreted to mean that cytochrome c oxidase is an enzyme that has evolved to approach its maximum efficiency. The steady-state kinetics were also examined at low ionic strength where multiphasic kinetics are exhibited. The effect of increased viscosity was exhibited over the whole experimentally accessible region indicating that there are effects due to diffusion control on both the high-affinity and low-affinity binding of ferrocytochrome c. Several models for diffusion control were examined and a comparison is made with other diffusion-controlled reactions of proteins.

Aerobiosis↗

PubMed1982

Role of diffusion potential on the flow of angiotensin II, bradykinin and related molecules through cellophane membranes.

The diffusion of electrically charged peptides (angiotensin II, bradykinin and [Suc1]angiotensin II) across tight cellophane membranes, obtained by different degrees of acetylation, shows a kinetic behaviour which was interpreted in the literature as indicative of the existence of different molecular conformations presenting slow interconversion velocities and different permeabilities across the membrane. A diffusion potential (delta psi) was found to be present across the membrane along diffusion experiments performed in low ionic strength. Upon annihilation of delta psi by chemical voltage clamping (by equally increasing the ionic strength on both bathing solutions) the diffusion rate was decreased and the flow followed first order kinetics, indicating a major role of delta psi in the process. As the ionic strength increase could also affect molecular conformation, the role of delta psi on the diffusion of those molecules was tested by fitting flux and delta psi experimental results by an integrated form of Nernst-Planck flux equation. It is concluded that the deviation from first order diffusion kinetics, observed in low ionic strength, is solely due to the diffusion potential, and not to the existence of more than one molecular conformation in aqueous solution. This study was extended to amino acids and other related charged molecules.

Angiotensin II↗

PubMed1976

Interaction of convection and diffusion in pulmonary gas transport.

Normal human subjects inspired various volumes of a normoxic argon mixture containing low concentrations of several biologically inert tracer gases with markedly different diffusivities (helium, neon, and sulfur hexafluoride). The behavior of Ne, Ar, and SF6 could be predicted on the basis of axial dispersion due to differences in diffusivity. For example, neon, having the highest diffusivity of the three, was more uniformly distributed within the bronchial tree than either argon or SF6. The behavior of helium, however, was not consistent with predictions based solely on axial diffusion. Contrary to expectation, the early portion of expiration was helium enriched while gas assumed to come from the alveolar regions contained relatively less helium than the other gases. Results of this study suggest that radial diffusion during convective bulk flow may play a significant role in intrapulmonary gas transport if relative diffusivity is extremely large. We conclude that diffusion gradients do exist within the bronchial tree during normal quiet breathing and that these gradients become less significant as inspired volume increases.

Argon↗

PubMed1991

Diffusion limitation of O2 supply to tissue in homogeneous and heterogeneous models.

The role of diffusion limitation in O2 supply was studied in cross-sectional elements of the Krogh cylinder model (with O2 supply from a central capillary) and of the solid cylinder model (with O2 supply from the outer surface). The effect of diffusion limitation was quantified in terms of the ratio O2 uptake/O2 requirement (= fraction of cross-sectional area supplied with O2), assuming local O2 requirement per unit volume to be constant and independent of PO2 at PO2 greater than 0. Calculations were performed for single cylinders of varied radius and O2 requirement (homogeneous models). Unequal distribution of diffusion conditions was represented by a model composed of three sorts of Krogh or solid cylinders, with radii in relation 3: square root of 3:1, but of equal cross-sectional area, i.e. number of cylinders of each sort in relation 1:3:9 (heterogeneous models). The results revealed the following main features. (1) At the same outer radius, diffusion limitation sets in at a smaller O2 requirement, and increases more steeply with increasing O2 requirement, in the homogeneous Krogh cylinder model compared with the homogeneous solid cylinder model. A similar behavior is observed when the radius of the cylinder section is increased at constant O2 requirement. (2) Diffusion limitation in the heterogeneous model sets in at a lower O2 requirement value, and increases more gradually with increasing O2 requirement, than in the corresponding homogeneous models with the same average cylinder diameter. This behavior is due to sequential onset, in the heterogeneous model, of anoxia in the cylinder sections of different radii. We conclude that diffusion heterogeneity has to be taken into account when the role of diffusion limitation in tissue O2 supply is investigated.

Animals↗

PubMedDate not supplied

Comparison of the enhanced steady-state diffusion of calcium by calbindin-D9K and calmodulin: possible importance in intestinal calcium absorption.

The diffusion of calcium was measured using the unidirectional flux of 45Ca across an aqueous layer. The aqueous layer was bounded by two dialysis membranes and convection was eliminated by gelling the aqueous layer with agarose. The apparent self-diffusion coefficient was determined by the dependence of the tracer flux on the diffusion distance. The apparent self-diffusion coefficient increased linearly with the concentration of calbindin-D9K and calmodulin, but the effect of calmodulin was markedly less than that of calbindin-D9K. This difference is attributed to the lower association constant for calmodulin. The ion-exchange resin Chelex-100 also increased the steady-state of 45Ca, but the effect of Chelex-100 was much less efficient than the effect of calbindin-D9K. The mechanism of enhanced diffusion was attributed to an enhanced gradient of total 45Ca. These results indicate that the steady-state unidirectional calcium flux is a superposition of free calcium diffusion and bound calcium diffusion, with only a small contribution due to a 'bucket brigade' mechanism. We suggest that this phenomenon may be important in calcium absorption across the intestine.

Animals↗

PubMed1992

Kinetics of diffusion in a spherical cell. I. No solute buffering.

Realistic neuron models that involve effects of concentration changes of second messengers on ion channels must include processes such as diffusion and solute buffering. These processes, which span a wide range of spatial and temporal scales, may impose a severe computational burden. In this paper and its companion, we examine the kinetics of diffusion and present methods for stimulating it accurately and efficiency. The problem of calcium diffusion in a spherical cell is used as a device to demonstrate the practical application of our analysis. However, the scope of these papers is not limited to this problem. The same analysis that we apply and concerns that we raise are germane to the spread of any second messenger, and can be adapted to other geometries. The focus of this paper is the simplest case: diffusion in the absence of solute buffering. This analysis also applies whenever buffering is so fast that it is instantaneous compared to diffusion, or so slow that concentration gradients have dissipated before substantial buffering takes place. The second paper investigates the more difficult situation where diffusion and buffering occur at comparable rates. In the absence of buffering, concentration changes produced by diffusion can be fit by an infinite series of exponential terms. We show how to design a model with N + 1 compartments that fits the N slowest terms of this series exactly in a shell just inside the cell membrane.

Animals↗

PubMed2006

Evidence-based approaches to dissemination and diffusion of physical activity interventions.

With the increasing availability of effective, evidence-based physical activity interventions, widespread diffusion is needed. We examine conceptual foundations for research on dissemination and diffusion of physical activity interventions; describe two school-based program examples; review examples of dissemination and diffusion research on other health behaviors; and examine policies that may accelerate the diffusion process. Lack of dissemination and diffusion evaluation research and policy advocacy is one of the factors limiting the impact of evidence-based physical activity interventions on public health. There is the need to collaborate with policy experts from other fields to improve the interdisciplinary science base for dissemination and diffusion. The promise of widespread adoption of evidence-based physical activity interventions to improve public health is sufficient to justify devotion of substantial resources to the relevant research on dissemination and diffusion.

Adolescent↗

PubMed2005

Outward fluid flow reduces inward diffusion of bacterial lipopolysaccharide across intact and demineralised dentine.

OBJECTIVE: To examine the ability of outward fluid flow (OF) on resisting the inward diffusion of bacterial lipopolysaccharide (LPS) across the demineralised dentine (DD) in comparison with that across the intact dentine (ID). DESIGN: Twenty ID discs were prepared from freshly extracted human third molars. After etching both dentine surfaces, hydraulic conductance (L(p)) of the dentine was measured. Ten dentine discs were then completely demineralised using 10% EDTA, and L(p) was re-measured. The diffusion of LPS through ID and DD was measured against the OF and compared to the non-outward flow (NF) (n = 5 for each group) at 0, 1, 4 and 8h. Longitudinal sections of ID and DD surfaces were observed under a scanning electron microscope (SEM). RESULTS: The L(p) of DD was significantly higher than that of ID (independent t-test, p < 0.001). The application of OF and demineralisation significantly affected LPS diffusion (two-way ANOVA, p < 0.05). In addition, the effect of OF depended on dentine demineralisation. SEM images of ID showed intact dentinal tubules, whereas those of DD showed expanded collagen fibres and enlarged dentinal tubules. CONCLUSIONS: The inward diffusion of LPS across DD differed from that of ID and the OF affected the inward diffusion of LPS. In the presence of the OF, the inward diffusion of LPS was reduced to near zero in both ID and DD. Nevertheless, when compared to that in the ID group, the OF produced the slightly greater effect to resist the inward LPS diffusion in the DD group.

Adolescent↗

PubMed2006

Spatially confined diffusion of calcium in dendrites of hippocampal neurons revealed by flash photolysis of caged calcium.

The extent of diffusion of a locally evoked calcium surge in dendrites of cultured hippocampal neurons was studied by flash photolysis of caged EGTA. Cells were transfected with pDsRed for visualization, preincubated with caged NP-EGTA (AM) and Fluo-4 (AM) at room temperature and imaged in a PASCAL confocal microscope. Pulses of UV laser light within an active sphere of about 1 micro m(2) produced a rise of Fluo-4 fluorescence transients in dendrites which peaked at 1 ms and decayed exponentially with a fast (8-10 ms) time constant. A slower decay component was uncovered following incubation with thapsigargin. Lateral diffusion of [Ca(2+)]i did not vary significantly among different size dendrites being symmetric and reaching about 3-3.5 micro mm at a diffusion rate of 0.8 micro mm/ms on both sides of the photolysis center. Fluo-4 was also replaced by the membrane-bound Fluo-NOMO (AM) or by the 'heavy' Calcium Green dextran (CGd) loaded through a patch pipette. Similar rates of diffusion were found in these cases, indicating that the diffusion is not of the dye complexed to calcium but of genuine free calcium ions. Interestingly, presence of a dendritic spine at the focus of photolysis significantly reduced [Ca(2+)]i spread while the focal transient remained unaffected. Finally, [Ca(2+)]i diffused about twice as far from the photolysis sphere in glass tubes of a similar diameter to that of a dendrite, indicating that intrinsic calcium uptake mechanisms in the dendrite determine the diffusion of calcium away from its original site of rise.

Aniline Compounds↗

PubMed2006

Evaluation of white matter integrity in ex vivo brains of amyloid plaque-bearing APPsw transgenic mice using magnetic resonance diffusion tensor imaging.

Magnetic resonance diffusion tensor imaging (DTI) was used to examine the integrity of midline white matter tracts in APPsw (Tg2576) transgenic mice, a mouse-model of cerebral amyloid deposition. Ex vivo DTI was performed on formalin-fixed brains from APPsw and age-matched transgene-negative control mice at the ages of 12, 15, and 17 months. The characteristics of water diffusion in six midline white matter tracts were quantified using four metrics: relative anisotropy (RA), mean diffusivity, axial diffusivity, and radial diffusivity. Two-way ANOVA analyses indicated a significant main effect of transgene on RA in the corpus callosum (CC) and ventral hippocampal commissure (VHC), due to small reductions (2-6%) in RA in APPsw mice relative to age-matched control mice. However, these reductions were not significant at any specific age group and were not progressive with increasing age. The other diffusion metrics exhibited no significant differences between APPsw and control mice in the CC and VHC, nor did any of the diffusion metrics exhibit significant differences between APPsw and control mice in other midline white matter tracts (anterior commissure, posterior commissure, fornix, and dorsal fornix). Overall, these results indicate that white matter integrity, as measured by ex vivo DTI, is predominately unaltered in formalin-fixed brains from amyloid plaque-bearing APPsw mice.

Age Factors↗