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At least 343 records · Page 19Linked to original sources

PubMed1980

Lateral diffusion in biological membranes. A normal-mode analysis of diffusion on a spherical surface.

A new approach is described for the analysis of lateral diffusion in biological membranes. It is shown that a suitably defined first moment of the concentration distribution on a spherical surface decays as a single exponential with a relaxation rate proportional to the diffusion coefficient and inversely proportional to the square of the radius of the sphere. The approach is illustrated with an example of fluorescence redistribution after photobleaching of membrane proteins in a spectrin-deficient spherocytic mouse erythrocyte membrane.

Diffusion↗

PubMed2003

Predictions of pulsed field gradient NMR echo-decays for molecules diffusing in various restrictive geometries. Simulations of diffusion propagators based on a finite element method.

Pulsed field gradient NMR diffusometry is a promising tool for investigating structures of porous material through determinations of dynamic displacements of molecules in porous systems. A problem with this approach is the lack of closed analytical expressions for echo-decays in anything but idealized pore geometries. We present here an approach based on calculating the appropriate diffusion propagator by means of finite element calculations. The suggested method is quite general, and can be applied to arbitrary porous systems. The protocol for the calculations is outlined and we show results from some different cases: diffusion in confined geometries and in systems that are spatially inhomogeneous with respect to concentration.

Diffusion↗

PubMed2006

Restricted diffusion of a freely diffusible second messenger: mechanisms underlying compartmentalized cAMP signalling.

It is becoming increasingly evident that the freely diffusible second messenger cAMP can transduce specific responses by localized signalling. The machinery that underpins compartmentalized cAMP signalling is only now becoming appreciated. Adenylate cyclases, the enzymes that synthesize cAMP, are localized at discrete parts of the plasma membrane, and phosphodiesterases, the enzymes that degrade cAMP, can be targeted to selected subcellular compartments. A-kinase-anchoring proteins then serve to anchor PKA (protein kinase A) close to specific targets, resulting in selective activation. The specific activation of such individual subsets of PKA requires that cAMP is made available in discrete compartments. In this presentation, the molecular and structural mechanisms responsible for compartmentalized PKA signalling and restricted diffusion of cAMP will be discussed.

Animals↗

PubMed1977

Diffusion rates of cell surface antigens of mouse-human heterokaryons. II. Effect of membrane potential on lateral diffusion.

The rate of appearance, in a population of mouse-human heterokaryons, of cells with intermixed mouse and human surface antigens may be used to estimate the rate of lateral diffusion of the antigens in a single cell. Most heterokaryons appear to restrict diffusion of their surface antigens. These restrictions are altered by exposing either heterokaryons or their parent cells to conditions that change cell surface membrane potential. Media containing unphysiological concentrations of potassium ion, drugs, affecting the Na+,K+ ATPase, or a channel-forming antibiotic, gramicidin, all affect lateral mobility of cell surface antigens in a manner consistent with a common effect on membrane potential.

Animals↗

PubMed2002

Apparent diffusion coefficient mapping predicts mortality and outcome in rats with intracerebral haemodynamic disturbance: potential role of intraoperative diffusion and perfusion weighted magnetic resonance imaging to detect cerebral ischaemia.

BACKGROUND: Usefulness and ability of diffusion and perfusion weighted magnetic resonance images (DWI and PWI) to detect intracerebral haemodynamic disturbance have not been fully evaluated. METHODS: After the right common carotid artery had been ligated, rats were exsanguinated to maintain a mean arterial pressure of 35, 42, or 50 mm Hg (n=6, each group). Apparent diffusion coefficient (ADC) maps were calculated from DWIs and lesion volume (area) was defined based on ADC values (ADC lesion volume (area)). RESULTS: ADC lesion volume during exsanguination in the 35 mm Hg group (417 (111) mm(3), P<0.01) was significantly larger than in the 42 mm Hg group (87 (84) mm(3)) and 50 mm Hg group (42 (58) mm(3)). The low relative cerebral blood flow area, calculated from PWI, was significantly larger during exsanguination in the 35 mm Hg group than in the other groups. ADC lesion volume in the six rats that died within 3 days of the MRI study was significantly larger (median 421 mm(3), range 205-476 mm(3), P<0.005) than in the 12 rats that survived for 3 days (median 26 mm(3), range 3-517 mm(3)). Rats with an ADC lesion area over 14 mm(2) on the coronal slice including the caudate putamen during exsanguination died within 3 days or revealed a more severe histopathological outcome than those that survived for 3 days. CONCLUSIONS: Incomplete cerebral ischaemia created by the combination of common carotid artery occlusion and exsanguination could be detected by DWI and PWI both qualitatively and quantitatively. The size of the lesion on ADC mapping was found to correlate with mortality and outcome.

Animals↗

PubMed1982

Diffusion in the vicinity of standard-design nuclear power plants--I. Wind-tunnel evaluation of diffusive characteristics of a simulated suburban neutral atmospheric boundary layer.

A large meteorological wind tunnel was used to simulate a suburban atmospheric boundary layer. The model-prototype scale was 1:300 and the roughness length was approximately 1.0 m full scale. The model boundary layer simulated full scale dispersion from ground-level and elevated release points over surfaces of comparable roughness length. This information should prove useful in a variety of transport and diffusion studies over short to moderate downwind distances. It will be used in Part II as the baseline data set with which to compare diffusion downwind of standard-design nuclear power plants.

Air Pollution, Radioactive↗

PubMedDate not supplied

Translational molecular self-diffusion in magnetic resonance imaging. II. Measurement of the self-diffusion coefficient.

By varying slice-selective gradients in successive data acquisitions, the first in vitro measurements of molecular self-diffusion coefficients were performed in a magnetic resonance imager at 0.35 Tesla. Reasonably accurate measurements were found by the MRI method in comparison with 2.3 T NMR spectrometer measurements on the same samples, and in comparison to reported literature values. Thus, in addition to T1, T2, mobile proton density, flow velocity, magnetic susceptibility, and chemical shift, molecular self-diffusion coefficients are now added to the list of biophysical parameters measurable by magnetic resonance imaging in the noninvasive characterization of biological systems.

Diffusion↗

PubMedDate not supplied

Diffusion-weighted echo planar imaging of ovarian tumors: is it useful to measure apparent diffusion coefficients?

PURPOSE: Our goal was to test the hypothesis, as previously reported in other studies, that apparent diffusion coefficients (ADCs) provide specific information to diagnose ovarian tumors, especially to discriminate between benign and malignant lesions. METHOD: T1-and T2-weighted spin echo imaging and diffusion-weighted echo planar imaging were performed in 31 women with 61 cystic components of ovarian tumors. RESULTS: The lesions that showed typical watery intensity, hypointensity in T1-weighted imaging, and hyperintensity in T2-weighted imaging had similar ADCs, ranging from 1.54 to 1.84 x 10(-3) mm2 /s. The lesions that showed signal intensity different from typical watery intensity in conventional MRI tended to have low ADCs. In endometrial cysts, the mean ADC of the subgroup that showed typical watery intensity was higher than that of other subgroups. CONCLUSION: With conventional MRI, a tendency of ADCs could be predicted. ADCs may not provide additional information, especially to discriminate benign from malignant lesions.

Adult↗

PubMedDate not supplied

Periodically rotated overlapping parallel lines with enhanced reconstruction-based diffusion tensor imaging. Comparison with echo planar imaging-based diffusion tensor imaging.

OBJECTIVE: To evaluate diffusion tensor imaging (DTI) based on periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) compared with DTI based on single-shot echo planar imaging (EPI). METHODS: Diffusion tensor data were acquired with PROPELLER (PROPELLER-DTI, 3 NEX), EPI (EPI-DTI2, 16 NEX) with the same acquisition time (11.4 minutes) and with EPI (EPI-DTI1, number of excitations = 4) with a shorter acquisition time (2.8 minutes). Regions of interest were set in the genu and splenium of the corpus callosum, as determined on T2-weighted fast spin echo images and fractional anisotropy (FA) maps, separately. Two neuroradiologists visually evaluated image distortion and quality in the supra- and infratentorial structures. RESULTS: In the genu, standard deviation determined by respective FA maps was decreased in order of PROPELLER-DTI, EPI-DTI1, and EPI-DTI2. Both EPI-DTI sequences were quantitatively superior in the splenium, but PROPELLER-DTI was less distorted. CONCLUSION: Periodically rotated overlapping parallel lines with enhanced reconstruction-DTI could become a complementary tool when qualitatively evaluating seriously distorted structures.

Adult↗

PubMedDate not supplied

Diffusion-weighted imaging of malignant breast tumors: the usefulness of apparent diffusion coefficient (ADC) value and ADC map for the detection of malignant breast tumors and evaluation of cancer extension.

The authors used breast diffusion-weighted imaging (DWI) to diagnose breast cancer and identify cancer extension. Isotropic DWI was performed with EPI. The apparent diffusion coefficient (ADC) value was calculated and displayed on an ADC map. The authors compared between the distribution of low ADC values and pathologic cancer extension. The mean ADC value of breast cancer was 1.12 +/- 0.24 x 10(-3) mm/s, which was lower than that of normal breast tissue. The ADC value for invasive ductal carcinoma was lower than that of noninvasive ductal carcinoma. The sensitivity of the ADC value for breast cancer using a threshold of less than 1.6 x 10(-3) mm/s was 95%. Seventy-five percent of all cases showed precise distribution of low ADC value as cancer extension. The causes of underestimation were susceptibility artifact from bleeding and the limit of spatial resolution. Benign proliferative change showed a low ADC value. The authors conclude that DWI has a potential for clinical appreciation in detecting breast cancer.

Adult↗

PubMed2000

Crossover from nonclassical to classical chemical kinetics in an initially separated A + B<-->C reaction-diffusion system with arbitrary diffusion constants.

The asymptotic long-time properties of the reaction front formed in a reversible reaction-diffusion process A + B<-->C with initially separated reactants are investigated. The case of arbitrary nonzero values of the diffusion constants DA, DB, DC of the components A, B, C and the initial concentrations a0 and b0 of A and B is considered. The system is studied in the limit of g-->0, where g is the backward reaction rate constant. In accordance with previous work, the dynamics of the reaction front is described as a crossover between the "irreversible" regime at times t << g-1 and the "reversible" regime at times t >> g-1. It is shown that through this crossover the macroscopic properties of the reaction front, such as the global rate of C production, the motion of the reaction zone center, and the concentration profiles of the components outside the reaction front, are unchanged. The concentration profiles of the components inside the reaction zone are described by quasistatic equations. The results of the theoretical consideration are confirmed by computing the mean-field kinetics equations.

Chemistry↗

PubMedDate not supplied

A diffusion theory model of spatially resolved, steady-state diffuse reflectance for the noninvasive determination of tissue optical properties in vivo.

A model based upon steady-state diffusion theory which describes the radial dependence of diffuse reflectance of light from tissues is developed. This model incorporates a photon dipole source in order to satisfy the tissue boundary conditions and is suitable for either refractive index matched or mismatched surfaces. The predictions of the model were compared with Monte Carlo simulations as well as experimental measurements made with tissue simulating phantoms. The model describes the reflectance data accurately to radial distances as small as 0.5 mm when compared to Monte Carlo simulations and agrees with experimental measurements to distances as small as 1 mm. A nonlinear least-squares fitting procedure has been used to determine the tissue optical properties from the radial reflectance data in both phantoms and tissues in vivo. The optical properties derived for the phantoms are within 5%-10% of those determined by other established techniques. The in vivo values are also consistent with those reported by other investigators.

Diffusion↗

PubMed2005

What has diffusion imaging in animals told us about diffusion imaging in patients with ischaemic stroke?

BACKGROUND AND PURPOSE: In acute ischaemic stroke, the amount (and type) of cellular damage underlying diffusion-weighted imaging (DWI) lesion appearances is unclear. We summarized all information from experimental studies of DWI in focal ischaemia models. METHODS: We systematically reviewed all published studies of DWI in focal ischaemic stroke models. We extracted key experimental details to determine CORRELATIONS between histological features and DWI lesion characteristics. RESULTS: Of 141 potentially eligible papers (including more than 2,817 animals, mostly rats), details of key experimental methods were unfortunately often omitted. Consistent findings amongst high-quality studies with blinded analysis included: neuronal damage persists or progresses despite early DWI lesion 'normalisation'; the apparent diffusion coefficient is not very sensitive to the amount of neuronal damage; the 'brighter' the DWI lesion, the greater the neuronal damage; and the DWI lesion may reflect glial more than neuronal changes. Anaesthesia and fixation techniques may inadvertently affect these findings. CONCLUSIONS: The relationship between cellular damage and DWI lesion appearance, particularly recovery patterns in reperfusion experiments, remains imprecise. Key experimental details could be reported more completely and consistently. Potential problems from repeated anaesthetics need to be addressed. Early DWI lesion 'recovery' in acute stroke patients may largely reflect glial rather than neuronal 'recovery'.

Animals↗

PubMed2002

Predictors of hemorrhagic transformation after intravenous recombinant tissue plasminogen activator: prognostic value of the initial apparent diffusion coefficient and diffusion-weighted lesion volume.

BACKGROUND AND PURPOSE: Hemorrhagic transformation (HT) is a potentially dangerous complication of thrombolytic therapy. Recent studies suggest that diffusion-weighted MRI (DWI) can help to predict the risk of intracerebral hemorrhage (ICH) after thrombolysis. We sought to examine which pretreatment DWI parameters and clinical data are predictive of ICH after intravenous thrombolysis. METHODS: We retrospectively reviewed our prospective stroke database for patients with ischemic stroke treated with intravenous recombinant tissue plasminogen activator (rtPA) within 3 hours from symptom onset who had DWI before treatment and MRI with T2* sequence or CT 24 to 48 hours later to assess for ICH over the past 4 years. We measured the volumes and voxel-by-voxel apparent diffusion coefficient (ADC) values of the initial DWI lesions and retrieved demographic data, risk factors, National Institutes of Health Stroke Scale (NIHSS) scores on admission, and blood tests results. We examined several variables using univariate and multivariate regression analyses to determine predictors of ICH. RESULTS: Twenty-nine patients fulfilled our inclusion criteria; 17 patients (58%) had ICH, and of these 4 (13%) had symptomatic ICH and fatal outcome. On univariate analysis, higher systolic blood pressure, NIHSS score, serum glucose level, volume of initial DWI lesion, and absolute number of voxels with ADC value < or =550x10(-6) mm2/s were statistically associated with ICH, and all were subjected to multivariate analysis. However, only the absolute number of voxels, ie, volume of ischemic tissue on DWI, with ADC < or =550x10(-6) mm2/s emerged as an independent predictor of ICH. CONCLUSIONS: Our findings suggest that volumetric ADC analysis can be used to assess ICH risk after thrombolysis. This may be particularly helpful if rtPA is to be given outside the 3-hour window.

Adult↗

PubMed1975

Diffusing capacity, membrane diffusing capacity, capillary blood volume, pulmonary tissue volume, and cardiac output measured by a rebreathing technique.

A rebreathing method for estimating diffusing capacity, membrane diffusing capacity, pulmonary capillary blood volume, pulmonary capillary blood flow, and pulmonary tissue volume consists of rebreathing into a bag for 15 sec while acetylene, (18O)-carbon monoxide, oxygen, and helium are continuously sampled by a mass spectrometer. Because the masses of carbon monoxide and nitrogen are nearly identical at 28, it was necessary to use a stable isotope, C18O, to distinguish this gas with the mass spectrometer. Comparison of the pulmonary capillary blood flow by the rebreathing technique with the simultaneously obtained indicator dilution measurement in anesthetized dogs revealed good agreement. Estimations of pulmonary tissue volume appeared to be quite reproducible and consistent; the values tended to be somewhat smaller and less variable among normal subjects than reported by other investigators. After subtraction of capillary blood volume, tissue volume was 311 plus or minus 73 ml at rest and increased significantly to 352 plus or minus 61 ml at 75 watts of exercise. Pulmonary tissue volume in dogs using the rebreathing method averaged 9.2 ml per kg of body weight, a mean comparable to previously reported estimates using the ether plethysmographic method. The slope of pulmonary capillary blood flow (cardiac output in normal subjects) as a function of oxygen consumption during exercise in normal subjects of 0.0060 times oxygen consumption in milliliter per min was identical to published values. The rebreathing technique provides a rapid, reliable, noninvasive method for estimating pulmonary hemodynamic parameters.

Acetylene↗

PubMed1996

Hemodialysis increases apparent diffusion coefficient of brain water in nephrectomized rats measured by isotropic diffusion-weighted magnetic resonance imaging.

The nature of brain edema in dialysis disequilibrium syndrome (DDS) was investigated by diffusion-weighted magnetic resonance imaging (DWI). DWI was performed on normal or bilaterally nephrectomized rats before, and immediately after, hemodialysis. Hemodialysis was performed with a custom-made dialyzer (surface area 150 cm2) against a bicarbonate-buffered bath for 90 min with or without 70 mM urea. Hemodialysis with non-urea bath decreased plasma urea by 21 mM, and plasma osmolality by 22 mosmol/kg H2O, and increased brain water content by 8.0% (all < 0.05), while hemodialysis with urea bath did not affect plasma urea, osmolality, or brain water content. Three sets of axial DWI images of the brain were obtained at different gradient weighing factors with an in-plane resolution of 0.39 mm2. The apparent diffusion coefficient (Dapp) of the brain water was not affected by bilateral nephrectomy, or by hemodialysis in normal rats. In nephrectomized rats, brain Dapp was significantly increased after dialysis with non-urea bath (1.15 +/- 0.08 vs 0.89 +/- 0.07 x 10(-9)m2/sec, P < 0.01). No significant changes of brain water Dapp could be observed after dialysis with urea bath. The increased Dapp associated with DDS indicates that brain extracellular water increases and/or intracellular water decreases after hemodialysis. Our results strongly suggest that the brain edema induced by hemodialysis in uremic rats is due to interstitial edema rather than cytotoxic edema. Furthermore, our results support a primary role for the "reverse urea effect" in the pathogenesis of brain edema in DDS.DWI may be a useful diagnostic tool for DDS in patients with end-stage renal disease.

Animals↗

PubMed2005

Diffusion-weighted imaging with calculated apparent diffusion coefficient of enhancing extra-axial masses.

BACKGROUND AND PURPOSE: Sometimes intracranial contrast-enhancing tumors like meningiomas, metastases, lymphomas, and schwannomas can mimic each other. It was the aim of the present study to investigate if intracranial contrast-enhancing lesions can be reliably differentiated with the help of diffusion-weighted imaging with calculated apparent diffusion coefficients (ADCs). METHODS: 29 patients (ages ranging from 22 to 82 years, mean age of 58.6 years) were included. Nine meningiomas, 7 metastases, 6 lymphomas, and 7 schwannomas were investigated. The ADC value in the lesions and in the perifocal edema was analyzed. RESULTS: For the lymphomas, the authors measured the lowest ADC values in the contrast-enhancing part (0.59 +/- 0.09 . 10(-3) mm2/sec). The meningiomas showed a mean ADC value of 0.98 +/- 0.18 . 10(-3) mm2/sec. The schwannomas and metastases showed higher ADC values of 1.33 +/- 0.28 . 10(-3) mm2/sec and 1.05 +/- 0.20 . 10(-3) mm2/sec. The authors saw a statistically significant difference between lymphomas, meningiomas, and metastases concerning the ADC values in the contrast-enhancing part. CONCLUSIONS: In spite of a small sample size and partly a wide range of values, the authors found statistically significant differences between meningiomas, metastases, and lymphomas concerning ADC values. Nevertheless, a differentiation of these lesions only with the help of ADC values seems questionable.

Adult↗

PubMed2004

Modeling of diffuse-diffuse photon coupling via a nonscattering region: a comparative study.

It is well established that diffusion approximation is valid for light propagation in highly scattering media, but it breaks down in nonscattering regions. The previous methods that manipulate nonscattering regions are essentially boundary-to-boundary coupling (BBC) methods through a nonscattering void region based on the radiosity theory. We present a boundary-to-interior coupling (BIC) method. BIC is based on the fact that the collimated pencil beam incident on the medium can be replaced by an isotropic point source positioned at one reduced scattering length inside the medium from an illuminated point. A similar replacement is possible for the nondiffuse lights that enter the diffuse medium through the void, and it is formulated as the BIC method. We implemented both coupling methods using the finite element method (FEM) and tested for the circle with a void gap and for a four-layer adult head model. For mean time of flight, the BIC shows better agreement with Monte Carlo (MC) simulation results than BBC. For intensity, BIC shows a comparable match with MC data compared with that of BBC. The effect of absorption of the clear layer in the adult head model was investigated. Both mean time and intensity decrease as absorption of the clear layer increases.

Computer Simulation↗