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PubMed2005

Comparison of diffusion approximation and Monte Carlo based finite element models for simulating thermal responses to laser irradiation in discrete vessels.

Both diffusion approximation (DA) and Monte Carlo (MC) models have been used to simulate light distribution in multilayered human skin with or without discrete blood vessels. However, no detailed comparison of the light distribution, heat generation and induced thermal damage between these two models has been done for discrete vessels. Three models were constructed: (1) MC-based finite element method (FEM) model, referred to as MC-FEM; (2) DA-based FEM with simple scaling factors according to chromophore concentrations (SFCC) in the epidermis and vessels, referred to as DA-FEM-SFCC; and (3) DA-FEM with improved scaling factors (ISF) obtained by equalizing the total light energy depositions that are solved from the DA and MC models in the epidermis and vessels, respectively, referred to as DA-FEM-ISF. The results show that DA-FEM-SFCC underestimates the light energy deposition in the epidermis and vessels when compared to MC-FEM. The difference is nonlinearly dependent on wavelength, dermal blood volume fraction, vessel size and depth, etc. Thus, the temperature and damage profiles are also dramatically different. DA-FEM-ISF achieves much better results in calculating heat generation and induced thermal damage when compared to MC-FEM, and has the advantages of both calculation speed and accuracy. The disadvantage is that a multidimensional ISF table is needed for DA-FEM-ISF to be a practical modelling tool.

Blood Vessels↗
PubMed1994

Water diffusivity in porcine stratum corneum measured by a thermal gravimetric analysis technique.

Water is a natural constituent of the stratum corneum (sc) affecting its plasticity and modulating its barrier function. Diffusion coefficients (D) were calculated by measuring the desorption rates of water from porcine sc and dermis samples by a thermal gravimetric analysis (TGA) technique at isothermal conditions in the range 30-80 degrees C. Water-loaded samples were exposed to a flow of dry air, and the change of weight and of temperature were monitored with time. Distinct abrupt decreases in rate of desorption marked three different phases of water in the sc, designated as free, bulk, and bound water. Concomitant with the sharp change in evaporation rate, an increase in temperature was observed, in accordance with the absorption of heat accompanied with the water desorption process. Desorption curves were plotted against time and optimized. Values of D were estimated from the ratio of the evaporated water to the initial sc water content, as a function of the square root of time. The "initial slope" and the t1/2 (time for which Mt/Mo = 0.5, where Mo is the equilibrium amount of water absorbed in the membrane and Mt is the amount of water released by the membrane in a time t) methods gave similar results. The water D values of sc at 30 degrees C calculated by the two methods were 3.3 +/- 0.6 x 10(-10) and 2.7 +/- 0.8 x 10(-10) cm2/s, respectively. These values were about two orders of magnitude lower than the calculated D value for water in the dermis.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2004

Magnetic resonance elastography and diffusion-weighted imaging of the sol/gel phase transition in agarose.

The dynamics of the sol/gel phase transition in agarose was analyzed with magnetic resonance elastography (MRE) and diffusion-weighted imaging, providing complementary information on a microstructural as well as on a macroscopic spatial scale. In thermal equilibrium, the diffusion coefficient of agarose is linearly correlated with temperature, independent of the sol/gel phase transition. In larger agarose samples, the transition from the sol to the gel state was characterized by a complex position and temperature dependency of both MRE shear wave patterns and apparent diffusion coefficients (ADC). The position dependency of the temperature was experimentally found to be qualitatively similar to the behavior of the ADC maps. The dynamics of the temperature could be described with a simplified model that described the heat exchange between sol and gel compartments. The experiments supported the approach to derive temperature maps from the ADC maps by a linear relationship. The spatially resolved dynamics of the temperature maps were therefore employed to determine the elasticities. For this reason, experimental MRE data were simulated using a model of coupled harmonic oscillators. The calculated images agreed well with the experimentally observed MRE wave patterns.

Elasticity↗
PubMed1987

A self-heated thermistor technique to measure effective thermal properties from the tissue surface.

A microcomputer based instrument to measure effective thermal conductivity and diffusivity at the surface of a tissue has been developed. Self-heated spherical thermistors, partially embedded in an insulator, are used to simultaneously heat tissue and measure the resulting temperature rise. The temperature increase of the thermistor for a given applied power is a function of the combined thermal properties of the insulator, the thermistor, and the tissue. Once the probe is calibrated, the instrument accurately measures the thermal properties of tissue. Conductivity measurements are accurate to 2 percent and diffusivity measurements are accurate to 4 percent. A simplified bioheat equation is used which assumes the effective tissue thermal conductivity is a linear function of perfusion. Since tissue blood flow strongly affects heat transfer, the surface thermistor probe is quite sensitive to perfusion.

Animals↗
PubMed2002

Evaluation of a new super-long-pulsed 810 nm diode laser for the removal of unwanted hair: the concept of thermal damage time.

BACKGROUND: Laser hair removal is based on the dual concepts of selective photothermolysis and thermal relaxation time. In most laser hair removal systems, light with emitted pulse durations of 2-50 msec targets hair follicles. A novel concept of laser hair removal uses the thermal damage time rather than the thermal relaxation time of the hair follicle. The follicle's thermal damage time is the amount of time required for diffusion of delivered laser energy from the treated hair to follicular-associated hair stem cells. This can range from 170 to 1000 msec. OBJECTIVE: To investigate the theory of thermal damage time, we performed a pilot study to evaluate the clinical efficacy and side effect profile of a modified 810 nm diode laser device operating in a super-long-pulse mode (200-1000 msec). METHODS: Ten female subjects with Fitzpatrick skin types I-VI received either one or two laser treatments at eight test sites. Super-long pulse durations of 200-1000 msec were evaluated with delivered fluences ranging from 23 to 115 J/cm2. Subjects were followed for 6 months after the first treatment. Subjects were evaluated for hair removal efficiency, optimal pulse duration and delivered fluence, and associated complication rate. RESULTS: The clinical results show that safe hair removal in all skin types can be accomplished with an 810 nm diode laser delivering super-long pulse durations. Pain and complications were greatest at the highest pulse duration (1000 msec) and the highest fluence (115 J/cm2). Optimal hair reduction at 6 months (31%) was achieved at a thermal diffusion time of 400 msec (46 J/cm2). CONCLUSION: The super-long pulsed 810 nm diode laser can safely remove unwanted hair in a full variety of skin types. Pain and increased risk of complications may preclude the use of the laser at very high fluences and pulse duration in the range of 1000 msec.

Adult↗
PubMed2006

Control of the metal-support interface of NiO-loaded photocatalysts via cold plasma treatment.

NiO-loaded semiconductors have been extensively used as the photocatalysts for water splitting. The metal-support interface is an important factor affecting the efficiency. In the present work, the pretreatment methods were studied to produce a more desirable metal-support interface using Ta2O5 and ZrO2 as the support. The traditional method includes a thermal decomposition, reduction at 773 K, and oxidation at 473 K (R773-O473). The thermal decomposition of Ni(NO3)2 makes the Ni atoms migrate into the bulk of the supports, resulting in a diffused interfacial region. Alternatively, a cold plasma treatment was used to replace the thermal decomposition. Metal salts are quickly decomposed by glow discharge plasma treatment at room temperature, avoiding the thermal diffusion of Ni atoms. With the sequent R773-O473 treatment, a clean metal-support interface is produced. Moreover, the metal particles have optimal shapes with a larger surface. In photocatalysis, the clean metal-support interface is more favorable for the charge separation and transfer, and the increased metal surface provides more active sites. NiO/Ta2O5 and NiO/ZrO2 prepared with the plasma treatment exhibit higher activity for photocatalytic hydrogen generation from pure water and methanol solution, respectively. This work shows the potential of cold plasma treatment in the preparation of metal-loaded catalysts and nanostructured materials.

Journal Article↗
PubMed1999

NMR imaging of thermally polarized helium-3 gas.

It is shown that thermally polarized 3He gas can be used to measure important physical parameters and to design, test, and tune imaging sequences. The bulk values of T1, T2, and the diffusion coefficient were measured in a glass cell containing a mixture of helium-3 (0.8 bar) and oxygen (0.2 bar). They were found to be T1 = 7 s, T2 = 2.4 s, and D = 1.6 cm2 s(-1). The relaxation times T2* and T1 and the apparent diffusion coefficient of thermally polarized helium-3 gas were measured in the rat lung, and these parameters were used to design a helium-3 optimized multi-spin-echo sequence which was shown to increase the signal-to-noise ratio sufficiently to obtain the first NMR-images of thermally polarized helium-3 in the rat lung.

Animals↗
PubMed2001

Giant acceleration of free diffusion by use of tilted periodic potentials.

The effective diffusion coefficient for the overdamped Brownian motion in a tilted periodic potential is calculated in closed analytical form. Universality classes and scaling properties for weak thermal noise are identified near the threshold tilt where deterministic running solutions set in. In this regime the diffusion may be greatly enhanced, as compared to free thermal diffusion with, for a realistic experimental setup, an enhancement of up to 14 orders of magnitude.

Journal Article↗
PubMedDate not supplied

H+ back diffusion and peptic activity related to ulcerogenesis following thermal injury.

This investigation was undertaken to study the effects of cutaneous burn on the integrity of the gastric mucosal barrier (GMB). H+ back diffusion and peptic activity of gastric aspirate were determined in burned and shamburned rats. Twenty-four hours after a 30 percent body surface contact burn (Brevital anesthesia) and a 48-hour fast with saline and ten percent dextrose ad libitim gastric secretions were collected for two hours following pyloric ligation under urethan anesthesia. Studied were: basal secretion, histamine stimulation, glycine instillation, saline instillation, and a combined glycine instillation with histamine augmentation. Significant reductions in acid secretion occurred in burned rats unless the secretion was stimulated by histamine and/or glycine. Following stimulation, acid values in the burned rats exceeded those of unburned rats. GMB disruption, as determined by glycine trapping, was not an apparent pathogenetic mechanism of gastric erosion in 24-hour postburn rats. Peptic activity was elevated in burned rats with mucosal erosion, suggesting a contributory role for this proteolytic enzyme.

Animals↗
PubMed1976

Diffusion frequency factors in some simple examples of transition-state rate theory.

In the Eyring rate theory, the rate constant is expressed as a product of a frequency factor (kT/h) and a quotient of partition functions. Continuing an earlier paper, it is shown here by means of simple examples that the Eyring formalism may be extended to include diffusion-controlled processes if a new frequency factor, D/Rlambda, is substituted for kT/h, where D = diffusion coefficient, lambda = thermal de Broglie wavelength, and R = a characteristic distance that depends on the particular case. The Eyring formalism is also applicable in hybrid cases, intermediate between diffusion (D/Rlambda) and dynamics (kT/h). Because these modified frequency factors are not "universal" (as kT/h is), their main use (other than conceptual) would appear to be in cases in which one considers a simple model (with calculable frequency factor) together with a related more complicated model. In the latter case, as an approximation, one would combine the "simple" frequency factor with the "complicated" quotient of partition functions in order to obtain the desired rate constant. Examples are given.

Diffusion↗
PubMed1983

CO2 lasers: beam patterns in relation to surgical use.

According to surgeons operating with a variety of CO2 lasers available at the National Cancer Institute of Milan (Coherent, Sharplan, Valfivre), these lasers have different cutting and coagulation properties. To identify what physical parameters might corroborate the subjective impression of the surgeons, a comparative study of the crater forms in perspex samples was performed. Perspex was chosen for its thermal properties (in fact, its thermal conductivity and diffusivity are similar to those of organic tissue) and because it allowed good visualization and measurement of crater characteristics. Depth of penetration, crater diameter, and extension of thermal damage were measured against power, focal length, and exposure time for each CO2 laser model. These results can be used as an index of behaviour of different surgical lasers. It appears that for fully characterizing the interaction of surgical lasers with the sample, it is necessary to specify either power, focal length, exposure time, or beam mode.

Biophysical Phenomena↗
PubMed2006

Monitoring of odor compounds produced by solid waste treatment plants with diffusive samplers.

Nuisance caused by odors is one of the most important problems for waste management plants. To control an odor nuisance, it must first be quantified. The analytical difficulties in odor measurements are related to the high number of volatile components (belonging to several chemical classes), above all when the concentration is lower than the detection limit of the technique used for the measurement. In this work, 2-butanone, alpha-pinene, tetrachloroethylene, dimethyldisulfide, beta-pinene, limonene, phenol and benzoic acid are determined, because they are representative of some important classes of compounds with higher odor impact. The compounds are sampled with thermal desorbable radial diffusive samplers Radiello containing Tenax cartridges. The analytical repeatability and the complete thermal desorption of the cartridges were verified for each odor compound; the relative standard deviations for repeated samples and the recovery percentage were, respectively, less than 7% and about 97% for all compounds. The measurements of the linearity of sampling showed no systematic difference according to the collection period. The comparison between the odor threshold and the limit of detection demonstrated that this method is reliable for the recognition and quantification of odor compounds, allowing Public Administration to impose legal limits and the control agencies to verify them.

Adsorption↗
PubMed1993

Assessment of pulmonary edema based on extravascular thermal volume in dogs.

To establish criteria of pulmonary edema by extravascular thermal volume (ETV), ETV and a roentgenogram of the thorax were compared in normal and experimental hydrostatic pulmonary edema dogs. ETV was measured by the double indicator dilution technique using heat as a diffusible indicator and sodium ions as a non-diffusible indicator (thermal-Na method). Mean ETV in 14 dogs under physiologic conditions was 8.20 +/- 1.44 ml/kg. Comparison of ETV with the roentgenogram indicated the density of thoracic the roentgenogram to increase when ETV exceeded 11 ml/kg. Pulmonary edema was recognized clearly in the thoracic roentgenogram at more than 15 ml/kg of ETV. Further increase in ETV did not occur with additional radiographic change. ETV was significantly (P < 0.01) correlated with arterial oxygen tension (PaO2) but not statistically correlated with colloid oncotic pressure (COP), pulmonary arterial wedge pressure (PAWP) or PAWP-COP. Measurement of ETV would thus, appear to be a reliable method for evaluating the severity of pulmonary edema, which has been difficult to diagnose by roentgenography and should facilitate the early diagnosis and management of pulmonary edema.

Animals↗
PubMed2001

Thermodynamics and diffusion of a lattice gas on a simple cubic lattice.

A lattice gas model with nearest neighbor attractive interactions on a simple cubic lattice is considered. The method of nonequilibrium statistical ensembles due to Zubarev is used to derive expressions for jump and chemical diffusion coefficients. For thermally activated hopping dynamics in the hydrodynamical (low frequency, long wavelength) limit, and neglecting specific memory effects, these expressions are represented in a simple form in terms of the zero concentration limit of the chemical diffusion coefficient and equilibrium characteristics, i.e., the chemical potential, and the probability for two nearest neighbor sites to be vacant. These equilibrium characteristics are calculated by means of the self-consistent diagram approximation. The equilibrium characteristics and diffusion coefficients are in a good agreement with extensive Monte Carlo simulation results.

Journal Article↗
PubMed2005

Xenon NMR measurements of permeability and tortuosity in reservoir rocks.

In this work we present measurements of permeability, effective porosity and tortuosity on a variety of rock samples using NMR/MRI of thermal and laser-polarized gas. Permeability and effective porosity are measured simultaneously using MRI to monitor the inflow of laser-polarized xenon into the rock core. Tortuosity is determined from measurements of the time-dependent diffusion coefficient using thermal xenon in sealed samples. The initial results from a limited number of rocks indicate inverse correlations between tortuosity and both effective porosity and permeability. Further studies to widen the number of types of rocks studied may eventually aid in explaining the poorly understood connection between permeability and tortuosity of rock cores.

Geological Phenomena↗
PubMed2006

Single-molecule dynamics in a self-assembled 2D molecular sieve.

A two-dimensional molecular sieve has been realized. It consists of a host matrix of molecularly engineered building blocks self-assembled at the liquid-solid interface. The simultaneous size- and shape-dependent dynamics of different guest molecules is observed in situ, in real time with submolecular resolution using a scanning tunneling microscope both at the liquid-solid interface and under vacuum. The temperature-dependent dynamics reveals that the diffusion proceeds through thermally activated channeling between single-molecule surface cavities.

Diffusion↗
PubMed1994

Basic optothermal diffusion theory for interstitial laser photocoagulation.

A theoretical basis for interstitial laser photocoagulation (ILP) practiced with point-emitting fiber tips has been established by solving the bioheat transfer equation, using basic Green's function methods, for steady and instantaneous point sources of both optical energy and direct heat. Three combination optical and thermal parameters have been identified that strongly influence temperature distributions during ILP. These are defined here as optothermal heat capacities and an optothermal diffusion length, all of which characterize how a thermal diffusion temperature profile is flattened and reduced when optical diffusion is added. Relevance and limitations of this theory for practical ILP are discussed. A useful result is a mathematical verification of previous empirical observations that point optical sources heat tissues less than point heat sources of the same power. A comparison of normalized theoretical temperature transients with published measurements suggests that in normal liver, blood perfusion cooling may exceed thermal conduction by a factor of 5.6 +/- 1.7.

Biophysical Phenomena↗
PubMed2004

Characterization of zinc hydroxynitrates by diffuse reflectance infrared spectroscopy--structural modifications during thermal treatment.

Four zinc hydroxynitrates are known, but only the Infrared spectrum of Zn5(OH)8(NO3)2.2H2O is reported in the literature. Here, we report the IR spectra of the other three zinc hydroxynitrates, namely Zn(OH)(NO3).H2O, Zn3(OH)4(NO3)2 and Zn5(OH)8(NO3)2. The positions of the bands of the nitrate groups between 1000 and 1600 cm(-1) are discussed in relation to the structure of these compounds. The modifications of these IR spectra with temperature are consistent with previous studies on the thermal decomposition of these compounds.

Nitrates↗