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PubMed1988

Diffusion model of the optical absorbance of whole blood.

Photon-diffusion theory has had limited success in modeling the optical transmittance of whole blood. Therefore we have developed a new photon-diffusion model of the optical absorbance of blood. The model has benefited from experiments designed to test its fundamental assumptions, and it has been compared extensively with transmittance data from whole blood. The model is consistent with both experimental and theoretical notions. Furthermore, when all parameters associated with a given optical geometry are known, the model needs no variational parameters to predict the absolute transmittance of whole blood. However, even if the exact value of the incident light intensity is unknown (which is the case in many situations), only a single additive constant is required to scale experiment to theory. Finally, the model is shown to be useful for simulating scattering effects and for delineating the relative contributions of the diffuse transmittance and the collimated transmittance to the total optical density of whole blood. Applications of the model include oximetry and measurements of the arteriovenous oxygen difference in whole, undiluted blood.

Absorption↗
PubMedDate not supplied

Ionic strength affects diffusive permeability to an inorganic phosphate ion of negatively charged dialysis membranes.

Electrolytes undergo electrostatic resistances in reaching membrane surfaces and passing through membrane pores when weakly charged dialysis membranes are used in hemodialysis treatments. Diffusive permeability to an electrolyte of weakly charged dialysis membranes depends upon the effective charge density of the membranes, which varies with ionic strength. The authors carried out dialysis experiments at a temperature of 310 K, with 32P-Na2HPO4 in aqueous NaCl and bovine serum, to obtain diffusive permeability to an inorganic phosphate ion of regenerated cellulose, and polymethylmethacrylate membranes of various fixed charge densities at varying ionic strengths ranging from 1.66 to 100 mol/m3. Diffusive permeability to an inorganic phosphate ion increased with ionic strength. Bovine serum, having an ionic strength of approximately 150 mol/m3, gave higher diffusive permeability to an inorganic phosphate ion. The internal structure of dialysis membranes, such as pore size, surface porosity, and tortuosity, also affects diffusive permeability. The electrolyte diffusion theory gives an effective charge density of -5.3 mol/m3 for the cellulosic membrane. In conclusion, ionic strength enhances diffusive permeability to an inorganic phosphate ion in both aqueous NaCl and bovine serum. Inorganic phosphate ion transport through negatively charged dialysis membranes depends upon the degree of dissociation of inorganic phosphate, the molecular size and valence of hydrated inorganic phosphate ion, the effective charge density and internal structure of the membrane, and ionic strength.

Animals↗
PubMed1997

Theoretical framework for implementing a managed care curriculum for continuing medical education--Part I.

Healthcare reform has created a new working environment for practicing physicians, as economic issues have become inseparably intertwined with clinical practice. Although physicians have recognized this change, and some are returning to school for formal education in business and healthcare administration, formal education may not be practical or desirable for the majority of practicing physicians. Other curriculum models to meet the needs of these professionals should be considered, particularly given the growing interest in continuing education for physicians in the areas of managed care and related aspects of practice management. Currently, no theory-based models for implementing a managed care curriculum specifically for working physicians have been developed. This paper will integrate diffusion theory, instructional systems design theory, and learning theory as they apply to the implementation of a managed care curriculum for continuing medical education. Through integration of theory with practical application, a CME curriculum for practicing physicians can be both innovative as well as effective. This integration offers the benefit of educational programs within the context of realistic situations that physicians can apply to their own work settings.

Curriculum↗
PubMed2005

Diffusion of breast conserving surgery in medical communities.

Excluding skin cancers, breast cancer is the most common form of cancer in women. Due to an increased focus on early detection, many more cases of breast cancer are now diagnosed at an early stage, which makes the use of breast conserving surgery (BCS) an efficacious and often more desirable treatment choice than mastectomy. An analysis of the variation in the use of BCS in the United States was performed using data from the years 1988 and 1994, and stratifying hospitals on the basis of teaching status. In both 1988 and 1994, BCS was highest in academic teaching hospitals and lowest in community hospitals. This finding is interpreted within the framework of classical diffusion theory. Social and cultural norms in local medical communities have a strong effect on the degree to which innovations diffuse rapidly or not. This analysis is useful in the understanding of geographic and hospital-based variations in treatment for early stage breast cancer and other illnesses that have long and strongly held traditions of treatment.

Academic Medical Centers↗
PubMed2000

Theory of solid-phase microextraction

The main objective of this contribution is to describe the fundamental concepts associated with solid-phase microextraction (SPME). Theory provides insight when developing SPME methods and identifies parameters for rigorous control and optimization. A mathematical model has been developed to understand the principal processes of SPME by applying basic fundamental principles of thermodynamics and diffusion theory. The model assumes idealized conditions and is limited to air, liquid, or headspace above liquid sampling. Theory for ideal cases can be quite accurate for trace concentrations in simple matrices such as air or drinking water at ambient conditions when secondary factors such as thermal expansion of polymers and changes in diffusion coefficients because of solutes in polymers can be neglected. When conditions are more complex, theory for ideal cases still efficiently estimates general relationships between parameters.

Journal Article↗
PubMed2000

Power law mass dependence of diffusion: A mode coupling theory analysis

The self-diffusion coefficient of a tagged molecule is known to exhibit a weak mass dependence, especially for solutes with size comparable to or larger than the size of the solvent molecules. Sometimes this mass dependence can be fitted to a power law, with a small exponent, less than 0.1. This weak mass dependence has often been considered as supportive of the hydrodynamic picture (that is, the Stokes-Einstein relation) of diffusion rather than the kinetic theory approach, which predicts a stronger mass dependence, for example, via the Enskog theory. Neither can explain the weak power-law mass dependence. In order to understand this, we have carried out a mode coupling theory (MCT) analysis of diffusion. It is found that a straightforward application of the existing mode coupling theory expressions lead to an inaccurate mass dependence-it predicts an increase of diffusion coefficient with an increase of the mass. We find that this is because of the inadequate description of the initial decay of the collective contributions to the friction. We have proposed a new prescription to accurately describe the short time dynamics of the density and the current term. In addition, we have modified the existing MCT by imposing the full self-consistency between the frequency-dependent friction and the mean square displacement over the whole time and frequency plane. Previously the self-consistency was performed only at the zero frequency level between the zero frequency friction and the diffusion coefficient. With these two generalizations, the mode coupling theory is found to provide a fairly accurate description of the mass dependence. In particular, the theory can correctly reproduce the power-law dependence of solvent-solute diffusion ratio on solute-solvent mass ratio, observed in computer simulations of Bearman and Jolly [Mol. Phys. 44, 665 (1981)]. Another important result is that the current mode is found to play no significant role in determining the diffusion. Thus the hydrodynamic argument of weak mass dependence has little validity for same size solute-solvent systems.

Journal Article↗
PubMed2002

Transport across 1,9-decadiene precisely mimics the chemical selectivity of the barrier domain in egg lecithin bilayers.

The barrier domain solubility-diffusion theory of lipid bilayer permeability relates the permeability coefficient (P(m)) to the solute's partition coefficient (PC(barrier/w)) and diffusion coefficient (D(barrier)) in the ordered chain region of the bilayer that serves as the barrier region for polar permeants. To select the best solvent to mimic the barrier domain, permeability coefficients across a layer of 1,9-decadiene were compared with permeability coefficients from bilayer transport. Rate constants for transport, k, of alpha-methyl substituted analogues of p-toluic and p-methylhippuric acid were measured across a layer of 1,9-decadiene embedded in a PTFE filter membrane placed between two aqueous solutions in side-by-side diffusion cells. Permeability coefficients (P(1,9-decadiene)) were normalized to that obtained for p-toluic acid, which was included in donor solutions. The correlation of log(P(bilayer)) versus log(P(1,9-decadiene)) was linear with a slope of 0.99 +/- 0.02 SD, indicating that 1,9-decadiene precisely mimics the egg lecithin bilayer barrier domain in its chemical selectivity. Using the decadiene membrane transport method to indirectly estimate partition coefficients for similarly sized permeants extended the range of measurable values beyond those readily attainable by the traditional shake-flask method, allowing measurement of 1,9-decadiene/water PCs as low as 3 x 10(-7).

Algorithms↗
PubMed2006

Influence of fiber optic probe geometry on the applicability of inverse models of tissue reflectance spectroscopy: computational models and experimental measurements.

Accurate recovery of tissue optical properties from in vivo spectral measurements is crucial for improving the clinical utility of optical spectroscopic techniques. The performance of inversion algorithms can be optimized for the specific fiber optic probe illumination-collection geometry. A diffusion-theory-based inversion method has been developed for the extraction of tissue optical properties from the shape of normalized tissue diffusion reflectance spectra, specifically tuned for a fiber probe that comprises seven hexagonally close-packed fibers. The central fiber of the probe goes to the spectrometer as the detecting fiber, and the surrounding six outer fibers are connected to the white-light source as illumination fibers. The accuracy of the diffusion-based inversion algorithm has been systematically assessed against Monte Carlo (MC) simulation as a function of probe geometry and tissue optical property combinations. By use of this algorithm, the spectral absorption and scattering coefficients of normal and cancerous tissue are efficiently retrieved. Although there are significant differences between the diffusion approximation and the MC simulation at short source-detector (SD) separations, we show that with our algorithm the tissue optical properties are well retrieved within the SD separation of 0.5-3 mm that is compatible with endoscopic specifications. The presented inversion method is computationally efficient for eventual real-time in vivo tissue diagnostics application.

Computer Simulation↗
PubMed1980

Membrane transport of alkyl homologs: role of fluid flow in aqueous diffusion region.

Passive drug transport across a membrane involves the resistances in series offered by the membrane and the liquid layer immediately adjacent to it. The liquid layers generally are referred to as unstirred aqueous diffusion layers, suggesting that solute transport across these layers occurs solely by molecular diffusion. In accord with convective diffusion theory, recent studies showed that the transport from either a dissolving surface or a membrane surface depends not only on molecular diffusion but also on fluid convection inthe liquid adjacent to the membrane. Membrane permeation rates were determined for a series of alkyl p-aminobenzoates, and the results correlated with a model comprised of membrane diffusion and convective diffusion in the adjacent liquid region.

Aminobenzoates↗
PubMed1987

Inverse diffusion methods for data peak separation.

Previous methods for separation of overlapping data peaks include geometrical assessment and Fourier deconvolution. On the basis of inverse diffusion theory, we present new separation methods suitable for convenient programming and rapid calculation of Gaussian area contributions. Both continuum and discrete inverse diffusion models are described. Example computations are given for biological data: density gradient centrifugation, isoelectric focusing electrophoresis, and high-pressure liquid chromatography.

Centrifugation, Density Gradient↗
PubMed1991

Residual monomer in acrylic polymers.

Three proprietary dental resins were found to include acrylates (as distinct from methacrylates). As acrylates are well known to have adverse skin reactions, the nature and content of the acrylates were obtained. One resin contained methyl acrylate, and the other two ethyl acrylate. Also, the extractable free monomer from one proprietary and one experimental bone cement was determined as a function of time. This was carried out: (a) on unpolymerized material as soon as it formed a coherent dough; and (b) on already polymerized material. The extraction of n-butyl methacrylate from the experimental bone cement was very much less than that of methyl methacrylate from the proprietary bone cement. In both cases, whilst the kinetics of loss for short times appeared to conform to diffusion theory, subsequent loss did not; this was attributable to both continued polymerization of the monomer and to the concentration dependence of the diffusion coefficient.

Acrylates↗
PubMed2005

Detection of cortical activation with time-resolved diffuse optical methods.

Simulations based on diffusion theory that use a finite-element method and rely on an magnetic resonance imaging head model suggest that time-resolved diffuse optical techniques could provide information about the depth at which variations in perfusion take place and improve the detection of cortical activation. Experimental investigations were performed with sequentially driven picosecond laser diodes and an eight-channel time-correlated single-photon-counting detection system. The experimental results obtained for activation in the motor cortex, and for the Valsalva maneuver, confirm our assumptions and are in good agreement with the simulated data.

Algorithms↗
PubMed1991

Noninvasive determination of respiratory ozone absorption: development of a fast-responding ozone analyzer.

We developed a chemiluminescent ozone analyzer and constructed an ozone bolus generator with the eventual goal of using a bolus-response method to measure noninvasively the longitudinal distribution of ozone absorption in human lungs. Because the analyzer will be used to sample gases within a single breath, it must have a sufficiently rapid response to monitor changes in ozone concentration during a four-second breathing period, yet its sampling flow must be small enough that it does not interfere with quiet respiratory flows of 300 mL/sec. Our analyzer, which is based on the chemiluminescent reaction between 2-methyl-2-butene and ozone, has favorable performance characteristics: a 90 percent step-response time of 110 msec; a linear calibration from 0.03 to 10 parts per million (ppm)2 with a sensitivity of 2.3 nA/ppm; a signal-to-noise ratio of 30 evaluated at 0.5 ppm; and a minimum detection limit of 0.017 ppm. At an airflow corresponding to quiet breathing, the ozone generator is capable of producing single boluses with a peak ozone fraction as high as 4 ppm, but containing only 0.35 micrograms of ozone dispersed over a small volume of 19 mL. To test the combination of ozone analyzer and bolus generator, we performed bolus-response experiments at steady airflows of 50 to 200 mL/sec in excised pig and sheep tracheas. In spite of the small surface area available for radial diffusion, we found that 25 to 50 percent of the ozone introduced into the trachea was absorbed. By comparing the mathematical moments of the bolus input and the response curves to the predictions of a diffusion theory, we computed an absorption coefficient (K). The values of K increased with increasing airflow, implying that ozone absorption is limited by diffusion processes in the airway lumen as well as in the surrounding tissue.

Absorption↗
PubMed1987

Concentration dependence of protein diffusion.

The concentration dependence of protein self-diffusion constants is described by a free volume diffusion theory which accounts for the effects of local protein concentration fluctuations.

Animals↗
PubMed1996

Integrating sphere effect in whole-bladder wall photodynamic therapy: III. Fluence multiplication, optical penetration and light distribution with an eccentric source for human bladder optical properties.

Whole-bladder-wall (WBW) photodynamic therapy (PDT) is performed using approximately 630 nm light emitted by an isotropic light source centered in the bladder cavity. The phenomenon of an increased fluence rate in this spherical geometry, due to light scattering, is denoted as the integrating sphere effect. The fluence rate and the optical penetration depth depend on a single tissue optical parameter, namely the reduced albedo. The optical properties of (diseased) human bladder tissue, i.e. absorption coefficient, scattering coefficient, anisotropy factor and refractive index, were determined in vitro in the wavelength range of 450-880 nm. The integrating sphere effect and optical penetration depth were calculated with diffusion theory and compared to Monte Carlo (MC) computer simulations using approximately 630 nm optical properties. With increasing albedo, the integrating sphere effect calculated with diffusion approximation is increasingly larger than that found with MC simulations. Calculated and simulated optical penetration depths are in reasonable agreement. The smaller the integrating sphere effect for a given tissue absorption, the larger the optical penetration depth into the bladder wall, as the effective attenuation coefficient decreases. Optical penetration depths up to approximately 7.5 mm (definition dependent) can be responsible for unintended tissue damage beyond the bladder tissue. MC simulations were also performed with an eccentric light source and the uniformity of the light distribution at the bladder wall was assessed. The simulations show that even for a small eccentricity, the extremes in deviation from the mean fluence rate are large. All these results indicate that WBW PDT should be performed with some kind of in situ light dosimetry.

Cystitis↗
PubMed2000

Smoluchowski dynamics of the vnd/NK-2 homeodomain from Drosophila melanogaster: second-order maximum correlation approximation.

The mode coupling diffusion theory is applied to the derivation of local dynamics in proteins in solution. The rotational dynamics of the bonds along the protein sequence are calculated and compared to the experimentally measured nmr (15)N spin-lattice relaxation time T(1), at 36.5, 60.8, and 81.1 MHz of the vnd/NK-2 homeodomain from Drosophila melanogaster. The starting point for the calculations is the experimental three-dimensional solution structure of the homeodomain determined by multidimensional nmr spectroscopy. The higher order mode-coupling computations are compared also with the recently published first-order approximation calculations. The more accurate calculations improve substantially the first-order ORZLD calculations and show that the role of the strength of the hydrodynamic interactions becomes crucial to fix the order of magnitude of the rotational dynanics for these very compact molecules characterized by partial screening of the internal atoms to water. However, the relative mobility of the bonds along the sequence and the differential fluctuations depend only weakly on the hydrodynamic strength but strongly on the geometry of the three-dimensional structure and on the statistics incorporated into the theory. Both rigid and fluctuating dynamic models are examined, with fluctuations evaluated using molecular dynamics simulations. The comparison with nmr data shows that mode coupling diffusion accounts for the T(1) relaxation pattern at low frequency where the rotational tumbling dominates. An important contribution of internal motions in the nanosecond time scale is seen at high frequencies and is discussed in terms of diffusive concepts.

Animals↗
PubMed2006

A convergent diffusion and social marketing approach for disseminating proven approaches to physical activity promotion.

Approaches from diffusion of innovations and social marketing are used here to propose efficient means to promote and enhance the dissemination of evidence-based physical activity programs. While both approaches have traditionally been conceptualized as top-down, center-to-periphery, centralized efforts at social change, their operational methods have usually differed. The operational methods of diffusion theory have a strong relational emphasis, while the operational methods of social marketing have a strong transactional emphasis. Here, we argue for a convergence of diffusion of innovation and social marketing principles to stimulate the efficient dissemination of proven-effective programs. In general terms, we are encouraging a focus on societal sectors as a logical and efficient means for enhancing the impact of dissemination efforts. This requires an understanding of complex organizations and the functional roles played by different individuals in such organizations. In specific terms, ten principles are provided for working effectively within societal sectors and enhancing user involvement in the processes of adoption and implementation.

Cooperative Behavior↗
PubMed1995

The influence of glucose concentration upon the transport of light in tissue-simulating phantoms.

The effect of glucose upon the transport of light in tissue-simulating phantoms is shown and its possible application for non-invasive glucose monitoring in diabetic patients is discussed. The aim of this paper is to investigate the physical background of this effect. The presence of glucose in an aqueous solution increases its refractive index and therefore has an influence upon the scattering properties of particles suspended in solution. Experimental data on the effect of glucose upon the scattering coefficient and the phase function of aqueous suspensions of spherical polystyrene particles are presented for near-infrared wavelengths and compared to values predicted by Mie theory. The subsequent effect upon light transport in multiple scattering, tissue-simulating phantoms is demonstrated experimentally in a slab geometry and theoretically by applying diffusion theory. It is furthermore shown that optional measurements in the frequency domain allow changes of absorption and scattering coefficient to be separately determined. The possible magnitude of this glucose effect in tissue in vivo is discussed.

Glucose↗