Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “diffusion theory”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

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

At least 199 records · Page 11Linked to original sources
PubMed2005

Predicting the adoption of electronic health records by physicians: when will health care be paperless?

OBJECTIVES: The purpose of this study was threefold. First, we gathered and synthesized the historic literature regarding electronic health record (EHR) adoption rates among physicians in small practices (ten or fewer members). Next, we constructed models to project estimated future EHR adoption trends and timelines. We then determined the likelihood of achieving universal EHR adoption in the near future and articulate how barriers can be overcome in the small and solo practice medical environment. DESIGN: This study used EHR adoption data from six previous surveys of small practices to estimate historic market penetration rates. Applying technology diffusion theory, three future adoption scenarios, optimistic, best estimate, and conservative, are empirically derived. MEASUREMENT: EHR adoption parameters, external and internal coefficients of influence, are estimated using Bass diffusion models. RESULTS: All three EHR scenarios display the characteristic diffusion S curve that is indicative that the technology is likely to achieve significant market penetration, given enough time. Under current conditions, EHR adoption will reach its maximum market share in 2024 in the small practice setting. CONCLUSION: The promise of improved care quality and cost control has prompted a call for universal EHR adoption by 2014. The EHR products now available are unlikely to achieve full diffusion in a critical market segment within the time frame being targeted by policy makers.

Attitude of Health Personnel↗
PubMed2003

Solid-state 13C and 1H spin diffusion NMR analyses of the microfibril structure for bacterial cellulose.

To obtain further information about the cause for the rather large splitting of the C4 resonance line into the downfield (C4D) and upfield (C4U) lines in CP/MAS 13C NMR spectra for native cellulose, 13C and 1H spin diffusion measurements have been conducted by using different types of bacterial cellulose samples. In 13C spin diffusion measurements, the C4D resonance line is selectively inverted by the Dante pi pulse sequence and the 13C spin diffusion is allowed to proceed from the C4D carbons to other carbons including the C4U carbons with use of the 13C4-enriched bacterial cellulose sample. The analysis based on the simple spin diffusion theory for the process experimentally observed reveals that the C4U carbons may be located at distances less than about 1 nm from the C4D carbons. In 1H spin diffusion measurements, poly(vinyl alcohol) (PVA) films in which ribbon assemblies of bacterial cellulose are dispersed are employed and the 1H spin diffusion process is examined from the water-swollen PVA continuous phase to the dispersed ribbon assemblies by the 13C detection through the 1H-13C CP technique. As a result, it is found that the C4D and C4U carbons are almost equally subjected to the 1H spin diffusion from the PVA phase, indicating that the C4U carbons are not localized in some limited area, e.g. in the surfacial region, but are distributed in the whole area in the microfibrils. These experimental results suggest that the C4U carbons may exist as structural defects probably due to conformational irregularity associated with disordered hydrogen bonding of the CH(2)OH groups in the microfibrils.

Acetobacter↗
PubMed2003

In vivo optical properties of normal canine prostate at 732 nm using motexafin lutetium-mediated photodynamic therapy.

The optical properties (absorption [mu(a)], transport scattering [mu('s)] and effective attenuation [mu(eff)] coefficients) of normal canine prostate were measured in vivo using interstitial isotropic detectors. Measurements were made at 732 nm before, during and after motexafin lutetium (MLu)-mediated photodynamic therapy (PDT). They were derived by applying the diffusion theory to the in vivo peak fluence rates measured at several distances (3, 6, 9, 12 and 15 mm) from the central axis of a 2.5 cm cylindrical diffusing fiber (CDF). Mu(a) and mu('s) varied between 0.03-0.58 and 1.0-20 cm(-1), respectively. Mu(a) was proportional to the concentration of MLu.Mu(eff) varied between 0.33 and 4.9 cm(-1) (mean 1.3 +/- 1.1 cm(-1)), corresponding to an optical penetration depth (8 = 1/(mu(eff)) of 0.5-3 cm (mean 1.3 +/- 0.8 cm). The mean light fluence rate at 0.5 cm from the CDF was 126 +/- 48 mW/cm2 (N = 22) when the total power from the fiber was 375 mW (150 mW/cm). This study showed significant inter- and intraprostatic differences in the optical properties, suggesting that a real-time dosimetry measurement and feedback system for monitoring light fluences during treatment should be advocated for future PDT studies. However, no significant changes were observed before, during and after PDT within a single treatment site.

Animals↗
PubMed1993

Optimization of multifiber light delivery for the photodynamic therapy of localized prostate cancer.

The understanding of light distribution within the target organ is essential in ensuring efficacy and safety in photodynamic therapy (PDT). A computer simulator of light distribution in prostatic tissue was employed for optimizing dosimetry for PDT in localized prostatic cancer. The program was based on empirically determined light distributions and optical constants and an assumed fluence rate differential from fiber source to necrosis periphery. The diffusion theory approximation to the Boltzmann transport equation was the applicable formulation relevant to prostatic tissue, which has a high albedo with forward-scattering characteristics. Solving this equation of diffusive transfer for the appropriate fiber geometry yielded the energy fluence distributions for cleaved fiber and cylindrical diffuser light delivery. These distributions, confirmed by our measurements, show a 1/r and 1/square root of r dependency (r = distance from light source) of the fluence phi (r) for the cleaved fiber and diffuser, respectively. This manifests itself by the tighter spacing of energy fluence isodoses in the case of the cleaved fiber. It was predicted that for a typical PDT regime a single interstitially placed cleaved fiber would treat 0.05-0.72 cm3. Four parallel fibers improved the uniformity of light distribution and treatment volume, and an interfiber separation of 12 mm would be necessary to provide optimal overlap of PDT necrosis, treating 0.26-3.6 cm3. The cylindrical diffuser, however, could treat larger volumes, and it was predicted that four 3 cm long diffusers at an optimal separation of 25 mm would treat 25-88 cm3 of prostatic tissue.

Computer Simulation↗
PubMed1993

Depth-dose equivalent relationship for cosmic rays at various solar minima.

Galactic cosmic rays (GCR) pose a serious radiation hazard for long-duration missions. In designing a lunar habitat or a Mars transfer vehicle, the radiation exposure determines the shielding thickness, and hence the weight of spacecraft. In designing a habitat one has to focus on the worst-case radiation flux and its uncertainties. Using the spherically symmetric diffusion theory of the solar modulation of GCR, and data on the differential energy spectra of hydrogen, helium, oxygen, and iron from 1965 to 1989, it has been shown that the flux is determined by the diffusion parameter which is a function of the time in the solar cycle. This analysis also showed that the fluxes in the 1954 and 1976-1977 solar minima were similar and higher than those in 1965. In this paper, we have extended the spherical solar modulation theory back to 1954. These results show that the 1954-1955 GCR flux was nearly the same as that from 1976 to 1977 and that the 1965 flux values were nearly the same as those in 1986. Using this theory we have obtained the GCR spectra for all the nuclei and calculated the depth dose as a function of aluminum thickness. Using the ICRP 26 value for the quality factor, and the 1976-1977 spectra, it is shown that the shielding required to stay below 0.5 Sv is 17.5(-3)+8 g cm-2 of aluminum, and 9(-1.5)+5 g cm-2 to stay below 0.6 Sv. The calculated dose equivalent using the ICRP 60 values for quality factors is about 15% higher than that calculated using the ICRP 26 value. However, the errors on the quality factor itself may be substantial and are not taken into account.

Aluminum↗
PubMed2005

In vitro measurements of absolute blood oxygen saturation using pulsed near-infrared photoacoustic spectroscopy: accuracy and resolution.

Pulsed photoacoustic spectroscopy was used to measure blood oxygen saturation in vitro. An optical parametric oscillator laser system provided nanosecond excitation pulses over the wavelength range 740-1040 nm which were used to generate photoacoustic signals in a cuvette through which a saline suspension of red blood cells was circulated. The signal amplitude and the effective attenuation coefficient were extracted from the photoacoustic signals as a function of wavelength to provide photoacoustic spectra of the blood. From these, the relative concentrations of oxy- and deoxyhaemoglobin, and therefore blood oxygen saturation (SO2), were determined using forward models of the absorbed energy distribution based on diffusion theory. A standard linear model of the dependence of absorbance on the concentration of chromophores was also used to calculate the blood oxygen saturation from the signal amplitude spectra. The diffusion approximation model was shown to produce the highest accuracy in blood SO2. The photoacoustically determined oxygen saturation was found to have an accuracy of +/-4% SO2 for signal amplitude data and +/-2.5% SO2 for effective attenuation spectra. The smallest change in oxygen saturation that can be measured using this technique was +/-1% SO2.

Acoustics↗
PubMed1973

Analysis of Mauthner cell responses to iontophoretically delivered pulses of GABA, glycine and L-glutamate.

1. The intracellularly recorded responses of goldfish Mauthner neurones to iontophoretically applied pulses of amino acids have been analysed: their time courses have been compared with each other, and with those predicted from diffusion theory.2. The rise time of the response to GABA is slower than that to glycine or L-glutamate. The response curves of the latter substances were very similar, and unlike that of GABA were markedly affected by increasing the distance of pipette-tip from the membrane. The results suggest that the time course of the responses to glycine and L-glutamate are determined mainly by free diffusion in the brain tissue (at least within about 200 mum of the cell), while that to GABA must be rate-limited by other factors, e.g. drug-receptor activation time.3. The possibility that the responses are influenced by some desensitizing process was investigated by applying a second (test) drug pulse during the response to a prior conditioning one. In the case of glycine and of L-glutamate there was no attenuation of the response to a second pulse at any time. With GABA, however, the second response was reduced during the period of the conditioning response; the reduction was progressively less marked the later the test pulse occurred. A similar effect with GABA was seen when glycine was used as the test pulse. The responses to long-maintained drug pulses also indicated that for GABA, but not for glycine or glutamate, there seems to be some desensitizing process present.4. Calculated time courses of responses to brief pulses of glycine and of L-glutamate (based upon diffusion theory) differed somewhat from the observed curves, largely during the falling phase. However, when the calculations were based upon second-order reactions (two molecules of drug per receptor) the diffusion model gave results very like the observed ones.5. Possible implications of these results for the role these three amino acids may have as neuro-transmitters are mentioned.

Aminobutyrates↗
PubMed2004

Diffusive-to-ballistic transition in dynamic light transmission through thin scattering slabs: a radiative transfer approach.

We study the deviation from diffusion theory that occurs in the dynamic transport of light through thin scattering slabs. Solving numerically the time-dependent radiative transfer equation, we obtain the decay time and the effective diffusion coefficient Deff. We observe a nondiffusive behavior for systems whose thickness L is smaller than 8l(tr), where l(tr) is the transport mean free path. We introduce a simple model that yields the position of the transition between the diffusive and the nondiffusive regimes. The size dependence of Deff in the nondiffusive region is strongly affected by internal reflections. We show that the reduction of approximately 50% of Deff that was observed experimentally [Phys. Rev. Lett. 79, 4369 (1997)] can be reproduced by the radiative transfer approach. We demonstrate that the radiative transfer equation is an appropriate tool for studying dynamic light transport in thin scattering systems when coherent effects play no significant role.

Diffusion↗
PubMed2004

Theories for analyzing polymorphism data in duplicated genes.

A simple model for the evolutionary process of a pair of duplicated genes under concerted evolution is developed. The model considers mutation, recombination and gene conversion between two genes in a finite population. Based on diffusion theory, the expected amount of DNA variation within and between two genes are obtained. To investigate the pattern of DNA polymorphism, a coalescent tool to simulate patterns of polymorphism is developed. The theoretical results are well in agreement with polymorphism data in duplicated genes. The effect of selection on the pattern of polymorphism is also considered.

Alleles↗
PubMed2003

Measurement and analysis of light distribution in intralipid-10% at 650 nm.

A method for measuring the distributions of light in the biological tissue phantom Intralipid by use of an optical fiber is presented and measurements of distributions of light in Intralipid-10% suspensions at 650 nm are described. This approach is complementary to that in which an optical fiber with an isotropic tip detects the distribution of light in tissue phantoms. The characteristics of the distance-dependent intensity of scattering light in different directions were revealed by the experimental results; the effects of the optical parameters and of the radius of the incident beam on the distributions of light in tissue phantoms were given; and the experimental results were analyzed by the diffusion theory. These studies will help in further understanding of the scattering characteristics of tissue.

Diffusion↗
PubMed2004

[Measurements of diffusion reflectance and transmittance of biological tissues by using CCD].

The CCD experimental setup and the CCD image analysis system that were developed by using C Language were presented, and the spatially resolved diffusion reflectance and transmittance of porcine muscle and fat were determined by using CCD experimental setup. The experimental results were compared with those of Monte Carlo simulations and those of the two models based on diffusion theory, and we found good agreement between theory and experiment for the range from the beam center to 12 mm. It i s shown that the CCD experimental setup and the CCD image analysis system can be used to measure the spatially resolved diffusion reflectance and transmittance of biological tissues, and this work offers the experimental bases for noninvasive and fast measurement of optical parameters of biological tissues.

Algorithms↗
PubMed1981

Nutrition of the intervertebral disc: solute transport and metabolism.

The metabolism of the canine nucleus pulposus was investigated at different oxygen tensions. It was found that even at high oxygen tensions the metabolism is mainly anaerobic, only approximately 1.5% of the glucose being converted to carbon dioxide. The concentration dependence of oxygen consumption is limited to very low oxygen tensions. Values of oxygen consumption and lactic acid production were used to calculate the concentration profiles of these substances within the nucleus pulposus, using a diffusion theory. The predicted concentration profiles were compared with the experimental measurements of concentration at various positions in the disc. The good agreement in these values found in the nucleus confirms that the main mechanism of metabolite transport is diffusion, and the main route of nutrient supply into the nucleus is via the endplate.

Anaerobiosis↗
PubMed1988

Fluoride ion diffusion from polyalkenoate (glass-ionomer) cements.

Long-term elution of fluoride ions has been studied for one silicate and six glass-ionomer cements over a period of some two and-a-half years. Because equilibrium was not reached, classical diffusion theory could not be applied to the process. It was postulated that two elution processes were taking place, one short-term and rapid, the other more gradual and prolonged. It was possible only to estimate the maximum value for the diffusion coefficients.

Dental Cements↗
PubMed1990

Diffusion of technology innovation.

Within the context of nursing informatics as a field that addresses the use of information technology by nurses as they care for patients, carry out administrative tasks in health facilities, and educate others in the discipline, this article presents the theoretic perspectives of the adoption and implementation of such technologies. Research in the area of diffusion of innovations is reviewed from the perspective of classical diffusion theory and organizational theory. The state of the art and methodological issues of research in this area are addressed. Areas for future study are proposed.

Communication↗
PubMed1989

Time-resolved reflectance spectroscopy in turbid tissues.

Monte Carlo simulations illustrate how various absorption mu a and scattering mu s coefficients influence time-dependent reflectance R (t) from a semi-infinite homogeneous turbid tissue following an impulse of narrow-beam irradiation. The tissue absorption coefficient mu a in cm -1 can be obtained from measurements of R (t) after the first 20-200 ps (depends on mu s) following an impulse by the expression: mu a = -(n/c) d In [R(t)]/dt - 3n/2ct where n is the tissue-refractive index and c is the in vacuo speed of light. Early data in the first 20-200 ps do not conform to this expression or to diffusion theory. Monte Carlo simulations allow study of the early R(t) behavior. The volume of tissue involved in a measurement is specified by a volume radius r that approximately equals (6Dtc/n)1/2 where t is the time of measurement and D is the optical diffusion constant D = (3 mu s (1 - g]-1. At 50 ps and typical values of mu s = 100 cm-1 and anisotropy equal to 0.9, r equals 5 mm. The upper limit for measurable mu a values is limited by how quickly the reflectance signal is attenuated, and is estimated for current streak camera technology to be mu a less than or equal to 21 cm-1, assuming several measurements are taken over a dynamic range of two orders of magnitude within a 10 ps period.

Absorption↗
PubMed1991

Simple photon diffusion analysis of the effects of multiple scattering on pulse oximetry.

Photon diffusion theory is used to derive analytical expressions that relate the ac-dc intensity ratios measured by transmission-mode and reflectance-mode pulse oximeters to arterial oxygen saturation (SaO2). The effects of multiple scattering are examined by comparing the results of the photon diffusion analysis with those obtained using an analysis based on the Beer-Lambert law which neglects scattering. We show that the difference between the average lengths of the paths travelled by red and infrared photons makes the calibration curve of oximeters sensitive to the total attenuation coefficients of the tissue in the two wavelength bands, as well as to absorption by the pulsating arterial blood. Therefore, the shape of the calibration curve is affected by tissue blood volume, source-detector placement, and other variables that change the wavelength dependence of the attenuation coefficient of the tissue. After evaluating the relationship between SaO2 and the red/IR ac-dc ratio (R) under a variety of physiological conditions, we conclude that, for oximeters utilizing fixed calibration curves based on measurements obtained from normal subjects, errors introduced by interfering variables should be less than a few percent when SaO2 exceeds 70%. Predicted errors at lower oxygen saturation values are substantially greater because R is much more sensitive to interfering variables in this measurement range.

Absorption↗
PubMed2002

Time and length scales for diffusion in liquids.

The first six even moments of the displacement of a molecule in water and an atom in liquid argon are found by molecular dynamics simulations and compared with the moments predicted by diffusion theory. We find a noticeable difference between the moments higher than the second. The ratio between predicted and calculated moments approaches unity as 1/t for times larger than 10 ps. Continuous time random walk is used to explain this slow approach of the moments to their diffusion limit.

Argon↗
PubMed1999

Mode-coupling smoluchowski dynamics of a double-stranded DNA oligomer

The local dynamics of a double-stranded DNA d(TpCpGpCpG)(2) is obtained to second order in the mode-coupling expansion of the Smoluchowski diffusion theory. The time correlation functions of bond variables are derived and the (13)C-nmr spin-lattice relaxation times T(1) of different (13)C along the chains are calculated and compared to experimental data from the literature at three frequencies. The DNA is considered as a fluctuating three-dimensional structure undergoing rotational diffusion. The fluctuations are evaluated using molecular dynamics simulations, with the ensemble averages approximated by time averages along a trajectory of length 1 ns. Any technique for sampling the configurational space can be used as an alternative. For a fluctuating three-dimensional (3D) structure using the three first-order vector modes of lower rates, higher order basis sets of second-rank tensor are built to give the required mode coupling dynamics. Second- and even first-order theories are found to be in close agreement with the experimental results, especially at high frequency, where the differences in T(1) for (13)C in the base pairs, sugar, and backbone are well described. These atomistic calculations are of general application for studying, on a molecular basis, the local dynamics of fluctuating 3D structures such as double-helix DNA fragments, proteins, and protein-DNA complexes. Copyright 1999 John Wiley & Sons, Inc.

Journal Article↗