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Results for “diffusion theory”

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At least 325 records · Page 18Linked to original sources
PubMed2005

In vivo spectroscopy of jaundiced newborn skin reveals more than a bilirubin index.

AIM: The aims of this study were to improve the algorithms for calculating a transcutaneous bilirubin index (TcB), to follow the bilirubin concentrations during phototherapy and to evaluate possible changes in skin optical parameters such as pigmentation and erythema during phototherapy. METHOD: Reflectance measurements were performed on 51 jaundiced newborns, of which 10 were subjected to phototherapy. The measurements were collected with a diode array spectrophotometer with an integrating sphere accessory, and a TcB was calculated from the measured spectra using algorithms based on diffusion theory. The newborns' birthweights were > or = 2000 g and their gestational age was > or = 35.5 wk. They had no substantial illnesses, and no newborns were submitted to the study until their second day. Heel prick blood samples were analysed for total serum bilirubin (Sbr) by the diazo reaction method. Phototherapy equipment was either an overhead lamp or lightbed. RESULTS: Measurements from the forehead gave the best correlation between TcB and Sbr (r = 0.81, p < 0.05). However, during phototherapy no significant correlation between TcB and Sbr was observed. A correlation (r = 0.45, p < 0.05) was found between phototherapy and melanin index obtained from the patients' back. CONCLUSIONS: Reflectance spectroscopy is useful in assessing bilirubin concentrations before phototherapy, and can also reveal changes in skin parameters such as pigmentation occurring as a result of phototherapy.

Algorithms↗
PubMed1992

Oxidation of phosphatidylcholine membranes by singlet oxygen generated in the gas phase.

Singlet-oxygen (1O2) was generated in the gas phase by heterogeneous photosensitization and bubbled into suspensions of phosphatidylcholine (PC) liposomes. Lipid peroxidation and membrane lysis were observed, and were dependent on the 1O2 concentration and the degree of unsaturation of the liposome. An analysis based on large target diffusion theory indicates that approximately 5000, 2800, and 1600 interactions were required for the lysis of large dioleoylPC, dilinoleoylPC and dilinolenoylPC liposomes, respectively.

Kinetics↗
PubMed1994

Absorption spectrum of hematoporphyrin derivative in vivo in a murine tumor model.

Time-resolved reflectance was used to measure the absorption spectrum of hematoporphyrin derivative (HpD) in vivo in a murine tumor model. Reflectance measurements were performed in the 600-640 nm range on mice bearing the L1210 leukemia. Then the animals were administered 25 mg/kg body weight of HpD intraperitoneally. One hour later the reflectance measurements were repeated. Fitting of the data using the diffusion theory allowed assessment of the absorption coefficient before and after the administration. As a difference between the latter and the former data, the in vivo absorption spectrum of HpD was evaluated. Maximum absorption was measured at 620-625 nm. Similar spectral behavior was obtained for HpD in solution in the presence of low-density lipoproteins.

Animals↗
PubMed1994

Investigating the surface properties and bioadhesion of buccal patches.

By using a two-roll milling method, a new bioadhesive polymer patch formulation for drug controlled delivery and consisting of Carbopol 934P (CP), polyisobutylene (PIB), and polyisoprene (PIP) was prepared. The effects of different ratios of CP:PIB:PIP on the surface properties, adhesion, and swelling of buccal patches were investigated. It was found that the surface properties of buccal patches were not only dependent on the CP content but also dependent on the PIB:PIP ratio. The strongest peeling strength was found on the buccal patches with a CP:PIB:PIP ratio of 50:43.75:6.25. The maximum bioadhesion of patches was found following 2- to 8-h contact with the test medium, and this observation could be explained by the interpenetration of macromolecular chains at the polymer-polymer interface (based on the diffusion theory of polymer adhesion).

Acrylic Resins↗
PubMed2001

In vitro optical characterization and discrimination of female breast tissue during near infrared femtosecond laser pulses propagation.

Ultrashort infrared laser pulses were transmitted through excised female breast tissue. The resulted signal was recorded by a streak camera with a time resolution of the order of a few ps. Experimental data of the temporal spread of the ultrashort pulse during the transmission through the tissue have been analyzed using the Patterson analytical expression derived from the diffusion theory. This resulted in the calculation of the absorption and reduced scattering coefficients, which are related to the optical characteristics of each type of tissue. The goal of the study was to use the theoretical values of the coefficients to discriminate different kinds of tissue.

Breast Neoplasms↗
PubMedDate not supplied

Analytical model to describe fluorescence spectra of normal and preneoplastic epithelial tissue: comparison with Monte Carlo simulations and clinical measurements.

Fluorescence spectroscopy has shown promise for the detection of precancerous changes in vivo. The epithelial and stromal layers of tissue have very different optical properties; the albedo is relatively low in the epithelium and approaches one in the stroma. As precancer develops, the optical properties of the epithelium and stroma are altered in markedly different ways: epithelial scattering and fluorescence increase, and stromal scattering and fluorescence decrease. We present an analytical model of the fluorescence spectrum of a two-layer medium such as epithelial tissue. Our hypothesis is that accounting for the two different tissue layers will provide increased diagnostic information when used to analyze tissue fluorescence spectra measured in vivo. The Beer-Lambert law is used to describe light propagation in the epithelial layer, while light propagation in the highly scattering stromal layer is described with diffusion theory. Predictions of the analytical model are compared to results from Monte Carlo simulations of light propagation under a range of optical properties reported for normal and precancerous epithelial tissue. In all cases, the mean square error between the Monte Carlo simulations and the analytical model are within 15%. Finally, model predictions are compared to fluorescence spectra of normal and precancerous cervical tissue measured in vivo; the lineshape of fluorescence agrees well in both cases, and the decrease in fluorescence intensity from normal to precancerous tissue is correctly predicted to within 5%. Future work will explore the use of this model to extract information about changes in epithelial and stromal optical properties from clinical measurements and the diagnostic value of these parameters.

Algorithms↗
PubMedDate not supplied

In vivo determination of optical properties of normal and tumor tissue with white light reflectance and an empirical light transport model during endoscopy.

Determination of tissue optical properties is fundamental for application of light in either therapeutical or diagnostics procedures. In the present work we implemented a spatially resolved steady-state diffuse reflectance method where only two fibers (one source and one detector) spaced 2.5 mm apart are used for the determination of the optical properties. The method relies on the spectral characteristics of the tissue chromophores (water, dry tissue, and blood) and the assumption of a simple wavelength dependent expression for the determination of the reduced scattering coefficient. Because of the probe dimensions the method is suited for endoscopic measurements. The method was validated against more traditional models, such as the diffusion theory combined with adding doubling for in vitro measurements of bovine muscle. Mean and standard deviation of the absorption coefficient and the reduced scattering coefficient at 630 nm for normal mucosa were 0.87+/-0.22 cm(-1) and 7.8+/-2.3 cm(-1), respectively. Cancerous mucosa had values 1.87+/-1.10 cm(-1) and 8.4+/-2.3 cm(-1), respectively. These values are similar to data presented by other authors. Blood perfusion was the main variable accounting for differences in the absorption coefficient between the studied tissues.

Algorithms↗
PubMedDate not supplied

Time-resolved imaging of optical coefficients through murine chest cavities.

As small animal optical imaging and tomography are gaining popularity for interrogating functional and molecular events in vivo, it becomes increasingly necessary to gain knowledge of the optical properties of the species investigated to better understand and describe photon propagation through their tissues. To achieve characterization of the spatial variation of average optical properties through murine chest cavities, time- and spatially resolved measurements of femto-second laser pulse transmission are performed through mice using a high-speed gated image intensifier. Application of time-resolved diffusion theory for finite slab geometry is first confirmed on phantoms and then applied to in vivo measurements for spatially resolving and quantifying mouse optical properties. Photon transmission images through mouse chest cavities are further obtained at different time gates to visualize the spatial variation observed and confirm the optical coefficient patterns calculated.

Animals↗
PubMed2003

A submillimeter resolution fluorescence molecular imaging system for small animal imaging.

Most current imaging systems developed for tomographic investigations of intact tissues using diffuse photons suffer from a limited number of sources and detectors. In this paper we describe the construction and evaluation of a large dataset, low noise tomographic system for fluorescence imaging in small animals. The system consists of a parallel plate-imaging chamber and a lens coupled CCD camera, which enables conventional planar imaging as well as fluorescence tomography. The planar imaging data are used to guide the acquisition of a Fluorescence Molecular Tomography (FMT) dataset containing more than 106 measurements, and to superimpose anatomical features with tomographic results for improved visual representation. Experimental measurements exhibited good agreement with the diffusion theory models used to predict light propagation within the chamber. Tests of the instrument's capacity to quantitatively reconstruct fluorochrome distributions in three dimensions showed less than 5% errors between actual fluorochrome concentrations and FMT findings, and suggested a detection threshold of approximately 100 femptomoles for small localized objects. Experiments to assess the instrument's spatial resolution demonstrated the ability of the system to resolve objects placed at clear distances of less than 1 mm. This is a significant resolution increase over previously developed systems for animal imaging, and is primarily due to the large dataset employed and the use of inversion methods. Finally, the in vivo imaging capacity is showcased. It is expected that the large dataset collected can enable superior imaging of molecular probes in vivo and improve quantification of fluorescence signatures.

Animals↗
PubMed2005

Noninvasive reflection spectra provide quantitative information about the spatial distribution of skin chromophores.

In this work, a new method of analyzing noninvasive reflection spectra is presented. The approach explicitly models the inhomogeneity of chromophore distributions in living tissues and thus extracts not only apparent chromophore concentrations but also relative chromophore distributions in tissues. Furthermore, it works with spectra obtained with short source-detector separations where the diffusion theory of light transport through turbid media is not valid, and formerly presented methods thus fail. The effect of inhomogeneously distributed chromophores in a multicompartment model of tissues on measured reflection spectra is explained and an algorithm to deconvolute tissue spectra based on this model is presented. It is evaluated using simulated spectra and measurements on phantoms, which are made up of partially printed pieces of paper to simulate inhomogeneous dye distributions. Its applicability to real tissue is proven using reflection spectra obtained with 130 microm source-detector separation from a hemoperfusion stop experiment. The proposed model accurately determines apparent chromophore concentrations and corresponding distributions in simulated spectra and phantoms. Regarding real tissue spectra, the results correspond to former publications and the spectral reconstruction yields only minimal residuals, indicating a complete and accurate spectral deconvolution. In conclusion, the presented approach is a suitable extension and amendment to existing models of light transport through inhomogeneous samples.

Algorithms↗
PubMedDate not supplied

A photon dose distribution model employing convolution calculations.

A three-dimensional photon beam calculation is described which models the primary, first-scatter, and multiple-scatter dose components from first principles. Three key features of the model are (1) a multiple-scatter calculation based on diffusion theory, (2) the demonstration of the modulation transfer function of the radiation dose transport process, and (3) the use of the finite fast Fourier transform to perform the required convolutions. The results of calculations for cobalt-60 in a homogeneous phantom are used to verify the accuracy of the model.

Fourier Analysis↗
PubMedDate not supplied

Optical dosimetry for interstitial photodynamic therapy.

An approach to photodynamic treatment of tumors is the interstitial implantation of fiber optic light sources. Dosimetry is critical in identifying regions of low light intensity in the tumor which may prevent tumor cure. We describe a numerical technique for calculating light distributions within tumors, from multiple fiber optic sources. The method was tested using four translucent plastic needles, which were placed in a 0.94 X 0.94 cm grid pattern within excised Dunning R3327-AT rat prostate tumors. A cylindrical diffusing fiber tip, illuminated by 630 nm dye laser light was placed within one needle and a miniature light detector was placed within another. The average penetration depth in the tumor region between the two needles was calculated from the optical power measured by the detector, using a modified diffusion theory. Repeating the procedure for each pair of needles revealed significant variations in penetration depth within individual tumors. Average values of penetration depth, absorption coefficient, scattering coefficient, and mean scattering cosine were 0.282 cm, 0.469 cm-1, 250 cm-1 and 0.964, respectively. Calculated light distributions from four cylindrical sources in tumors gave reasonable agreement with direct light measurements using fiber optic probes.

Adenocarcinoma↗
PubMed1996

Experimental test of theoretical models for time-resolved reflectance.

Four different expressions, derived from the diffusion theory or the random walk model, were used to fit time-resolved reflectance data for the evaluation of tissue optical properties. The experimental reflectance curves were obtained from phantoms of known optical parameters (absorption and transport scattering coefficients) covering the range of typical values for biological tissues between 600 and 900 nm. The measurements were performed using an instrumentation for time-correlated single-photon counting. The potential of the four methods in the assessment of the absorption and transport scattering coefficients was evaluated in terms of absolute error, linearity error, and dispersion of data. Each method showed different performances depending on the optical properties of the sample and the experimental conditions. We propose some criteria for the optimal choice of the fitting method to be used in different applications.

Biophysical Phenomena↗
PubMed1998

Equilibration of air temperature inside the thimble of a Farmer-type ion chamber.

Ionization chambers are frequently moved from one environment to another, sometimes with significant differences in temperature between the chamber and measurement phantom. To obtain reliable ionization data, the temperature of the air in the chamber must be allowed to equilibrate with the measuring phantom. The air temperature inside a thimble of a Farmer-type ion chamber was measured as a function of time for various phantom materials (air, water, and plastic). Equilibration rates for the various conditions are presented. Heat-diffusion theory is presented to explain the characteristics of the measured data. Waiting times for temperature equilibration down to 10% of the initial temperature difference ranges from 1 to 18 min, depending on the phantom material and use of bare or covered thimble. Radiation measurements confirm the temperature data.

Equipment Design↗
PubMed1983

Conservation in soil of h(2) liberated from n(2) fixation by hup nodules.

Pigeon peas (Cajanus cajan) were grown in large soil columns (90-cm length by 30-cm diameter) and inoculated with four different strains of cowpea rhizobia, which varied with respect to hydrogen uptake activity (Hup). Despite the profuse liberation of H(2) from Hup nodules in vitro, H(2) gas was not detected in any of the soil columns. When H(2) was injected into the columns, the rates of consumption were highest in the treatments (including control) containing Hup nodules (218 and 177 nmol . h . cm) and lowest in the Hup treatments (158, 92, and 64 nmoles . h . cm). In situ H(2) uptake rates in small soil cores at fixed distances from the nodules decreased exponentially with distance from the nodule (R = 0.99). This decrease in H(2) consumption was associated with a similar decrease in numbers of H(2)-oxidizing chemolithotrophic bacteria as determined by the most-probable-number method. On the basis of two equations derived separately upon diffusive theory (Fix's Law) and kinetic theory (Michaelis-Menten), the empirically derived rate constants and coefficients indicated that all of the H(2) emitted from Hup nodules would be consumed by H(2)-oxidizing bacteria within a 3- to 4.5-cm radius of the nodule surface. It is concluded that H(2) is not lost from the soil-plant ecosystem during N(2) fixation in C. cajan but is conserved by H(2)-oxidizing bacteria.

Journal Article↗
PubMed1983

Effect of sodium taurocholate on gastric secretion in patients with duodenal ulceration.

The effect of sodium taurocholate instillation on the stimulated stomachs of patients with duodenal ulceration has been investigated. Instillation into the stomach of sodium taurocholate significantly reduced pentagastrin stimulated volume and outputs of both acid and sodium, with no change in the calculated volume of duodenogastric reflux. These observations are not consistent with the back-diffusion theory, but suggest direct parietal cell inhibition of gastric secretion by sodium taurocholate.

Duodenal Ulcer↗
PubMed1987

Reflectance measurements of hematocrit and oxyhemoglobin saturation.

Fiberoptic oximeters measure oxyhemoglobin saturation from the optical reflectance of whole blood, but the calibration of such oximeters is hematocrit dependent. Therefore, using photon-diffusion theory and an empirical approach, we have developed a new reflectance method that determines hematocrit and correspondingly corrects the oxyhemoglobin-saturation measurement. Our method employs four fiber-optic light guides, a photodetector, and three inexpensive light-emitting diodes (one with emissions at 660 nm and two at 813 nm). Hematocrit is determined from the ratio of reflectances from the differently spaced emitting fibers at 813 nm and is used to correct the 813-660 nm measurement of oxyhemoglobin saturation. In red cell suspensions, the mean difference between reflectance measurements of hematocrit and conventional determinations was only 2.09% (r = 0.99), and when compared with conventional gasometric measurements of oxyhemoglobin saturation, the reflectance method yielded the same calibration curve for different hematocrits and gave a mean difference of only 2.67%. Although the technique is demonstrated with a cuvette appropriate for an extracorporeal circulation in animal experiments, it could possibly be further developed for fiber-optic catheter oximeters.

Animals↗
PubMed1995

Age-related changes in perimicrovascular protein distribution.

The diffusion hypothesis for physiological aging proposes that an increase in interstitial matrix fiber-to-gel ratio causes a decrease in nutrient diffusion to the cells. This hypothesis predicts a decrease in interstitial matrix protein with age. The objective was to test this hypothesis by determining age-related changes in plasma protein distribution in perimicrovascular and distal regions of rat mesentery interstitial matrix. Rats that were 77, 140, 210, 315, 455, and 630 days old were anesthetized with pentobarbital sodium, and a mesenteric loop was exteriorized. Intravital video microspectrophotometry was performed using wavelengths of 280, 320, and 700 nm. Perimicrovascular protein concentrations from the protein absorbance images were used to obtain the histogram, mean, and skewness of the proximal and distal protein concentration distributions. An exponential gradient model was also used to obtain the proximal and distal protein concentrations and gradient decay constants. Proximal protein concentration increased from 77- to 140-day-old rat and then decreased gradually through 210-, 315-, 455-, and 630-day-old rats. Distal concentration decreased gradually from 140- to 630-day-old rats. There was an increase in positive skewness of the proximal protein distributions from 140- through 630-day-old rats. We found an age-related decrease in perimicrovascular protein and propose that this is due to a decrease in protein permeability with age. The results support the diffusion theory of aging.

Aging↗