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PubMed2003

Forest reflectance modeling: theoretical aspects and applications.

Forest reflectance models are able to contribute to the interpretation of satellite images over forested areas. A multipurpose forest reflectance model, its basic ideas, model input and output options are briefly described. The possibility to apply the model in forest remote sensing is demonstrated through the following simulation examples: the age dependence of forest reflectance, the seasonal course of forest reflectance, defoliation effects on forest reflectance, sensitivity of chlorophyll and water indices with respect to leaf chlorophyll and water content, respectively, and uncertainties in these indices introduced by the variation in stand parameters. When used together with the atmospheric radiative transfer and sensor calibration models, the forest reflectance model is applicable in satellite-imagery-aided forest ecology and health assessment, forest inventory and management.

Calibration↗
PubMed1988

A theoretical model of DNA curvature.

Distortions from the uniform idealized B-DNA structure are investigated in terms of differential interactions between adjacent nucleotide pairs on the basis of conformational energy calculations. A theoretical model of DNA curvature is proposed based on the evaluation of the curvature vector defined in the complex plane and the corresponding variance. The model appears to contain the basic physical features for translating the deterministic fluctuations of DNA sequences in superstructure elements. It allows the quantitative reproduction of all the available gel electrophoresis experiments on both periodical polynucleotides and tracts of DNAs as well as the theoretical prediction of the sequence dependent DNA writhing in good agreement with the experimental data. The general pattern of agreement between the theoretical and experimental data and the biological significance of the results obtained allow an extensive application of the model for the screening of DNA regions which are possible candidates for protein recognition.

DNA↗
PubMed1995

How do GPs discuss subjects other than illness? Formulating and evaluating a theoretical model to explain successful and less successful approaches to discussing psychosocial issues.

A theoretical model was formed, according to grounded theory, to understand how discussions about psychosocial problems might be designed. It was used in 19 videotaped consultations where it was considered relevant for the physician to take up psychosocial issues. 'Concern' i.e. that the patient could express that which was most pressing, was used as an indicator of outcome. A uniform pattern was observed in those cases where the patients expressed 'concern', in that the physician encouraged the patient by using open-ended questions, by following up the information received, and by having an empathic approach. On the other hand, in consultations where 'concern' was not expressed, it was noticed that the physician often asked close-ended, leading or negative questions, thus putting an end to the dialogue and, as a consequence, to the follow-up phase. Important therapeutic skills would appear to be the ability to reassure and support the patient as well as to be able to explain the connection between the patient's symptoms and psychosocial problems, rather than to solve these problems.

Adult↗
PubMed2006

Quantitative time-resolved fluorescence spectra of the cortical sarcoma and the adjacent normal tissue determined with an in vivo experimental method and theoretical model.

In this paper, a time-resolved fluorescence spectrum method is proposed to study the difference between the cortical sarcoma and the adjacent normal tissue. This is a fluorescence-light-intensity-independent method, which makes it more reliable in the presence of interference light emitted by nonfluorescent light absorbers and scatterers. In the implementation, a model of the tumor tissue and a model of light transport in turbid media using improved Monte Carlo simulations are proposed. In this method, the time-resolved fluorescence is collected in vivo. The improved theoretical model can interpret the fluorescence lifetime measurement. The simulation results show that the decay constant of the tumor tissue spectrum is larger than that of the adjacent normal tissue because of hypoxia and hyperplasia, which fits the theoretical analysis very well.

Animals↗
PubMed2002

Theoretical modeling of oral absorption of barbiturates.

The structural characteristics governing gastric absorption of barbiturates were studied using QSAR methodology. The results showed that the gastric absorption rate constant could be modeled using the theoretical parameters, accessible surface area, atomic charges and electrostatic potentials. Using molecular connectivity indexes was also satisfactory in modeling the absorption.

Absorption↗
PubMed1990

Theoretical model for assessing haemodynamics in arterial networks which include bypass grafts.

The paper presents a theoretical model which can be used to simulate a vascular network which includes loops and bypass grafts, a feature not possible with previous models. Using the linearised Navier-Stokes equations, the linearised equation of a uniform thick-walled viscoelastic tube, and the equation of continuity, the model is applied to a vascular network which includes a bypass graft. This method represents each segment of an artery or graft by a four-terminal-network whose A, B, C, D parameters are functions of the frequency and physical characteristics of the segment. The model predicts the flow and pressure waveforms at any point in the human arterial network very accurately when compared with data obtained from normal patients, patients with arterial stenoses and for hypertensive patients. The model also gives results which are in close agreement with hydraulic experimental data for the input impedance of systems with bypass loops.

Arteries↗
PubMed1999

A theoretical model of compliance in young diabetics.

The purpose of this paper is to present a theoretical model of compliance in young diabetics. According to the model, compliance is an active, intentional and responsible process in which young diabetics work to maintain their health in collaboration with the physician. Factors which improved compliance were motivation, the results of care, a sense of normality and adequate energy and will-power for care. Motivation was improved by support and encouragement given by parents and was found to influence energy and will-power for care. Energy and will-power for care were increased through the support of parents, encouragement and the results of care. Furthermore it was found that when young diabetics were not afraid of complications, their energy and will-power for care were better. Fear may, however, improve compliance, which is connected with the control of diabetes. The results of the study can be used to plan and implement care in young diabetics. In addition to this, the model is a good starting-point for further development and for developing an instrument to measure compliance and the factors related to it.

Adolescent↗
PubMed1990

Transport mechanisms in iontophoresis. I. A theoretical model for the effect of electroosmotic flow on flux enhancement in transdermal iontophoresis.

Bulk fluid flow or volume flow in the direction of counterion flow is a probable mechanism for enhanced flux of uncharged species by iontophoresis. Both the electrical volume force effect, resulting from the interaction of the "ion atmosphere" and the electric field, and an induced osmotic pressure effect produce volume flow in the same direction as counterion flow through the membrane. Since each of these effects is proportional to the membrane charge and the imposed electric field, we classify both as electroosmotic flow. This research develops a detailed theoretical model which allows the effect of volume flow on flux enhancement to be evaluated. A detailed theoretical result for the electroosmotic flow coefficient also results from the analysis. The model assumes that transport occurs in three types of aqueous pores: positively charged, neutral, and negatively charged. For hairless mouse skin (HMS), pore size, charge, and number are evaluated from transference number, volume flow, and electrical resistance data. The flux enhancement ratio is J1/J1D = sigma Ai alpha i/[1-exp(-alpha i)], where i = pore type, and the summation runs over the three pore types. Ai is the area fraction of pore type i effective for transport; J1 and J1D are flux of species 1 with and without the electric field, respectively; and alpha i is given by alpha i = F(-delta phi/RT)[zeta 1 + (-zeta mi)Bari2Cmi(Gi + F)].(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Cutaneous↗
PubMed2002

Theoretical models of wound healing: past successes and future challenges.

The complex biology of wound healing is an area in which theoretical modelling has already made a significant impact. In this review article, the authors describe the key features of wound healing biology, divided into four components: epidermal wound healing, remodelling of the dermal extracellular matrix, wound contraction, and angiogenesis. Within each of these categories, previous modelling work is described, and the authors identify what they regard as the main challenges for future theoretical work.

Epidermis↗
PubMed1989

A theoretical model for perceived intensity in human taste and smell. II. Temporal integration and reaction times.

The theoretical model for perceived intensity in human taste and smell published previously was extended by incorporating the concept of signal detectability and temporal integration phenomena at low and high stimulus levels. The processes involved in human taste and smell perception are divided into three stages: a) An "internal meter" registers outside signals. Adaptation occurs at this stage. b) The meter is read for two possible purposes: to detect a weak signal or to estimate the signal's intensity. Both tasks require the smoothing of noise, which is accomplished by integration of the meter over time. c) The internal estimate is expressed through the use of a scaling method.

Humans↗
PubMed2005

Control of blood vessel structure: insights from theoretical models.

Blood vessels are capable of continuous structural adaptation in response to changing local conditions and functional requirements. Theoretical modeling approaches have stimulated the development of new concepts in this area and have allowed investigation of the complex relations between adaptive responses to multiple stimuli and resulting functional properties of vascular networks. Early analyses based on a minimum-work principle predicted uniform wall shear stress in all segments of vascular networks and led to the concept that vessel diameter is controlled by a feedback system based on responses to wall shear stress. Vascular reactions to changes in transmural pressure suggested feedback control of circumferential wall stress. However, theoretical simulations of network adaptation showed that these two mechanisms cannot, by themselves, lead to stable and realistic network structures. Models combining reactions to fluid shear stress, circumferential stress, and metabolic status of tissue, with propagation of stimuli upstream and downstream along vascular segments, are needed to explain stable and functionally adequate adaptation of vascular structure. Such models provide a basis for predicting the response of vascular segments exposed to altered conditions, as, for example, in vascular grafts.

Animals↗
PubMed1998

Estimation of acoustical streaming: theoretical model, Doppler measurements and optical visualisation.

An approximate solution for the streaming velocity generated by flat and weakly focused transducers was derived by directly solving the Dirichlet boundary conditions for the Poisson equation, the solution of the Navier-Stokes equation for the axial components of the streaming velocity. The theoretical model was verified experimentally using a 32 MHz pulsed Doppler unit. The experimental acoustical fields were produced by three different 4 mm diameter flat and focused transducers driven by the transmitter generating the average acoustic power within the range from 1 microW to 6 mW. The streaming velocity was measured along the ultrasonic beam from 0 to 2 cm. Streaming was induced in a solution of water and corn starch. The experimental results showed that for a given acoustic power the streaming velocity was independent of the starch density in water, changed from 0.3 to 40 grams of starch in 1 l of distilled water. For applied acoustic powers, the streaming velocity changed linearly from 0.2 to 40 mm/s. Both, the theoretical solutions for plane and focused waves and the experimental results were in good agreement. The streaming velocity field was also visualised using the particle image velocimetry (PIV) and two different evaluation methods. The first based on the FFT-based cross-correlation analysis between small sections for each pair of images and the second employing the algorithm of searching for local displacements between several images.

Acoustics↗
PubMed1987

Verification of theoretical modeling of heat transmission in teeth by in vivo experiments.

A theoretical axisymmetric tooth model, simulating the conditions involved in the drinking of a liquid of a certain temperature, was compared with results of a similar in vivo experiment. The temperature changes as a result of one draught of a hot/cold liquid were recorded within the model as well as in the surrounding environment. The experimental data obtained were compared with the calculated results for the theoretical model as determined by the Finite Element Analysis. The temperatures recorded experimentally agreed reasonably with the calculated results. It can be concluded that the assumptions which have been made concerning the described thermal loading conditions lead to a good approximation of the physical reality.

Cold Temperature↗
PubMed2004

Vibrational states of tetrahedral molecular species in electric fields. A theoretical model for quantitative description of the latent symmetry effects in vibrational spectroscopy of matrix isolated tetrahedral species.

A theoretical model that enables a quantitative description of latent symmetry effects in the case of matrix isolated tetrahedral species is developed. Several particular cases are considered, that are of special interest from vibrational spectroscopic viewpoint. It is shown that the observed appearances of the IR spectra of various XY4n- anions isomorphously isolated in solid matrices may be systematically explained in terms of the orientation of the effective local crystalline field vector. The model also principally enables calculation of the magnitude of the effective local homogeneous field vector as well as its direction with respect to the symmetry elements of the dopant species, and it can be successively corrected accounting for the field non-homogeneity. Also, a solid theoretical basis is for the first time presented for the significantly smaller site group splitting of the nu4 modes of dopant XY4n- species in comparison to the splitting of nu3 ones, which is often observed in the IR spectra. However, the presented approach has potentially a much wider applicability, as it in facts considers the vibrational states of tetrahedral molecular species in electric fields, which may be of a different origin, for example, electrodes in electrochemical systems, external fields in Stark-type spectroscopies, etc.

Electricity↗
PubMedDate not supplied

[Theoretical model of the predator-prey interaction kinetics between "Bdellovibrio bacteriovorus" and "escherichia coli" (author's transl)].

A theoretical model is suggested in order to explain the main features of the interaction kinetics between the micropredator Bdellovibrio bacteriovorus and its prey Escherichia coli. Three parametes are used in this model: the incubation time T, the fixation rate constant k, and the predator multiplication factor a. Their values can be determined from the experimental variations of the total predator concentration p, and the total density of preys (c + c'). An experimental study of the predation kinetics was performed at various temperatures (25-40 degrees C) and in media with different Ca++ and Mg++ concentrations. From the values of parameters obtained, theoretical prey and predator density curves were computer-simulated; they agree satisfactorily with the experimental curves. The parameters values are quite reasonable and in good agreement with previous findings.

Bdellovibrio↗
PubMed2004

Mechanical properties of desiccated ragweed pollen grains determined by micromanipulation and theoretical modelling.

The mechanical properties of desiccated ragweed pollen grains were determined using a micromanipulation technique and a theoretical model. Single pollen grains with a diameter of approximately 20 microm were compressed and held, compressed and released, and compressed to rupture at different speeds between two parallel surfaces. Simultaneously, the force being imposed on the pollen grains was measured. It has been found that the rupture force of pollen grains increased linearly with their displacement at rupture on average, but was independent of their diameter. The mean rupture force was 1.20 +/- 0.03 mN, and mean deformation (the ratio between the displacement and diameter) at rupture was 22 +/- 0.6%. Single pollen grains were modeled as a capsule with a core full of air and a non permeable wall. A constitutive equation based on Hookean law was used to determine the mechanical property parameters Eh (product of the Young's modulus and wall thickness), and the mean value of Eh of desiccated pollen gains was estimated to be 1653 +/- 36 N/m.

Ambrosia↗
PubMed1993

Dielectric energy of orientation in dead and living cells of Schizosaccharomyces pombe. Fitting of experimental results to a theoretical model.

Using the experimental data obtained with killed cells of Schizosaccharomyces pombe (1), we have formulated a theoretical model that is able to predict cell orientation for microorganisms with ellipsoidal or cylindrical shapes as a function of the frequency of the electric field and of the conductivity of the external medium. In this model, comparison of the difference in potential energy for both orientations parallel-perpendicular with the thermal agitation energy allows one to interpret the intervals where these orientations occur. The model implies that the conductivity of the cytoplasm is slightly higher than that of the external medium. This assumption is easy to understand taking into account that not all the intracytoplasmic material is released to the exterior during cell death.

Biophysical Phenomena↗