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At least 145 records · Page 8Linked to original sources
PubMed1994

A theoretical model of infant incubator dynamics.

A spatially lumped mathematical model was developed and used for a computer simulation of the neonate-incubator system for parametric analysis of the factors that influence neonatal thermo-regulation. The simulation examined the effects of the following parameters: (1) size of the infant; (2) respiratory rate; (3) metabolic rate; (4) heart rate; (5) thermal properties of the mattress; (6) specific heat capacity of the incubator wall; (7) air flow rate; (8) heater control mechanisms.

Air Movements↗
PubMed1987

A theoretical model for water flux through the artery wall.

A two layer model for water flux through the artery is studied using a mathematical model based on the theory for the consolidation of water saturated soils. The matrix is considered to be constituted by two layers with different permeabilities and different elastic constants and the two systems of equations are coupled with the condition of continuity of pressure, total stress, solid displacement and fluid seepage velocity at the interface. The luminal pressure is considered to be harmonic in time. Exact solutions are obtained for displacements and pressures in both the layers. For large consolidation times, large pressure gradients are found to exist near the boundaries and at the interface. The heterogeneous model may not only be useful to understand the mechanics of transport in the physiological system but it will also help the bioengineers to choose proper implant materials to design artificial vascular organs for the purpose of prosthesis.

Arteries↗
PubMed2001

A theoretical model for event statistics in microdosimetry. I: Uniform distribution of heavy ion tracks.

In this work we describe a novel approach to solving microdosimetry problems using conditional probabilities and geometric concepts. The intersection of a convex site with a field of randomly oriented straight track segments is formulated in terms of the relative overlap between the chord associated with the action line of the track and the track itself. This results in a general formulation that predicts the contribution of crossers, stoppers, starters, and insiders in terms of two separate functions: the chord length distribution (characteristic of the site geometry and the type of randomness) and an independent set of conditional probabilities. A Monte Carlo code was written in order to validate the proposed approach. The code can represent the intersection between an isotropic field of charged particle tracks and a general ellipsoid of unrestricted geometry. This code was used to calculate the event distribution for a sphere as well as the expected mean value and variance of the track length distribution and to compare these against the deterministic calculations. The observed agreement was shown to be very good, within the precision of the Monte Carlo approach. The formulation is used to calculate the event frequency, lineal energy, and frequency mean specific energy for several monoenergetic and isotropic proton fields in a spherical site, as a function of the site diameter, proton energy, and the event type.

Biophysical Phenomena↗
PubMed2001

A theoretical model for event statistics in microdosimetry. II: Nonuniform distribution of heavy ion tracks.

A microdosimetry model, described in Part I, applies to the case of a convex site immersed in a uniform distribution of heavy particle tracks, and assumes no restrictions in site geometry or the kind of randomness. In Part II, this model is extended to include nonuniform distributions of particle tracks. This situation is relevant to the study of microdosimetry, for example, in boron neutron capture, in irradiation experiments using heavy ion particle beams, where the sources of particle tracks are external to the cell, or in irradiation from internally incorporated particle-emitting radionuclides, such as environmental radon or occupational exposure to radioactive materials. The formalism developed permits the calculation of statistical properties, track length distributions, and microdosimetric spectra for convex sites where the "inner" and "outer" concentrations of sources may be different, or for tracks originating on the surface of a convex site. Expressions applicable to the case of surface-distributed sources of tracks are presented that may represent situations such as boron compounds bound to the membrane of a cellular nucleus in boron neutron capture. A series of Monte Carlo calculations and analytical solutions, illustrating the case of spherical site geometry, are presented and compared. Finally, microdosimetric spectra and specific energy averages are calculated for alpha and lithium particles originating from thermal neutron capture in 10B, showing their dependence on 10B localization (extra-site, uniform, intra-site, or surface-distributed).

Biophysical Phenomena↗
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↗
PubMed1990

A theoretical model of acoustoelectric transducer with a nonuniform distribution of piezoelectric coefficient: application to transducer optimization.

A general equation is derived that describes the behavior of a piezoelectric transducer with a nonuniform distribution of piezoelectric coefficient within its bulk, when submitted to an arbitrary distribution of acoustic pressure. Based on this equation, an expression for the receiving transfer function of the transducer is calculated. The results demonstrate the dependence of the transfer function on the distribution of piezoelectric coefficient, and that it is possible to benefit from a nonuniform distribution to optimize the transfer function. The general equation also describes the influence of the external electric circuit loading the transducer, which leads to another independent means of optimizing the transfer function. The proposed model combines effects of piezoelectric material characteristics, acoustic backing, and electric loading, without resorting to Mason or other equivalent circuits for the transducer.

Models, Theoretical↗
PubMed1989

Flutter in collapsible tubes: a theoretical model of wheezes.

A mathematical analysis of flow through a flexible channel is examined as a model of flow-induced flutter oscillations that pertain to the production of wheezing breath sounds. The model provides predictions for the critical fluid speed that will initiate flutter waves of the wall, as well as their frequency and wavelength. The mathematical results are separated into linear theory (small oscillations) and nonlinear theory (larger oscillations). Linear theory determines the onset of the flutter, whereas nonlinear theory determines the relationships between the fluid speed and both the wave amplitudes and frequencies. The linear theory predictions correlate well with data taken at the onset of flutter and flow limitation during experiments of airflow in thick-walled collapsible tubes. The nonlinear theory predictions correlate well with data taken as these flows are forced to higher velocities while keeping the flow rate constant. Particular ranges of the parameters are selected to investigate and discuss the applications to airway flows. According to this theory, the mechanism of generation of wheezes is based in the interactions of fluid forces and friction and wall elastic-restoring forces and damping. In particular, a phase delay between the fluid pressure and wall motion is necessary. The wave speed theory of flow limitation is discussed with respect to the specific data and the flutter model.

Humans↗
PubMed2000

A theoretical model of benign external hydrocephalus that predicts a predisposition towards extra-axial hemorrhage after minor head trauma.

INTRODUCTION: There is controversy over whether there exists a predisposition towards bleeding into the subdural space in infants with benign external hydrocephalus (BEH) or other enlargement of the extra-axial space (e.g. subdural hygroma). The presumed etiology implicates shear forces in over-stretching the extra-axial blood vessels. We have created a model of the intracranial space that approximates certain aspects of BEH. Using this model, we predict situations where children with BEH will bleed into the extra-axial space when normal infants will not. METHODS: The cranial model consists of two spheres representing the brain and the skull. The distance between them represents the width of the extra-axial space. The spheres are concentric (with interspheric distance equal to N) in the normal condition and nonconcentric in BEH. In BEH, the distance between the two spheres varies from N to Q (0 < N </= Q) over a 906 M </= Q, with the translation orthogonal to the vector of N and parallel to that of M or vice versa), then the final length V(f) of a vein V is V(f) = (N(2) + M)(1/2), and the stretch ratio V(f)/V(i) is: V(f)/V(i) = (N(2) + M(2))(1/2)/N.S(i), with S(i) the slack factor, where i represents either n for normal or b for BEH and M = N in the normal condition. RESULTS: Given an equivalent capacity of veins to resist stretch injury (based on the proportion of change in length), for brain translations after a low-impact head injury, stretch ratios for BEH veins range from 1.677 to 3.436, whereas in the normal condition they range from 1.061 to 1.179. Therefore, for an increase in subarachnoid space from 3 (normal) to 6 mm (BEH), brain translocation in BEH will stretch veins beyond an average breaking point when the translation for the normal condition will not. CONCLUSIONS: Mathematical modeling of the cranial vault produces a relationship between venous stretch and the width of the extra-axial space. These equations predict an increased frequency of venous stretch injury in the situation of widened extra-axial space. Such venous injury is consistent with forces generated by minor trauma. This relationship, as predicted by our model, could underlie a predisposition towards extra-axial bleeding after minor head trauma in infants with BEH.

Biomechanical Phenomena↗