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At least 973 records · Page 54Linked to original sources
PubMed2006

Identification and evaluation of computer models for predicting environmental concentrations of pharmaceuticals and veterinary products in the Nordic environment.

According to European Union Council directive 2001/83, an application for the marketing authorization of a medicinal product shall be accompanied by an environmental risk assessment, including an exposure assessment. Computerized exposure models constitute an important tool in predicting environmental exposure to substances yet to be introduced on the market. This paper reports the process of identifying appropriate exposure models for estimating PECs (Predicted Environmental Concentrations) for pharmaceuticals and veterinary products, focusing on emissions to Swedish aquatic and terrestrial environments via water and sludge from sewage treatment plants. From a large number of information sources, a set of 181 potentially relevant exposure models was identified. A process of scrutinizing and testing these models resulted in a final selection of two models, namely SimpleTreat 3.1 that is used to estimate distribution and elimination of chemicals in sewage treatment plants (resulting in a PEC), and VetPec, suited for veterinary products, that estimates PEC in soil (including pore water), groundwater, and surface water. It is concluded that there is still potential for further development of exposure model(s) specifically designed for pharmaceutical emissions to the Nordic environment and climate. Furthermore, increased regulatory data requirements would facilitate the use of existing models, and improve the quality of the output data from these models.

Computer Simulation↗
PubMed1997

A hybrid computational model for ultrasound phased-array heating in presence of strongly scattering obstacles.

A computationally efficient hybrid ray-physical optics (HRPO) model is presented for the analysis and synthesis of multiple-focus ultrasound heating patterns through the human rib cage. In particular, a ray method is used to propagate the ultrasound fields from the source to the frontal plane of the rib cage. The physical-optics integration method is then employed to obtain the intensity pattern inside the rib cage. The solution of the matrix system is carried out by using the pseudo inverse technique to synthesize the desired heating pattern. The proposed technique guides the fields through the intercostal spacings between the solid ribs and, thus, minimal intensity levels are observed over the solid ribs. This simulation model allows for the design and optimization of large-aperture phased-array applicator systems for noninvasive ablative thermal surgery in the heart and liver through the rib cage.

Algorithms↗
PubMedDate not supplied

Evaluation of computational models for hemolysis estimation.

The potential for mechanical erythrocyte damage, or hemolysis, in heart valves and blood pumps has been estimated using computational fluid dynamics (CFD) analysis, combined with mathematical models of red cell damage mechanisms. To date, these prediction approaches have not been compared with each other in common benchmark cases, nor have they been evaluated in radically different flow geometries. In this study, four test devices, including a hemoresistometer, a spinning disk, a capillary tube, and a concentric cylinder viscometer were hemolysis tested and computationally simulated. A number of existing models were used to predict blood damage and these models were compared with each other and with actual measurement of hemolysis. The study indicates that the effectiveness of blood damage prediction from the existing models is similar and can potentially be improved with the consideration of a proposed repeated flow passage effect. It also indicates that the improved models can be used to more effectively predict blood damage in widely different situations.

Animals↗
PubMed2005

Computer modeling of red blood cell rheology in the microcirculation: a brief overview.

One of the major functions of the cardiovascular system is to deliver blood to the microcirculation where exchange of mass and energy can take place. In the present article, we will provide an overview of the state-of-the-art computational methods for modeling of red blood cell (RBC) rheology and dynamics in the microcirculation. While significant progress has been made in simulation of single-file motion of deformable RBCs in capillaries and of diluted sheared suspensions of RBCs in infinite domains, detailed understanding of the mechanics of blood flow in intermediate diameter microvessels (8-1,000 microm) has presented formidable challenges. The difficulties are largely due to modeling the motion of multiple, interacting, highly deformable particles. The current computational tools consist mainly of three-dimensional (3D) boundary-integral methods for single RBC dynamics and deformation; and for rheology of large systems of droplets at large volume fractions using periodic boundary conditions and novel adaptive computational meshes. Further advances will result from combination of these tools to produce new algorithms capable of describing the motion and deformation of large systems of RBCs in microvessels at physiologically relevant volume fractions.

Animals↗
PubMed1998

Biomechanical considerations of mandibular lengthening and widening by gradual distraction using a computer model.

PURPOSE: Experience using distraction osteogenesis for limb lengthening has shown the importance of appliance orientation. Although successful results of mandibular lengthening using osteodistraction have been reported, optimal orientation of the distractors relative to the mandible has not been determined. The purpose of this study was to evaluate the biomechanical effects of linear distractors placed parallel to the body of the mandible or parallel to the axis of distraction. MATERIALS AND METHODS: A two-dimensional model of the human mandible was generated for computer simulation of mandibular osteodistraction. Linear distractors were then analyzed based on their orientation, either parallel to the body of the mandible or parallel to the axis of distraction. In addition, two types of distraction osteogenesis procedures for mandibular reconstruction were analyzed: 1) bilateral mandibular lengthening, and 2) bilateral mandibular lengthening in combination with midline mandibular widening. RESULTS: Distractors oriented parallel to the body of the mandible caused a lateral displacement of the posterior components of the distraction devices and a reduction of the midline distraction gap during mandibular lengthening. These effects were eliminated when the device was oriented parallel to the axis of distraction. Midline symphyseal widening created axial rotation of the mandibular condyles regardless of the orientation of the distractors. CONCLUSIONS: Distraction appliances must be oriented parallel to the axis of distraction to prevent adverse biomechanical effects during bilateral mandibular lengthening. Additional ramus osteotomies, using hinged devices for angular correction, may be necessary to compensate for rotational movements of the mandibular condyles secondary to midline osteodistraction.

Biomechanical Phenomena↗
PubMed2006

Application of kinetic-based computer modelling to evaluate the efficacy of HI 6 in percutaneous VX poisoning.

The rife use of organophosphorus compounds (OP) as pesticides and the exertion of highly toxic OP-type chemical warfare agents (nerve agents) during military conflicts and terrorist attacks in the past emphasize the necessity of the development of effective therapeutic countermeasures. Presently, standard treatment of poisoning by OP includes administration of atropine as an antimuscarinic agent and of oximes, e.g. obidoxime or pralidoxime, as reactivators of OP-inhibited acetylcholinesterase (AChE), but is considered to be rather ineffective with certain nerve agents. The evaluation of new oximes as antidotes is only possible by implementation of animal experiments for ethical reasons and therefore complicated by a limited extrapolation of animal data to humans due to marked species differences. A computer simulation based on combination of AChE kinetic data (inhibition, reactivation, aging) with OP toxicokinetics and oxime pharmacokinetics allows the calculation of AChE activities at different scenarios and may facilitate to define effective oxime concentrations and to optimize oxime dosage in OP poisoning. On the base of species-specific kinetic data this model was used to calculate AChE activities in humans and pigs after percutaneous exposure to 5 x LD50 VX and treatment with HI 6. Due to marked species differences between human and pig AChE the HI 6 dose that is necessary to cause a comparable reactivation of VX-inhibited pig AChE is conspicuously higher. Hence, designing animal experiments with the aid of computer modeling may reduce the number of animal experiments and allow a more reliable extrapolation of animal data to humans.

Administration, Cutaneous↗
PubMed1979

A computer model of intermediate cerebellum dynamic operations in motor control.

An intermediate cerebellum theoretical model for processing central programming discharges and muscle force signals is described which can perform a correct motor task under different peripheral perturbations (loads). An indispensable condition is that the simulated interpositus nucleus cells controlling a given effector (muscle) are inhibited by impulses coming from that effector (negative feedback from muscle force detectors). The hypothesis is proposed that the intermediate cerebellum can act via the rubrospinal tract as an interface between programming and executing motor structures.

Cerebellum↗
PubMed2003

Computational modeling of arterial biomechanics: insights into pathogenesis and treatment of vascular disease.

We review how advances in computational techniques are improving our understanding of the biomechanical behavior of the healthy and diseased cardiovascular system. Numerical modeling of biomechanics is being used in a wide variety of ways, including assessment of effects of mural and hemodynamically induced stresses on atherogenesis, development of risk measures for aneurysm rupture, improvement in interpretation of medical images, and quantification of oxygen transport in diseased and healthy arteries. Although not amenable to routine clinical use, numerical modeling of cardiovascular biomechanics is a powerful research tool.

Arteries↗
PubMed2004

Computational models of hair cell bundle mechanics: I. Single stereocilium.

A distributed parameter model for describing the response of a stereocilium to an applied force is presented. This model is based on elasticity theory, plus the geometry and material properties of the stereocilium. The stereocilia shaft above the taper is not assumed to be perfectly rigid. It is assumed to be deformable and that two separate mechanisms are involved in its deformation: bending and shear. The influence of each mode of deformation is explored in parametric studies. Results show that the magnitude of tip deflection depends on the shear compliance of the stereocilium material, the degree of base taper, and stereocilium height. Furthermore, the deformation profiles observed experimentally will occur only if there are constraints on the geometry and material properties of the stereocilium.

Animals↗
PubMedDate not supplied

A computer model for intracellular pH regulation in Chinese hamster ovary cells.

A single-cell model for Chinese hamster ovary (CHO) cells has been extended to study the regulation of intracellular pH. This model provides for the first time a direct mechanistic linkage between intracellular pH control and cellular metabolism. Among the known mechanisms that regulate and disturb pHi, the Na/H antiport, effect of lactate and ammonia, proton leakage, and external pH have been included in the model. The ability of the model to predict both steady-state and transient pHi has been tested against experimental studies. The "regular model" that has been used to predict cell growth in suspension and batch cultures can predict the steady-state pHi and the transient response of pHi to an alkaline load well, but it had to be modified in terms of the maximum activity of the Na/H antiport in order to simulate the transient response of pHi to an acid load. The modification indicates the possibility that a cell could activate the Na/H antiport under acute acid load but return to its basal level of Na/H antiport activity under steady-state conditions, even when the cell interior is under low-pH conditions (6.7-7.0).

Amino Acids↗
PubMed1986

Financial planning and computer modeling in dental practice.

The financial plan describes the practice's financial strategy, projects the strategy's future effect on the practice, and establishes goals by which the practice's manager can measure subsequent performance. The act of putting together a financial plan is called the financial planning process. It is a process that consists of analyzing the practice; projecting future outcomes of decisions that have to be made regarding finances, investments, and day to day operations; deciding which alternatives to undertake; and measuring performance against goals that are established in the financial plan. Computer financial planning models can aid the practice manager in projecting future outcomes of various financial, investment, and operational decisions. These models can be created inexpensively by noncomputer programmers with the aid of computer software on the market today. The financial planning process for a hypothetical practice was summarized, and the financial model used to test out various alternatives available to the practice was described.

Computer Simulation↗
PubMed2006

Putting the computation back into computational modeling.

Many different varieties of modeling coexist in theoretical neuroscience. Here, we consider the positive and negative implications, for theories of schizophrenia, of a crucial distinction between computational and mathematical modeling.

Brain↗
PubMed2026

Implicit and explicit statistical learning in reading: Evidence from a randomized controlled-learning study and computational modeling.

A key challenge in reading acquisition is understanding how learners extract the complex probabilistic mappings between print, meaning, and sound. Statistical learning (SL) theory offers a mechanistic account of how such mappings are acquired, whether implicitly through exposure or explicitly through instruction. We conducted a randomized controlled-learning study in Chinese, a writing system characterized by multiple sub-lexical regularities linking orthography, semantics, and phonology. Ninety-five 2nd-3rd graders with or at risk for dyslexia were randomly assigned to one of three groups: an implicit-SL training group exposed to repeated lexical and sublexical orthography-semantics-phonology associations, an explicit-SL training group receiving the same input plus explicit instruction on the sublexical print-sound mapping, and a no-SL control group. Both SL groups outperformed controls on the characters they were trained on, as well as on untrained characters that required generalization. However, only the explicit group demonstrated abstraction of print-sound mapping to novel items. Neural network simulations further revealed distinct mechanisms supporting implicit and explicit SL, consistent with a dual-system account of reading acquisition. Together, these findings (1) clarify how implicit and explicit learning distinctly support the discovery of statistical structure in written language and (2) underscore the implicit-explicit dual learning mechanism underlying reading acquisition.

Humans↗
PubMed1996

A computer model for predicting image quality after photorefractive keratectomy.

BACKGROUND: Accurately predicting visual performance remains a concern in refractive surgery. The effects of the eye's optics on retinal image quality were investigated using computer ray tracing to model the human eye after photorefractive keratectomy (PRK). METHODS: Ray-tracing analysis was used with an anatomically realistic model of the human eye including aspheric surfaces and crystalline lens gradient index distributions. The contribution of corneal curvature to refractive error was investigated using data of axial length, corneal power, anterior chamber depth, and lens power from 318 eyes from the literature. The computer interface was specifically designed for use with PRK and provides graphical plots of the remodeled eye, ray paths and retinal image formation. RESULTS: Modeling the optical contribution of corneal curvature resulted in an improvement in predicted refractive state of the eye as a function of axial length expressed as the R2 value of the regression analysis from 0.88 to 0.96. Subsequently, analyses were conducted for single and multizone treatment areas of differing diameter and with varying pupil size. Retinal image quality following PRK for the human cornea was found to be affected by not only the corneal parameters of anterior curvature and thickness, but also by axial length, pupil size, and anterior chamber depth. CONCLUSIONS: The inclusion of multiple interdependent optical parameters showed differences from conventional methods in predicting refractive outcome following PRK and revealed factors affecting image quality may account for some imperfections in visual performance based on simpler optical modeling.

Anterior Chamber↗
PubMed1998

A computational model of ameboid deformation and locomotion.

Traditional continuum models of ameboid deformation and locomotion are limited by the computational difficulties intrinsic in free boundary conditions. A new model using the immersed boundary method overcomes these difficulties by representing the cell as a force field immersed in fluid domain. The forces can be derived from a direct mechanical interpretation of such cell components as the cell membrane, the actin cortex, and the transmembrane adhesions between the cytoskeleton and the substratum. The numerical cytoskeleton, modeled as a dynamic network of immersed springs, is able to qualitatively model the passive mechanical behavior of a shear-thinning viscoelastic fluid (Bottino 1997). The same network is used to generate active protrusive and contractile forces. When coordinated with the attachment-detachment cycle of the cell's adhesions to the substratum, these forces produce directed locomotion of the model ameba. With this model it is possible to study the effects of altering the numerical parameters upon the motility of the model cell in a manner suggestive of genetic deletion experiments. In the context of this ameboid cell model and its numerical implementation, simulations involving multicellular interaction, detailed internal signaling, and complex substrate geometries are tractable.

Actins↗
PubMed2006

The mechanism of involuntary visual spatial attention revealed by a new linear computation model.

EEG reveals brain electrical activities with high temporal resolution. Yet, multiple implicit variables may be involved in limited event related potential (ERP) measures. Special computation techniques are needed to recover these parameters. In the study of involuntary visual spatial attention, we may obtain the ERP in valid cued (V), invalid cued (I) and neutral cued (N) conditions. Usually, the effect of involuntary attention is computed by the subtraction model with the assumption that V/I and N are independent. Yet, they should be related. Treating V/I as a function of N, a linear model V(I) = W + GN is assumed, where W and G are implicit in the ERP measures. G is the gain control on the neutral function. Provided G and W are constant over a local brain region, we may use the Total Least Square (TLS) algorithm to compute their values. The values of W and G computed from an involuntary attention experiment data show that multiple implicit variables are involved in obtained ERPs. Here G acts as a "top-down" sensory modulator on the neutral ERPs and W is related to possible newly involved neural activities. The parameters derived from the new linear model also suggest that there are different mechanisms involved in involuntary attention and voluntary allocation of attention.

Adult↗