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At least 1,297 records · Page 72Linked to original sources
PubMed1999

Constrained customization of non-coplanar beam orientations in radiotherapy of brain tumours.

A methodology for the constrained customization of non-coplanar beam orientations in radiotherapy treatment planning has been developed and tested on a cohort of five patients with tumours of the brain. The methodology employed a combination of single and multibeam cost functions to produce customized beam orientations. The single-beam cost function was used to reduce the search space for the multibeam cost function, which was minimized using a fast simulated annealing algorithm. The scheme aims to produce well-spaced, customized beam orientations for each patient that produce low dose to organs at risk (OARs). The customized plans were compared with standard plans containing the number and orientation of beams chosen by a human planner. The beam orientation constraint-customized plans employed the same number of treatment beams as the standard plan but with beam orientations chosen by the constrained-customization scheme. Improvements from beam orientation constraint-customization were studied in isolation by customizing the beam weights of both plans using a dose-based downhill simplex algorithm. The results show that beam orientation constraint-customization reduced the maximum dose to the orbits by an average of 18.8 (+/-3.8, ISD)% and to the optic nerves by 11.4 (+/-4.8, ISD)% with no degradation of the planning target volume (PTV) dose distribution. The mean doses, averaged over the patient cohort, were reduced by 4.2 (+/-1.1, ISD)% and 12.4 (+/-3.1, ISD)% for the orbits and optic nerves respectively. In conclusion, the beam orientation constraint-customization can reduce the dose to OARs, for few-beam treatment plans, when compared with standard treatment plans developed by a human planner.

Adenoma↗
PubMed1996

Skill maintenance in extended spaceflight: a human factors analysis of space and analogue work environments.

This paper discusses the implications of increasing mission lengths of manned spaceflight for the design of future space systems from a human factors point of view. It is argued that the increase in mission duration has brought about a number of new problems, which have not been sufficiently addressed in space research. Therefore, a review of analogue work environments is carried out to make up for the paucity of space research found in the area of human performance in long-duration spaceflight. This resulted in an evaluation of seven analogue environments concerning their similarity to space with industrial process control and nuclear submarines coming out as the closest match on the technical dimension. Finally, some recommendations are given from the lessons learned in spaceflight, simulation studies and appropriate analogue environments.

Aerospace Medicine↗
PubMed1983

Spatial cross-correlation. A proposed mechanism for acoustic pitch perception.

We propose in this paper a new class of model processes for the extraction of spectral information from the neural representation of acoustic signals in mammals. We are concerned particularly with mechanisms for detecting the phase-locked activity of auditory neurons in response to frequencies and intensities of sound associated with speech perception. Recent psychophysical tests on deaf human subjects implanted with intracochlear stimulating electrodes as an auditory prosthesis have produced results which are in conflict with the predictions of the classical place-pitch and periodicity-pitch theories. In our model, the detection of synchronicity between two phase-locked signals derived from sources spaced a finite distance apart on the basilar membrane can be used to extract spectral information from the spatiotemporal pattern of basilar membrane motion. Computer simulations of this process suggest an optimal spacing of about 0.3-0.4 of the wavelength of the frequency to be detected. This interval is consistent with a number of psychophysical, neurophysiological, and anatomical observations, including the results of high resolution frequency-mapping of the anteroventral cochlear nucleus which are presented here. One particular version of this model, invoking the binaurally sensitive cells of the medial superior olive as the critical detecting elements, has properties which are useful in accounting for certain complex binaural psychophysical observations.

Auditory Pathways↗
PubMed1992

Study of vibratory pattern of the vocal folds in the excised canine larynx.

The effect of simulated thyroarytenoid and cricothyroid muscle contraction on the vibratory pattern of the vocal folds was studied in the excised canine larynx. To simulate the action of the thyroarytenoid muscle, small balloons were inflated in the paraglottic space at the level of the vocal folds. To simulate the action of the cricothyroid muscle, longitudinal tension was applied to the anterior commissure of the vocal folds. The photoglottographic and electroglottographic signals, sound intensity, and airflow rate were measured. This study showed that balloon inflation simulating thyroarytenoid muscle contraction produced an elevation of frequency of vibration with a decrease in open quotient, and that an increase in longitudinal tension simulating cricothyroid muscle contraction produced an elevation of frequency with an increase in open quotient. Vocal resistance decreased with increasing open quotient, amplitude of the photoglottographic waveform, and the frequency of vibration. Vocal efficiency increased with increasing photoglottographic amplitude and decreased with increasing frequency. The vocal efficiency peaked when the open quotient was approximately 0.5. This study suggests that glottographic parameters may be useful in assessing the effect of intrinsic laryngeal muscle activity on vocal efficiency and glottic resistance.

Animals↗
PubMed2005

Space-frequency model of ultrawide-band interactions in free-electron lasers.

The principle of operation of intense radiation devices such as microwave tubes, free-electron lasers, and masers, is based on a distributed interaction between an electron beam and electromagnetic radiation. Some of the effects emerging during the interaction involve a continuum of frequencies in their broadband spectrum. We developed a three-dimensional, space-frequency theory for the analysis and simulation of radiation excitation and propagation in electron devices and free-electron lasers operating in an ultrawide range of frequencies. The total electromagnetic field (radiation and space-charge waves) is presented in the frequency domain as an expansion in terms of transverse eigenmodes of the (cold) cavity, in which the field is excited and propagates. The mutual interaction between the electron beam and the electromagnetic field is fully described by coupled equations, expressing the evolution of mode amplitudes and electron beam dynamics. The approach is applied in a numerical particle code WB3D, simulating wideband interactions in free-electron lasers operating in the linear and nonlinear regimes.

Journal Article↗
PubMed2001

Disorder and plasticity in the fragmentation of coatings.

Using a one-dimensional model that takes into account ideal plasticity of the surface layer, we investigate the fragmentation of thin coatings under uniaxial tension. The coating is modeled as a chain of plastically deforming elements that are connected via leaf springs to a uniformly stretched substrate. Each coating element can only withstand a maximum elongation, which is randomly distributed. From simulations of the fragmentation process we find that the average crack spacing scales with applied strain epsilon, i.e., proportional to epsilon(-kappa). Simulations and analytical arguments show that the scaling exponent kappa depends on the disorder parameters of the model.

Journal Article↗
PubMed1999

Performance evaluation in analogue space environments: adaptation during an 8-month Antarctic wintering-over expedition.

BACKGROUND: This paper reports a study which examined the impact of long-term isolation and confinement on multiple-task performance. METHODS: A group of 10 scientists and 6 technicians from a French wintering-over expedition in the Antarctic participated in the 8-mo study. The group was tested 8 times on a computerized simulation of a complex life support system. The task environment allowed the measurement of primary and secondary task performance, system control activities and subjective operator state. RESULTS: No signs of serious performance decrements were observed but a number of subtle indications of hidden decrements emerged. The data also revealed strong differences in performance between the professional groups. Finally, the paper discusses the implications of the work for extended spaceflight.

Adult↗
PubMed1999

Extradural cavernous haemangioma simulating a disc protrusion.

Cavernous haemangiomas confined to the epidural space are rare and are therefore infrequently considered in the differential diagnosis of spinal epidural masses. In order to draw attention to this diagnosis, a case in which an epidural cavernous haemangioma simulates a lateral/foraminal disc protrusion is presented.

Adult↗
PubMed1998

Artificial pigs in space: using artificial intelligence and artificial life techniques to design animal housing.

Computer simulations have been used by us since the early 1970s to gain an understanding of the spacing and movement patterns of confined animals. The work has progressed from the early stages, in which we used randomly positioned points, to current investigations of animats (computer-simulated animals), which show low levels of learning via artificial neural networks. We have determined that 1) pens of equal floor area but of different shape result in different spatial and movement patterns for randomly positioned and moving animats; 2) when group size increases under constant density, freedom of movement approaches an asymptote at approximately six animats; 3) matching the number of animats with the number of corners results in optimal freedom of movement for small groups of animats; and 4) perimeter positioning occurs in groups of animats that maximize their distance to first- and second-nearest neighbors. Recently, we developed animats that move, compete for social dominance, and are motivated to obtain resources (food, resting sites, etc.). We are currently developing an animat that learns its behavior from the spatial and movement data collected on live pigs. The animat model is then used to pretest pen designs, followed by new pig spatial data fed into the animat model, resulting in a new pen design to be tested, and the steps are repeated. We believe that methodologies from artificial-life and artificial intelligence can contribute to the understanding of basic animal behavior principles, as well as to the solving of problems in production agriculture in areas such as animal housing design.

Animals↗
PubMed1994

Calcium signaling in restricted diffusion spaces.

One- and two-dimensional models of Ca2+ diffusion and regulation were developed and used to study the magnitudes and the spatial and temporal characteristics of the Ca2+ transients that are likely to develop in smooth muscle cells in restricted diffusion spaces between the plasma membrane and intracellular organelles. Simulations with the models showed that high [Ca2+] (on the order of several microM) can develop in such spaces and persist for 100-200 ms. These Ca2+ transients could: 1) facilitate the coupling of Ca2+ influx to intracellular Ca2+ release; 2) provide a mechanism for the regulation of stored Ca2+ that does not affect the contractile state of smooth muscle; 3) locally activate specific signal transduction pathways, before, or without activating other Ca2+ dependent pathways in the central cytoplasm of the cell. The latter possibility suggests that independent enzymatic processes in cells could be differentially regulated by the same intracellular second messenger.

Animals↗
PubMed2005

Multicanonical schemes for mapping out free-energy landscapes of single-component and multicomponent systems.

Multicanonical (MUCA) sampling is a powerful approach for simulating large domains of thermodynamic macrostate space that relies on mapping out either the density of states or a free energy of the system as a function of a suitable "order parameter." The purpose of this study is to extend and apply to more complex systems the method introduced in a previous paper [M. K. Fenwick and F. A. Escobedo, J. Chem. Phys. 120, 3066 (2004)] that uses Bennett's acceptance ratio method for estimating MUCA free energies. Four types of MUCA schemes are considered according to what order parameter is adopted and how the macrostate space is traversed: a la grand canonical ensemble, a la semigrand canonical ensemble, a la semigrand isothermal-isobaric ensemble, and a la isothermal-isobaric ensemble. Two types of systems are studied, the first is a two-component Lennard-Jones mixture that exhibits a vapor-liquid transition, and the second is a hard-cuboid containing system that exhibits an isotropic-liquid crystalline transition. These systems are simulated with different MUCA schemes and the resulting free-energy profiles are used to determine phase-coexistence conditions. For the Lennard-Jones systems, it is also demonstrated that different types of MUCA simulations can be conveniently performed over different macrostate regions and the results can be subsequently pieced together into a continuous weighting function.

Journal Article↗
PubMedDate not supplied

The effect of nonhomogeneous clone length distribution on the progress of an STS mapping project.

We provide both theoretical and simulation results on the progress of an STS mapping project in the presence of clone length inhomogeneity. For an example in which the genome comprises alternating regions of clones with short and long average length, the main conclusion is that the efficiency of the project is clearly decreased in the presence of such inhomogeneity. The case of deterministic clone length gives the worst progress. The general simulation algorithm we propose shows that strategies that space the anchors as regularly as possible do best: fewer contigs of larger average length are expected. The simulation algorithm can be used to study many statistical properties of the progress of any anchoring project.

Algorithms↗
PubMed2000

Comparative spinal distribution and clearance kinetics of intrathecally administered morphine, fentanyl, alfentanil, and sufentanil.

BACKGROUND: Despite widespread use, little is known about the comparative pharmacokinetics of intrathecally administered opioids. The present study was designed to characterize the rate and extent of opioid distribution within cerebrospinal fluid, spinal cord, epidural space, and systemic circulation after intrathecal injection. METHODS: Equal doses of morphine and alfentanil, fentanyl, or sufentanil were administered intrathecally (L3) to anesthetized pigs. Microdialysis probes were used to sample cerebrospinal fluid at L2, T11, T7, T3, and the epidural space at L2 every 5-10 min for 4 h. At the end of the experiment, spinal cord and epidural fat tissue were sampled, and each probe's recovery was determined in vitro. Using SAAM II pharmacokinetic modeling software (SAAM Institute, University of Washington, Seattle, WA), the data were fit to a 16-compartment model that was divided into four spinal levels, each of which consisted of a caternary arrangement of four compartments representing the spinal cord, cerebrospinal fluid, epidural space, and epidural fat. RESULTS: Model simulations revealed that the integral exposure (area under the curve divided by dose) of the spinal cord (i.e., effect compartment) to the opioids was highest for morphine because of its low spinal cord distribution volume and slow clearance into plasma The integral exposure of the spinal cord to the other opioids was relatively low, but for different reasons: alfentanil has a high clearance from spinal cord into plasma, fentanyl distributes rapidly into the epidural space and fat, and sufentanil has a high spinal cord volume of distribution. CONCLUSIONS: The four opioids studied demonstrate markedly different pharmacokinetic behavior, which correlates well with their pharmacodynamic behavior.

Alfentanil↗
PubMed1993

The Habitability Mini-Laboratory: testing the tools of space habitat architecture.

Living in the closed, confined environment of a space station for a long period and under microgravity conditions, crew members can encounter problems of a physiological and also a psychological nature. The architecture of their living quarters can greatly influence their well-being and their efficiency. Simulation of a proposed architecture and of human movement within that architecture is the most effective way to evaluate the design. Two simulation tools, Computer-Aided Design (CAD) software tools and a mock-up on a smaller scale, were used to 'construct' several proposed architectures. Those models were then evaluated to determine the investigation methods that should be used in future architectural development projects.

Computer Simulation↗
PubMed1979

Measurements of the dead space volume.

The "anatomical" dead space is commonly measured by sampling an inert gas (N2) and volume in the exhalation following a large breath of oxygen (VD(F)). It may also be measured from an inert gas washout (VD(O)) that describes both volume and the delivery of VD(O) throughout the expiration. VD(O) is known to increase with age and is enlarged in some obstructive syndromes. VD(O) was appreciably larger than VD(F) in our normal subjects. Both measures increased with lung volume, the increase being entirely due to an increase in the volume of phase I. Physiological dead space (VD(p)) however, did not change significantly with lung volume, showing "alveolar" dead space to diminish as a result. An increase in VD(O) occurred with increasing respiratory frequency that was explained by the increase in volume of phase I. Although an increase in VD(F) occurred with frequency, this was significantly less than that seen by VD(O), i.e., VD(F) did not see the progressive increase in phase I volume with frequency. No lung volume or frequency changes, parasympatholytic or sympathomimetic drugs, or altered patterns of breathing simulated the late delivery of dead space seen in age and some obstructive syndromes.

Aging↗
PubMed2000

Duct detection and wall spacing estimation in breast tissue.

The relationship between duct tissue and several types of malignant disease suggests that methods for characterizing duct structures may be useful tools in ultrasonic tissue characterization. This paper presents performance results from ultrasonic phantom experiments and Monte Carlo simulations for detecting and estimating duct wall spacings on the order of those typically found in breast tissue using methods based on the generalized spectrum (GS) and cepstrum. A performance comparison demonstrates the advantages of each method and examines the effects of various signal processing options, including a special normalization technique for the GS that effectively whitens the data spectrum and reduces interfering spectral influences with little overall performance loss. Experimental results (for both simulation and phantom) indicate that the GS typically achieves detection rates of over 90% (at 10% false alarm rates) over a broad range of SNR values (3-21 dB). The GS detection performance exceeds that of the cepstrum and exhibits more robustness to noise and signal processing parameters. Simulation results with fixed system effects indicate better estimation performance for cepstral-based methods, while experimental phantom results show the GS estimation performance to be the same or better than the cepstral-based method.

Algorithms↗
PubMed2005

Preflight virtual reality training as a countermeasure for space motion sickness and disorientation.

INTRODUCTION: Research suggests that preflight training in virtual reality devices can simulate certain aspects of microgravity and may prove to be an effective countermeasure for space motion sickness (SMS) and spatial disorientation (SD). It is hypothesized that exposing subjects preflight to variable virtual orientations, similar to those encountered during spaceflight, will reduce the incidence and/or severity of SMS and SD. METHODS: Subjects were assigned to either a variable training (VT) or nonvariable training (NVT) condition to perform a simple navigation and switch activation task in a virtual space station. VT subjects performed the task starting in several different orientations, whereas NVT subjects always performed the task starting in the same orientation. On a separate day, all subjects then performed the same task in a transfer of training session starting from a novel orientation. RESULTS: When exposed to the novel test orientation, VT subjects performed the tasks more quickly (12%) and with fewer nausea symptoms (53%) than during the training session, compared with NVT subjects who performed more slowly (6%) and with more nausea symptoms (28%). Both VT and NVT conditions were effective in reducing the number of wall hits in the novel orientation (39% and 34%, respectively). DISCUSSION: These results demonstrate the effectiveness of using variable training in a virtual environment for reducing nausea and improving task performance in potentially disorienting surroundings, and suggest that such training may be developed into an effective countermeasure for SMS, SD, and associated performance decrements that occur in spaceflight.

Adult↗
PubMed1988

Conformational differences between alpha-cyclodextrin in aqueous solution and in crystalline form. A molecular dynamics study.

The computer simulation technique of molecular dynamics is a powerful tool to delineate the conformational differences between a molecule in different environments. As an illustration, the difference between an alpha-cyclodextrin molecule in aqueous solution and in crystalline form is determined. Two molecular dynamics simulations are compared. In one simulation, one alpha-cyclodextrin form in a "truncated octahedron box" containing 611 water molecules is simulated over 90 picoseconds to mimic the solution structure. In the other simulation, the crystalline form is modelled by a molecular dynamics simulation of four unit cells in space group P2(1)2(1)2(1) containing 16 alpha-cyclodextrin molecules and 96 water molecules over a period of 15 picoseconds. The solution structure of alpha-cyclodextrin deviates by about 0.1 nm from that in the crystal and shows twice as much mobility of the atoms. The experimentally observed twist of glucose unit 5 out of alignment with the other five glucose units in the alpha-cyclodextrin torus that is present in the crystal simulation, disappears in the simulation in solution, but the glucosidic torsion angles around the ring remain asymmetric. The hydrogen-bonding patterns in crystal and in solution are rather different. This means that in a crystal structure, the molecule and its (hydration) hydrogen-bonding scheme represent only one static minimum energy picture, whereas the molecular dynamics simulations yield a description of all the many hydrogen-bonding configurations that can occur in solution.

Animals↗