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At least 469 records · Page 26Linked to original sources
PubMed1997

The cardiovascular helix: a three-dimensional view of blood pressure and heart rate over time.

BACKGROUND: The baroreflex system plays an important role in the fast regulation of blood pressure. Conventional determination of baroreflex sensitivity either by pharmacological or by physical methods has proven cumbersome. Alternative approaches are based on continuously recording data of systolic arterial blood pressure (SAP) and heart rate, with evaluation by regression assessment or spectral analysis. Data selection for both methods results in various conflicts. We therefore developed a non-invasive method to evaluate baroreceptor functioning on the basis of a visual representation that would be independent of the restrictions related to regression and spectral analysis.METHODS: By plotting SAP and heart rate (measured by using a Finapres device) of 33 young men against time in a three-dimensional space, a helical structure could be visualized for subjects at rest as well as under stress. The shape of the helix was determined by the phase lag between SAP and heart rate and by the amplitude ratio. On the basis of this helix a model using two sinusoidal representations of SAP and heart rate parameterized the interaction between blood pressure and heart rate. The transfer function analysis was studied by means of a curve-fitting method similar to the Monte Carlo method. RESULTS: We found baroreceptor gain mode to be 9 ms/mmHg for subjects under stress and 11 ms/mmHg for subjects at rest. The values of phase lag between SAP and heart rate tended to gather around approximately 90 degrees. CONCLUSION: This method can be applied using even small sets of data over random periods of time.

Journal Article↗
PubMed1999

Experimental verification of a CT-based Monte Carlo dose-calculation method in heterogeneous phantoms.

To further validate the Monte Carlo dose-calculation method [Med. Phys. 25, 867-878 (1998)] developed at the Memorial Sloan-Kettering Cancer Center, we have performed experimental verification in various inhomogeneous phantoms. The phantom geometries included simple layered slabs, a simulated bone column, a simulated missing-tissue hemisphere, and an anthropomorphic head geometry (Alderson Rando Phantom). The densities of the inhomogeneity range from 0.14 to 1.86 g/cm3, simulating both clinically relevant lunglike and bonelike materials. The data are reported as central axis depth doses, dose profiles, dose values at points of interest, such as points at the interface of two different media and in the "nasopharynx" region of the Rando head. The dosimeters used in the measurement included dosimetry film, TLD chips, and rods. The measured data were compared to that of Monte Carlo calculations for the same geometrical configurations. In the case of the Rando head phantom, a CT scan of the phantom was used to define the calculation geometry and to locate the points of interest. The agreement between the calculation and measurement is generally within 2.5%. This work validates the accuracy of the Monte Carlo method. While Monte Carlo, at present, is still too slow for routine treatment planning, it can be used as a benchmark against which other dose calculation methods can be compared.

Electrons↗
PubMed1993

The relationship between cell and population growth.

A steady-state model of cell volume frequency distribution using the method of Williams (1971) is derived. Results are compared to a Monte Carlo simulation of cell growth and division. It is suggested that the Monte Carlo method might be of value for investigating cell and population properties for which analytic methods are not currently available.

Bacteria↗
PubMedDate not supplied

Comparison of two statistical techniques for the surveillance of birth defects through a Monte Carlo simulation.

In a multi-hospital surveillance system for detecting increases in congenital malformation rates, the performance of the set technique applied individually to hospitals and of the cumulative sum (Cusum) technique applied to the aggregate of hospitals has been compared using a Monte Carlo method. The Cusum technique appears to be both more sensitive to real increases and less likely to issue false alarms.

Computers↗
PubMed2005

Determining the error of dose estimates and minimum and maximum acceptable concentrations from assays with nonlinear dose-response curves.

A method is described here that uses a modified Monte-Carlo method to provide an improved estimate of the confidence bounds of concentration estimates. This method accommodates even strongly nonlinear curve models, such as the five parameter logistic model, in contrast to the common but often poor approach of linearizing the regression problem and using linear theory to obtain the confidence bounds. The method uses an interpolation technique to reduce artifacts in the precision profile due to small simulation sample sizes and proximity to horizontal asymptotes in the curve model. The paper also describes how to define and calculate the minimum and maximum acceptable concentrations of dose-response curves by locating the concentrations where the size of the error, defined in terms of the size of the concentration confidence interval, exceeds the threshold of acceptability determined for the application.

Algorithms↗
PubMed2005

Improving protein structure prediction with model-based search.

MOTIVATION: De novo protein structure prediction can be formulated as search in a high-dimensional space. One of the most frequently used computational tools to solve such search problems is the Monte Carlo method. We present a novel search technique, called model-based search. This method samples the high-dimensional search space to build an approximate model of the underlying function. This model is incrementally refined in areas of interest, whereas areas that are not of interest are excluded from further exploration. Model-based search derives its efficiency from the fact that the information obtained during the exploration of the search space is used to guide further exploration. In contrast, Monte Carlo-based techniques lack memory and exploration is performed based on random walks, ignoring the information obtained in previous steps. RESULTS: Model-based search is applied to protein structure prediction, where search is employed to find the global minimum of the protein's energy landscape. We show that model-based search uses computational resources more efficiently to find lower-energy conformations of proteins than one of the leading protein structure prediction methods, which relies on a tailored Monte Carlo method to perform a search. The performance improvements become more pronounced as the dimensionality of the search problem increases. We argue that model-based search will enable more accurate protein structure prediction than was previously possible. Furthermore, we believe that similar performance improvements can be expected in other problems that are currently solved using Monte Carlo-based search methods. AVAILABILITY: An implementation of model-based search can be obtained by contacting the authors.

Amino Acids↗
PubMed2004

A space radiation transport method development.

Improved spacecraft shield design requires early entry of radiation constraints into the design process to maximize performance and minimize costs. As a result, we have been investigating high-speed computational procedures to allow shield analysis from the preliminary design concepts to the final design. In particular, we will discuss the progress towards a full three-dimensional and computationally efficient deterministic code for which the current HZETRN evaluates the lowest-order asymptotic term. HZETRN is the first deterministic solution to the Boltzmann equation allowing field mapping within the International Space Station (ISS) in tens of minutes using standard finite element method (FEM) geometry common to engineering design practice enabling development of integrated multidisciplinary design optimization methods. A single ray trace in ISS FEM geometry requires 14 ms and severely limits application of Monte Carlo methods to such engineering models. A potential means of improving the Monte Carlo efficiency in coupling to spacecraft geometry is given in terms of re-configurable computing and could be utilized in the final design as verification of the deterministic method optimized design.

Cosmids↗