Biomedical subjects

Robert M Kirby

Publications and source records attributed to Robert M Kirby.

7 recordsLinked to original sources

PubMed2006

Stochastic Markovian modeling of electrophysiology of ion channels: reconstruction of standard deviations in macroscopic currents.

Markovian models of ion channels have proven useful in the reconstruction of experimental data and prediction of cellular electrophysiology. We present the stochastic Galerkin method as an alternative to Monte Carlo and other stochastic methods for assessing the impact of uncertain rate coefficients on the predictions of Markovian ion channel models. We extend and study two different ion channel models: a simple model with only a single open and a closed state and a detailed model of the cardiac rapidly activating delayed rectifier potassium current. We demonstrate the efficacy of stochastic Galerkin methods for computing solutions to systems with random model parameters. Our studies illustrate the characteristic changes in distributions of state transitions and electrical currents through ion channels due to random rate coefficients. Furthermore, the studies indicate the applicability of the stochastic Galerkin technique for uncertainty and sensitivity analysis of bio-mathematical models.

Electric Conductivity↗

PubMed2006

Simulation of soft tissue failure using the material point method.

Understanding the factors that control the extent of tissue damage as a result of material failure in soft tissues may provide means to improve diagnosis and treatment of soft tissue injuries. The objective of this research was to develop and test a computational framework for the study of the failure of anisotropic soft tissues subjected to finite deformation. An anisotropic constitutive model incorporating strain-based failure criteria was implemented in an existing computational solid mechanics software based on the material point method (MPM), a quasi-meshless particle method for simulations in computational mechanics. The constitutive model and the strain-based failure formulations were tested using simulations of simple shear and tensile mechanical tests. The model was then applied to investigate a scenario of a penetrating injury: a low-speed projectile was released through a myocardial material slab. Sensitivity studies were performed to establish the necessary grid resolution and time-step size. Results of the simple shear and tensile test simulations demonstrated the correct implementation of the constitutive model and the influence of both fiber family and matrix failure on predictions of overall tissue failure. The slab penetration simulations produced physically realistic wound tracts, exhibiting diameter increase from entrance to exit. Simulations examining the effect of bullet initial velocity showed that the anisotropy influenced the shape and size of the exit wound more at lower velocities. Furthermore, the size and taper of the wound cavity was smaller for the higher bullet velocity. It was concluded that these effects were due to the amount of momentum transfer. The results demonstrate the feasibility of using MPM and the associated failure model for large-scale numerical simulations of soft tissue failure.

Animals↗

PubMed2006

Ballistic injury simulation using the material point method.

The Material Point Method is used in the computational simulation of ballistic projectile damage to the heart. A transversely isotropic hyperelastic material model is used to model the myocardium. Computed estimates of tissue damage are used to characterize damaged tissue. The method's potential to estimate the damaged tissue in the complete torso is considered.

Computer Simulation↗

PubMed2005

Computational simulation of penetrating trauma in biological soft tissues using the material point method.

The objective of this research was to develop realistic computational models for soft tissues subjected to finite deformation and failure, and to test these models in the context of numerical simulations of penetrating trauma injuries. A transversely isotropic hyperelastic model with strain-based failure criteria was used to represent the behavior of anisotropic soft tissue. The constitutive model was implemented into an existing numerical code based on the Material Point Method (MPM). The penetration of a low-speed bullet through a myocardium material slab was simulated and several wounding scenarios were analyzed and compared. The material symmetry, the type of contact modeled between the bullet and the soft tissue and the bullet speed were shown to have a significant influence on the wound profile.

Computer Simulation↗

PubMed2004

Longitudinal trends in trauma mortality and survival in Stoke-on-Trent 1992-1998.

OBJECTIVES: To identify factors related to mortality and to test the null hypothesis of no longitudinal trend in mortality in patients admitted to the North Staffordshire Hospital (NSH) with an Injury Severity Score (ISS) greater than 15, between April 1992 and March 1998. DESIGN: Longitudinal prospective study of 18 factors, including age, sex, mechanism of injury, anatomical injury scores and year of admission. Outcome, based on mortality at discharge, was analysed in two ways: alive or dead at discharge (mortality) and time to death or discharge (survival). RESULTS: A decreasing trend (P < 0.01 ) in mortality with year of admission was detected on the log-odds scale. The trend could not be explained by a case-mix analysis, which allowed for the 17 other factors. Using multiple logistic regression analysis (mortality) and Cox proportional hazards analysis (survival), eight factors were identified as determinants of outcome: age, head AIS score, chest AIS score, abdominal AIS score, calendar year of admission, external injury AIS score, mechanism of the injury and primary receiving hospital. CONCLUSIONS: The observed improvement in survival in severely injured patients must result from the interplay of factors not controlled in this analysis or improvements in patient care or both.

Adult↗

PubMedDate not supplied

Comparing 2D vector field visualization methods: a user study.

We present results from a user study that compared six visualization methods for two-dimensional vector data. Users performed three simple but representative tasks using visualizations from each method: 1) locating all critical points in an image, 2) identifying critical point types, and 3) advecting a particle. Visualization methods included two that used different spatial distributions of short arrow icons, two that used different distributions of integral curves, one that used wedges located to suggest flow lines, and line-integral convolution (LIC). Results show different strengths and weaknesses for each method. We found that users performed these tasks better with methods that: 1) showed the sign of vectors within the vector field, 2) visually represented integral curves, and 3) visually represented the locations of critical points. Expert user performance was not statistically different from nonexpert user performance. We used several methods to analyze the data including omnibus analysis of variance, pairwise t-tests, and graphical analysis using inferential confidence intervals. We concluded that using the inferential confidence intervals for displaying the overall pattern of results for each task measure and for performing subsequent pairwise comparisons of the condition means was the best method for analyzing the data in this study. These results provide quantitative support for some of the anecdotal evidence concerning visualization methods. The tasks and testing framework also provide a basis for comparing other visualization methods, for creating more effective methods and for defining additional tasks to further understand the tradeoffs among the methods. In the future, we also envision extending this work to more ambitious comparisons, such as evaluating two-dimensional vectors on two-dimensional surfaces embedded in three-dimensional space and defining analogous tasks for three-dimensional visualization methods.

Algorithms↗

PubMedDate not supplied

Ray-tracing polymorphic multidomain spectral/hp elements for isosurface rendering.

The purpose of this paper is to present a ray-tracing isosurface rendering algorithm for spectral/hp (high-order finite) element methods in which the visualization error is both quantified and minimized. Determination of the ray-isosurface intersection is accomplished by classic polynomial root-finding applied to a polynomial approximation obtained by projecting the finite element solution over element-partitioned segments along the ray. Combining the smoothness properties of spectral/hp elements with classic orthogonal polynomial approximation theory, we devise an adaptive scheme which allows the polynomial approximation along a ray-segment to be arbitrarily close to the true solution. The resulting images converge toward a pixel-exact image at a rate far faster than sampling the spectral/hp element solution and applying classic low-order visualization techniques such as marching cubes.

Algorithms↗