Worcester Polytechnic Institute

About: Worcester Polytechnic Institute is a education organization based out in Worcester, Massachusetts, United States. It is known for research contribution in the topics: Population & Data envelopment analysis. The organization has 6270 authors who have published 12704 publications receiving 332081 citations. The organization is also known as: WPI.

3D MRI-based anisotropic FSI models with cyclic bending for human coronary atherosclerotic plaque mechanical analysis.

Abstract: Heart attack and stroke are often caused by atherosclerotic plaque rupture, which happens without warning most of the time. Magnetic resonance imaging (MRI)-based atherosclerotic plaque models with fluid-structure interactions (FSIs) have been introduced to perform flow and stress/strain analysis and identify possible mechanical and morphological indices for accurate plaque vulnerability assessment. For coronary arteries, cyclic bending associated with heart motion and anisotropy of the vessel walls may have significant influence on flow and stress/strain distributions in the plaque. FSI models with cyclic bending and anisotropic vessel properties for coronary plaques are lacking in the current literature. In this paper, cyclic bending and anisotropic vessel properties were added to 3D FSI coronary plaque models so that the models would be more realistic for more accurate computational flow and stress/strain predictions. Six computational models using one ex vivo MRI human coronary plaque specimen data were constructed to assess the effects of cyclic bending, anisotropic vessel properties, pulsating pressure, plaque structure, and axial stretch on plaque stress/strain distributions. Our results indicate that cyclic bending and anisotropic properties may cause 50-800% increase in maximum principal stress (Stress-P1) values at selected locations. The stress increase varies with location and is higher when bending is coupled with axial stretch, nonsmooth plaque structure, and resonant pressure conditions (zero phase angle shift). Effects of cyclic bending on flow behaviors are more modest (9.8% decrease in maximum velocity, 2.5% decrease in flow rate, 15% increase in maximum flow shear stress). Inclusion of cyclic bending, anisotropic vessel material properties, accurate plaque structure, and axial stretch in computational FSI models should lead to a considerable improvement of accuracy of computational stress/strain predictions for coronary plaque vulnerability assessment. Further studies incorporating additional mechanical property data and in vivo MRI data are needed to obtain more complete and accurate knowledge about flow and stress/strain behaviors in coronary plaques and to identify critical indicators for better plaque assessment and possible rupture predictions.

Maximum likelihood methods for cure rate models with missing covariates.

Abstract: We propose maximum likelihood methods for parameter estimation for a novel class of semiparametric survival models with a cure fraction, in which the covariates are allowed to be missing. We allow the covariates to be either categorical or continuous and specify a parametric distribution for the covariates that is written as a sequence of one-dimensional conditional distributions. We propose a novel EM algorithm for maximum likelihood estimation and derive standard errors by using Louis's formula (Louis, 1982, Journal of the Royal Statistical Society, Series B 44, 226-233). Computational techniques using the Monte Carlo EM algorithm are discussed and implemented. A real data set involving a melanoma cancer clinical trial is examined in detail to demonstrate the methodology.

Orientational coupling enhancement in a carbon nanotube dispersed liquid crystal.

Abstract: We present a detailed study of a dilute suspension of carbon nanotubes (CNTs) in a pentylcyanobiphenyl (5CB) liquid crystal (LC) by probing the dielectric properties as a function of applied ac voltage and frequency. In principle, to minimize the elastic distortion in the nematic matrix, the monodispersed CNTs follow the nematic director without disturbing the director field significantly. A strong anchoring energy due to π-π electron stacking between LC-CNT molecules results in an increase in the dielectric anisotropy for the hybrid system, indicating a significant enhancement in the orientational order parameter. The frequency-dependent dielectric anisotropy for the composite system reveals the intrinsic frequency response of the LC-CNT anchoring mechanism. As a matter of consequence, the extracted value of splay elastic constant suggests that LC-CNT anchoring has an impact on the structural modification of the hybrid LC+CNT system. This strong anchoring energy stabilizes local pseudonematic domains, giving rise to a nonzero dielectric anisotropy in the isotropic phase that also shows an intrinsic frequency response.

Bayesian analysis and model selection for interval-censored survival data.

Abstract: Summary. Interval-censored data occur in survival analysis when the survival time of each patient is only known to be within an interval and these censoring intervals differ from patient to patient. For such data, we present some Bayesian discretized semiparametric models, incorporating proportional and nonproportional hazards structures, along with associated statistical analyses and tools for model selection using sampling-based methods. The scope of these methodologies is illustrated through a reanalysis of a breast cancer data set (Finkelstein, 1986, Biometrics42, 845–854) to test whether the effect of covariate on survival changes over time.