Matthew D. Sinnott

Carotid Artery Anatomy and Geometry as Risk Factors for Carotid Atherosclerotic Disease

Background and Purpose— Traditional vascular risk factors do not completely explain the asymmetry, racial, and sex differences in carotid artery disease. Carotid anatomy and geometry may play a role in the pathogenesis of internal carotid artery (ICA) stenosis, but their effects are unknown. We hypothesized that carotid artery anatomy and geometry would be independently associated with ICA stenosis. Method— This is a retrospective study of patients with CT angiography at Monash Medical Centre, 2006 to 2007. Carotid arteries were segmented using semiautomated methods to estimate measures of carotid anatomy and geometry. Measurements of carotid artery geometry were performed according to the recent article by Thomas and colleagues. ICA stenosis was dichotomized as <30% or ≥30% stenosis. Cluster logistic regression was used to examine the associations of anatomy and geometry with stenosis accounting for the paired arteries within subjects, adjusting for age, sex, and vascular risk factors. Results— Mean age of the sample (n=178) was 68.4 years (SD, 14 years). The following were independently associated with ICA stenosis: ICA radius at the bifurcation (OR, 0.20; 95% CI, 0.14–0.29), ICA angle (OR, 1.05 per degree increment; 95% CI, 1.04–1.07), age (OR, 1.05 per year increment; 95% CI, 1.03–1.07), male sex (OR, 1.72; 95% CI, 1.08–2.8), and ever-smoker (OR, 1.85; 95% CI, 1.15–2.96). Conclusions— Carotid anatomy and geometry may enhance the risk of stenosis independent of traditional vascular risk factors and may be of help in very early identification of patients at high risk of developing carotid artery atherosclerosis for aggressive intervention.

Prediction of industrial, biophysical and extreme geophysical flows using particle methods

Purpose – The purpose of this paper is to show how simulation of the flow of particulates and fluids using discrete element modelling (DEM) and smoothed particle dynamics (SPH) particle methods, offer opportunities for better understanding the dynamics of flow processes.Design/methodology/approach – DEM and SPH methods are demonstrated in a broad range of computationally‐demanding applications including comminution, biomedical, geophysical extreme flow events (risk/disaster modelling), eating of food by humans and elite water‐based sports.Findings – DEM is ideally suited to predicting industrial and geophysical applications where collisions between particles are the dominant physics. SPH is highly suited to multi‐physics fluid flow applications in industrial, biophysical and geophysical applications. The advantages and disadvantages of these particle methods are discussed.Research limitations/implications – Research results are limited by the numerical resolution that can currently be afforded.Practical i...

Investigating the relationships between peristaltic contraction and fluid transport in the human colon using Smoothed Particle Hydrodynamics

Complex relationships exist between gut contractility and the flow of digesta. We propose here a Smoothed Particle Hydrodynamics model coupling the flow of luminal content and wall flexure to help investigate these relationships. The model indicates that a zone of muscular relaxation preceding the contraction is an important element for transport. Low pressures in this zone generate positive thrust for low viscosity content. The viscosity of luminal content controls the localization of the flow and the magnitude of the radial pressure gradient and together with contraction amplitude they control the transport rate. For high viscosity content, high lumen occlusion is required for effective propulsion.

Effect of rotor blade angle and clearance on blood flow through a non-pulsatile, axial, heart pump

This study involves simulation of blood flow through a non-pulsatile, axial pump consisting of a complex multi-sectioned impeller with rotating and stationary components, and a stationary housing. We use a particle-based method, Smoothed Particle Hydrodynamics (SPH), to investigate the effects of varying the impeller blade pitch angle and the gap size between the rotor and housing. We show that the impeller blade inclination affects pump performance and the level of shear-related blood damage. The inclusion of a rotor-housing clearance results in flow losses that reduce the efficiency of the pump but also reduce the viscous energy losses in the pump.

Does the principle of minimum work apply at the carotid bifurcation: a retrospective cohort study

BackgroundThere is recent interest in the role of carotid bifurcation anatomy, geometry and hemodynamic factors in the pathogenesis of carotid artery atherosclerosis. Certain anatomical and geometric configurations at the carotid bifurcation have been linked to disturbed flow. It has been proposed that vascular dimensions are selected to minimize energy required to maintain blood flow, and that this occurs when an exponent of 3 relates the radii of parent and daughter arteries. We evaluate whether the dimensions of bifurcation of the extracranial carotid artery follow this principle of minimum work.MethodsThis study involved subjects who had computed tomographic angiography (CTA) at our institution between 2006 and 2007. Radii of the common, internal and external carotid arteries were determined. The exponent was determined for individual bifurcations using numerical methods and for the sample using nonlinear regression.ResultsMean age for 45 participants was 56.9 ± 16.5 years with 26 males. Prevalence of vascular risk factors was: hypertension-48%, smoking-23%, diabetes-16.7%, hyperlipidemia-51%, ischemic heart disease-18.7%.The value of the exponent ranged from 1.3 to 1.6, depending on estimation methodology.ConclusionsThe principle of minimum work (defined by an exponent of 3) may not apply at the carotid bifurcation. Additional factors may play a role in the relationship between the radii of the parent and daughter vessels.

10 – Modelling food digestion

Food digestion involves the flow of material and its transformation from entry into the mouth, during passage along the gastrointestinal tract, to its eventual exit from the body. The range of food materials and their complex structures and the range of processes that are used by the human body to liberate nutrients for absorption are varied and complex. Their effectiveness is critical to human nutrition and health. An understanding of the operation of these processes can be improved substantially using advanced computational models. The complexity of this system presents many challenges for such modelling to be realistic and effective. This chapter describes the nature of these challenges and the current status of such modelling.

How arterial pressures affect the consideration of internal carotid artery angle as a risk factor for carotid artherosclerotic disease

Patient–specific geometric factors together with traditional risk factors may aid the early identification of patients at high risk of developing carotid artery disease requiring surgical intervention. Recent studies have linked aspects of carotid geometry to the pathogenesis of internal carotid artery (ICA) stenosis. Abnormal wall shear stress (WSS) is found for large ICA angles. Low WSS is believed to correspond to plaque formation whereas high WSS may result in plaque rupture and clotting. Here, the meshless method, smoothed particle hydrodynamics, is used to simulate Newtonian flow through a clinical, rigid walled, carotid bifurcation. The resulting flow field and WSS are reported for a range of different ICA angles. Varying the angle without changing boundary pressure conditions produces minimal change in flow and WSS. Greater ICA downstream pressures appear important for maintaining well–behaved flow through the bifurcation by suppressing flow separation downstream of the stenosis resulting in more uniform wall stress.

Computer Modeling of Anterior Circulation Stroke: Proof of Concept in Cerebrovascular Occlusion

Background: Current literature emphasizes the role of the Circle of Willis (CoW) in salvaging ischemic brain tissue but not that of leptomeningeal anastomoses (LA). We developed a computational model of the cerebral circulation to (1) evaluate the roles of the CoW and LA in restoring flow to the superficial compartment of the middle cerebral artery (MCA) territory and (2) estimate the size of the LA required to maintain flow above the critical ischemic threshold (>30% of baseline) under simulated occlusion. Methods: Cerebral vasculature was modeled as a network of junctions connected by cylindrical pipes. The experiments included occlusion of successive distal branches of the intracranial arteries while the diameters of LA were varied. Results: The model showed that the region of reduced flow became progressively smaller as the site of occlusion was moved from the large proximal to the smaller distal arteries. There was no improvement in flow in the MCA territory when the diameters of the inter-territorial LA were varied from 0.0625 to 0.5 mm while keeping the intra-territorial LA constant. By contrast, the diameter of the inter-territorial LA needed to be >1.0 mm in order to provide adequate (>30%) flow to selected arteries in the occluded MCA territory. Conclusion: The CoW and inter-territorial LA together play important supportive roles in intracranial artery occlusion. Computational modeling provides the ability to experimentally investigate the effect of arterial occlusion on CoW and LA function.

Rogue wave impact on a semi-submersible offshore platform

Full three-dimensional simulation of the impact of a rogue wave on a semi-submersible platform is undertaken using the Smoothed Particle Hydrodynamics (SPH) technique. Two different mooring configurations are considered: A Tension Leg Platform (TLP) system and a Taut Spread Mooring (TSM) system. It is seen that for a wave impact normal to the platform side, the heave and surge responses of the platform are significantly different for the two mooring systems. The TLP system undergoes large surge but comparatively smaller heave motions than the TSM system. The degree of pitch is very similar. The total tension in the mooring cables is approximately four times higher in the TSM system and exceeds the strength of the cables used in the simulation. SPH is seen to be an attractive alternative to standard methods for simulating the coupled interaction of highly non-linear breaking waves and structural motion.Copyright

Role of Descending Inhibition in Transport of Fluid Contents in the Colon

Constipation, a common cause of morbidity, is estimated to affect between 15 and 27% of the western world. While often perceived as a benign, easily treated condition, a number of studies confirm a significant adverse impact on health-related quality of life which correlates with severity of constipation. Available evidence implicates abnormal colonic contractility in severe constipation. However, measurement of human colonic motility poses substantial methodological challenges.