Ben Aldham

Movement and Dislocation of Modular Stent-Grafts Due to Pulsatile Flow and the Pressure Difference Between the Stent-Graft and the Aneurysm Sac

Purpose: To investigate the stability and movement of modular aortic stent-grafts subjected to oscillating forces from pulsatile blood flow, with particular reference to the thoracic aorta. Methods: Analytical mathematical modeling was used to understand the forces on modular grafts. In a benchtop experiment, a transparent acrylic box was filled with water to mimic an aneurysm. Two stent-grafts were placed inside the box in a nested, arched configuration where one component was partly inside the other. A pump produced a pulsatile !5-L/min flow of water through the stent-grafts at a mean inlet pressure of !100 mmHg (∼13,330 Pa), with systolic and diastolic pressures of ∼130 and ∼80 mmHg, respectively (pulse pressure 50 mmHg). The movement of the 2 modular stent-grafts was observed. Results: The curved stent-graft system oscillated transversely when there was zero mean pressure difference between the stent-graft and the aneurysm. As the mean pressure difference was increased, this transverse graft movement was damped and then disappeared. A relatively large pressure difference caused the stent-graft to inflate and become sturdier. In terms of stability, the analytical mathematical model for a 30-mm-diameter Zenith modular stent-graft curved through 90° (with the ends of the graft fixed in place) showed that the modular components will separate at a pressure difference of 0 mmHg for 1 stent segment overlap (20 mm) and at an average 59 mmHg pressure difference for 2 stent overlaps, but the device would not separate at a pressure difference of 90 mmHg for 3 stent overlaps. Conclusion: Transverse cyclic movement of the curved stent-graft system with pulsation indicates a pressurized sac. When the pressure difference is large and there is a blood-tight seal between the aneurysm and the stent-graft, then the transverse movement of the stent-graft is minimal, but the risk for modular separation is highest. Curved thoracic endografts are subject to forces that may cause migration or separation, the latter being more likely if the seal between the graft and the sac is blood tight, if the blood pressure is high, and if the diameter of the graft is small and the sac large. Operators should plan for maximum overlap of modular components when treating large or long thoracic aneurysms.

Density segregation of granular material in a rotating cylindrical tumbler

Many mining operations use large quantities of water to separate valuable minerals from less valuable gangue. This dependence on liquid separation has an environmental impact in terms of energy and water use and also implies a cap on production due to the availability of water. To address these problems, the CSIRO has developed the CSIRO Rotational Classifier, which - by using the phenomena of rotational segregation - can quickly separate dry granular material in terms of size and/or density without the use of any liquids. The purpose of this paper is to obtain a deeper understanding of how rotational segregation can separate particles of different densities in a rotating cylinder, free from any interstitial fluids. This was accomplished by analyzing a cross section at the 20% fill level in a 50% full classifier, which contained a 50-50 ratio of glass and lead beads. The granular bed was sampled at different time intervals over a 60 second period with a classifier rotation rate of 2 rpm. These experiments resulted in a high segregation level of 0.9 in 20 seconds and 0.95 by 60 seconds (where a level of 1 implies full segregation). The results then underwent image analysis and were subsequently compared to results from a discrete element method (DEM) model where similar segregation ratios, albeit at longer timescales, were obtained. This study gave a further insight into the segregation process particularly in terms of axial formation of the segregated core which may one day be used in the separation of minerals.

Quantitative Guidelines for the Prediction of Ultrasound Contrast Agent Destruction During Injection

Experiments and theory were undertaken on the destruction of ultrasound contrast agent microbubbles on needle injection, with the aim of predicting agent loss during in vivo studies. Agents were expelled through a variety of syringe and needle combinations, subjecting the microbubbles to a range of pressure drops. Imaging of the bubbles identified cases where bubbles were destroyed and the extent of destruction. Fluid-dynamic calculations determined the pressure drop for each syringe and needle combination. It was found that agent destruction occurred at a critical pressure drop that depended only on the type of microbubble. Protein-shelled microbubbles (sonicated bovine serum albumin) were virtually all destroyed above their critical pressure drop of 109 ± 7 kPa Two types of lipid-shelled microbubbles were found to have a pressure drop threshold above which more than 50% of the microbubbles were destroyed. The commercial lipid-shelled agent Definity was found to have a critical pressure drop for destruction of 230 ± 10 kPa; for a previously published lipid-shelled agent, this value was 150 ± 40 kPa. It is recommended that attention to the predictions of a simple formula could preclude unnecessary destruction of microbubble contrast agent during in vivo injections. This approach may also preclude undesirable release of drug or gene payloads in targeted microbubble therapies. Example values of appropriate injection rates for various agents and conditions are given.

Detection of small vessel knock signals by Doppler ultrasound in a laboratory model simulating a penetrating branch occlusion

not to be published. Concurrent 1 – Platform Session Wednesday, 13 August 2008: 1430–1445 h CEREBRAL INFARCT SIZE IS GENDER-DEPENDANT FOLLOWING TRANSIENT BUT NOT PERMANENT MIDDLE CEREBRAL ARTERY OCCLUSION IN MICE: DIFFERENTIAL EFFECTS ON NOX1, NOX2 AND NOX4 EXPRESSION CG Sobey, VH Brait, KA Jackman, AE Mast, TM DeSilva, AA Miller, GR Drummond Department of Pharmacology, Monash University, Melbourne, VIC Females are strongly protected against cerebral ischemia-reperfusion (I-R) damage but the underlying mechanisms are poorly understood. Moreover, it is not clear whether females are similarly protected during ischemia with no reperfusion (I-NR). Here we have tested whether females are protected to a similar degree during both cerebral I-R and I-NR, and whether the outcomes are associated with changes in expression of NADPH oxidase Nox subunits in the ischemic brain. Stroke was induced by 0.5 h middle cerebral artery occlusion in 6–8 week-old female and male C57Bl6J mice. After 24 h, brain infarct volume was measured in thionin-stained coronal sections. After I-R, males had a larger infarct volume than females (55 ± 11 vs 24 ± 6 mm; P < 0.05), whereas there was no gender difference following I-NR (88 ± 11 vs 88 ± 9 mm). After I-R in males, expression of Nox2 increased by ∼30% and Nox4 decreased by ∼20%, whereas in females neither isoform was altered by I-R. After I-NR in males, changes in Nox2 and Nox4 were slightly more pronounced than after I-R (+60% and −25%, respectively). However in females, in contrast to the lack of effects on brain Nox2 and Nox4 expression after I-R, marked changes occurred after I-NR (+70% and −40%, respectively). There were no differences in Nox1 expression between any of the groups. Thus, cerebral infarct size is gender-dependent following I-R but not I-NR in mice. The lack of protection in females after I-NR is associated with changes in brain expression of Nox2 and/or Nox4 not present after I-R. Concurrent 1 – Platform Session Wednesday, 13 August 2008: 1445–1500 h OPTIMIZING MISMATCH DEFINITIONS IN ACUTE STROKE MRI – AN EPITHET POST HOC STUDY Soren Christensen, Mark W Parsons, Deidre A De Silva, Martin Ebinger, Ken Butcher, John Fink, Patricia Desmond, P Alan Barber, C Bladin, C Levi, Geoffrey A Donnan, Stephen M Davis Royal Melbourne Hospital, University of Melbourne, VIC Background: The recently published EPITHET trial tested whether tissue plasminogen activator (tPA) attenuated growth in patients with a perfusion/diffusion weighted imaging mismatch, believed to identify potential treatment responders.The trial was negative for the primary growth outcome measure. Secondary outcome measures consistently showed a tendency for improved outcomes in the tPA group. Aims: Eighty-six percent of patients had mismatch, and we hypothesized that a more restrictive mismatch definition would identify an enriched population with larger treatment response. In this descriptive post-hoc analysis we investigated the effect of changing the Tmax and mismatch ratio definitions in order to identify effect sizes, definitions and outcome measures for use in future studies. Methods: Mismatch definitions were varied to identify subpopulations with mismatch frequencies from 10 to 100%. We recorded effect and significance level of the differences between treatment arms at each definition. Outcome measures were infarct growth and mRS 0–1. Results: More restrictive mismatch definitions were significantly correlated with larger growth differences between treatment arms R = 0.40 (P < 0.001) and identified a population with higher treatment response. The odds ratio for mRS 0–1 in the tPA arm showed a much weaker correlation R = 0.09 (P < 0.001) with mismatch definitions and although there was a constant trend for benefit of tPA, the group differences were not significant at any relevant threshold. Conclusion: More restrictive mismatch definitions identify subgroups of patients in EPITHET with higher and significant response to treatment. Based on EPITHET 360 patients are required to demonstrate effect on clinical outcomes.