Patrick H. Geoghegan

Experimental investigation of the mechanical properties of brain simulants used for cranial gunshot simulation

The mechanical properties of the human brain at high strain rate were investigated to analyse the mechanisms that cause backspatter when a cranial gunshot wound occurs. Different concentrations of gelatine and a new material (M1) developed in this work were tested and compared to bovine brain samples. Kinetic energy absorption and expansion rate of the samples caused by the impact of a bullet from .22 air rifle (AR) (average velocity (uav) of 290m/s) and .22 long rifle (LR) (average velocity (uav) of 330m/s) were analysed using a high speed camera (24,000fps). The AR projectile had, in the region of interest, an average kinetic energy (Ek) of 42±1.3J. On average, the bovine brain absorbed 50±5% of Ek, and the simulants 46-58±5%. The Ek of the .22 LR was 141±3.7J. The bovine brain absorbed 27% of the .22LR Ek and the simulants 15-29%. The expansion of the sample, after penetration, was measured. The bovine brain experienced significant plastic deformation whereas the gelatine solution exhibited a principally elastic response. The permanent damage patterns in the M1 material were much closer to those in brain tissue, than were the damage patterns in the gelatine. The results provide a first step to developing a realistic experimental simulant for the human brain which can produce the same blood backspatter patterns as a human brain during a cranial gunshot. These results can also be used to improve the 3D models of human heads used in car crash and blast trauma injury research.

A PIV COMPARISON OF THE FLOW FIELD AND WALL SHEAR STRESS IN RIGID AND COMPLIANT MODELS OF HEALTHY CAROTID ARTERIES

Certain systems relevant to circulatory disease have walls which are neither rigid nor static, for example, the coronary arteries, the carotid artery and the heart chambers. In vitro modeling allows the fluid mechanics of the circulatory system to be studied without the ethical and safety issues associated with animal and human experiments. Computational methods in which the equations are coupled governing the flow and the elastic walls are maturing. Currently there is a lack of experimental data in compliant arterial systems to validate the numerical predictions. Previous experimental work has commonly used rigid wall boundaries, ignoring the effect of wall compliance. Particle Image Velocimetry is used to provide a direct comparison of both the flow field and wall shear stress (WSS) observed in experimental phantoms of rigid and compliant geometries representing an idealized common carotid artery. The input flow waveform and the mechanical response of the phantom are physiologically realistic. The results show that compliance affects the velocity profile within the artery. A rigid boundary causes severe overestimation of the peak WSS with a maximum relative difference of 61% occurring; showing compliance protects the artery from exposure to high magnitude WSS. This is important when trying to understand the development of diseases like atherosclerosis. The maximum, minimum and time averaged WSS in the rigid geometry was 2.3, 0.51 and 1.03Pa and in the compliant geometry 1.4, 0.58 and 0.84Pa, respectively.

Visualization of the air ejected from the temporary cavity in brain and tissue simulants during gunshot wounding

One hypothesis for the physical mechanism responsible for backspatter during cranial gunshot wounding is that air is ejected by the collapse of the temporary cavity formed around the bullet path. Using bovine and ovine heads and simulant materials, evidence of this ejection was sought by measuring the velocity of the air that was drawn in and ejected from the cavity in front of the wound channel after bullet impact. A laminar flow of fog-laden air was arranged in front of the wound channel and two high speed cameras recording at 30,000 frames/second captured the air motion. All samples were shot with standard 9 mm × 19 mm FMJ ammunition. Different concentrations of ballistic gelatine were used to characterize the effect of elasticity of the material on the velocity of the air. Fresh bovine and ovine heads were shot with the same experimental set up to investigate if there was induction of air into, and ejection of air from the entrance wounds. The results show, for the first time, that the temporary cavity does eject air in gelatine. The velocity of in-drawn air for 3, 5 and 10% concentration of gelatine was 81, 76 and 65 m/s respectively and the velocity of ejected air for 5 and 10% concentration of gelatine were 43 and 72 m/s respectively. The results show that when the concentration of gelatine is increased, the velocity of the air drawn into the cavity decreases and the velocity of the ejected air increases. However, no ejection was observed in 3% gelatine, ovine or bovine heads. Although ejection of air was not observed, ejection of brain from the wound channel was seen. Using the velocity of the ejected brain, the minimum intracranial pressure required to eject the brain tissue was estimated to be 712 kPa and 468 kPa for the sheep and bovine heads respectively.

Respiratory airway resistance monitoring in mechanically ventilated patients

Physiological models of respiratory mechanics can be used to optimise mechanical ventilator settings to improve critically ill patient outcomes. Models are generally generated via either physical measurements or analogous behaviours that can model experimental outcomes. However, models derived solely from physical measurements are infrequently applied to clinical data. This investigation assesses the efficacy of a physically derived airway branching model (ABM) to capture clinical data. The ABM is derived via classical pressure-flow equations and branching based on known anatomy. It is compared to two well accepted lumped parameter models of the respiratory system: the linear lung model (LLM) and the Dynostatic Model (DSM). The ABM significantly underestimates the total pressure drop from the trachea to the alveoli. While the LLM and DSM both recorded peak pressure drops of 17.8 cmH2O and 10.2 cmH2O, respectively, the maximum ABM modelled pressure drop was 0.66 cmH2O. This result indicates that the anatomically accurate ABM model does not incorporate all of the airway resistances that are clinically observed in critically ill patients. In particular, it is hypothesised that the primary discrepancy is in the endotracheal tube. In contrast to the lumped parameter models, the ABM was capable of defining the pressure drop in the deep bronchial paths and thus may allow further investigation of alveoli recruitment and gas exchange at that level given realistic initial pressures at the upper airways.

Rheometry based on free surface velocity

ABSTRACT This paper explores the possibility of identifying the rheology of a fluid by monitoring how the free surface velocity field is affected by a perturbation in the flow. The dam-break problem is considered which results from the release of a gate initially separating two fluid pools of different depth. The flow velocity is measured by seeding the free surface with buoyant particles and using Particle Tracking Velocimetry. In parallel, a mathematical model based on the lubrication approximation for fluids with a power-law rheology is developed. The model is validated against a similarity solution which is obtained for the spreading of a gravity current under its own weight and neglecting surface tension. Minimizing the difference between the free surface velocity fields obtained numerically and measured experimentally enables the identification of rheological parameters. The methodology is tested on ideal and noisy synthetic data sets and experimental data obtained with aqueous glycerol.

Regressive cross-correlation of pressure signals in the region of stenosis: Insights from particle image velocimetry experimentation

Abstract Various anomalies in arterial geometry can cause serious hemodynamic dysfunction. In particular, stenosed arteries can cause reduced blood flow, excess stress on the heart, and elements can shear off causing blockage, which in the brain leads to stroke. This research assesses whether pressure signals obtained close to a stenosis are distinct from signals observed in other areas of the artery. Particle image velocimetry was used to determine the fluid velocity field within a compliant phantom that mimicked a stenosis in the carotid artery during physiological pulsatile pressure waves. The Navier-Stokes representation of the velocity fields were used to determine the pressure responses across the domain. A three-parameter regressive cross-correlation was used to calibrate the output pressure responses against the pressure input signal. The transform between the input-output pressure signals allowed detection of the region immediately downstream of the stenosis. In particular, if the cross correlative parameter that relates the instantaneous transfer across the input-output signals was greater than the delayed transfer parameter a stenosis is present. In contrast, the delayed transfer parameter was larger for the region upstream of the stenosis. This outcome is particularly valuable as it does not require calibration of the absolute pressure, which can be difficult to determine physiologically due to factors such as arterial geometry and intrathoracic pressure. However, the outcomes need to be validated in more geometries prior to clinical validation.

Fabrication of a compliant phantom of the human aortic arch for use in Particle Image Velocimetry (PIV) experimentation

Abstract Compliant phantoms of the human aortic arch can mimic patient specific cardiovascular dysfunctions in vitro. Hence, phantoms may enable elucidation of haemodynamic disturbances caused by aortic dysfunction. This paper describes the fabrication of a thin-walled silicone phantom of the human ascending aorta and brachiocephalic artery. The model geometry was determined via a meta-analysis and modelled in SolidWorks before 3D printing. The solid model surface was smoothed and scanned with a 3D scanner. An offset outer mould was milled from Ebalta S-Model board. The final phantom indicated that ABS was a suitable material for the internal model, the Ebalta S-Model board yielded a rough external surface. Co-location of the moulds during silicone pour was insufficient to enable consistent wall thickness. The resulting phantom was free of air bubbles but did not have the desired wall thickness consistency.

Modelling nasal high flow therapy effects on upper airway resistance and resistive work of breathing

AIM The goal of this paper is to quantify upper airway resistance with and without nasal high flow (NHF) therapy. For adults, NHF therapy feeds 30-60 L/min of warm humidified air into the nose through short cannulas which do not seal the nostril. NHF therapy has been reported to increase airway pressure, increase tidal volume (Vt) and decrease respiratory rate (RR), but it is unclear how these findings affect the work done to overcome airway resistance to air flow during expiration. Also, there is little information on how the choice of nasal cannula size may affect work of breathing. In this paper, estimates of airway resistance without and with different NHF flow (applied via different cannula sizes) were made. The breathing efforts required to overcome airway resistance under these conditions were quantified. METHOD NHF was applied via three different cannula sizes to a 3-D printed human upper airway. Pressure drop and flow rate were measured and used to estimate inspiratory and expiratory upper airway resistances. The resistance information was used to compute the muscular work required to overcome the resistance of the upper airway to flow. RESULTS NHF raises expiratory resistance relative to spontaneous breathing if the breathing pattern does not change but reduces work of breathing if peak expiratory flow falls. Of the cannula sizes used, the large cannula produced the greatest resistance and the small cannula produced the least. The work required to cause tracheal flow through the upper airway was reduced if the RR and minute volume are reduced by NHF. NHF has been observed to do so in COPD patients (Bräunlich et al., 2013). A reduction in I:E ratio due to therapy was found to reduce work of breathing if the peak inspiratory flow is less than the flow below which no inspiratory effort is required to overcome upper airway resistance. CONCLUSION NHF raises expiratory resistance but it can reduce the work required to overcome upper airway resistance via a fall in inspiratory work of breathing, RR and minute volume.

A response to Marquis et al. (2017) What is the error margin of your signature analysis

Marquis et al (2017) [What is the error margin of your signature analysis? Forensic Science International, 281, e1–e8] ostensibly presents a model of how to respond to a request from a court to state an “error margin” for a conclusion from a forensic analysis. We interpret the court’s request as an explicit request for meaningful empirical validation to be conducted and the results reported. Marquis et al (2017), however, recommends a method based entirely on subjective judgement and does not subject it to any empirical validation. We believe that much resistance to the adoption of the likelihood ratio framework is not to the idea of assessing the relative probabilities (or likelihoods) of the evidence under prosecution and defence hypotheses per se, but to what is perceived to be unwarranted subjective assignment of those probabilities. In order to maximize transparency, replicability, and resistance to cognitive bias, we recommend the use of methods based on relevant data, quantitative measurements, and statistical models. If the method is based on subjective judgement, the output should be empirically calibrated. Irrespective of the basis of the method, its implementation should be empirically validated under conditions reflecting those of the case at hand.

A Review of Arterial Phantom Fabrication Methods for Flow Measurement Using PIV Techniques

Cardiovascular diseases (CVD) are the leading cause of morbidity and mortality in the western world. In the last three decades, fluid dynamics investigations have been an important component in the study of the cardiovascular system and CVD. A large proportion of studies have been restricted to computational fluid dynamic (CFD) modeling of blood flow. However, with the development of flow measurement techniques such as particle image velocimetry (PIV), and recent advances in additive manufacturing, experimental investigation of such flow systems has become of interest to validate CFD studies, testing vascular implants and using the data for therapeutic procedures. This article reviews the technical aspects of in-vitro arterial flow measurement with the focus on PIV. CAD modeling of geometries and rapid prototyping of molds has been reviewed. Different processes of casting rigid and compliant models for experimental analysis have been reviewed and the accuracy of construction of each method has been compared. A review of refractive index matching and blood mimicking flow circuits is also provided. Methodologies and results of the most influential experimental studies are compared to elucidate the benefits, accuracy and limitations of each method.