Dynamically scaled phantom phase contrast MRI compared to true-scale computational modeling of coronary artery flow.

PURPOSE: To examine the feasibility of combining computational fluid dynamics (CFD) and dynamically scaled phantom phase-contrast magnetic resonance imaging (PC-MRI) for coronary flow assessment. MATERIALS AND METHODS: Left main coronary bifurcations segmented from computed tomography with bifurcation angles of 33°, 68°, and 117° were scaled-up ∼7× and 3D printed. Steady coronary flow was reproduced in these phantoms using the principle of dynamic similarity to preserve the true-scale Reynolds number, using blood analog fluid and a pump circuit in a 3T MRI scanner. After PC-MRI acquisition, the data were segmented and coregistered to CFD simulations of identical, but true-scale geometries. Velocities at the inlet region were extracted from the PC-MRI to define the CFD inlet boundary condition. RESULTS: The PC-MRI and CFD flow data agreed well, and comparison showed: 1) small velocity magnitude discrepancies (2-8%); 2) with a Spearman's rank correlation ≥0.72; and 3) a velocity vector correlation (including direction) of r(2) ≥ 0.82. The highest agreement was achieved for high velocity regions with discrepancies being located in slow or recirculating zones with low MRI signal-to-noise ratio (SNRv ) in tortuous segments and large bifurcating vessels. CONCLUSION: Characterization of coronary flow using a dynamically scaled PC-MRI phantom flow is feasible and provides higher resolution than current in vivo or true-scale in vitro methods, and may be used to provide boundary conditions for true-scale CFD simulations. J. MAGN. RESON. IMAGING 2016;44:983-992.

A deformable template method for describing and averaging the anatomical variation of the human nasal cavity.

BACKGROUND: Understanding airflow through human airways is of importance in drug delivery and development of assisted breathing methods. In this work, we focus on development of a new method to obtain an averaged upper airway geometry from computed tomography (CT) scans of many individuals. This geometry can be used for air flow simulation. We examine the geometry resulting from a data set consisting of 26 airway scans. The methods used to achieve this include nasal cavity segmentation and a deformable template matching procedure. METHODS: The method uses CT scans of the nasal cavity of individuals to obtain a segmented mesh, and coronal cross-sections of this segmented mesh are taken. The cross-sections are processed to extract the nasal cavity, and then thinned ('skeletonized') representations of the airways are computed. A reference template is then deformed such that it lies on this thinned representation. The average of these deformations is used to obtain the average geometry. Our procedure tolerates a wider variety of nasal cavity geometries than earlier methods. RESULTS: To assess the averaging method, key landmark points on each of the input scans as well as the output average geometry are located and compared with one another, showing good agreement. In addition, the cross-sectional area (CSA) profile of the nasal cavities of the input scans and average geometry are also computed, showing that the CSA of the average model falls within the variation of the population. CONCLUSIONS: The use of a deformable template method for aligning and averaging the nasal cavity provides an improved, detailed geometry that is unavailable without using deformation.

Landmark-free Shape Analysis of the Human Duodenum.

The primary function of the duodenum is to undertake chemical digestion by ensuring that the partially digested food received from the stomach is well-mixed with the enzymes and chemicals secreted into it. However, little is known about the anatomical variations in the shape of the duodenum within humans, and thus the effect of duodenum shape on the flow and mixing occurring within the lumen has not been studied. In this work, a methodology for analyzing shape variations in the normal duodenal anatomy has been developed and applied to a publicly available dataset of abdominal CT images. This method does not require the placement of landmarks as it is based on the underlying tubular 'C' shape of the duodenum. The average duodenal length and radius of this dataset (consisting of 34 subjects) were 212.8 ± 38 mm and 10.8 ± 2.5 mm respectively. A Principal Component Analysis (PCA) was conducted on a sample of 34 duodenums after normalizing their lengths and the first five principal components were found to contribute to 82 % of the total variation. The first shape component (accounting for 42 % of overall variation) consisted of variations in the radius along the duodenum with no deformations normal to the central plane, and the subsequent shape modes consisted of twists in the centerline either in and out of the central plane, and radial variations at either the inlet or outlet. This is the first study to analyze shape variations in the human duodenum and the results can be combined with flow modeling to analyze the effect of shape on the flow and mixing occurring within the duodenum.Clinical relevance- The methods developed in this study can be used by clinicians to diagnose abnormalities in an individual's duodenum shape.

Modelling Flow and Mixing in the Proximal Small Intestine.

The small intestine is the primary site of enzymatic digestion and nutrient absorption in humans. Intestinal contractions facilitate digesta mixing, transport and contact with the absorptive surfaces. These motility patterns are regulated by an underlying electrical activity, termed slow waves. In this study, we use computational fluid dynamics simulation of flow and mixing of intestinal contents in the human duodenum with anatomically realistic geometry and contraction patterns. Parameters including the amplitude of contraction (10-50% reduction of radius) and the rheology of the digesta (Newtonian vs Non-Newtonian power law fluid) were altered in-order to study their effects on mixing. Interesting flow features such as stagnation points and reversed flow were observed with digesta. Increases in the amplitude of contraction lead to increased propulsion of digesta along the intestine and increased mixing.

The Effect of Heated CO2 Insufflation in Minimising Surgical Wound Contamination During Open Surgery.

The primary source of infections in open surgeries has been found to be bacteria and viruses carried into the surgical wound on the surfaces of skin particles shed by patients and surgical staff. In open cardiac surgeries, insufflation of the wound with carbon dioxide is used to limit the quantity of air able to enter into the heart, avoiding air embolisms when the heart is restarted. This surgical technique has been evaluated as a method of limiting the number of skin particles able to enter into the wound, using computational fluid dynamics (CFD) simulations and experimental testing. Spherical particles of 5.0 and 13.5 µm in diameter were used to simulate skin particles falling above a wound, travelling in air ventilation velocities of either 0.2 or 0.4 m/s, and with or without CO2 insufflation. The CFD simulations with CO2 included a diffuser placed in the wound and supplied with CO2 at a rate of 10 L/min. Experimental testing was completed under similar conditions. The results of CFD simulations and experimental testing showed CO2 insufflation can significantly limit the number of particles able to enter into the wound.

Simulation of carbon dioxide insufflation via a diffuser in an open surgical wound model.

Flow within a model surgical opening during insufflation with heated carbon dioxide was studied using computational fluid dynamics. A volume of fluid method was used to simulate the mixture of ambient air and carbon dioxide gas. The negative buoyancy of the carbon dioxide caused it to fill the wound and form a protective layer on the internal surfaces for a range of flow rates, temperatures, and angles of patient inclination. It was observed that the flow remained attached to the surface of the model due to the action of the Coanda effect. A flow rate of 10 L/min was sufficient to maintain a warm carbon dioxide barrier for a moderately sized surgical incision for all likely angles of inclination.

Large eddy simulation of high frequency oscillating flow in an asymmetric branching airway model.

The implementation of artificial ventilation schemes is necessary when respiration fails. One approach involves the application of high frequency oscillatory ventilation (HFOV) to the respiratory system. Oscillatory airflow in the upper bronchial tree can be characterized by Reynolds numbers as high as 10(4), hence, the flow presents turbulent features. In this study, transitional and turbulent flow within an asymmetric bifurcating model of the upper airway during HFOV are studied using large eddy simulation (LES) methods. The flow, characterized by a peak Reynolds number of 8132, is analysed using a validated LES model of a three-dimensional branching geometry. The pressures, velocities, and vorticity within the flow are presented and compared with prior models for branching flow systems. The results demonstrate how pendelluft occurs at asymmetric branches within the respiratory system. These results may be useful in optimising treatments using HFOV methods.