Numerical simulation of blood flow in the left ventricle and aortic sinus using magnetic resonance imaging and computational fluid dynamics

Understanding cardiac blood flow patterns has many applications in analysing haemodynamics and for the clinical assessment of heart function. In this study, numerical simulations of blood flow in a patient-specific anatomical model of the left ventricle (LV) and the aortic sinus are presented. The realistic 3D geometry of both LV and aortic sinus is extracted from the processing of magnetic resonance imaging (MRI). Furthermore, motion of inner walls of LV and aortic sinus is obtained from cine-MR image analysis and is used as a constraint to a numerical computational fluid dynamics (CFD) model based on the moving boundary approach. Arbitrary Lagrangian–Eulerian finite element method formulation is used for the numerical solution of the transient dynamic equations of the fluid domain. Simulation results include detailed flow characteristics such as velocity, pressure and wall shear stress for the whole domain. The aortic outflow is compared with data obtained by phase-contrast MRI. Good agreement was found between simulation results and these measurements.     

Regional and systemic hemodynamic responses following the creation of a murine arteriovenous fistula

The study of hemodynamic alterations following the creation of an arteriovenous fistula (AVF) is relevant to vascular adaptive responses and hemodialysis access dysfunction. This study examined such alterations in a murine AVF created by anastomosing the carotid artery to the jugular vein. AVF blood flow was markedly increased due to reduced AVF vascular resistance. Despite such markedly increased basal blood flow, AVF blood flow further increased in response to acetylcholine. This AVF model exhibited increased cardiac output and decreased systemic vascular resistance; the kidney, in contrast, exhibited decreased blood flow and increased vascular resistance. Augmentation in AVF blood flow was attended by increased arterial heme oxygenase-1 (HO-1) mRNA and protein expression, the latter localized to smooth muscle cells of the AVF artery; AVF blood flow was substantially reduced in HO-1(-/-) mice compared with HO-1(+/+) mice. Finally, in a murine model of a representative disease known to exhibit impaired hemodynamic responses (sickle cell disease), the creation of an AVF was attended by decreased AVF flow and impaired AVF function. We conclude that this AVF model exhibits markedly increased AVF blood flow, a vasodilatory reserve capacity, increased cardiac output, decreased renal blood flow, and a dependency on intact hemodynamic responses, in general, and HO-1 expression, in particular, in achieving and maintaining AVF blood flow. We suggest that these findings support the utility of this model in investigating the basis for and the consequences of hemodynamic stress, including shear stress, and the pathobiology of hemodialysis AVF dysfunction.     

Sawfishes stealth revealed using computational fluid dynamics

Detailed computational fluid dynamics simulations for the rostrum of three species of sawfish (Pristidae) revealed that negligible turbulent flow is generated from all rostra during lateral swipe prey manipulation and swimming. These results suggest that sawfishes are effective stealth hunters that may not be detected by their teleost prey's lateral line sensory system during pursuits. Moreover, during lateral swipes, the rostra were found to induce little velocity into the surrounding fluid. Consistent with previous data of sawfish feeding behaviour, these data indicate that the rostrum is therefore unlikely to be used to stir up the bottom to uncover benthic prey. Whilst swimming with the rostrum inclined at a small angle to the horizontal, the coefficient of drag of the rostrum is relatively low and the coefficient of lift is zero.     

High resolution proton magnetic resonance of sonicated phospholipids

We have recorded high resolution proton magnetic resonance spectra of sonicated phospholipid vesicles. The following lipids were used in separate experiments: phosphatidylglycerol, phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine from egg yolk as well as dimyristoyl phosphatidylcholine. Mixed lipid vesicles were also investigated. Assignments of the peaks associated with the various protons of the different lipids are presented. It is shown that in favorable cases, it is possible to resolve the different phospholipid head groups of mixed lipid samples. Spin lattice relaxation times (T₁) of each peak were collected at 500 MHz and 90 MHz. The influence of the addition of a small concentration of spin labeled phospholipid on i) the linewidths ii) the spin lattice relaxation times, was determined. It is shown that nitroxide radicals selectively broaden the peaks associated with the protons localized at a comparable depth of the bilayer. On the other hand, T₁ are less selectively perturbed. Potential applicability of ¹H-NMR for the investigation of lipid-proton specificity in membranes is discussed.     

High resolution nuclear magnetic resonance studies of nigeran

Polymer motion in solution can be studied by ¹³CNMR relaxation methods, which provide information about the correlation time for C-H vectors. ¹³C-Relaxation and Nuclear Overhauser Enhancement (NOE) data may frequently be combined to determine the dipole-dipole relaxation contribution. An alternative method is proposed based on a comparison of the proton spin-lattice relaxation rates of the centre proton resonances of an unlabelled molecule with the relaxation rates of the ¹³C satellites (from ¹³C labelled molecules).Selectively labelled nigeran which is an alternating $\text{1 → 3 and 1 → 4 α-}\text{d}\text{-glucan}$ has been investigated. The discussion in terms of the occurrence of different motions for each of the two units of the polymer requires an unambiguous assignment of the two anomeric carbons. For this reason a detailed assignment of the ¹H and ¹³C Nuclear Magnetic Resonance (NMR) spectra of nigeran in dimethylsulphoxide-d₆ is described, based on T₁ and NOE measurements in addition to selective homonuclear and heteronuclear spin decoupling experiments. These values are correlated with a conformation estimated by HSEA hard-spheres calculation. The measurements of the relaxation parameters for labelled and unlabelled compounds which provide an alternative determination of the ¹³C-¹H dipole-dipole relaxation contribution in a macromolecule agree well with ¹³C-{¹H} NOE experiments.     

High-throughput magnetic resonance imaging in murine colonic inflammation

The aim of this study was to investigate if a rapid magnetic resonance imaging (MRI) screening protocol (<5min/mouse) could characterize colonic inflammation in a chronic experimental colitis model. No respiratory triggering or spasmolytic agent was used during MRI-acquisition. Biomarkers assessed in vivo were colon wall thickness and T2w signal intensity (reflecting oedema) and ex vivo inflammatory score, colon weight, and plasma haptoglobin. The inflammation was characterised by significantly higher local and systemic inflammatory markers in the colitic mice compared to healthy mice. MRI-colon wall thickness and T2w signal intensity correlated well with inflammatory score (r=0.95 and 0.94), colon weight (r=0.92 and 0.93) and plasma haptoglobin (r=0.89 and 0.95). Thus, the data showed that in vivo MRI screening could be used to assess colon wall inflammation, suggesting that high-throughput MRI can be used to follow the potential efficacy of new IBD therapies in individual animal in longitudinal studies.     

Inductively-overcoupled coil design for high resolution magnetic resonance imaging

Background  

Maintaining the quality of magnetic resonance images acquired with the current implantable coil technology is challenging in longitudinal studies. To overcome this challenge, the principle of 'inductive overcoupling' is introduced as a method to tune and match a dual coil system. This system consists of an imaging coil built with fixed electrical elements and a matching coil equipped with tuning and matching capabilities. Overcoupling here refers to the condition beyond which the peak of the current in the imaging coil splits.     

Optimization of cardiovascular stent design using computational fluid dynamics

Coronary stent design affects the spatial distribution of wall shear stress (WSS), which can influence the progression of endothelialization, neointimal hyperplasia, and restenosis. Previous computational fluid dynamics (CFD) studies have only examined a small number of possible geometries to identify stent designs that reduce alterations in near-wall hemodynamics. Based on a previously described framework for optimizing cardiovascular geometries, we developed a methodology that couples CFD and three-dimensional shape-optimization for use in stent design. The optimization procedure was fully-automated, such that solid model construction, anisotropic mesh generation, CFD simulation, and WSS quantification did not require user intervention. We applied the method to determine the optimal number of circumferentially repeating stent cells (N(C)) for slotted-tube stents with various diameters and intrastrut areas. Optimal stent designs were defined as those minimizing the area of low intrastrut time-averaged WSS. Interestingly, we determined that the optimal value of N(C) was dependent on the intrastrut angle with respect to the primary flow direction. Further investigation indicated that stent designs with an intrastrut angle of approximately 40 deg minimized the area of low time-averaged WSS regardless of vessel size or intrastrut area. Future application of this optimization method to commercially available stent designs may lead to stents with superior hemodynamic performance and the potential for improved clinical outcomes.     

Quantifying mixing using magnetic resonance imaging

Mixing is a unit operation that combines two or more components into a homogeneous mixture. This work involves mixing two viscous liquid streams using an in-line static mixer. The mixer is a split-and-recombine design that employs shear and extensional flow to increase the interfacial contact between the components. A prototype split-and-recombine (SAR) mixer was constructed by aligning a series of thin laser-cut Poly (methyl methacrylate) (PMMA) plates held in place in a PVC pipe. Mixing in this device is illustrated in the photograph in Fig. 1. Red dye was added to a portion of the test fluid and used as the minor component being mixed into the major (undyed) component. At the inlet of the mixer, the injected layer of tracer fluid is split into two layers as it flows through the mixing section. On each subsequent mixing section, the number of horizontal layers is duplicated. Ultimately, the single stream of dye is uniformly dispersed throughout the cross section of the device. Using a non-Newtonian test fluid of 0.2% Carbopol and a doped tracer fluid of similar composition, mixing in the unit is visualized using magnetic resonance imaging (MRI). MRI is a very powerful experimental probe of molecular chemical and physical environment as well as sample structure on the length scales from microns to centimeters. This sensitivity has resulted in broad application of these techniques to characterize physical, chemical and/or biological properties of materials ranging from humans to foods to porous media (1, 2). The equipment and conditions used here are suitable for imaging liquids containing substantial amounts of NMR mobile (1)H such as ordinary water and organic liquids including oils. Traditionally MRI has utilized super conducting magnets which are not suitable for industrial environments and not portable within a laboratory (Fig. 2). Recent advances in magnet technology have permitted the construction of large volume industrially compatible magnets suitable for imaging process flows. Here, MRI provides spatially resolved component concentrations at different axial locations during the mixing process. This work documents real-time mixing of highly viscous fluids via distributive mixing with an application to personal care products.     

High resolution proton nuclear magnetic resonance spectroscopy of lactoperoxidase

The first high resolution proton nuclear magnetic resonance spectra are reported for the native ferric and ferric cyano complexes of bovine lactoperoxidase. The spectrum of the native species exhibits broad heme signals in a far downfield region characteristic of the high-spin ferric state. The low-spin cyano complex yields a proton nuclear magnetic resonance spectrum with signals as far as 68.5 ppm downfield and as far as -28 ppm upfield of the tetramethylsilane reference. These peak positions are anomalous with respect to those seen only as far as 35 ppm downfield in other cyano hemoprotein complexes. An extreme asymmetry in the unpaired spin delocalization pattern of the iron porphyrin is suggested. The unusual proton nuclear magnetic resonance properties parallel distinctive optical spectral properties and the exceptional resistance to heme displacement from the enzyme. Lactoperoxidase utilized in these studies was isolated from raw milk and purified by an improved, rapid chromatographic procedure.     

Gastric gas and fluid emptying assessed by magnetic resonance imaging

Magnetic resonance imaging (MRI) was used to characterize the volumes and rates of gastric emptying of both liquid and gas following the ingestion of beverages of varying carbonation and carbohydrate levels. Eight subjects drank 800 ml each of four test beverages in a counterbalanced order: water, a non-carbonated carbohydrate-electrolyte solution (NC), a lightly carbonated carbohydrate-electrolyte solution (PC), and a carbonated cola (CC). T2-weighted, echoplanar images (25-30 contiguous slices, 1 cm thick, 256 x 128 matrix, TE = 80, 40 cm FOV) of the abdomen were collected at minutes 3,110, 20, 30, 45, and 60 following beverage ingestion. Images were analyzed for gas and liquid volumes. Water and NC emptied the most rapidly, with half times of 21(3) and 31(3) min, respectively [mean (SE)]. PC emptied significantly slower [47 (6) min] and CC slower yet [107 (8) min]. The carbonation content of the beverage accounted for 84% of the variation in emptying time, whereas carbohydrate content did not account for any significant variation. The gastric gas volume of the CC was higher at 2 min post-ingestion compared with all other drinks; however, the rate of emptying of the gas was the same among all beverages. Significantly greater total gastric volumes (gas+ liquid) were associated with the ingestion of CC, and accordingly produced a greater severity of gastric distress, as evaluated with a gastric distress inventory. The high gastric gas volumes (approximately 600 ml) after ingestion of CC suggested a potential source of error in body composition using standard hydrostatic weighing methods. This prediction was tested in nine additional subjects. Ingestion of 800 ml of CC prior to hydrostatic weighing resulted in a 0.7% underestimate of body density and thus an 11% overestimate of percentage body fat compared to measurements made before beverage consumption.     

Cardiac dysfunction in the diabetic rat: quantitative evaluation using high resolution magnetic resonance imaging

Background

There have been scant reports on the cumulative effects of atherosclerotic risk factors on steatohepatitis.

Methods

We defined cases of steatohepatitis (n = 124) from one health examination center at National Taiwan University Hospital from January to December 2002. We selected controls, matched by age, gender and drinking status. Metabolic syndrome was defined by the modified ATP-III guidelines. High-dimensional interactions of risk factors for steatohepatitis were evaluated.

Results

Steatohepatitis cases had the highest C-reactive protein, lymphocytes, Framingham scores and predicted coronary risks. The odds ratio (OR) of metabolic syndrome for steatohepatitis was the highest (OR = 9.9), followed by high glucose status (OR = 4.5) and obesity (OR = 3.6). The highest area under the ROC curve was metabolic syndrome (area = 0.80), followed by obesity (0.75) and high glucose level (0.73). Metabolic syndrome was the highest population-attributable risk factor (0.59). Significant interaction was found with a three-factor model, including obesity, metabolic syndrome and Framingham risk status, with lesser average prediction error (22.6%), higher average cross-validation consistency (6.3) and lower average prediction error (24.3%). Compared with persons with no risk factors, OR increased as the number of risk factors increased (OR = 3.0 with one risk factor, 17.5 with two risk factors, 10.8 with three risk factors, respectively).

Conclusion

Metabolic syndrome, inflammation markers and atherosclerotic risk scores are significantly related to steatohepatitis status among the healthy examinee population in Taiwan.     

Functional imaging using computational fluid dynamics to predict treatment success of mandibular advancement devices in sleep-disordered breathing

Mandibular advancement devices (MADs) have emerged as a popular alternative for the treatment of sleep-disordered breathing. These devices bring the mandibula forward in order to increase upper airway (UA) volume and prevent total UA collapse during sleep. However, the precise mechanism of action appears to be quite complex and is not yet completely understood; this might explain interindividual variation in treatment success. We examined whether an UA model, that combines imaging techniques and computational fluid dynamics (CFD), allows for a prediction of the treatment outcome with MADs. Ten patients that were treated with a custom-made mandibular advancement device (MAD), underwent split-night polysomnography. The morning after the sleep study, a low radiation dose CT scan was scheduled with and without the MAD. The CT examinations allowed for a comparison between the change in UA volume and the anatomical characteristics through the conversion to three-dimensional computer models. Furthermore, the change in UA resistance could be calculated through flow simulations with CFD. Boundary conditions for the model such as mass flow rate and pressure distributions were obtained during the split-night polysomnography. Therefore, the flow modeling was based on a patient specific geometry and patient specific boundary conditions. The results indicated that a decrease in UA resistance and an increase in UA volume correlate with both a clinical and an objective improvement. The results of this pilot study suggest that the outcome of MAD treatment can be predicted using the described UA model.     

Investigation of fluid flow patterns in a hollow fiber module using magnetic resonance velocity imaging

Summary Magnetic Resonance Imaging (MRI) was used to obtain new information about fluid flow patterns in hollow fiber reactors. Significant changes in inlet flow distribution were observed as a function of Reynolds number. Images taken at the tube bundle entrance and exit showed that maldistribution of flow persists throughout the module. Furthermore, the results suggest that individual fibers act in a mixed degree as feeders or collectors depending upon radial position. These effects must be considered when modelling or designing hollow fiber reactors.     

Radiation dosimetry using Fricke-infused gels and magnetic resonance imaging

We have witnessed the advancements of MRI-Fricke-infused gel dosimetry since its commencing in 1984. Over the years, many efforts have been spent to improve the method's efficacy, i.e., to improve its dose-response sensitivity, reproducibility and measurement accuracy. In this article, we give a review of the development of this relatively new dosimetric method. An example of applying this method to gamma knife stereotactic radiosurgery dose distribution mapping is also given.     

Effects of wall distensibility in hemodynamic simulations of an arteriovenous fistula

Arteriovenous fistulae are created surgically to provide adequate access for dialysis patients suffering from end-stage renal disease. It has long been hypothesized that the rapid blood vessel remodeling occurring after fistula creation is in part a process to restore the mechanical stresses to some preferred level, i.e., mechanical homeostasis. The current study presents fluid–structure interaction (FSI) simulations of a patient-specific model of a mature arteriovenous fistula reconstructed from 3D ultrasound scans. The FSI results are compared with previously published data of the same model but with rigid walls. Ultrasound-derived wall motion measurements are also used to validate the FSI simulations of the wall motion. Very large time-averaged shear stresses, 10–15 Pa, are calculated at the fistula anastomosis in the FSI simulations, values which are much larger than what is typically thought to be the normal homeostatic shear stress in the peripheral vasculature. Although this result is systematically lower by as much as 50 % compared to the analogous rigid-walled simulations, the inclusion of distensible vessel walls in hemodynamic simulations does not reduce the high anastomotic shear stresses to “normal” values. Therefore, rigid-walled analyses may be acceptable for identifying high shear regions of arteriovenous fistulae.     

Differentiation between hemosiderin- and ferritin-bound brain iron using nuclear magnetic resonance and magnetic resonance imaging.

MRI is an optimal clinical (research) tool to provide information on brain morphology and pathology and to detect metal ions that possess intrinsic magnetic properties. Non-heme iron is abundantly present in the brain in three different forms: "low molecular weight" complexes, iron bound to "medium molecular weight complexes" metalloproteins such as transferrin, and "high molecular weight" complexes as ferritin and hemosiderin. The total amount and form of iron may differ in health and disease, and MRI can possibly quantify and monitor such changes. Ferritin-bound iron is the main storage form of iron and is present predominantly in the extrapyramidal nuclei where its amounts normally increase as a function of age. Ferritin is water soluble and shortens both, T1 and T2 relaxation, with as result a signal change on the MR images. Hemosiderin, a degradation product of ferritin, is water-insoluble with a stronger T2 shortening effect than ferritin. The larger cluster size of hemosiderin and its water-insolubility also explain a lack of significant T1-shortening effect on T1-weighted images. Using both in vitro specimens and intact brain tissue in vivo we demonstrate here that MRI may be able to distinguish between ferritin- and hemosiderin-bound iron.     

Two-dimensional intraventricular flow pattern visualization using the image-based computational fluid dynamics

The image-based computational fluid dynamics (IB-CFD) technique, as the combination of medical images and the CFD method, is utilized in this research to analyze the left ventricle (LV) hemodynamics. The research primarily aims to propose a semi-automated technique utilizing some freely available and commercial software packages in order to simulate the LV hemodynamics using the IB-CFD technique. In this research, moreover, two different physiological time-resolved 2D models of a patient-specific LV with two different types of aortic and mitral valves, including the orifice-type valves and integrated with rigid leaflets, are adopted to visualize the process of developing intraventricular vortex formation and propagation. The blood flow pattern over the whole cardiac cycle of two models is also compared to investigate the effect of utilizing different valve types in the process of the intraventricular vortex formation. Numerical findings indicate that the model with integrated valves can predict more complex intraventricular flow that can match better the physiological flow pattern in comparison to the orifice-type model.