This paper deals with the problem of abdominal aortic aneurysms (AAA), taking into account the possibility of using mechanical tests of aortic tissues for improvement of level of their constitutive models. First part of thesis deals with the introduction into the problem, description of the structure of the wall of the healthy aorta, its main components and the degenerative changes which lead to formation of AAA. This is followed by a brief excursion into constitutive modeling, which focuses closely on the description of the models used to describe the mechanical behavior of soft tissues. The theoretical part is then supplemented by a narrower selection of constitutive models used for modeling aortic wall and intraluminal thrombus, together with published results, which are reviewed and discussed at the end of this section. The main part of this thesis is devoted to tests of mechanical properties of arterial tissues. First, the methodology is presented together with the description of the customizations of the laboratory equipments together with the test rig. In addition, attention is focused on the results of mechanical tests of intraluminal thrombus, where the results of both uniaxial tensile tests and equbiaxial testing are presented. Also the influence of distance ILT from the lumen on the mechanical properties of the thrombus is examined. Another area of interest is the investigation of the effect of elastase on the chnage of mechanical properties of pig aorta. In this case, porcine aortas are experimentally tested only by biaxial testing, and the time of elastase action to alter the mechanical properties is analyzed so that the resulting tissue has a similar stress-strain response as aneurysmal tissue. Finally, the results of experimental measurements, limitations and other possible ways of research are summarized.

The aim of this diploma thesis is to assess the applicability of pulse wave propagation monitoring in the cardiovascular system in the field of prediction and early diagnosis of abdominal aortic aneurysm (AAA). The very first part is focused on description of heart and blood vessels with its pathological changes in presence of aneurysm. For this reason, current methods of monitoring and surgical treating of AAA were mentioned. Due to their difficult clinical use widely in the population, new methods based on pulse wave monitoring were presented. Using an analytical approach we estimated the difference in the arrival of the pulse wave at measurable locations between healthy and pathological aorta in the order of miliseconds. By experimental monitoring using photoplethysmographic sensors, we observed significant changes of pulse wave velocity with respect to the mechanical properties of the artery wall (mainly associated with age), which we tried to implement by hyperelastic material models used in computational simulations of pulse wave proagation on simplified geometries by fluid structure interaction method. These analyzes should verify applicability of FSI simulations in further development of diagnostic methods of AAA.

This thesis deals with computational modeling of the influence of residual stresses in idealized geometry of blood vessels and subsequent application of acquired knowledge to abdominal aortic aneurysm. In the terms of quality of the computational model, we reduced the uncertainties that are included in the computational model without considering the influence of residual stresses. The basic assumption of homogenization significant peaks of the stress between inner and outer vessel wall was met for each level of the computational model Methods that have been used are: deformation method (opening angle method), inverse mechanics of large deformations, fictitious temperature – for linear elastic material and hyperelastic material defined by the constitutive model. Numerical verification was carried out using program ANSYS.

No description available.

Crocodiles are ectothermic, intermittent breathing reptiles with low metabolic rates. Additionally, they are diving animals that can stay submerged for long periods, which poses special demands on their cardiovascular system. The crocodilian heart is four-chambered with completely separated ventricles, making it unique among other reptiles. It has special anatomical structures that give it the capability to shunt blood away from the lungs, which results in mixing of deoxygenated and oxygenated blood. The purpose of this study was to morphologically describe the heart and review its functions and the significance of its circulatory patterns. Dissection of two crocodilian hearts was performed for morphological characterization of the heart, including the special features that contribute to the shunting: the left aorta, the foramen of Panizza and the cog-teeth-like valves. Obstruction of the pulmonary outflow tract by the cog-teeth-like valves decreases pulmonary blood flow and generates an increased right ventricular pressure, diverting venous blood into the ‘extra’ left aorta and the systemic circulation. Thus, during shunting conditions, venous blood from the left aorta and arterial blood from the right aorta is mixed. The functional significance of this shunting pattern has been extensively discussed and several hypotheses have been proposed. A recent study showed that the absence of the right-to-left shunt does not affect the diving physiology of crocodiles negatively, but it did result in cardiac hypertrophy, indicating that the shunt might have a significance for the circulation and physiology of extant crocodiles.

<p>  </p> <p>Cardiovascular disease has been the leading cause of death in the United States for over 70 years. To evaluate the extent and progression of cardiovascular disease, non-invasive imaging techniques are frequently used clinically and pre-clinically. Current echocardiographic and cine magnetic resonance approaches rely on measurements that are typically obtained from two-dimensional images, which assumes uniformity of the structure being evaluated. To explore methods to potentially address these shortcomings, our group has developed and validated high frequency four-dimensional ultrasound techniques as well as created a software toolbox that allows for measurement of myocardial kinematics. In this thesis, I assisted in the application of these methods to two murine models of disease states: myocardial infarction and aortic aneurysm. Another study I aided in focused on cardiac magnetic resonance imaging data from patients with Duchenne muscular dystrophy. From our software, we are able to obtain various strain and strain rate estimates that reveal significant functional changes in infarction and Duchenne muscular dystrophy earlier than standard measurement techniques. Furthermore, we are able to identify vascular expansion, transmural thickening, and changes in hemodynamics prior to aneurysm development. Earlier detection and localization allows for more targeted surveillance and interventions, which ultimately may result in improved clinical outcomes. Ideally, these findings can be used to expand the capabilities of cardiac research and the development of clinically applicable imaging techniques and treatments to better address underlying cardiovascular pathophysiology. </p>

This dissertation addresses the problem of elevation of the T wave of an electrocardiogram (ECG) signal in the magnetic resonance imaging (MRI). In the MRI, due to the strong static magnetic field the interaction of the blood flow with this strong magnetic field induces a voltage in the body. This voltage appears as a superimposition at the locus of the T wave of the ECG signal. This looses important information required by the doctors to interpret the ST segment of the ECG and detect diseases such as myocardial infarction. This dissertation aims at finding a solution to the problem of elevation of the T wave of an ECG signal in the MRI. The first step is to simulate the entire situation and obtain the magnetic field dependent T wave elevation. This is achieved by building a model of the aorta and simulating the blood flow in it. This model is then subjected to a static magnetic field and the surface potential on the thorax is measured to observe the T wave elevation. The various parameters on which the T wave elevation is dependent are then analyzed. Different approaches are used to reduce this T wave elevation problem. The direct approach aims at computing the magnitude of T wave elevation using magneto-hydro-dynamic equations. The indirect approach uses digital signal processing tools like the least mean square adaptive filter to remove the T wave elevation and obtain artifact free ECG signal in the MRI. Excellent results are obtained from the simulation model. The model perfectly simulates the ECG signal in the MRI at all the 12 leads of the ECG. These results are compared with ECG signals measured in the MRI. A simulation package is developed in MATLAB based on the simulation model. This package is a graphical user interface allowing the user to change the strength of magnetic field, the radius of the aorta and the orientation of the aorta with respect to the heart and observe the ECG signals with the elevation at the 12 leads of the ECG. Also the artifacts introduced due to the magnetic field can be removed by the least mean square adaptive filter. The filter adapts the ECG signal in the MRI to the ECG signal of the patient outside the MRI. Before the adaptation, the heart rate of the ECG outside the MRI is matched to the ECG in the MRI by interpolation or decimation. The adaptive filter works excellently to remove the T wave artifacts. When the cardiac output of the patient changes, the simulation model is used along with the adaptive filter to obtain the artifact free ECG signal.

No description available.

No description available.

No description available.