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Session: Cardiac Diagnostics
Session starts: Friday 25 January, 10:30
Presentation starts: 11:30
Room: Lecture room 559

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Richard Lopata (Eindhoven University of Technology)
Marcel Rutten (Eindhoven University of Technology)
Frans van de Vosse (Eindhoven University of Technology)

Abstract:
The size and growth of abdominal aortic aneurysms (AAAs) are currently monitored with ultrasound. Intervention of AAAs is not without risk and is performed only when rupture seems inevitable. However, clinical decision making is based on diameter (growth) measurements, which are retrospective and prone to observer errors. Novel techniques, such as wall stress analysis, dive deeper into the underlying mechanical causes of AAA rupture, but require CT imaging. Ultrasound strain imaging and elastography have been proposed for mechanical characterization of the aortic wall. The benefits are the lack of ionizing radiation and high resolution. Although applied successfully in vivo, the accuracy of these techniques is not easily quantified. In this study, an experimental framework is proposed, closely mimicking in vivo circumstances, to evaluate elastography of the abdominal aorta. The mechanical properties of porcine aorta’s are measured and compared with static experiments. Three porcine aortas, obtained from a local slaughterhouse, were prepared and kept in phosphate-buffered saline solution (l = 15 ± 2 cm). The vessels were mounted in a hemodynamic setup, in which physiological blood flow and pressure were closely mimicked in both shape and magnitude. The vessels were subjected to 30% pre-stretch in the direction of the vessel axis [1]. Water was circulated with a computer-controlled pump, and blood pressure was regulated by adjusting the peripheral resistance and impedance of the setup. Raw ultrasound data (RF) were obtained in the longitudinal direction of the vessel with a MyLab70 (Esaote Europe, NL), equipped with a 2D linear array transducer (fc = 4-11 MHz, fr = 65 Hz) for several cardiac cycles. The pressure was measured with a pressure sensor (St Jude Medical, USA). Next, the aorta was subjected to static pressures, ranging from 80 to 120mmHg, in 5mmHg increments. RF-data were acquired at each pressure level. The data of three cardiac cycles (dynamic) or three experiments (static) were processed with a 2D coarse-to-fine displacement estimation algorithm [2]. By assuming axisymmetry, the volume change in time was calculated, from which the compliance per length (Cl), distensibility (D) and Young’s modulus (E) were estimated. The average parameter estimates for all three aortas, as measured during the dynamic experiments, were Cl = 3.0 - 3.8 x 10-9 m2/Pa, D = 124 - 175 kPa-1 and E = 359 - 525 kPa. An average intra-subject variability of 5.7 - 5.9% was found for all parameters. The static measurements were only successful for aortas 1 en 2. However, a good correspondence was found between the dynamic (E1 = 359 kPa; E2 = 453 kPa) and the static measurements (E1 = 366 kPa; E2 = 424 kPa). The porcine aortas appear to be stiffer than those of normal human volunteers, which could be caused by the freezing process, denaturation or small leaks. In future work, these in vitro experiments will be validated further by means of mechanical testing.