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Session: Poster session II
Session starts: Thursday 24 January, 16:00

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Mathijs Peters (Eindhoven University of Technology)
Jan Nijs (UZ Brussels)
Bastiaan Kietselaer (Maastricht University Medical Centre)
Frans van de Vosse (Eindhoven University of Technology)
Richard Lopata (Eindhoven University of Technology)

Abstract:
The elastic behavior of the ascending aorta has been extensively studied in vitro [1]. Non-invasive in vivo characterization of elastic behavior of the aortic aneurysmal wall may provide substantial improvement in adequate patient selection and preoperative workup. The goal of this study is to investigate and estimate the mechanical properties of ascending thoracic aortic aneurysms (ATAAs) using an in vivo ultrasound technique and validate the results with in vitro mechanical testing on resected aortic specimens. However, before testing on human data, the proposed method was validated experimentally using porcine tissue. Porcine aortas (n = 3) were mounted in an experimental setup in which the vessels were pressurized from 0 to 130 mmHg in incremental steps of 10 mmHg. Raw ultrasound data were obtained at a single cross-section with a MyLab 70 (Esaote Europe, NL) with a linear array transducer. Data were processed using in-house developed 2D displacement estimation and tracking software [2]. The physiological circumferential strain range and Young’s modulus (E) were estimated. To examine the influence of longitudinal pre-stretch (LP), all measurements were performed for two previously reported values, LP = 1.1 and 1.3 [3, 4]. For validation of the US method, rectangular samples of the porcine tissue were tested in a bi-axial tensile testing device (CellScale, Canada). The samples (15 mm x 15 mm) were tested for circumferential strains of 0.1 to 0.5, for LP = 1.1 and 1.3. To investigate temperature dependency, all experiments were conducted at both room (21ºC) and body temperature (37ºC). In an ongoing in vivo study, biplane data were obtained in five patients with an ATAA using an iE33 with an X7-2t transesophageal probe (Philips, USA). Two-dimensional speckle tracking was performed on these data to estimate the mechanical behavior. Next, resected samples were stored in phosphate-buffered saline solution at -80ºC awaiting tensile testing. In the ultrasound experiment, the estimated physiological strain range was 0.21 ± 0.02 (end diastole) to 0.27 ± 0.01 (end systole) at LP = 1.1. For LP = 1.3, the strain was 0.25 ± 0.01 (end diastole) to 0.30 ± 0.02 (end systole). For the US-data, Eu = 224 ± 20 kPa (LP = 10%) and Eu = 258 ± 12 kPa (LP = 30%). The corresponding tensile test data revealed Et = 289 ± 82 kPa (LP = 10%) and Et = 296 ± 45 kPa (LP = 30%). The large variation was caused by one aorta that seemed considerably stiffer during the tensile test (348 – 383 kPa), whereas the other data were in good agreement (Eu = 201 – 257 kPa vs. Et = 234 – 276 kPa). Although different results were observed at both temperatures, no conclusions could be drawn. The in vivo ultrasound results revealed peak circumferential strains of 5 – 10% in the aortic root. However, the speckle tracking technique needs further improvement and revealed drift in the direction of the aortic wall. Tensile testing on human samples will be performed in the near future for validation of the US results. Other future work includes the assessment of local variability of Young’s modulus and strain and comparison with tensile test data.