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Technical Brief

A Novel Method for Optical High Spatiotemporal Strain Analysis for Transcatheter Aortic Valves In Vitro

[+] Author and Article Information
Simon Heide-Jørgensen, Sellaswasmy Kumaran Krishna, Jonas Taborsky

Department of Engineering,
Faculty of Science and Technology,
Aarhus University,
Aarhus 8000, Denmark

Tommy Bechsgaard

Department of Cardiothoracic Surgery,
Aarhus University Hospital,
Aarhus N8200, Denmark

Rachid Zegdi

Hôpital Européen Georges Pompidou,
Service de Chirurgie Cardiovasculaire,
Paris 75015, France

Peter Johansen

Department of Engineering
Faculty of Science and Technology,
Aarhus University
Aarhus 8000, Denmark
e-mail: pj@eng.au.dk

1Corresponding author.

Manuscript received May 14, 2015; final manuscript received December 30, 2015; published online January 29, 2016. Assoc. Editor: Thao (Vicky) Nguyen.

J Biomech Eng 138(3), 034504 (Jan 29, 2016) (7 pages) Paper No: BIO-15-1238; doi: 10.1115/1.4032501 History: Received May 14, 2015; Revised December 30, 2015

The transcatheter aortic valve implantation (TAVI) valve is a bioprosthetic valve within a metal stent frame. Like traditional surgical bioprosthetic valves, the TAVI valve leaflet tissue is expected to calcify and degrade over time. However, clinical studies of TAVI valve longevity are still limited. In order to indirectly assess the longevity of TAVI valves, an estimate of the mechanical wear and tear in terms of valvular deformation and strain of the leaflets under various conditions is warranted. The aim of this study was, therefore, to develop a platform for noncontact TAVI valve deformation analysis with both high temporal and spatial resolutions based on stereophotogrammetry and digital image correlation (DIC). A left-heart pulsatile in vitro flow loop system for mounting of TAVI valves was designed. The system enabled high-resolution imaging of all three TAVI valve leaflets simultaneously for up to 2000 frames per second through two high-speed cameras allowing three-dimensional analyses. A coating technique for applying a stochastic pattern on the leaflets of the TAVI valve was developed. The technique allowed a pattern recognition software to apply frame-by-frame cross correlation based deformation measurements from which the leaflet motions and the strain fields were derived. The spatiotemporal development of a very detailed strain field was obtained with a 0.5 ms time resolution and a spatial resolution of 72 μm/pixel. Hence, a platform offering a new and enhanced supplementary experimental evaluation of tissue valves during various conditions in vitro is presented.

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Figures

Grahic Jump Location
Fig. 1

A schematic overview of the in vitro model consisting of a piston pump connected to the ventricular chamber, a valve section where the TAVI valve is positioned, connecting the ventricular chamber to the arterial compliance section where pneumatic compliance is added to the system. The compliance section is connected to the atrial reservoir with a rubber hose where a transit time flowmeter measures the peripheral flow and a clamp adds the peripheral systemic resistance to the system. The atrial reservoir acts as a reservoir for fluid in the system and provides the ventricular filling pressure. The atrial reservoir and the ventricular chamber are connected by a mechanical mitral valve which ensures unidirectional flow. Pressures in the ventricular and compliance section are monitored with mikro-tip pressure catheters.

Grahic Jump Location
Fig. 2

Pressure and flow data acquired from the in vitro system including data from two full heart cycles. Pressures are acquired using mikro-tip pressure catheters in the ventricular chamber (solid) and the aortic chamber (dotted). The peripheral flow (dashed) is acquired using a transit time flow probe placed between the aortic chamber and the atrial reservoir.

Grahic Jump Location
Fig. 3

The closed TAVI valve surrounded by the nitinol stent seen from the aortic point of view. The surface of the each leaflet is covered with a fine pattern of ink speckles with a particle size less than 1 μm. The ink is applied directly to the surface of the leaflets by an airbrush. The contrast between the black color of the ink and the pale tissue texture of the leaflets is sufficient for the DIC to be implemented, thereby obviating the need to apply a base color to the leaflets.

Grahic Jump Location
Fig. 4

A photograph of the experimental setup used in the study consisting of the two high-speed cameras (in the front), a pulsatile left heart in vitro model and a piston pump attached to the system. The photo depicts the calibration of the system where the calibration panel (black rectangle with white dots) is placed in the middle of the system between the ventricular and the compliance chamber. By conducting a series of images of the calibration panel at different angles the camera configuration in terms of relative angle and distance is determined. After the calibration the TAVI valve replaces the calibration panel and the experiment is executed once the system is tuned to simulate physiological conditions in terms of pressure and flow.

Grahic Jump Location
Fig. 5

Post processing of the experimental data of from the TAVI valve seen by the left camera. The applied facet field is marked by the squares, which each contains a unique signature based onthe gray level intensity of the pixels inside it. Using stereophotogrammetry, the facet field is transformed into a three-dimensional surface. To reduce computational time areas with no interest is masked out and no facets were applied. Since the facets need to be visual for both cameras computation of the 3D structure along edges, such as the coaptation lines, is not possible.

Grahic Jump Location
Fig. 6

The figure depicts the strain assessment along the profile of the leaflet across the section line. The top left figure shows the leaflet from a frontal view where the black line is the section line along which the strain is assessed. The top right figure shows the leaflet from the side and it is seen that the section line is not straight but follows the profile of the leaflet. The graph at the bottom shows the strain along the section line of the leaflet. The x-axis shows the length of the section line where x = 0 mm is at the coaptation line of the leaflet and x = 8.84 mm is at the commissure. The graph shows that the leaflet experiences the largest strains around 4.32% across the belly region. The lowest strain values around 1.5% are found at the coaptation line. The commissure region experiences strains around 2.7%.

Grahic Jump Location
Fig. 7

Visualization of the strain distribution of the TAVI valve leaflets on the surface created from the DIC. The empty space separating the leaflets represents the coaptation line where data is absent. The left column (panel I) depicts the von Mises or equivalent strain and its development at the reference stage (time = 0), which is early diastole where the leaflets margins touch; at frame 8 (time = 4 ms); and at frame 16 (time = 8 ms). After this frame and to the final frame of the dataset there is no notable change in the strain. The legend shows the distribution of the strain values at each stage. To the right (panel II) is the major principal strain depicted in the same manner as the von Mises strain. Due to edge geometry, high strain concentration along the edges is false registered. The histogram shows that the strain concentrations along the edges represent a negligible part of all the strains.

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