Tissue layer and calcification characteristics of the aortic valve and pericardium prostheses defined via polarization-sensitive optical coherence tomography

J. Golde (Dresden)¹, A. Starcke (Dresden)², G. Steiner (Dresden)³, A. Jannasch (Dresden)⁴, F. Sonntag (Dresden)², S. Tugtekin (Dresden)⁴, C. Dittfeld (Dresden)^5
1Fraunhofer Institute for Material and Beam Technology IWS, Dresden Dresden, Deutschland; ²Fraunhofer Institute for Material and Beam Technology IWS, Dresden, Germany Dresden, Deutschland; ³Dresden University of Technology, Faculty of Medicine Carl Gustav Carus, Clinical Sensoring and Monitoring Dresden, Deutschland; ⁴Department of Cardiac Surgery, Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden, Heart Centre Dresden Dresden, Deutschland; ⁵Faculty of Medicine and University Hospital Carl Gustav Carus, Technische Universität Dresden Department of Cardiac Surgery Dresden, Deutschland

Introduction: Calcific aortic valve (AV) disease results in stenosis, the most treated heart valve disease and is caused by fibrosis and calcification of the ECM rich valve tissues. Healthy three-layered AV tissue structure perfectly fits the challenging biomechanical requirements of the valve position between left ventricle and aorta. Mostly collagen rich tunica fibrosa is origin for dystrophic hydroxyapatite intercalation and pathological differentiation of valvular interstitial cells leads to fibrosis and osteogenesis. Also, main ECM component of AV bioprostheses fabricated from xenogeneous, glutaraldehyde (GA)-fixed pericardium is collagen and ECM orientation varies depending on subsegment localization. Polarization-sensitive optical coherence tomography (PS-OCT) is applied on the one hand to define healthy AV and pericardium layer structures or pathological remodeling features and on the other hand to monitor calcification characteristics. Aim is the correlation of ECM attributes with the pathological calcification in AV tissue and, based on the source collagen layer orientation, also the correlation of degenerative calcification in explanted bioprostheses.
Materials and methods: Healthy porcine and stenotic human AV tissue but also porcine and bovine pericardium (native and GA-fixed) was investigated using PS-OCT. The spectrometer-based 1300 nm PS-OCT system delivered ~5 µm axial and 20 µm lateral resolution. Custom MATLAB processing extracted four complementary contrasts, i.e. intensity, local retardation, optic axis orientation and degree of polarization (DOP) from 3D averaged Stokes parameters to visualize birefringent collagen and elastin layers, quantify retardation and map depolarizing calcifications. Subsequently samples were investigated via histology using HE, van Kossa, Picrosiriusred, Elastica-van Giesson and Movat Pentachrom stain for verification of the remodeled ECM and calcification characteristics. FTIR-spectroscopy was used for correlation of calcification using cryosectioned parallel tissue subsegments.
Results: PS-OCT allows the optical differentiation of three-layered AV structure and in addition the visualization of microcalcification in respective sublocalizations. Discrimination of single ECM fibers is initially confirmed, quantitative analysis is under development. Xenogeneous pericardium possesses two layers with different orientations of ECM revealing a dichotomy of the cross sections. These PS-OCT characteristics are verified using histological analyses for definition of tunica fibrosa, spongiosa and elastin-rich ventricularis in AV and different collagen layer orientations in pericardium. The latter is especially traceable via Elastica-van-Giesson stain to monitor and follow the orientation of the elastin fibers in the collagen matrix. Explanted pericardial bioprostheses sections exhibit widespread calcification areas in the valve matrix reflected in PS-OCT signals and chemically verified using FTIR-spectroscopy.
Conclusion: PS-OCT allows the label-free, non-destructive, volumetric assessment of collagen organization, elastin alignment and early calcific deposits in AV tissues and pericardium for AV replacement. Therefore PS-OCT is a powerful tool to detect degeneration mechanisms in biological valve prostheses, but also for the optimization of fabrication process and should furthermore accompany the advancement of micro-physiological test systems for AV culture.