Full-field characterisation of epicardial deformation using three dimensional digital image correlation (3D-DIC)

Imaging of the heart provides valuable insight into its functionality and the progression of diseases affecting the cardiac muscle. Currently, ultrasound 2D speckle tracking echocardiography (US-2D-STE) is the most frequently used clinical technique to detect and monitor the progression of cardiovascular disease through changes in strain of the cardiac muscle. However, this imaging modality has several limitations, which reduce the accuracy and reproducibility of the measurement. As a result, the complex behaviour of heart deformation including contraction and twisting cannot be accurately detected by this technique. This thesis describes the development of an optical method based on 3D digital image correlation (3D-DIC) to enable full-field deformation analysis in the heart. The hypothesis of this project is that 3D measurement of local strain in experimental in vitro and ex vivo models of the heart will provide a detailed characterisation of the behaviour of the heart and provide reference measurements for comparison with clinical imaging modalities. The experimental method requires a robust stereo optical system ensuring high-quality and synchronised imaging during heart deformation. The developed methodology was validated through multiple experimental and numerical tests in a zero-strain configuration, which provided an estimate of the error in the reconstruction of strain on the cardiac surface (approximately 1%). Applications in experimental in vitro and ex vivo models of the heart are described. Moreover, a comparison of the performance of 3D-DIC and US-2D-STE under the same conditions of the heart is investigated, demonstrating the superiority of 3D-DIC for dynamic, high-resolution strain measurements (approximately 1.5 mm). However, being an optical technique, 3D-DIC is limited to only surface measurements on the epicardium and requires an effective speckle pattern to be applied on the heart surface, which may pose biocompatible problems and important challenges in its application in Vivo. This experimental work has led to the development of a robust tool for localised and detailed measurement of strain at a high temporal and spatial resolution, with the latter one order of magnitude improved with respect to existing optical techniques. Interpretation of the full-field results can be used to show the non-uniform and inhomogeneous strain distribution on the epicardial surface and identify changes in strain within ex vivo models of cardiac disease.