Optical Coherence Elastography of the Vitreous Humour

Abstract

The vitreous humour is the biggest part of the eye situated behind the crystalline lens. Various pathogenic changes influence its biomechanics, which is why a clinical tool that would measure and monitor biomechanical properties of the vitreous humour would aid in detecting the early stages of different eye diseases. Finding such a tool is challenging since this part of the eye is near-transparent, highly viscous and shape variable. The stiffness is a biomarker often used to assess a sample biomechanical condition. Several modalities, such as ultrasound elastography, Magnetic Resonance Elastography or Optical Coherence Elastography, were developed to provide direct quantitative visualisation of the tissue stiffness. These imaging techniques often use mechanical piezoelectric transducer or focused ultrasound to induce mechanical waves and monitor their propagation to assess the biomechanics of a sample. Optical Coherence Elastography (OCE) is relatively new in comparison to other modalities. It combines high-resolution, non-contact quantitative measurements with clinical applicability not achievable by other methods. The aim of this thesis is to implement OCE for biomechanical analysis of vitreous humour. Due to the specific properties of the vitreous humour, three different approaches, each based on the generation and detection of a particular mechanical wave, were tested to quantify the viscoelastic properties of the vitreous humour. In the first approach, the detection of mechanically induced surface waves was implemented in the extracted vitreous humour. The second approach tested the possibility of visualising shear waves generated in the same manner in the intact vitreous humour. Lastly, the feasibility of employing laser-induced longitudinally polarised shear waves for an all-optical assessment of the intact vitreous humour was studied in the third approach. The first two approaches provided an interesting insight into OCE studies of the vitreous humour and helped develop new analytical tools: a model-independent analysis of viscoelastic parameters and a Singular-Value-Decomposition-based analysis of scattering. The feasibility study showed that the third approach is capable of probing the viscoelastic parameters of a vitreous humour phantom and, unlike the other tested approaches, can be advanced into the OCE-based method for viscoelastic analysis of the intact vitreous humour.