Optical Coherence Elastography

In biomedical science, changes in tissue biomechanical properties serve as the initial bioindicators of disease progression, such as systemic sclerosis. It is an autoimmune disease, which is characterised by fibrosis of the skin. It might increase the risk of life-threatening complications and organ failures such as lung, heart, kidney, gastrointestinal tract and skin cancer. The exact cause of the development of systemic sclerosis is still unknown but involves an interplay between external environmental events such as virus infection, and genetic factors. According to a study, women are more likely than men to develop systemic sclerosis, usually in their fifth decade of life. Diagnosing this disease in an early stage is very important. Early detection and accurate measurement can help reduce the risk of certain complications and rapid disease progression. Assessment of the mechanical properties of the tissue plays a significant role in diagnosing the disease at an earlier stage because these are associated with the biological functionality of the tissue against pathological conditions. Optical coherence elastography (OCE) is a newly emerging technique which can provide qualitative and quantitative measurement of the mechanical properties of the tissue with greater precision. In this thesis, I used a series of experimental approaches to assess tissue mechanical properties to understand the nature, properties, and capabilities of surface acoustics waves for the elastography of tissues using a wave-based OCE method. Wave-based OCE has become a promising and most studied method among all OCE techniques. This approach uses mechanical waves for non-destructive and quantitative measurements of the elastic properties of soft tissue. OCE is fundamentally based on the principles of Optical Coherence Tomography (OCT ) technique and has the ability to measure tissue motion (displacement) with sub-nanometer accuracy by using phase sensitive OCT method. OCT generates both structural images based on tissue light scattering, and time-sequenced displacement images, revealing detailed tissue movement and elasticity. The current systems are restricted to lab setups with limited clinical use of this technique. To enable clinical applications of OCE, I developed a portable swept-source OCE (SS-OCE) system to measure skin elastic properties at different locations using a phase-stable fibre-based common-path OCT probe. This approach significantly reduces phase noise in the system, improving its performance compared to traditional non-common path systems. The results evaluated the system effectiveness by assessing the mechanical properties of agar-based tissue phantoms and human hand skin. Later, I recognised the need for a higher phase stability measurement system and also a method which is more suitable for calculating the deeper layer elastic properties of the skin. For this purpose, I developed another portable system based on the spectral domain OCE (SD-OCE) using the same common path OCT probe used in the previous system. Here, I performed three different wave excitation methods such as pulse wave, sine wave and resonant frequency. From the results, I concluded that the pulsed wave excitation method is more suitable for assessing the elastic properties of the skin's deeper layer than the other two methods. After evaluating this method, there was another challenge: tissue bulk movement due to some external and internal conditions of the tissue. Then, I worked on how I could minimise the bulk tissue movement. For that, I modified the SD-OCE system common path to a non-common path, where I divided a single OCT beam into two beams. Using a dual-OCT beam method is a novel approach in OCE, where both beams are focused simultaneously to reduce common noise from both points caused by bulk tissue movement. This approach allowed me to measure skin elasticity at different locations (palm and wrist) with minimal effect from tissue bulk motion. This approach can be readily adapted for clinical applications. In my thesis, I also analysed the structural images of the tissue with the OCT method, where I minimised polarization-dependent artefacts induced by reflected and scattered light from the tissue. In this work, the newly developed analysis methods can improve our knowledge in this field, leading to new diagnostic and treatment approaches previously missed in clinical practice.