Imaging of fiber structures in the posterior eye
Authors:   , Willemse Joy 
Published: November 2023

Imaging the human body is the most important diagnostic tool in healthcare. Therefore, there is an endless need for improvement of imaging techniques. In this thesis, an extension of a medical imaging technique is further developed to provide extra image contrast based on polarization-changing properties of the tissue. This technique, called polarization-sensitive optical coherence tomography (PS-OCT), is an extension of optical coherence tomography (OCT), which is often used in ophthalmology to create high resolution images of the retina of the human eye. Retinal imaging is the main application of PS-OCT chosen for this thesis.
Extra image contrast in PS-OCT includes imaging of birefringence, depolarization, and optic axis of the tissue. An in-house built PS-OCT setup is presented and improved. This setup is then used throughout the thesis to create 3D in vivo retinal images of healthy volunteers and patients.
Improvements of PS-OCT data analysis are presented and demonstrated in healthy volunteers. A new imaging metric, optic axis uniformity (OAxU), is introduced. OAxU is defined as the length of the spatially averaged optic axis and varies between 0 and 1. Differential Mueller matrix calculations have been implemented and improved to visualize birefringent structures and their optic axes locally. A deep-learning model is developed as a tool to segment the PS-OCT images.
Improvement of PS-OCT analysis allows for orientation images of the collagen in the peripapillary sclera, revealing a radially oriented fiber layer preceding a circumferentially orientated fiber layer. Close to the optic nerve head (ONH), the circumferentially oriented layer displaces the radially oriented fiber layer. PS-OCT is used to image patients with high myopia, glaucoma, osteogenesis imperfecta (OI) or wet age-related macular degeneration (wAMD) to compare the visualization of these new findings in ocular fibrillar architecture to healthy volunteers. These diseases are relevant for PS-OCT imaging to detect changes in scleral collagen, damage to the RNFL, or subretinal fibrosis.
Multiple parameters are investigated in this study, including birefringence of the sclera and its correlation with axial length, orientations of RNFL in glaucoma patients, dimensions of the circularly oriented sclera (COS) ring, and thickness of the radially oriented sclera (ROS) layer. The ROS is thinner in myopia patients and OI patients. Besides a thinner ROS layer, birefringence of the sclera was lower in OI patients.
Fibrosis can be detected and its orientation can be determined in 3D in AMD patients. Fibrosis shows a higher organization than expected and was often observed in large patches of similar orientations which can vary in depth.
The applicability of (PS-)OCT goes beyond ophthalmology. In this thesis, it is investigated whether PS-OCT would be valuable in the imaging of airway. Multiple diseased lungs and healthy lungs are imaged ex vivo to investigate differences in alveoli size/presence and ASM thickness. Optic axis orientation images proved very useful to discriminate different birefringent structures, which is not possible in standard OCT imaging. This allowed for automatic ASM segmentation which is relevant for diseases were this layer is thickened such as asthma, fibrotic sarcoidosis (FS), and fibrotic hypersensitivity pneumonitis (fHP).