title: Simultaneous Optical Coherence Tomography and Scheimpflug Imaging Using the Same Incident Light
Creators: name: Li, Xiaoran
Publisher: name: University of Liverpool Repository
Year: 2024

description: Medical imaging technologies can improve the diagnosis and clinical management of many ophthalmic diseases. Cross-sectional ophthalmic imaging devices are essential clinical tools for diagnosing and monitoring medical conditions that affect the anterior segment of the eye. Optical imaging technologies such as optical coherence tomography (OCT) and Scheimpflug imaging (SI) are well developed and widely used in the field of ophthalmology due to its non-invasive and non-contact characteristics. For any single anterior chamber cross-sectional (tomographic) imaging technology, there is a practical compromise between image size and image resolution. In order to obtain large field-of-view (FOV) cross-sectional images of the whole anterior chamber and high-resolution cross-sectional images of the fine corneal layers, measurements by multiple devices are currently required, e.g., OCT and SI devices. This incurs a significant device capital cost, time cost of technicians and clinicians and patients as well as inconvenience to the patients. This project presents a novel raster scanning tomographic imaging device that acquires simultaneously large FOV Scheimpflug and high-resolution spectral domain OCT (SD-OCT) cross-sectional images using the same illuminating photons to overcome the need of multiple devices for cross-sectional imaging. Based on this new idea, two imaging modalities have been developed: a Point-Scanning (PS) device and Line-Field (LF) device. First, a novel imaging device that combines PS-OCT and Scheimpflug image has been developed using fibre optics due to the advantages of its compactness and portability. This new imaging device can take a large-FOV Scheimpflug image covering the whole anterior chamber with an image depth of 12.5 mm and a high-resolution OCT image of cornea with an axial resolution of 2.7µm (nG=1.37). This device allows both Scheimpflug image and OCT images to be obtained simultaneously by raster scanning of the same infrared illuminating light. A pair of polarisers are used in Scheimpflug images during raster scanning in order to address the issues related to the surface reflection. Twelve porcine and fourteen bovine eyes were measured ex-vivo using the device. It was demonstrated that the device has the capability of acquiring simultaneously large FOV SI images covering whole anterior segment of eye and high axial resolution OCT images of cornea resolving thin corneal layers including the epithelium, stroma, and the Descemet’s and Endothelium complex. Second, another new device that combines LF-OCT and SI imaging has been developed. This enables entire high-resolution OCT B-scan image and large FOV SI cross-sectional image to be obtained simultaneously in parallel in a single shot. This imaging configuration with its increased imaging speed has the potential to minimise the image artefacts caused by patients’ involuntary motion or blinking during an in-vivo examination. Third, a new thickness correction method has been developed to correct the thickness distortion in the Scheimpflug image caused by the refraction of the light beam in the cornea. There is a reasonable agreement in the overall central corneal thicknesses (CCT) obtained from the simultaneous OCT and Scheimpflug measurements, which demonstrates the accuracy and reliability of the thickness correction method for the Scheimpflug device. In addition, using the proposed new device both OCT and SI images were taken simultaneously at the same cornea position using the same illumination light, the physical thickness of the cornea obtained by OCT and SI should be the same. This provides a unique method for directly measuring the refractive index of each individual cornea samples. To be concluded, there are three significant contributions of this thesis: the idea of combining a SD-OCT and SI to a single device is proposed and implemented using scanning-point configuration in order to obtain OCT and Scheimpflug images simultaneously. Then, the imaging speed is largely improved by further developing line-field configuration. Consequently, the motion artefact will be reduced for in-vivo measurement, which paves the way to develop further this technology to a commercial medical imaging device. In addition, a modified Scheimpflug thickness correction method is proposed and developed to correct thickness error caused by light refraction. As this correction method only works for central cornea region, a new correction method that is suitable for the whole corneal is proposed and implemented.