David F. Wiley

Advances in Fourier-domain optical coherence tomography (Fd-OCT) permit visualization of three-dimensional morphology of in-vivo retinal structures in a way that promises to revolutionize clinical and experimental imaging of the retina. The relevance of these advances will be further increased by the recent introduction of several commercial Fd-OCT instruments that can be used in clinical practice. However, due to some inherent limitations of current Fd-OCT technology (e.g., lack of spectroscopic information, inability to measure fluorescent signals), it is important to co-register Fd-OCT data with images obtained by other clinical imaging modalities such as fundus cameras and fluorescence angiography to create a more complete interpretation and representation of structures imaged. The co-registration between different imaging platforms becomes even more important if small retinal changes are monitored for early detection and treatment. Despite advances in volume acquisition speed with FD-OCT, eye/head motion artifacts can be still seen on acquired data. Additionally high-sampling density, large field-of-view (FOV) Fd-OCT volumes may also be needed for comparison with conventional imaging. In standard Fd-OCT systems, higher sampling density and larger imaging FOV (with constant sampling densities) lead to longer acquisition time which further increases eye/head motion artifacts. To overcome those problems, we tested 3D stitching of multiple, smaller retinal volumes which can be acquired in a less time (reduction of motion artifacts) and/or when stitched create a larger FOV representation of the retina. Custom visualization software that makes possible manual co-registration and simultaneous visualization of volumetric Fd-OCT data sets is described. Volumetric visualizations of healthy retinas with corresponding fundus pictures are presented followed by examples of retinal volumes of high sampling density that are created from multiple "standard" Fd-OCT volumes.

The ability to obtain true three-dimensional (3D) morphology of the retinal structures is essential for future clinical and experimental studies. It becomes especially critical if the measurements acquired with different instruments need to be compared, or precise volumetric data are needed for monitoring and treatment of retinal disease. On the other hand, it is well understood that optical coherence tomography (OCT) images are distorted by several factors. Only limited work has been performed to eliminate these problems in ophthalmic retinal imaging, perhaps because they are less evident in the more common 2D representation mode of time-domain OCT. With recent progress in imaging speed of Fourier domain - OCT (Fd-OCT) techniques, however, 3D OCT imaging is more frequently being used, thereby exposing problems that have been ignored previously. In this paper we propose possible solutions to minimize and compensate for artifacts caused by subject eye and head motion, and distortions caused by the geometry of the scanning optics. The first is corrected by cross-correlation based B-scan registration techniques; the second is corrected by incorporating the geometry of the scanning beam into custom volume rendering software. Retinal volumes of optical nerve head (ONH) and foveal regions of healthy volunteer, with and without corrections, are presented. Finally, some common factors that may lead to increased distortions of the ophthalmic OCT image such as refractive error or position of the subject's head are discussed.

Best quadratic simplicial spline approximations can be computed, using quadratic Bernstein-Bezier basis functions, by identifying and bisecting simplicial elements with largest errors. Our method begins with an initial triangulation of the domain; a best quadratic spline approximation is computed; errors are computed for all simplices; and simplices of maximal error are subdivided. This process is repeated until a user-specified global error tolerance is met. The initial approximations for the unit square and cube are given by two quadratic triangles and five quadratic tetrahedra, respectively. Our more complex triangulation and approximation method that respects field discontinuities and geometrical features allows us to better approximate data. Data is visualized by using the hierarchy of increasingly better quadratic approximations generated by this process. Many visualization problems arise for quadratic elements. First tessellating quadratic elements with smaller linear ones and then rendering the smaller linear elements is one way to visualize quadratic elements. Our results show a significant reduction in the number of simplices required to approximate data sets when using quadratic elements as compared to using linear elements.