Kenneth R. Sloan

Surfaces from contours

This paper is concerned with the problem of reconstructing the surfaces of three-dimensional objects, given a collection of planar contours representing cross-sections through the objects. This problem has important aplications in biomedical research and instruction, solid modeling, and industrial inspection. The method we describe produces a triangulated mesh from the data points of the contours which is then used in conjunction with a piecewise parametric surface-fitting algorithm to produce a reconstructed surface. The problem can be broken into four subproblems: the correspondence problem (which contours should be connected by the surface?), the tiling problem (how should the contours be connected?), the branching problem (what do we do when there are branches in the surface?), and the surface-fitting problem (what is the precise geometry of the reconstructed surface?) We describe our system for surface reconstruction from sets of contours with respect to each of these subproblems. Special attention is given to the correspondence and branching problems. We present a method that can handle sets of contours in which adjacent contours share a very contorted boundary, and we describe a new approach to solving the correspondence problem using a Minimum Spanning Tree generated from the contours.

Subretinal Drusenoid Deposits In Non-Neovascular Age-Related Macular Degeneration: Morphology, Prevalence, Topography, And Biogenesis Model

Purpose: To characterize the morphology, prevalence, and topography of subretinal drusenoid deposits, a candidate histological correlate of reticular pseudodrusen, with reference to basal linear deposit (BlinD), a specific lesion of age-related macular degeneration, and to propose a biogenesis model for both lesion. Methods: Donor eyes with median death-to-preservation of 2:40 hours were postfixed in osmium tannic acid paraphenylenediamine and prepared for macula-wide high-resolution digital sections. Annotated thicknesses of 21 chorioretinal layers were determined at standard locations in sections through the fovea and the superior perifovea. Results: In 22 eyes of 20 white donors (83.1 ± 7.7 years), SDD appeared as isolated or confluent drusenoid dollops punctuated by tufts of retinal pigment epithelium apical processes and associated with photoreceptor perturbation. Subretinal drusenoid deposits and BlinD were detected in 85 and 90% of non-neovascular age-related macular degeneration donors, respectively. Subretinal drusenoid deposit was thick (median, 9.4 &mgr;m) and more abundant in the perifovea than in the fovea (P < 0.0001). BlinD was thin (median, 2.1 &mgr;m) and more abundant in the fovea than in the perifovea (P < 0.0001). Conclusion: Subretinal drusenoid deposits and BlinD prevalence in age-related macular degeneration eyes are high. Subretinal drusenoid deposits organized morphology, topography, and impact on surrounding photoreceptors imply specific processes of biogenesis. Contrasting topographies of subretinal drusenoid deposits and BlinD suggest relationships with differentiable aspects of rod and cone physiology, respectively. A 2-lesion 2-compartment biogenesis model incorporating outer retinal lipid homeostasis is presented.

Human chorioretinal layer thicknesses measured in macula-wide, high-resolution histologic sections.

PURPOSE To provide a comprehensive description of chorioretinal layer thicknesses in the normal human macula, including two-layer pairs that can produce a combined signal in some optical coherence tomography (OCT) devices (ganglion cell [GCL] and inner plexiform [IPL] layers and outer plexiform [OPL] and outer nuclear [ONL] layers). METHODS In 0.8-μm-thick, macula-wide sections through the foveola of 18 donors (age range, 40-92 years), 21 layers were measured at 25 locations by a trained observer and validated by a second observer. Tissue volume changes were assessed by comparing total retinal thickness in ex vivo OCT and in sections. RESULTS Median tissue shrinkage was 14.5% overall and 29% in the fovea. Histologic laminar boundaries resembled those in SD-OCT scans, but the shapes of the foveolar OPL and ONL differed. Histologic GCL, IPL, and OPLHenle were thickest at 0.8. to 1, 1.5, and 0.4 mm eccentricity, respectively. ONL was thickest in an inward bulge at the foveal center. At 1 mm eccentricity, GCL, INL, and OPLHenle represented 17.3% to 21.1%, 18.0% to 18.5%, and 14.2% to 16.6% of total retinal thickness, respectively. In donors ≥ 70 years of age, the RPE and choroid were 17.1% and 29.6% thinner and OPLHenle was 20.8% thicker than in donors <70 years. CONCLUSIONS In this study, the first graphic representation and thickness database of chorioretinal layers in normal macula were generated. Newer OCT systems can separate GCL from IPL and OPLHenle from ONL, with good agreement for the proportion of retinal thickness occupied by OPLHenle in OCT and histology. The thickening of OPLHenle in older eyes may reflect Müller cell hypertrophy associated with rod loss.

Two-Part Texture Mappings

Most published techniques for mapping two-dimensional texture patterns onto three-dimensional curved surfaces assume that either the texture pattern has been predistorted to compensate for the distortion of the mapping or the curved surfaces are represented parametrically. We address the problem of mapping undistorted planar textures onto arbitrarily represented surfaces. Our mapping technique is done in two parts. First the texture pattern is embedded in 3-space on an intermediate surface. Then the pattern is projected onto the target surface in a way that depends only on the geometry of the target object (not on its parameterization). Both steps have relatively low distortion, so the original texture need not be predistorted. We also discuss interactive techniques that make two-part mapping practical.

Packing geometry of human cone photoreceptors: variation with eccentricity and evidence for local anisotropy.

Disorder in the packing geometry of the human cone mosaic is believed to help alleviate spatial aliasing effects. To characterize cone packing geometry, we gathered positions of cone inner segments at seven locations along four primary and two oblique meridians in an adult human retina. We generated statistical descriptors based on the distribution of distances and angles to Voronoi neighbors. Parameters of a compressed-jittered model were fit to the actual mosaic. Local anisotropies were investigated using correlograms. We find that (1) median distance between Voronoi neighbors increases with eccentricity, but the minimum distance is constant (6-8 microns) across peripheral retina; (2) the cone mosaic is least compressed and jittered at the edge of the foveal rod-free zone; (3) disorder in the foveal center resembles that described by Pum et al. (1990); (4) cone spacing is 10-15% less in one direction than in the orthogonal direction; and (5) cone spacing is greater in the radial direction (along meridians) than in the tangential direction (along lines of isoeccentricity). The nearly constant minimum distance implies that high spatial frequencies may be sampled even in peripheral retina. Local anisotropy of the cone mosaic is discussed in relation to the growth of the primate retina during development and to the orientation biases of retinal ganglion cells.

Quantitative Autofluorescence and Cell Density Maps of the Human Retinal Pigment Epithelium

PURPOSE Lipofuscin (LF) accumulation within RPE cells is considered pathogenic in AMD. To test whether LF contributes to RPE cell loss in aging and to provide a cellular basis for fundus autofluorescence (AF) we created maps of human RPE cell number and histologic AF. METHODS Retinal pigment epithelium-Bruchs membrane flat mounts were prepared from 20 donor eyes (10 ≤ 51 and 10 > 80 years; postmortem: ≤4.2 hours; no retinal pathologies), preserving foveal position. Phalloidin-binding RPE cytoskeleton and LF-AF (488-nm excitation) were imaged at up to 90 predefined positions. Maps were assembled from 83,330 cells in 1470 locations. From Voronoi regions representing each cell, the number of neighbors, cell area, and total AF intensity normalized to an AF standard was determined. RESULTS Highly variable between individuals, RPE-AF increases significantly with age. A perifoveal ring of high AF mirrors rod photoreceptor topography and fundus-AF. Retinal pigment epithelium cell density peaks at the fovea, independent of age, yet no net RPE cell loss is detectable. The RPE monolayer undergoes considerable lifelong re-modeling. The relationship of cell size and AF, a surrogate for LF concentration, is orderly and linear in both groups. Autofluorescence topography differs distinctly from the topography of age-related rod loss. CONCLUSIONS Digital maps of quantitative AF, cell density, and packing geometry provide metrics for cellular-resolution clinical imaging and model systems. The uncoupling of RPE LF content, cell number, and photoreceptor topography in aging challenges LFs role in AMD.

Dynamic Quantization: Two Adaptive Data Structures for Multidimensional Spaces

Two new data structures are defined for use in multidimensional histogramming. Their purpose is to cover a parameter space with a limited number of histogram bins so that fine precision is maintained where it is needed. The original motivation for these data structures was to implement Hough-like transforms in high-dimensional parameter spaces. The two data structures share the ability to adapt to distributions that change with time.

Accelerated volume rendering using homogeneous region encoding

Previous accelerated volume rendering techniques have used auxiliary hierarchical datastructures to skip empty and homogeneous regions. Although some recent research has taken advantage of more efficient direct encoding techniques to skip empty regions, no work has been done to directly encode homogeneous but not empty regions. 3D distance transforms previously used to encode empty space can be extended to preprocess homogeneous regions as well, and these regions can be efficiently encoded and incorporated into volume ray-casting and back projection algorithms with a high degree of flexibility.

Computer methods for sampling, reconstruction, display and analysis of retinal whole mounts

We are quantifying the distribution of photoreceptors and ganglion cells in human retina with the goal of establishing a reliable anatomical database which may be compared to information about visual function. We required a representation of retinal cell distributions which facilitated collection, analysis, and display of morphometric data from the entire retina of a large number of eyes. We report computer methods to (1) reconstruct the original retinal sphere from a three-piece whole mount preparation; (2) sample the retina in a manner which allowed description of approximately radially symmetrical cell distributions and avoided both undersampling (which produces interpolation artifacts) and oversampling (which wastes time); (3) interpolate between data points in order to produce plots of cell density along arbitrary meridians and maps of average cell density from several eyes; (4) specify locations on the retinal surface using a spherical coordinate system with its primary axis through the fovea; and (5) produce color-coded maps of cell distributions in a standard perimetric projection.

A computer system for combined neuronal mapping and morphometry.

A computer system has been developed which allows both mapping and planimetry of histological preparations. Data consist of cytoarchitectonic boundaries defined in low-power micrographs of thick sections, and nuclear and vascular profiles defined in high-power micrographs of semi-thin sections taken from the original thick tissue, and landmarks common to both. Data from these different preparations and magnifications are entered into the computer on a digitizing tablet and are stored as points in a real-world coordinate system, along with profile labels, landmarks for alignment purposes, and section depth. Subsequent programs determine parameters such as area and perimeter for each profile, as well as its geometric center and relationship to the cytoarchitectonic boundary. This mapping parameter allows morphometric analysis of profiles as a function of their position. Other programs allow display and manipulation of data in 3 dimensions, cell counts, and stereology. Data acquisition is done in the laboratory, using a micro-computer; analysis and display are done on a large time-sharing computer connected to the small machine by a telephone interface.