Optimal scheduling of tracing computations for real-time vascular landmark extraction from retinal fundus images.

Recently, this group published fast algorithms for automatic tracing (vectorization) of the vasculature in live retinal angiograms, and for the extraction of visual landmarks formed by vascular bifurcations and crossings. These landmarks are used for feature-based image matching for controlling a computer-assisted laser retinal surgery instrument currently under development. This paper describes methods to schedule the vascular tracing computations to maximize the rate of growth of quality of the partial tracing results within a frame cycle. There are two main advantages. First, progressive image matching from partially extracted landmark sets can be faster, and provide an earlier indication of matching failure. Second, the likelihood of successful image matching is greatly improved since the extracted landmarks are of the highest quality for the given computational budget. The scheduling method is based on quantitative measures for the computational work and the quality of landmarks. A coarse grid-based analysis of the image is used to generate seed points for the tracing computations, along with estimates of local edge strengths, orientations, and vessel thickness. These estimates are used to define criteria for real-time preemptive scheduling of the tracing computations. It is shown that the optimal schedule can only be achieved in perfect hindsight, and is thus unrealizable. This leads to scheduling heuristics that approximate the behavior of the optimal algorithm. One such approximation produced approximately 400% improvement in the quality of the partial results at a defined milestone, as compared to random scheduling. The resulting algorithm can be readily implemented on conventional and multiple-processor systems, and is being applied to computer-assisted laser retinal surgery.

Rapid automated tracing and feature extraction from retinal fundus images using direct exploratory algorithms.

Algorithms are presented for rapid, automatic, robust, adaptive, and accurate tracing of retinal vasculature and analysis of intersections and crossovers. This method improves upon prior work in several ways: 1) automatic adaptation from frame to frame without manual initialization/adjustment, with few tunable parameters; 2) robust operation on image sequences exhibiting natural variability, poor and varying imaging conditions, including over/under-exposure, low contrast, and artifacts such as glare; 3) does not require the vasculature to be connected, so it can handle partial views; and 4) operation is efficient enough for use on unspecialized hardware, and amenable to deadline-driven computing, being able to produce a rapidly and monotonically improving sequence of usable partial results. Increased computation can be traded for superior tracing performance. Its efficiency comes from direct processing on gray-level data without any preprocessing, and from processing only a minimally necessary fraction of pixels in an exploratory manner, avoiding low-level image-wide operations such as thresholding, edge detection, and morphological processing. These properties make the algorithm suited to real-time, on-line (live) processing and is being applied to computer-assisted laser retinal surgery.

Image processing algorithms for retinal montage synthesis, mapping, and real-time location determination.

Although laser retinal surgery is the best available treatment for choridal neovascularization, the current procedure has a low success rate (50%). Challenges, such as motion-compensated beam steering, ensuring complete coverage and minimizing incidental photodamage, can be overcome with improved instrumentation. This paper presents core image processing algorithms for 1) rapid identification of branching and crossover points of the retinal vasculature; 2) automatic montaging of video retinal angiograms; 3) real-time location determination and tracking using a combination of feature-tagged point-matching and dynamic-pixel templates. These algorithms tradeoff conflicting needs for accuracy, robustness to image variations (due to movements and the difficulty of providing steady illumination) and noise, and operational speed in the context of available hardware. The algorithm for locating vasculature landmarks performed robustly at a speed of 16-30 video image frames/s depending upon the field on a Silicon Graphics workstation. The montaging algorithm performed at a speed of 1.6-4 s for merging 5-12 frames. The tracking algorithm was validated by manually locating six landmark points on an image sequence with 180 frames, demonstrating a mean-squared error of 1.35 pixels. It successfully detected and rejected instances when the image dimmed, faded, lost contrast, or lost focus.

Naka-Rushton equation parameters in electroretinogram analysis of daunomycin effects on retinal function.

Intraocular daunomycin may inhibit intravitreal cellular proliferation in proliferative vitreoretinopathy after rhegmatogenous retinal detachments and retinal reattachment surgery. Doses of 15 nmol or more have shown histological retinal toxicity when injected into the intact vitreous of rabbit eyes. We injected 30, 20, 10 and 0 nmol doses of daunomycin into rabbit eyes with gas-compressed vitreous to better simulate the clinical conditions in which it would be used. We then evaluated effects on retinal function by examining scotopic b-wave amplitudes, measured for a four log unit set of intensities. We used the three independent parameters Rmax, log K, and n of the Naka-Rushton equation to measure changes in retinal function. All three doses of daunomycin failed to measurably depress retinal function (i.e., there were no dose-related losses of Rmax, log K or n) in this experimental model. We used this approach to monitor alterations in retinal function since it can show selective changes in each parameter. This selectivity is an advantage over monitoring retinal function with single intensities or comparisons limited solely to electroretinogram amplitudes.

White centered retinal hemorrhages in diabetic retinopathy.

Two hundred fifteen diabetic patients (430 eyes) with early proliferative retinopathy, moderate-to-severe nonproliferative retinopathy, and/or diabetic macular edema in each eye were examined for the presence of white centered retinal hemorrhages (WCRHs). Of the eyes examined, 15.6% demonstrated at least one WCRH and 4.9% showed five or more WCRHs. The prevalence of WCRHs was not significantly related to hypertension, anemia, leukocytosis, thrombocytopenia, thrombocytosis, abnormalities of coagulation, increased serum creatinine, or ingestion of oral contraceptives. Local factors associated with microvascular damage and repair are likely responsible for WCRHs in diabetics. Microaneurysms are at the center of many diabetic WCRHs, and the pale centers of many WCRHs in diabetic retinopathy may be platelet-fibrin aggregates.

Endophthalmitis and pars plana vitrectomy.

A review of all cases of suspected endophthalmitis following cataract extraction seen at one referral hospital between 1979 and 1983 was conducted. Of the 13 cases, 7 were proven by culture to be bacterial endophthalmitis when organisms were grown from aspirates of the aqueous, the vitreous or both. Antibiotic therapy was administered by intravitreal, periocular, topical and systemic routes. In some cases vitrectomy was performed immediately after the diagnostic aspiration or within 24 hours. Poor visual outcome was associated with a long interval between onset of symptoms and admission to hospital, expulsive hemorrhage, retinal detachment or resistance of the microorganism to the intraocularly administered antibiotic. The two eyes that yielded gentamicin-resistant Streptococcus viridans and ended up with no light perception demonstrate the need for simultaneous intravitreal injection of two antibiotics.

Retention of wooden foreign bodies in the orbit.

Two cases of retention of wooden foreign bodies in the orbit are presented. Both patients had a draining fistula. A brain abscess developed in one and a foreign body granuloma in the other. In both cases computerized tomography was very helpful in localizing the foreign bodies and the sequelae. In such cases a subperiosteal surgical approach provides good exposure and minimizes the damage to the orbital contents. A high degree of suspicion of penetration of the orbit is essential in tree branch injuries of the eyelids.

Retinal detachment and pseudophakia.

Retinal detachments occur in up to 3.6% of pseudophakic patients. The intraocular lens makes it hard to see the peripheral fundus but the small pupil ophthalmoscope is a valuable help. The functional and anatomic success rates after operation are about 5% less than those of patients with aphakic retinal detachments matched for age and sex. I present fifteen cases of pseudophakic detachment. Any manipulation likely to induce contact between lens and cornea such as collapse of the globe after drainage of fluid or gas injections should be avoided. Careful examination of the fundus before and after cataract extraction and careful selection of paients for lens implantation are the only means of reducing the incidence of retinal detachment.

Adaptation of Jena 5000 photocoagulator to continuous wave laser power source.

We describe the modifications made in adapting a Jena 5,000 photocoagulator to a tunable dye laser power source and discuss the principles of the tunable dye laser.

Gelatin implants in retina surgery.

Gelatin absorbable implants are a practical and effective material for scleral buckling. They may be used in segmental scleral buckling, meridional implants, aphakic detachments, lower temporal dialyses and macular holes. We describe the technique and the results obtained in 53 patients.