Biomedical subjects

D Robert Iskander

Publications and source records attributed to D Robert Iskander.

22 records · Page 2Linked to original sources

PubMed2002

Ocular microfluctuations and videokeratoscopy.

PURPOSE: We aimed to minimize the effect of ocular microfluctuations during videokeratoscopy to improve the qualitative and quantitative analysis of corneal topography. METHODS: A technique was developed to measure the tilt, displacement, and cyclotorsion in multiple videokeratographs from the same cornea. This information was used to reposition each videokeratograph according to the average position of a sample of multiple measurements. The corneal topography of 10 subjects was measured 20 times each, using videokeratoscopy. RESULTS: The root mean square error calculated from difference between single videokeratographs and the average videokeratograph decreased by an average of 24.6% for the 10 subjects' data. The standard deviation of some terms of Zernike polynomials fitted to the cornea was significantly reduced after the procedure. CONCLUSION: The method that we propose can improve the precision performance of videokeratoscopy in multiple measurements of corneal topography.

Adult↗

PubMed2002

Modeling of corneal surfaces with radial polynomials.

We consider analytical modeling of the anterior corneal surface with a set of orthogonal basis functions that are a product of radial polynomials and angular functions. Several candidate basis functions were chosen from the repertoire of functions that are orthogonal in the unit circle and invariant in form with respect to rotation about the origin. In particular, it is shown that a set of functions that is referred herein as Bhatia-Wolf polynomials, represents a better and more robust alternative for modeling corneal elevation data than traditionally used Zernike polynomials. Examples of modeling corneal elevation are given for normal corneas and for abnormal corneas with significant distortion.

Computer Simulation↗

PubMed2002

Automatic estimation of the corneal limbus in videokeratoscopy.

An algorithm for estimating the corneal limbus from videokeratoscopic images is proposed. After the image is transformed to a polar grid, a novel edge-detection procedure, suitable for the detection of the soft edge produced by the limbus, is used to locate the limbus. Outliers due to the eyelids, eyelashes, and videokeratoscopic rings are removed by taking advantage of the approximate circularity of the cornea. An ellipse which minimizes the sum of the squared algebraic errors is fitted to the remaining edge points. Comparisons between the proposed algorithm, a manual computer-based technique and an algorithm which uses conventional edge-detection techniques demonstrate the accuracy of the proposed algorithm.

Algorithms↗

PubMedDate not supplied

Monochromatic aberrations and characteristics of retinal image quality.

BACKGROUND: Patients use a wide variety of terms to describe the characteristics of their vision. These descriptions encompass the effects of their eyes' monochromatic aberrations. METHODS: To illustrate the effect of monochromatic aberrations on the quality of the retinal image, we mathematically reconstructed the image falling on the retina. This has been achieved by combining the properties of various scenes with the optical characteristics of the eye. RESULTS: The effects of some common monochromatic aberrations are illustrated. We also show examples of the retinal image characteristics for two eyes, one with a decentred corneal apex and a second with a decentred refractive surgery ablation. CONCLUSIONS: The image reconstruction technique provides a powerful tool for investigating the quality of the retinal image. It provides the capacity for clinicians to better understand a patient's visual performance. The image reconstruction technique can also broaden our knowledge of the effects of various forms of aberrations on retinal image quality for complex real-world scenes.

Journal Article↗