Vincent Nourrit

Instrument for measuring the misalignments of ocular surfaces

A compact and robust instrument for measuring the alignment of ocular surfaces has been designed and used in living eyes. It is based on recording Purkinje images (reflections of light at the ocular surfaces) at nine different angular fixations. A complete analysis on the causes of misalignments of Purkinje images and its relations with those physical variables to be measured (global eye tilt, lens decentration and lens tilt) is presented. A research prototype based on these ideas was built and tested in normal and pseudophakic eyes (after cataract surgery). The new analysis techniques, together with the semicircular extended source and multiple fixation tests that we used, are significant improvements towards a robust approach to measuring the misalignments of the ocular surfaces in vivo. This instrument will be of use in both basic studies of the eyes optics and clinical ophthalmology.

Spatial and Spectral Imaging of Retinal Laser Photocoagulation Burns

PURPOSE To correlate in vivo spatial and spectral morphologic changes of short- to long-pulse 532 nm Nd:YAG retinal laser lesions using Fourier-domain optical coherence tomography (FD OCT), autofluorescence (AF), fluorescein angiography (FA), and multispectral imaging. METHODS Ten eyes with treatment-naive preproliferative or proliferative diabetic retinopathy were studied. A titration grid of laser burns at 20, 100, and 200 milliseconds was applied to the nasal retina and laser fluence titrated to produce four grades of laser lesion visibility: subvisible (SV), barely visible (BV, light-gray), threshold (TH, gray-white), and suprathreshold (ST, white). The AF, FA, FD-OCT, and multispectral imaging were performed 1 week before laser, and 1 hour, 4 weeks, and 3 and 6 months post-laser. Multispectral imaging measured relative tissue oxygen concentration. RESULTS Laser burn visibility and lesion size increased in a linear relationship according to fixed fluence levels. At fixed pulse durations, there was a semilogarithmic increase in lesion size over 6 months. At 20 milliseconds, all grades of laser lesion were reduced significantly in size after 6 months: SV, 51%; BV, 54%; TH, 49%; and ST, 50% (P < 0.001), with retinal pigment epithelial proliferation and photoreceptor infilling. At 20 milliseconds, there was healing of photoreceptor inner segment/outer segment junction layers compared with 100- and 200-millisecond lesions. Significant increases in mean tissue oxygenation (range, four to six units) within the laser titration area and in oxygen concentration across the laser lesions (P < 0.01) were detected at 6 months. CONCLUSIONS For patients undergoing therapeutic laser, there may be improved tissue oxygenation, higher predictability of burn morphology, and more spatial localization of healing responses of burns at 20 milliseconds compared with longer pulse durations over time.

High-resolution hyperspectral imaging of the retina with a modified fundus camera

PURPOSE to examine the practical feasibility of developing a hyperspectral camera from a Zeiss fundus camera and to illustrate its use in imaging diabetic retinopathy and glaucoma patients. METHODS the original light source of the camera was replaced with an external lamp filtered by a fast tunable liquid-crystal filter. The filtered light was then brought into the camera through an optical fiber. The original film camera was replaced by a digital camera. Images were obtained in normals and patients (primary open angle glaucoma, diabetic retinopathy) recruited at the Manchester Royal Eye Hospital. RESULTS a series of eight images were captured across 495- to 720-nm wavelengths, and recording time was less than 1.6s. The light level at the cornea was below the ANSI limits, and patients judged the measurement to be very comfortable. Images were of high quality and were used to generate a pixel-to-pixel oxygenation map of the optic nerve head. Frame alignment is necessary for frame-to-frame comparison but can be achieved through simple methods. CONCLUSIONS we have developed a hyperspectral camera with high spatial and spectral resolution across the whole visible spectrum that can be adapted from a standard fundus camera. The hyperspectral technique allows wavelength-specific visualization of retinal lesions that may be subvisible using a white light source camera. This hyperspectral technique may facilitate localization of retinal and disc pathology and consequently facilitate the diagnosis and management of retinal disease.

Propagation and diffraction of truncated Gaussian beams

The Fresnel diffraction of a truncated Gaussian beam is investigated in detail. Our aim is to provide usable analytical expressions of the diffracted field with respect to the variation of propagation and diffraction parameters. Particular attention is paid to the determination of a trade-off between computing accuracy and simplicity. An illustration of the use of the various expressions describing the beam diffraction and propagation is given for the case of mode coupling in a single-mode fiber.

Relationships between visual field sensitivity and spectral absorption properties of the neuroretinal rim in glaucoma by multispectral imaging.

PURPOSE To investigate the relationship between neuroretinal rim (NRR) differential light absorption (DLA, a measure of spectral absorption properties) and visual field (VF) sensitivity in primary open-angle glaucoma (POAG). METHODS Patients diagnosed with (n = 22) or suspected of having (n = 7) POAG were imaged with a multispectral system incorporating a modified digital fundus camera, 250-W tungsten-halogen lamp, and fast-tuneable liquid crystal filter. Five images were captured sequentially within 1.0 second at wavelengths selected according to absorption properties of hemoglobin (range, 570-610 nm), and a Beer-Lambert law model was used to produce DLA maps of residual NRR from the images. Patients also underwent VF testing. Differences in NRR DLA in vertically opposing 180° and 45° sectors either side of the horizontal midline were compared with corresponding differences in VF sensitivity on both decibel and linear scales by Spearmans rank correlation. RESULTS The decibel VF sensitivity scale showed significant relationships between superior-inferior NRR DLA difference and sensitivity differences between corresponding VF areas in 180° NRR sectors (Spearman ρ = 0.68; P < 0.0001), superior-/inferior-temporal 45° NRR sectors (ρ = 0.57; P < 0.002), and superior-/inferior-nasal 45° NRR sectors (ρ = 0.59; P < 0.001). Using the linear VF sensitivity scale significant relationships were found for 180° NRR sectors (ρ = 0.62; P < 0.0002) and superior-inferior-nasal 45° NRR sectors (ρ = 0.53; P < 0.002). No significant difference was found between correlations using the linear or decibel VF sensitivity scales. CONCLUSIONS Residual NRR DLA is related to VF sensitivity in POAG. Multispectral imaging may provide clinically important information for the assessment and management of POAG.

Forward light scatter and contrast sensitivity in keratoconic patients

PURPOSE The aim of this study was to obtain intraocular forward light scatter (FLS) data in keratoconus patients and to explore possible relationships with contrast sensitivity. METHOD Intraocular straylight was assessed in the right eye of 10 visually-normal subjects (31±5 years) and 5 keratoconus patients (34±9 years) using the van den Berg straylight meter. Contrast sensitivity was also measured in both groups, with and without a glare source provided by the Brightness Acuity Tester (BAT), using a Pelli-Robson chart. Inclusion criteria for the keratoconic patients were chosen to avoid natural causes of increased scatter and to limit the influence of factors that could affect the straylight measurements. RESULTS Intraocular light scatter was significantly greater in the keratoconic patients than in normal subjects for each of the three eccentricities tested (Mann-Whitney U test (2-tailed): Z≤-2.2, p≤0.027). Baseline Pelli-Robson contrast sensitivity was also found to be lower in the keratoconus group than in the normal subjects (Mann-Whitney U test: Z=-2.2, p=0.023). Under glare conditions contrast loss was significantly greater in the keratoconic patients than in the normal subjects (Mann-Whitney U test: Z=-2.5, p=0.013). High levels of negative correlation were found between the magnitudes of intraocular straylight and baseline contrast sensitivity in the keratoconus group (Spearmans correlation coefficient (RS)≤-0.87, p≤0.027). CONCLUSION The high values of straylight measured in the keratoconic patients cannot be explained by age or scarring and could be related to the structural degradation of the cornea. Further investigation of intraocular scatter in keratoconus is warranted to improve our current understanding of the visual deficits associated with this pathology.

Blind deconvolution for high-resolution confocal scanning laser ophthalmoscopy*

We investigate the potential of image deconvolution techniques, either in combination or as a substitute for adaptive optics, in a high-resolution confocal scanning laser ophthalmoscope (SLO). After reviewing the validity of standard hypotheses and the a priori information, we implement two deconvolution algorithms to be applied to experimental retinal images recorded with our own high-resolution research SLO. Despite the important assumptions, standard non-iterative or iterative techniques can improve on images (subtending from 1° to 5°), although the best results were obtained when deconvolution techniques were used in combination with adaptive optics. Low signal-to-noise ratio and blurring caused by eye movements are the main limiting factors for deconvolution techniques in this application.

Nonlinear registration for scanned retinal images: application to ocular polarimetry

Retinal images of approximately 1 degrees of visual field were recorded with a homemade scanning laser ophthalmoscope. The benefit of using a nonlinear registration technique to improve the summation process when averaging frames, rather than a standard approach based on correlation, was assessed. Results suggest that nonlinear methods can surpass linear transformations, allowing improved contrast and more uniform image quality. The importance of this is also demonstrated with specific polarization measurements to determine the degree of polarization across an imaged retinal area. In such a context, where this parameter of polarization is extracted from a combination of registered images, the benefit of the nonlinear method is further increased.

Differences in spectral absorption properties between active neovascular macular degeneration and mild age related maculopathy

This study examines the differences in spectral absorption properties between the maculae of patients with active neovascular macular degeneration and those with early age related maculopathy (ARM). Patients attending for management of neovascular age related macular degeneration (AMD) underwent multispectral imaging with a system comprising of a modified digital fundus camera coupled with a 250-W tungsten-halogen lamp and a liquid crystal fast-tuneable filter. Images were obtained at 8 wavelengths between 496 and 700 nm. Aligned images were used to generate a DLA (differential light absorption, a measure of spectral absorption properties) map of the macular area. DLA maps were generated for both eyes of 10 sequential patients attending for anti-vascular endothelial growth factor injections. Each of these patients had active leaking neovascular AMD in one eye and early ARM or milder disease in the fellow eye. Eyes with neovascular AMD demonstrated lower average levels of DLA compared with their fellow eyes with early ARM (p=0.037, t test). The significant difference in DLA demonstrates the potential of multispectral imaging for differentiating the two pathologies non-invasively.

Comparison of forward light scatter estimations using Shack-Hartmann spot patterns and a straylight meter.

Purpose To determine whether an unmodified commercial wavefront aberrometer (irx3) can be used to estimate forward light scattering and how this assessment matches estimations obtained from the C‐Quant straylight meter. Setting University of Manchester, Manchester, United Kingdom. Design Prospective comparative study. Methods Measurements obtained with a straylight meter and with Shack‐Hartmann spot patterns using a previously reported metric were compared. The method was first validated in a model eye by spraying an aerosol over 4 contact lenses to generate various levels of scattering. Measurements with both methods were subsequently obtained in healthy eyes. Results The study comprised 33 healthy participants (mean age 38.9 years ± 13.1 [SD]). A good correlation was observed between the density of droplets over the contact lenses and the objective scatter value extracted from the hartmanngrams (r = 0.972, P < .05). However, in eyes of subjects, no significant relationship was found between measurements from the straylight meter and the metric derived from the Shack‐Hartmann method (r = 0.133, P = .460). Conclusions The hartmanngrams provided a valid objective measurement of the light scatter in a model eye; the measurements in human eyes were not significantly correlated with those of the light scatter meter. The straylight meter assesses large‐angle scattering, while the Shack‐Hartmann method collates information from a narrow angle around the center of the point‐spread function; this could be the reason for the difference in measurements. Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned.