Scanning Laser Ophthalmoscope Images Research Articles

Use of a wide field scanning laser ophthalmoscope for screening purposes

Abstract Purpose: The wide field (200°) imaging system (Optos, Great Britain) using a scanning laser ophthalmoscope (SLO) enables to visualize a large part of the retina including the periphery. The present study was designed to evaluate the imaging system for screening purposes. Methods: The SLO system was used in 2 groups of patients:1) Outpatient clinic:An undilated retinal biomicroscopy was performed by a resident, and an undilated SLO image was taken by an orthoptist in training. The images were reviewed by another resident masked to results of the previous retinal biomicroscopy.2) Screening for diabetic retinopathyAn undilated and dilated retinal biomicroscopy was performed by a senior ophtalmologist. An undilated SLO image was taken by an orthoptist in training. The images were reviewed by a resident masked to results of the previous retinal biomicroscopy.The results of the clinical examination were compared with the results of the SLO image analysis. Results: In the outpatient group (n=123 eyes), the undilated retinal biomicroscopy missed 18% of the retinal lesions detected on the undilated SLO images. Less than 1% of the retinal lesions detected with the undilated retinal biomicroscopy were missed on the SLO images.In the diabetes group (n=134 eyes), the undilated retinal biomicroscopy missed 10% and the dilated retinal biomicroscopy missed 5% of the retinal lesions detected on the undilated SLO images. None of the retinal lesions detected with the undilated retinal biomicroscopy were missed on the SLO images. Conclusions: In both groups of patients, the diagnostic rate of undilated SLO images was significantly higher when compared with the diagnostic rate of undilated retinal biomicroscopy.

Electrophysiology and autofluorescence imaging in differential diagnosis of retinal and optic nerve disease

Abstract Purpose: Electrophysiological methods enable exact layer by layer diagnosis of the retina and help to answer frequent diagnostic ambiguities between macular and optic nerve diseases. Methods: New multifocal techniques enable topographic representation of the retinal function and can provide accurate diagnosis of macular diseases in early stages. Current trends with emphasis on non‐invasive recording of EOG, ERG, pattern ERG, VEP and multifocal testing, with regards to conventional perimetry, microperimetry and scanning laser ophthalmoscope imaging including autofluorescence of the RPE will be reviewed in representative clinical cases. The importance of standardisation on the performing of all tests according to the recommendations of the International Society for Clinical Electrophysiology of Vision (ISCEV) is emphasised. Results: Global retinal function is well assessed by flash evoked ERGs, but these may be normal in macular disease. PERG and multifocal ERG in combination with autofluorescence imaging may delineate macular diseases in very early stages. In such cases, visual loss may, due to the resulting delay in VEP, erroneously be attributed to optic nerve disease. On the other hand, optic nerve diseases usually do not affect multifocal ERG but can affect PERG, especially its N95 component. Combination of reduced N95 component and delay in VEP is strongly suggestive for optic nerve or ganglion cell disease in which autofluorescence imaging would usually be normal. Conclusions: By judging the cause of visual loss, combination of morphological features by RPE autofluorescence with electrophysiological and psychophysical methods usually leads to correct diagnosis.

Nonmydriatic screening for diabetic retinopathy by ultra-widefield scanning laser ophthalmoscopy (Optomap)

To compare the diagnostic properties of a nonmydriatic 200 degrees ultra-widefield scanning laser ophthalmoscope (SLO) versus onsite mydriatic ophthalmologic examination for diabetic retinopathy. A consecutive series of 51 eyes of 51 patients with different levels of diabetic retinopathy (DR) was examined. Grading of diabetic retinopathy and macular edema obtained on stereoscopic dilated funduscopy by a retina specialist was compared against grading obtained from Optomap Panoramic200 SLO images. All SLOs were performed with an undilated pupil, and no additional clinical information was used for evaluation of the Optomap images by three independent, masked expert graders. A total of five images (9.8%) were not gradable due to insufficient quality. Clinically 4 eyes had proliferative diabetic retinopathy (PDR), while 9 had none, 5 mild, 19 moderate and 14 severe nonproliferative diseases (NPDR). Of the gradable 46 images, a clinically significant macular edema (CSME) was present in 28 eyes clinically. On Optomap, all eyes with PDR were detected as being proliferative, and a sensitivity of 94% at a specificity of 100% was obtained for all graders to detect more than mild DR. Agreement between Optomap retinopathy grading and clinical assessment was good with unweighted kappas of 0.68, 0.68 and 0.51. Assessment of CSME yielded sensitivities of 93, 93 and 89% at specificities of 89, 72 and 83%. The Optomap Panoramic200 nonmydriatic images are of sufficient quality to assess DR and CSME validly and therefore fulfill the basic requirements for telescreening programs.

Autofluorescence Imaging in Age-Related Macular Degeneration Complicated by Choroidal Neovascularization: A Prospective Study

To determine if integrity of the retinal pigment epithelium (RPE)/photoreceptor complex as assessed by autofluorescence imaging can be predicted on the basis of visual acuity (VA), size, or fluorescein angiographic characteristics of the lesion in the early stage of choroidal neovascularization in age-related macular degeneration (AMD). Prospective, observational, consecutive case series. Seventy-nine eyes of 78 patients with untreated early-stage subfoveal neovascular AMD. Digital color fundus photography and fluorescein angiography were carried out by certified photographers using the same camera throughout the study. Confocal scanning laser ophthalmoscopy images were obtained using a retinal angiograph. Autofluorescence images were compared with digital fluorescein angiography and fundus color photographs using IMAGEnet. Autofluorescence at the macula was correlated with VA, angiographic lesion characteristics, lesion size, and length of symptoms. Of the 79 eyes studied, 40 had classic and predominantly classic choroidal neovascularization, 10 had minimally classic, 29 had occult, 75 were subfoveal, and 4 were juxtafoveal. In 54 eyes, autofluorescence was continuous at the central macula, and this correlated significantly with VA, lesion size, and symptom length but not choroidal neovascularization type. However, there was considerable overlap between the 2 groups such that the integrity of RPE autofluorescence could not be predicted on the basis of these criteria. Intact autofluorescence at the macula in early choroidal neovascularization correlates with VA, lesion size, and symptom length but not lesion type. None predict with any certainty the integrity of the outer retina. Our data suggest that the RPE/photoreceptor complex may be intact at the macula for several months in the presence of choroidal neovascularization, suggesting that VA might be rescued if treatment were effective in suppressing neovascular growth without damaging the RPE/retina complex, although this remains to be tested. It would be sensible to assess autofluorescence in treatment protocols to test this concept because it may be a marker for earlier disease and predict outcomes of treatment.

Characterizing image quality in a scanning laser ophthalmoscope with differing pinholes and induced scattered light

We quantify the effects on scanning laser ophthalmoscope image quality of controlled amounts of scattered light, confocal pinhole diameter, and age. Optical volumes through the optic nerve head were recorded for a range of pinhole sizes in 12 subjects (19-64 years). The usefulness of various overall metrics in quantifying the changes in fundus image quality is assessed. For registered and averaged images, we calculated signal-to-noise ratio, entropy, and acutance. Entropy was best able to distinguish differing image quality. The optimum confocal pinhole diameter was found to be 50 microm (on the retina), providing improved axial resolution and image quality under all conditions.

Intereye Spatial Relationship of Abnormal Neuroretinal Rim Locations in Glaucoma Patients from the Diagnostic Innovations in Glaucoma Study

To determine the spatial relationship in neuroretinal rim parameter values between eyes in ocular hypertensives, glaucoma suspects, and glaucoma patients. Observational-cross-sectional study. A total of 334 eyes of 167 patients were included. All patients underwent confocal scanning laser ophthalmoscopy imaging in both eyes. Intereye concordance ratio of abnormal rim sectors as classified by the Moorfield regression analysis (MRA) was calculated as the ratio of matching abnormal rim areas to the total number of abnormal rim areas. The amount of agreement was calculated for each sector separately. Of the right (left) eyes, 39 (39) eyes were classified "borderline" (BL) by MRA, 62 (65) eyes were "within normal limits" (WNL), and 66 (63) eyes "outside normal limits" (ONL). A total of 78 patients had a normal overall MRA result in both eyes (BL = WNL). Of the 89 patients with a defect in at least one sector, mean intereye concordance was 0.28 +/- 0.04 (BL = WNL). When considering BL as ONL, concordance increased to 0.43 +/- 0.03. The lowest intereye agreement in rim abnormality was found in the temporal-inferior sector with 29.0% (Kappa +/- SE [standard error], 0.25 +/- 0.09, BL = WNL) matching. The highest agreement was found in the nasal- inferior sector with 47% (Kappa +/- SE, 0.53 +/- 0.1, BL = WNL) matching. There is a moderate intereye agreement in abnormal neuroretinal rim locations in eyes of ocular hypertensives, glaucoma suspects, and glaucoma patients. Longitudinal studies are needed to elucidate the agreement over time and to better understand the morphological pattern of progression between eyes in glaucoma.

Agreement Between Stereophotographic and Confocal Scanning Laser Ophthalmoscopy Measurements of Cup/Disc Ratio: Effect on a Predictive Model for Glaucoma Development

To evaluate risk estimates obtained by incorporating confocal scanning laser ophthalmoscopy measurements of cup/disc (C/D) ratio into a previously described and validated predictive model to estimate the risk of glaucoma development in ocular hypertension. These risk estimates were compared with those obtained by the original model in which vertical C/D ratio was estimated from stereophotographs. Cross-sectional study. The study included 118 eyes of 59 patients with normal optic discs, normal visual fields, and high intraocular pressure. The original predictive model contained information on 6 baseline factors: age, intraocular pressure, central corneal thickness, vertical C/D ratio, visual field pattern standard deviation, and presence of diabetes. Information regarding these baseline factors was collected for each patient. For the original model, vertical C/D ratio was estimated from stereophotographs. The Heidelberg Retina Tomograph (HRT) model used an identical model, except that the parameter linear C/D ratio was used to provide C/D ratio estimates. All patients underwent confocal scanning laser ophthalmoscopy imaging HRT II within 6 months of stereophotographs. The agreement between stereophotograph and HRT II risk estimates was evaluated by Bland-Altman plots. The difference between HRT II and stereophotograph estimates of C/D ratio was within 0.2 in 95% of the patients. When incorporated into the predictive model, estimates of risk using the HRT II parameter linear C/D ratio were highly correlated to those obtained using stereophotographs (rho=0.954; P<0.001). The 95% limits of agreement were -4.57% to 4.65%. HRT II and stereophotograph estimates of C/D ratio can be used interchangeably when incorporated into a predictive model to estimate the risk of conversion from ocular hypertension to glaucoma.

Combining Structural and Functional Testing for Detection of Glaucoma

To assess whether the combined use of both structural and functional tests improves discrimination between healthy and glaucomatous eyes over either type of testing alone. Observational cross-sectional study. One hundred twenty-three eyes of 123 participants enrolled in the Diagnostic Innovations in Glaucoma Study. Because both structural and functional tests were evaluated, 2 definitions of glaucoma were used: glaucomatous visual field (VF) damage based on repeatable abnormal standard automated perimetry results (n = 43) and glaucomatous optic disc (glaucomatous optic neuropathy [GON]) based on masked assessment of optic disc stereophotographs (n = 65). Participants had scanning laser polarimetry, optical coherence tomography (OCT), and confocal scanning laser ophthalmoscopy imaging in addition to frequency-doubling technology (FDT) perimetry and short-wavelength automated perimetry (SWAP) testing completed within a 6-month interval. For each glaucoma definition, sensitivities and specificities were calculated for the best structural parameters, FDT perimetry pattern standard deviation (PSD), and SWAP PSD/glaucoma hemifield test, and then for each possible pairwise combination of one structural and one functional parameter. The best structural parameters for discriminating between glaucoma and control eyes were nerve fiber indicator for scanning laser polarimetry, inferior average retinal nerve fiber layer thickness for OCT, and Moorfields regression classification for confocal scanning laser ophthalmoscopy. Sensitivities and specificities for detecting glaucomatous VF damage for scanning laser polarimetry, OCT, confocal scanning laser ophthalmoscopy, FDT perimetry, and SWAP were 41.9% and 98.3%, 58.1% and 98.3%, 58.1% and 84.5%, 44.2% and 98.3%, and 65.1% and 86.2%, respectively. Adding FDT perimetry to each of the best structural parameters led to a significant (P<0.05) increase in sensitivity without a significant change in specificity compared with structural parameters alone. Adding SWAP to each of the best structural parameters led to a significant increase in sensitivity and also a significant decrease in specificity compared with each structural parameter alone. A combination of parameters from structural tests and functional tests can improve the sensitivity of glaucoma detection. The sensitivity and specificity desired for a particular clinical application will determine the selection of the particular diagnostic tests.

In Vivo Imaging and Counting of Rat Retinal Ganglion Cells Using a Scanning Laser Ophthalmoscope

To determine whether a scanning laser ophthalmoscope (SLO) is useful for in vivo imaging and counting of rat retinal ganglion cells (RGCs). RGCs of Brown Norway rats were retrogradely labeled bilaterally with the fluorescent dye 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodine (DiA). The unilateral optic nerve was crushed intraorbitally with a clip. RGCs were imaged in vivo with an SLO with an argon blue laser (488 nm) and optical filter sets for fluorescein angiography, before and 1, 2, and 4 weeks after the crush. Fluorescent cells were also counted in retinal flatmounts at baseline and 1, 2, and 4 weeks after the crush. An image overlay analysis was performed to check cell positions in the SLO images over time. Lectin histochemical analysis was performed to determine the relationship of microglia to the newly emerged DiA fluorescence detected by image overlay analysis after the optic nerve crush. Fluorescent RGCs were visible in vivo with an SLO. RGC survival decreased gradually after the crush. In the retina after the optic nerve crush, newly emerged DiA fluorescence detected by image overlay analysis corresponded to fluorescent cells morphologically different from RGCs in the retinal flatmount and was colocalized mostly with lectin-stained microglial processes. RGC counts by SLO were comparable to those in retinal flatmounts. The SLO is useful for in vivo imaging of rat RGCs and therefore may be a valuable tool for monitoring RGC changes over time in various rat models of RGC damage.

In Vivo Imaging and Quantitative Evaluation of the Rat Retinal Nerve Fiber Layer Using Scanning Laser Ophthalmoscopy

To determine whether scanning laser ophthalmoscopy (SLO) is useful for in vivo imaging and quantitative evaluation of rat retinal nerve fiber layer (RNFL) using an optic nerve crush model. The optic nerve of the right eye was crushed intraorbitally with a clip. The left eye served as the untreated control. Fundus images of both eyes were recorded by SLO using an argon blue laser before and 1, 2, and 4 weeks after optic nerve crush. The focused plane was sequentially moved by changing the refractive values in the SLO setting. The range of refractive values (DeltaF) in which the RNFL reflex was clearly observed was determined. The RNFL thickness in retinal sections was measured and compared to the DeltaF value from SLO images taken before histologic preparation. Striations of RNFL radiating from the optic disc were clearly visible by SLO. No obvious changes in the RNFL reflex were observed 1 week after optic nerve crush. However, striations of RNFL became uniformly darker and thinner 2 weeks after the crush and were barely visible 4 weeks after the crush. The DeltaF value was unchanged 1 week after the crush, but then decreased significantly and progressively after the second week. DeltaF was unchanged in the control eyes during the experimental period. The DeltaF value correlated significantly with the histologically determined RNFL thickness. SLO is a useful and valuable tool for in vivo imaging and quantitative evaluation of rat RNFL.

ISSUES IN QUANTIFYING ATROPHIC MACULAR DISEASE USING RETINAL AUTOFLUORESCENCE

In Brief Purpose: To demonstrate the potential and limits of autofluorescence imaging in identifying and delineating areas of atrophy. Methods: Fundus photographs and infrared scanning laser ophthalmoscope (SLO) imaging, SLO macular perimetry, and SLO autofluorescence imaging results were compared for two patients with geographic atrophy (GA) from age-related macular degeneration, one patient with pigmentary alteration of the retina, and two patients with Stargardt disease. The main outcome measure in this case series was the presence of reduced autofluorescence. Results: Drusen may become undetectable during autofluorescence imaging for some patients, allowing simple identification of areas of GA with areas of reduced autofluorescence. In other patients, drusen themselves have decreased autofluorescence, despite having intact retinal function in the retina overlying them. Some patients may have areas of reduced autofluorescence that persist for many years, without evidence of the development of atrophy. In Stargardt disease, decreased autofluorescence can easily detect and delineate areas of scotoma. Areas with mottled autofluorescence may have overlying function, but the function may not be adequate to support a fixation locus in that area. Conclusions: Using decreased autofluorescence to delineate areas of atrophy may be helpful in atrophic macular disorders. For GA, correlation with fundus photographs or macular perimetry findings may be necessary to differentiate between drusen and atrophy. For Stargardt disease, the nature of areas of decreased autofluorescence may help explain visual function of those areas. A reduction in retinal autofluorescence is a valuable tool for delineating areas of atrophy in age–related macular degeneration and Shargardt disease.

ISSUES IN QUANTIFYING ATROPHIC MACULAR DISEASE USING RETINAL AUTOFLUORESCENCE

To demonstrate the potential and limits of autofluorescence imaging in identifying and delineating areas of atrophy. Fundus photographs and infrared scanning laser ophthalmoscope (SLO) imaging, SLO macular perimetry, and SLO autofluorescence imaging results were compared for two patients with geographic atrophy (GA) from age-related macular degeneration, one patient with pigmentary alteration of the retina, and two patients with Stargardt disease. The main outcome measure in this case series was the presence of reduced autofluorescence. Drusen may become undetectable during autofluorescence imaging for some patients, allowing simple identification of areas of GA with areas of reduced autofluorescence. In other patients, drusen themselves have decreased autofluorescence, despite having intact retinal function in the retina overlying them. Some patients may have areas of reduced autofluorescence that persist for many years, without evidence of the development of atrophy. In Stargardt disease, decreased autofluorescence can easily detect and delineate areas of scotoma. Areas with mottled autofluorescence may have overlying function, but the function may not be adequate to support a fixation locus in that area. Using decreased autofluorescence to delineate areas of atrophy may be helpful in atrophic macular disorders. For GA, correlation with fundus photographs or macular perimetry findings may be necessary to differentiate between drusen and atrophy. For Stargardt disease, the nature of areas of decreased autofluorescence may help explain visual function of those areas.

Adaptive optics scanning laser ophthalmoscope for stabilized retinal imaging

A retinal imaging instrument that integrates adaptive optics (AO), scanning laser ophthalmoscopy (SLO), and retinal tracking components was built and tested. The system uses a Hartmann-Shack wave-front sensor (HS-WS) and MEMS-based deformable mirror (DM) for AO-correction of high-resolution, confocal SLO images. The system includes a wide-field line-scanning laser ophthalmoscope for easy orientation of the high-magnification SLO raster. The AO system corrected ocular aberrations to <0.1 mum RMS wave-front error. An active retinal tracking with custom processing board sensed and corrected eye motion with a bandwidth exceeding 1 kHz. We demonstrate tracking accuracy down to 6 mum RMS for some subjects (typically performance: 10-15 mum RMS). The system has the potential to become an important tool to clinicians and researchers for vision studies and the early detection and treatment of retinal diseases.

Line-scanning laser ophthalmoscope

Scanning laser ophthalmoscopy (SLO) is a powerful imaging tool with specialized applications limited to research and ophthalmology clinics due in part to instrument size, cost, and complexity. Conversely, low-cost retinal imaging devices have limited capabilities in screening, detection, and diagnosis of diseases. To fill the niche between these two, a hand-held, nonmydriatic line-scanning laser ophthalmoscope (LSLO) is designed, constructed, and tested on normal human subjects. The LSLO has only one moving part and uses a novel optical approach to produce wide-field confocal fundus images. Imaging modes include multiwavelength illumination and live stereoscopic imaging with a split aperture. Image processing and display functions are controlled with two stacked prototype compact printed circuit boards. With near shot-noise limited performance, the digital LSLO camera requires low illumination power (<500 microW) at near-infrared wavelengths. The line-scanning principle of operation is examined in comparison to SLO and other imaging modes. The line-scanning approach produces high-contrast confocal images with nearly the same performance as a flying-spot SLO. The LSLO may significantly enhance SLO utility for routine use by ophthalmologists, optometrists, general practitioners, and also emergency medical personnel and technicians in the field for retinal disease detection and other diverse applications.

Correspondence: The Effect of Compression on Clinical Diagnosis of Glaucoma Based on Non-analyzed Confocal Scanning Laser Ophthalmoscopy Images

Knowledge of the effect of compression of ophthalmic images on diagnostic reading is essential for effective tele-ophthalmology applications. It was therefore with great anticipation that I read the article “The Effect of Compression on Clinical Diagnosis of Glaucoma Based on Non-analyzed Confocal Scanning Laser Ophthalmoscopy Images,” by Belair et al. in the July/August 2005 issue of Ophthalmic Surgery, Lasers & Imaging. 1 The authors compared diagnostic accuracy of independent observers based on HRT images in different compressed and uncompressed formats, including lossy JPEG compression, lossy JPEG 2000 compression, lossy GIF compression, and lossless PNG and TIFF compression. Their choice to include PNG, a format not supported by most popular image viewers, allows an unexpected verification of their results.

Ultra-Wide-Field Fluorescein Angiography of the Ocular Fundus

To evaluate the clinical applicability of a novel scanning laser ophthalmoscopy imaging system for ultra-wide-field fluorescein angiography. Observational case report. Wide-field fluorescein angiography images were captured in normal volunteers and in patients with a variety of clinical disorders with a newly developed scanning laser ophthalmoscope (Optomap; Optos PLC., Dunfermline, Scotland, United Kingdom) that produces high-resolution images of the fundus up to a 200-degree field. Fluorescein angiograms were performed using standard sequence times. Peripheral vascular perfusion of all four quadrants of a normal fundus was observed with good detail of the capillary architecture. Patients with diabetes and ocular inflammatory eye disease showed evidence of peripheral retinal nonperfusion. The relative degrees of retinal perfusion and nonperfusion could be compared in single images. Ultra-wide-field fundus fluorescein angiography offers the possibility of evaluating and quantitating peripheral retinal perfusion and vascular pathology in fundus disease.

The Effect of Compression on Clinical Diagnosis of Glaucoma Based on Non-analyzed Confocal Scanning Laser Ophthalmoscopy Images

To evaluate the effect of different image compression formats of non-analyzed Heidelberg Retina Tomography (HRT; Heidelberg Engineering, Heidelberg, Germany) images on the diagnosis of glaucoma by ophthalmologists. Thirty-three topographic and reflectance images taken with the HRT representing different levels of disease were transformed using nine different compression formats. Three independent ophthalmologists, masked as to contour line and stereometric parameters, classified the original and compressed HRT images as normal, suspected glaucoma, or glaucoma, and Kappa agreement coefficients were calculated. The Tagged Image File Format had the largest file size and the Joint Photographic Experts Group (JPEG) 2000 format had the smallest size. The highest Kappa coefficient value was 1.00 for all ophthalmologists using the Tagged Image File Format. Kappa values for JPEG formats were all in the range of good to excellent agreement. Kappa values were lower for Portable Network Graphic and Graphics Interchange Format compression formats. Image compression with JPEG 2000 at a ratio of 20:1 provided sufficient quality for glaucoma analysis in conjunction with a relatively small image size format, and may prove to be attractive for HRT telemedicine applications. Further clinical studies validating the usefulness of interpreting non-analyzed HRT images are required.

SPECTRAL IMAGING OF THE AREA OF INTERNAL LIMITING MEMBRANE PEELING

To evaluate a spectral imaging technique to detect the area of internal limiting membrane (ILM) peeling after vitrectomy for idiopathic macular hole. In a prospective study, 15 eyes of 15 patients with idiopathic macular holes were tested. After vitrectomy with ILM peeling, retinal images were taken with color fundus photography, red-free fundus photography, and scanning laser ophthalmoscope imaging at 488 nm, 514 nm, 633 nm, and 780 nm. We calculated the Michelson contrast at the margin of ILM peeling, and each image was rank ordered for the ability to discern the margin of ILM peeling. The Michelson contrasts in scanning laser ophthalmoscope images at 488 nm and 514 nm were significantly larger than those in images at 633 nm and 780 nm and in the red-free fundus photograph. The scanning laser ophthalmoscope images at 488 nm and 514 nm were rated superior to images at 633 nm and 780 nm, the color fundus photograph, and the red-free fundus photograph. The scanning laser ophthalmoscope images at 488 nm and 514 nm provide a better tool than some of the common clinical means for detection of the area of ILM peeling. This may assist with rapid, noninvasive assessment of ILM peeling.

Retinal Location of the Preferred Retinal Locus Relative to the Fovea in Scanning Laser Ophthalmoscope Images

It is difficult to determine the position of a preferred retinal locus (PRL) relative to the fovea in scanning laser ophthalmoscope (SLO) images as a result of disease-related retinal morphologic changes that obscure the fovea. To overcome this problem, we developed a method for determining retinal foveal position based on normal fixation position relative to the optic disk. The normal foveal position measurements can then be used to estimate the distance between a PRL and the fovea. Using the SLO, foveal position was determined for 50 normal subjects by measuring the retinal locus of fixation relative to the optic disk in undistorted SLO images. The resulting normal foveal fixation area is described by a bivariate normal ellipse that can be plotted on any undistorted SLO image. Measurement reliability was assessed by repeated measurements. The PRL relative to the normal foveal fixation area was determined for 24 subjects with macular degeneration and bilateral central scotomas. The normal foveal fixation area based on all 50 subjects is described by a p = 0.9 bivariate ellipse whose centroid is located 12.6 degrees temporal to the temporal optic disk edge and 1.4 degrees inferior to a horizontal line bisecting the disk. PRL area is shown to increase with distance from the foveal fixation ellipse centroid. The shape of the PRL, characterized by the ratio of PRL ellipse major to minor axis, was found to depend on whether the PRL was vertically or horizontally aligned with the foveal fixation centroid. PRL position relative to the fovea can be reliably estimated by plotting the normal foveal fixation bivariate ellipse on undistorted SLO images of retinas in which the fovea is obscured as a result of the disease process.

A multiple sector search method for extracting viewing strategy from fundus images obtained through a scanning laser ophthalmoscope

The scanning laser ophthalmoscope (SLO) is an instrument of great value for observing viewing behaviour and characteristics of fixation in macula disorders; it also may have potential in assessing visual rehabilitation in affected individuals. It is important to understand the viewing strategies used by individuals with a central scotoma and eccentric fixation and to determine whether these strategies have changed as a result of any eccentric viewing training. We describe a method based on searching for multiple sectors in relative correspondence in consecutive SLO fundus images. In this method, successive fixation positions, and consequently viewing strategy, during the reading of isolated words and text passages can be extracted from SLO images. We found the maximum cumulative error in determining fixation position over 1000 consecutive frames to be 5 pixels (approximately 15 min arc) using fundus images from an individual with a macular lesion and clearly visible retinal features. The reconstruction of fixation sequence was found to closely resemble real time or frame-by-frame viewing of the videotaped fundus images from a series of reading tasks in over 50 patients with central scotomas. This method can also be used to assess reading behavior through an SLO in ocularly-healthy individuals.