D. J. Mordant

Spectral imaging of the retina

IntroductionThe work described here involved the use of a modified fundus camera to obtain sequential hyperspectral images of the retina in 14 normal volunteers and in 1 illustrative patient with a retinal vascular occlusion.MethodsThe paper describes analysis techniques, which allow oximetry within retinal vessels; these results are presented as retinal oximetry maps.ResultsUsing spectral images, with wavelengths between 556 and 650 nm, the mean oxygen saturation (OS) value in temporal retinal arterioles in normal volunteers was 104.3 (±16.7), and in normal temporal retinal venules was 34.8 (±17.8). These values are comparable to those quoted in the literature, although, the venular saturations are slightly lower than those values found by other authors; explanations are offered for these differences.DiscussionThe described imaging and analysis techniques produce a clinically useful map of retinal oximetric values. The results from normal volunteers and from one illustrative patient are presented. Further developments, including the recent development of a ‘snapshot’ spectral camera, promises enhanced non-invasive retinal vessel oximetry mapping.

Validation of Human Whole Blood Oximetry, Using a Hyperspectral Fundus Camera with a Model Eye

PURPOSE To assess the accuracy of human blood oximetry measurements in a model eye with a hyperspectral fundus camera. METHODS Seven human whole blood samples (two arterial, five venous) were obtained, the oxygen saturations measured with a CO oximeter, and the samples inserted into quartz tubes with internal diameters of 100 and 150 μm. The tubes (n = 20; ten 100 μm and ten 150 μm) were placed within a model eye in front of a background reflectance surface with reflectivities of 20%, 60%, and 99%. Spectral images at wavelengths between 500 and 650 nm were acquired with a hyperspectral fundus camera and analyzed with an oximetric model to calculate the oxygen saturation of blood within the tubes. The calculated oxygen saturations were compared with the measured oxygen saturations. The effects of the background reflectivity and tube size on the accuracy of the calculated oxygen saturations were evaluated. RESULTS Background reflectivity and tube size had no significant effect on the mean oxygen saturation difference (P = 0.18 and P = 0.99, respectively; repeated-measures, two-way ANOVA). The mean differences (SD) between the measured and calculated oxygen saturations in segments of the 100 and 150 μm tubes overlying the 20%, 60%, and 99% background reflectivities were (100 μm) -4.0% (13.4%), -6.4% (9.9%), and -5.5% (10.2%) and (150 μm) -5.3% (10.8%), -5.2% (10.7%), and -5.2% (10.9%), respectively. CONCLUSIONS There was reasonable agreement between the measured oxygen saturation values and those calculated by the oximetry model. The oximetry model could be used to determine the functional health of the retina.

Non-contact ultra-widefield imaging of retinopathy of prematurity using the Optos dual wavelength scanning laser ophthalmoscope

AimsThe purpose of this report is to demonstrate that a non-contact ultra-widefield dual wavelength laser camera (Optos) is able to capture high-quality images in retinopathy of prematurity (ROP).Materials and methodsWe conducted a retrospective review of patients attending the Oxford Eye Hospital with ROP between 1 August 2012 and 16 November 2012 that underwent standard clinical assessment. Anterior segment imaging, where relevant, was performed with Retcam. Retinal imaging was then performed with Optos, using a modified ‘flying baby position’.ResultsThe Optos scanning laser ophthalmoscope was able to acquire ultra-widefield fundal images in nine ROP subjects. The images obtained show clear views of the different stages of ROP features at the posterior pole and peripheral retina. Regression of ROP features were identified, following laser and intravitreal bevacizumab treatment. Additionally, ‘skip areas’ missed by initial laser treatment could be identified in the peripheral retina.ConclusionThe Optos ultra-widefield scanning laser ophthalmoscope is capable of acquiring clinically useful high-quality images of the fundus in ROP subjects. The imaging technique could potentially be used in monitoring ROP progression and documenting ROP regression following treatment.

Oxygen saturation measurements of the retinal vasculature in treated asymmetrical primary open-angle glaucoma using hyperspectral imaging

PurposeTo determine whether there are differences in retinal vascular oxygen saturation measurements, estimated using a hyperspectral fundus camera, between normal eyes and treated eyes of subjects with asymmetrical primary open-angle glaucoma (POAG).MethodsA noninvasive hyperspectral fundus camera was used to acquire spectral images of the retina at wavelengths between 556 and 650 nm in 2-nm increments. In total, 14 normal eyes and both eyes of 11 treated POAG subjects were imaged and analyzed using algorithms that use the spectral variation of the optical densities of blood vessels to estimate the oxygen saturation of blood within the retinal vasculature. In the treated POAG group, each of the eyes were categorized, based on the mean deviation of the Humphrey visual-field analyzer result, as either more-advanced or less-advanced, glaucomatous eyes. Unpaired t-tests (two-tailed) with Welch’s correction were used to compare the mean oxygen saturation between the normal subjects and the treated POAG subgroups.ResultsIn less-advanced and more-advanced-treated POAG eyes, mean retinal venular oxygen saturations (48.2±21.6% and 42.6±18.8%, respectively) were significantly higher than in normal eyes (27.9±9.9%; P=0.03 and 0.01, respectively). Arteriolar oxygen saturation was not significantly different between normal eyes and treated POAG eyes.ConclusionsThe increased oxygen saturation of the retinal venules in advanced-treated POAG eyes may indicate reduced metabolic consumption of oxygen in the inner retinal tissues.

New spectral imaging techniques for blood oximetry in the retina

Hyperspectral imaging of the retina presents a unique opportunity for direct and quantitative mapping of retinal biochemistry - particularly of the vasculature where blood oximetry is enabled by the strong variation of absorption spectra with oxygenation. This is particularly pertinent both to research and to clinical investigation and diagnosis of retinal diseases such as diabetes, glaucoma and age-related macular degeneration. The optimal exploitation of hyperspectral imaging however, presents a set of challenging problems, including; the poorly characterised and controlled optical environment of structures within the retina to be imaged; the erratic motion of the eye ball; and the compounding effects of the optical sensitivity of the retina and the low numerical aperture of the eye. We have developed two spectral imaging techniques to address these issues. We describe first a system in which a liquid crystal tuneable filter is integrated into the illumination system of a conventional fundus camera to enable time-sequential, random access recording of narrow-band spectral images. Image processing techniques are described to eradicate the artefacts that may be introduced by time-sequential imaging. In addition we describe a unique snapshot spectral imaging technique dubbed IRIS that employs polarising interferometry and Wollaston prism beam splitters to simultaneously replicate and spectrally filter images of the retina into multiple spectral bands onto a single detector array. Results of early clinical trials acquired with these two techniques together with a physical model which enables oximetry map are reported.

Light path-length distributions within the retina

Abstract. A Monte Carlo simulation of light propagation through the retina has been developed to understand the path-length distributions within the retinal vessel. For full-field illumination, the path-length distribution within the vessel comprises directly backscattered light and light that has passed once or twice through the vessel. The origins of these light path-length distributions can be better understood by investigating different combinations of single-point illumination and detection positions. Perhaps the most significant observation is that illumination at the edges of the vessel, rather than over the whole field of view, and detection directly above the vessel capture only the light that has taken a single pass through the vessel. This path-length distribution is tightly constrained around the diameter of the vessel and can potentially provide enhancements for oxygen saturation imaging. The method could be practically implemented using an offset-pinhole confocal imaging system or structured light illumination.