Through a series of simulations and experiments, we demonstrate that the frequently cited criterion of matching speckle size to detector element (pixel) size in laser speckle contrast imaging (LSCI) has the detrimental effect of reducing the contrast and thereby decreasing the variation in the laser speckle contrast image. Unlike quasi-elastic light scattering, where this matching condition has been shown to maximize the signal-to-noise ratio, in LSCI, the minimum speckle size must exceed the Nyquist criterion in order to maximize the contrast of the speckle patterns.
Purpose To characterize the rod and cone photoreceptor mosaic at retinal locations spanning the central 60°in vivo using adaptive optics scanning laser ophthalmoscopy (AO-SLO) in healthy human eyes. Methods AO-SLO images (0.7 × 0.9°) were acquired at 680 nm from 14 locations from 30°nasal retina (NR) to 30°temporal retina (TR) in 5 subjects. Registered averaged images were used to measure rod and cone density and spacing within 60 × 60 μm regions of interest. Voronoi analysis was performed to examine packing geometry at all locations. Results Average peak cone density near the fovea was 164 000 ± 24 000 cones/mm 2 and decreased to 6700 ± 1500 and 5400 ± 700 cones/mm 2 at 30°NR and 30°TR, respectively. Cone-to-cone spacing increased from 2.7 ± 0.2 μm at the fovea to 14.6 ± 1.4 μm at 30°NR and 16.3 ± 0.7 μm at 30°TR. Rod density peaked at 25°NR (124 000 ± 20 000 rods/mm 2 ) and 20°TR (120 000 ± 12 000 rods/mm 2 ) and decreased at higher eccentricities. Center-to-center rod spacing was lowest nasally at 25°(2.1 ± 0.1 μm). Temporally, rod spacing was lowest at 20°(2.2 ± 0.1 μm) before increasing to 2.3 ± 0.1 μm at 30°TR. Conclusions Both rod and cone densities showed good agreement with histology and prior AO-SLO studies. The results demonstrate the ability to image at higher retinal eccentricities than reported previously. This has clinical importance in diseases that initially affect the peripheral retina such as retinitis pigmentosa.
We have designed and implemented a dual-mode adaptive optics (AO) imaging system that combines spectral domain optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO) for in vivo imaging of the human retina. The system simultaneously acquires SLO frames and OCT B-scans at 60 Hz with an OCT volume acquisition time of 4.2 s. Transverse eye motion measured from the SLO is used to register the OCT B-scans to generate three-dimensional (3-D) volumes. Key optical design considerations include: minimizing system aberrations through the use of off-axis relay telescopes, conjugate pupil plane requirements, and the use of dichroic beam splitters to separate and recombine the OCT and SLO beams around the nonshared horizontal scanning mirrors. To demonstrate system performance, AO-OCT-SLO images and measurements are taken from three normal human subjects ranging in retinal eccentricity from the fovea out to 15-deg temporal and 20-deg superior. Also presented are en face OCT projections generated from the registered 3-D volumes. The ability to acquire high-resolution 3-D images of the human retina in the midperiphery and beyond has clinical importance in diseases, such as retinitis pigmentosa and cone-rod dystrophy.
Background and Objective:
Subthreshold micropulse laser (SML) treatment at 577 nm has been proposed as a safe and efficacious therapy for diabetic macular edema (DME). The study objective was to evaluate the integrity of individual cone photoreceptors after SML treatment using high resolution retinal imaging.
Methods:
An observational cohort study of four DME subjects treated using SML were followed over time. Cone inner and outer segment lengths (ISL/OSL) and total retinal thicknesses (TRT) were measured as the edema resolved. The primary outcome was the detection of any laser induced photoreceptor damage/change following the SML treatment using adaptive optics (AO) imaging.
Results:
Individual cones observed pre-treatment remained visible, while cones that were initially obscured by the DME became more discernable after the treatment. TRT showed statistically significant thinning in half of the subjects. One subject showed no significant change while one showed a statistically significant increase in TRT despite the treatment. No subject was found to have photoreceptor damage following treatment.
Conclusions:
SML at 577 nm did not result in measureable structural damage to the underlying photoreceptor layer supporting previous work that SML is a safe alternative for treating DME.
We demonstrate the ability to image microcystic lesions early in their development and have quantified longitudinal changes. The presence of small hyperreflective structures at a layer midway through the INL seems to be a precursor to their formation and is a potential biomarker for assessing POAG severity and progression. The adaptive optics imaging system is also able to visualize retinal ganglion cells in this subject, despite severe thinning of the GCL.
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