Efficient scan pattern and chromatic multiplexing improves performance of low-cost portable confocal scanning laser ophthalmoscopy

Abstract

Purpose : Confocal scanning laser ophthalmoscopy (cSLO) offers improvements over handheld fundus photography in telemedicine applications, such as screening for diabetic retinopathy. Here we report on a portable cSLO design that uses a novel scan pattern to reduce acquisition times over traditional cSLOs and allows widefield color imaging at reduced cost due to its chromatically multiplexed topology.

Methods : A handheld patient interface receives light from a green laser diode and NIR superluminescent diode, operating at 520 and 785nm, respectively. The light is collimated, scanned by galvanometers, and relayed into the patient's pupil using off-the-shelf optics and custom optomechanics. A tunable liquid lens compensates for patient refraction. The two light sources are temporally multiplexed so that only one is active at any time, allowing light collected from the patient eye to be detected on a single photomultiplier tube with no filtering. The optical design achieves diffraction-limited performance over a 50° FOV at a working distance of 16mm. Patient safety is ensured by limiting incident power on the subject eye to <200μW in NIR and <100μW in green, well below ANSI Z80.36-2016 limits. The galvanometers use a novel hybrid spiral scan pattern composed of concentric constant angular velocity (CAV) and constant linear velocity (CLV) portions. The spiral pattern provides a convenient fixation target, and its circular outline is a natural fit for ophthalmic imaging. The CAV/CLV hybridization offers performance advantages over a traditional raster, as well as scan fidelity improvements over CAV or CLV spirals alone. The scan operates at 5.6Hz with 30μm radial resolution. GPU-based algorithms produce contrast-enhanced frames in real time, which are then are registered and colorized in post-processing.

Results : Feasibility of the prototype cSLO device in screening applications was demonstrated by imaging a healthy human volunteer under a protocol approved by the Duke University IRB.

Conclusions : We have presented an optimized portable confocal scanning laser ophthalmoscope that offers speed and cost improvements over current designs. Performance of the system was demonstrated by imaging a healthy human volunteer.

This is a 2021 ARVO Annual Meeting abstract.

 

50° FOV color SLO image. 4 frame average (2 NIR, 2 green) acquired in 0.8s.

 

Example hybrid spiral scan pattern, with instantaneous linear speed denoted by color.