Multispectral Scanning Light Ophthalmoscope (MSLO) using optimized illumination schemes

We present an MSLO capable of recording spectral information at each spatial imaging point to enable quantitative mapping of vital physiological parameters indicating retinal health condition

Parallel scanning light ophthalmoscope (PSLO) for retinal imaging

Introduction The eye is constantly in motion even when fixating on a target. These so-called fixational eye movements exist to maintain a sharp vision and they can easily extend to frequencies above 100 Hz. However, they are also the major source of artefacts in retinal imaging systems where the imaging is typically done 30 Hz. In order to reduce eye motion related artifacts in retinal image data we are developing a high-speed imaging system using digital light projection (DLP) technology. Methods To achieve high imaging speeds, retinal area is illuminated with multiple spots/lines in parallel within the whole field of view (FOV) instead of using a single focused spot/line like in traditional scanning laser ophthalmoscopes. These multiple lines/spots patterns are generated with a digital light projector (Lightcrafter 4500, Texas Instrument) and by slightly altering spot/line patterns that we are projecting to the retina, a scanning effect is created. The back-scattered light patterns from the retinal layers are collected via the beamsplitter (PBS) and imaged on to the camera. After every pattern is projected, the final frame is generated by combining these back-reflected illumination patterns. To compensate the lack of physical pinholes, out-of-focus light is removed in the post-processing. Results The fovea of a healthy subject was imaged using 72 patterns. On the left all recorded line patterns were combined to form a non-confocal fundus image showing negligible visible structure. On the right the same lines undergo image processing to remove the out-of-focus light and the corneal scattering. This leads to improved contrast and better lateral resolution in the fundus image. The typical Henle’s fiber layer bowtie is observed around the fovea seen as two brighter areas. Image size is approximately 2.3 mm × 2.3 mm. Conclusions It is possible to create confocal images with the PSLO system. In theory the projector can achieve higher frame rates than traditional scanner-based systems (> 100 Hz) by illuminating the sample with multiple spots/lines. In retinal imaging, such a setup will provide better images because higher imaging speeds reduce motion artifacts

Parallel line scanning ophthalmoscope for retinal imaging

A parallel line scanning ophthalmoscope (PLSO) is presented using a digital micromirror device (DMD) for parallel confocal line imaging of the retina. The posterior part of the eye is illuminated using up to seven parallel lines, which were projected at 100 Hz. The DMD offers a high degree of parallelism in illuminating the retina compared to traditional scanning laser ophthalmoscope systems utilizing scanning mirrors. The system operated at the shot-noise limit with a signal-to-noise ratio of 28 for an optical power measured at the cornea of 100 μW. To demonstrate the imaging capabilities of the system, the macula and the optic nerve head of a healthy volunteer were imaged. Confocal images show good contrast and lateral resolution with a 10° × 10° field of view

Parallel scanning laser ophthalmoscope (PSLO) for high-speed retinal imaging

Purpose High-speed imaging of the retina is crucial for obtaining high quality images in the presence of eye motion. To improve the speed of traditional scanners, a high-speed ophthalmic device is presented using a digital micro-mirror device (DMD) for confocal imaging with multiple simultaneous spots. Methods The PSLO consists of three parts: an illumination, an imaging and a detector arm (Fig. 1). The DMD is uniformly illuminated with a near-infrared (850 nm) LED. The separation between ON positioned mirror elements was made large enough to eliminate cross-talk between neighboring virtual pinholes, and therefore allowed multi-spot confocal imaging across the whole field of view (FOV). The DMD is programmed to project series of shifted point pattern configurations, effectively scanning the spots over the sample surface. The DMD was imaged onto a sample and the returning light was tapped of via a beam-splitter and imaged on a CMOS camera. Multiple point illuminated frames are combined to form one confocal wide-field image. As a proof of principle images of a resolution target were acquired with the PSLO system. Results The resolution target was imaged with a pattern with virtual pinhole size of 2x2 mirrors and the separation between two pinholes was 4 mirror elements. Figure 1B shows the results for combining 9 illumination patterns to form the final image. Conclusions It is possible to create wide-field confocal images with the PSLO system. In theory the DMD can achieve higher frame rates than traditional scanner-based systems by illuminating the sample with multiple spots. In retinal imaging, such a setup will provide better images because higher imaging speeds reduce motion artifacts

Digital micromirror device based ophthalmoscope with concentric circle scanning

Retinal imaging is demonstrated using a novel scanning light ophthalmoscope based on a digital micromirror device with 810 nm illumination. Concentric circles were used as scan patterns, which facilitated fixation by a human subject for imaging. An annular illumination was implemented in the system to reduce the background caused by corneal reflections and thereby to enhance the signal-to-noise ratio. A 1.9-fold increase in the signal-to-noise ratio was found by using an annular illumination aperture compared to a circular illumination aperture, resulting in a 5-fold increase in imaging speed and a better signal-to-noise ratio compared to our previous system. We tested the imaging performance of our system by performing non-mydriatic imaging on two subjects at a speed of 7 Hz with a maximum 20° (diameter) field of view. The images were shot noise limited and clearly show various anatomical features of the retina with high contrast

Parallel scanning laser ophthalmoscope for retinal imaging

Introduction High-speed imaging of the retina is crucial for obtaining high quality images in the presence of eye motion. To improve the speed of traditional scanners, a high-speed ophthalmic device is presented using a digital micro-mirror device (DMD) for confocal imaging with multiple simultaneous spots. Methods An experimental ophthalmic imaging system was constructed based on an 850 nm LED and a DMD containing 1024 x 768 micro-mirrors. Single mirror elements are sparsely turned ON to create multiple spots over the whole field of view. The DMD is programmed to project series of shifted point pattern configurations, effectively scanning the spots over the sample surface. The backscattered light from the retina is tapped off via a beam-splitter and imaged onto a CMOS camera. A confocal image is constructed by applying an image mask of virtual pinholes to each recorded frame. A wide-field confocal image is then created by combining all frames in a single image. Results In the figure a dollar note was imaged with all mirrors ON (widefield) and with multiple spots configuration (every 100th mirror ON). In widefield mode light is detected from different planes above and below the focal plane. When using multiple spots and virtual pinholes, only light from the focal plane is detected. The image on the right shows clearly the microstructure of the bank note. Conclusions It is possible to create confocal images with the PSLO system. In theory the DMD can achieve higher frame rates than traditional scanner-based systems (> 2 kHz) by illuminating the sample with multiple spots