Optimal wavelengths for subdiffuse scanning laser oximetry of the human retina

Retinal blood vessel oxygenation is considered to be an important marker for numerous eye diseases. Oxygenation is typically assessed by imaging the retinal vessels at different wavelengths using multispectral imaging techniques, where the choice of wavelengths will affect the achievable measurement accuracy. Here, we present a detailed analysis of the error propagation of measurement noise in retinal oximetry, to identify optimal wavelengths that will yield the lowest uncertainty in saturation estimation for a given measurement noise level. In our analysis, we also investigate the effect of hemoglobin packing in discrete blood vessels (pigment packaging), which may result in a nonnegligible bias in saturation estimation if unaccounted for under specific geometrical conditions, such as subdiffuse sampling of smaller blood vessels located deeper within the retina. Our analyses show that using 470, 506, and 592 nm, a fairly accurate estimation of the whole oxygen saturation regime [0 1] can be realized, even in the presence of the pigment packing effect. To validate the analysis, we developed a scanning laser ophthalmoscope to produce high contrast images with a maximum pixel rate of 60 kHz and a maximum 30-deg imaging field of view. Confocal reflectance measurements were then conducted on a tissue-mimicking scattering phantom with optical properties similar to retinal tissue including narrow channels filled with absorbing dyes to mimic blood vessels. By imaging at three optimal wavelengths, the saturation of the dye combination was calculated. The experimental values show good agreement with our theoretical derivations

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 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

Dual wavelength Scanning Light Ophthalmoscope with concentric circle scanning

Abstract Purpose :Multispectral imaging helps in gathering important physiological parameters about the retina. We present a novel and compact scanning light ophthalmoscope (SLO) using a digital micromirror device (DMD) capable of imaging the retina at 7 Hz at two different wavelengths with a maximum 20° × 20° field of view (FOV). Methods :The dual-wavelength SLO (Fig.1) used DMD to create concentric circle scanning on the retina. The concentric circles were centred around the fovea and provided fixation. By shifting the centre of the circles to different locations on the DMD, we imaged different regions of the retina. An annulus was placed in conjugate to the pupil plane in the illumination arm to create an annular illumination on the cornea. In the detection arm, a circular aperture was used to block corneal reflections and pass only the signal reflected from the retina onto the camera. Blocking the corneal reflections reduced the background and increased the signal to noise ratio. Polarisation optics were used to discard the stray reflections within the system. Virtual pinholes were implemented in the digital to create confocal images (Heintzmann et al, 2006). To demonstrate the capabilities of our system, we imaged the right eye of a healthy volunteer with a DMD pattern projection speed of 140 Hz and a fill-factor of 1/20. We used 660 nm and 810 nm illuminations to record confocal images. Results :Fig. 2A shows the fundus photograph of the subject. Fig. 2B & 2C shows the images of the macular region with the fovea in the centre imaged using 810 nm and 660 nm. Since we used polarisation optics, the macular bowtie structure is visible in Figs. 2B & 2C. The optic nerve head region is shown in Figs. 2D & 2E for both wavelengths. Blood vessels in the perifoveal inferior region were also imaged (Fig.2F & 2G). These images show high contrast details of the retina with big and small blood vessels at two different wavelengths. Conclusions :We have demonstrated multispectral retinal imaging using a DMD to create high contrast retinal images. The DMD enables fixating the eye to different locations allowing us to image different parts of peri- and parafoveal regions. This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017. Figure1: Optical layout of the system Figure 2: Imaging in the right eye of a healthy volunteer (A) : Fundus photo. (B-G) : Confocal images of different peri- and para-foveal regions imaged by 810 nm and 660 nm denoted by coloured dotted circles. Scale bar is 5 degrees

Parallel line scanning ophthalmoscope for retinal imaging

Purpose: To visualize retinal structures using a newly developed parallel line scanning ophthalmoscope (PLSO). Methods: A PLSO was built using a digital micromirror device (DMD) instead of traditional scanning mirrors to scan lines over the field of view (FOV). The DMD consists of 912 × 1140 micromirrors which can be individually switched on/off based on a programmed binary pattern. By switching on multiple (parallel) two-element wide lines in the DMD, the corresponding lines on the retina are imaged on a CMOS camera. After acquisition of each frame, the micromirrors are turned off and the mirrors for the next set of adjacent lines are turned on. This is repeated until the whole FOV is imaged. Confocal images are generated from the data by subtracting the maximum and minimum intensity values for each pixel in the sequence. The fovea and optic nerve head (ONH) of a healthy subject were imaged using 10º × 10º FOV at 100 Hz with 7 parallel lines resulting in a full image frame rate of 1.4 fps. The images were acquired through a dark-adapted pupil without any dilatation. The acquired data were processed, as mentioned earlier, into confocal images; but also non-confocal images were obtained by averaging all frames. Results: Figure 1A shows the imaged areas. In the non-confocal images (Fig. 1B&C), the corneal scattering is dominant and makes the retinal structures covered in haze. In the confocal images (Fig. 1D&E), confocality and contrast are improved. The foveal avascular zone and smaller blood vessels are visible in the fovea image (Fig. 1D). Also the quality of the ONH image is improved and many of the main features can be distinguished such as small blood vessels (Fig. 1E). Conclusions: The PLSO provided high contrast images of the fovea and ONH and detailed retinal structures could be observed. The DMD eliminates moving parts from the system and exposure time for each frame is potentially shorter than in full-field imaging, which reduces intra-frame motion. In retinal imaging, such a setup will provide better images because higher imaging speeds reduce motion artifacts

In vivo retinal imaging for fixational eye motion detection using a high-speed digital micromirror device (DMD)-based ophthalmoscope

Retinal motion detection with an accuracy of 0.77 arcmin corresponding to 3.7 µm on the retina is demonstrated with a novel digital micromirror device based ophthalmoscope. By generating a confocal image as a reference, eye motion could be measured from consecutively measured subsampled frames. The subsampled frames provide 7.7 millisecond snapshots of the retina without motion artifacts between the image points of the subsampled frame, distributed over the full field of view. An ophthalmoscope pattern projection speed of 130 Hz enabled a motion detection bandwidth of 65 Hz. A model eye with a scanning mirror was built to test the performance of the motion detection algorithm. Furthermore, an in vivo motion trace was obtained from a healthy volunteer. The obtained eye motion trace clearly shows the three main types of fixational eye movements. Lastly, the obtained eye motion trace was used to correct for the eye motion in consecutively obtained subsampled frames to produce an averaged confocal image correct for motion artefacts

In vivo subdiffuse scanning laser oximetry of the human retina

Scanning laser ophthalmoscopes (SLOs) have the potential to perform high speed, high contrast, functional imaging of the human retina for diagnosis and follow-up of retinal diseases. Commercial SLOs typically use a monochromatic laser source or a superluminescent diode for imaging. Multispectral SLOs using an array of laser sources for spectral imaging have been demonstrated in research settings, with applications mainly aiming at retinal oxygenation measurements. Previous SLO-based oximetry techniques are predominantly based on wavelengths that depend on laser source availability. We describe an SLO system based on a supercontinuum (SC) source and a double-clad fiber using the single-mode core for illumination and the larger inner cladding for quasi-confocal detection to increase throughput and signal-to-noise ratio. A balanced detection scheme was implemented to suppress the relative intensity noise of the SC source. The SLO produced dual wavelength, high-quality images at 10  frames  /  s with a maximum 20 deg imaging field-of-view with any desired combination of wavelengths in the visible spectrum. We demonstrate SLO-based dual-wavelength oximetry in vessels down to 50  μm in diameter. Reproducibility was demonstrated by performing three different imaging sessions of the same volunteer, 8 min apart. Finally, by performing a wavelength sweep between 485 and 608 nm, we determined, for our SLO geometry, an approximately linear relationship between the effective path length of photons through the blood vessels and the vessel diameter.

Visualization 1

A confocal video generated from the data (left) and the corresponding eye movement is drawn in the plot (right)