The use of forward scatter to improve retinal vascular imaging with an adaptive optics scanning laser ophthalmoscope
Spotlight summary: The development of techniques to image and examine the retinal microvasculature has important implications for better understanding vessel structure and blood flow in normal eyes and the pathological changes that occur during ocular disease. Standard clinical techniques used to assess the integrity of retinal vasculature and blood flow dynamics include wide-field fundus photography (often following the injection of contrast agents into the bloodstream) and, more recently, scanning laser ophthalmoscopy and optical coherence tomography. While these techniques can readily image larger arteries and veins, they are invasive and/or limited in their ability to resolve microvasculature due to their low lateral resolution. Confocal adaptive optics scanning laser ophthalmoscopes (AOSLOs) have been used to overcome the eye’s optical aberrations and noninvasively yield high-resolution images of retinal capillaries and blood flow in living eyes. However, partly due to the strong backscattered signal originating from the retinal nerve fiber layer and other non-neural structures, it has remained challenging to directly image blood flow and the organization of the retinal microvasculature with high resolution outside of the central macular region.
In this paper, Chui et al. cleverly applied a previously described technique used to enhance the detection of multiply scattered light in the retina to better elucidate retinal microvasculature in living eyes. Traditionally, confocal pinholes with diameters less than or approximately equal to the diameter of Airy’s disk are placed on-axis in a retinal conjugate plane immediately before the imaging detector in confocal scanning laser ophthalmoscopes to improve optical sectioning and reject out-of-focus light. Chui et al. used a large diameter pinhole (approximately 10 Airy disk diameters) and placed it off-axis to block directly backscattered light and collect forward and multiply scattered light, thereby increasing the visibility of retinal microvasculature and blood flow in their AOSLO images. Using this technique, the authors were able to visualize vascular walls in retinal arterioles (including the tunica adventitia, media and intima layers) and quantify vessel wall thickness and lumen diameter. When combined with a variance mapping technique, Chui et al. also produced excellent noninvasive, reflectance images of the radial peripapillary capillaries (structures which nourish the retinal ganglion cell axons and had been previously imaged invasively in fluorescence using an AOSLO). Moreover, the authors include impressive videos demonstrating their ability to detect the single file flow of erythrocytes in capillaries and large vessels.
The non-traditional use of large, decentered pinholes in an AOSLO could play an important role in improving our understanding of the static and dynamic properties of the retinal microvasculature in living eyes. This technique can be used to directly and noninvasively quantify the integrity of a broad range of vessels (from arteries to arterioles to daughter branches and capillaries) through measurements of vessel density, vessel wall thickness, lumen diameter, and the velocities of red and white blood cell flow. It will be exciting to see whether the use of multiply scattered light in high-resolution imaging systems can reveal increased information on vessel characteristics in normal eyes and in diseased eyes believed to have a strong vascular component (e.g., hypertension, diabetes, macular degeneration, glaucoma and others).
ToC Category: Ophthalmology Applications
|OCIS Codes:||(110.1220) Imaging systems : Apertures|
|(170.1470) Medical optics and biotechnology : Blood or tissue constituent monitoring|
|(170.4460) Medical optics and biotechnology : Ophthalmic optics and devices|
|(110.1085) Imaging systems : Adaptive imaging|
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