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).
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