Development of a LED-based imaging system for high-frequency functional hyperemia assessment.

Abstract

Purpose : Diffuse light flicker stimulation of the retina drives neural activity alters vascular oxygen saturation (SO₂). This process of “functional hyperemia” has been assessed in previous studies using a relatively limited set of stimulus characteristics. As the first step toward determining how functional hyperemia may depend on stimulus characteristics including flicker rate, luminance, and contrast in health and disease, a LED-based optical vascular imaging system was developed.

Methods : A slit lamp was modified to incorporate a high-power white LED and a custom motorized optical filter wheel. The filter wheel contains three optical filters: a 530 nm filter to elicit flicker-driven vasodilatory responses (with temporal frequency, luminance, and contrast control); a 570 nm filter to obtain oxygen-insensitive images the retina; and a 606 nm filter to obtain oxygen-sensitive images the retina. Retinal images were acquired using a CMOS camera. Images centered on the optic nerve head allow quantification of oxygen saturation of hemoglobin, and inner-retinal oxygen extraction fraction.

Results : Data obtained from a model eye indicate that the system can image the large vessels of the retina while delivering stimuli that differ in flicker frequency, luminance, and contrast. Acquisition at 12 frames per second provided good image quality, which was improved by binning frames and increasing image gain. The total acquisition time for 10 images through the 570 and 606 nm filters was determined to be 3 seconds. The ratio of reflectance within and outside of vessels defines the vessel optical density. The ratio of optical density measured through the 570 and 606 nm filters provides an index of SO₂. Artery and vein SO₂ values differ, which allows estimating the oxygen extraction fraction (OEF) as: (SO_2artery - SO_2vein / SO_2artery), the value of which depends on the presence or absence of light flicker. The approach used to obtain the images from the model eye will be applied to human subjects.

Conclusions : The LED-based slit lamp instrument allows flexible control over stimulus characteristics, and will provide measures of oxygen saturation, and oxygen extraction fraction. In addition to studying functional hyperemia in clinical populations, data from this instrument can be paired with direct measures of neural function, such as the flicker electroretinogram, to inform our understanding of neurovascular coupling.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.