Abstract
Purpose: :
To measure retinal blood velocity changes due to excited neural activity under diffuse luminance flicker condition.
Methods: :
Blood velocity measurements were obtained using the Indiana adaptive optics scanning laser ophthalmoscope (AOSLO) system. AO control was maintained using two deformable mirrors (BMC MEMS DM and Mirao 52d DM) and a Shack Hartmann sensor. The laser beam steering was performed by a 15 kHz line scanner and a programmable vertical scanner. The vertical scan was programmed to briefly pause while the line scan repeatedly crossed the target blood vessel, allowing 15 kHz sampling of the same intersectional area. As an erythrocyte passes through the scanned area, it introduces intensity changes. Because the line scans are obtained rapidly, the movement of the erythrocyte produces a diagonal streak spanning several lines in the image. Offline image processing software developed in MATLAB (Mathworks) was used to measure the streak slope which represents the erythrocyte velocity in the scan direction. Flicker stimulation with a frequency of 8 Hz was delivered by a green LED with current control. For any target vessel, blood velocity with flicker off was firstly measured for several cardiac cycles as the baseline data. Then the flicker was turned on, and blood velocity in the same blood vessel was measured after waiting for 30 seconds to get stable response. The cardiac cycle was determined using a separate cardiac pulse monitor synchronized to the AOSLO system.
Results: :
Blood velocity in both retinal arteries and veins were measured. Arteries and veins were readily differentiated by the direction of flow. As expected, the blood velocity fluctuated with the subject’s cardiac cycle, with more pulsatility in arteries than in veins. In median sized arteries, the centerline blood velocity increased 30% on average during the systolic cardiac phase and 36% during the diastolic phase under flicker condition. In the smallest artery measured (25 µm), the centerline blood velocity increment was 27% during the systolic cardiac phase and 22% during the diastolic phase. For a 94 um vein, the centerline blood velocity increased 10%.
Conclusions: :
Diffuse Luminance Flicker increases retinal blood flow possibly due to neuro-vascular coupling. The physiological changes in retinal blood velocity are measurable with the assistance of adaptive optics and high speed imaging.
Keywords: blood supply • retina • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)