Grin lens endoscopy in the cat retina: Quantitative measurement of capillary red blood cell velocity

Purpose. We have developed techniques to follow the motion of blood cells though the capillary bed of the cat retina using a robot-controlled system to manipulate a GRIN lens based endoscope. Methods. The head and eye of an anesthetized, spontaneously breathing cat were immobilized according to our standard procedures (IOVS 35(3):1019-1025, 1994). We used a GRIN lens endoscope system (Volpi Mfg, Zurich, CH), held by a goniometric robotic micromanipulator of our own design to acquire high resolution video images of the retinal circulation. The 20 gauge probe, 68 mm long and incorporating a 0.5 mm GRIN lens surrounded by a fiber optic bundle used for illumination, was placed in the vitreal chamber through a sclerotomy. We viewed the RGB output from the endoscope on a monitor and saved selected events for off-line analysis on a video recorder. Because GRIN lens magnification is a function of focal depth, and because even the slightest jitter in this system causes unacceptable results, the manipulator was used to place and hold the probe tip in the region of interest, sufficiently near the retina so that individual blood could be resolved. Results. In trials from five eyes, the motion of red blood cells were readily observed. Red cell velocities were variable and ranged from 0.4 mm/s in a tight capillary loop to 2.7 mm/s in straight segments near the optic disk. Conclusions. The GRIN lens endoscope system allows simultaneous, direct and accurate measurement of red blood cell velocity in retinal capillaries.