Super-Resolution Ultrasound Localization Microscopy using High-Frequency Ultrasound to Measure Ocular Perfusion Velocity in the Rat Eye

Imaging of the ocular vasculature can provide new insights into the pathophysiology of 1 ocular diseases. This study proposes a novel high-frequency super-resolution ultrasound localization 2 microscopy (SRULM) technique and evaluates its ability to measure in vivo perfusion changes in the 3 rat eye at elevated intraocular pressure (IOP). A 38.4 MHz center frequency linear array transducer on 4 a VisualSonics Vevo F2 imaging platform was used to collect high frame rate (1 kHz) radiofrequency 5 data of the posterior rat eye following systemic microbubble contrast injection. Following clutter and 6 spatiotemporal non-local means filtering, individual microbubbles were localized and tracked. The 7 microbubble tracks were accumulated over 10,000 frames to generate vascular images quantifying 8 perfusion velocity and direction. Experiments were performed using physiologic relevant controlled 9 flow states for algorithm validation and subsequently performed in vivo on the rat eye at 10 mm 10 Hg IOP increments from 10 to 60 mm Hg. The posterior vasculature of the rat eye, including the 11 ophthalmic artery, long posterior ciliary arteries and their branches, central retinal artery and retinal 12 arterioles and venules were successfully visualized, and velocities quantified at each IOP level. 13 Significant reductions in arterial flow were measured as IOP was elevated. High-frequency SRULM 14 can be used to visualize and quantify perfusion velocity of the rat eye in both the retrobulbar and 15 intraocular vasculature simultaneously. The ability to detect ocular perfusion changes throughout the 16 depth of the eye may help elucidate the role ischemia has in the pathophysiology of ocular diseases 17 such as glaucoma. 18