Abstract
Purpose::
To apply an easily configured adaptive optics scanning laser ophthalmoscope (AOSLO) for improved imaging of the retinal vasculature and for the study of retinal hemodynamics without the assistance of contrast dyes.
Methods::
The adaptive optics control of our system was maintained using a BMC MEMS deformable mirror and Shack Hartmann sensor operating in closed loop at 8 Hz. By adjusting the shape of the deformable mirror and/or the position of a badal optometry, we could move the focus plane to optimally image blood vessels in different retinal layers. A set of precisely controlled confocal apertures was fixed in the retinal plane of the system, with different sizes of 0.6x, 1.2x, 1.7x, 12x, and 120x the Airy disc. These apertures were interchanged using an aperture wheel translated by a pair of high precision linear translation stages. The rapid scanning was achieved by a resonant galvanometer at 8k Hz. The slow scan could be programmed to provide full frame images (512x512) at 15 Hz, or sub images at higher rate (for instance 54x512 images at 150 Hz). For high speed imaging the slow scan galvanometer was scanned in a triangle pattern, and the frames were separated in post processing. A full frame field size of 2 degrees by 1.3 deg was used, and an 840 nm (center wavelength) SLD was used as the light source.
Results::
Excellent images of the blood vessels and blood flows were obtained, including the smallest capillaries. The contrast of the blood vessels content depended on confocal aperture size. The intensity of the interior of the blood vessels increased when in focus for larger confocal apertures, confirming the expected increase in forward scattering for blood. We also measured rapidly moving, very bright cells in all vessels. These features were a few microns. and the intensity were changing during their movement. These had intensity 4 to 5 times the background level, and did not depend markedly on confocal aperture size, suggesting that these represent specular reflections from either oriented cells, or a sparse sample of cells. In the large vessels the bright reflection from these cells made up the central bright streak in the vessels. Capillary velocities measured using the speed of moving particles ranged from 0.8 mm/second to 1.2 mm/second, in agreement with other studies.
Conclusions::
The contrast of the blood vessels as seen with the AOSLO in near infrared light is controlled by the reflecting and scattering properties of cells in the blood, more than by the properties of the vessel walls. By controlling the imaging conditions, it is possible to improve the image quality of the small capillaries.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • imaging/image analysis: non-clinical • blood supply