Background and Objective: To evaluate the effect of an acute elevated intraocular pressure (IOP) on oxygen saturation
of structures of the optic nerve head.
Study Design/Materials and Methods: In the cynomolgus monkey eye, IOP was set to 10 mm Hg, and then raised to
30, 45, and 55 mm Hg. The ONH and overlying vessels were imaged using a fundus camera attached to a hyperspectral
imaging system (HSI) at 10 and 30 minutes after IOP elevation. Results: Raising IOP from 10 to 30 mm Hg did not
significantly (P < 0.0001) change saturation in vessels or ONH tissue structures but at 55 mm Hg, all structures showed
significant reduction. Conclusions: Quantitative assay of the blood oxygen saturation in structures on the surface and
overlying the optic nerve head is possible using hyperspectral imaging techniques.
We present an automated method to perform accurate, rapid, and objective measurement of the blood oxygen saturation over each segment of the retinal vascular hierarchy from dual-wavelength fundus images. Its speed and automation (2 s per entire image versus 20 s per segment for manual methods) enables detailed level-by-level measurements over wider areas. An automated tracing algorithm is used to estimate vessel centerlines, thickness, directions, and locations of landmarks such as bifurcations and crossover points. The hierarchical structure of the vascular network is recovered from the trace fragments and landmarks by a novel algorithm. Optical densities (OD) are measured from vascular segments using the minimum reflected intensities inside and outside the vessel. The OD ratio (ODR=OD600/OD570) bears an inverse relationship to systemic HbO2 saturation (SO2). The sensitivity for detecting saturation change when breathing air versus pure oxygen was calculated from the measurements made on six subjects and was found to be 0.0226 ODR units, which is in good agreement with previous manual measurements by the dual-wavelength technique, indicating the validity of the automation. A fully automated system for retinal vessel oximetry would prove useful to achieve early assessments of risk for progression of disease conditions associated with oxygen utilization.
We describe a non-invasive in vivo hyperspectral imaging technique for visualizing the spatial distribution of retina and optic nerve head (ONH) tissue oxygenation. Real time images of the fundus are acquired with continuous wavelengths (410-918 nm) to generate a data cube consisting of one spectral and two spatial dimensions. Reflected light from the one-dimensional (1-D) area of the sample is first passed through a grating and is then imaged onto a 12-bit silicone charge- coupled device (CCD) detector. A scanner then proceeds to the next 1-D area of the sample. Acquired image frames contain 256 spatial pixels and 256 wavelengths along rows and columns. Image sequences are scanned along the perpendicular spatial dimension using the push-broom method, whereby the spectrograph and camera are translated under constant velocity with respect to the fundus camera image over 6.6 mm of travel. This set of acquired images contains the full reflected light spectrum at each pixel of a two dimensional area of the retina and ONH. The system employs a focal plane scanner (FPS) using a linear actuator to provide motion. An algorithm processes spectral information at each pixel to represent the varying spatial distribution of retina and ONH tissue oxygenation. Imaging data are obtained from ONH tissue at both normal intraocular pressure (IOP) and acutely raised IOP.