Hyperspectral Imaging of Oxygen Saturation in the Optic Nerve Head, Retina, and Choroid

Abstract

Abstract: : Purpose: Develop an innovative hyperspectral imaging technique for the non-invasive evaluation of oxygen saturation in the retina, optic nerve head (ONH), and choroid. Methods: A focal plane scanning-prism grating prism (FPS-PGP) apparatus was interfaced to a Topcon fundus camera with infrared capability via a c-mount. FPS-PGP uses a transmission spectrometer to achieve a highly resolved (<3 nm resolution) spectrum at all the points along the entrance slit. The resulting FPS-PGP spectrometer fundus camera, equipped with a tungsten illumination light, was used to acquire separate hyperspectral images of the ONH and the macular area in two anesthetized cynomolgus monkeys. The hyperspectral images contained spatial dimensions of 640x250 pixels and 256 spectral bands; the 256 spectral bands were scanned from 410 nm to 918 nm with 2 nm resolution. Results: From the ONH hyperspectral data, the presence of hyperspectral reflectance from the major retinal vessels emerging from the ONH, as well hyperspectral reflectance from the ONH itself, was observed. The left-right symmetry exhibited in the horizontal spectrographic lines that pass through the ONH parallel to the horizontal meridian which bisects the optic nerve (known as the median raphe) indicates that the signature of oxygen tissue saturation is, in fact, present. At the 628-nm wavelength, the maximal reflectance from both blood vessels and ONH tissue was observed. From macular hyperspectral reflectance data, the maximal reflectance intensity was evident at 618 nm, indicating that the signature obtained was that of oxy-hemoglobin and deoxy-hemoglobin. A 10 nm shift between ONH and macular images was observed at maximal reflectance. Conclusions. The hyperspectral technique measures spectral changes within the visible and infrared spectrum and provides information on the molecular state of hemoglobin. The hyperspectral imaging device will allow non-invasive measurement in real time of reductions and elevations in oxygen saturation. The distinct optical signature of the biological material such as oxy- and deoxy-hemoglobin as a function of the reflectance spectrum will enable the determination of the relative concentrations. These studies were motivated by the potential for clinical application of this innovative technology in the early diagnosis of and monitoring of therapy for ocular vascular diseases in which the associated hypoxia may eventually lead to loss of vision.

Keywords: optic flow • hypoxia • retina