The Oxymap Retinal Oximeter
The Oxymap Retinal Oximeter measures retinal vessel oxygen saturation and vessel width.
The oximeter is built on the Topcon TRC-50DX fundus camera.
It is composed of a custom-made optical adapter and two high resolution digital cameras (1600 x 1200 square pixels) which produce quality images in a 50° field of view without sacrificing pixel resolution across the vessels.
The Oxymap Analyzer software stores fundus images and subjects’ data in a database and allows users to study retinal oxygenation in groups of individuals, in health and eye diseases, or compare oxygenation before and after treatment. Color-coded oxygen saturation values give a quick overview of the oxygenation of the retina. Users can also select vessel segments of interest, exclude vessel areas from calculations, get detailed analyses and record numerical results.
How Does the Oxymap Retinal Oximeter Work?
The oximetry measurements are based on the fact that the color of blood depends on the oxygen saturation of hemoglobin.
The Oxymap Retinal Oximeter is based on a conventional fundus camera and is operated in a very similar way. It simultaneously acquires two images of the same area of the fundus at two different wavelengths of light. One of the two wavelengths is sensitive to oxygen saturation, i.e. the light absorbance changes with the oxygen saturation, while the other is insensitive to oxygen saturation and is used to calibrate the light intensity.
The two spectral images are automatically processed by the Oxymap Analyzer software. Oxymap Analyzer detects blood vessels and estimates the light absorbance (optical density) at each point along the vessels at each wavelength.
The ratio of the optical densities (optical density ratio) has been shown to be linearly related to hemoglobin oxygen saturation.1 The parameters for the linear equation were calibrated so that the average oxygen saturation of arterioles and venules was 92,2% and 57,9% respectively, for a group of normal subjects.2
The vessel width is evaluated based on average cross sections of vessel segments which are detected by the vessel detection algorithm.
The Oxymap Retinal Oximeter is based on a conventional fundus camera and is operated in a very similar way. It simultaneously acquires two images of the same area of the fundus at two different wavelengths of light. One of the two wavelengths is sensitive to oxygen saturation, i.e. the light absorbance changes with the oxygen saturation, while the other is insensitive to oxygen saturation and is used to calibrate the light intensity.
The two spectral images are automatically processed by the Oxymap Analyzer software. Oxymap Analyzer detects blood vessels and estimates the light absorbance (optical density) at each point along the vessels at each wavelength.
The ratio of the optical densities (optical density ratio) has been shown to be linearly related to hemoglobin oxygen saturation.1 The parameters for the linear equation were calibrated so that the average oxygen saturation of arterioles and venules was 92,2% and 57,9% respectively, for a group of normal subjects.2
The vessel width is evaluated based on average cross sections of vessel segments which are detected by the vessel detection algorithm.
References
1. Beach JM, Schwenzer K, Srinivas S, Kim D, Tiedeman JS. Oximetry of retinal vessels by dual-wavelength fundus imaging: calibration and influence of pigmentation. J Appl Physiol. 1999 Feb;86(2):748-58.
2. Schweitzer D, Hammer M, Kraft J, et al. In vivo measurement of the oxygen saturation of retinal vessels in healthy volunteers. IEEE Trans Biomed Eng. 1999;46:1454–1465.
