The Oxymap T1
The Oxymap T1 was developed to fit the Topcon TRC-50DX/EX fundus cameras.
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 its database and allows users to study retinal oxygenation and vessel width 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 T1 Work?
The oximetry measurements are based on the fact that the color of blood depends on the oxygen saturation of hemoglobin.
The Oxymap T1 is installed on a conventional fundus camera which is operated as for color photography. The Oxymap T1 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 or ODR) 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 retinal vessel width is evaluated based on average cross sections of vessel segments which are detected by the vessel detection algorithm.
Sensitivity of the Oxymap T1 retinal Oximeter
The device has been tested and shown good agreement between values accepted in the scientific literature and currently accepted state of the art methods used to measure oxygen saturation in healthy individuals where the oxygen saturation is expected to be 92% and 57% in the retinal arteries and veins respectively, humans whose systemic arterial oxygen saturation 74–92% and in the animal model where the systemic arterial oxygen saturation was in the range 10%-100%. This is discussed further in the following publications:
- In Retinal Vessel Oxygen Saturation during 100% Oxygen Breathing in Healthy Individuals it is shown that there was a significant difference in the arteries and veins of individuals between the following: (1) Breathing room air (2) breathing 100% oxygen and (3) breathing room air again. It shows that the instrument is sensitive to changes in oxygen saturation.
- In Retinal Oximetry Images Must Be Standardized: A Methodological Analysis the repeatability of the measurements was evaluated The results showed the standard deviation of repeated saturation measurements on the same vessel segment to be 1.0% in arterioles and 1.4% in venules.
- In Retinal Vessel Oxygen Saturation in Healthy Individuals the oxygen saturation in retinal vessels of healthy eyes was measured to determine the effects of age, sex and cardiovascular parameters, as well as the reliability of the measurements and topographic differences. The oxygen saturation was 92.2 +- 3.7% in retinal arterioles and 55.6 +- 6.3% in venules when measured in eyes from 120 healthy individuals aged 18–80.
- In Spectrophotometric Retinal Oximetry in Pigs, the validity of the measurements from the Oxymap T1 were compared to blood gas analysis and intravitreal measurements of partial pressure of oxygen (pO2) in pigs. The conclusion, with the caveat that there are differences between humans and pigs, was that oximetry is sensitive to changes in oxygen saturation in pigs and correlated with intravitreal pO2 measurements and with femoral artery pO2.
- In Retinal Oxygen Saturation in Patients with Systemic Hypoxemia, oximetry was assessed across a broad range of arterial saturation levels. The results showed that retinal and femoral artery oxygen saturation were correlated ( 0.82; P 0.001), with the former approximating the latter at least as well as fingertip oximetry.