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T. Mihashi, Y. Hirohara, H. Kanda, T. Morimoto, T. Miyoshi, T. Fujikado; Oxygen Saturation Analysis With the Pulsation of Choroid Observed by a Dynamic Two-Spectral-Band Fundus Camera. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1027. doi: https://doi.org/.
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We have been working on the functional imaging of the retina using a two-spectral-band fundus camera (730-780 nm, 820-880 nm). When we analyzed the spatio-temporal independent components (IC), we noticed the IC having choroidal spatial pattern in the image and fluctuation caused by the pulse in the time course. We investigated if this IC may be used for estimating oxygen saturation in the choroid.
Two eyes of two cats were studied under general anesthesia. The total time of the experiment was six minutes. For the first cat, we provided artificial ventilation with 50 % oxygen for 30 seconds at the beginning of the experiment, then no ventilation for three minutes, and again provided the artificial ventilation for 2 minutes 30 seconds. For the second cat the durations of three ventilation conditions were 30 seconds, 1 minute 20 seconds, and 4 minutes 10 seconds. We obtained images from a cat retina using the fundus camera during the experiment. We repeated one-second imaging every 2.5 seconds for 150 times. The one-second imaging consisted of 40 images for each spectral band. We analyzed the 40 images using an independent component analysis (ICA). We manually chose a time-invariant pattern IC (TIIC) and a choroidal spatio-temporal pattern IC (CIC) from the ICs obtained by the ICA. Because the spatial pattern in the IC was normalized in power spectrum, we could calculate the power ratio in the pulse portion of the choroidal pattern by comparing the power spectrum of the time course at pulse frequency in the CIC to the average value of the time course in the TIIC. Finally, we calculated a ratio of these in the two spectral bands and investigated if the values could be used for possible oxygen saturation measurement.
It was easy to select a TIIC and a CIC from each one-second duration. The ratios of pulsation portion in the two spectral bands were 0.604 and 0.657 for two cats under the regular 50% oxygen condition. After the three-minute non-oxygen condition in the first cat, the ratio was 0.825 for one cat. In the other cat with one-minute-20-second non-oxygen condition, the ratio did not vary and stayed around 0.65. Theory of the pulse oximetry predicted that the ratio is the larger when the oxygen saturation is less, so the tendency of the first cat results was consistent with the theoretical prediction.
The results suggested possibility of choroidal oximetry using pulsation observed by a multi-spectral-band fundus camera.
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