Serial dual-wavelength illumination for retinal vessel oximetry using a conventional fundus camera: a proof-of-concept study

Abstract

Abstract Purpose Retinal vessel oximetry requires fundus images at two wavelengths with different hemoglobin absorption characteristics. Simultaneous acquisition using one-chip color cameras may be affected by incomplete spectral separation of detector channels, whereas multi-chip systems such as 3-chip CCD cameras can achieve improved spectral separation. We present a serial dual-wavelength illumination approach integrated into a conventional fundus camera and compare oxygen saturation measurements with those obtained from a commercial retinal oximeter. Methods A custom LED illumination unit was fiber-coupled into a fundus camera to provide serial illumination at 548 nm (isosbestic hemoglobin wavelength) and 605 nm (oxygen-sensitive wavelength with differential absorption between oxy- and deoxyhemoglobin), each for 250 ms. Corneal irradiance was 33 mW/cm² at 548 nm and 7 mW/cm² at 605 nm. Sequentially acquired images were registered and analyzed using established retinal oximetry algorithms. Mean arterial and venous oxygen saturation values were compared with measurements from a commercial retinal oximeter in nine participants undergoing routine ophthalmic examination. Linear regression analysis was performed to assess the association between methods, and agreement was additionally evaluated using Bland–Altman analysis. Results Mean arterial oxygen saturation was 99.4 ± 5.7% with serial illumination and 100.4 ± 3.0% with standard oximetry. Mean venous oxygen saturation was 71.5 ± 7.0% and 69.5 ± 11.6%, respectively. The mean absolute differences between both techniques were 3.6 ± 2.7% for arteries and 8.4 ± 9.3% for veins. Bland–Altman analysis showed a small arterial bias of 0.94% with limits of agreement from − 7.9% to + 9.8%. For venous measurements, bias was − 1.98% with wider limits of agreement (− 26.9% to + 22.9%). Arterial measurements showed borderline non-significant correlation ( R = 0.606, p = 0.084). Venous measurements showed no correlation. Conclusions Serial illumination is feasible for retinal vessel oximetry and mitigates detector-level spectral crosstalk associated with simultaneous dual-wavelength imaging using one-chip color cameras. Arterial measurements showed only a small systematic bias compared with the commercial retinal oximeter, whereas venous measurements demonstrated greater variability, indicating the need for further optimization and validation.