An optical evaluation of the phenomenon of red out in neuroendoscopic surgery: what is the physical background?

The loss of visualization in neuroendoscopy due to intraoperative bleeding is called "red out". Although red out is a well-known problem during endoscopy, clear physical descriptions of this phenomenon are lacking. The aim of this study was to investigate the optical properties of red out. In particular, the attenuation coefficient, comprising scattering, and absorption, of various blood/Ringer solutions was quantified and measured spectrally resolved. Small amounts of blood virtually preclude neuroendoscopic visualization. In blood/Ringer solutions with a dilution of 1:101, it was not possible to distinguish characters (font size 10, Arial) at a distance of 5 mm. We have concluded, from our physical investigations, that the problem of red out is dominated by scattering rather than by absorption. Accordingly, technical developments aimed at increasing information acquisition under red out conditions should be based on optical measurement concepts for scattering media.