A light-scattering signal, which is sensitive to cellular/subcellular structural integrity, is a potential
indicator of brain tissue viability because metabolic energy is used in part to maintain the structure of cells. We previously observed a unique triphasic scattering change (
TSC) at a certain time after
oxygen/
glucose deprivation for blood-free rat brains;
TSC almost coincided with the cerebral
adenosine triphosphate (
ATP) depletion. We examine whether such
TSC can be observed in the presence of blood in vivo, for which transcranial diffuse reflectance measurement is performed for rat brains during
hypoxia induced by
nitrogen gas inhalation. At a certain time after
hypoxia, diffuse reflectance intensity in the near-infrared region changes in three phases, which is shown by spectroscopic analysis to be due to scattering change in the tissue. During
hypoxia, rats are reoxygenated at various time points. When the
oxygen supply is started before
TSC, all rats survive, whereas no rats survive when the
oxygen supply is started after
TSC. Survival is probabilistic when the
oxygen supply is started during
TSC, indicating that the period of
TSC can be regarded as a critical time zone for rescuing the brain. The results demonstrate that light scattering signal can be an
indicator of brain tissue reversibility.