Light scattering signal is a potential indicator of tissue viability in brain because cellular and subcellular structural
integrity should be associated with cell viability in brain tissue. We previously performed multiwavelength diffuse
reflectance measurement for a rat global ischemic brain model and observed a unique triphasic change in light scattering
at a certain time after oxygen and glucose deprivation. This triphasic scattering change (TSC) was shown to precede
cerebral ATP exhaustion, suggesting that loss of brain tissue viability can be predicted by detecting scattering signal. In
the present study, we examined correlation between light scattering signal and tissue reversibility in rat brain in vivo. We
performed transcranial diffuse reflectance measurement for rat brain; under spontaneous respiration, hypoxia was
induced for the rat by nitrogen gas inhalation and reoxygenation was started at various time points. We observed a TSC,
which started at 140 ± 15 s after starting nitrogen gas inhalation (mean ± SD, n=8). When reoxygenation was started
before the TSC, all rats survived (n=7), while no rats survived when reoxygenation was started after the TSC (n=8).
When reoxygenation was started during the TSC, rats survived probabilistically (n=31). Disability of motor function was
not observed for the survived rats. These results indicate that TSC can be used as an indicator of loss of tissue
reversibility in brains, providing useful information on the critical time zone for treatment to rescue the brain.