Identifying the appropriate measurement environment for laser speckle flowmetry of cerebral blood flow in rats

Laser speckle flowmetry (LSF) is a noninvasive tool for cerebral blood flow (CBF) measurement via a cranial bone window. LSF is influenced by various factors including the extent of removal of bone and dura mater and tissue wetness in the bone window. In this study, we aimed to characterize the effect of these conditions on LSF signals and identify optimal measurement conditions for CBF LSF measurements in rats. Three bone windows were created over the Sprague-Dawley rat brains including (i) bone removal until the brain surface was visible through the thin skull, (ii) complete bone removal for dura mater exposure, and (iii) dura mater removal for cortical surface exposure. We investigated the difference in the LSF signals of these windows under dry and wet conditions. The differences between signals obtained using artificial cerebrospinal fluid (aCSF) and mineral oil for wet conditions were also examined. Furthermore, we investigated the stability of repeated CBF measurements in thinned skulls over 15 days and the effects of gentamicin ointment. No significant difference was observed in the LSF values of the three bone windows under dry and wet conditions. Moreover, mineral oil may provide better LSF signal stability. CBF LSF measurements with minimum signal fluctuation were possible for 15 days using the thinned skull window with gentamicin ointment. In conclusion, CBF LSF measurements are feasible in rats using thinned skulls in wet environments, preferably with mineral oils. Relatively repetitive CBF LSF measurements were possible for long duration using gentamicin ointment for daily wound closure.