Laser system refinements to reduce variability in infarct size in the rat photothrombotic stroke model

•Reproducible photothrombosis requires monitoring and control of laser intensity.•Intensity of a polarized DPSS laser beam can be varied by a rotatable polarizer.•Laser beam intensity can also be varied using a variable neutral density filter.•A beamsplitter allows continuous beam intensity monitoring during stroke induction.

BackgroundThe rat photothrombotic stroke model can induce brain infarcts with reasonable biological variability. Nevertheless, we observed unexplained high inter-individual variability despite using a rigorous protocol. Of the three major determinants of infarct volume, photosensitive dye concentration and illumination period were strictly controlled, whereas undetected fluctuation in laser power output was suspected to account for the variability.New methodThe frequently utilized Diode Pumped Solid State (DPSS) lasers emitting 532 nm (green) light can exhibit fluctuations in output power due to temperature and input power alterations. The polarization properties of the Nd:YAG and Nd:YVO4 crystals commonly used in these lasers are another potential source of fluctuation, since one means of controlling output power uses a polarizer with a variable transmission axis. Thus, the properties of DPSS lasers and the relationship between power output and infarct size were explored.ResultsDPSS laser beam intensity showed considerable variation. Either a polarizer or a variable neutral density filter allowed adjustment of a polarized laser beam to the desired intensity. When the beam was unpolarized, the experimenter was restricted to using a variable neutral density filter.Comparison with existing method(s)Our refined approach includes continuous monitoring of DPSS laser intensity via beam sampling using a pellicle beamsplitter and photodiode sensor. This guarantees the desired beam intensity at the targeted brain area during stroke induction, with the intensity controlled either through a polarizer or variable neutral density filter.ConclusionsContinuous monitoring and control of laser beam intensity is critical for ensuring consistent infarct size.