Thermal effects of transcranial near-infrared laser irradiation on rabbit cortex

Transcranial near-infrared laser therapy (TLT) improves stroke outcome in animal models. Adequate laser doses are necessary to exert therapeutic effects. However, applying higher laser energy may cause cortical tissue heating and exacerbate stroke injury. The objective of this study is to examine the thermal effect and safety of transcranial near-infrared laser therapy. Diode laser with a wavelength of 808 nm was used to deliver different power densities to the brain cortex of rabbits. Cortical temperature was monitored and measured using a thermal probe during the 2 min transcranial laser irradiation. Neuro-pathological changes were examined with histological staining 24 h after laser treatment. Transcranial laser irradiation for 2 min at cortical power densities of 22.2 and 55.6 mW/cm2 with continuous wave (CW) did not increase cortical temperature in rabbits. With the same treatment regime, cortical power density at 111.1 mW/cm2 increased brain temperature gradually by 0.5 °C over the 2 min exposure and returned to baseline values within 1-2 min post-irradiation. Separately, histological staining was evaluated after triple laser exposure of 22.2 mW/cm2 CW and 111.1 mW/cm2 pulse wave (PW) and showed normal neural cell morphology. The present study demonstrated that the TLT powers currently utilized in animal stroke studies do not cause cortical tissue heating and histopathological damage.