  The study of the interaction between laser and brain tissue has important
theoretical and practical significance for brain imaging. A two-dimensional
simulation model that studies the propagation of light and heat transfer in
brain tissue based on finite element has been developed by using the commercial
finite element simulation software COMSOL Multiphysics. In this study, the
model consists of three parts of 1) a layer of water on the surface of the
brain, 2) brain tissue and 3) short pulsed laser source (the wavelength is
840nm). The laser point source is located in the middle of the layer of water
above the brain tissue and irradiates the brain tissue. The propagation of
light in brain tissue was simulated by solving the diffusion equation. And the
temperature changes of gray matter and blood vessels were achieved by solving
the biological heat transfer equation. The simulation results show that the
light energy in the brain tissue decreases exponentially with the increase of
penetration depth. Since the cerebral blood vessels have a stronger absorption
on light compared with the surrounding tissues, the remaining light energy of
the blood vessels in the cerebral cortex is ~ 85.8% of the remaining light
energy in the surrounding gray matter. In the process of biological heat
transfer, due to more light deposition in blood vessels, the temperature of
blood vessels is 0.15 K higher than that of gray matter, and the temperature of
gray matter hardly changes. This research is helpful to understand the
propagation of light in the brain and the interaction between them, and has
certain theoretical guiding for the optical imaging of the brain.
