Green’s function representation of laser induced thermal dynamics and determination of thermal criteria for optically induced neural activation

Infrared nerve stimulation (INS) is rapidly becoming an important tool for basic research and a promising new clinical technology to selectively activate nerves to restore function, map the nervous system, and perform diagnostic procedures. To the best of our understanding, the mechanism of stimulation is photothermal; thus, describing the laserinduced heat distribution is fundamental to determining the relationship between stimulation pulse and neural response. This work develops both a framework describing the time evolution of the heat distribution induced by optical fluence and a novel method to extract thermal criteria for neural activation. We are first concerned with the general problem of describing the temperature distribution in a homogenous medium. To this end, we determine a Green’s function solution to the heat diffusion equation and convolve it with the optical fluence. This provides a general solution to the thermal problem of interest in the form of a single integral over time. Other useful closed form solutions can be determined for interesting special cases. This pursuit also yields an expression for the thermal relaxation time, which provides a rigorous description of thermal confinement for INS applications. The insight we gain from this framework allows us to extract thermal criteria for neural activation from experimental data. Our work provides both insight into the mechanism for stimulation and understanding sufficient to aid in the development of INS devices. Thermal criteria values will prove useful for choosing parameters such as spot size, pulse width, stimulation spacing, and stimulation depth in future INS applications.