In-vivo assessment of unwanted tissue heating during near infrared laser light emission

Sammanfattning: In this work, potential adverse tissue heating in conjunction with near infrared (NIR) laser spectroscopy was studied. The tissue heating from laser light within the NIR optical window is currently a relatively unexplored field. Further knowledge of this heating is essential for the development of safe medical devices utilising laser spectroscopic diagnostics methods on living tissue. Surface temperature increase from laser emission at wavelengths 761, 937 and 971 nm was measured on the forearms of 12 healthy volunteers. The temperature was measured with a heat camera as well as an infrared thermometer. By varying the size of the laser spot between 1-20 mm^2 and the power between 80-120 mW the dependence of both parameters were tested. The measured skin temperature increase from the laser illumination on all participants was below 6 °C. Furthermore a simulation of the stationary laser induced heating was produced by combining a Monte-Carlo simulation of photon migration with a finite element method simulation of tissue heating. The results of the simulations show a linear increase of the temperature as a function of the power and non-linear decrease as a function of the inverse squared radius of laser spot. By combining these dependencies a model for the predicted induced heating as a function of both the power and radius was produced. This model with the fitted coefficients from the simulations is meant to be used in aiding the development of non-invasive medical devices that utilise the optical window for diagnostics or monitoring. All but one of the volunteers included in the project had light skin categorised as 1-3 on the Fitzpatrick scale. However a few measurements were conducted with the 761 nm laser on a volunteer with more pigmented skin of type 5. The results showed a much higher temperature increase than from lighter skin. Heating was also measured on mucosa on the inside of the lower lip on three volunteers. The measured surface temperature increases on mucosa was on the same order as for light skin.