Laser-induced changes in autofluorescence of in-vivo skin

Abstract

Autofluorescence is a known property of skin, however, its temporal dynamics are not well known. In this study, we measured the autofluorescence changes in vivo skin during continuous exposure to 442 nm He-Cd laser light. The measurements were performed using a computerized autofluorescence and diffuse reflectance spectroanalyzer system developed in our laboratory. A novel calibration procedure was used to reduce the autofluorescence signal fluctuations over the exposure time of 11 minutes. Integral intensities were calculated for various wavelength regions from the spectra and plotted as a function of time. Non-linear regression fitting of the time function revealed a double exponential decay process [I(t) equals a exp(-t/(tau) <SUB>1</SUB>) + b exp(-t/(tau) <SUB>2</SUB>) + c] in which (tau) <SUB>1</SUB> and (tau) <SUB>2</SUB> differed by an order of magnitude. Autofluorescence imaging demonstrated that a complete recovery of the exposed areas requires approximately 6 days. The variations of parameters a, b, c, (tau) <SUB>1</SUB>, (tau) <SUB>2</SUB> with exposure intensity were measured. A hypothesis for the physical meanings of the double exponential decay process is proposed.