Measurement of optical path length change in response to pulsed laser irradiation using phase sensitive OCT
Optical excitation of a sample produces photothermal effects such as a temperature rise, pressure change, refractive index change, and thermoelastic displacement due to absorption of laser radiation in a short pulse or continuous train of pulses. Change of optical path length in tissue in response to pulsed laser irradiation is a photothermal response associated with refractive index change. The detection of laser induced optical path length change may provide useful information for diagnostic biomedical applications such as imaging and sensing. Detection of optical path length change in response to pulsed laser irradiation is associated with a relative strain or stress change. DP-OCT can measure change of relative optical path length between light backscattered from two spatially separated sites. Dye and Superparamagnetic iron oxide nanoparticles (SPION) solutions were used in basic studies to investigate optical path length change in response to pulsed laser irradiation. DP-OCT detection of thermoelastic surface displacement and thermal wave radiometric imaging (TWRI) of arteries of SPIONinjected hyperlipidemic rabbits were conducted to detect atherosclerotic plaques. Studies using both DP-OCT and TWRI showed that pulsed laser irradiation of nanoparicles can improve detection capability of atherosclerotic plaque. Analytical solutions for DP-OCT experiments and TWRI were developed to investigate optical path length change in response to pulsed laser irradiation. Homogeneous planar absorbers and point heat sources in an elastic medium were assumed for dye solutions and SPION in a rabbit artery, respectively. Refractive index change and thermal expansion coefficient of dye solutions were obtained by correlating measured optical path length and temperature increase using the developed analytical solution. Measured thermoelastic surface displacements of rabbit arteries in response to pulsed laser irradiation were consistent with the derived analytical solution. Laser spot size, penetration depth, and nanoparticle distribution determined the rate of thermoelastic surface displacement and decay during and after laser irradiation respectively. In addition, magnitude and phase difference between plaque and background for TWRI was examined using a derived analytical solution. Thermoelastic surface displacement of neural tissue in response to pulsed Ho:YAG laser irradiation was measured by DP-OCT. Thermoelastic displacement does not account for optical stimulation of neural tissue.