Changes in optical properties of ex vivo rat prostate due toheating

Abstract

This study examines the effectiveness of a single, first-order Arrheniusprocess in accurately modelling the thermally induced changes in the opticalproperties, particularly the reduced scattering coefficient, µ's, and theabsorption coefficient, µa, of ex vivo rat prostate. Recent workhas shown that µ's can increase as much as five-fold due to thermalcoagulation, and the observed change in µ's has been modelled wellaccording to a first-order rate process in albumen. Conversely, opticalproperty measurements conducted using pig liver suggest that this change inµ's cannot suitably be described using a single rate parameter. In canineprostate, measurements have indicated that while the absorption coefficientvaries with temperature, it does not do so according to first-order kinetics.A double integrating sphere system was used to measure the reflectance andtransmittance of light at 810 nm through a thin sample of prostate. Usingprostate samples collected from Sprague-Dawley rats, optical properties weremeasured at a constant elevated temperature. Tissue samples were measured overthe range 54-83 °C. The optical properties of the sample weredetermined through comparison with reflectance and transmittance valuespredicted by a Monte Carlo simulation of light propagation in turbid media. Afirst-order Arrhenius model was applied to the observed change in µ's andµa to determine the rate process parameters for thermal coagulation. Themeasured rate coefficients were Ea = (7.18±1.74)×104 J mol-1 and Afreq = 3.14×108 s-1 for µ's. Itwas determined that the change in µ's is well described by a singlefirst-order rate process. Similar analysis performed on the changes in µadue to increased temperatures yielded Ea = (1.01±0.35)×105 J mol-1 and Afreq = 8.92×1012 s-1. The resultsfor µa suggest that the Arrhenius model may be applicable to the changesin absorption.