AbstractID: 9100 Title: Modeling a light detection system used to determine tissue optical properties during esophageal photodynamic theory We have developed a method to measure the tissue optical properties during esophageal PDT. This is desirable clinically because the tissue optical properties determine the light penetration depth. The light detection system consists of two parallel light transmitting fibers. A fiber with a spherical scattering tip is used to measure light signal at varying distances from another fiber with a 2mm cylindrical diffusing tip emitting 630nm red light. Correction is necessary because (1) the geometry of esophagus is different from an infinite uniform turbid medium and (2) there is significant attenuation of the catheters for both the light source and the detector. Verification of this system was performed in liquid and solid phantoms with known reduced scattering (µ s’) and absorption (µ a) coefficients. Measurements in the liquid phantom were made using 2 concentrations of intralipid (0.25 and 0.5 %) and two concentrations of green ink (0.006 and 0.03%) with µ s’ = 3.5 and 7.1 cm-1 and µ a = 0.1 and 0.5 cm-1. The optical properties of the phantoms were calculated from the light measurements and compared to the known values. A substantial difference was observed between measurements performed in a full phantom and on the surface of the phantom. A distance dependent correction factor was required to correct for the isotropic detector signal, presumably due to anisotropy of catheter transmission. A theory based upon a light source in a semi-infinite medium has been developed to interpret the measured data. We can successfully extrapolate the optical properties using a light detection system.