Knowledge of optical properties is required to determine light dose in photodynamic therapy. We have designed an
interstitial optical probe, consisting of six helically arranged side-firing fibers enclosed in a 1.1 mm diameter
encapsulant, that can be used to determine these values. White light is delivered by one fiber and detected by the others.
Based on a Monte Carlo (MC) model of the probe, the absorption (μa) and reduced scattering (μs') coefficients of the
sample are determined. Recovery was verified in tissue-simulating phantoms containing MnTPPS or intact human
erythrocytes as absorbers and Intralipid as scatterer. Mean errors in recovery of μa and μs' were 9% and 19%,
respectively. In phantoms containing erythrocytes, hemoglobin oxygen saturation was recovered with mean error of
Using the MC model, we mapped the volumes sampled by particular spectroscopy fibers. For μa = 0.1 cm-1 and μs' = 20cm-1, 49% of photon packets detected at the fiber adjacent to the source sampled a radius further than 5 mm from the probe, while 24% of photon packets sampled further than 7.5 mm. When μs' was reduced to 10 cm-1, 54% of photon packets traversed a radius greater than 5 mm from the probe and 29% sampled further than 7.5 mm. Changing the value of μa to 0.2 cm-1 did not have an effect on the sampled volume.
We also provide a new probe design that aims to improve upon the accuracy of the current probe by incorporating a
wider range of source-detector separations.