Abstract
A Monte Carlo model has been developed for optical coherence tomography (OCT). A geometrical optics implementation of the OCT probe with low-coherence interferometric detection was combined with three-dimensional stochastic Monte Carlo modelling of photon propagation in the homogeneous sample medium. Optical properties of the sample were selected to simulate intralipid and blood, representing moderately ( g = 0.7) and highly ( g = 0.99) anisotropic scattering respectively. For shallow optical depths in simulated intralipid (<3 scattering mean free path (mfp) units), the number of detected backscattered photons followed the extinction-single-backscatter model, and OCT was found to detect only minimally scattered photons. Within this depth range the backscatter positions of detected photons corresponded well with the nominal focus position of the probe. For propagation to deeper positions in intralipid, localization of backscattering was quickly lost due to detection of stray photons, and the number of detected photons remained constant with increasing depth in the non-absorbing medium. For strongly forward-directed scattering in simulated blood, the number of detected photons approached the extinction-single-backscatter model only for very shallow depths (<2 mfp units). However, backscattering positions for detected photons correlated well with the nominal focus position of the probe even for optical depths greater than 40 mfp units.
Highlights
Optical coherence tomography (OCT) is a point-scanning method which permits imaging in highly turbid media and yields high-resolution three-dimensional backscatter information from depths down to 1–2 mm in biological tissues
The stochastic Monte Carlo model for photon propagation within the medium was combined with a geometrical optics model of the OCT probe, and interferometric detection, which was implemented according to the antenna theorem of a heterodyne receiver (Schmitt et al 1994b, Siegman 1966)
This high selectivity translates into very low photon yields in the Monte Carlo simulation
Summary
Optical coherence tomography (OCT) is a point-scanning method which permits imaging in highly turbid media and yields high-resolution three-dimensional backscatter information from depths down to 1–2 mm in biological tissues (reviewed by Fercher 1996). Multiple scattering effects have been included into such analytical models (Yadlowsky et al 1995b), but the models are still of limited value for comparison with experiment since only simple geometries can be considered. Monte Carlo simulation provides a method for implementing realistic models of the experimental situation based on specified geometries, characteristic parameters of source and detector, as well as full stochastic modelling of light propagation in the scattering medium to be probed. The OCT detector was modelled with low-coherence interferometric path length gating and confocal selectivity determined by the antenna theorem for a heterodyne receiver (Siegman 1966, Schmitt et al 1994b) By this theorem the effective size of the OCT detector is limited to twice the diameter of an ideal confocal detector.
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