Accuracy and noise in optical Doppler tomography studied by Monte Carlo simulation.

A Monte Carlo model has been developed for optical Doppler tomography (ODT) within the framework of a model for optical coherence tomography (OCT). A phantom situation represented by blood flowing in a horizontal 100 microm diameter vessel placed at 250 microm axial depth in 2% intralipid solution was implemented for the Monte Carlo simulation, and a similar configuration used for experimental ODT measurements in the laboratory. Simulated depth profiles through the centre of the vessel of average Doppler frequency demonstrated an accuracy of 3-4% deviation in frequency values and position localization of flow borders, compared with true values. Stochastic Doppler frequency noise was experimentally observed as a shadowing in regions underneath the vessel and also seen in simulated Doppler frequency depth profiles. By Monte Carlo simulation, this Doppler noise was shown to represent a nearly constant level over an investigated 100 microm interval of depth underneath the vessel. The noise level was essentially independent of the numerical aperture of the detector and angle between the flow velocity and the direction of observation, as long as this angle was larger than 60 degrees. Since this angle determines the magnitude of the Doppler frequency for backscattering from the flow region, this means that the signal-to-noise ratio between Doppler signal from the flow region to Doppler noise from regions underneath the flow is improved by decreasing the angle between the flow direction and direction of observation. Doppler noise values from Monte Carlo simulations were compared with values from statistical analysis.