The macroscopic sources of contrast in a relatively low numerical aperture confocal imaging system were computed using a Monte Carlo model, and found to originate primarily from mismatches in index of refraction. Comparison of the model with measurements of the background signals from scattering tissue phantoms illustrates the dependence of the confocal signal on the scattering phase function of the medium.
The FDTD method was used to study the scattering properties of inhomogeneous cells. The scattering properties of cells were found to be highly dependent on cell morphology, particularly small organelles and inhomogeneities within the nucleus. The phase function for most cells cannot be adequately described by a single parameter, such as g in the Henyey Greenstein approximation. However, a three-term summation of exponentials provides an accurate fit to the cell scattering pattern.
Measurements of the scattering properties of cell suspensions compared favorably with the FDTD predictions. Both the measurements and the FDTD model were used to present a possible explanation for the acetowhitening effect used in colposcopy.
The effects of an inhomogeneous cell on the propagation of a focused Gaussian beam were examined using the FDTD model, and used as a possible explanation for the loss of lateral resolution in confocal imaging. Comparison of the beam spreading due to tissue phantoms and cells indicates that polystyrene based tissue phantoms may not be accurate models for tissue in high resolution imaging applications.