Evaluation of Spatial Variations in the Time and Frequency Dependence of Imaging Operators for Diffusion Tomography

Employing the framework of a perturbation model for optical diffusion tomography, the sensitivity and selectivity attainable from optical time- and frequency-domain, including phased array, measurements, are compared and contrasted. Monte Carlo simulations were used to calculate impulse-response functions in the interior of several homogeneous media. From the results, the impact on detected light due to small localized changes in absorption cross-section were computed. A feature unique to the frequency domain is the ability to qualitatively modify the depth profile of the weight amplitude by employing several sources in a phased array. In the case of single-source transmission measurements, a time-resolved measurement with a short integration time leads to enhanced ability to resolve deep-lying structures, by increasing the weight in deep regions relative to those near the surface. In contrast, as the source modulation frequency is increased in a frequency-domain measurement, the weight amplitude drops off most rapidly in regions farthest from the boundaries, and more slowly in more superficial regions. The significance of these findings for perturbation-based image recovery schemes is discussed.