Effect of tissue heterogeneity on detectability of optical signals

Vascularization (hemoglobin concentration) is the main intrinsic contrast in near infrared optics and may be considered as the background heterogeneity or biological noise. This noise causes complexity of the assessment of boundary measurements and uncertainty of the detectability of an optical anomaly in the medium. We have investigated the influence of this biological noise on boundary measurements with its varying degrees. We have studied an infinite-slab medium hosting an absorbing anomaly in transmission mode. In order to model this optical heterogeneity in the volume of interest we have employed a spatially random distribution pattern of absorption and observed that the amplitude of measured signals at air-tissue boundary is even decreased by 26% for low contrast heterogeneity having 40% volume fraction of heterogeneity. Furthermore optical signal measured along the detectors has been flattened as the amount of heterogeneity in the medium increases. As a measure of flatness, we have defined the contrast of the measured signals along the detectors and observed that the contrast decreases approximately linearly with an increasing heterogeneity. It is 0.79 for the heteroueneity having 40% volume fraction (VF), 0.82 for VF = %20 whereas it is 0.85 for VF = 0% suggesting that as heterogeneity (aka biological noise) increases, the detectability of an inhomogeneity diminishes.