To validate the application of the instrument in clinical problems related to a localized change in absorption properties, such as functional brain imaging or muscle metabolism monitoring, we performed measurements applying the nEUROPT protocol.22 In particular, we assessed the “sensitivity,” expressed by the contrast as a function of depth, and the “spatial resolution,” based on a lateral scan. The nEUROPT tests were taken on an inhomogeneous solid phantom with switchable black polyvinyl chloride inclusions of different dimensions, which mimics a realistic absorption perturbation.23 The inclusions (black cylinders with diameter and height of 3, 4, 5, and 7 mm, corresponding to an equivalent absorption variation of 0.05, 0.10, 0.17, and , for a volume of , respectively) were placed inside a movable cylinder (solid epoxy resin with same optical properties of the host phantom). Both lateral (Y-scan, measuring surface parallel to the inclusion movement) and depth scans (Z-scan, measuring surface perpendicular to the inclusion movement) were performed in 1-mm steps. The time-dependent contrast as a function of the inclusion position , at 690 nm, was calculated as Display Formulawhere represents the photon counts in the time windows located at time delay , with respect to the IRF peak, and width of 350 ps; is either the lateral or depth coordinate in the Y- or Z-scan, respectively; and is the photons’ count in the same time window with the inclusion at the reference position (i.e., outside the sensitivity region of the technique). The contrast in the time windows at short temporal delays (early gates) represents early detected photons, which traveled principally on the surface of the samples (UP layer). Conversely, with longest delays (late gates), we are considering late detected photons, which probed the sample more in depth (DW layer).