Spatial localization using interfering photon density waves: contrast enhancement and limitations

摘要

Abstract: In most of the optical methods proposed for imaging an absorbing object embedded in a turbid medium, data is collected using a single source and detector scanned mechanically across the surface of the medium. In this study we exploited destructive interference of diffusive photon-density waves originating from two sources to localize one absorbing (or fluorescent) object in a scattering medium. A frequency-domain instrument is described for scanning several laser-beam spots across the surface of a turbid medium using 1D (or 2D) acousto-optical deflectors and detecting the signals with a gated, intensified CCD camera at a modulation frequency of 246 MHz. The localization of multiple objects arranged in the form of a spatial grating was investigated theoretically with an analytic model by combining the magnitude and phase of the signals detected from the objects. A novel grating pattern comprising several destructively interfering lines, which acts as spatial frequency filter, is discussed. The results were compared with those obtained using a single-source/single-detector scanning configuration. We show that the FWHM (full-width half-maximum) of the signal detected using the single-source/single-detector configuration establishes a limiting spatial scale over which multiple objects can be resolved. Beyond this limit the resolution can only be increased under severe penalty of contrast and signal loss. !18