Imaging of turbid media using trajectory filter methods

摘要

Optical imaging through biological tissues and other scattering media is challenging, as the scattered light creates an extremely high background noise level that makes it difficult to detect objects that are embedded within the media. This thesis examines a relatively unexplored method of separating scattered light from unscattered light that has application to optical imaging through turbid media. The method creates an optical filter that blocks photons based upon their exit trajectory direction. Such a trajectory filter can be used with a collimated beam that transmissively illuminates a scattering medium to create an imaging system in which a shadowgram is formed from those photons that pass through the filter and have a trajectory close to that of the collimated beam. Experiments have shown that such a system is effective up to measured optical depths of 18 to 21 and scattering ratios of 1E8 to 1E9 using both coherent and incoherent sources. A micromachined linear array of 50 µm x 10 mm collimating holes was developed earlier as a photon trajectory filter and was used to successfully image through media in which the ratio of scattered to unscattered light is extremely high (1E7). These results are much better than simple theory would predict. This thesis provides a theoretical basis for the trajectory filter system to allow its performance to be characterized and compared against other optical imaging methods, such as time-domain imaging. Using Monte Carlo simulations, it is found that the trajectory filter method is more effective than pathlength-based methods for imaging through turbid media with moderate levels of scattering, up to ~20 optical depths, and that it can be combined with other imaging methods to further improve contrast. Advantages of the trajectory filter method include coherence and wavelength invariance and the ability to perform either wide beam, full-field or narrow beam, scanned imaging. Experimental results are presented for laser and incoherent beams using two types of trajectory filters: spatiofrequency and linear collimating hole array. It is found that the trajectory filter method offers a viable means of transmissively imaging through moderately scattering media at optical and near infrared wavelengths.