Angular distribution of quasi-ballistic light measured through turbid media using angular domain optical imaging

摘要

We experimentally characterized the angular distribution and proportion of minimally deviated quasi-ballistic photons versus multiply scattered photons in a turbid medium. The study examined the angular distribution of photons propagating through and exiting the highly scattering medium over a narrow range about the axis of a collimated light source in trans-illumination mode. The measurements were made using an angular domain imaging system that employed one of three silicon micro-machined arrays of micro-tunnels each with a different range of acceptance angles. The balance between quasi-ballistic photons and unwanted multiply scattered photons accepted by the micro-machined angular filters was measured in order to determine the optimum range of acceptance angles for the system. The experiments were performed in tissue mimicking phantoms using a 1-cm thick optical cell with 0.7% Intralipid™ and an 808 nm diode laser. A maximum spatial resolution and contrast of 150 μm and 6% were observed by selecting the proper acceptance angle, respectively. Image contrast was improved further (about 10 times) by subtraction of the background signal representative of the multiply scattered photons by inserting an optical wedge into the light path. The measurements indicated that the highest spatial resolution and contrast for trans-illumination images through the sample were obtained for the angular filter with an angular acceptance range of 0.4° to 0.5°. It was concluded that this range of acceptance angles was small enough to attenuate the majority of multiply scattered photons, but still accept a significant proportion of quasi-ballistic photons, thereby optimizing both image resolution and contrast.