Simulation of laser tomoscopy in a heterogeneous biological medium

A Monte Carlo model has been developed to study the propagation of an ultrashort light pulse through a heterogeneous thick biological specimen. A circular blood vessel is moved within a tissular slab to simulate biological specimen scanning using a picosecond laser source and a collimated ultrafast multichannel opticl shutter. Features of the transmitted light are computed for each position of the blood vessel. The computer program gives an account of the transmitted photons, the flight time which does not exceed straightforward crossing time plus time gate of known duration. A small blood vessel (radius R=2 mm) placed in a 40 mm thick slab is easily located when a time gate of 10 ps duration is employed. Such a time gate also allows the detection of a middle-sized vessel (R=4 mm) embedded in a thicker sample (80 mm). The contrast computed for the transmittance profile is greatly improved when a time gate is used. In addition, shifting of the blood vessel towards the unilluminated side of the sample decreases the contrast. We demonstrate that the time selection process may provide a substantial improvement to the laser tomoscopy technique when used for imaging biological media.