Time Resolved Systems
Initial studies have focused on measuring the attenuation of an absorbing phantom made from solid diffusing resin immersed in diffusing intralipid and ink matching fluid. Those studies have found that distributed diffuse objects with complex shapes can be successfully imaged using early photons and appropriate forward models to describe the propagation of the early photons in the diffusive medium.
The above schematic describes an advanced time-resolved small animal imaging system based on an ultrafast Ti:Sapphire laser and a gated image-intensified CCD camera with 200 ps temporal resolution. Ultrashort laser pulses are transmitted through the diffusing medium of interest, and only the earliest photons to be emitted from the opposite side of the chamber are collected by the gated CCD. These early photons have experienced the fewest scattering events as they traversed the chamber, and as such maintain higher spatial information than do photons collected with a standard cw imaging system.
Shown above are reconstructions of an absorbing phantom of a triangle and square in cross section. On the left is an image obtained using the inverse Radon transform commonly used in x-ray CT. An admittedly simplified photon propagation theory, it still does quite well at reconstructing the objects. On the right is an image reconstructed using a more physical model that gives greater width to the photon beams as they propagate through the medium. Work is underway to define better forward models and to extend this technique to the imaging of fluorescent phantoms.