Optical tomographic image reconstruction from ultrafast time-sliced transmission measurements

Optical imaging and localization of objects inside a highly scattering medi um, such as a tumor in the breast, is a challenging problem with many pract ical applications. Conventional imaging methods generally provide only two- dimensional (2-D) images of limited spatial resolution with little diagnost ic ability. Here we present an inversion algorithm that uses time-resolved transillumination measurements in the form of a sequence of picosecond-dura tion intensity patterns of transmitted ultrashort light pulses to reconstru ct three-dimensional (3-D) images of an absorbing object located inside a s lab of a highly scattering medium. The experimental arrangement used a 3-mm -diameter collimated beam of 800-nm, 150-fs, 1-kHz repetition rate light pu lses from a Ti:sapphire laser and amplifier system to illuminate one side o f the slab sample. An ultrafast gated intensified camera system that provid es a minimum FWHM gate width of 80 ps recorded the 2-D intensity patterns o f the light transmitted through the opposite side of the slab. The gate pos ition was varied in steps of 100 ps over a 5-ns range to obtain a sequence of 2-D transmitted. light intensity patterns of both less-scattered and mul tiple-scattered light for image reconstruction. The inversion algorithm is based on the diffusion approximation of the radiative transfer theory for p hoton transport in a turbid medium. It uses a Green's function perturbative approach under the Rytov approximation and combines a 2-D matrix inversion with a one-dimensional Fourier-transform inversion to achieve speedy 3-D i mage reconstruction. In addition to the lateral position, the method provid es information about the axial position of the object as well, whereas the 2-D reconstruction methods yield only lateral position. (C) 1999 Optical So ciety of America.