Abstract
Optical imaging and localization of objects inside a highly scattering
medium, such as a tumor in the breast, is a challenging problem with
many practical applications. Conventional imaging methods generally
provide only two-dimensional (2-D) images of limited spatial resolution
with little diagnostic ability. Here we present an inversion algorithm
that uses time-resolved transillumination measurements in the form
of a sequence of picosecond-duration intensity patterns of transmitted
ultrashort light pulses to reconstruct three-dimensional (3-D) images
of an absorbing object located inside a slab 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 pulses from a
Ti:sapphire laser and amplifier system to illuminate one side of
the slab sample. An ultrafast gated intensified camera system that
provides a minimum FWHM gate width of 80 ps recorded the 2-D intensity
patterns of the light transmitted through the opposite side of the
slab. The gate position 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 multiple-scattered light for image reconstruction.
The inversion algorithm is based on the diffusion approximation of
the radiative transfer theory for photon 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 image reconstruction.
In addition to the lateral position, the method provides information
about the axial position of the object as well, whereas the 2-D reconstruction
methods yield only lateral position. (C) 1999 Optical Society of
America.
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