  Diffuse optical tomography (DOT) use near-infrared light for imaging optical
properties of biological tissues. Time-domain DOT systems use pulsed lasers and
measure time-varying temporal point spread function (TPSF), carrying
information from both superficial and deep layers of imaged target. In this
work, feasibility of nanosecond scale light pulses as sources for time-domain
DOT is studied. Nanosecond sources enable using relatively robust measurement
setups with standard analog-to-digital converter waveform digitizers, such as
digital oscilloscopes. However, this type of systems have variations in source
pulses and limited temporal sampling, that could limit their usage. In this
work, these different aspects and possible limitations were studied with
simulations and experiments.
  Simulations showed that information carried by time-domain data of diffuse
medium is on low frequencies. This enables usage of relatively slow response
time measurement electronics, and image processing using Fourier-transformed
time-domain data. Furthermore, the temporal sampling in measurements needs to
be high enough to capture the TPSF, but this rate can be achieved with standard
digital oscilloscopes. It was shown that, although variations in light pulses
of nanosecond lasers are larger than those of picosecond sources, they do not
affect significantly on image quality. In this work, a prototype time-domain
DOT experimental system utilising a high-energy nanosecond laser was
constructed. The system consisted of a nanosecond Nd-YAG laser combined with
optical parametric oscillator for light input, and avalanche photodetector and
high-bandwidth oscilloscope for TPSF measurements. The system was used in both
absolute and difference imaging of two phantoms. The experiments verified that
both absorbing and scattering objects can be reconstructed with time-domain DOT
using a nanosecond laser.
