  It has been shown that with the use of ultra-wideband (UWB) electromagnetic
signal and time of arrival (ToA) principle, it is possible to locate medical
implants given the permittivity distribution of the body. We propose a new
imaging modality using the reverse process to acquire permittivity
distributions as a surrogate of human anatomy. In the proposed systems, the
locations of the signal source, receiver, and signal shapes are assumed to be
known exactly. The measured data is recorded as the time it takes for the
signal to travel from the signal source to the signal receiver. The
finite-difference-time-domain (FDTD) method is used for the modeling of signal
propagation within the phantom, which is used for both simulation and image
reconstruction. Image reconstruction is achieved using linear regression on the
training pairs, which includes randomly generated images and its corresponding
arrival times generated using the FDTD approach. The linear weights of the
training images are generated to minimize the difference between the arrival
time of the reconstruction image and the measured arrival time. A simulation
study using UWB signal with the central frequency of 300 MHz and the
Shepp-Logan phantom was carried out. Ten-picosecond timing resolution is used
for the simulation and image reconstruction. The quantitative difference
between the arrival times of the phantom and the reconstructed image reduced
with an increased iteration number. The quantitative error of the reconstructed
image reached below 10% after 900 iterations, and 8.4% after 1200 iterations.
With additional post-smoothing to suppress the introduced noise pattern through
reconstruction, 6.5% error was achieved. In this paper, an approach that
utilizes the ToA principle to achieve transmission imaging with radio waves is
proposed and validated using a simulation study.
