  Magnetic Particle Imaging (MPI) is a novel imaging modality with important
applications such as angiography, stem cell tracking, and cancer imaging.
Recently, there have been efforts to increase the functionality of MPI via
multi-color imaging methods that can distinguish the responses of different
nanoparticles, or nanoparticles in different environmental conditions. The
proposed techniques typically rely on extensive calibrations that capture the
differences in the harmonic responses of the nanoparticles. In this work, we
propose a method to directly estimate the relaxation time constant of the
nanoparticles from the MPI signal, which is then used to generate a multi-color
relaxation map. The technique is based on the underlying mirror symmetry of the
adiabatic MPI signal when the same region is scanned back and forth. We
validate the proposed method via extensive simulations, and via experiments on
our in-house Magnetic Particle Spectrometer (MPS) setup at 550 Hz and our
in-house MPI scanner at 9.7 kHz. Our results show that nanoparticles can be
successfully distinguished with the proposed technique, without any calibration
or prior knowledge about the nanoparticles.
