  Magnetic Particle Imaging (MPI) has been shown to provide remarkable contrast
for imaging applications such as angiography, stem cell tracking, and cancer
imaging. Recently, there is growing interest in the functional imaging
capabilities of MPI, where color MPI techniques have explored separating
different nanoparticles, which could potentially be used to distinguish
nanoparticles in different states or environments. Viscosity mapping is a
promising functional imaging application for MPI, as increased viscosity levels
in vivo have been associated with numerous diseases such as hypertension,
atherosclerosis, and cancer. In this work, we propose a viscosity mapping
technique for MPI through the estimation of the relaxation time constant of the
nanoparticles. Importantly, the proposed time constant estimation scheme does
not require any prior information regarding the nanoparticles. We validate this
method with extensive experiments in an in-house magnetic particle spectroscopy
(MPS) setup at four different frequencies (between 250 Hz and 10.8 kHz) and at
three different field strengths (between 5 mT and 15 mT) for viscosities
ranging between 0.89 mPa.s to 15.33 mPa.s. Our results demonstrate the
viscosity mapping ability of MPI in the biologically relevant viscosity range.
