  Visualization of the in vivo spatial distribution of superparamagnetic iron
oxide nanoparticles (SPIONs) is crucial to biomedicine. Magnetic particle
imaging (MPI) is one of the most promising approaches for direct measurements
of the SPION distribution. In this paper, we systematically investigate a
single-harmonic-based narrowband MPI approach. Herein, only the 3rd harmonic at
15 kHz of the SPION signal induced in an excitation magnetic field of 5 kHz is
measured via a narrowband detection system for imaging during scanning a
field-free-point in a field of view. Experiments on spot and line phantoms are
performed to evaluate the spatial distribution by the assessment of the full
width at half maximum and modulation transfer function at different excitation
magnetic fields from 4 to 10 mT. Experimental results demonstrate that
reconstructed images have a spatial resolution of 1.6 and 1.5 mm for a gradient
field of 2.2 T/m and 4.4 T/m in x- and z-direction, respectively, at an
excitation magnetic field of 4 mT. In terms of line gap, two lines with a gap
of 0.5 mm are resolved. With increasing the excitation magnetic field to 10 mT,
the spatial resolution gets worse to 2.4 and 2.0 mm in x- and z-direction,
respectively. Moreover, the custom-built MPI scanner allows a limit of
detection of 53 microgram (Fe)/mL (500 ng Fe weight) using perimag SPIONs. In
addition, the excellent performance is demonstrated by imaging experiments on
an "emg" logo phantom. We believe that the proposed narrowband MPI approach is
a promising approach for SPION imaging.
