Biomedical applications such as cell tracking and angiography require the detection of low concentrations of superparamagnetic iron oxide nanoparticles (SPIOs) for imaging purposes. Magnetic particle imaging (MPI) is a new technology which enables the quantitative and time-resolved localization of SPIO distributions. However, the minimum concentration at which the SPIOs can be reconstructed with a suitable quality still remains to be investigated. In this work we examine the background signals in raw data that were measured without any SPIOs in the scanner tube. We show that a background subtraction in combination with a frequency cutoff for the dynamic part of the background signal lowers the detection limit for SPIOs in MPI up to a factor of ten. In-vivo mouse experiments show that for early time points from when the tracer enters the vena cava a reconstructed image of sufficient quality can only be obtained when a background subtraction is performed.
