Surface coatings are important components of Magnetic Particle Imaging (MPI) tracers - they preserve their key properties responsible for optimum tracer performance in physiological environments. , surface coatings form a physical barrier between the hydrophobic SPION cores and the physiological environment, and their design dictates the blood half-life and biodistribution of MPI tracers. Here we show the effect of tuning poly(ethylene glycol) (PEG)-based surface coatings on both and (mouse model) MPI performance of SPIONs. Our results showed that varying PEG molecular weight had a profound impact on colloidal stability, characterized using Dynamic Light Scattering (DLS), and the response of SPIONs, measured in a 25 kHz/20 mTμ Magnetic Particle Spectrometer (MPS). Increasing PEG molecular weight from 5 kDa to 20 kDa preserved colloidal stability and response of ~25 nm SPIONs - the optimum core diameter for MPI - in serum-rich cell culture medium for up to 24 hours. Furthermore, we compared the circulation time of SPIONs as a function of hydrodynamic diameter and showed that clustered SPIONs can adversely affect blood half-life; critically, SPIONs with clusters had 5 times shorter blood half-life than individually coated SPIONs. We anticipate that the development of MPI SPION tracers with long blood half-lives have potential not only in vascular imaging applications, but also enable opportunities in molecular targeting and imaging - a critical step towards early cancer detection using the new MPI modality.