@@ ntroduction Magnetic resonance(MR)molecular imaging enhanced by superparamagnetic iron oxide(SPIO)nanoparticles is of great clinical and research interest for its potential to non．invasively monitor physiological processes in vivo．The low detection concentrations of SPIO nanoparticles compared to other MR contrast agents provide the opportunity to label cells with the particles and track them within a subject using MR imaging and attach labels to the particle surface to promote particle accumulation in tissues of interest(1)．While these contrast agents have conventionally been used for their ability to shorten T2 and T2*，a number of techniques have been proposed to generate increased signal from tissues containing the particles(2)．Howeve~many of these techniques rely on changes in the macroscopic magnetic field that may not be ideal for particle localization and quantification in vivo，especially in a continuous concentration gradient of SPIO nanoparticles．