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To use contrast based on longitudinal relaxation times (T1) or rates (R1) to quantify the biodistribution of iron oxide nanoparticles (IONPs), which are of interest for hyperthermia therapy, cell targeting, and drug delivery, within primary clearance organs.Mesoporous silica-coated IONPs (msIONPs) were intravenously injected into 15 naïve mice. Imaging and mapping of the longitudinal relaxation rate constant at 24 h or 1 week postinjection were performed with an echoless pulse sequence (SWIFT). Alternating magnetic field heating measurements were also performed on ex vivo tissues.Signal enhancement from positive T1 contrast caused by IONPs was observed and quantified in vivo in liver, spleen, and kidney at concentrations up to 3.2 mg Fe/(g tissue wt.) (61 mM Fe). In most cases, each organ had a linear correlation between the R1 and the tissue iron concentration despite variations in intra-organ distribution, degradation, and IONP surface charge. Linear correlation between R1 and volumetric SAR in hyperthermia therapy was observed.The linear dependence between R1 and tissue iron concentration in major organs allows quantitative monitoring of IONP biodistribution in a dosage range relevant to magnetic hyperthermia applications, which falls into the concentration gap between CT and conventional MRI techniques.