T1 relaxometry or EPR signal intensity – Which is best for quantifying iron oxide nanoparticles in tissues non-destructively?

Motivation: Currently there is no low-cost method to nondestructively quantify iron oxide nanoparticles (IONPs) in tissue across a wide concentration range (0.05-100 mg Fe/mL). Goal(s): Our lab has developed a low-cost, LOngitudinally Detected Electron Paramagnetic Resonance (LOD-EPR) system. This work aims to evaluate LOD-EPR IONP quantification accuracy. Approach: We compare IONP Fe quantification accuracy of R1 (=1/T1) from MR relaxometry versus LOD-EPR signal in solution and IONP-perfused rat kidney sections used in cryopreservation. Results: LOD-EPR signal vs Fe concentration is linear in 0.05-10 mg Fe/mL IONP solutions and in IONP-perfused tissue, whereas R1 vs Fe concentration is linear in solution but not in tissue. Impact: Accurate quantification of IONPs in tissues at room temperature can be done using low-cost, benchtop LOD-EPR. Our primary application is in IONP rewarming of cryopreserved organs, however other applications such as dosimetry and oxygen sensing should also be possible.