Abstract Unfortunately, ~20% of kidneys donated for transplant are discarded due to short storage viability (~24-36 h). Cryopreservation decreases storage temperatures below -140oC, enabling storage of kidneys for months to years. To re-warm cryopreserved kidneys without ice formation and cracking, fast and even warming is needed. For this, iron oxide nanoparticles (IONPs) can be perfused into them prior to cooling, then radiofrequency waves can be applied to rapidly rotate their magnetic moments, causing local heating around IONPs and kidney re- warming. Despite initial success of this method in rat kidneys, the complex behavior of IONPs (uneven particle distribution, aggregation, etc.) limits reproducibility and is a major bottleneck in its translation to human organs. Currently there is no low-cost method to nondestructively track IONPs in organs across concentrations of interest. Thus, there is a need for precise monitoring of IONP concentrations (1) after perfusion, to ensure even IONP distribution and therefore even kidney heating and viability, and (2) prior to transplant, to prevent potential IONP- related iron toxicity post-transplant. To address this, our lab has developed a low-cost, longitudinally detected electron paramagnetic resonance (LOD-EPR) system that can directly detect electrons in IONPs. LOD-EPR is sensitive to IONP concentration in solution and in IONP-loaded kidney biopsies with improved accuracy in tissue compared to nuclear magnetic resonance (NMR) based on data under review. This project aims to develop LOD-EPR into an IONP quantification and imaging system for cryopreserved rat kidneys and compare its accuracy to similar techniques such as magnetic particle imaging, and micro-CT. Aim 1 will improve the sensitivity of LOD-EPR through hardware changes on receive and transmit and increase the bore size to fit whole rat kidneys. Aim 2 will develop the LOD-EPR system into an imaging system primarily by implementing permanent magnet-based linear gradients. Its IONP quantification accuracy and imaging resolution will be compared to magnetic particle imaging and micro-CT in IONP-perfused whole rat kidneys. By enabling IONP monitoring during the cryopreservation process, IONP synthesis, kidney perfusion, and radiofrequency heating parameters can be fully optimized for increased kidney viability during re-warming. This will ultimately expedite development of kidney cryopreservation to increase transplant availability.