Towards optimized MRI contrast agents for implant engineering: Clustering and immobilization effects of magnetic nanoparticles

Abstract

Abstract Magnetic nanoparticles (MNP) have been successfully used as additives for the fabrication of implants such as hernia implants or vascular grafts in order to enable their in-vivo visualization with magnetic resonance imaging (MRI). However, functionality and long term stability of such implants were not quantitatively assessed until now. Reliable assessment of functionality is related not only to the determination of MNP concentration, but also to the impact of aggregation and immobilization of MNP inside the implant material on the MRI signal. In this regard, novel models must be developed which describe the relation between proton relaxation and both MNP clustering and mobility of MNP inside the implants. In this study, we experimentally quantify the transverse relaxation dependence on MNP size and MNP clusters, confirming theoretical descriptions. We identify three MNP size ranges for which different proton relaxation trends occur: One for which relaxivity increases with size (up to approx. 75 nm), a second for which relaxivity is constant (from 75 nm to 130 nm) and a third for which relaxivity decreases (from 130 nm to 220 nm). Further, we describe the impact of gradual MNP immobilization in agarose gels on relaxivity for three MNP types representing either of the identified size ranges. For all MNP types, we observe an increase of relaxivity with agarose content up to an MNP type specific maximum value. The relative rise of relaxivity is higher for MNP with larger sizes. The highest increase of the transverse relaxivity from 240 mM-1s−1 to 1000 mM-1 s−1 is achieved for MNP clusters after immobilization in a gel with 7%(w/w) agarose. The effects of MNP clustering and immobilization on relaxivity are valuable information for the engineering of implants with different contrast properties in MRI. Further, the relation between MNP immobilization and relaxivity values may serve as a basis for future non-invasive assessment of changes in implant functionality by MRI measurements.