Temperature dependent magnetorelaxometry of magnetic nanoparticle ensembles.

Magnetorelaxometry imaging (MRXI) is a non-invasive, quantitative, imaging technique for magnetic nanoparticles (MNPs). The image resolution of this technique significantly depends on the relaxation amplitude (ΔB). For this work, we measured the room temperature (299 K) relaxation signals of eight commercial MNP sample systems with different magnetic properties, in both fluid and immobilized states, in order to select the most suitable sample for a particular MRXI setting. Additionally, the effect of elevated temperatures (up to hyperthermia temperature, 335 K) on the relaxation signals of four different MNP systems (Synomag, Perimag, BNF and Nanomag), in both states were investigated. The ΔB values of fluid samples significantly decreased with increasing temperature, and the behaviour for immobilized samples depended on their blocking temperature (TB). For samples with TB < 299 K, ΔB also decreased with increasing temperature. Whereas for samples with TB > 299 K, the opposite behaviour was observed. These results are beneficial to improving the image resolution in MRXI and show, among the investigated systems, and for our setup, Synomag is the best candidate for future in vitro and in vivo studies. This is due to its consistently high ΔB between 299 K and 335 K in both states. Our findings demonstrate the feasibility of temperature imaging by MRXI.