Characterization of commercial iron oxide clusters with high transverse relaxivity

Abstract

Clusters of iron oxide particles are excellent contrast agents for molecular and cellular MRI because of their large effect on water proton transverse relaxation. Their efficiency depends on the magnetization of the particles, the size of the cluster and the intra-aggregate volume fraction occupied by the iron oxide particles inside the cluster. After optimization of these different parameters, a relaxivity of ∼ 750 s−1mM−1 can theoretically be achieved. Polymag࣪™ clusters are initially intended for magnetofection but are here shown to present excellent relaxation properties. Magnetometry shows that the particles constituting the clusters have a high saturation magnetization (Mv = 371000 A/m) and present a broad log-normal size distribution (d0,c = 4.9 and σ = 0.53). The clusters have a hydrodynamic diameter of 180 nm and also present a large polydispersity index (PDI = 0.15). The transverse relaxivity of the clusters is remarkably high: r2 = 470 s−1mM−1 at 1.41 T and 37°C. As their relaxation properties are independent of temperature and interecho time, the clusters should be mainly in the Static Dephasing Regime (SDR), even if some large clusters are probably closer to the Partial Refocusing Model (PRM), as suggested by the large estimated ΔωtτD ∼ 55. Since they present a large magnetic moment, the Polymag™ clusters are sensitive to the magnetic field of the NMR device, in which they probably form linear chains: a reversible increase of the relaxation times is observed after the insertion in the NMR system. They could be good candidates for cellular imaging since they should be easily internalized in cells using the magnetofection protocol and then easily detected by MRI thanks to their high relaxivity.

Keywords

• Iron oxide particles
• Cluster
• SPIO
• Contrast agent
• Relaxivity