Effect of Hollow Structures on T₁ and T₂ Relaxivities and Their Application in Accurate Tumor Imaging

Great progress in precisely controlling the structures of magnetic nanoparticles has been made to investigate structure–relaxivity relationships in recent years. However, the investigation of the influence of hollow structures with unique interior structures on the relaxation rate of magnetic nanoparticles is rare. Herein, we obtained a series of hollow manganese-doped iron oxide nanoparticles (MnIONs) with different void and dopant ratios through a controllable etch process and systemically investigated the influence of hollow structures on the T1/T2 relaxation rate. Due to the increased surface-to-volume (S/V) ratio, hollow MnIO nanoparticles (HMNs) show increased T1 relaxivity compared to solid MnIONs. The T1 relaxivities of HMNs with different void ratios are proportion to the number of exposed magnetic ions and electronic relaxation time value, which are determined by the S/V ratio and dopant level. More importantly, HMNs exhibit reduced saturated magnetization values with increased T2 relaxivities compared to solid MnIONs. The elevated T2 relaxivities of HMNs are attributed to the increased number of magnetic cores per unit volume and magnetic field inhomogeneity induced by hollow structures. These parameters are highly dependent on their void ratios, thus eventually determining their T2 relaxivities. In vivo studies demonstrate that HMNs with relatively high T1 or T2 relaxivity show superior sensitivity in tumor detection to traditional T1 or T2 contrast agents. This work summarized the effects and mechanisms of hollow structures on the T1 and T2 relaxation rates of magnetic nanoparticles, providing examples in vivo for the design of excellent T1 or T2 contrast agents (CAs) for early cancer diagnosis.