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The deposition time was controlled to prepare Mn nanowires of different lengths and diameters on templates of anodic aluminum oxide (AAO) with different pore sizes. The surface of as-prepared Mn nanowires was modified with SiO 2 using the sol-gel method to improve their dispersion in aqueous solution. The effects of the diameter and length of the as-prepared Mn nanowires coated with SiO 2 on the relaxivity were investigated. It was found that the Mn@SiO 2 nanowires have smaller diameters and a higher longitudinal relaxivity ( r 1 ) with an increased length. Mn3@SiO 2 nanowires had the highest r 1 value of 5.8 mM -1 s -1 among the Mn@SiO 2 nanowires (Mn3 nanowires have a diameter of about 30 nm and a length of about 0.5 μm length). Additionally, the biocompatibility and in vivo imaging ability of the Mn3@SiO 2 nanowires were evaluated. The Mn3@SiO 2 nanowires had good cytotoxicity and biocompatibility, and the kidney of SD rats showed a positive enhancement effect during small animal imaging at 1.5 T. This study showed that the Mn3@SiO 2 nanowires could potentially become contrast agents (CAs) of longitudinal relaxation time ( T 1 ).