Physical properties of magnetic NWs and conventional spherical nanoparticles (nanospheres; NSs) preparations. a) TEM image showing the wormlike nanostructure. More than 80% of the particles have a contorted linear appearance with a hydrodynamic length of 50–80 nm. Scale bar is 30 nm. (Inset: atomic force microscopy image showing the elongated shape. Scale bar is 100 nm). b) Magnetization curves for NWs, NSs, and Feridex. c) T2 relaxation rates as a function of iron concentration (mm Fe) for NWs, NSs, and Feridex. d) T2-weighted MR images of NWs, NSs, and Feridex with different concentrations.