NMR relaxation and magnetic properties of superparamagnetic nanoworms

Abstract

Maghemite particles are used as T₂ contrast agents for magnetic resonance imaging, especially for molecular and cellular imaging. Linear clusters of particles - called nanoworms - were recently developed to enhance the targeting efficiency. In this work, the magnetic and NMR relaxation properties of these nanoworms are studied at multiple magnetic fields. After the usual saturation at 0.5 T, the magnetization of the worms is still increasing, which results in an appreciable increase of the transverse relaxivity at high magnetic fields. The obtained relaxivities are typical of superparamagnetic particles of iron oxide (SPIOs). The transverse relaxation of the worms is clearly more efficient than for the isolated grains, which is confirmed by computer simulations. At high field, the longitudinal relaxation of the worms is less pronounced than for the grains, as expected for SPIOs. The nanoworms thus constitute a promising T₂ agent for cellular and molecular imaging.

Figure 1.

Transmission electron microscopy of the nanoworms, illustrating their elongated shape. The bar represents 50 nm.

Figure 2.

SAXS of iron oxide nanoworms. The simulated scattering form factor of spherical particles with a diameter of 5 nm is presented in blue. The two observed power laws are shown in red.

Figure 3.

Magnetometric curve of nanoworms obtained at 37°C between –8.5 and 8.5 T. The inset shows the residuals of the fittings of the Langevin function and equation (1) to the data.

Figure 4.

NMRD profiles of an aqueous solution of magnetic nanoworms obtained at 5, 20 and 37°C. The solid line is a fitting of the Roch model (15) to the data at 37°C. The dashed line represents the predicted theoretical NMRD curve for isolated particles, as described in the text.