Nano-sized ferrite particles for magnetic resonance imaging thermometry

Recently, we reported the application of magnetic particles as temperature sensors for use in magnetic resonance imaging thermometry (tMRI). In this novel method, the brightness of the magnetic resonance (MR) image changes with temperature due to a temperature-dependent local magnetic field inhomogeneity caused by the dipolar field of the magnetic particles. Ferrites are new and promising compounds for tMRI applications because of their biocompatibility and because their magnetic properties can be varied by changing composition. Earlier studies used micrometer sized ferrite particles in a proof-of-concept demonstration. However, such large particles cannot be administered intravenously for in-vivo use. In this report, we establish the use of nanoscale ferrite particles as temperature sensors for tMRI. Scanning transmission electron microscopy and X-ray diffraction demonstrate the synthesis of 210 nm Co0.3Zn0.7Fe2O4 clusters comprised of 10-30 nm crystallites. Temperature-dependent magnetization measurements reveal a Curie temperature of around 275 K. We conducted nuclear magnetic resonance (NMR) and MRI studies of samples with different concentrations of ferrite nanoparticles suspended in agar gel. The relative MR image intensity shows a near-linear temperature dependence. At concentrations as low as 0.12 g/L these ferrite nanoparticles provide sufficient image contrast to determine temperature changes with accuracy of ±1.0 K at 310 K, bolstering the potential viability of this material for bio-medical applications.