Superparamegnetic particles as MRI temperature contrast agent

A method of MRI temperature contrast developed by our group, which employs temperature dependent local field inhomogeneities due to a presence of micrometer sized magnetic particles and corresponding changes in image intensity (T₂* contrast), faces limitations due to particle size. For human applications, large particles cannot be used as they face problems with delivery and secretion. Smaller particles are required. However, spin transverse relaxation process for such particles is no longer governed by a static dephasing regime and T₂* temperature dependent inhomogeneity contributions vanish due to averaging by spin motion. We hypothesize that superparamagnetic particles may provide contrast mechanisms only if nuclear relaxation is temperature dependent. To test this hypothesis, we used Mn-Zn ferrite superparamagnetic particles with an average size of 7.8 ± 2.1 nm. Particles with a 2 mM concentration were embeded in agar gel for NMR and MRI measurements at temperature 5 - 50 ^oC. At 3.0 T, NMR results show that T₁ is temperature independent, while observed NMR linewidth drops from 130 Hz to 70 Hz and T₂ increases from 1.3 ms to 2.8 ms. Intensity of T₂ weighted MR images depends linearly on temperature, providing the contrasting mechanism necessary for temperature determination.