MR imaging differentiation of Fe2+ and Fe3+ based on relaxation and magnetic susceptibility properties.

PURPOSE: The aim of this study is to evaluate the MR imaging behavior of ferrous (Fe2+) and ferric (Fe3+) iron ions in order to develop a noninvasive technique to quantitatively differentiate between both forms of iron.
METHODS: MRI was performed at 3 T in a phantom consisting of 21 samples with different concentrations of ferrous and ferric chloride solutions (between 0 and 10 mmol/L). A multi-echo spoiled gradient-echo pulse sequence with eight echoes was used for both T 2* and quantitative susceptibility measurements. The transverse relaxation rate, R 2* = 1/T 2*, was determined by nonlinear exponential fitting based on the mean signals in each sample. The susceptibilities, χ, of the samples were calculated after phase unwrapping and background field removal by fitting the spatial convolution of a unit dipole response to the measured internal field map. Relaxation rate changes, ΔR 2*(c Fe), and susceptibility changes, Δχ(c Fe), their linear slopes, as well as the ratios ΔR 2*(c Fe) / Δχ(c Fe) were determined for all concentrations.
RESULTS: The linear slopes of the relaxation rate were (12.5 ± 0.4) s-1/(mmol/L) for Fe3+ and (0.77 ± 0.09) s-1/(mmol/L) for Fe2+ (significantly different, z test P < 0.0001). The linear slopes of the susceptibility were (0.088 ± 0.003) ppm/(mmol/L) for Fe3+ and (0.079 ± 0.006) ppm/(mmol/L) for Fe2+. The individual ratios ΔR 2*/Δχ were greater than 40 s-1/ppm for all samples with ferric solution and lower than 20 s-1/ppm for all but one of the samples with ferrous solution.
CONCLUSION: Ferrous and ferric iron ions show significantly different relaxation behaviors in MRI but similar susceptibility patterns. These properties can be used to differentiate ferrous and ferric samples.