Accurate T2* Quantification with Susceptibility Correction at 3.0 T

Purpose: Quantitative imaging by means of T2*-relaxometry is important in many high field MRI applications such as in BOLD contrast studies for oxygen mapping as well as for the determination of iron content in brain and liver. Also the sensitive detection of iron oxide particle-based contrast agents is becoming a growing application, especially within the scope of molecular imaging MR.

Methods: T2*-relaxometry for quantitative MR imaging at 3T is strongly hampered by large-scale field inhomogeneities ΔB₀ (susceptibility), which lead to signal losses and an overestimation of the relaxation rate R2*. To derive an accurate measurement of a voxel based T2*-map, a susceptibility correction was developed: ΔB₀ is derived from multislice T2*-relaxometry data and used as an initial value for an iterative optimization, by which the relaxation signal is corrected for each voxel. This correction leads to a significantly reduced influence of the local field variation and allows a quantification of the T2*-relaxometry data. The method was tested on a clinical 3T scanner (Philips Intera).

Results: In phantoms the concentration of an iron oxide based contrast agent was correctly determined even in regions of strong susceptibilities. In volunteer studies T2*-maps of brain and liver tissue were acquired. It could be shown that the uncorrected value of R2* was overestimated in parts of the tissue and that the error was minimized by the correction method.

Conclusion: This demonstrates that the proposed approach is essential for T2*-relaxometry at high field-strength, and thus allows a better quantification of physiological parameters like oxygenation and/or contrast agent concentration.