Prospective Evaluation of an R2*Method for Assessing Liver Iron Concentration (LIC) Against FerriScan: Derivation of the Calibration Curve and Characterization of the Nature and Source of Uncertainty in the Relationship

Background FerriScan is the method-of-choice for noninvasive liver iron concentration (LIC) quantification. However, it has a number of drawbacks including cost and expediency. Purpose/Hypothesis To characterize an R2*-based MRI technique that may potentially be used as an alternative to FerriScan. This was accomplished through the derivation of a calibration curve that characterized the relationship between FerriScan-derived LIC and R2*. The nature and source of uncertainty in this curve were investigated. It was hypothesized that the source of uncertainty is heterogeneity of LIC across the liver. Study Type Prospective. Subjects In all, 125 patients (69 women, 56 men) undergoing chelation treatment for iron overload prospectively underwent FerriScan and R2* MRI during the same exam. Field Strength/Sequence Pulse sequences included 2D multislice spin-echo pulse for FerriScan, and a prototype 3D 6-echo gradient echo acquisition for R2* mapping at 1.5T. Assessment A linear calibration curve was derived from the relationship between FerriScan-derived LIC estimates and R2* through least-squares fitting. Statistical Tests The nature of the uncertainty in the curve was characterized through tests of normality and homoscedasticity. The source of uncertainty was tested by comparing the magnitude of LIC variation over the FerriScan ROI to the observed uncertainty in the R2*-derived LIC estimates. Results A linear relationship between logarithmically transformed FerriScan-derived LIC and R2* (log{FerriScan-derived LIC} = 1.029 log{R2*} - 3.822) was confirmed. Uncertainty was random, with a behaviour that was normal and homoscedastic. The source of uncertainty was confirmed as iron heterogeneity across the liver. The nontransformed calibration curve was: FerriScan-derived LIC = 0.0266. R2*, with a constant coefficient-of-variation of 0.32. Data Conclusion FerriScan and R2* techniques were found to provide equivalent quantification of LIC in this study. Any difference in accuracy or precision was at a level lower than the uncertainty caused by variation in LIC over the liver.