Relaxivity-iron calibration in hepatic iron overload: Predictions of a Monte Carlo model

Abstract

Purpose R2∗ (1/T2∗) and single echo R2 (1/T2) have been calibrated to liver iron concentration (LIC) in patients with thalassemia and transfusion-dependent sickle cell disease at 1.5T. The R2∗-LIC relationship is linear, whereas that of R2 is curvilinear. However, the increasing popularity of high-field scanners requires generalizing these relationships to higher field strengths. In this study, we tested the hypothesis that numerical simulation can accurately determine the field dependence of iron-mediated transverse relaxation rates. Methods We previously replicated the calibration curves between R2 and R2∗ and iron at 1.5T using Monte Carlo models incorporating realistic liver structure, iron deposit susceptibility, and proton mobility. In this paper, we extend our model to predict relaxivity-iron calibrations at higher field strengths. Predictions were validated by measuring R2 and R2∗ at 1.5T and 3T in six β-thalassemia major patients. Results Predicted R2∗ increased twofold at 3T from 1.5T, whereas R2 increased by a factor of 1.47. Patient data exhibited a coefficient of variation of 3.6% and 7.2%, respectively, to the best-fit simulated data. Simulations over the range 0.25T-7T showed R2∗ increasing linearly with field strength, whereas R2 exhibited a concave-downward relationship. Conclusion A model-based approach predicts alterations in relaxivity-iron calibrations with field strength without repeating imaging studies. The model may generalize to alternative pulse sequences and tissue iron distribution. Magn Reson Med 74:879-883, 2015.