Quantitative R₂∗ MRI of the liver with rician noise models for evaluation of hepatic iron overload: Simulation, phantom, and early clinical experience

Purpose To compare Rician and non-Rician noise models for quantitative R₂∗ magnetic resonance imaging (MRI), in a simulation, phantom, and human study. Materials and Methods Synthetic 12-echo spoiled GRE (SGRE) datasets were generated with various R₂∗ rates (0-2000 sec^-1) at a signal-to-noise ratio (SNR) of 50, 20, 10, and 5. Phantoms of different MnCl₂ concentrations (0-25 mM) were constructed and imaged using a 12-echo 3D SGRE sequence at 1.5T. Increasing levels of synthetic noise was added to the original data to simulate sequentially lower SNR conditions. Sixteen patients with suspected or known iron overload were imaged using 12-echo 3D SGRE at 1.5T. Various R₂∗ quantification methods, based on Rician and non-Rician noise models, were compared in the simulation, phantom, and human datasets. Results Non-Rician R₂∗ estimates were variably inaccurate in the high R₂∗ range (>500 sec^-1), with SNR-dependent linear goodness-of-fit statistic (R²) of 0.373-0.999. Rician R₂∗ estimates were accurate even in the high R₂∗ range, with high R² of 0.940-0.999 regardless of SNR. Non-Rician R₂∗ estimates were variably nonlinear at high MnCl₂ concentrations, with SNR-dependent R² of 0.345-0.994. Rician R₂∗ estimates were linear even at high MnCl₂ concentrations, with high R² of 0.923-0.994 regardless of SNR. Between-method agreement of the R₂∗ estimates was excellent in patients with low ferritin but poor in patients with high ferritin. Rician R₂∗ estimates had excellent correlation with ferritin (r = 0.966 P < 0.001). Conclusion Rician R₂∗ estimates were most consistent in the high R₂∗ conditions and under varying SNR, and may be more reliable when high iron load is suspected.