• Aaryani Tipirneni-Sajja, Ralf B Loeffler, Axel J Krafft, Andrea N Sajewski, Robert J Ogg, Jane S Hankins, and Claudia M Hillenbrand.
• Department of Diagnostic Imaging, St. Jude Children's Research Hospital, Memphis, Tennessee, USA.
• J Magn Reson Imaging. 2019 May 1; 49 (5): 1475-1488.

BackgroundCurrent R2*-MRI techniques for measuring hepatic iron content (HIC) use various acquisition types and fitting models.PurposeTo evaluate the accuracy and precision of R2*-HIC acquisition and fitting methods.Study TypeSignal simulations, phantom study, and prospective in vivo cohort.PopulationIn all, 132 patients (58/74 male/female, mean age 17.7 years).Field Strength/Sequence2D-multiecho gradient-echo (GRE) and ultrashort echo time (UTE) acquisitions at 1.5T.AssessmentSynthetic MR signals were created to mimic published GRE and UTE methods, using different R2* values (25-2000 s-1 ) and signal-to-noise ratios (SNR). Phantoms with varying iron concentrations were scanned at 1.5T. In vivo data were analyzed from 132 patients acquired at 1.5T. R2* was estimated by fitting using three signal models. Accuracy and precision of R2* measurements for UTE acquisition parameters (SNR, echo spacing [ΔTE], maximum echo time [TEmax ]) and fitting methods were compared for simulated, phantom, and in vivo datasets.Statistical TestsR2* accuracy was determined from the relative error and by linear regression analysis. Precision was evaluated using coefficient of variation (CoV) analysis.ResultsIn simulations, all models had high R2* accuracy (error <5%) and precision (CoV <10%) for all SNRs, shorter ΔTE (≤0.5 msec), and longer TEmax (≥10.1 msec); except the constant offset model overestimated R2* at the lowest SNR. In phantoms and in vivo, all models produced similar R2* values for different SNRs and shorter ΔTEs (slopes: 0.99-1.06, R2 > 0.99, P < 0.001). In all experiments, R2* results degraded for high R2* values with longer ΔTE (≥1 msec). In vivo, shorter and longer TEmax gave similar R2* results (slopes: 1.02-1.06, R2 > 0.99, P < 0.001) for the noise subtraction model for 25≤R2*≤2000 s-1 . However, both quadratic and constant offset models, using shorter TEmax (≤4.7 msec) overestimated R2* and yielded high CoVs up to ∼170% for low R2* (<250 s-1 ).Data ConclusionUTE with TEmax ≥ 10.1 msec and ΔTE ≤ 0.5 msec yields accurate R2* estimates over the entire clinical HIC range. Monoexponential fitting with noise subtraction is the most robust signal model to changes in UTE parameters and achieves the highest R2* accuracy and precision.Level Of Evidence2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;49:1475-1488.© 2018 International Society for Magnetic Resonance in Medicine.

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