Effects of B₁⁺ Heterogeneity on Spin Echo‐Based Liver Iron Estimates

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Background Liver iron concentration (LIC) measured by MRI has become the clinical reference standard for managing iron overload in chronically transfused patients. Transverse relaxivity (R 2 or R 2 * ) measurements are converted to LIC units using empirically derived calibration curves. Hypothesis That flip angle (FA) error due to B 1 + spatial heterogeneity causes significant LIC quantitation error. B 1 + scale ( b 1 , [FA actual /FA specified ]) variation is a major problem at 3 T which could reduce the accuracy of transverse relaxivity measurements. Study Type Prospective. Population Forty‐seven subjects with chronic transfusional iron overload undergoing clinically indicated LIC assessment. Field Strength/Sequence 5 T/3 T dual‐repetition time B 1 + mapping sequence Assessment We quantified the average/standard deviation b 1 in the right and left lobes of the liver from B 1 + maps acquired at 1.5 T and 3 T. The impact of b 1 variation on spin echo LIC estimates was determined using a Monte Carlo model. Statistical Tests Mean, median, and standard deviation in whole liver and right and left lobes; two‐sided t ‐test between whole‐liver b 1 means. Results Average b 1 within the liver was 99.3% ± 12.3% at 1.5 T versus 69.6% ± 14.6% at 3 T and was independent of iron burden ( P < 0.05). Monte Carlo simulations demonstrated that b 1 systematically increased R 2 estimates at lower LIC (<~25 mg/g at 1.5 T, <~15 mg/g at 3 T) but flattened or even inverted the R 2 ‐LIC relationship at higher LIC (≥~25 mg/g to 1.5 T, ≥~15 mg/g to 3 T); changes in the R 2 ‐LIC relationship were symmetric with respect to over and under excitation and were similar at 1.5 T and 3 T (for the same R 2 value). The R 2 * ‐LIC relationship was independent of b 1 . Conclusion Spin echo R 2 measurement of LIC at 3 T is error‐prone without correction for b 1 errors. The impact of b 1 error on current 1.5 T spin echo‐based techniques for LIC quantification is large enough to introduce measurable intersubject variability but the in vivo effect size needs a dedicated validation study. Level of Evidence 1. Technical Efficacy Stage 2.