Effects of B Heterogeneity on Spin Echo-Based Liver Iron Estimates.

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 or R ) measurements are converted to LIC units using empirically derived calibration curves.

Hypothesis: That flip angle (FA) error due to B spatial heterogeneity causes significant LIC quantitation error. B scale (b , [FA /FA ]) 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 mapping sequence ASSESSMENT: We quantified the average/standard deviation b in the right and left lobes of the liver from B maps acquired at 1.5 T and 3 T. The impact of b 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 means.

Results: Average b 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 systematically increased R estimates at lower LIC (<~25 mg/g at 1.5 T, <~15 mg/g at 3 T) but flattened or even inverted the R -LIC relationship at higher LIC (≥~25 mg/g to 1.5 T, ≥~15 mg/g to 3 T); changes in the R -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 value). The R -LIC relationship was independent of b .

Conclusion: Spin echo R measurement of LIC at 3 T is error-prone without correction for b errors. The impact of b 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.

Technical Efficacy Stage: 2.