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

• Published in: Journal of magnetic resonance imaging Vol. 55; no. 5; pp. 1419 - 1425
• Main Authors: Doyle, Eamon K., Thornton, Samuel, Ghugre, Nilesh R., Coates, Thomas D., Nayak, Krishna S., Wood, John C.
• Format: Journal Article
• Language: English
• Published: Hoboken, USA John Wiley & Sons, Inc 01-05-2022; Wiley Subscription Services, Inc
• Subjects: B1+ inhomogeneity; Calibration; Error correction; Estimates; Field strength; Heterogeneity; In vivo methods and tests; Iron; Liver; Lobes; Magnetic resonance imaging; Mathematical models; Monte Carlo; Overloading; Population studies; Quantitation; Spatial heterogeneity; spin echo; Standard deviation; Statistical analysis; Statistical tests
• ISSN: 1053-1807; 1522-2586
• DOI: 10.1002/jmri.27928

Background
Liver iron concentration (LIC) measured by MRI has become the clinical reference standard for managing iron overload in chronically transfused patients. Transverse relaxivity (R2 or R2*) measurements are converted to LIC units using empirically derived calibration curves.
Hypothesis
That flip angle (FA) error due to B1+ spatial heterogeneity causes significant LIC quantitation error. B1+ scale (b1, [FAactual/FAspecified]) 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 B1+ mapping sequence
Assessment
We quantified the average/standard deviation b1 in the right and left lobes of the liver from B1+ maps acquired at 1.5 T and 3 T. The impact of b1 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 b1 means.
Results
Average b1 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 b1 systematically increased R2 estimates at lower LIC (<~25 mg/g at 1.5 T, <~15 mg/g at 3 T) but flattened or even inverted the R2‐LIC relationship at higher LIC (≥~25 mg/g to 1.5 T, ≥~15 mg/g to 3 T); changes in the R2‐LIC relationship were symmetric with respect to over and under excitation and were similar at 1.5 T and 3 T (for the same R2 value). The R2*‐LIC relationship was independent of b1.
Conclusion
Spin echo R2 measurement of LIC at 3 T is error‐prone without correction for b1 errors. The impact of b1 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.