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

Abstract

BACKGROUND\nLiver 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.\n\n\nHYPOTHESIS\nThat 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\u2009T which could reduce the accuracy of transverse relaxivity measurements.\n\n\nSTUDY TYPE\nProspective.\n\n\nPOPULATION\nForty-seven subjects with chronic transfusional iron overload undergoing clinically indicated LIC assessment.\n\n\nFIELD STRENGTH/SEQUENCE\n5\u2009T/3\u2009T 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\u2009T and 3\u2009T. The impact of b1 variation on spin echo LIC estimates was determined using a Monte Carlo model.\n\n\nSTATISTICAL TESTS\nMean, median, and standard deviation in whole liver and right and left lobes; two-sided t-test between whole-liver b1 means.\n\n\nRESULTS\nAverage b1 within the liver was 99.3%\u2009±\u200912.3% at 1.5\u2009T versus 69.6%\u2009±\u200914.6% at 3\u2009T and was independent of iron burden (P\xa0<\u20090.05). Monte Carlo simulations demonstrated that b1 systematically increased R2 estimates at lower LIC (<~25\u2009mg/g at 1.5\u2009T, <~15\u2009mg/g at 3\u2009T) but flattened or even inverted the R2 -LIC relationship at higher LIC (≥~25\u2009mg/g to 1.5\u2009T, ≥~15\u2009mg/g to 3\u2009T); changes in the R2 -LIC relationship were symmetric with respect to over and under excitation and were similar at 1.5\u2009T and 3\u2009T (for the same R2 value). The R2 * -LIC relationship was independent of b1 .\n\n\nCONCLUSION\nSpin echo R2 measurement of LIC at 3\u2009T is error-prone without correction for b1 errors. The impact of b1 error on current 1.5\u2009T 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.\n\n\nLEVEL OF EVIDENCE\n1.\n\n\nTECHNICAL EFFICACY STAGE\n2.