Conversion of 3 T liver, spleen, pancreas, and kidney R2* measurements to 1.5 T R2* equivalents: Validation of a theoretical framework.

Iron quantification of the liver and heart at 1.5 T have become the standard of care for patients with transfusional siderosis [1]. However some institutions must rely on higher field systems to perform iron quantitation. While studies performed at both field strengths have cross-calibrated measurements for liver and heart, a similar validation has not been performed for other abdominal organs. The purpose of this study was to validate a formula to convert 3 T abdominal R2* measurements to 1.5 T R2* equivalents; we hypothesized similar accuracy in spleen, kidney, and pancreas as previously shown in liver and heart. This was a prospective study in 64 patients with transfusional siderosis, including thalassemia, sickle cell disease, and rare anemias. We performed paired multiecho, gradient echo imaging at 1.5 T and 3.0 T. All 3 T R2* values were converted to 1.5 T equivalents using the following relationship, R2*1.5T = (R2*3T - R2*3T) * (1.5/3.0) + R2*1.5T, where R2*3T and R2*1.5T are organ R2* values derived from an ensemble average of non‑iron overloaded subjects. Lin's Concordance Correlation Coefficient and Bland-Altman analysis was measured between 1.5 T R2* equivalents and natively acquired R2* values in the liver, spleen, pancreas, and kidney. Bland Altman analysis demonstrated no significant bias for any of the organs. Limits of agreement for liver (-19.3%-16%), spleen (-21.5%-26.5%), and kidney (-25%-24.8%) were lower than for pancreas (-56.2%-52.1%). This reflects the heterogeneous nature of iron distribution, irregular gland shape, fatty infiltration, and increased vulnerability to regional susceptibility artifacts in the pancreas. CONCLUSION: We validated a generalizable cross-field R2* conversion formula in abdominal organs, allowing 3 T R2* measurements to be projected onto validated risk thresholds [5-7], and iron calculations [2]. These data suggest that while it is always better not to switch field strengths when quantifying iron, 3 T abdominal R2* estimates can be converted to equivalent R2* measurements at 1.5 T to facilitate their interpretability and clinical utility.