Magnetic susceptibility as a B₀ field strength independent MRI biomarker of liver iron overload

Purpose MR-based quantification of liver magnetic susceptibility may enable field strength-independent measurement of liver iron concentration (LIC). However, susceptibility quantification is challenging, due to nonlocal effects of susceptibility on the B₀ field. The purpose of this work is to demonstrate feasibility of susceptibility-based LIC quantification using a fat-referenced approach. Methods Phantoms consisting of vials with increasing iron concentrations immersed between oil/water layers, and 27 subjects (9 controls/18 subjects with liver iron overload) were scanned. Ferriscan (1.5 T) provided R2-based reference LIC. Multiecho three-dimensional-SPGR (1.5 T/3 T) enabled fat-water, B₀- and R2*-mapping. Phantom iron concentration (mg Fe L^-1) was estimated from B₀ differences (ΔB₀) between vials and neighboring oil. Liver susceptibility and LIC (mg Fe g^-1 dry tissue) was estimated from ΔB₀ between the lateral right lobe of the liver and adjacent subcutaneous adipose tissue. Results Estimated phantom iron concentrations had good correlation with true iron concentrations (1.5 T:slope = 0.86, intercept = 0.72, r² = 0.98; 3 T:slope = 0.85, intercept = 1.73, r² = 0.98). In liver, ΔB₀ correlated strongly with R2* (1.5 T:r² = 0.86; 3 T:r² = 0.93) and B₀-LIC had good agreement with Ferriscan-LIC (slopes/intercepts nearly 1.0/0.0, 1.5 T:r ² = 0.67, slope = 0.93 ± 0.13, P ≈ 0.50, intercept = 1.93 ± 0.78, P ≈ 0.02; 3 T:r² = 0.84, slope = 1.01 ± 0.09, P ≈ 0.90, intercept = 0.23 ± 0.52, P ≈ 0.68). Discussion Fat-referenced, susceptibility-based LIC estimation is feasible at both field strengths. This approach may enable improved susceptibility mapping in the abdomen.