Whole liver phase-based R2 mapping in liver iron overload within a breath-hold.

Abstract

Purpose: Diagnosis and treatment monitoring of iron overload increasingly relies on non-invasive MRI-based measurement of liver iron concentration (LIC). Liver R₂ mapping is known to correlate well with LIC. However, traditional spin-echo based R₂ mapping methods have drawbacks such as long acquisition times and limited volumetric coverage. In this work, we present an optimized phase-based R₂ mapping method to quantify whole-liver R₂ in iron overload patients within a single breathhold.

Theory and methods: A recently developed phase-based R₂ mapping method is optimized in this study to improve estimation of high R₂ values using reduced TR, spatial averaging, and R₁ correction. Using Bloch equation simulations, the proposed optimization method was evaluated. Furthermore, the impact of fat on R₂ bias was investigated through simulations. The feasibility of the optimized phase-based R₂ method was assessed in healthy volunteers and patients with iron overload and compared to reference STEAM-MRS R₂ measurements.

Results: Simulations demonstrate that a shorter TR extends the dynamic range of R₂ estimation to higher values and that averaging of signal phase before R₂ estimation is necessary when R₂ is high. Phantom experiments also demonstrate reduced phase-based R₂ bias using R₁ correction. Good agreement (1.5 T: r² = 0.76, 3.0 T: r² = 0.70) between the modified phase-based R₂ method and reference STEAM R₂ was found in healthy volunteers and iron overload patients over a wide range of LIC values.

Conclusion: This study demonstrates the feasibility of the proposed phase-based R₂ method to accurately measure whole-liver R₂ mapping in severe iron overloaded patients during a single breathhold.