Hepatic Iron Quantification Using a Free-Breathing 3D Radial Gradient Echo Technique and Validation With a 2D Biopsy-Calibrated R₂^* Relaxometry Method

Background: Hepatic iron content (HIC) is an important parameter for the management of iron overload. Non-invasive HIC assessment is often performed using biopsy-calibrated two-dimensional breath-hold Cartesian gradient echo (2D BH GRE) R₂^* -MRI. However, breath-holding is not possible in most pediatric patients or those with respiratory problems, and three-dimensional free-breathing radial GRE (3D FB rGRE) has emerged as a viable alternative.

Purpose: To evaluate the performance of a 3D FB rGRE and validate its R₂^* and fat fraction (FF) quantification with 3D breath-hold Cartesian GRE (3D BH cGRE) and biopsy-calibrated 2D BH GRE across a wide range of HICs.

Study type: Retrospective.

Subjects: Twenty-nine patients with hepatic iron overload (22 females, median age: 15 [5-25] years).

Field strength/sequence: Three-dimensional radial and 2D and 3D Cartesian multi-echo GRE at 1.5 T.

Assessment: R₂^* and FF maps were computed for 3D GREs using a multi-spectral fat model and 2D GRE R₂^* maps were calculated using a mono-exponential model. Mean R₂^* and FF values were calculated via whole-liver contouring and T₂^* -thresholding by three operators.

Statistical tests: Inter- and intra-observer reproducibility was assessed using Bland-Altman and intraclass correlation coefficient (ICC). Linear regression and Bland-Altman analysis were performed to compare R₂^* and FF values among the three acquisitions. One-way repeated-measures ANOVA and Wilcoxon signed-rank tests, respectively, were used to test for significant differences between R₂^* and FF values obtained with different acquisitions. Statistical significance was assumed at P < 0.05.

Results: The mean biases and ICC for inter- and intra-observer reproducibility were close to 0% and >0.99, respectively for both R₂^* and FF. The 3D FB rGRE R₂^* and FF values were not significantly different (P > 0.44) and highly correlated (R² ≥ 0.98) with breath-hold Cartesian GREs, with mean biases ≤ ±2.5% and slopes 0.90-1.12. In non-breath-holding patients, Cartesian GREs showed motion artifacts, whereas 3D FB rGRE exhibited only minimal streaking artifacts.

Data conclusion: Free-breathing 3D radial GRE is a viable alternative in non-breath-hold patients for accurate HIC estimation.

Level of evidence: 3 TECHNICAL EFFICACY: Stage 2.