Improved free-breathing liver fat and iron quantification using a 2D chemical shift-encoded MRI with flip angle modulation and motion-corrected averaging

Abstract

Objectives: 3D chemical shift-encoded (CSE) MRI enables accurate and precise quantification of proton density fat fraction (PDFF) and R2*, biomarkers of hepatic fat and iron deposition. Unfortunately, 3D CSE-MRI requires reliable breath-holding. Free-breathing 2D CSE-MRI with sequential radiofrequency excitation is a motion-robust alternative but suffers from low signal-to-noise-ratio (SNR). To overcome this limitation, this work explores the combination of flip angle-modulated (FAM) 2D CSE imaging with a non-local means (NLM) motion-corrected averaging technique.

Methods: In this prospective study, 35 healthy subjects (27 children/8 adults) were imaged on a 3T MRI-system. Multi-echo 3D CSE ("3D") and 2D CSE FAM ("FAM") images were acquired during breath-hold and free-breathing, respectively, to obtain PDFF and R2* maps of the liver. Multi-repetition FAM was postprocessed with direct averaging (DA)- and NLM-based averaging and compared to 3D CSE using Bland-Altmann and regression analysis. Image qualities of PDFF and R2* maps were reviewed by two radiologists using a Likert-like scale (score 1-5, 5 = best).

Results: Compared to 3D CSE, multi-repetition FAM-NLM showed excellent agreement (regression slope = 1.0, R² = 0.996) for PDFF and good agreement (regression slope 1.08-1.15, R² ≥ 0.899) for R2*. Further, multi-repetition FAM-NLM PDFF and R2* maps had fewer artifacts (score 3.8 vs. 3.2, p < 0.0001 for PDFF; score 3.2 vs. 2.6, p < 0.001 for R2*) and better overall image quality (score 4.0 vs. 3.5, p < 0.0001 for PDFF; score 3.4 vs. 2.7, p < 0.0001 for R2*).

Conclusions: Free-breathing FAM-NLM provides superior image quality of the liver compared to the conventional breath-hold 3D CSE-MRI, while minimizing bias for PDFF and R2* quantification.

Key points: • 2D CSE FAM-NLM is a free-breathing method for liver fat and iron quantification and viable alternative for patients unable to hold their breath. • 2D CSE FAM-NLM is a feasible alternative to breath-hold 3D CSE methods, with low bias in proton density fat fraction (PDFF) and no clinically significant bias in R2*. • Quantitatively, multiple repetitions in 2D CSE FAM-NLM lead to improved SNR.

Keywords: Flip angle modulation; Free-breathing; NLM; PDFF; R2*.