Multi-echo Quantitative Susceptibility Mapping: How to Combine Echoes for Accuracy and Precision at 3 T

Abstract

Purpose: To compare different multi-echo combination methods for MRI quantitative susceptibility mapping (QSM), aiming to elucidate, given the current lack of consensus, how to optimally combine multi-echo gradient-recalled echo (GRE) signal phase information, either before or after applying Laplacian-base methods (LBMs) for phase unwrapping or background field removal. Methods: Multi-echo GRE data were simulated in a numerical head phantom, and multi-echo GRE images were acquired at 3 T in 10 healthy volunteers. To enable image-based estimation of GRE signal noise, 5 volunteers were scanned twice in the same session without repositioning. Five QSM processing pipelines were designed: one applied nonlinear phase fitting over echo times (TEs) before LBMs; two applied LBMs to the TE-dependent phase and then combined multiple TEs via either TE-weighted or signal-to-noise ratio (SNR)-weighted averaging; two calculated TE-dependent susceptibility maps via either multi-step or single-step QSM and then combined multiple TEs via magnitude-weighted averaging. Results from different pipelines were compared using visual inspection; summary statistics of susceptibility in deep gray matter, white matter and venous regions; phase noise propagation maps (error propagation theory); and, in the healthy volunteers, regional fixed bias analysis (Bland-Altman) and regional differences between the means (non-parametric tests). Results: Nonlinearly fitting the multi-echo phase over TEs before applying LBMs provided the best compromise between regional accuracy/precision and mitigation of phase noise propagation compared to averaging the LBM-processed TE-dependent phase. This result was especially true in high-susceptibility venous regions. Conclusion: For multi-echo QSM, combining the signal phase by nonlinear fitting before applying LBMs is recommended.