A Flexible Approach for Fat-Water Separation With Bipolar Readouts and Correction of Gradient-Induced Phase and Amplitude Effects.

PURPOSE: To develop a fat-water separation approach that corrects bipolar readout gradient-induced effects without additional scans and is compatible with any fat-water separation method. THEORY AND METHODS: The proposed approach combines joint fat-water separation of the odd and even echoes of a bipolar multi-echo gradient echo acquisition with an inverse problem to find least-squares estimates for phase and amplitude corrections to eliminate bipolar-induced effects. Optimization of sequence parameters through the calculation of the equivalent number of signal averages (NSA) with Cramér-Rao Bound theory (CRB) is presented. The proposed approach is demonstrated with a graph-cut fat-water separation. Characterization of the accuracy was performed via Monte Carlo Simulations (MC). The approach was tested in phantoms and in vivo. Proton density fat fraction (PDFF) and effective transverse relaxation rate ( R 2 * $$ {R}_2^{\ast } $$ ) maps were evaluated to quantify performance. RESULTS: NSA calculations suggest short TE1 and ΔTE = 1.5 ms as optimal alternatives for fat-water separation. MC simulations demonstrated accurate estimation of fat and water complex signals, main field frequency heterogeneity, and R 2 * $$ {R}_2^{\ast } $$ with mean relative error within 1%. In phantoms and in vivo at 3 T, the proposed approach effectively eliminated effects induced by bipolar readout gradients in fat-water separation, notably reducing the error in PDFF estimates in vivo from 0.110 to 0.180 (without correction) to -0.013 to 0.009 (with correction). CONCLUSION: We proposed an approach to correct bipolar readout-induced effects that are detrimental for fat-water separation. This approach can extend the use of existing fat-water separation techniques designed for data acquired using unipolar readout gradients to data collected with bipolar readout gradients.