Abstract
PURPOSE: To develop and validate a method for B0 mapping for knee imaging using the quantitative Double-Echo in Steady-State (qDESS) exploiting the phase difference ( Δθ ) between the two echoes acquired. Contrary to a two-gradient-echo (2-GRE) method, Δθ depends only on the first echo time. METHODS: Bloch simulations were applied to investigate robustness to noise of the proposed methodology and all imaging studies were validated with phantoms and in vivo simultaneous bilateral knee acquisitions. Two phantoms and five healthy subjects were scanned using qDESS, water saturation shift referencing (WASSR), and multi-GRE sequences. ΔB0 maps were calculated with the qDESS and the 2-GRE methods and compared against those obtained with WASSR. The comparison was quantitatively assessed exploiting pixel-wise difference maps, Bland-Altman (BA) analysis, and Lin's concordance coefficient ( ρc ). For in vivo subjects, the comparison was assessed in cartilage using average values in six subregions. RESULTS: The proposed method for measuring ΔB0 inhomogeneities from a qDESS acquisition provided ΔB0 maps that were in good agreement with those obtained using WASSR. ΔB0 ρc values were ≥ 0.98 and 0.90 in phantoms and in vivo, respectively. The agreement between qDESS and WASSR was comparable to that of a 2-GRE method. CONCLUSION: The proposed method may allow B0 correction for qDESS T2 mapping using an inherently co-registered ΔB0 map without requiring an additional B0 measurement sequence. More generally, the method may help shorten knee imaging protocols that require an auxiliary ΔB0 map by exploiting a qDESS acquisition that also provides T2 measurements and high-quality morphological imaging.