B_1 Field inhomogeneity correction for qDESS T_2 mapping: application to rapid bilateral knee imaging

Purpose T_2 mapping is a powerful tool for studying osteoarthritis (OA) changes and bilateral imaging may be useful in investigating the role of between-knee asymmetry in OA onset and progression. The quantitative double-echo in steady-state (qDESS) can provide fast simultaneous bilateral knee T_2 and high-resolution morphometry for cartilage and meniscus. The qDESS uses an analytical signal model to compute T_2 relaxometry maps, which require knowledge of the flip angle (FA). In the presence of B_1 inhomogeneities, inconsistencies between the nominal and actual FA can affect the accuracy of T_2 measurements. We propose a pixel-wise B_1 correction method for qDESS T_2 mapping exploiting an auxiliary B_1 map to compute the actual FA used in the model. Methods The technique was validated in a phantom and in vivo with simultaneous bilateral knee imaging. T_2 measurements of femoral cartilage (FC) of both knees of six healthy participants were repeated longitudinally to investigate the association between T_2 variation and B_1 . Results The results showed that applying the B_1 correction mitigated T_2 variations that were driven by B_1 inhomogeneities. Specifically, T_2 left–right symmetry increased following the B_1 correction ( ρ _c = 0.74 > ρ _c = 0.69). Without the B_1 correction, T_2 values showed a linear dependence with B_1 . The linear coefficient decreased using the B_1 correction (from 24.3 ± 1.6 ms to 4.1 ± 1.8) and the correlation was not statistically significant after the application of the Bonferroni correction ( p value > 0.01). Conclusion The study showed that B_1 correction could mitigate variations driven by the sensitivity of the qDESS T_2 mapping method to B_1 , therefore, increasing the sensitivity to detect real biological changes. The proposed method may improve the robustness of bilateral qDESS T_2 mapping, allowing for an accurate and more efficient evaluation of OA pathways and pathophysiology through longitudinal and cross-sectional studies.