Optimal Control-Based RF Pulses Versus Classical Motion-Encoding Strategies for In Vivo Hepatic MR Elastography...

This study compared an optimal control (OC)-based motion-encoding approach using radiofrequency (RF) pulses with the conventional motion-encoding gradient (MEG) method for in vivo hepatic magnetic resonance elastography (MRE) across multiple excitation frequencies in mice, evaluating signal quality and temporal reproducibility. Eight healthy C57BL/6 mice, including four males and four females, underwent hepatic MRE at 7T at 300, 500 and 600 Hz using RARE-based sequences: classical MEG-MRE and optimized OC-MRE with GRAPE-designed RF pulses. In both sequences, motion was encoded only along the slice direction. Each mouse was scanned on days 1, 15 and 90. Reconstruction involved phase unwrapping, directional filtering and inversion with the Algebraic Inversion of Differential Equation algorithm to estimate the shear storage modulus ( $G^{\text{'}}$ ) on the manual liver segmentation and on an automatic circular region of interest designed for statistical analyses. Signal-to-noise ratio (SNR), phase-to-noise ratio (PNR) and intra/interanimal reproducibility were quantified using ANOVA, intraclass correlation coefficients (ICC), Bland-Altman analysis and coefficients of variation (CV). OC-MRE provided a mean twofold increase in SNR compared with MEG-MRE (OC/MEG = 1.8-2.1 across frequencies) and higher PNR values. $G^{\text{'}}$ increased with frequency for both methods, consistent with viscoelastic behavior and showed strong intermethod agreement (mean bias < 0.05 kPa; r > 0.96, p < 0.001). Intra-animal reproducibility was moderate at 300 Hz (ICC = 0.31-0.50) and high at 500-600 Hz (ICC > 0.75). No significant temporal changes in hepatic stiffness were observed across sessions. OC-MRE improves signal quality while maintaining quantitative consistency with conventional MEG-MRE. Its short echo time and absence of motion encoding gradients enhance robustness and make it a promising tool for longitudinal small-animal studies and future clinical translation, particularly in scenarios with short transverse relaxation times ( $T_2$ / $T_2^{*}$ ) such as those encountered in hepatic iron overload.