Generalized inhomogeneity-resilient relaxation along a fictitious field (girRAFF) for improved robustness in rotating...

To optimize Relaxation along a Fictitious Field (RAFF) pulses for rotating frame relaxometry with improved robustness in the presence of $B_0$ and $B_1^{+}$ field inhomogeneities.

The resilience of RAFF pulses against $B_0$ and $B_1^{+}$ inhomogeneities was studied using Bloch simulations. A parameterized extension of the RAFF formulation was introduced and used to derive a generalized inhomogeneity-resilient RAFF (girRAFF) pulse. RAFF and girRAFF preparation efficiency, defined as the ratio of the longitudinal magnetization before and after the preparation ( $M_z\left(T_p\right)/M_0$ ), were simulated and validated in phantom experiments. $T_{\text{RAFF}}$ and $T_{\text{girRAFF}}$ parametric maps were acquired at 3T in phantom, the calf muscle, and the knee cartilage of healthy subjects. The relaxation time maps were analyzed for resilience against artificially induced field inhomogeneities and assessed in terms of in vivo reproducibility.

Optimized girRAFF preparations yielded improved preparation efficiency (0.95/0.91 simulations/phantom) with respect to RAFF (0.36/0.67 simulations/phantom). $T_{\text{girRAFF}}$ preparations showed in phantom/calf 6.0/4.8 times higher resilience to $B_0$ inhomogeneities than RAFF, and a 4.7/5.3 improved resilience to $B_1^{+}$ inhomogeneities. In the knee cartilage, $T_{\text{girRAFF}}$ (53 $\pm$ 14 ms) was higher than $T_{\text{RAFF}}$ (42 $\pm$ 11 ms). Moreover, girRAFF preparations yielded 7.6/4.9 times improved reproducibility across $B_0$ / $B_1^{+}$ inhomogeneity conditions, 1.9 times better reproducibility across subjects and 1.2 times across slices compared with RAFF. Dixon-based fat suppression led to a further 15-fold improvement in the robustness of girRAFF to inhomogeneities.

RAFF pulses display residual sensitivity to off-resonance and pronounced sensitivity to $B_1^{+}$ inhomogeneities. Optimized girRAFF pulses provide increased robustness and may be an appealing alternative for applications where resilience against field inhomogeneities is required.