Generalized inhomogeneity‐resilient relaxation along a fictitious field (girRAFF) for improved robustness in rotating frame relaxometry at 3T

• Published in: Magnetic resonance in medicine Vol. 92; no. 6; pp. 2373 - 2391
• Main Authors: Coletti, Chiara, Naaktgeboren, Roeland, Tourais, Joao, Van De Steeg‐Henzen, Christal, Weingärtner, Sebastian
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.12.2024
• Subjects: Adult; Algorithms; B0$$ {\mathrm{B}}_0 $$/B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities; Cartilage; Computer Simulation; Humans; Image Processing, Computer-Assisted - methods; Inhomogeneity; Knee; Magnetic Resonance Imaging; Male; Muscle contraction; Muscle, Skeletal - diagnostic imaging; Muscle, Skeletal - physiology; Phantoms, Imaging; RAFF; relaxation mapping; Relaxation time; Reproducibility; Reproducibility of Results; Resilience; RF pulse design; Robustness; rotating‐frame of reference; Rotation; Sensitivity; Simulation; rotating‐frame of reference; inhomogeneities; RAFF; relaxation mapping; RF pulse design; /
• ISSN: 0740-3194; 1522-2594; 1522-2594
• DOI: 10.1002/mrm.30219

Purpose To optimize Relaxation along a Fictitious Field (RAFF) pulses for rotating frame relaxometry with improved robustness in the presence of B0$$ {\mathrm{B}}_0 $$ and B1+$$ {\mathrm{B}}_1^{+} $$ field inhomogeneities. Methods The resilience of RAFF pulses against B0$$ {\mathrm{B}}_0 $$ and B1+$$ {\mathrm{B}}_1^{+} $$ 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 (Mz(Tp)/M0$$ {M}_z\left({T}_p\right)/{M}_0 $$), were simulated and validated in phantom experiments. TRAFF$$ {T}_{\mathrm{RAFF}} $$ and TgirRAFF$$ {T}_{\mathrm{girRAFF}} $$ 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. Results Optimized girRAFF preparations yielded improved preparation efficiency (0.95/0.91 simulations/phantom) with respect to RAFF (0.36/0.67 simulations/phantom). TgirRAFF$$ {T}_{\mathrm{girRAFF}} $$ preparations showed in phantom/calf 6.0/4.8 times higher resilience to B0$$ {\mathrm{B}}_0 $$ inhomogeneities than RAFF, and a 4.7/5.3 improved resilience to B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities. In the knee cartilage, TgirRAFF$$ {T}_{\mathrm{girRAFF}} $$ (53 ±$$ \pm $$ 14 ms) was higher than TRAFF$$ {T}_{\mathrm{RAFF}} $$ (42 ±$$ \pm $$ 11 ms). Moreover, girRAFF preparations yielded 7.6/4.9 times improved reproducibility across B0$$ {\mathrm{B}}_0 $$/B1+$$ {\mathrm{B}}_1^{+} $$ 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. Conclusions RAFF pulses display residual sensitivity to off‐resonance and pronounced sensitivity to B1+$$ {\mathrm{B}}_1^{+} $$ inhomogeneities. Optimized girRAFF pulses provide increased robustness and may be an appealing alternative for applications where resilience against field inhomogeneities is required.