Simultaneous T₁, T₂, and T_1ρ relaxation mapping of the lower leg muscle with MR fingerprinting

Purpose: To develop a novel MR-fingerprinting (MRF) pulse sequence that is insensitive to (Formula presented.) and B₀ imperfections for simultaneous T₁, T₂, and T_1ρ relaxation mapping. Methods: We implemented a totally balanced spin-lock (TB-SL) module to encode T_1ρ relaxation into an existing MRF framework that encoded T₁ and T₂. The spin-lock module used two 180° pulses with compensatory phases to reduce T_1ρ sensitivity to B₁ and B₀ inhomogeneities. We compared T_1ρ measured using TB-SL MRF in Bloch simulations, model agar phantoms, and in vivo experiments to those with a self-compensated spin-lock preparation module (SC-SL). The TB-SL MRF repeatability was evaluated in maps acquired in the lower leg skeletal muscle of 12 diabetic peripheral neuropathy patients, scanned two times each during visits separated by about 30 days. Results: The phantom relaxation times measured with TB-SL and SC-SL MRF were in good agreement with reference values in regions with low B₁ inhomogeneities. Compared with SC-SL, TB-SL MRF showed in experiments greater robustness against severe B₁ inhomogeneities and in Bloch simulations greater robustness against B₁ and B₀. We measured with TB-SL MRF an average T₁ = 950.1 ± 28.7 ms, T₂ = 26.0 ± 1.2 ms, and T_1ρ = 31.7 ± 3.2 ms in skeletal muscle across patients. Bland-Altman analysis demonstrated low bias between TB-SL and SC-SL MRF and between TB-SL MRF maps acquired in two visits. The coefficient of variation was less than 3% for all measurements. Conclusion: The proposed TB-SL MRF sequence is fast and insensitive to (Formula presented.) and B₀ imperfections. It can simultaneously map T₁, T₂, T_1ρ, and (Formula presented.) in a single scan and can potentially be used to study muscle composition.