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 B 1 + and B 0 imperfections for simultaneous T 1 , T 2 , 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 T 1 and T 2 . The spin-lock module used two 180° pulses with compensatory phases to reduce T 1ρ sensitivity to B 1 and B 0 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 B 1 inhomogeneities. Compared with SC-SL, TB-SL MRF showed in experiments greater robustness against severe B 1 inhomogeneities and in Bloch simulations greater robustness against B 1 and B 0 . We measured with TB-SL MRF an average T 1 = 950.1 ± 28.7 ms, T 2 = 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 B 1 + and B 0 imperfections. It can simultaneously map T 1 , T 2 , T 1ρ , and B 1 + in a single scan and can potentially be used to study muscle composition.