Accurate myocardial T1 mapping at 5 T using an improved MOLLI method: A validation study.

Background: Accurate myocardial T1 mapping at 5T remains a technical challenge due to field inhomogeneity and prolonged T1 values. The aim of this study is to develop an accurate and clinically applicable myocardial T1 mapping technique for 5T magnetic resonance imaging (MRI) systems and validate its performance in a multicenter study.

Methods: The proposed method is based on a 5-(3)-3 Modified Look-Locker Inversion Recovery (MOLLI) sequence, dubbed combined-correction MOLLI (coMOLLI), which corrects for both inversion efficiency and readout-induced signal disturbances. Specifically, coMOLLI employs a radiofrequency spoiled gradient recalled echo (GRE) readout rather than the commonly used balanced steady-state free precession (bSSFP) readout. Its signal evolution is modeled to estimate T1 values, which incorporates both inversion efficiency and readout disturbances to improve fitting accuracy. To further enhance accuracy, the inversion pulse was redesigned under hardware constraints and the observed B₀ and B₁ variations over the heart at 5T, using adiabatic hyperbolic secant (HSn) and tangent/hyperbolic tangent (Tan/Tanh) pulses. The method was validated in phantom experiments, as well as in 21 healthy volunteers and 9 patients.

Results: The optimized inversion pulse at 5T is the Tan/Tanh pulse with A = 10kHz, K_s = 4, k = 22, and T_p = 8 ms. In phantom studies, coMOLLI showed high accuracy versus reference inversion recovery - fast spin echo (IR-FSE), yielding relative errors within 5% for all nine vials. In vivo studies, the average native myocardial T1 values across 21 healthy volunteers were 1468 ± 48 ms, 1514 ± 39 ms, and 1545 ± 50 ms, and blood T1 values were 2182 ± 132 ms, 2124 ± 153 ms, and 2131 ± 158 ms for apical, middle, and base slices, respectively.

Conclusion: The coMOLLI method demonstrated high accuracy in phantom studies and feasibility in vivo studies. By adopting the widely used 5-(3)-3 MOLLI acquisition scheme, it shows potential for clinical cardiac imaging at 5T.