PLANET: An Ellipse Fitting Approach for Simultaneous T1 and T2 Mapping Using Phase-Cycled Balanced Steady-State Free Precession

Purpose: To demonstrate the feasibility of a novel, ellipse fitting approach, named PLANET, for simultaneous estimation of relaxation times T ₁ and T ₂ from a single 3D phase-cycled balanced steady-state free precession (bSSFP) sequence. Methods: A method is presented in which the elliptical signal model is used to describe the phase-cycled bSSFP steady-state signal. The fitting of the model to the acquired data is reformulated into a linear convex problem, which is solved directly by a linear least squares method, specific to ellipses. Subsequently, the relaxation times T ₁ and T ₂, the banding free magnitude, and the off-resonance are calculated from the fitting results. Results: Maps of T ₁ and T ₂, as well as an off-resonance and a banding free magnitude can be simultaneously, quickly, and robustly estimated from a single 3D phase-cycled bSSFP sequence. The feasibility of the method was demonstrated in a phantom and in the brain of healthy volunteers on a clinical MR scanner. The results were in good agreement for the phantom, but a systematic underestimation of T ₁ was observed in the brain. Conclusion: The presented method allows for accurate mapping of relaxation times and off-resonance, and for the reconstruction of banding free magnitude images at realistic signal-to-noise ratios. Magn Reson Med 79:711–722, 2018.