Transverse relaxation and flip angle mapping: Evaluation of simultaneous and independent methods using multiple spin echoes.

PURPOSE: To evaluate transverse relaxation (T2) and flip angle maps derived from signal pathway modeling of multiple spin echoes using simultaneous or independent T2 and flip angle fitting.
METHODS: We examined different approaches to indirect and stimulated echo compensated T2 relaxometry from multiple spin echoes to evaluate both T2 and flip angle accuracy in simulation, phantom, and human brain. Signal pathways were modeled with or without independent flip angle maps using either Bloch simulations, or Extended Phase Graph (EPG) with Fourier or Shinnar-Le Roux approximation of slice profiles.
RESULTS: Slice-selective decay curves differ substantially between models. Inaccurate flip angles are obtained with EPG methods, although T2 values are relatively accurate. Providing measured flip angles to EPG methods yields erroneous T2. Bloch methods improve both T2 and flip angle results. Simultaneous fitting can suffer from flip angle redundancy yielding multiple T2 solutions, particularly in low signal-to-noise ratio cases.
CONCLUSION: EPG fitting provides reasonably accurate T2, but is limited by poor accuracy in resulting flip angles, and T2 errors increase when flip angles are provided. Bloch simultaneous fitting of T2 and flip angle provides excellent results, but can be limited by multiple solutions which can be overcome by including a flip angle map. Magn Reson Med 77:2057-2065, 2017.