Determination of Optimal Fat Suppression in LOW-TIDE B-SSFP Imaging using Eigenvalue Analysis

Introduction: Linear filter-based Optimal Window Transition to Driven Equilibrium (LOW-TIDE) is a preparation scheme for T2 weighting and intrinsic fat suppression with balanced SSFP imaging when partial Fourier encoding in the phase direction is used [1]. Both TIDE [2] and LOW-TIDE use a (α/2)-(TR/2) preparation pulse followed by a train of π (180°) excitation pulses. This is followed by a smooth ramp down to the final asymptotic flip angle train. The relationship between imaging parameters and fat suppression has previously only been determined through numerical simulations of the Bloch equations [3]. No inverse solution has been offered to predict the partial Fourier factor (PFF) necessary for fat suppression with a particular combination of imaging parameters. Here we explore a semi-analytical form (to determine optimal fat suppression) suitable for implementation in pulse programming software. The effective echo time to achieve optimal suppression can then be predicted and the partial Fourier factor (PFF) adjusted in a real-time fashion during scan prescription. Materials and Methods: A 4-term Blackman-Harris window was used for ramp down to the final flip angle [1]. The exact characterization of fat signal evolution can only be predicted through numerical solution of Bloch equations [3]. However, it has been shown [3] that signal evolution in bSSFP can be considered to be a third order linear system that can be analyzed using eigenvalue decomposition. Accordingly, the magnetization at the k iteration can be expressed as