A preparation pulse for fast steady state approach in Actual Flip angle Imaging

Background: Actual Flip angle Imaging (AFI) is a sequence used for B-1 mapping, also embedded in the Variable flip angle with AFI for simultaneous estimation of T-1, B-1 and equilibrium magnetization. Purpose: To investigate the design of a preparation module for AFI to allow a fast approach to steady state (SS) without requiring the use of dummy acquisitions. Methods: The features of a preparation module with a B-1 insensitive adiabatic pulse, spoiler gradients, and a recovery time Trec$T_{rec}$ were studied with simulations and validated via experiments and acquired with different k-space traveling strategies. The robustness of the flip angle of the preparation pulse on the acquired signal is studied. Results: When a 90 degrees DEG; adiabatic pulse is used, the forthcoming Trec$T_{rec}$ can be expressed as a function of repetition times and AFI flip angle only as TR1(n+cos & alpha;)/(1-cos2 & alpha;)$\mathrm{TR_1}(n+\cos \alpha )/(1-\cos <^>2\alpha )$, where n represents the ratio between the two repetition times of AFI. The robustness of the method is demonstrated by showing that using the values further away from 90 & DEG; still allows for a faster approach to SS than the use of dummy pulses. Conclusions: The preparation module is particularly advantageous for low flip angles, as well as for AFI sequences that sample the center of the k-space early in the sequence, such as centric ordering acquisitions, and for ultrafast EPI-based AFI methods, thus allowing to reduce scanner overhead time.