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

Actual Flip angle Imaging (AFI) is a sequence used for B₁ mapping, also embedded in the Variable flip angle with AFI for simultaneous estimation of T₁ , B₁ and equilibrium magnetization.

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.

The features of a preparation module with a B₁ insensitive adiabatic pulse, spoiler gradients, and a recovery time $$ 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.

When a 90° adiabatic pulse is used, the forthcoming $$ can be expressed as a function of repetition times and AFI flip angle only as $$ , 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° still allows for a faster approach to SS than the use of dummy pulses.

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.