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

Abstract

Background: 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.

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 B₁ insensitive adiabatic pulse, spoiler gradients, and a recovery time $T_{rec}$ 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° adiabatic pulse is used, the forthcoming $T_{rec}$ can be expressed as a function of repetition times and AFI flip angle only as ${\mathrm{TR}}_1(n+\cos \alpha )/(1-{\cos }^2\alpha )$ , 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° 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.

Keywords: Actual Flip angle Imaging; B1 field; EPG simulations; T1 mapping; preparation pulse.