Imperfect spoiling in variable flip angle T1 mapping at 7T: Quantifying and minimizing impact

Abstract

Purpose: The variable flip angle (VFA) approach to T₁ mapping assumes perfectly spoiled transverse magnetisation at the end of each repetition time (TR). Despite radiofrequency (RF) and gradient spoiling, this condition is rarely met, leading to erroneous T₁ estimates ( $T_1^{\text{app}}$ ). Theoretical corrections can be applied but make assumptions about tissue properties, for example, a global T₂ time. Here, we investigate the effect of imperfect spoiling at 7T and the interaction between the RF and gradient spoiling conditions, additionally accounting for diffusion. We provide guidance on the optimal approach to maximise the accuracy of the T₁ estimate in the context of 3D multi-echo acquisitions.

Methods: The impact of the spoiling regime was investigated through numerical simulations, phantom and invivo experiments.

Results: The predicted dependence of $T_1^{\text{app}}$ on tissue properties, system settings, and spoiling conditions was observed in both phantom and in vivo experiments. Diffusion effects modulated the dependence of $T_1^{\text{app}}$ on both $B_1^{+}$ efficiency and T₂ times.

Conclusion: Error in $T_1^{\text{app}}$ can be minimized by using an RF spoiling increment and gradient spoiler moment combination that minimizes T₂ -dependence and safeguards image quality. Although the diffusion effect was comparatively small at 7T, correction factors accounting for this effect are recommended.

Keywords: 7T; EPG; MPM; T1 mapping; VFA; imperfect spoiling.