Introduction

1Variable Flip Angle T₁ Mapping¶

Variable flip angle (VFA) T₁ mapping Christensen et al., 1974Fram et al., 1987Gupta, 1977, also known as Driven Equilibrium Single Pulse Observation of T₁ (DESPOT1) Homer & Beevers, 1985Deoni et al., 2003, is a rapid quantitative T₁ measurement technique that is widely used to acquire 3D T₁ maps (e.g. whole-brain) in a clinically feasible time. VFA estimates T₁ values by acquiring multiple spoiled gradient echo acquisitions, each with different excitation flip angles ($\theta_{n}$ for n = 1, 2, .., N and $\theta_{i}$ ≠ $\theta_{j}$). The steady-state signal of this pulse sequence (Figure 2.7) uses very short TRs (on the order of magnitude of 10 ms) and is very sensitive to T₁ for a wide range of flip angles.

VFA is a technique that originates from the NMR field, and was adopted because of its time efficiency and the ability to acquire accurate T₁ values simultaneously for a wide range of values Christensen et al., 1974Gupta, 1977. For imaging applications, VFA also benefits from an increase in SNR because it can be acquired using a 3D acquisition instead of multislice, which also helps to reduce slice profile effects. One important drawback of VFA for T₁ mapping is that the signal is very sensitive to inaccuracies in the flip angle value, thus impacting the T₁ estimates. In practice, the nominal flip angle (i.e. the value set at the scanner) is different than the actual flip angle experienced by the spins (e.g. at 3.0 T, variations of up to ±30%), an issue that increases with field strength. VFA typically requires the acquisition of another quantitative map, the transmit RF amplitude (B₁⁺, or B₁ for short), to calibrate the nominal flip angle to its actual value because of B₁ inhomogeneities that occur in most loaded MRI coils Sled & Pike, 1998. The need to acquire an additional B₁ map reduces the time savings offered by VFA over saturation-recovery techniques, and inaccuracies/imprecisions of the B₁ map are also propagated into the VFA T₁ map Boudreau et al., 2017Lee et al., 2017.