Rapid 2D variable flip angle method for accurate and precise T₁ measurements over a wide range of T₁  values

To perform dynamic T₁ mapping using a 2D variable flip angle (VFA) method, a correction for the slice profile effect is needed. In this work we investigated the impact of flip angle selection and excitation RF pulse profile on the performance of slice profile correction when applied to T₁ mapping over a range of T₁ values.

A correction of the slice profile effect is proposed, based on Bloch simulation of steady-state signals. With this correction, Monte Carlo simulations were performed to assess the accuracy and precision of 2D VFA T₁ mapping in the presence of noise, for RF pulses with time-bandwidth products of 2, 3 and 10 and with flip angle pairs in the range [1°-90°]. To evaluate its performance over a wide range of T₁ , maximum errors were calculated for six T₁ values between 50 ms and 1250 ms. The method was demonstrated using in vitro and in vivo experiments.

Without corrections, 2D VFA severely underestimates T₁ . Slice profile errors were effectively reduced with the correction based on simulations, both in vitro and in vivo. The precision and accuracy of the method depend on the nominal T₁ values, the FA pair, and the RF pulse shape. FA pairs leading to <5% errors in T₁ can be identified for the common RF shapes, for T₁ values between 50 ms and 1250 ms.

2D VFA T₁ mapping with Bloch-simulation-based correction can deliver T₁ estimates that are accurate and precise to within 5% over a wide T₁ range.