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

Abstract

To perform dynamic T 1 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 T 1 mapping over a range of T 1 values. Methods: 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 T 1 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 T 1 , maximum errors were calculated for six T 1 values between 50 ms and 1250 ms. The method was demonstrated using in vitro and in vivo experiments.Results: Without corrections, 2D VFA severely underestimates T 1 . 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 T 1 values, the FA pair, and the RF pulse shape. FA pairs leading to <5% errors in T 1 can be identified for the common RF shapes, for T 1 values between 50 ms and 1250 ms.Conclusions: 2D VFA T 1 mapping with Bloch-simulation-based correction can deliver T 1 estimates that are accurate and precise to within 5% over a wide T 1 range.

Simultaneous proton resonance frequency T1 ‐ MR shear wave elastography for MR‐guided focused ultrasound multiparametric treatment monitoring

“…As showed by Lena et al, the best T 1 precision is achieved when the Ernst angle for the middle of the expected temperature range is used. 90 This study was performed for a single tissue type, so the relationship will be more complicated when imaging, for example, both muscle and fat. A high in-plane resolution is important to accurately detect the shear wave fronts.…”

“…It can be noted that the Ernst angle depends on T 1 , which in turn depends on temperature, so the Ernst angle will also be changing with temperature, and the most accurate approach is to select the Ernst angle that corresponds to the middle of the temperature range of interest. 89,90…”

Interleaved water and fat MR thermometry for monitoring high intensity focused ultrasound ablation of bone lesions

“…Since the method was applied in 2D, a correction for the slice profile effect was applied. ​ 24 , 25 We corrected the VFA T 1 maps by means of a T 1 look‐up table (LUT), obtained by Bloch simulations including slice profile effects. Briefly, for an input T1 value and two FAs, the steady state signals were calculated and used to find the apparent longitudinal relaxation time using the conventional VFA T 1 estimation.…”