Brain Imaging Center, Goethe University Frankfurt, Frankfurt/Main, Germany.
Department of Neurology, Goethe University Frankfurt, Frankfurt/Main, Germany.
Magn Reson Med. 2018 Jun;79(6):3082-3092. doi: 10.1002/mrm.26979. Epub 2017 Oct 20.
The variable flip angle method derives T maps from radiofrequency-spoiled gradient-echo data sets, acquired with different flip angles α. Because the method assumes validity of the Ernst equation, insufficient spoiling of transverse magnetization yields errors in T estimation, depending on the chosen radiofrequency-spoiling phase increment (Δϕ). This paper presents a versatile correction method that uses modified flip angles α' to restore the validity of the Ernst equation.
Spoiled gradient-echo signals were simulated for three commonly used phase increments Δϕ (50°/117°/150°), different values of α, repetition time (TR), T , and a T of 85 ms. For each parameter combination, α' (for which the Ernst equation yielded the same signal) and a correction factor C (α, TR, T ) = α'/α were determined. C was found to be independent of T and fitted as polynomial C (α, TR), allowing to calculate α' for any protocol using this Δϕ. The accuracy of the correction method for T values deviating from 85 ms was also determined. The method was tested in vitro and in vivo for variable flip angle scans with different acquisition parameters.
The technique considerably improved the accuracy of variable flip angle-based T maps in vitro and in vivo.
The proposed method allows for a simple correction of insufficient spoiling in gradient-echo data. The required polynomial parameters are supplied for three common Δϕ. Magn Reson Med 79:3082-3092, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
可变翻转角方法从具有不同翻转角α的射频扰相梯度回波数据集中得出 T 图。由于该方法假设 Ernst 方程有效，因此横向磁化的不足会导致 T 估计的误差，这取决于所选的射频扰相相位增量（Δϕ）。本文提出了一种通用的校正方法，该方法使用修正的翻转角α'来恢复 Ernst 方程的有效性。
模拟了三种常用的相位增量（50°/117°/150°）、不同的α值、重复时间（TR）、T 和 T 为 85 ms 的扰相梯度回波信号。对于每个参数组合，确定了（在 Ernst 方程产生相同信号的情况下的）α'和校正因子 C（α，TR，T）=α'/α。发现 C 与 T 无关，并拟合为多项式 C（α，TR），允许使用该Δϕ计算任何协议的α'。还确定了该校正方法对偏离 85 ms 的 T 值的准确性。该方法在体外和体内进行了具有不同采集参数的可变翻转角扫描测试。
该技术显著提高了体外和体内基于可变翻转角的 T 图的准确性。
所提出的方法允许对梯度回波数据中的不足扰相进行简单校正。为三个常见的Δϕ提供了所需的多项式参数。磁共振医学 79：3082-3092，2018。© 2017 国际磁共振学会。
