Dowell Nicholas G, Tofts Paul S
NMR Research Unit, Institute of Neurology, University College London, London, UK.
Magn Reson Med. 2007 Sep;58(3):622-30. doi: 10.1002/mrm.21368.
RF field B1 nonuniformity is the largest cause of error in the quantitative measurement of many clinically relevant parameters in MR images and spectra. Knowledge of the absolute flip angles at every region will improve the accuracy and precision of such parameters. This method uses the 180 degrees signal null to construct a flip angle map of the entire brain in less than 4 min, independent of T1, T2, and proton density. Three spoiled gradient echo volume acquisitions of the whole brain were made with three different flip angles. The optimum choice of flip angles was determined to be 145 degrees, 180 degrees, and 215 degrees. Linear regression analysis was used to determine the nominal (system calibrated) flip angle required for a signal null at every pixel and thence determine the absolute flip angle at that location. The experiment utilizes an existing MR sequence supplied by the scanner manufacturer. The technique is validated experimentally and a theoretical investigation into the optimum experimental parameters is presented.
射频场B1不均匀性是磁共振图像和光谱中许多临床相关参数定量测量误差的最大原因。了解每个区域的绝对翻转角将提高此类参数的准确性和精度。该方法利用180度信号零点在不到4分钟的时间内构建全脑的翻转角图，与T1、T2和质子密度无关。用三个不同的翻转角对全脑进行了三次扰相梯度回波容积采集。确定的最佳翻转角选择为145度、180度和215度。使用线性回归分析来确定每个像素处信号零点所需的标称（系统校准）翻转角，从而确定该位置的绝对翻转角。该实验利用了扫描仪制造商提供的现有磁共振序列。该技术经过实验验证，并对最佳实验参数进行了理论研究。
