Thomas David L, De Vita Enrico, Deichmann Ralf, Turner Robert, Ordidge Roger J
Wellcome Trust High Field MR Research Laboratory, Department of Medical Physics and Bioengineering, University College London, London WC1N 3AR, UK.
Magn Reson Med. 2005 Jun;53(6):1452-8. doi: 10.1002/mrm.20482.
A modification to the 3D modified driven equilibrium Fourier transform (MDEFT) imaging technique is proposed that reduces its sensitivity to RF inhomogeneity. This is especially important at high field strengths where RF focusing effects exacerbate B(1) inhomogeneity, causing significant signal nonuniformity in the images. The adiabatic inversion pulse used during the preparation period of the MDEFT sequence is replaced by a hard (nonadiabatic) pulse with a nominal flip angle of 130 degrees. The spatial inhomogeneity of the hard pulse preparation compensates for the inhomogeneity of the excitation pulses. Uniform signal intensity is obtained for a wide range of B(1) amplitudes and the high CNR characteristic of MDEFT is retained. The new approach was validated by numerical simulations and successfully applied to human brain imaging at 4.7 T, resulting in high-quality T(1)-weighted images of the whole human brain at high field strength with uniform signal intensity and contrast, despite the presence of significant RF inhomogeneity.
提出了一种对三维修正驱动平衡傅里叶变换（MDEFT）成像技术的改进方法，该方法降低了其对射频不均匀性的敏感性。这在高场强下尤为重要，因为射频聚焦效应会加剧B(1)不均匀性，导致图像中出现明显的信号不均匀性。在MDEFT序列准备期使用的绝热反转脉冲被一个标称翻转角为130度的硬（非绝热）脉冲所取代。硬脉冲准备的空间不均匀性补偿了激发脉冲的不均匀性。对于广泛的B(1)幅度范围都能获得均匀的信号强度，并且保留了MDEFT的高对比度噪声比（CNR）特性。新方法通过数值模拟得到验证，并成功应用于4.7 T的人脑成像，尽管存在显著的射频不均匀性，但仍能在高场强下获得全脑高质量的T(1)加权图像，且信号强度和对比度均匀。
