Wood M L, Zur Y, Neuringer L J
Department of Diagnostic Imaging, St. Michael's Hospital, Toronto, Ontario, Canada.
Med Phys. 1991 Sep-Oct;18(5):1038-44. doi: 10.1118/1.596739.
Steady-state free precession (SSFP) pulse sequences can produce magnetic resonance (MR) images rapidly, in which cerebrospinal fluid (CSF) is several times more intense than the other tissues. However, motion in the presence of magnetic field gradients reduces the intensity of CSF drastically, unless the time integral of the gradient waveform between each radio-frequency (rf) pulse vanishes. The consequences of motion on SSFP are explored here in detail theoretically and experimentally. The principle of gradient moment nulling is applied with the objective of giving CSF in SSFP images uniformly high intensity everywhere, in spite of motion. Theoretical analysis of the phase of the transverse magnetization from a group of isochromats, with a trajectory described by a Taylor series, reveals how motion along each direction disrupts SSFP and also causes ghost artifacts. Images of CSF in the cervical spine are found to have less extensive flow voids and weaker ghosts from pulsation if the first moment calculated from the rf pulse to the center of the gradient echo vanishes for both the frequency encoding and slice selection gradient waveforms. However, first-order moment nulling of the phase encoding gradient waveform is unnecessary for SSFP imaging of CSF.
稳态自由进动（SSFP）脉冲序列能够快速生成磁共振（MR）图像，在这类图像中，脑脊液（CSF）的信号强度比其他组织高出数倍。然而，在存在磁场梯度的情况下，运动将大幅降低脑脊液的信号强度，除非每个射频（rf）脉冲之间梯度波形的时间积分消失。本文从理论和实验两方面详细探讨了运动对SSFP的影响。应用梯度矩归零原理的目的是，即便存在运动，也能使SSFP图像中的脑脊液在各处都具有均匀的高强度。对一组等色团横向磁化强度相位的理论分析（其轨迹由泰勒级数描述）揭示了沿各个方向的运动如何干扰SSFP并产生重影伪影。如果从射频脉冲到梯度回波中心计算出的一阶矩对于频率编码和层面选择梯度波形均消失，那么在颈椎部位的脑脊液图像中会发现流动空洞范围更小，且由脉动产生的重影也更弱。然而，对于脑脊液的SSFP成像，相位编码梯度波形的一阶矩归零并无必要。
