Wheeler-Kingshott C A, Crémillieux Y, Doran S J
Department of Physics, University of Surrey, Guildford, GU2 5XH, United Kingdom.
J Magn Reson. 2000 Mar;143(1):161-71. doi: 10.1006/jmre.1999.1978.
The effect of coherent rotational motion on images acquired with the ultrafast single-shot spin-echo Burst sequence has been analyzed. Previous experience has demonstrated that sample rotation during Burst experiments has the potential to cause severe image artifacts. In this paper we show that no distortions are visible when the readout gradient is parallel to the rotation axis, but that there is a very distinctive behavior for the case of the rotation axis orthogonal to the imaging plane. The mathematical expression that describes the resulting signal is presented and is used as a basis for a method of correcting the k-space data. The conditions under which undistorted images may be recovered are discussed. It is shown that there is an asymmetry, dependent on the rotation direction, in both the manifestation of the artifact and the range of angular velocities over which one can correct the images. Data from an agar gel phantom rotating at a known rate are used to show how the theory is successful at reconstructing images, with no free parameters. The range of angular velocities over which correction is possible depends on the timing parameters of the pulse sequence, but for these data was -0.016 < omega less, similar 0.1 revolutions/s. Volunteer experiments have confirmed that the theory is applicable to patient motion and can correct motional distortion even when the exact rate is not known a priori. By optimizing the reconstruction to restore a known sample geometry/aspect ratio, an estimate of the rotation angular frequency is obtained with a precision of +/-10%.
分析了相干旋转运动对超快单次自旋回波脉冲串序列采集图像的影响。以往经验表明，在脉冲串实验过程中样品旋转有可能导致严重的图像伪影。在本文中，我们表明当读出梯度与旋转轴平行时没有可见的失真，但对于旋转轴与成像平面正交的情况存在非常独特的行为。给出了描述所得信号的数学表达式，并将其用作校正k空间数据方法的基础。讨论了可以恢复无失真图像的条件。结果表明，在伪影的表现以及可以校正图像的角速度范围内，存在依赖于旋转方向的不对称性。来自以已知速率旋转的琼脂凝胶体模的数据用于展示该理论如何成功重建图像，且无需自由参数。可以进行校正的角速度范围取决于脉冲序列的定时参数，但对于这些数据，范围是-0.016 < ω ≤ 0.1转/秒。志愿者实验证实该理论适用于患者运动，并且即使事先不知道确切速率也可以校正运动失真。通过优化重建以恢复已知的样品几何形状/纵横比，可以获得精度为±10%的旋转角频率估计值。
