Duerk J L, Simonetti O P, Hurst G C
Department of Radiology, MetroHealth Medical Center, Cleveland, Ohio 44109.
Magn Reson Imaging. 1990;8(2):141-51. doi: 10.1016/0730-725x(90)90247-y.
A linear algebra based deprivation is presented to demonstrate that linearly time scaling an entire gradient waveform by a factor "R" exponentially increases its sensitivity to time derivatives of position by R(i + 1), where i refers to the i-th derivative of position (e.g., i = 1 is velocity). Thus, time scaling will preserve zero valued refocussing moments associated with artifact reduction techniques designed for motion occurring between excitation and detection. Typically, gradient waveforms for artifact reduction techniques are derived for use only at specific echo times. The time scaling described here allows for simple modification of refocussing gradient waveforms for use at variable echo times. Motion sensitivity associated with non-zero moment gradient waveforms can be easily predicted and modified using this technique, with consideration for field of view, resolution, and bandwidth. A clinical example is presented showing the predicted changes in sensitivity to nonrefocussed derivatives of position as the imaging gradients are time scaled. Further, trade-offs and alternatives in sensitivity to motion, slice thickness, image bandwidth, field of view and resolution will be discussed in conjunction with time scaling. This technique will have applicability in many situations involving MRI of moving tissue and a clinical example in cardiac imaging is presented.
提出了一种基于线性代数的剥夺方法，以证明将整个梯度波形按因子“R”进行线性时间缩放会使其对位置的时间导数的敏感度呈指数级增加，增加倍数为R(i + 1)，其中i指位置的第i阶导数（例如，i = 1时为速度）。因此，时间缩放将保留与为激发和检测之间发生的运动设计的伪影减少技术相关的零值重聚焦矩。通常，用于伪影减少技术的梯度波形仅在特定回波时间导出。这里描述的时间缩放允许简单修改重聚焦梯度波形，以便在可变回波时间使用。使用该技术可以轻松预测和修改与非零矩梯度波形相关的运动敏感度，同时考虑视野、分辨率和带宽。给出了一个临床示例，展示了随着成像梯度进行时间缩放，对位置的非重聚焦导数的敏感度的预测变化。此外，将结合时间缩放讨论在运动敏感度、切片厚度、图像带宽、视野和分辨率方面的权衡与替代方案。该技术将适用于许多涉及移动组织MRI的情况，并给出了心脏成像的临床示例。
