MI Lab, Department of Medical Imaging, St. Olavs Hospital HF, N-7006 Trondheim, Norway.
J Magn Reson. 2011 Sep;212(1):74-85. doi: 10.1016/j.jmr.2011.06.012. Epub 2011 Jul 19.
The physiological noise in 3D image acquisition is shown to depend strongly on the sampling scheme. Five sampling schemes are considered: Linear, Centric, Segmented, Random and Tuned. Tuned acquisition means that data acquisition at k-space positions k and -k are separated with a specific time interval. We model physiological noise as a periodic temporal oscillation with arbitrary spatial amplitude in the physical object and develop a general framework to describe how this is rendered in the reconstructed image. Reconstructed noise can be decomposed in one component that is in phase with the signal (parallel) and one that is 90° out of phase (orthogonal). Only the former has a significant influence on the magnitude of the signal. The study focuses on fMRI using 3D EPI. Each k-space plane is acquired in a single shot in a time much shorter than the period of the physiological noise. The above mentioned sampling schemes are applied in the slow k-space direction and noise propagates almost exclusively in this direction. The problem then, is effectively one-dimensional. Numerical simulations and analytical expressions are presented. 3D noise measurements and 2D measurements with high temporal resolution are conducted. The measurements are performed under breath-hold to isolate the effect of cardiac-induced pulsatile motion. We compare the time-course stability of the sampling schemes and the extent to which noise propagates from a localized source into other parts of the imaging volume. Tuned and Linear acquisitions perform better than Centric, Segmented and Random.
三维图像采集的生理噪声强烈依赖于采样方案。我们考虑了五种采样方案：线性、中心、分段、随机和调谐。调谐采集意味着在 k 空间位置 k 和 -k 处的数据采集用特定的时间间隔隔开。我们将生理噪声建模为物理物体中具有任意空间幅度的周期性时间振荡，并开发了一个通用框架来描述它在重建图像中的呈现方式。重建噪声可以分解为与信号同相（平行）的分量和 90°异相（正交）的分量。只有前者对信号幅度有显著影响。该研究使用 3D EPI 聚焦于 fMRI。在比生理噪声周期短得多的时间内，单个拍摄中获取每个 k 空间平面。上述采样方案应用于缓慢的 k 空间方向，噪声几乎仅在该方向上传播。因此，问题实际上是一维的。提出了数值模拟和解析表达式。进行了 3D 噪声测量和具有高时间分辨率的 2D 测量。这些测量是在屏住呼吸的情况下进行的，以隔离心脏引起的脉动运动的影响。我们比较了采样方案的时间稳定性以及噪声从局部源传播到成像体积其他部分的程度。调谐和线性采集的性能优于中心、分段和随机采集。
