Ip W R, Holden J E, Winkler S S
Phys Med Biol. 1983 Aug;28(8):953-62. doi: 10.1088/0031-9155/28/8/007.
In conventional computed tomography (CT) imaging of a point object, projection filtering causes the back-projected contributions to image positions away from the point to sum to zero. If the point object intensity is time-dependent, and all the projections are not acquired simultaneously, this cancellation cannot be complete and artefacts result. Loss of spatial invariance makes a general linear-systems approach to the problem impossible. We have studied the properties of such artefacts by the computer simulation of decaying exponential time-dependence in three different spatial distributions and four transmission and emission CT geometries. Spatially complex time-dependent objects typically produce artefacts that can be treated as an additional broad-spectrum noise source with a power comparable to that of other CT noises. Artefacts from broad ranges of similar time-dependence can add coherently to cause patches of artefact, particularly in geometries with a strong correlation between projection acquisition time and projection angle. As expected, artefacts are reduced for all geometries as scan duration is reduced. In our model, with a most rapid decay constant of 1.2 min-1, negligible artefacts were observed for a six second scan duration.
在对一个点状物体进行传统计算机断层扫描（CT）成像时，投影滤波会使反向投影到远离该点的图像位置的贡献之和为零。如果点状物体的强度随时间变化，并且所有投影不是同时采集的，这种抵消就无法完全实现，从而产生伪影。空间不变性的丧失使得用一般线性系统方法解决该问题变得不可能。我们通过计算机模拟三种不同空间分布以及四种透射和发射CT几何结构中指数衰减的时间依赖性，研究了此类伪影的特性。空间复杂的随时间变化的物体通常会产生伪影，这些伪影可被视为一种额外的广谱噪声源，其功率与其他CT噪声相当。来自广泛相似时间依赖性的伪影可能会相干叠加，导致出现伪影斑块，特别是在投影采集时间和投影角度之间有很强相关性的几何结构中。正如预期的那样，随着扫描持续时间的减少，所有几何结构中的伪影都会减少。在我们的模型中，对于1.2分钟⁻¹的最快衰减常数，在6秒的扫描持续时间内观察到的伪影可忽略不计。
