Mutaf Yildirim D, Brinkmann Debra H
Division of Medical Physics, Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota, U.S.A.
J Appl Clin Med Phys. 2008 Oct 29;9(4):172-180. doi: 10.1120/jacmp.v9i4.2819.
The accuracy of four-dimensional computed tomography (4DCT) imaging depends on temporal characteristics of the acquisition protocol--for example, the temporal spacing of the reconstructed images (also known as cine duration between images) and the gantry rotation speed. These parameters affect the temporal resolution of 4DCT images, and a single default acquisition protocol, as commonly used in most clinics, may be suboptimal for a subset of respiratory motion characteristics. It could lead to substantial inaccuracies in target delineation. The aim of the present study was to evaluate the interplay between parameters affecting temporal resolution and the accuracy of the resulting images. We acquired 4DCT images of cylindrical phantoms under repetitive motion induced by a translation platform. Acquisition settings varied with respect to temporal spacing, gantry rotation speed, and motion period of the phantoms. Reconstructed images were sorted into 10 phase bins and were compared to static phantom images acquired at corresponding positions of the respiration phase. Acquisitions with different temporal spacing did not play a significant role in the amount of motion observed in full-cycle maximum intensity projection images. Target delineation accuracy at end-of-inhalation phase was observed to be constant up to a threshold in the value of the reconstruction interval, beyond which it varied arbitrarily. This threshold was found to be correlated with the number of phase bins and the motion period. No observable variations were noted with images from the end of exhalation when temporal spacing was varied. Target delineation accuracy was observed to be enhanced in acquisitions using faster gantry rotation speeds. An evaluation of the acquisition parameters needs to be performed depending on the period of the motion and limiting factors such as the availability of acquisition settings, X-ray tube workload, image storage, and processing power.
四维计算机断层扫描（4DCT）成像的准确性取决于采集协议的时间特性——例如，重建图像的时间间隔（也称为图像间的电影持续时间）和机架旋转速度。这些参数会影响4DCT图像的时间分辨率，而大多数诊所常用的单一默认采集协议可能对一部分呼吸运动特征而言并非最优。这可能导致靶区勾画出现显著误差。本研究的目的是评估影响时间分辨率的参数与所得图像准确性之间的相互作用。我们在平移平台诱导的重复运动下获取了圆柱形体模的4DCT图像。采集设置在时间间隔、机架旋转速度和体模的运动周期方面有所不同。重建图像被分类到10个相位区间，并与在呼吸相位相应位置获取的静态体模图像进行比较。不同时间间隔的采集在全周期最大强度投影图像中观察到的运动量方面没有起到显著作用。在吸气末期相位的靶区勾画准确性在重建间隔值达到一个阈值之前保持恒定，超过该阈值后则任意变化。发现该阈值与相位区间数量和运动周期相关。当改变时间间隔时，呼气末期图像未观察到明显变化。在使用更快机架旋转速度的采集中，观察到靶区勾画准确性有所提高。需要根据运动周期以及诸如采集设置的可用性、X射线管工作量、图像存储和处理能力等限制因素来对采集参数进行评估。
