求翻译：在二维 CT 系统中存在平行扫描和扇束扫描两种扫描模式，而基于平行光投影的重建算法最为简单，运算量最小，但是扫描时间长，仅在第一代 CT 中得到应用。然而基于平行投影的重建算法却是之后的扇束及锥束三维重建算法的基础，它们的核心算法都是滤波反投影（Filtered Backprojection，FBP） ，该算法由于其易于理解、便于实现、重建效果好，在目前 CT 中广泛使用。是什么意思？

The existence of parallel two-dimensional CT system scan and two scanning fan-beam scan mode, and parallel light projection based reconstruction algorithm is most simple, minimal computation, but the long scan time, only the first generation of CT has been applied. However, the reconstruction algori

In 2 -dimensional CT systems exist in parallel scanning and fan beam scanning two scanning mode and the reconstruction algorithm based on parallel optical projector computing volume is the easiest, most small, but scanning a long time and only in the first generation in CT are applied. However, as a

Has the parallel scan and the fan in the two-dimensional CT system ties scans two kind of scanning patterns, but is simplest based on the parallel light projection reconstruction algorithm, the operand is smallest, but the scanning time is long, only obtains the application in first generation of CT

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