The fan-beam short-scan FBP algorithm is not exact

The fan-beam short-scan filtered backprojection (FBP) algorithm is popular in the computed tomography (CT) industry and in clinical applications. This paper points out that the fan-beam short-scan FBP algorithm is not exact, even in an ideal noiseless situation. The error depends on the focal length of the imaging geometry. When the focal length is long enough, the error is small and can therefore be ignored.