CT fan beam reconstruction with a nonstationary axis of rotation

To reconstruct an image using computed tomography (CT), the axis of rotation must pivot at the same point on the reconstruction plane that the X-ray source and the CT detector assembly rotate about around the imaged object. This pivot point is used as a reference point for backprojecting pixel values to their proper coordinates. Reconstructing an image with a nonstationary axis of rotation would backproject pixel values to incorrect coordinate points. A convolution filtered backprojection algorithm has been derived for correcting images that were acquired with a nonstationary axis of rotation using the fan beam geometry with a collinear (flat) detector. The correction method accounts for the vertical displacements of the axis of rotation as the CT scanner rotates around the imaged object, as may be the case when sagging occurs. Software simulations are performed to show how the algorithm corrects for the shift in the axis of rotation