Affiliations: Doubletask North York, Ontario, Canada | University of Toronto, Toronto, Canada | Northwestern Poly Technical University Xian, Xian, China
Note: [] Corresponding author: Shuangren Zhao, Doubletask North York,
Ontario, Canada. E-mail: shrzhao@rogers.com
Abstract: In computer tomography (CT), truncated projections are produced due
to scanning large objects with a detector that is limited in width. Applying
filtered back-projection(FBP) method directly to truncated projections, the
reconstructed image will contain truncation artifacts – bright rings on the
boundary of region of interest (ROI). Extrapolation algorithms can be used to
reduce the truncation artifacts; however extrapolations are usually double the
length of the projection data; resulting in an increased calculation time. This
paper introduces mixed extrapolation, which is a combination of exponential and
quadratic extrapolation. It is proven that doubling the length of the
projection data for the mixed extrapolation can be avoided. The projections
were extrapolated according to the boundary values and their derivatives. The
algorithm achieves equivalence to the extrapolation approach with negligible
increased calculation time. Supplementary functions are introduced in order to
simplify the calculations. These functions can be calculated prior to
extrapolation process, hence the calculation time is significantly reduced. The
calculation times are compared between fast extrapolation introduced in this
paper and normal extrapolation with doubling the length of projection data.
Keywords: Extrapolation, truncation artifacts, truncated projections, local tomography, limit field of view, reconstruction, incomplete projections, quadratic, exponential
