Basic Definition
Beam shape refers to the shape of the area illuminated by x-rays emanating from the source. It may be determined by the engineering of the source or by post-source collimators. Common beam shapes include parallel beam, fan beam, and cone beam. Pencil beam is sometimes used in synchrotron settings, but not in lab microCT systems. Reconstruction algorithms are sensitive to beam shape and trajectory.
More Information
Further reading on beam shapes and their relation to detector geometry is available at https://www.ctlab.geo.utexas.edu/about-ct/essentials-of-computed-tomography/, under “Scanning Configuration”