  For a plane symmetric object we can find two views - mirrored at the plane of
symmetry - that will yield the exact same image of that object. In consequence,
having one image of a plane symmetric object and a calibrated camera, we can
automatically have a second, virtual image of that object if the 3-D location
of the symmetry plane is known. In this work, we show for the first time that
the above concept naturally extends to transmission imaging and present an
algorithm to estimate the 3-D symmetry plane from a set of projection domain
images based on Grangeat's theorem. We then exploit symmetry to generate a
virtual trajectory by mirroring views at the plane of symmetry. If the plane is
not perpendicular to the acquired trajectory plane, the virtual and real
trajectory will be oblique. The resulting X-shaped trajectory will be
data-complete, allowing for the compensation of in-plane motion using epipolar
consistency. We evaluate the proposed method on a synthetic symmetric phantom
and, in a proof-of-concept study, apply it to a real scan of an anthropomorphic
human head phantom.
