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Sampling effects for emerging cone-beam CT systems and scan trajectories: from Tuy’s condition to system design and routine image quality tests

Principles of cone-beam sampling are considered in terms of an analytical figure of merit for data incompleteness [tan(ψ𝑚𝑖𝑛)] and experimental measurements of cone-beam artifact magnitude in an image quality phantom. The phantom incorporates disk-pairs oriented parallel to the axial plane (perpendicular to the z-axis) at various locations throughout the field of view. Experiments were conducted in which the phantom was imaged using a mobile C-arm with various, tilted and untilted source-detector trajectories (orbits). In the current work, scan orbits were limited to circular orbits – viz., a semicircular arc spanning ~196°. The magnitude of cone-beam artifacts was assessed in terms of the signal modulation (zmod) in the narrow region between disk-pairs, and the relationship of zmod and tan(ψ𝑚𝑖𝑛) was evaluated for each orbit. The results show visually and quantitatively the effect of scan orbit and object orientation on cone-beam sampling effects and demonstrate the relationship between analytical tan(Ψ𝑚𝑖𝑛) and empirical zmod figures of merit. The analysis suggests a means by which cone-beam sampling effects (incompleteness) can be quantifiably considered in the development of new cone-beam CT system geometries, multi-source configurations, and protocols involving novel source-detector trajectories.