Evaluation of Stair-step Artifact on Virtual CT Endoscopy : Use of a "Y"-shaped Acrylic Phantom

Stair-step artifacts in helical computed tomography(CT)are associated with inclined surfaces in the Zaxis direction. We evaluated stair-step artifacts on virtual CT endoscopy. A "Y"-shaped phantom was scanned with a helical CT scanner, and the images were shown by using virtual CT endoscopy with combinations of x-ray beam collimation(1, 3, 5 and 10 mm), table velocity(1, 2, 3, 5 and 10 mm/s), and reconstruction interval(0.5, 1.5, 2, 2.5, 3, 5 and 10 mm). Stair-step artifacts arose from two sources:aliasing effect and rotation effect, forming the rugged wall of isoclosed curves and the rugged wall of spiral-like patterns. To eliminate these artifacts, the reconstruction interval should be about half of the table velocity. To eliminate artifacts of similar circles at the "Y"-point, the reconstruction interval should be less than one-third the internal diameter of the phantom. To eliminate the torsional fold at the "Y"-point, the table velocity sould be less than 5 mm/s.