CT measurement of the diameter of spinal and other bony canals: effects of section angle and thickness.

The effects of section angle and thickness on the measurement of spinal canal diameter by computed tomography (CT) were tested. A phantom of three spinal canals was constructed of three Lucite rods of different diameters, each covered with 1.8-mm-thick Teflon. A second phantom was constructed of two Lucite rods of the same diameter covered with 1.8- and 8.8-mm-thick Teflon, respectively. CT sections 10, 5, and 1.5 mm thick were obtained at various angles relative to the transverse plane of each canal. Anteroposterior diameters were measured at bone window settings using a standard software package and a method that uses a CT-number profile. Results show that sections obtained at an angle to the transverse plane of the canal do not always overestimate its diameter, as previously suggested, but can make it appear artifactually stenotic. The effect is from volume averaging and is most marked when thick, severely angled sections are used.