Accurate CT measurement of the spinal cord using metrizamide: physical factors.

Computed tomography (CT) using metrizamide is a valuable tool in the neuroradiologic evaluation of the spinal cord, but CT must be used with care when measuring object size because size may vary with different window center (level) settings. A simple method to determine appropriate window center (level) settings during CT object measurement is described. A polystyrene core with a Teflon outer cylinder designed to simulate the spinal column, spinal cord, and subarachnoid space was scanned with varying concentrations of metrizamide with an EMI 1010 unit. Computer printouts of the CT pixel matrix were analyzed and a constant relationship of the phantom cord true size to the CT numbers of the metrizamide concentration and the phantom spinal cord was found. Window center (level) selection greatly influences measurement of cord size, whereas window width dose not. The appropriate window center (level) selection is the mean between the metrizamide CT attenuation number and the cord CT attenuation number.