Imaging factors influencing spine and cord measurements by CT: a phantom study.

Metrizamide computed tomography (CT) of the spine allows evaluation of the contents and measurement of the size of intracanalicular structures. The relative size (linear and area measurements) of spinal structures can be changed by varying imaging factors or the density of intrathecal contrast material. Two phantoms, one consisting of rods embedded in a plastic cylinder and the other of a vertebral body with a central rod simulating spinal cord, were evaluated with varying imaging factors (window width and window level) and different contrast concentrations within the surround. It was found that wide window widths allowed the most consistent measurements independent of window level, that a window level midway between the CT number of rod and surround would facilitate uniform measurements independent of window width, and that the use of high concentrations of contrast material (high CT number) in the surround, in combination with a wide window width, was most effective in establishing consistent measurements.