Imaging sequences consist of sagittal and axial T1-weighted images (TR 500-650/TE 15), sagittal turbo spin-echo T2-weighted images. A specialized set of axial turbo spin-echo T2-weighted (TR 4000/TE (eff) 120) images, using a 256 x 256 matrix with a 22 cm field of view, are obtained in planes that are perpendicular to the spinal cord, approximated by visual inspection. Single axial images are obtained at each disk space level from C2/3 through C7/T1 with a slice thickness of 4 mm. Axial images are also obtained at the level of the base of the odontoid and the foramen magnum, perpendicular to the upper spinal cord, approximated by visual inspection (Figure 1). The AP spinal cord, transverse spinal cord and spinal canal diameters are measured on these specialized T2-weighted axial images using the MR scanner software (Figure 2).

To avoid ambiguity and maximize reproducibility, all measures are indexed to the canal level at the disc space, i.e., the C2-3 disc becomes the C2 canal. Note that the spinal cord level is one level above the "canal level." The AP compression ratio is computed from these last two measures; cord area is calculated as the area of an ellipse (a = p/ 4 d1d2).