Changes in cross-sectional measurements of the spinal canal and intervertebral foramina as a function of body position: in vivo studies on an open-configuration MR system.

Objective: The purpose of this study was to evaluate physiologic changes of the cross-sectional area of the spinal canal and neural foramina in young asymptomatic volunteers.
Subjects and Methods: Twelve asymptomatic volunteers were examined in a 0.5-T open-configuration MR system. T2-weighted fast spin-echo sequences were obtained in upright neutral, upright flexed, upright extended, and supine extended positions. The cross-sectional area of the spinal canal and the thickness of the ligamentum flavum were measured on angled axial images at the L4-L5 level. The anteroposterior diameter of the spinal canal and cross-sectional areas of the neural foramina were measured on sagittal images from L1 to S1.
Results: At disk level, the cross-sectional area of the spinal canal varied significantly between body positions, most notably between the upright flexed (mean, 268 mm2) and the upright extended (mean, 224 mm2) positions (p < .0001). The maximum thickness of the ligamenta flava increased in the extended positions (p < .0001). The cross-sectional area of the neural foramina underwent position-dependent variations of as much as 44.4%. The smallest cross-sectional areas were found in the extended positions.
Conclusion: In asymptomatic volunteers, MR imaging is able to show position-dependent changes in the cross-sectional areas of the spinal canal and the intervertebral foramina. The extended positions best reveal important findings.