Measuring the axial rotation of lumbar vertebrae in vivo with MR imaging

Abstract

Background and purpose: Flexion-extension radiography is neither sensitive nor specific in the diagnosis of degenerative spinal instability, a presumed cause of back pain and an indication for spinal fusion. We tested the hypothesis that with MR imaging and a device to rotate the torso, axial rotations of lumbar vertebrae can be measured with sufficient accuracy and that significantly different rotations can be detected between lumbar segments with degenerated disks and those with normal disks.

Methods: We studied five volunteers without back pain (group 1), five patients who underwent MR imaging because of back pain but were not considered candidates for fusion (group 2), and five patients in whom diskography identified one or more disks with concordant pain (group 3). Each participant was placed on a specially built table that provided separate supports for the torso and for the hips and legs. Series of sagittal images were acquired with a T2-weighted fast spin-echo sequence, with the torso rotated clockwise and then counterclockwise. The amount of rotation was calculated from axial images with use of an automated program.

Results: In the five volunteers, rotations of the lumbar motion segments varied between -1.8 degrees and 5.7 degrees, with an average of 0.8 degrees. The abnormal disks in five patients in group 2 rotated from -0.9 degrees to 5.6 degrees, with an average of 3.2 degrees. In group 3, the disks in which concordant pain was elicited rotated from 0.8 degrees to 4.4 degrees, with an average of 2.2 degrees. Difference in rotation between abnormal and normal disks was statistically significant.

Conclusion: Measurements of rotations of lumbar vertebrae with MR imaging may have value for determining levels that move abnormally in axial rotation.

Fig 1.

Table insert that provides clockwise and counterclockwise rotation at the lumbar spine. The insert is placed on the MR gantry. The patient is positioned on the insert with head and thorax on the longer segment and hips on the shorter segment. The segments are on rollers that permit them to rotate 8° in a clockwise and a counterclockwise direction, with the axis of rotation centered at a point 10 cm above the segment, so that the spine is at the isocenter of rotation.

Fig 2.

A–G, Images illustrate the application of the pixel shift program to measure rotation. The first step is to choose a vertebral level from each of the image sets with the thorax rotated clockwise (A) and counterclockwise (B). Note the vertical reference line to the left of each image. The next step is to choose the pivot point and region of interest for the pixel shift analysis (crosshairs and circular cursor in A and B). All voxels outside the cursor are excluded from analysis of motion (C and D). Alignment of the one image with the other before pixel shifting is illustrated by a subtraction image (E), which reveals a mismatch. When the one image is rotated with respect to the other to maximize the correlation, the angle of rotation (illustrated by the reference lines in F) is recorded. Alignment of the images after rotation is illustrated by a subtraction image (G), which shows no mismatch for the vertebral bodies.

Fig 3.

Sagittal T2-weighted image in a 29-year-old woman with chronic back pain. Intervertebral disks at L4–L5 and L5–S1 have diminished height, diminished signal intensity, and bulging of the posterior anulus fibrosus. No herniations are evident. Diskography subsequently showed concordant pain at the L4–L5 level only. The rotations were 0.3° at L1–L2, 1.3° at L2–L3, -0.1° at L3–L4, 2.2° at L4–L5, and 1.9° at L5–S1.

Fig 4.

Average rotation occurring at the five lumbar levels in the five control subjects as the gantry table is rotated left and right.

Fig 5.

Average rotation at the two lower lumbar levels with normal disks (gray line) and those with concordant pain (black line) elicited at diskography in group 3 patients.