1) Description of Measurement

Lumbar spinal canal cross-sectional area (CSA) quantifies the total area available for the cauda equina within the central canal. It is a sensitive morphologic marker for lumbar spinal stenosis and correlates more closely with symptom severity than single-dimension measurements such as AP canal diameter.

CSA incorporates the combined effects of disc bulge, facet hypertrophy, ligamentum flavum thickening, and spondylolisthesis, providing a global assessment of central canal compromise.

2) Instructions to Measure

• Identify the lumbar level of maximal stenosis on sagittal MRI.

• Scroll to the corresponding axial T2 slice at the disc level or mid-vertebral body level.

• Using a freehand or polygon ROI tool, trace the inner bony and ligamentous margins of the spinal canal, including thecal sac and excluding osteophytes or epidural fat.

• Close the region of interest to generate the cross-sectional area (mm²).

• Record the smallest CSA across all lumbar levels.

3) Normal vs. Pathologic Ranges

• Normal: > 100 mm2

• Relative (moderate) stenosis: 75 - 100 mm2

• Severe stenosis: 50 - 75 mm2

• Critical stenosis: < 50 mm2

Key points:

• Symptomatic neurogenic claudication is most commonly seen when CSA is < 75 mm².

• CSA < 50 mm² is strongly associated with severe clinical stenosis.

4) Important References

Abel F, Tan ET, Chazen JL, et al. MRI after Lumbar Spine Decompression and Fusion Surgery: Technical Considerations, Expected Findings, and Complications. Radiology. 2023 Jul;308(1):e222732. doi: 10.1148/radiol.222732.

Hansen BB, Nordberg CL, Hansen P, et al. Weight-bearing MRI of the Lumbar Spine: Spinal Stenosis and Spondylolisthesis. Semin Musculoskelet Radiol. 2019 Dec;23(6):621-633. doi: 10.1055/s-0039-1697937.

Genevay S, Atlas SJ. Lumbar spinal stenosis. Best Pract Res Clin Rheumatol. 2010 Apr;24(2):253-65. doi: 10.1016/j.berh.2009.11.001.

5) Other info....

CSA is preferred over AP diameter when:

• Compression is asymmetric

• There is significant facet hypertrophy or ligamentum flavum infolding

Should be interpreted with:

• Lateral recess depth

• Foraminal dimensions

Dynamic factors (extension-induced narrowing) are not captured on routine MRI; consider upright or flexion-extension imaging if symptoms and CSA are discordant.