1) Description of Measurement

Lumbar AP canal diameter on CT quantifies the anteroposterior bony dimension of the spinal canal and is a classic metric for diagnosing developmental or degenerative lumbar spinal stenosis. It reflects the space available for the thecal sac and cauda equina and is particularly useful when MRI is unavailable or contraindicated.

2) Instructions to Measure

• Identify the lumbar level of maximal canal narrowing on sagittal CT.

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

• Identify:

• Posterior vertebral body cortex (anterior boundary), and

• Anterior cortex of the lamina / base of the spinous process (posterior boundary).
  
  

• Draw a straight line between these two points along the midline of the spinal canal.

• Record the distance in millimeters (mm) — this is the AP canal diameter.

3) Normal vs. Pathologic Ranges

• Normal AP diameter: > 12 mm

• Relative stenosis: 10-12 mm

• Absolute stenosis: < 10 mm

• Critical stenosis: < 7 mm

Key point:

Symptoms of neurogenic claudication are most likely when diameter is < 10 mm.

4) Important References

Tallroth K. Plain CT of the degenerative lumbar spine. Eur J Radiol. 1998 Jul;27(3):206-13. doi: 10.1016/s0720-048x(97)00168-x.

Jentzsch T, Mantel KE, Slankamenac K, et al. CT-based surrogate parameters for MRI-based disc height and endplate degeneration in the lumbar spine. BMC Med Imaging. 2024 Aug 13;24(1):213. doi: 10.1186/s12880-024-01395-1.

Khurana B, Prevedello LM, Bono CM, et al. CT for thoracic and lumbar spine fractures: Can CT findings accurately predict posterior ligament complex injury? Eur Spine J. 2018 Dec;27(12):3007-3015. doi: 10.1007/s00586-018-5712-z.

5) Other info....

AP diameter alone may underestimate severity when compression is asymmetric; combine with:

• Dural sac CSA

• Lateral recess width

• Foraminal dimensions

CT measurements reflect static bony stenosis and do not capture dynamic narrowing seen on extension.