How is lumbar spinal stenosis defined and described on CT and MRI

• Lumbar spinal stenosis refers to any type of bone or soft tissue pathology that results in narrowing or constriction of the spinal canal, nerve root canal, or both

• Central spinal stenosis refers to compression in the region of the spinal canal occupied by the thecal sac

• Lateral stenosis involves the nerve root canal and is described in terms of three zones, using the pedicle as a reference point (Fig. 12-5). Spinal stenosis may involve a single spine segment or multiple spinal segments. It may or may not be associated with instability of the spine.

Figure 12-5. Anatomy of spinal stenosis. Axial (A) and sagittal (B) views demonstrate anatomic relationships of the thecal sac and nerve roots to the surrounding lumbar osseous structures and intervertebral disc. The nerve root may be compressed along its course through the subarticular zone (zone 1), the foraminal zone (zone 2), or the extraforaminal zone (zone 3). In zone 3, the nerve root may be compressed as it exits the nerve root canal or further laterally in the so-called far-lateral region. (From Devlin VJ. Degenerative lumbar spinal stenosis and decompression. Spine State Art Rev 1997;11:107-28, with permission.)

12. A 65-year-old woman presents with symptoms of back pain and neurogenic claudication. Available imaging studies include a lateral myelogram image (Fig. 12-6A) and an axial CT image (Fig. 12-6B). What is the patient's diagnosis? Explain what neural structures are compressed.

The clinical and radiographic findings are classic for L4-L5 degenerative spondylolisthesis (grade 1). The lateral myelogram image shows an intact neural arch at the level of spondylolisthesis, leading to the diagnosis of degenerative spondylolisthesis. Disc degeneration and subluxation with subsequent facet joint and ligamentum flavum hypertrophy result in central spinal stenosis (open arrow and opposing arrows) and zone 1 (subarticular) lateral canal stenosis (small arrows). L4-L5 degenerative spondylolisthesis typically results in central spinal stenosis at the L4-L5 level associated with compression of the traversing L5 nerve roots bilaterally. The exiting L4 nerve roots are not typically involved unless there is advanced loss of disc space height, at which time the L4 nerve roots become compressed in the region of the neural foramen. Degenerative spondylolisthesis does not progress beyond a grade 2 (50%) slip unless prior surgery has been performed at the level of listhesis.

Figure 12-6. A, Lateral myelographic view shows L4-5 spondylolisthesis and spinal stenosis. B, Axial computed tomography (CT) scans at the L4-5 level shows central spinal stenosis (open arrow and opposing arrows) and zone 1 (subarticular) lateral canal stenosis (small arrows). (From Cole AJ, Herring SA. The Low Back Pain Handbook. Philadelphia: Hanley & Belfus; 1997, with permission.)

13. What is the role of CT in evaluation of a patient following spinal decompression and spinal fusion with instrumentation? When should a myelogram be added?

A CT scan can provide critical information following spinal decompression and fusion procedures. A myelogram should be performed in conjunction with the CT scan if it is necessary to assess the spinal canal and nerve root canals at the operative site. Problems that can be diagnosed with CT with or without myelography include:

• Persistent neural compression

• Adjacent level spinal stenosis or instability

• Nonunion following attempted spinal fusion (pseudarthrosis) (Fig. 12-7A, B)

• Incorrect placement of spinal implants including pedicle screws, interbody grafts, or fusion cages (Figure 12-7C). Although MRI may provide meaningful information in the presence of titanium spinal implants, significant artifact may persist and CT remains the best imaging test. MRI in the presence of stainless steel spinal implants will not provide useful information regarding the instrumented spinal segments due to artifact.

Figure 12-6. A, Lateral myelographic view shows L4-5 spondylolisthesis and spinal stenosis. B, Axial computed tomography (CT) scans at the L4-5 level shows central spinal stenosis (open arrow and opposing arrows) and zone 1 (subarticular) lateral canal stenosis (small arrows). (From Cole AJ, Herring SA. The Low Back Pain Handbook. Philadelphia: Hanley & Belfus; 1997, with permission.)

Figure 12-7. A, Coronal computed tomography (CT). B, Saggital CT image shows nonunion following attempted L1-L2 posterior interbody fusion. Note the lucencies around the interbody cages (superior arrows) and subsidence of cages into the L2 vertebral body (inferior arrows). C, Right-sided pedicle screw is improperly placed because it is not contained within bone and impinges on the adjacent nerve root. (A, B, From Fogel GR, Toohey JS, Neidre A, Brantigan JW. The Spine Journal 6: 421-427, 2006. C, From Devlin VJ. Spine Secrets. Philadelphia: Hanley & Belfus; 2003, with permission.)

Figure 12-7. A, Coronal computed tomography (CT). B, Saggital CT image shows nonunion following attempted L1-L2 posterior interbody fusion. Note the lucencies around the interbody cages (superior arrows) and subsidence of cages into the L2 vertebral body (inferior arrows). C, Right-sided pedicle screw is improperly placed because it is not contained within bone and impinges on the adjacent nerve root. (A, B, From Fogel GR, Toohey JS, Neidre A, Brantigan JW. The Spine Journal 6: 421-427, 2006. C, From Devlin VJ. Spine Secrets. Philadelphia: Hanley & Belfus; 2003, with permission.)

Key Points

1. Multiplanar CT is the imaging study of choice for evaluating the complex osseous anatomy of the spine.

2. Contrast agents may be injected into the thecal sac or intravenously to enhance CT visualization of the spinal cord, nerve roots, and vascular structures.

3. The radiation dosage associated with CT is an important concern and may be minimized by following appropriate protocols.

Website

Principles of CT and CT Technology: http://tech.snmjournals.Org/cgi/content/full/35/3/115

BiBLiOGRAPHY

1. Haaga JR, Dogra VS, Forsting M, et al., editors. Haaga: CT and MRI of the Whole Body. 5th ed. Philadelphia: Mosby; 2008.

2. Resnick D, Kransdorf MJ, editors. Resnick: Bone and Joint Imaging. 3rd ed. Philadelphia: Saunders; 2005.

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