Residency · Residency · Diagnostic Radiology
Spine Imaging: Degenerative Disease and Red Flags
Overview
Clinical Context
Low back pain is the most common reason for spine imaging. However, the majority of low back pain is self-limited and does not require imaging. The ACR Appropriateness Criteria recommend imaging only when red flags are present or symptoms persist beyond 6 weeks of conservative management. MRI is the modality of choice for evaluating disc herniation, neural compression, and spinal cord pathology.
Normal Disc Anatomy
The nucleus pulposus is the central, gelatinous portion of the disc and appears T2 hyperintense due to its high water content. The annulus fibrosus is the peripheral structure consisting of concentric collagen lamellae and appears T2 hypointense. Normal disc height decreases with age, and loss of T2 signal reflects disc desiccation and degeneration. The endplates are thin cartilaginous layers between the disc and the vertebral body.
Disc Nomenclature (Combined Task Force Standardization)
Disc Contour Abnormalities
A bulge is a generalized extension of disc tissue beyond the vertebral body margins involving more than 50% of the disc circumference and is not considered a herniation. A herniation is a localized displacement of disc material beyond the normal margins. There are three types of herniation. A protrusion has a base wider than its apex and is contained by the outer annular fibers. An extrusion has a base narrower than its dome and extends through the annulus; it may be subligamentous (contained by the posterior longitudinal ligament) or transligamentous. A sequestration (free fragment) is an extruded fragment that has separated from the parent disc and may migrate cranially or caudally.
Direction of Herniation
A central (midline) herniation may compress the thecal sac or cauda equina. A paracentral (posterolateral) herniation is the most common type and compresses the traversing nerve root (for example, an L4-5 disc compresses the L5 root). A foraminal (lateral) herniation compresses the exiting nerve root (for example, an L4-5 disc compresses the L4 root). A far lateral (extraforaminal) herniation also compresses the exiting nerve root and is often missed on routine axial images.
Key Point: Which Root Is Compressed?
In the lumbar spine, a paracentral herniation affects the traversing root (numbered one level below; for example, an L4-5 paracentral herniation affects the L5 root), while a foraminal or far lateral herniation affects the exiting root (same level; for example, an L4-5 foraminal herniation affects the L4 root). In the cervical spine, nerve roots exit above the corresponding pedicle, so a C5-6 disc herniation affects the C6 root.
Spinal Stenosis
Central Stenosis
Central stenosis is narrowing of the central spinal canal caused by disc herniation, facet hypertrophy, ligamentum flavum hypertrophy, or spondylolisthesis. It is graded qualitatively as mild, moderate, or severe based on CSF effacement and compression of neural elements. In the lumbar spine, an AP diameter of less than 12 mm represents relative stenosis and less than 10 mm represents absolute stenosis.
Lateral Recess Stenosis
Lateral recess stenosis involves narrowing of the lateral recess (between the posterior vertebral body or disc and the superior articular process) and compresses the traversing nerve root. An AP diameter of less than 3 mm is considered stenotic.
Foraminal Stenosis
Foraminal stenosis is narrowing of the neural foramen caused by disc height loss, foraminal disc herniation, uncovertebral joint hypertrophy (in the cervical spine), or facet hypertrophy. It is graded based on normal fat surrounding the nerve root versus partial effacement versus complete effacement.
Modic Endplate Changes
Classification
| Modic Type | T1 Signal | T2 Signal | Pathology | Clinical Correlation |
|---|---|---|---|---|
| 1 | Hypointense | Hyperintense | Edema/inflammation | Correlates with active pain |
| 2 | Hyperintense | Hyperintense/isointense | Fatty marrow replacement | Most common; usually stable |
| 3 | Hypointense | Hypointense | Sclerosis | Least common |
Type 1 changes are T1 hypointense and T2 hyperintense, representing edema and inflammation, and correlate with active back pain. Type 2 changes are T1 hyperintense and T2 hyperintense or isointense, representing fatty marrow replacement; they are the most common type and are typically stable. Type 3 changes are T1 hypointense and T2 hypointense, representing sclerosis, and are the least common.
Clinical Significance
Type 1 changes correlate most strongly with low back pain. The types can convert over time, with conversion from Type 1 to Type 2 being the most common progression. Type 1 changes must be distinguished from discitis-osteomyelitis; infection crosses the disc space and destroys the endplate, whereas Modic 1 changes respect the endplate more cleanly.
Spondylolisthesis
Definition and Grading
| Meyerding Grade | Percent Slippage |
|---|---|
| I | 0-25% |
| II | 25-50% |
| III | 50-75% |
| IV | 75-100% |
| V (Spondyloptosis) | >100% |
Spondylolisthesis is forward displacement of one vertebral body relative to the one below. The Meyerding grading system classifies this as Grade I (0-25%), Grade II (25-50%), Grade III (50-75%), Grade IV (75-100%), and Grade V or spondyloptosis (greater than 100%).
Types
Isthmic (spondylolytic) spondylolisthesis results from a pars interarticularis defect (spondylolysis), is most common at L5-S1, and is best seen on CT or sagittal MRI. Degenerative spondylolisthesis results from facet joint degeneration and incompetence, is most common at L4-L5, and the pars is intact. Less common categories include traumatic, pathologic, and dysplastic spondylolisthesis.
Red Flags Requiring Urgent Imaging
Infection (Discitis-Osteomyelitis, Epidural Abscess)
On MRI, discitis-osteomyelitis produces a T2 hyperintense disc with loss of the intranuclear cleft, T1 hypointense endplates with enhancement, and paravertebral or epidural phlegmon or abscess (a rim-enhancing fluid collection). Disc space narrowing with endplate erosion is characteristic. The key difference from Modic 1 changes is that infection destroys the endplate and crosses the disc space, with more irregular enhancement. An epidural abscess appears as a T2 hyperintense collection in the epidural space with peripheral enhancement and may cause cord compression.
Malignancy (Metastatic Disease, Myeloma)
On MRI, metastatic disease produces T1 hypointense marrow replacement (darker than adjacent normal marrow or muscle), variable T2 signal, and enhancement with contrast. A pathologic compression fracture is characterized by a convex posterior cortex, an enhancing soft tissue mass, pedicle involvement, and signal abnormality in non-fractured levels. By contrast, a benign osteoporotic compression fracture shows retropulsion of a fragment, a linear low-signal fracture line, no pedicle involvement, a fluid sign (T2 bright cleft), and normal marrow signal in other vertebral bodies.
Cauda Equina Syndrome
Cauda equina syndrome results from compression of the cauda equina nerve roots, usually below L1-L2 at the level of the conus. Symptoms include bilateral leg weakness, saddle anesthesia, and bowel or bladder dysfunction. MRI shows a large central disc herniation, epidural mass, or hemorrhage compressing the cauda equina. This is a surgical emergency requiring imaging within hours.
Myelopathy (Cervical Cord Compression)
T2 hyperintensity within the spinal cord on sagittal images represents cord signal change and indicates myelopathy. T1 hypointensity, if present, suggests chronic, irreversible myelomalacia. Causes include severe stenosis (from spondylosis or ossification of the posterior longitudinal ligament), disc herniation, tumor, and epidural abscess.
<image>A sagittal T2-weighted MRI of the lumbar spine showing disc nomenclature. Four inset axial T2-weighted images at the L4-5 level demonstrate: (1) Normal disc. (2) Broad-based disc bulge with mild thecal sac effacement. (3) Paracentral disc protrusion compressing the left L5 traversing nerve root. (4) Disc extrusion with caudal migration (sequestered fragment). Each type is labeled and arrows indicate the nerve root involvement.</image>
<image>A sagittal MRI panel comparing benign vs. malignant vertebral compression fractures. Left panel: T1 and T2 sagittal images of a benign osteoporotic compression fracture showing a linear T1 dark fracture line, T2 fluid cleft sign, no posterior cortex convexity, and normal marrow signal in adjacent vertebral bodies. Right panel: T1 and T2 sagittal images of a pathologic compression fracture showing diffuse T1 hypointense marrow replacement, convex posterior cortex with epidural soft tissue mass, pedicle involvement, and signal abnormality in additional non-fractured vertebral bodies. Key differentiating features are labeled with arrows.</image>
<image>A sagittal T2 and post-contrast T1 fat-saturated MRI of the lumbar spine showing discitis-osteomyelitis. The L3-4 disc is T2 hyperintense with loss of the intranuclear cleft. The adjacent L3 and L4 endplates are eroded and show enhancement. A pre-vertebral phlegmon and a posterior epidural abscess (rim-enhancing collection compressing the thecal sac) are visible. An inset shows a normal disc for comparison. Arrows label the infected disc, eroded endplates, paravertebral collection, and epidural abscess.</image>
Clinical Pearls
Paracentral disc herniations compress the traversing root (one level below), while foraminal herniations compress the exiting root (same level) -- this distinction is critical for clinical-radiologic correlation. Far lateral (extraforaminal) disc herniations are easily missed, and the sagittal images lateral to the foramen should be scrutinized along with coronal reformats when available. Modic Type 1 endplate changes can closely mimic discitis-osteomyelitis; key differentiators include endplate destruction, paraspinal or epidural collections, and disc T2 hyperintensity in infection. The fluid cleft sign (T2 bright linear signal within a compression fracture) strongly favors a benign osteoporotic fracture over a pathologic fracture. The presence of cord signal change (T2 hyperintensity) in the setting of cervical stenosis should always be reported, as this finding changes management urgency and surgical timing. Imaging is not recommended for routine low back pain in the first 6 weeks without red flags (fever, weight loss, neurologic deficit, history of cancer, trauma, or immunosuppression).
References
- Fardon DF, et al. "Lumbar Disc Nomenclature: Version 2.0." The Spine Journal, 2014
- Modic MT, et al. "Degenerative Disk Disease: Assessment of Changes in Vertebral Body Marrow with MR Imaging." Radiology, 1988
- ACR Appropriateness Criteria: Low Back Pain, 2021
- Shah LM, Ross JS. "Imaging of Degenerative and Infectious Conditions of the Spine." Neurosurgery, 2016
