Imaging of the Aging Spine

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16 Imaging of the Aging Spine

Colin S. Poon, Navid Zenooz, Gordon Sze

KEY POINTS

• Radiography is suited for evaluation of spine alignment. Lateral flexion and extension views are commonly used for assessment of segmental instability.
• MRI provides the most comprehensive imaging evaluation of the aging spine. Fat-suppressed imaging sequences are particularly valuable for imaging of spine trauma, inflammation, infection, and neoplasm. For inflammation, infection, and neoplasm, contrast enhanced T1-weighted imaging sequences can provide additional information.
• Nuclear bone scan is sensitive but not specific for evaluation of most active spine diseases.
• Radiography with lateral flexion and extension views is most commonly used for follow-up of postoperative spine. CT provides better assessment of hardware placement and postoperative complications. MRI and nuclear bone scan can be used for problem solving in patients with postoperative complications, and persistent or new symptoms.

Although the aging spine can be affected by a wide spectrum of diseases including neoplasm, infection, trauma, and degenerative disease, the latter by far is the most important in terms of disease burden and socioeconomic impact in the aging population. Back pain, with or without radiculopathy, is the most common indication for imaging of the spine. Patients with debilitating degenerative disease are often treated by surgery or other interventional procedures. Many of these patients will continue to have active complaints and require imaging follow-up. For these reasons, this chapter will focus on imaging of degenerative disease. Many other pathological conditions including trauma, infection and neoplasm can also affect the aging spine. An awareness of the imaging application in these diseases is important because a major role for early imaging of back pain is the exclusion of these “red flag” conditions. The imaging of these other diseases and postoperative spine, as well as a discussion of imaging techniques, are included in the Appendix (on the website) to serve as an introduction to these topics.

Imaging of Degenerative Spine Disease

Correlation between imaging morphology of degenerative disease and clinical symptoms can be poor, particularly for the most common complaint of pain. The reasons of the discrepancy are not clear, but several factors may come into play. Subjective complaints such as pain may be due to inflammatory response in the surrounding soft tissues, rather than mass effect that can be visualized directly on imaging. In addition, degenerative changes may indirectly compress the nerve roots by distorting their normal surrounding soft tissue structures, such as epidural fat, rather than compress the nerves directly. Imaging usually provides only a static snapshot of the anatomical structures. For example, most imaging studies are acquired with the patient supine, which is most likely different from the posture of the patients when they experience their symptoms. Although specialized units such as upright MRI scanners are now available to address these issues, their use is not yet widely adopted. Notwithstanding its limitations, imaging provides an important means for evaluation of the spine.

Degenerative disease of the spine most commonly involves the lumbar spine, followed by the cervical spine. Manifestations of degenerative spine disease include intervertebral disc degeneration, disruption of the annulus fibrosus, herniation of the nucleus pulposus, vertebral endplate changes, osteophyte formations, facet arthropathy, formation of juxta-articular cysts, degenerative spondylolisthesis, and spinal stenosis.

Intervertebral Disc Degeneration

On radiography (Figure 16-1), intervertebral disc degeneration is indirectly inferred from loss of the normal disc space height. Gas may be seen in the disc space, due to a negative pressure within the degenerative disc causing extraction of nitrogen from extracellular space. This is commonly referred to as vacuum phenomenon. The vacuum phenomenon can be accentuated during extension of the spine and reduced during flexion. Vertebral endplate irregularity is often seen, with or without associated sclerotic changes at the endplates.

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FIGURE 16-1 Radiographic features of degenerative disc disease.

Disc space narrowing and subtle cartilaginous endplate sclerosis are present at L4-L5.

With the wide availability of MRI, CT is rarely requested for the primary evaluation of degenerative disc disease, except in patients with contraindications for MRI examination. Similar to radiography, CT can demonstrate disc space loss, endplate irregularity or sclerotic changes, and vacuum phenomenon. However, CT also allows direct visualization of disc bulging and disc herniation (Figure 16-2), although with a lesser soft tissue contrast compared to MRI. When more accurate depiction of disc bulging and disc herniation is required, CT myelography can be performed (Figure 16-3) .

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FIGURE 16-2 CT of intervertebral disc degeneration.

A, Reformatted sagittal CT image in bone window showing severe disc degeneration including severe disc space loss, lucency within the disc space consistent with gas (vacuum phenomenon), and sclerosis at the adjacent endplates (white arrow). The facet joint also demonstrates irregular hypertrophy, osteophytes and loss of joint space (open arrow). Degeneration of these structures lead to instability, resulting in anterolisthesis of L4 over L5. B, Axial image in soft tissue window demonstrates diffuse disc bulging (white arrows), thickening of the ligamentum flavum (black arrows), and facet arthropathic changes that include joint space narrowing, facet hypertrophy, and vacuum phenomenon in the facet joints (open arrow). These changes lead to severe spinal canal stenosis, with the thecal sac (T) severely compressed anteriorly and posterolaterally.

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FIGURE 16-3 CT myelogram.

A, Axial image at the L3-L4 intervertebral level demonstrates a left central disc protrusion (open arrow), causing stenosis and impingement of the nerve roots at the left lateral recess. By comparison, the right L4 nerve root at right lateral recess (white arrow) is floating freely within the thecal sac. B, At the L2-L3 level, there is severe spinal stenosis as a result of disc bulging, ligamentum flavum hypertrophy, and facet arthropathic changes that include facet hypertrophy and sclerosis (circle), resulting in almost complete obliteration of the cerebrospinal fluid space (arrow).

MRI provides the best soft tissue details of degenerative disc disease. In young healthy patients, the intervertebral discs demonstrate hyperintensity on T2-weighted images. With aging, there is loss of this hyperintensity due to a decrease of water content and changes in proteoglycan composition (Figure 16-4). There is decreased disc height and the endplates may become irregular. Gas from vacuum phenomenon may fill the space of a degenerative disc, which may demonstrate hypointensity on both T1- and T2-weighted images. Alternatively, the space may be filled with fluid, which is seen as hyperintensity on T2-weighted images. A degenerative disc may also calcify, which can give hypointensity or hyperintensity on T1-weighted images, depending on the type and concentration of calcification. A degenerative disc may also enhance secondary to the presence of granulation tissues.

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FIGURE 16-4 Disc degeneration seen on MRI (T2-weighted image) (same patient as Figure 16-1). There is disc space narrowing and loss of the normal T2 hyperintensity of the L4-L5 disc. Bulging of the disc with a small protrusion into the spinal canal is also shown (arrow). Compare the L4-L5 disc with the normal appearance of the discs at L2-L3 and L3-L4 levels.

Fissures of the annulus fibrosus may be seen in the intervertebral discs. On MRI, annular disruptions (also referred as fissures) may be seen as a small high intensity zone within the outer annulus (Figure 16-5) .

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FIGURE 16-5 Annular disruption seen as a high intensity zone (arrow) on T2 weighted images.

One of the primary advantages of MRI is the direct visualization of disc bulging or herniation, and its associated mass effect on the nervous structures. At a particular disc level, a disc can have bulging and one or more areas of herniation seen on the same occasion. In 2001, multiple societies reached a consensus to standardize the nomenclature and classification of disc pathology.1 This work is currently being revised (A. Williams, S. Rothman, R. Murtagh, G. Sze, in progress). The consensus was initially developed for lumbar disc disease but is generalized to disc disease in the rest of the spine. Normal disc space is defined craniocaudally by the vertebral body endplates, and circumferentially by the ring apophysis of the vertebral bodies. In the newly revised consensus, a disc bulge refers to diffuse displacement of disc material beyond the normal disc space, and covers greater than 25% of the normal disc space circumference (i.e, greater than 90 degrees of the circumference) (Figure 16-6, A) . Disc displacement covering 25% or less of the circumference is called herniation. When the width of the base of the disc herniation is greater than any other measurements in the same plane of the herniation, it is called a protrusion (Figure 16-6, B). When any of the measurements of the herniation is greater than the width at its base, the herniation is described as an extrusion (Figure 16-6, C and D

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