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Fig. 11. Epidural abscess. Sagittal T2-weighted (A) and sagittal postcontrast Tl-weighted (B) images show a dorsal epidural abscess that anteriorly displaces and compresses the spinal cord. The abscess is centrally bright (white arrowheads) on the T2-weighted image and peripherally enhancing (black arrowheads) on the Tl-weighted post-contrast image.

Gadolinium is essential in evaluating for resolution of the infection. The first MR indication of resolution (early healing) is a decrease in the amount of soft tissue enhancement or abnormal soft tissue (56). These findings are indicative of healing regardless of whether there is improving or worsening bone/disc changes. Late healing has a high peripheral T1 signal, which extends centrally with decreasing gadolinium enhancement (reliable sign). These findings may be due to a healing edge with fat deposition. An increase in enhancement does not necessarily mean treatment failure, as some patients with clinical improvement will initially have increasing or persistent gadolinium enhancement before showing decreased enhancement. The lack of gadolinium enhancement indicates that there is no longer active inflammation (56). The use of fat saturation following the administration of gadolinium increases the conspicuity of marrow involvement.

Differentiating pyogenic infections from granuloma-tous infections such as tuberculosis and Brucella is often difficult on MRI although there are certain findings that when present are more suggestive of one type of infection (44). Pyogenic infections tend to result in loss of disc height which is thought to be due to the release of proteolytic enzymes that digest the nucleus pulposus (49). Nonpyogenic organisms, such as Mycobacterium tuberculosis, usually do not affect the discs because they do not release the proteolytic enzymes (39,46,49,50,57). Disc space narrowing can, however, occur secondary to bone destruction which results in disc herniation into the vertebral body (49).

Tuberculosis often involves the middle column which is hypothesized to be secondary to the high blood flow to the posterior aspect of the vertebral body. This provides the mycobacterium the high concentration of oxygen it needs to survive. In addition, tuberculosis can affect the pedicles and posterior elements which is usually not seen with pyogenic infections or Brucella (50,55) (Fig. 12).

Tuberculosis often causes severe vertebral body damage with associated spinal deformity/collapse (gibbus) and frequently results in paraspinal and significant epidural involvement with abscess extension, meningeal involvement or bony fragment extension (55). These findings are atypical with Brucella. Paraspinal masses have been shown to be larger in tuberculosis than with pyogenic infections (49,58,59).

Tuberculosis tends to spread to adjacent vertebral bodies beneath the longitudinal ligaments and therefore can result in skip lesions (44,49). The presence of skip lesions or posterior element or multiple vertebral body involvement may hinder differentiation of this type of infection from neoplasms (49).

Vertebral Body
Fig. 12. Tuberculous infection. Axial post-contrast T1-weighted image in a patient with tuberculous spondylitis reveals a paravertebral abscess (arrowhead), dorsal epidural abscess, and involvement of the posterior elements.

addition, the loss of vertebral endplate definition, contiguous vertebral involvement, and obscuration of adjacent fat planes are most suggestive of infection, while tumors tend to not affect or only focally obscure the adjacent fat planes (60).

The differential for the MRI features seen with spinal infections in addition includes seronegative spondyloarthropathies and Modic type I degenerative changes. The spondyloarthropathies including ankylosing spondylitis, psoriatic arthritis, and Reiter's disease can present with similar findings to infection including disc space loss and mixed endplate lysis and sclerosis (39,46,61-63). One must therefore rely on additional clinical information and characteristic radiographic features seen with these processes. Modic type I degenerative changes exhibit decreased T1 signal and increased T2 signal which is often seen in infection. MRI differentiation between these two processes may be difficult and therefore one must try to determine if there is increased T2 disc signal and enhancement of the vertebral metaphysis and disc, which are reliable indicators of infection. Unlike infection, type I Modic discs rarely enhance (27,39,45,49,64,65). An understanding of the MRI findings seen with spinal infections is important as the early diagnosis and treatment may prevent permanent neuralgic deficits (50).

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