INFECTION

Diskitis and Osteomyelitis

      Spine infections involve the vertebral body and adjacent disk interspace. Bacterial infections (Staphylococcus and Streptococcus) are most common and are spread by a hematogenous route. Risk factors include intravenous drug abuse, diabetes mellitus, immunocompromised states, and recent surgical procedures on or near the spine. Tuberculous and fungal infections result from systemic spread of pulmonary infections.

      The early diagnosis of spinal diskitis and osteomyelitis has important clinical implications. Diskitis and osteomyelitis have a typical appearance on T1- and T2-weighted images. On short TR/TE sequences, the earliest signs of infection include loss of signal in both the disk space and the end-plate marrow of the contiguous vertebral bodies. There is also disruption of the low-signal end-plate band separating the marrow cavity from the nucleus pulposus of the disk. The bacteria produce a proteolytic enzyme that destroys the disk material, so narrowing of the disk space also occurs in the early stages. On long TR/TE images, there is an increase in signal in the disk and in the contiguous vertebral body marrow cavities that may appear more extensive than the changes seen on the T1-weighted studies. Endnote Tuberculous and fungal infections have a propensity to extend beneath the longitudinal ligaments to involve multiple vertebral levels. Since these organisms don’t produce any proteolytic enzyme, the disk space is preserved during the early stages of infection. Gadolinium-enhanced T1-weighted images with fat suppression are the most sensitive for detecting spinal infections and for delineating the extent of disease. Endnote

      While these signal alterations are also seen in the marrow of patients with primary or metastatic neoplastic disease, as a rule tumor does not breach the end plate. Also, although extensive involvement of the marrow cavity may be present with neoplasia, including vertebral destruction and collapse, the disk space often remains intact or is only minimally affected. Based on the morphologic features of vertebral diskitis and osteomyelitis, a more accurate diagnosis of spinal infection can be made with MR imaging than conventional plain film radiography, nuclear medicine, and computed tomography. When early diskitis and osteomyelitis go undetected and untreated, epidural or paravertebral abscess may develop, requiring surgical drainage. Endnote

Epidural Abscess

       Most epidural abscesses are associated with diskitis or osteomyelitis, however, isolated infections of the epidural space can occur. The diagnosis of epidural abscess can be a challenge for both the clinician and radiologist. Endnote Patients may present with back pain or radicular pain. Fever and leukocytosis may be mild. Early diagnosis and prompt therapy are critical for favorable patient outcomes.

      The imaging findings can be quite subtle on plain T1 and T2-weighted images. During the cellulitis stage, the first sign of infection is thickening of the epidural tissues, which is initially isointense on T1-weighted images and moderately hyperintense on T2-weighted images. When liquefaction occurs, the abscess cavity becomes hypointense and more hyperintense on T1 and T2-weighted images, respectively. Detection of the infectious process is easier on gadolinium-enhanced scans. The inflamed tissues are very vascular and enhance with gadolinium. On both the T2-weighted images and the enhanced T1-weighted images, fat suppression increases the contrast between the infectious process and normal tissues. The abscess cavity does not enhance, and appears as thin linear region of hypointensity surrounded by the enhancing cellulitis on sagittal images. The abscess cavity has an oval configuration on axial images. Endnote  

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