Introduction

Spinal infections are serious disorders, and it is imperative that the diagnosis be achieved as early as possible. Delay in appropriate treatment can result in irreversible sequelae such as instability, incapacitating deformity, and neurologic deficit. ¹ Imaging plays a vital role, as it provides valuable information regarding the presence and location of infection, the severity of disease, associated morphological changes, and clues toward the causative organism. ²

Although plain radiography remains the basic modality of investigation, advanced imaging techniques aid in early and accurate detection of radiographically occult, multifocal, and skip lesions. ³ , ⁴ They also enable tissue sampling at the most representative site within the lesion. In recent times, imaging is also increasingly used to assess the response and healing status of spinal infections after treatment. ⁵

The imaging techniques include plain radiography, ultrasonography (USG), computed tomography (CT), magnetic resonance imaging (MRI), and nuclear scintigraphy scans. Each technique has advantages and limitations ( Table 3.1 ). It is imperative that the techniques be skillfully used to the benefit of the patient. The treating physician and radiologist must also be aware of conditions such as degenerative spondylitis, inflammatory spondylitis, and neuropathic spondylo-arthropathy that can often mimic infection; in those situations, imaging should be interpreted with caution.

Pathophysiology

The commonest mode of infection in the spine is by hematogenous seeding of microorganisms, either through arterial arcades or venous plexus. ⁶ Retrograde flow of blood from the pelvic venous plexus to the perivertebral venous plexus has been demonstrated and is believed to be a major conduit for spread of infections. ⁷ Other modes of infection include iatrogenic direct inoculation and spread of infection from adjacent sites. ⁸ The frequent sites of pyogenic spondylodiskitis are the lower thoracic and lumbar spine followed by the upper thoracic region. ⁹ For tuberculosis (TB), the thoracic spine (37.5%) is the commonest location, followed by the thoracolumbar region (27.5%). ¹⁰

The intervertebral disks in children remain vascular up to 7 years of age, allowing primary inoculation of infection in the disk. After 7 to 8 years of age, hematogenous infections primarily infect the subchondral bone, with secondary involvement of the disks. Paradiskal infection on either side of the disk is the most common pattern of infection, as embryologically the disk and the adjacent bodies are supplied by the branches of the same vessel. ¹¹ Marrow signal alteration due to increased water content by exudates and edema is the earliest radiological finding detected by MRI, much before the appearance of radiographic changes. ¹² In TB, four types have been described based on the location of the lesion: paradiskal, central, anterior, and appendicular. ¹³

Pyogenic organisms release proteolytic enzymes such as hyaluronic acid, which destroy the disk substance early. With advancing subchondral infection, the intervening disk loses its nutrition, and this leads to secondary destruction with loss of disk height. ¹⁴ In contrast, disk involvement is late in mycobacterial infections due to the lack of proteolytic enzymes. The hypersensitive response of the host leading to marked exudation with an abscess containing inflammatory cells, caseous material, bone fragments, and occasionally tubercle bacilli can cause a large abscess in TB. ¹⁵

The constant increase in life expectancy and the ever-increasing incidence of medical comorbidities such as diabetes mellitus, hypertension, and HIV infections, along with the high prevalence of immune-deficient survivors, have resulted in an overall global increase in spinal infections. ¹⁶ In addition, with increasing reliance on surgical management of spinal disorders, iatrogenic and postoperative spinal infections have also become a cause of concern. ¹⁷

Plain Radiography

Plain radiography is the primary investigation of choice, but it has the limitation of delayed diagnosis of even up to 6 months, as at least 30% of bone destruction is required for identification. ⁵ Furthermore, lesions of the sacrum, neural arch/facet joints, and craniovertebral and cervicothoracic junctions could go undetected on radiographs.

Radiographic findings suggestive of infection include reduction in disk space, demineralization with blurring, resorption/erosion of end plates, and destruction and collapse of the vertebral body, resulting in deformity ¹⁸ ( Figs. 3.1 and 3.2 ). Indirect evidence of infections is the presence of prevertebral soft tissue shadows of abscesses in the cervical spine and thickened paravertebral shadows at the thoracolumbar region. Erosions in the anterior and lateral aspect of the vertebral body, called the Gouge defect, occur due to an anterior subperiosteal lesion under the anterior longitudinal ligament. ¹⁹ The aneurysmal phenomenon or syndrome represents anterior marginal scalloping, which is due to the mass effect. ²⁰ Although a large anterior collection of abscesses is the usual cause, it can also be due to lymphomas or aortic aneurysms. In advanced thoracic spine lesions with collapse, a bird’s-nest appearance due to radiodense globular and fusiform shadows can be seen. ²¹ There are no concrete radiographic features to differentiate pyogenic and TB spondylitis. However, the involvement of more than two vertebral bodies and the presence of perivertebral abscesses are more common in TB.

Computed Tomography Scan

The CT scan has the advantage of early detection of infection when compared with plain radiography, but it cannot detect the subchondral marrow changes that are seen on MRI, which are the earliest signs of infection. CT clearly demonstrates the bony morphology, the extent of osseous destruction, bone fragmentation, calcification in soft tissues, and the extent of deformity ( Fig. 3.3 ). Since CT provides irrefutable detail in evaluating the anatomy of the facets, pedicles and lamina, it helps the surgeon to decide on the need for instrumentation based on the stability of the spine. Bone fragments can be detected in the epidural soft tissue collections too ( Fig. 3.3f ). Air pockets due to gas collection can be seen at the site of infection ( Fig. 3.3e ) and is referred to as emphysematous osteomyelitis of the spine.

A plain CT does not demonstrate epidural involvement or spinal cord compression. CT myelogram is useful in detecting spinal canal compromise. Contrast-enhanced CT defines the extent of tubercular abscess, but the extent of epidural involvement cannot be determined. The CT features that suggest pyogenic spondylodiskitis include moderate circumferential paravertebral soft tissue involvement, sparing of posterior appendages, gas within the bone, disk space narrowing, and diffuse bone destruction. ²² Four patterns of bone destruction have been described in TB: osteolytic, fragmentary, subperiosteal, and localized sclerotic lesions ( Fig. 3.3 ). The fragmentary type is most common (47%), followed by the lytic type (24.1%). ²³

Magnetic Resonance Imaging

Magnetic resonance imaging is the most sensitive of all diagnostic investigations and is also the imaging modality of choice in spinal infections. ²⁴ It is noninvasive, and it is safer than both CT and plain radiography in that MRI does not employ harmful ionizing radiations. Although whole spine CT is harmful, whole spine MRI is safe and aids in detecting satellite and skip lesions efficiently. In addition, it enables detailed evaluation of the vertebral marrow, the disk space, the neural arch, the facet joint, paravertebral and epidural soft tissue, and the intraspinal structures including the dura, nerve roots, and the cord. MRI offers the benefit of analyzing the sacroiliac joints too, which if involved do not produce obvious radiographic changes.

The MRI sequences taken during evaluation of spinal infections include T1-weighted imaging (T1WI), T2-weighted imaging (T2WI), short tau inversion recovery (STIR) sequences, and contrast-enhanced T1WI. Marrow edema is the earliest sign that can be detected, and it appears hypointense on T1WI, isointense on T2WI, and hyperintense on STIR sequences ( Fig. 3.4 ). ²⁵ The STIR sequence (a type of fat-suppression sequence) is the most sensitive in detecting marrow edema ( Fig. 3.5 ). ³ In spinal infections, the usual pattern observed is the involvement of two adjoining vertebrae and the intervening disk. However, the collapse of a vertebral body with intraosseous abscess or bony destruction is not uncommon. Altered morphology of the disk can also be seen with loss of differentiation of the annulus fibrosus and nucleus pulposus, effacement of the intradiskal nuclear cleft, the appearance of intradiskal collection, and progressive reduction in its height ( Figs. 3.6 and 3.7 ).

Tuberculous versus Pyogenic Infection

Differentiation of tubercular and pyogenic lesions can be challenging. The thoracolumbar spine is the most common region affected in TB as compared with lumbar spine involvement in pyogenic infections. ²⁶ In TB, the involvement of the cervical spine and the isolated neural arch or facet joint is extremely rare. Involvement of the unilateral lamina and spinous process of the cervical and upper thoracic regions, although rare, is relatively frequent in TB, whereas facet joint and adjacent articular facets are more often involved in pyogenic spondylitis. ²⁷ The diagnostic clues in differentiating TB from pyogenic infection are listed in Table 3.2 . ²⁸

The MRI features of TB may be typical or atypical, and with the increasing incidence of multidrug-resistant strains, atypical clinical and radiological presentations are on the rise. Subligamentous spread of inflammatory tissue and initial preservation of the disk spaces are characteristic features of TB. Typical TB spondylitis is seen affecting a single region in ~ 65% of patients. Multiple contiguous-level infections are seen in 20% and multiple noncontiguous skip levels of involvement in ~ 10% ( Table 3.2 ). Atypical radiological presentations of spinal TB include osseous destruction of the vertebral body with sparing of the disks ( Fig. 3.8 ); vertebra plana (advanced collapse of the vertebral body), which is more common in children; ivory vertebra; isolated involvement of the neural arch/facet joint; a solid soft tissue component; and noncontiguous bony lesions ( Fig. 3.8c ).