The sagittal plane provides extensive longitudinal spinal coverage and permits the simultaneous evaluation of vertebral bodies, intervertebral disks, ligaments, the spinal canal, neural foramina, and spinal cord. When special phased-array coils are used, almost the entire spinal column can be imaged in one view.

Sagittal imaging is supplemented by axial images at selected levels. Axial images are particularly useful for assessing the relationship of a lesion to the dural sac and cord. The imaging plane is oriented parallel to the intervertebral disk surfaces.

The coronal plane rarely adds information in spinal examinations. It is occasionally used to investigate paraspinal abscesses or tumors and to evaluate the joints at the craniocervical junction. Paracoronal (oblique) images may be helpful in evaluations of the cervical intervertebral foramina (see ▶ Fig. 11.18).

The standard slice thickness for spinal MRI is 3 to 4 mm. Axial images with a smaller slice thickness (1–2 mm) may be helpful for evaluating degenerative changes in the cervical spine (e.g., foraminal stenosis).

11.1.2 MRI Sequences

T1w and T2w spin echo (SE) or turbo spin echo (TSE) sequences are most commonly used for routine sagittal imaging of the spine. Axial imaging parallel to the disk space may employ two- or three-dimensional gradient echo (GRE) sequences, depending on the region of interest and clinical question:

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  GRE sequences: These were often used in the past to shorten the acquisition time. Since the advent of TSE sequences this rationale no longer applies, but GRE sequences are still useful for acquiring thin axial slices of the cervical spine. Also, they are generally less susceptible to cerebrospinal fluid (CSF) flow artifacts than TSE sequences are. GRE sequences cannot supply a true T2w image and can provide only T2*w contrast. This makes them much more sensitive to field inhomogeneities and susceptibility artifacts than TSE sequences. This disadvantage can, however, be utilized for the detection or exclusion of blood breakdown products. On the other hand, GRE images may overestimate bony narrowing of the spinal canal or intervertebral foramina, depending on the degree of T2* weighting.

  
  
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  Fat-suppressed sequences: These sequences are particularly helpful for detecting vertebral body edema, which may otherwise be obscured by the high signal from fatty bone marrow. The following two techniques are most commonly used:

  
  
  
  
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    In the STIR sequence, a modification of the inversion-recovery technique, the null point is adjusted in a way that suppresses the signal from fat. The STIR sequence combines high sensitivity with good anatomic resolution. It should be noted, however, that STIR images must be acquired before intravenous contrast administration because any enhancement could mask signal changes due to edema.

    
    
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    Fat signals can also be suppressed by adding a special presaturation pulse. This technique can be used with both T1- and T2-weighted SE and GRE sequences.

  
  
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  FLAIR sequences: FLAIR sequences are rarely used for spinal imaging. They are less sensitive to intramedullary lesions (e.g., in multiple sclerosis) than they are to supratentorial pathology.

  
  
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  Three-dimensional sequences with very heavy T2 weighting: These sequences are used for the high-resolution imaging of spinal nerves and for detecting thin septations or cysts in the spinal subarachnoid space (e.g., TrueFISP, CISS, FIESTA). With these sequences, the slice thickness can be reduced to the range of approximately 0.5 mm. Their main disadvantage is poor soft-tissue contrast. With proper reformatting, images can be produced that have a myelogram-like appearance (MR myelography).

  
  
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  Magnetic resonance angiography (MRA) sequences: Despite many technical attempts, available MRA sequences do not yet have the necessary resolution for imaging very small spinal vessels.

  
  
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  GRE–echo planar imaging (EPI) sequences: These sequences are used mainly to screen for ischemically induced diffusion changes in the brain. With their very short acquisition time, they can provide useful images even in restless patients. They are, however, very sensitive to local magnetic field inhomogeneities like those occurring at interfaces between bone and soft tissue. For this reason they are not suitable for routine spinal imaging.

The underlying clinical question is an important consideration in selecting the proper sequence:

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  Degenerative diseases: The imaging of degenerative spinal changes mainly requires high anatomic resolution and acceptable contrast between CSF, bone, and soft tissues. For example, TSE sequences with a high matrix (anatomic resolution), high echo train length (shorter acquisition time), and a long effective echo time (T2 weighting) are a good choice for sagittal T2w imaging. They can provide visually compelling images that clearly differentiate the intervertebral disk, subarachnoid space, and spinal cord.

  
  
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  Intramedullary lesions: The detection of intramedullary lesions requires a different strategy. For example, the sensitivity of T2w TSE sequences to multiple sclerosis lesions is reduced by increasing the echo train length (turbo factor) or increasing the degree of T2 weighting. Use of the magnetization transfer (MT) technique is helpful, however. STIR images (see above) also have high sensitivity to intramedullary lesions in multiple sclerosis.

  
  
  
  

  
  

  Pitfall

  
  

  
  

  A good general rule in intramedullary diseases is that the contrast between diseased and healthy spinal cord tissue is much less important than high anatomic resolution. Sequences that provide the most visually appealing images are not necessarily the first choice when it comes to lesion contrast.

11.1.3 Contrast Agents

Gadolinium contrast is generally unnecessary in the diagnosis of degenerative or traumatic disorders but may be helpful in differentiating between postoperative scar tissue and a herniated disk. Contrast administration is mandatory in searching for neoplastic or inflammatory diseases in the spinal canal ( ▶ Table 11.1). Areas of faint enhancement in the vertebral bodies are sometimes obscured by the bright signal from fatty bone marrow. The use of fat-suppressed T1w sequences may be necessary in these cases.

11.2 Spinal Column

11.2.1 Vertebrae

The human spinal column consists of 33 vertebrae that are numbered sequentially by region from above downward. It is subdivided into cervical, thoracic, lumbar, sacral, and coccygeal regions. There are seven vertebrae in the cervical spine (C1–C7), 12 in the thoracic spine (T1–T12), and five in the lumbar spine (L1–L5). The sacrum is formed by the fusion of five vertebrae (S1–S5) and articulates caudally with the four fused vertebrae of the coccyx (Co1–Co4). The spine has a physiologic lordosis at the cervical and lumbar levels and kyphosis in the thoracic region. The sacrum and coccyx together form a kyphotic curve. ▶ Fig. 11.1 illustrates the “double S-shaped” curvature of the normal spine.

Each vertebra from C3 to L5 consists of an anterior body and a posterior arch, which together enclose the spinal canal ( ▶ Fig. 11.2). The vertebral body is shaped like a short cylinder whose flat surfaces are formed by the upper and lower endplates. Each vertebral arch consists of paired roots called pedicles, lateral masses with the superior and inferior articular processes, transverse processes, posterior laminae, and spinous processes. The cross-sectional shape of the bony spinal canal is triangular in the cervical and lumbar vertebrae and round in the thoracic vertebrae. The inner diameter of the bony spinal canal decreases in the craniocaudal direction. The normal sagittal diameter in adults is approximately 15 to 16 mm at the T1–T2 level and approximately 12 mm in the lumbar spine.

The superior and inferior articular processes of adjacent vertebrae form the facet joints. The orientation of the articular facets varies at different levels.

The pedicles of two adjacent vertebrae together form an opening on each side called the intervertebral foramen. Located posterolateral to the vertebral body, these foramina are the openings through which the spinal nerves exit the spinal canal ( ▶ Fig. 11.3). The intervertebral foramina are bounded anteriorly by the vertebral body and elsewhere by the pedicles of the two adjacent vertebral bodies, the facet joint, and partly by the ▶ ligamentum flavum.

11.2.1.1 Cervical Vertebrae

The first and second cervical vertebrae (C1 and C2) present special features. The C1 vertebra (the atlas) consists basically of a ring-shaped vertebral arch; it lacks a body in the true sense ( ▶ Fig. 11.4). The reinforced sides of this arch, the lateral masses, support the skull and articulate with the occipital condyles to form the atlanto-occipital joint. Its inferior articular processes articulate with the axis to form the atlantoaxial joint ( ▶ Fig. 11.5). The C2 vertebra (the axis) differs from other vertebral bodies by the presence of a large process, the dens, that projects upward from the body. The dens arises from the anterior part of the vertebral body, and its tip extends almost to the level of the foramen magnum. The anterior part of the upper dens articulates with the anterior arch of the axis (atlantodental joint).

The bodies of the C3–C7 vertebrae have a roughly rectangular overall shape ( ▶ Fig. 11.6). They gradually increase in size from C3 to C7. Unlike other vertebrae, they have superior projections called uncinate processes that arise from the upper lateral margin of the vertebral bodies and articulate with the vertebral bodies above to form the uncovertebral joints (see ▶ Fig. 11.5). Another distinctive feature of the cervical vertebrae is the presence of transverse foramina (foramina transversaria) in the transverse processes. These foramina transmit the vertebral arteries and the plexus of vertebral veins ( ▶ Fig. 11.7, ▶ Fig. 11.8). The C7 vertebra is also called the “vertebra prominens” because of its large spinous process. Viewed in the sagittal plane, the articular surfaces of the facet joints in the cervical spine are angled 45º downward and backward from the horizontal.

11.2.1.2 Thoracic Vertebrae

The thoracic vertebral bodies are roughly triangular in outline and are slightly wedge-shaped when viewed in sagittal section. They gradually increase in height from above downward. The articular surfaces of the facet joints in the thoracic spine are oriented almost in the frontal plane ( ▶ Fig. 11.9). The thoracic vertebrae articulate with the ribs on the vertebral body itself (superior and inferior costal facets) and on the transverse process (transverse costal facet). The spinous processes in the midthoracic spine are long, slant downward, and overlap one another like shingles on a roof.

11.2.1.3 Lumbar Vertebrae

The five vertebral bodies in the lumbar spine are roughly bean-shaped. They are larger than in the cervical and thoracic vertebrae in conformance with the greater weight-bearing loads at the lumbar level (see ▶ Fig. 11.2). Because the transverse processes of the lumbar vertebrae are actually rudimentary ribs, they are also called “costal processes” ( ▶ Fig. 11.10). The articular surfaces of the facet joints have an approximately sagittal orientation in the upper lumbar spine and face obliquely outward in the lower lumbar region (see ▶ Fig. 11.3 and ▶ Fig. 11.10). The spinous processes of the lumbar vertebrae are powerfully developed and project straight backward.

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