The spinal vertebral bodies are stabilized by the anterior and posterior longitudinal ligaments (ALL, PLL). The ALL runs from the basiocciput to the sacrum along the anterior and anterolateral aspect of the vertebral bodies and intervertebral discs. The PLL runs from C2 to the sacrum along the posterior surfaces of the vertebral bodies and discs. The PLL is firmly attached to the intervertebral discs but is slightly separated from the center of the vertebral bodies by connective tissue and the basivertebral veins. The PLL extends cephalad to the foramen magnum, merging into the tectorial membrane and dura matter. The ALL and PLL, owing to their fibrous nature, theoretically should show decreased MR signal and thus are not routinely visualized.

Ossification of the PLL is a type of degenerative disease that most of the time affects the cervical spine and the thoracic region in up to 25% of cases, leading to severe spinal stenosis, sometimes with severe compression of the cord and/or nerve roots. This entity has been mostly described in the Japanese population, where it is more common. Its etiology is unknown. In the cervical spine, the areas most commonly affected are C4, C5 and C6. For PLL ossification, MR provides excellent detail about the relationship between the calcified-ossified PLL, cord and nerve roots.

The characteristic MR finding in calcified-ossified PLL is that of an irregular, vertically oriented, low intensity signal structure in the anterior aspect of the spinal canal immediately behind the epidural venous plexus. Differentiation between calcified PLL and prominent osteophytes may be difficult on T1-weighted sequences; however, T2-weighted or field echo MR sequences are the most valuable sequences to demonstrate this pathology because of the excellent contrast between the epidural venous plexus, the signal void ossified PLL and the bright SAS.