Anatomy:
The epidural space is located between the dura mater and the vertebral body.There is no communication between the cervical and intracranial epidural space. (1) [Figure1.]
Space of Okada [Figure3] was first described in the cervical spine (2) and spans approximately 5 mm length (3) It connects facets joints, neural foramen, paraspinal musculature and spinous process bursa on same level. This space spreads injected material and infection, into the paraspinal musculature and soft tissues without involvement of the vertebral body, disc, or epidural space. In case of pars defect, extension from one level to another might happens. (3) The space of Okada is visible when contrast is injected.
Contents:
Fat: the dominant component of posterior spinal epidural space, it protects the cord within the dural sac and spinal nerves.
Venous drainage: Batson’s plexus and intraosseous veins.
The route of spread of infection and malignancy to the epidural space is usually hematogenous because the venous system is valveless and the lymphatics are spared.
Arteries: Anterior and posterior spinal arterial arcades arise from vertebral arteries, thoracic and lumbar aorta.
Normal Structures and Artifacts Mimicking Spinal Disorders on MRI
Epidural veins
Sometimes cannot be distinguished from a herniated disc. especially if its located in the anterior epidural space.
Motion artifact
Patient movement, CSF pulsation and swallowing, produce turbulence in blood and CSF resulting in mixture of increased and decreased signal intensity. [Figure 4,5]
Incomplete Fat Saturation
Frequency-selective is the most popular method of eliminating unwanted fat signal. This technique is prone to focal failure, which results in high signal intensity at fat-air interfaces. This artifact arises from the magnetic susceptibility differences between fat and air, or from magnetic field inhomogenisity in large field of view. [Figure6]
Artifacts Due to Metal
Susceptibility artifacts due (e.g. pedicular screws and intervertebral spacers) occur because these metals become highly magnetized when placed in the magnet. Use of fat saturated sequences may exaggerate this artifact.