Spinal MRI Anatomy

Overview

Spinal MRI is the definitive modality for evaluating the spinal cord, nerve roots, and surrounding meninges. Analyzing neuroanatomy, from the termination of the conus medullaris to the intricate gray and white matter tracts, is a fundamental competency in neuroradiology.

Spinal Cord Termination

The precise level where the solid spinal cord transitions into the floating cauda equina is critical for identifying tethered cords and predicting surgical outcomes.

• Adult Termination (Conus Medullaris): In normal adults, the spinal cord tapers and terminates faithfully at the level of the L1-L2 intervertebral disc. A cord termination below the L2 inferior endplate is highly suspicious for a tethered cord syndrome, whereas neonatal cords naturally terminate lower (around L3) before ascending during growth.
• Cauda Equina: Below L1/L2, the nerve roots travel inferiorly within the expansive lumbar cistern (the subarachnoid space) wrapped exclusively by a thin arachnoid and dura.

Internal Architecture: Gray & White Matter

The spinal cord exhibits a geographic H-shaped gray matter core surrounded by distinct white matter ascending and descending tracts.

• Lateral Gray Horns: Unique to the thoracic and upper lumbar regions (T1 to L2). These horns house the vital preganglionic sympathetic cell bodies that dictate systemic autonomic outflow.
• Substantia Gelatinosa: Situated at the very apex of the posterior (dorsal) gray horn, this specialized nucleus serves as the primary relay center for crude touch, pain, and temperature pathways (the spinothalamic tract).

Nerve Root Exit Pathways

Radiologists must decisively map compressed nerve roots based on precisely which neuroforamina they exit.

• Cervical Spine: Cervical nerve roots exit above their corresponding vertebra. Thus, the C5 nerve root exits through the C4-C5 foramen. Because there are 8 cervical roots but 7 vertebrae, the C8 root exits below C7 (at C7-T1).
• Lumbar Spine: Below T1, all nerve roots exit below their corresponding vertebral body. Consequently, the massive L5 spinal root leaves via the exquisitely vulnerable L5-S1 intervertebral foramen. A classic L4/L5 paracentral disc herniation will typically spare the exiting L4 root but brutally compress the descending L5 root in the lateral recess.

Vascular Supply

The spinal cord is highly susceptible to ischemic insults resulting from inadequate collateral arterial networks.

• Anterior Spinal Artery: A single midline artery that comprehensively supplies the anterior two-thirds of the cord. This extensive territory includes the anterior gray horns (motor neurons) and the lateral white columns (corticospinal tracts).
• Posterior Spinal Arteries: Paired arteries that strictly supply the posterior one-third of the cord, localized predominantly to the dorsal columns (proprioception and fine touch).

High Yield Facts