Overview
The spine is a complex axio-skeletal structure housing the spinal cord and its investing meninges, nerve roots, paraspinal musculature, and an intricate vascular network. MRI is the modality of choice for neural structures; CT is preferred for bony detail in trauma and degeneration. Plain radiography is appropriate for minor injuries and alignment assessment. CT-myelography is used when MRI is contraindicated.
Vertebrae, Sacrum, and Associated Joints
Vertebral Column Overview
The vertebral column comprises 33 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused into the sacrum), and 3-4 coccygeal (rudimentary; often fuse into a single coccyx).
| Region | Count | Key Feature |
|---|---|---|
| Cervical | 7 (C1-C7) | Transverse foramina, bifid spinous processes (C3-C6); uncovertebral joints (C3-C7) |
| Thoracic | 12 (T1-T12) | Costal facets, long oblique spinous processes |
| Lumbar | 5 (L1-L5) | Large bodies, horizontal spinous processes |
| Sacrum | 5 (fused S1-S5) | Sacral foramina, sacroiliac joints, lateral masses |
| Coccyx | 3-4 (often fused) | Rudimentary; variable number and fusion |
Joints of the Spine
| Joint Type | Location | Imaging Notes |
|---|---|---|
| Intervertebral disc (fibrocartilaginous) | Between vertebral bodies C2-S1 | Nucleus pulposus: T2 hyperintense; annulus fibrosus: T2 hypointense |
| Zygapophyseal (facet) joints | Between articular processes | Synovial; best seen on CT axial/sagittal; degeneration = sclerosis, joint space narrowing, osteophytes |
| Atlantoaxial joint | C1-C2 | Median pivot + paired lateral gliding joints; odontoid-anterior arch gap ≤3 mm in adults, ≤5 mm in children |
| Atlanto-occipital joint | Occiput-C1 | Condylar; nodding (flexion/extension) motion |
| Uncovertebral joints (of Luschka) | Cervical only (C3-C7) | Lateral neurocentral joints; osteophytes cause foraminal stenosis |
| Sacroiliac joints | Sacrum-ilium | Anterior synovial portion, posterior ligamentous portion; CT/MRI for sacroiliitis |
| Costovertebral joints | Thoracic | Rib head to vertebral body (costocentral) and transverse process (costotransverse) |
Imaging Appearances of Vertebral Bodies
- Plain radiograph: Vertebral body height, endplate integrity, disc space, alignment (lordosis/kyphosis), pedicle silhouette
- CT: Cortical bone ~400-1000 HU; cancellous bone ~150-300 HU; disc ~50-100 HU; disc is hypodense relative to adjacent bone
- MRI: Normal marrow T1 hyperintense (fatty); red marrow conversion in disease → T1 hypointensity. Disc: nucleus T2 bright, annulus T2 dark. Endplate changes classified by Modic (I: T1↓T2↑, oedema/vascularisation; II: T1↑T2↑, fatty; III: T1↓T2↓, sclerosis)
Ligaments
| Ligament | Location | Function | MRI Signal |
|---|---|---|---|
| Anterior longitudinal ligament (ALL) | Anterior vertebral body surface | Limits extension | T1/T2 hypointense band |
| Posterior longitudinal ligament (PLL) | Posterior vertebral body within canal | Limits flexion | T1/T2 hypointense; may ossify (OPLL) |
| Ligamentum flavum | Between adjacent laminae | Limits flexion; closes posterior canal | T1/T2 intermediate-low; thickens and may ossify in stenosis |
| Interspinous ligament | Between spinous processes | Limits flexion | T2 intermediate |
| Supraspinous ligament | Overlying spinous process tips | Limits flexion | T2 hypointense |
| Tectorial membrane | C0-C2 (posterior; continues cranially as dura, caudally as PLL) | Limits atlantoaxial flexion | T2 hypointense |
Paraspinal Muscles
| Group | Muscles | Function |
|---|---|---|
| Superficial (erector spinae) | Iliocostalis, longissimus, spinalis | Extension and lateral flexion |
| Deep (transversospinalis) | Semispinalis, multifidus, rotatores | Rotation, fine postural stabilisation |
| Suboccipital | Rectus capitis posterior major/minor; obliquus capitis superior/inferior | Craniocervical positioning and rotation |
| Prevertebral | Longus colli, longus capitis, scalenes | Cervical flexion |
| Psoas major | Originates L1-L5 transverse processes and vertebral bodies | Hip flexion; forms iliopsoas with iliacus |
MRI: Muscles are intermediate signal on T1 and T2. Acute denervation → T2 hyperintensity (oedema). Chronic denervation → T1 hyperintensity (fatty replacement). CT: normal muscle ~40-60 HU; fat ~−100 HU.
Sinuvertebral (recurrent meningeal) nerves: Each spinal nerve gives rise to 2-4 small sinuvertebral branches that re-enter the intervertebral foramen to supply the dura, PLL, disc annulus fibrosus, facet joint capsules, and intraspinal blood vessels.
Spinal Cord: Structure and Internal Anatomy
Gross Anatomy
The spinal cord extends from the foramen magnum (medullary continuation) to the conus medullaris, which terminates at L1-L2 in adults (range T12 to the L2-L3 disc). At birth the conus lies at approximately L3; it reaches the adult level by around age 20 due to differential growth of the vertebral column relative to the spinal cord.
Two enlargements correspond to the limb plexuses:
| Enlargement | Cord Segments | Vertebral Level | Plexus |
|---|---|---|---|
| Cervical | C5-T1 | Approximately C3-T1 vertebrae | Brachial plexus |
| Lumbosacral | L2-S3 | Approximately T9-L1 vertebrae | Lumbar and sacral plexuses |
Both enlargements reflect a greatly increased mass of anterior horn motor cells.
External surface landmarks:
- Anterior median fissure: Deep midline anterior groove, extends entire cord length
- Posterior median sulcus: Shallow posterior midline groove
- Posterolateral sulci (bilateral): Entry points of posterior nerve rootlets
Internal Structure: Grey and White Matter
In cross-section, grey matter (cell bodies) forms an H-shaped (butterfly) column surrounding the central canal, enclosed by white matter (fibre tracts).
The cord is almost divided into two symmetric halves by the anterior median fissure and the posterior median septum. A white commissure lies anterior to the grey commissure; the grey commissure (central limb of the H) contains the central canal, the ependyma-lined downward continuation of the fourth ventricle, extending into the upper few millimetres of the filum terminale.
| Grey Matter Horn | Content | Region of Prominence |
|---|---|---|
| Anterior (ventral) horn | Lower motor neurones (somatic); medial cells → trunk; lateral cells → limbs (ventral = proximal, dorsal = distal) | Largest at cervical and lumbosacral enlargements |
| Posterior (dorsal) horn | Sensory relay neurones; extends to cord surface | All levels |
| Lateral horn | Preganglionic sympathetic neurones (T1-L2); preganglionic parasympathetic neurones (S2-S4) | T1-L2 and S2-S4 only |
White matter surrounds grey matter and is organised into three paired funiculi (columns):
| Column | Location | Major Tracts |
|---|---|---|
| Posterior (dorsal) | Between posterior horns | Fasciculus gracilis (medial; lower limb/sacral); fasciculus cuneatus (lateral; upper limb/cervical), vibration, proprioception, discriminative touch; largest in cervical region |
| Lateral | Between anterior and posterior horns | Lateral corticospinal tract (descending, voluntary limb motor); spinothalamic (ascending, pain/temperature); spinocerebellar tracts |
| Anterior (ventral) | Anterior to anterior horns | Anterior corticospinal tract (bilateral axial/girdle muscles); vestibulospinal; reticulospinal; tectospinal |
Key Ascending and Descending Tracts
| Tract | Column | Direction | Function | Decussation |
|---|---|---|---|---|
| Fasciculus gracilis/cuneatus | Posterior | Ascending | Vibration, proprioception, discriminative touch | Medulla (medial lemniscus) |
| Spinothalamic | Anterolateral | Ascending | Pain, temperature, crude touch | Within 1-2 spinal segments of entry |
| Lateral corticospinal | Lateral | Descending | Voluntary limb movement | Pyramidal decussation in caudal medulla |
| Anterior corticospinal | Anterior | Descending | Bilateral axial and girdle muscles | At level of termination (cervical/upper thoracic) |
| Lateral vestibulospinal | Lateral/anterior | Descending | Postural tone, balance; entire cord | Ipsilateral (no decussation) |
| Medial vestibulospinal | Near anterior median fissure (cervical) | Descending | Head and neck positioning | Ipsilateral (no decussation) |
| Rubrospinal | Lateral | Descending | Limb movement | Dorsal tegmental decussation (midbrain) |
| Reticulospinal | Anterior/lateral | Descending | Automatic posture and gait | Entire cord |
| Tectospinal | Anterior | Descending | Head orientation to visual/auditory stimuli | Dorsal tegmental decussation |
Clinical pearl, sacral sparing: The lateral corticospinal and anterolateral (spinothalamic) tracts are somatotopically laminated with sacral fibres most peripheral. Anterior spinal artery infarction may spare these sacral fibres (supplied by peripheral pial vessels), preserving perianal sensation and sacral motor function despite an otherwise severe anterior cord syndrome.
MRI Appearance of the Spinal Cord
- T2: Modality of choice for internal cord structure; CSF markedly hyperintense (myelographic effect); the H-shaped grey matter is subtly more hyperintense than surrounding white matter at higher field strengths; axial T2 best demonstrates the butterfly pattern
- T1: Cord isointense to grey matter; CSF hypointense; cord outlined by T1-bright epidural fat
- Cord dimensions (approximate): Cervical AP diameter ~8-9 mm; thoracic ~7 mm
- STIR: Sensitive for oedema/contusion (intrinsic T2 hyperintensity)
- High-resolution CISS/FIESTA: Cord herniation, CSF leak, vascular malformations
Cauda Equina and Nerve Roots
Below the conus medullaris, lumbar, sacral, and coccygeal nerve roots descend nearly vertically through the subarachnoid space to reach their respective intervertebral foramina, this collection is the cauda equina (nerve roots only; the filum terminale is not part of the cauda equina).
- The filum terminale is a slender pia mater extension from the conus apex, passing centrally through the cauda equina; it pierces the dural sac at S2, then attaches to the posterior aspect of the coccyx
- The subarachnoid space (thecal sac) ends at S2 and contains approximately 75 mL of the total ~150 mL CSF volume
- Lumbar puncture is performed at L3/4 or L4/5 (below the conus), safely within the cauda equina
Nerve root exit angles by level:
| Level | Direction of Root Exit |
|---|---|
| Cervical | Relatively horizontal |
| Upper thoracic | Briefly angle upward upon emerging from the theca, then curve downward to their foramen |
| Lower thoracic | Increasingly oblique downward |
| Lumbar | Steeply descending; pass through the lateral recess before exiting the foramen (clinically important in lateral recess stenosis) |
MRI: Cauda equina roots appear as discrete T2-hypointense linear/rounded structures within T2-hyperintense CSF. On axial T2, roots appear as small rounded hypointense dots. Post-contrast root enhancement is abnormal (radiculitis, lymphoma, carcinomatous meningitis, sarcoidosis).
Cauda equina syndrome is a surgical emergency caused by compression of the cauda (disc herniation, haematoma, tumour, abscess, severe central stenosis); MRI demonstrates the compressing lesion with nerve root clumping, displacement, or enhancement.
Spinal Meninges and Spaces
Three Meningeal Layers
| Layer | Characteristics | Imaging Notes |
|---|---|---|
| Dura mater | Thick outer fibrous layer; forms thecal sac; ends at S2; blends with epineurium of mixed spinal nerves distally | T1/T2 hypointense; gadolinium enhancement when inflamed |
| Arachnoid mater | Middle layer; sends delicate trabeculae to pia across subarachnoid space (lace-like arrangement over cord) | Not separately visualised; arachnoiditis = clumping/matting of roots |
| Pia mater | Innermost; closely invests cord surface; lines anterior median sulcus; forms filum terminale and denticulate ligaments | Not separately visible on routine MRI; leptomeningeal gadolinium enhancement = pathological |
Meningeal Spaces
| Space | Contents | Normal Imaging Appearance |
|---|---|---|
| Epidural space | Fat, internal vertebral venous plexus (Batson's), dural sleeves of nerve roots | Fat: T1 hyperintense; haematoma/abscess: T1/T2 variable; biconvex/localised morphology |
| Subdural space | Potential space between dura and arachnoid | Haematoma: crescentic T1 hyperintensity circumferential within thecal sac; roots displaced centrally ("inverted Mercedes-Benz sign" on axial T1 lumbar MRI) |
| Subarachnoid space | CSF (~75 mL), nerve roots, arachnoid trabeculae; ends at S2 | T1 hypointense, T2 hyperintense; filled by contrast on myelography |
Denticulate Ligaments
Lateral projections of pia mater that cross the subarachnoid space between anterior and posterior nerve roots, pierce the arachnoid, and attach to the inner surface of the dura. There are 21 paired denticulate ligaments on each side, extending from the foramen magnum to the T12/L1 level (the lowest lies between the T12 and L1 nerve roots). They stabilise the cord within the dural tube.
Arterial Supply to the Spinal Cord
Longitudinal Arteries
| Artery | Origin | Territory | Character |
|---|---|---|---|
| Anterior spinal artery (ASA) | Union of two branches from the vertebral arteries (intracranial) | Anterior ~two-thirds of cord: anterior horns, lateral corticospinal tract, anterolateral tracts | Single continuous vessel in anterior median fissure; reinforced by segmental radiculomedullary feeders throughout its course |
| Posterior spinal arteries (PSA, paired) | Vertebral artery or PICA (intracranial) | Posterior ~one-third of cord: dorsal columns, posterior horns | Paired but discontinuous plexiform network along posterolateral cord surface; reinforced by radiculopial feeders |
Intrinsic Arterial Distribution
- Sulco-commissural (central) arteries: Arise from the ASA; penetrate the anterior median fissure; supply one hemicord alternately, anterior grey horns, central grey matter, and central white matter
- Pial arterial plexus: Circumferential anastomosis around the cord; supplies peripheral white matter including the sacral fibres of the lateral corticospinal and anterolateral tracts (basis of sacral sparing)
Segmental Reinforcement by Region
| Region | Predominant Segmental Supply |
|---|---|
| Cervicothoracic | Vertebral, ascending cervical, deep cervical, costocervical trunk arteries |
| Midthoracic (T4-T8; watershed zone) | Small aortic perforators; dependent on collateral from above and below; most vulnerable to ischaemia |
| Thoracolumbar | Artery of Adamkiewicz (arteria radicularis magna) |
As fetal development proceeds, most segmental radicular arteries regress; those that remain form anastomoses with the anterior and posterior spinal arteries. The remaining radicular vessels are variable in number and position, and blood may flow up or down the cord from them.
Artery of Adamkiewicz (Arteria Radicularis Magna)
- Dominant radiculomedullary feeder to the thoracolumbar cord
- Arises from a left-sided lower intercostal or upper lumbar aortic branch, most commonly T9-L2
- Characteristic hairpin turn (ascending then descending loop) on angiography or CTA
- Occlusion during aortic or spinal surgery risks anterior cord ischaemia (anterior cord syndrome, paraplegia with preserved posterior column function)
- Preoperative identification by CTA (high injection rate ~6 mL/s with high-concentration contrast) or gadolinium-enhanced MRA; DSA is gold standard but invasive and reserved for experienced centres
Venous Drainage of the Spinal Cord
Intrinsic Perimedullary Veins
- Anterior and posterior midline longitudinal veins
- A pair of lateral longitudinal veins on each side (anterior and posterior to nerve roots)
- These form a loose coronal perimedullary venous plexus
- On myelography: tortuous curvilinear shadows on the posterior cord surface (supine films)
- On MRI T2: flow voids may notch the dorsal cord surface when enlarged; small pial surface T1 enhancement with gadolinium even when normal
Vertebral Venous Plexus (Batson's Plexus)
| Component | Location | Clinical Significance |
|---|---|---|
| Internal vertebral venous plexus | Epidural space (anterior and posterior internal plexuses) | Route of haematogenous metastatic spread; epidural haematoma |
| External vertebral venous plexus | Outside vertebral column | Communicates with pelvic, abdominal, and thoracic veins |
| Intervertebral veins | Through intervertebral foramina | Connect internal and external plexuses; drain to segmental veins (intercostal, lumbar, lateral sacral) |
Batson's plexus is valveless, permitting bidirectional flow and providing a haematogenous pathway for spinal metastases from pelvic and abdominal malignancies (prostate, breast, bladder, colorectal).
Neurovascular and Lymphatic Supply of the Vertebral Column
- Arterial supply: Segmental arteries (posterior intercostal, lumbar, lateral sacral) give dorsal branches to vertebrae; vertebral bodies receive supply via anterior and posterior nutrient arteries (metaphyseal arterioles)
- Venous drainage: Basivertebral veins drain the vertebral body into the internal vertebral venous plexus; the basivertebral vein produces a midline notch on CT (do not mistake for fracture)
- Lymphatics: Lateral aortic (para-aortic/lumbar) nodes for thoracolumbar regions; posterior mediastinal nodes for thoracic spine; lateral cervical nodes for the cervical spine
- Nerve supply: Sinuvertebral (recurrent meningeal) nerves, 2-4 per spinal level, re-enter the foramen to supply dura, PLL, annulus fibrosus, facet capsules, and intraspinal vessels; all major somatic plexuses (cervical, brachial, lumbar, sacral) are formed by anterior rami
Imaging Modalities: Comparative Summary
| Structure | Plain Radiograph | CT | MRI |
|---|---|---|---|
| Vertebral body | Outline, height, alignment | Cortex, cancellous detail, fracture morphology | Marrow signal; oedema T2/STIR hyperintense |
| Intervertebral disc | Disc height only | ~50-100 HU; calcification visible | Nucleus T2 bright; degeneration = T2 dark, height loss |
| Spinal cord | Not visible | Not visible (poor soft tissue contrast) | T2 best; axial T2 shows H-shaped grey matter |
| Cauda equina | Not visible | CT-myelography: filling defects | Axial T2: discrete hypointense root dots in hyperintense CSF |
| Epidural space | Not visible | Fat negative HU; haematoma/abscess variable | T1: fat bright; pathological contents variable |
| Meninges | Not visible | Thin; may calcify | Gadolinium enhancement = leptomeningeal disease |
| Vasculature | Not visible | CTA: Adamkiewicz (~6 mL/s injection rate) | Gadolinium-enhanced MRA; DSA gold standard for AVMs |
| Ligamentum flavum | Not visible | Thickening, ossification | T1/T2 hypointense; stenosis best assessed on sagittal T2 |
Clinical and Radiological Pitfalls
- Conus level: Normal at L1-L2; conus below the L2-L3 disc level raises concern for tethered cord; always report in relation to vertebral landmarks
- Transitional lumbosacral anatomy: Partial sacralization of L5 or lumbarisation of S1 risks wrong-level surgery; explicit communication of variant anatomy is mandatory
- Basivertebral vein: Midline posterior notch on CT vertebral body, do not mistake for burst fracture fragment
- Sacral sparing: Preserved perianal sensation and toe movement in apparent complete cord injury implies an incomplete lesion; reflects pial supply to peripheral sacral fibres
- Watershed infarction: T4-T8 midthoracic cord most vulnerable during hypoperfusion or aortic surgery
- Cauda equina syndrome: Surgical emergency; urgent MRI identifies the compressing pathology (disc, haematoma, tumour, abscess) with root clumping, displacement, or enhancement
- Flow voids vs. vascular malformation: Posterior cord T2 flow voids may be normal perimedullary veins or enlarged veins from spinal AVF/AVM; gadolinium-enhanced MRI and high-resolution CISS sequences help differentiate; DSA is gold standard but
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