Spine Radiology

A practical guide to reading cervical and lumbar spine X-rays and MRI for NEET PG. The exam loves correlating imaging findings with the clinical lesion — disc prolapse vs. canal stenosis, Meyerding grades of spondylolisthesis, Pott's spine erosion patterns, and cord/marrow signal changes. Master the "first investigation vs. investigation of choice" dichotomy and you will reliably bag these single-best-answer questions.

Imaging modalities: what to order and why

The spine can be imaged by plain radiograph, CT, MRI, myelography and bone scan. NEET PG repeatedly tests which is the first/screening investigation versus the best/confirmatory one.

Modality	Best for	Limitation
Plain X-ray (AP + lateral ± oblique)	Alignment, fractures, listhesis, gross bone destruction, disc-space narrowing	Soft tissue, cord and disc not seen
CT	Bony detail, fracture lines, posterior element/facet, calcified disc, OPLL	Radiation; poor for cord & ligaments
MRI	Cord, disc, nerve roots, marrow, abscess, ligaments — overall IOC for spine	Cost; contraindicated with old pacemakers/metal
Bone scan (Tc-99m MDP)	Detecting occult mets, infection, stress fracture; whole-skeleton survey	Non-specific (hot in any high-turnover lesion)
Myelography / CT-myelo	Arachnoiditis, when MRI contraindicated	Invasive; largely replaced by MRI

High-yield: MRI is the investigation of choice for almost the entire spine — disc prolapse, cord compression, Pott's spine, cord signal change, intramedullary tumours, and nerve-root compression. Plain X-ray remains the first/initial investigation for trauma and listhesis.

Approach to any spine film → 1) Alignment (3 lordotic curves) → 2) Bone (vertebral body height, pedicles) → 3) Cartilage/disc spaces → 4) Soft tissue (prevertebral) → 5) Special — count levels.

Normal radiographic anatomy and key lines

On a lateral cervical spine film, three smooth lordotic curves must be traced: the anterior vertebral line, the posterior vertebral line, and the spinolaminar line. Loss/step in any line signals instability or listhesis.

• Prevertebral soft tissue: at C2 should be < 7 mm; at C6 should be < 22 mm (the classic "7 at 2, 22 at 6" rule). Widening suggests a retropharyngeal haematoma/abscess or fracture.
• Atlanto-dental interval (ADI): normal < 3 mm in adults, < 5 mm in children. Widening = atlanto-axial subluxation (rheumatoid arthritis, Down syndrome, trauma).
• Pedicles on AP view should be intact and symmetric — a "winking owl"/absent pedicle sign suggests a lytic metastasis.

High-yield: Absent pedicle ("winking owl sign") on AP spine X-ray = metastasis (commonly lytic). The pedicle is the first structure destroyed in vertebral mets, hence the eye that "winks."

Disc prolapse (herniated nucleus pulposus)

The intervertebral disc has a central nucleus pulposus and a peripheral annulus fibrosus. With degeneration the nucleus herniates posteriorly/posterolaterally (path of least resistance, since the posterior longitudinal ligament is strong centrally), compressing the traversing nerve root.

Nomenclature (commonly confused)

• Bulge: > 50% (180°) of disc circumference extends beyond margins — diffuse, usually degenerative.
• Protrusion: focal; base wider than the dome of the herniated material.
• Extrusion: dome wider than the base (neck narrower than the herniated portion).
• Sequestration: free fragment that has lost continuity with the parent disc — can migrate.

Root compression rule (very high-yield)

In the lumbar spine, a paracentral (posterolateral) disc — the commonest type — compresses the traversing (lower) nerve root, whereas a far-lateral/foraminal disc compresses the exiting root.

Disc level	Paracentral herniation compresses	Clinical
L4–L5	L5 root (traversing)	Weak big-toe dorsiflexion (EHL), dorsum-of-foot sensory loss
L5–S1	S1 root (traversing)	Weak plantar flexion, absent ankle jerk, lateral-foot sensory loss
L3–L4	L4 root (traversing)	Weak knee extension, reduced knee jerk

High-yield: Commonest site of lumbar disc prolapse = L4–L5 (closely followed by L5–S1). A paracentral L4–L5 disc hits the L5 root, not the L4 root.

Imaging: MRI is the IOC. Disc material is T2 hyperintense if hydrated, and the herniation is seen indenting the thecal sac/root. Loss of the normal T2-bright nucleus = disc desiccation ("black disc"). A Modic change describes vertebral endplate marrow signal: Type 1 (oedema — T1 dark, T2 bright), Type 2 (fatty — T1 bright, T2 bright/iso), Type 3 (sclerotic — T1 & T2 dark).

Management: Most resolve with conservative therapy (analgesia, physiotherapy, short rest). Surgery (microdiscectomy) is indicated for cauda equina syndrome, progressive neurological deficit, or refractory radiculopathy. Cauda equina (saddle anaesthesia, bladder/bowel dysfunction, bilateral sciatica) is a surgical emergency — urgent MRI then decompression.

Spondylolisthesis & Meyerding grading

Spondylolisthesis = forward slip of one vertebra over the one below (commonest L5 over S1). Spondylolysis is a defect in the pars interarticularis, classically the "Scotty dog with a collar" on oblique lumbar X-ray (the collar = pars defect).

Wiltse classification (types)

1. Dysplastic (congenital)
2. Isthmic (pars defect — commonest in young adults/athletes)
3. Degenerative (older, facet arthropathy; commonest overall, esp. L4–L5)
4. Traumatic
5. Pathologic

Meyerding grading (percentage slip of vertebral body)

Grade	% Slip
I	0–25%
II	25–50%
III	50–75%
IV	75–100%
V (spondyloptosis)	> 100% (vertebra falls off)

High-yield: Meyerding grading divides the sagittal AP diameter of the top of the lower vertebra into quarters and measures how far the upper vertebra has slipped. Grade V = spondyloptosis. Grades I–II are usually managed conservatively; III–V or neurological deficit/instability → surgical fusion.

The "Scotty dog" mnemonic: nose = transverse process, eye = pedicle, ear = superior articular facet, neck = pars interarticularis (defect = collar/decapitation), front leg = inferior articular facet.

Spinal canal stenosis

Narrowing of the central canal, lateral recess, or neural foramen, compressing the cord/cauda equina. Most often degenerative (facet hypertrophy, ligamentum flavum buckling, disc bulge, osteophytes); congenitally short pedicles (achondroplasia) predispose.

• Cervical canal stenosis → myelopathy (spastic gait, hyperreflexia, clumsy hands, Lhermitte sign). Cord signal may show T2 hyperintensity (myelomalacia/oedema).
• Lumbar canal stenosis → neurogenic claudication: pain on walking/standing, relieved by sitting or flexion ("shopping-cart sign"), preserved distal pulses (distinguishing it from vascular claudication).

Feature	Neurogenic claudication	Vascular claudication
Relief	Sitting/bending forward	Standing still
Provoked by	Standing & walking	Walking only
Pulses	Normal	Diminished
Walking uphill	Better tolerated	Worse

Cut-offs: AP cervical canal diameter < 13 mm (Torg–Pavlov ratio < 0.8) suggests congenital cervical stenosis; lumbar canal AP diameter < 10 mm is absolute stenosis (10–12 mm relative). MRI IOC. Management: conservative first; decompressive laminectomy for progressive deficit/intractable symptoms.

High-yield: Neurogenic claudication is relieved by flexion (sitting, leaning on a trolley) because flexion enlarges the canal — the single most discriminating clinical clue from vascular claudication.

Vertebral collapse / compression patterns

A key exam theme is distinguishing benign (osteoporotic) from malignant (metastatic) vertebral collapse on MRI.

Feature	Osteoporotic (benign)	Malignant collapse
Marrow signal	Preserved fat in spared marrow; band-like T1 ↓	Diffuse T1 ↓ replacing whole body
Pedicle	Spared	Often destroyed (winking owl)
Posterior cortex	Retropulsed angular fragment, concave	Convex posterior bulge/soft-tissue mass
Other levels	May have other osteoporotic wedges	Multiple discrete lesions, paraspinal mass
"Fluid sign"	Present (Kummell disease/avascular cleft)	Absent

• Wedge (anterior) collapse: osteoporosis, trauma.
• Biconcave ("codfish/fish-mouth") vertebra: osteoporosis, osteomalacia, sickle cell.
• "H-shaped/Lincoln-log" vertebra: sickle-cell disease (central endplate infarction).
• "Picture-frame" vertebra & ivory vertebra: Paget disease; a single dense ivory vertebra also seen in osteoblastic mets (prostate, breast), lymphoma.
• "Rugger-jersey spine": renal osteodystrophy / secondary hyperparathyroidism (dense endplate bands).
• Vertebra plana (uniform pancake collapse) in a child = Langerhans cell histiocytosis (eosinophilic granuloma) — Calvé disease.

High-yield: A child with vertebra plana → Langerhans cell histiocytosis. An adult ivory (sclerotic) vertebra → think osteoblastic metastasis (prostate), Paget, or lymphoma.

Pott's disease (tuberculous spondylitis)

The classic and most NEET-tested infective lesion of the spine. Spread is haematogenous (Batson venous plexus) from a pulmonary/visceral focus; the thoracolumbar junction (D12–L1) is most commonly affected.

Imaging hallmarks

• Paradiscal lesion is the commonest pattern: infection begins in the anteroinferior corner of two adjacent vertebral bodies near the endplates, spreading under the anterior longitudinal ligament to the next vertebra.
• Disc space is relatively preserved early (TB lacks proteolytic enzymes) — a crucial point distinguishing it from pyogenic spondylitis where the disc is destroyed early.
• Gibbus deformity: sharp, angular kyphosis from anterior wedge collapse of multiple bodies — the visible knuckle/hump.
• Cold abscess: paravertebral soft-tissue shadow; can track to the psoas (psoas abscess) or present in the groin. Calcification within a paravertebral abscess is virtually pathognomonic of TB.
• "Aneurysmal/scalloped" anterior vertebral erosion from subligamentous abscess.

Feature	TB (Pott) spondylitis	Pyogenic spondylitis
Disc space	Preserved early	Destroyed early
Spread	Subligamentous, multilevel, skip lesions	Localised, 2 vertebrae
Abscess	Large, cold, may calcify	Smaller, hot/painful
Onset	Insidious (weeks–months)	Acute
Sclerosis	Late	Earlier reactive sclerosis

High-yield: Paradiscal erosion with relative disc preservation + a calcified paravertebral cold abscess + gibbus = Pott's spine. MRI is the IOC (detects marrow oedema, abscess, cord compression earliest); biopsy/GeneXpert confirms. Treat with ATT; surgery (decompression/fusion) for cord compression — Pott's paraplegia.

Pott's paraplegia is the dreaded complication; "early-onset" (during active disease, from abscess/oedema — better prognosis) versus "late-onset" (from bony bridge/gibbus mechanical compression — worse). Hueston's/Tuli's "middle-path regimen" balances conservative ATT with selective surgery.

MRI cord signal changes — the language of the report

• T2 hyperintensity within the cord = oedema, myelomalacia, demyelination, infarct, or contusion — a marker of cord injury in compressive myelopathy.
• T1 hypointensity with cord expansion = tumour/oedema; with enhancement consider neoplasm or active demyelination.
• "Owl's eye"/"snake eye" sign (bilateral T2-bright anterior horns) = cord infarct or chronic compressive myelopathy.
• "Pencil-like/longitudinally extensive" cord T2 signal (≥ 3 segments) = neuromyelitis optica spectrum (vs. short segment in MS).
• Syrinx: central CSF-signal cavity (Chiari, post-traumatic).
• Gibbs/truncation artefact can mimic a syrinx — beware.

High-yield: Persistent cord T2 hyperintensity in cervical spondylotic myelopathy predicts poorer surgical recovery — a favourite correlation question.

Named signs & eponyms worth memorising

• Scotty dog with collar — spondylolysis (pars defect), oblique lumbar X-ray.
• Winking owl / absent pedicle — vertebral metastasis.
• Rugger-jersey spine — renal osteodystrophy.
• Fish/codfish vertebra — osteoporosis/osteomalacia.
• H-shaped vertebra — sickle-cell disease.
• Ivory vertebra — osteoblastic mets / Paget / lymphoma.
• Bamboo spine + dagger sign + shiny corners (Romanus lesion) — ankylosing spondylitis.
• Vertebra plana (Calvé) — Langerhans cell histiocytosis (child).
• Gibbus + paradiscal erosion — Pott's disease.

Ankylosing spondylitis (commonly paired with Pott's)

Seronegative HLA-B27 spondyloarthropathy. First and most sensitive site = sacroiliac joints (bilateral symmetric sacroiliitis). X-ray progression: erosions → "pseudo-widening" → sclerosis → fusion. Spinal features: squaring of vertebrae, shiny corners (Romanus lesions), syndesmophytes (vertical, marginal — vs. horizontal/bulky osteophytes of degeneration), culminating in the "bamboo spine." The "dagger sign" is a single central radiodense line from ossified supraspinous/interspinous ligaments. MRI shows active sacroiliitis (STIR/T2 marrow oedema) before X-ray changes — MRI is most sensitive for early disease.

High-yield: Syndesmophytes are thin, vertical, marginal and bridge adjacent vertebrae (AS); osteophytes are horizontal, non-marginal beaks (spondylosis). The earliest AS imaging change is bilateral sacroiliitis, best seen early on MRI.

Key differentials at a glance

• Radiculopathy vs. myelopathy: root compression → dermatomal pain, LMN signs; cord compression → UMN signs below the level, gait/sphincter involvement.
• Disc prolapse vs. canal stenosis: acute dermatomal radiculopathy in a younger patient vs. positional neurogenic claudication in an older one.
• TB vs. pyogenic vs. metastasis of vertebra: disc-space preservation + cold abscess (TB), disc destruction + acute fever (pyogenic), pedicle/posterior-element destruction with disc sparing + multiple levels (mets).
• Osteoporotic vs. malignant collapse: use the marrow-signal/pedicle/posterior-cortex table above.

Recently asked / exam angle

• "Paradiscal lesion with preserved disc space and paravertebral calcified abscess" → Pott's spine (single most repeated radiology-orthopaedics overlap).
• "Forward slip of L5 on S1 measured by quarters" → Meyerding grading; Grade V = spondyloptosis.
• "Absent pedicle on AP X-ray" → metastasis (winking owl).
• "L4–L5 paracentral disc compresses which root?" → L5 (traversing).
• "Investigation of choice for spinal cord compression / disc prolapse / Pott's spine" → MRI.
• "Child with vertebra plana" → Langerhans cell histiocytosis.
• "Earliest radiological site in ankylosing spondylitis" → sacroiliac joints; modality most sensitive early = MRI.
• "Neurogenic claudication relieved by flexion" → lumbar canal stenosis.
• Image-based: identify "Scotty dog with collar," "bamboo spine," "rugger-jersey spine," "ivory vertebra."

Rapid revision

1. MRI = investigation of choice for cord, disc, Pott's spine, nerve-root compression; X-ray = first investigation in trauma/listhesis.
2. Commonest lumbar disc prolapse = L4–L5; paracentral disc compresses the traversing (lower) root.
3. L4–L5 → L5 root; L5–S1 → S1 root (absent ankle jerk).
4. Meyerding grading: I ≤25%, II ≤50%, III ≤75%, IV ≤100%, V = spondyloptosis (>100%).
5. Spondylolysis = pars defect = "Scotty dog with a collar."
6. Neurogenic claudication → relieved by flexion/sitting, pulses normal.
7. Pott's spine: paradiscal erosion, disc preserved early, gibbus, calcified cold abscess — TB hallmark.
8. Pyogenic spondylitis destroys the disc early (opposite of TB).
9. Winking owl / absent pedicle = vertebral metastasis.
10. Vertebra plana in a child = Langerhans cell histiocytosis; ivory vertebra = osteoblastic mets/Paget/lymphoma.
11. Ankylosing spondylitis: earliest = bilateral sacroiliitis; spine → squaring, syndesmophytes, bamboo spine, dagger sign.
12. Cord T2 hyperintensity = oedema/myelomalacia; persistent signal predicts poor surgical recovery.