MRI Spine - Basics

Why MRI?

Standard Protocol (Routine Sequences)

Core Sequences Explained

T1-Weighted Imaging

• Short TR (300-700 ms) and short TE (<30 ms)
• Fat = bright (high signal) - bone marrow appears bright
• CSF = dark (low signal)
• Excellent for: bone marrow changes, anatomical detail, osseous structures, discs, soft tissue
• Before/after gadolinium: detects neoplastic and infectious processes

T2-Weighted Imaging

• Long TR (2000-3000 ms) and long TE (80-120 ms)
• CSF = bright ("myelographic effect") - acts as natural contrast for the cord
• Healthy nucleus pulposus = bright (high water content)
• Degenerated disc = dark (loss of water/proteoglycans)
• Used as Fast Spin Echo (FSE) in practice to reduce acquisition time
• Best for: disc assessment, cord compression, edema, tumor, infection

STIR (Short Tau Inversion Recovery)

• Fat-suppression sequence using an inversion pulse
• Fat signal is nulled; pathological fluid and edema become very bright
• Excellent for: bone marrow edema, occult fractures (acute vs. chronic), ligament injury, Modic changes, vertebral metastases, multiple sclerosis lesions
• More robust fat suppression than frequency-selective saturation in large fields of view
• Preferred over T2 fat-sat in trauma because it is more reliable

Gradient Echo (GRE / T2*)

• Shorter acquisition times, less CSF pulsation artifact
• Best demonstrates hemorrhage and blood products - important prognostic indicator in neurological injury
• More susceptible to metallic susceptibility artifacts (avoid in post-surgical spines with implants)
• Useful in cervical and thoracic spine

Contrast-Enhanced T1 (Gadolinium)

• Gd chelates shorten T1 - enhancement = bright on T1
• Blood-brain/spinal barrier breakdown = intra-axial enhancement
• Hypervascularity = extra-axial enhancement
• Key uses:
  • Distinguish post-op scar (enhances immediately and uniformly) from recurrent disc (peripheral enhancement)
  • Neoplastic and infectious workup
  • Fat suppression after Gd improves conspicuity of epidural enhancement

Sagittal T1 pre (A) and post-gadolinium (B) - Grainger & Allison's Diagnostic Radiology

DWI (Diffusion-Weighted Imaging)

• High signal on DWI + low ADC = restricted diffusion (cell death)
• Uses in spine: abscess, cord infarction, metastatic disease
• Increasingly added to routine spine protocols

Signal Characteristics - Quick Reference

Modic Changes (Vertebral Endplate)

Intervertebral Disc on MRI

• Young adult normal disc: low-intermediate T1, high T2 (bright nucleus pulposus indistinguishable from inner annulus)
• Outer annulus fibrosus: low signal on both T1 and T2 (high collagen)
• Degeneration: T2 signal decreases with loss of water/proteoglycans; disc height loss; annular tears
• Third decade change: intranuclear cleft appears as a dark horizontal band on T2 - normal aging
• Disc herniation is best assessed on both sagittal and axial images; foraminal herniation confirmed on sagittal T1 (loss of bright epidural fat around nerve root)

Artifacts to Know

Regional Notes

Clinical Indications by Sequence

• Grainger & Allison's Diagnostic Radiology, Chapter 47 (Spine Imaging)
• Campbell's Operative Orthopaedics 15th Ed 2026 (MRI sections)
• Schwartz's Principles of Surgery 11th Ed (Diagnostic Studies, Ch. 42)
• PMC review - MRI of the spine

MRI Spine: Interpreting Specific Findings

PART 1 — DISC PATHOLOGY SPECTRUM

1.1 Normal Disc

• Sagittal T2: Bright (high signal) nucleus pulposus surrounded by dark outer annulus
• T1: Low-intermediate signal disc against bright bone marrow
• Disc margins should not extend beyond the vertebral apophyses (ring edges)
• In the 3rd decade, an intranuclear cleft (dark horizontal band on T2) is normal ageing

1.2 Disc Bulge

• Symmetric bulge: Equal extension in all directions (seen normally at L5-S1 or with ligamentous laxity)
• Asymmetric bulge: More on one side (e.g., in scoliosis)
• May cause mild lateral recess narrowing

Disc bulge types - Grainger & Allison's Diagnostic Radiology

Key rule: Bulge = >50% circumference. Herniation = <50% (focal). This distinction is critical and often confused.

1.3 Disc Herniation

A. Protrusion

• Base (neck where it contacts the disc) is wider than the apex in ALL planes
• Think of a blister that hasn't fully popped
• Most common and mildest form
• Focal protrusion: <25% of circumference
• Broad-based protrusion: 25-50% of circumference

B. Extrusion

• Base is narrower than the herniated material in at least one plane - the "toothpaste sign"
• Material has squeezed through a narrow neck
• Can migrate cranially or caudally within the spinal canal (migrated extrusion)
• Important: Any herniation that extends above or below the disc level is an extrusion by definition
• Rarely asymptomatic - extrusion in an asymptomatic patient is uncommon

C. Sequestration / Free Fragment

• Disc material is completely disconnected from the parent disc
• Also called a "free fragment"
• High T2 signal and peripheral gadolinium enhancement → predicts spontaneous regression
• On imaging, often difficult to confirm discontinuity → the term "migration" is more practical (disc material displaced away from extrusion site regardless of continuity)

Contained vs. Uncontained

• Contained: Annulus fibrosus still covers the herniated material
• Uncontained: No annular cover; disc material is in direct contact with epidural space
• Usually impossible to distinguish on MRI (requires discography)

Massive disc extrusion L3-L4 - Grainger & Allison's Diagnostic Radiology

1.4 Summary Comparison Table

PART 2 — DISC LOCATION ZONES (Axial Classification)

← Posterior ←
  Central (midline)
  Paracentral (right/left of centre)
  Subarticular / Lateral recess (right/left)
  Foraminal (neural foramen, right/left)
  Extraforaminal / Far lateral (outside foramen, right/left)
  Anterior zone

• Pedicle level
• Infrapedicle level
• Disc level
• Suprapedicle level

PART 3 — TRAVERSING vs. EXITING NERVE ROOTS

The Key Concept

The Lumbar Rule (Most Important)

Memory rule: Central/paracentral/subarticular = ONE LEVEL DOWN (traversing root). Foraminal/extraforaminal = SAME LEVEL (exiting root).

Practical Example (L4-L5 disc):

• Paracentral herniation → L5 radiculopathy (foot drop, weak EHL)
• Foraminal herniation → L4 radiculopathy (weak quad, knee jerk reduced)

How to Identify on MRI

• Seen on axial T2 as a dot of intermediate signal in the lateral corner of the spinal canal, between the posterior vertebral body and the facet joint
• Surrounded by bright CSF
• Normally distinct and separate; compression = effacement of surrounding CSF, deviation, or loss of root outline

• Seen on sagittal T1 as a structure surrounded by bright epidural fat within the "keyhole"-shaped foramen
• Normal: fat clearly visible around the root = open foramen
• Abnormal: fat effaced = foraminal stenosis

PART 4 — NERVE ROOT RELATIONSHIP TERMINOLOGY

PART 5 — SPINAL STENOSIS

Central Canal Stenosis

• Cord signal change on T2 (in cervical/thoracic) = severe stenosis regardless of morphology

• Mild: <1/3 narrowing
• Moderate: 1/3 to 2/3
• Severe: >2/3

Neural Foraminal Stenosis

• Causes: disc protrusion/extrusion, osteophytes, facet hypertrophy, loss of disc height
• AP diameter <3 mm = diagnostic for foraminal stenosis

Lateral Recess Stenosis

• Lateral recess height <3 mm = significant stenosis
• Lateral recess angle <30° = further evidence of stenosis

PART 6 — ANNULAR FISSURE / TEAR

• T2: Linear bright signal (hyperintense zone / HIZ) within the disc, usually posteriorly
• Post-gadolinium T1: Enhancement from vascularized granulation tissue ingrowth
• Represents a chronic degenerative process (the term "annular tear" is still in common use but "annular fissure" is preferred to avoid implying acute trauma)

PART 7 — QUICK INTERPRETATION CHECKLIST

• Normal (bright NP)? Mildly degenerate (reduced signal)? Severely degenerate (dark/collapsed)?

• Bulge / protrusion / extrusion / sequestration?
• Focal or broad-based?
• Any annular fissure (HIZ)?

• Central / paracentral / subarticular / foraminal / extraforaminal?

• CSF space preserved? Roots clumped? Grade 0-3?
• In cervical/thoracic: cord deformity? T2 cord signal change?

• Fat surrounding root preserved bilaterally?
• Any effacement = foraminal stenosis

• Abutting / indenting / displacing / compressing?
• Which root: traversing or exiting?
• Correlate with clinical level

• Modic changes? (I = edema, II = fat, III = sclerosis)
• Endplate erosion (infection)?

• Grainger & Allison's Diagnostic Radiology, Chapters 47 & 48 (Spine)
• PMC MRI Spine Review (Safriel et al.)
• Radsource Spine Nomenclature (Combined Task Forces)
• RACGP - Making Sense of MRI Lumbar Spine
• Lee Grading System for Stenosis (KJR/PMC)

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MRI Spine: Interpreting Specific Findings (with Images)

PART 1 — DISC PATHOLOGY SPECTRUM

Normal Disc (T2 signal)

• Bright nucleus pulposus on T2 = healthy, hydrated disc
• Dark outer annulus on T2 = normal collagen-rich ring
• Disc margin must NOT extend beyond the vertebral apophyses

Disc Bulge vs. Herniation - The Core Distinction

Fig. A = Normal disc. B = Symmetric disc bulge (>50% circumference, uniform). C = Asymmetric disc bulge. - Grainger & Allison's Diagnostic Radiology

PART 2 — HERNIATION SUBTYPES

2.1 Protrusion

• Base (neck) wider than apex in all planes
• Like a blister that hasn't burst
• Focal (<25% disc circumference) or broad-based (25-50%)

2.2 Extrusion

• Base narrower than the herniated part = "toothpaste sign"
• Can migrate cranially or caudally
• Any herniation extending above/below disc level = extrusion by definition
• Rarely asymptomatic

2.3 Sequestration / Free Fragment

• Completely disconnected from parent disc
• Best term on imaging: "migration" (displacement away from extrusion site)
• Peripheral Gd enhancement on T1 = ongoing resorption → predicts spontaneous regression

Massive disc extrusion L3-L4. Sagittal (A) and axial (B) T2. Note displacement of nerve roots and loss of lateral recess. - Grainger & Allison's Diagnostic Radiology

Summary Table

PART 3 — LOCATION ZONES (Axial Plane)

POSTERIOR
     ┌─────────────────────────────────┐
     │     CENTRAL (midline)           │
     │   PARACENTRAL (R/L of centre)   │
     │  SUBARTICULAR / LATERAL RECESS  │
     │    FORAMINAL (neural foramen)   │
     │  EXTRAFORAMINAL (far lateral)   │
     └─────────────────────────────────┘
ANTERIOR

PART 4 — TRAVERSING vs. EXITING NERVE ROOTS

The Anatomy

Thecal sac (dural tube)
        │
        │ ← Traversing (transiting) root descends inside thecal sac
        │    occupies the LATERAL RECESS
        │
        └──► Exiting root leaves sac, travels through NEURAL FORAMEN
             at its own numbered level

Worked Example at L4-L5 Disc

L4 vertebra
  └── L4-L5 DISC ──────────────────────────────────
           │                                        │
    CENTRAL/PARACENTRAL                        FORAMINAL/
    SUBARTICULAR zone                          EXTRAFORAMINAL
           │                                        │
    L5 root (traversing                      L4 root (exiting
    through L4-L5 recess)                    through L4-L5 foramen)
           │                                        │
    → L5 RADICULOPATHY                       → L4 RADICULOPATHY
    (foot drop, EHL weak)                    (knee jerk reduced,
                                              quad weak)

How to Identify on MRI

• Small dot of intermediate signal in the lateral recess
• Should be surrounded by bright CSF
• Sign of compression: CSF effaced around root, root deviated or indistinct

• Seen passing through the "keyhole"-shaped foramen
• Should be surrounded by bright epidural fat
• Sign of foraminal stenosis: fat effaced, root contacted or morphologically deformed

Sagittal T1 pre (A) and post-Gd (B). On T1, bright epidural fat is the landmark for identifying exiting nerve roots in the foramen. - Grainger & Allison's Diagnostic Radiology

PART 5 — NERVE ROOT RELATIONSHIP TERMINOLOGY

Critical caveat: Many herniations seen on MRI are asymptomatic. A disc "indenting" a root in a pain-free patient does not mean that finding is the cause of symptoms. Always correlate with the clinical level.

PART 6 — SPINAL STENOSIS

Central Canal Stenosis - Lee Grading (Axial T2)

Neural Foraminal Stenosis (Sagittal T1)

Lateral Recess Stenosis - Bartynski Grading (Axial T2)

• Lateral recess height <3 mm = stenosis
• Lateral recess angle <30° = further evidence

PART 7 — ANNULAR FISSURE / TEAR

• T2: Bright linear signal (HIZ = High Intensity Zone) in posterior disc
• Post-Gd T1: Enhancement (granulation tissue / vascular ingrowth)

Preferred modern term: "annular fissure" (chronic degenerative) rather than "tear" (which implies acute trauma). Both terms are in common use.

PART 8 — SYSTEMATIC READING CHECKLIST

1. DISC SIGNAL (sag T2)
   → Normal bright? Mild degenerate? Severe dark/collapsed?

2. DISC CONTOUR (sag + axial T2)
   → Bulge / protrusion / extrusion / sequestration?
   → Focal or broad-based?
   → Any annular fissure (HIZ)?

3. LOCATION (axial T2)
   → Central / paracentral / subarticular / foraminal / extraforaminal?

4. CENTRAL CANAL (axial T2)
   → CSF preserved? Root clumping? Grade 0-3?
   → Cord signal change? (cervical/thoracic)

5. NEURAL FORAMINA (sag T1 both sides)
   → Fat around exiting root preserved?
   → Effaced = foraminal stenosis → grade it

6. NERVE ROOT RELATIONSHIP
   → Abutting / indenting / displacing / compressing?
   → Traversing or exiting root?
   → Does it match the clinical dermatomal level?

7. ENDPLATES (sag T1 + T2 + STIR)
   → Modic type I/II/III?
   → Erosion (infection)? Fracture? Schmorl's node?

• Grainger & Allison's Diagnostic Radiology (Chapters 47 & 48)
• PMC - MRI of the Spine
• Radsource Spine Nomenclature (Combined Task Forces)
• RACGP - Making Sense of Lumbar Spine MRI
• Lee Grading System for Stenosis - KJR