Understanding Disc Desiccation at L5-S1: MRI Assessment

MRI Evaluation of L5-S1 Disc Desiccation: Key Findings

Disc desiccation (degenerative dehydration) at L5-S1 is assessed via a constellation of MRI features. No single sign is diagnostic; radiologists synthesize multiple parameters.

Primary MRI Signs

1. T2 Signal Loss: Normal nucleus pulposus = high T2 signal (bright). Desiccation → low/intermediate signal (dark). Graded: Pfirrmann I–V (I=normal, V=severe).
2. Disc Height Reduction: Measured mid-sagittally; compare to adjacent levels (L4-L5) or normative data. >20% loss = significant.
3. Annular Fissures/Tears: High-intensity zones (HIZ) on T2 = radial tears; may leak contrast (if post-contrast) or correlate with discogenic pain.
4. Disc Bulge/Protrusion/Extrusion: Loss of structural integrity → contour abnormality. May compress S1 nerve root (lateral recess/foraminal).
5. Modic Changes (Vertebral Endplate Signal):
   • Type 1: Low T1 / High T2 (edema/inflammation) — acute/active
   • Type 2: High T1 / High T2 (fatty replacement) — chronic
   • Type 3: Low T1 / Low T2 (sclerosis) — late
6. Facet Joint Degeneration: Joint space narrowing, osteophytes, subchondral cysts, sclerosis—often coexists at L5-S1.
7. Ligamentum Flavum Hypertrophy / Spinal Stenosis: Secondary to disc height loss & facet arthropathy.
8. Vacuum Phenomenon: Gas (nitrogen) in degenerated disc—indicates advanced collapse.

Quantitative Measures

• Disc Height Index (DHI)
• Signal Intensity Ratio (NP vs. CSF)
• T2 Relaxometry (research)

Clinical Correlation Is Essential

Imaging findings ≠ symptoms. Many asymptomatic adults have Pfirrmann III–IV. Treatment targets clinical syndrome (radiculopathy, axial pain, instability), not MRI alone.