What type of MRI (Magnetic Resonance Imaging) of the spine is best for diagnosing discitis?

MRI for Discitis: Optimal Protocol and Technique

MRI without and with IV contrast is the gold standard imaging modality for diagnosing discitis, with a sensitivity of 96%, specificity of 94%, and accuracy of 92%. 1

Optimal MRI Protocol for Discitis

Essential Sequences

• T1-weighted sagittal and axial images
• T2-weighted sagittal and axial images with fat suppression (STIR or T2 fat-sat)
• Post-contrast T1-weighted sagittal and axial images with fat suppression

The examination should include:

• Complete coverage of the affected spinal region
• Fat suppression techniques for both pre- and post-contrast imaging
• Diffusion-weighted imaging (DWI) sequences

Key Technical Considerations

1. Fat suppression is critical - STIR or T2 fat-sat sequences are necessary to identify marrow edema, paraspinal inflammation, and soft tissue abnormalities 1
2. Both pre- and post-contrast imaging - Pre-contrast images are required for comparison to accurately assess enhancement patterns 1
3. Contrast enhancement - IV contrast increases lesion conspicuity and helps define the extent of infection 1
4. Diffusion-weighted imaging - Helps differentiate infectious spondylitis from degenerative (Modic type 1) changes 1

Diagnostic Value and Findings

Primary MRI Findings in Discitis

• Vertebral endplate erosions with adjacent marrow edema
• Disc space narrowing with abnormal signal intensity
• Paraspinal and epidural soft tissue inflammation/abscess
• Enhancement of affected vertebrae, disc space, and adjacent soft tissues

Specific Diagnostic Signs

• The superior-inferior paraspinal edema ratio (SI-PER) ≥2.5 has 96% sensitivity and 75% specificity for discitis 2
• Epidural enhancement on contrast-enhanced MRI combined with abnormal lab values predicts positive biopsy results for spondylodiscitis 1

Alternative Imaging When MRI Contraindicated

If MRI cannot be performed (e.g., implantable devices, claustrophobia):

1. Combination spine gallium/Tc99 bone scan - Second-line option 1
2. CT with contrast - Less sensitive but useful for bony detail and guiding biopsy 1
3. PET/CT - May be used as a complementary examination in select cases 1

Common Pitfalls to Avoid

1. Inadequate sequences - Standard protocols for disc disease without fat suppression are insufficient for detecting inflammatory changes 3
2. MRI with contrast only - Pre-contrast images are essential for comparison 1
3. Early imaging limitations - Very early MRI may show atypical findings with involvement limited to the disc, anterior soft tissues, or epidural space rather than the vertebral body 4
4. Mimics - Degenerative endplate changes can mimic discitis; diffusion-weighted imaging helps differentiate 1

Follow-up Imaging Considerations

• MRI findings often lag behind clinical improvement 1
• Resolution of subcutaneous fluid collections or decreased signal abnormality in paraspinal or epidural locations suggests treatment response 1
• Consider repeat MRI if initial imaging is negative but clinical suspicion remains high 4

For suspected discitis, radiographs are of limited value in early disease as changes may take 2-8 weeks to appear, and should not delay MRI when clinical suspicion is high 1.