What lumbar spine MRI protocol should be ordered for a patient with suspected sciatica?

MRI Protocol for Sciatica

Order a lumbar spine MRI without IV contrast as the first-line imaging study for patients with sciatica and suspected nerve root compression. 1

Standard Protocol Sequences

The recommended lumbar spine MRI protocol for sciatica should include:

• Sagittal T2-weighted sequences at 3-4 mm thickness to visualize disc morphology, spinal canal patency, and nerve root compression 1
• Sagittal T1-weighted spin echo sequences at 4-5 mm thickness to assess vertebral marrow and anatomic detail 1
• Axial T2-weighted sequences at 3-4 mm thickness through the lower lumbar spine (typically L3-S1) to evaluate neural foramina, lateral recesses, and nerve root compression 1
• Fat-suppressed T2-weighted sequences (STIR or fat-saturated T2) to detect perineural edema/inflammation and bone marrow pathology 1, 2

When to Add IV Contrast

Contrast is NOT needed for routine sciatica evaluation. 1 Add gadolinium contrast (MRI without and with IV contrast) only in specific circumstances:

• Prior lumbar surgery with new or progressive symptoms to differentiate recurrent disc herniation from postoperative scar tissue 1, 3
• Suspected infection (discitis/osteomyelitis) or epidural abscess when noncontrast sequences are nondiagnostic 1, 3
• Suspected malignancy or metastatic disease to characterize tumor extent and enhancement patterns 1, 3
• Suspected inflammatory conditions such as axial spondyloarthropathy when initial imaging is equivocal 1

Timing Considerations

Immediate MRI (no waiting period) is indicated when red flags are present: 1, 3

• Suspected cauda equina syndrome (urinary retention/incontinence, saddle anesthesia, bilateral leg weakness)
• Progressive or severe neurological deficits (worsening motor weakness, reflex loss)
• Clinical suspicion of malignancy, infection, or vertebral fracture
• Significant trauma with neurological symptoms

For uncomplicated sciatica without red flags, wait 6 weeks of conservative management before ordering MRI, and only if the patient is a potential surgical or epidural injection candidate. 1, 3 The natural history shows improvement within 4 weeks in the majority of patients. 3, 4

Common Pitfalls to Avoid

• Do not order MRI for acute low back pain without radicular symptoms – imaging does not improve outcomes and leads to unnecessary interventions 1, 3
• Do not skip the 6-week conservative trial unless red flags are present – premature imaging increases intervention rates without proven benefit 3
• Remember that MRI findings correlate poorly with symptoms – up to 20-28% of asymptomatic individuals have disc herniations on MRI 3
• Ensure anatomical correlation – MRI abnormalities must match the patient's dermatomal distribution of symptoms to be considered causative 3

Alternative Imaging When MRI is Unavailable

CT lumbar spine without IV contrast is appropriate when MRI cannot be performed (non-MRI-compatible implants, severe claustrophobia, or prolonged MRI delays >2-4 weeks). 1, 3

• CT demonstrates >80% sensitivity and specificity for canal stenosis, foraminal stenosis, and degenerative changes 3
• Thecal sac effacement ≥50% on CT predicts significant spinal stenosis; <50% effacement reliably excludes cauda equina impingement 1
• CT lacks adequate soft-tissue resolution to visualize most disc herniations – a normal CT does not exclude nerve root compression 3

CT myelography evaluates spinal canal patency and neural foramina but requires lumbar puncture and intrathecal contrast, limiting its use to surgical planning scenarios. 1

Additional Technical Considerations

• Fat-suppressed sequences (STIR or fat-saturated T2) may reveal perineural edema/inflammation in 29.5% of patients with unexplained sciatica on routine sequences, though this finding's clinical utility remains limited 2
• Axial loading/dynamic MRI in slight extension can unmask positional nerve root compression missed on supine imaging, particularly in patients with neurogenic claudication 5
• Follow-up MRI at 1 year does not distinguish favorable from unfavorable outcomes – 35% with favorable outcomes and 33% with unfavorable outcomes still show visible disc herniation 6