MRI Lumbar Spine Without Contrast
For a patient presenting with back pain and right leg weakness, order an MRI of the lumbar spine without intravenous contrast as the initial imaging study. 1, 2
Rationale for MRI Without Contrast
Leg weakness represents a neurological deficit that constitutes a "red flag" requiring immediate imaging without waiting for a trial of conservative therapy. 1, 2 The American College of Radiology designates MRI lumbar spine without contrast as the gold standard because it:
- Directly visualizes the spinal cord, nerve roots, intervertebral discs, and ligaments with superior soft-tissue contrast 1
- Accurately depicts nerve root compression, disc herniation, spinal stenosis, and canal compromise that cause radicular symptoms and motor deficits 1, 2
- Demonstrates vertebral marrow pathology using T1-weighted and fat-suppressed sequences to detect infection, malignancy, or fracture 1, 3
- Avoids ionizing radiation exposure 2, 3
When to Obtain Imaging Immediately (No 6-Week Wait)
Do not delay imaging when any of these red-flag features are present: 1, 2, 3
- Motor weakness (as in this patient) 1, 2
- Progressive or rapidly worsening neurological deficits 1, 2, 3
- Suspected cauda equina syndrome (urinary retention/incontinence, saddle anesthesia, bilateral leg weakness) 1, 2
- Clinical suspicion of malignancy, infection, or vertebral fracture 1, 2, 3
- History of cancer with new back pain 1, 2
Critical Diagnostic Considerations
Motor weakness in the right leg suggests nerve root compression at a specific lumbar level—most commonly L5 or S1—that must be correlated with the dermatomal distribution of weakness and sensory changes. 2 The MRI findings must anatomically match the clinical presentation:
- L5 radiculopathy produces weakness of ankle dorsiflexion and great toe extension 2
- S1 radiculopathy produces weakness of ankle plantarflexion 2
MRI demonstrates 96% sensitivity and 94% specificity for detecting disc herniation, spinal stenosis, or other compressive lesions causing radicular symptoms. 2
When to Add Intravenous Contrast
Contrast is not needed for initial evaluation of radiculopathy with motor weakness. 1, 2 Add gadolinium contrast only in specific circumstances:
- Prior lumbar surgery with new or progressive symptoms (to differentiate recurrent disc herniation from epidural scar tissue) 2, 4
- Suspected infection (discitis, osteomyelitis, epidural abscess) when noncontrast images are nondiagnostic 2, 4
- Suspected malignancy or metastatic disease 2, 4
- Fever, elevated inflammatory markers (ESR/CRP), or immunosuppression 4
Alternative Imaging When MRI Cannot Be Performed
If MRI is contraindicated (non-MRI-compatible implants, severe claustrophobia) or delayed beyond 2–4 weeks, obtain CT lumbar spine without contrast. 2 CT demonstrates:
- Thecal sac effacement ≥50% predicts clinically significant spinal stenosis 1, 2
- Thecal sac effacement <50% reliably excludes cauda equina impingement 1, 2
- High-resolution bony anatomy for surgical planning 2
CT myelography evaluates spinal canal patency and neural foramina but requires lumbar puncture and intrathecal contrast administration. 1, 2
Common Pitfalls to Avoid
Do not order plain radiographs (X-rays) as the initial study for a patient with motor weakness. 1, 2 Radiographs cannot visualize nerve root compression, disc herniation, or spinal stenosis—the most common causes of radiculopathy with weakness. 1, 2
Do not order MRI with contrast alone (without noncontrast sequences), as interpretation requires correlation with standard T1- and T2-weighted images. 1, 4
Do not delay imaging for 6 weeks of conservative management when motor weakness is present, as this represents a neurological deficit requiring urgent evaluation. 1, 2, 3
Surgical Referral Threshold
Refer to spine surgery when: 2