How to Order MRI for Numbness of Right Leg
For numbness of the right leg, order an MRI of the lumbar spine without IV contrast as the primary imaging study if initial radiographs are normal or if there are concerning neurological findings suggesting nerve root or spinal cord pathology. 1
Initial Imaging Approach
Start with plain radiographs (X-rays) of the lumbar spine as the first-line imaging study. 1 This provides:
- Basic anatomic evaluation of vertebral alignment
- Detection of obvious degenerative changes or fractures
- Exclusion of alternative diagnoses like tumor or severe spondylolisthesis 1
However, radiographs have significant limitations—they cannot adequately assess the spinal cord, nerve roots, or soft tissue structures that are the most common causes of leg numbness. 1
When to Proceed Directly to MRI
Order MRI lumbar spine without IV contrast immediately (bypassing radiographs) if any red flag symptoms are present: 2
- Saddle anesthesia or perianal numbness - suggests cauda equina syndrome requiring urgent MRI within 12 hours 2
- New-onset urinary retention or bowel dysfunction with back pain 2
- Progressive bilateral leg weakness or numbness 2
- Numbness ascending from the leg upward - may indicate spinal cord pathology requiring full spinal column MRI 2
MRI Ordering Specifications
Standard Protocol
Order: "MRI lumbar spine without IV contrast" 1, 3
This protocol is preferred because:
- MRI is the modality of choice for evaluating spinal cord and nerve root pathology 1
- It provides superior soft-tissue contrast for detecting disc herniations, spinal stenosis, and nerve root compression 1
- IV contrast is not routinely needed for initial evaluation of radiculopathy or numbness 1, 3
Anatomic Coverage
- For isolated leg numbness: Focus on lumbar spine (L1-S1) 3
- If numbness involves both legs or has ascending pattern: Order full spinal column MRI (cervical, thoracic, and lumbar) to exclude multilevel or cord pathology 2
Special Considerations for Positioning
Standard supine MRI may miss dynamic pathology. Consider requesting axial loaded MRI or upright/positional MRI if: 4, 5
- Symptoms occur only with specific positions (standing, walking, extension) 4, 5
- Initial supine MRI is negative but clinical suspicion remains high 5
- Suspected dynamic spondylolisthesis or positional stenosis 5
Critical pitfall: Conventional recumbent MRI may underestimate disease because the supine position takes stress off the spine and may not reproduce the pathology causing symptoms. 4, 5
When to Add IV Contrast
Add IV contrast (order "MRI lumbar spine without and with IV contrast") only if: 1
- Suspected infection (osteomyelitis, discitis, epidural abscess) 1
- Concern for neoplasm or metastatic disease 1
- Evaluation of postoperative complications 1
- Initial non-contrast MRI shows abnormal enhancement patterns requiring characterization 1
For routine evaluation of leg numbness from suspected radiculopathy or stenosis, contrast is not necessary and adds cost without diagnostic benefit. 1, 3
Alternative Imaging if MRI Contraindicated
If MRI cannot be performed (pacemaker, severe claustrophobia, metallic implants): 1
- CT lumbar spine without IV contrast - reasonable alternative for bony evaluation and severe stenosis 1
- CT myelogram - can delineate nerve root compression when MRI is not possible 1
However, these modalities are inferior to MRI for soft tissue and nerve root evaluation. 1
Expected Diagnostic Findings
Based on the level of pathology, leg numbness patterns correlate with specific nerve roots: 3
- L3-L4 disc herniation: Anterior/medial thigh numbness, diminished patellar reflex 3
- L4-L5 disc herniation: Lateral leg and dorsal foot numbness 3
- L5-S1 disc herniation: Posterior leg and plantar foot numbness 3
Urgent Imaging Timeframe
Do not delay MRI when cauda equina syndrome is suspected—obtain within 12 hours of presentation. 2 Conventional x-rays, CT scans, or bone scans cannot exclude spinal cord compression or cauda equina syndrome. 2