Is MRI with magnetic resonance angiography (MRA) the same as MRI with MR neurography for evaluating pudendal‑nerve injury causing persistent lower urinary‑tract dysfunction after anorectal surgery?

MRI with MRA vs. MR Neurography: Different Techniques for Different Purposes

No, MRI with magnetic resonance angiography (MRA) and MRI with MR neurography are not the same—they are distinct imaging techniques that visualize different anatomical structures. MRA focuses on blood vessels, while MR neurography is specifically designed to visualize peripheral nerves. For evaluating pudendal-nerve injury causing persistent lower urinary-tract dysfunction after anorectal surgery, MR neurography combined with contrast-enhanced pelvic MRI is the appropriate imaging approach, not MRA. 1

Key Differences Between MRA and MR Neurography

MRA (Magnetic Resonance Angiography)

• MRA is designed to visualize blood vessels (arteries and veins) and detect vascular pathology such as stenosis, thrombosis, or arterial injury. 2
• MRA uses specialized sequences to highlight flowing blood and vascular structures, not nerves. 2
• In pelvic imaging, MRA would be appropriate for evaluating renal artery stenosis or vascular complications, but it does not visualize peripheral nerves like the pudendal nerve. 2

MR Neurography

• MR neurography uses high-resolution sequences specifically optimized to directly visualize peripheral nerves, including the pudendal nerve, and can identify compression, entrapment, edema, inflammation, or structural injury. 1, 3
• The American College of Radiology recommends MR neurography as the optimal imaging modality for evaluating nerve injury due to its superior soft-tissue contrast and ability to detect intraneural signal abnormalities. 3
• Standard pelvic MRI protocols lack the specialized sequences required to adequately visualize nerves—neurography sequences must be specifically requested. 1

Correct Imaging Protocol for Pudendal-Nerve Injury

When to Order Imaging

• Imaging should be obtained only after 3–4 weeks of appropriate conservative care if symptoms persist unchanged, pain worsens while sitting (meeting Nantes criteria for pudendal neuralgia), red-flag signs appear, or symptoms progressively deteriorate. 1
• Imaging before 3–4 weeks leads to unnecessary tests, as most pelvic-floor guarding symptoms resolve with conservative management. 1

What to Order

• Request "MRI pelvis with gadolinium contrast AND MR neurography" to ensure both nerve visualization and detection of post-surgical complications (hematoma, abscess, scar tissue) that may compress the pudendal nerve. 1
• Gadolinium contrast improves visualization of fluid collections, fistulas, mesh complications, and inflammatory changes around the nerve. 1, 4
• Ordering only "MRI pelvis" without specifying neurography will miss pudendal nerve pathology, as routine sequences cannot adequately depict the nerve and will result in false-negative results. 1

Clinical Context: Pudendal-Nerve Injury vs. Pelvic-Floor Guarding

Features Suggesting Nerve Injury (Requires Imaging)

• Persistent, unchanged sensory loss beyond 3–4 weeks despite relaxation techniques or positional changes. 1
• Pain that intensifies while sitting, meeting Nantes criteria for pudendal neuralgia. 1
• Associated fecal urgency or incontinence, suggesting broader S2–S4 root involvement. 1
• Progressive worsening of sensory loss after >2 weeks, indicating possible evolving nerve compression. 1

Features Suggesting Pelvic-Floor Guarding (Conservative Management)

• Symptoms improve within days to 2–3 weeks with conscious relaxation techniques (warm baths, diaphragmatic breathing). 1
• Diffuse, poorly localized discomfort rather than sharp, dermatomal pain. 1
• Sensation fluctuates with stress, anxiety, or body position. 1
• Immediate relief with pelvic-floor relaxation exercises; symptoms worsen with Kegel exercises. 1

Red-Flag Signs Requiring Emergency Evaluation

• Bilateral leg weakness, saddle anesthesia, or loss of anal sphincter tone → emergency MRI within 12–48 hours for possible cauda-equina syndrome. 1
• Complete urinary retention with absent bladder sensation → urgent work-up to exclude cauda-equina syndrome. 1

Common Pitfalls to Avoid

• Ordering MRA instead of MR neurography—MRA visualizes blood vessels, not nerves, and will not detect pudendal nerve pathology. 2, 1
• Ordering standard pelvic MRI without specifying neurography—this will miss nerve injury and lead to diagnostic delays. 1
• Imaging too early (<3–4 weeks)—most guarding-related symptoms resolve with conservative care, making early imaging unnecessary. 1
• Initiating Kegel/strengthening exercises when hypertonicity is present—this worsens symptoms; prioritize relaxation-focused pelvic-floor physiotherapy first. 1