Magnetic resonance neurography (MRN) plays a key role in assessing disorders of the lumbosacral plexus (LSP), a complex network of nerves serving the pelvis and lower limbs. Due to the anatomical depth and intricacy of this region, traditional electrodiagnostic methods often fall short. MRN, particularly when supported by advanced imaging techniques, allows for clear visualisation of the LSP and can assist in identifying a broad range of pathological conditions. With ongoing advancements in MRI technology, including higher resolution and improved contrast capabilities, the diagnostic potential of MRN continues to grow. A clear understanding of imaging protocols, hardware requirements and common disease presentations is crucial for optimising examination outcomes.
Technical Foundations and Imaging Protocols
Successful MRN of the LSP depends on optimised hardware, patient preparation and imaging protocols. The preferred magnetic field strength is 3.0 T, offering enhanced spatial resolution and signal-to-noise ratio compared to 1.5 T. Wide-bore MRI systems with robust field homogeneity and high-channel coil arrays are ideal, particularly for patients with larger body habitus. Attention to patient positioning is critical, with bladder emptying and fasting before the scan recommended to reduce motion artefacts. In some cases, antispasmodic agents may be considered to further limit bowel motion.
The MRN protocol typically integrates two-dimensional and three-dimensional sequences. High-resolution, fat-suppressed, fluid-sensitive intermediate-weighted sequences provide structural clarity. Three-dimensional sequences such as STIR-FSE or DESS offer isotropic resolution and enable multiplanar reformation, although trade-offs exist between image quality and acquisition time. Dixon fat suppression is often used to enhance contrast between nerve signal and surrounding tissues, though its use may introduce fat-water swapping artefacts. Additional tools such as PROPELLER techniques and respiratory gating are used in select scenarios to mitigate motion interference.
Targeted imaging depends on clinical presentation, with protocols tailored to visualise specific nerves, such as the sciatic, femoral or pudendal nerves. Smaller field-of-view oblique sagittal sequences can enhance fascicular detail. In cases involving potential neoplastic or inflammatory conditions, contrast-enhanced studies and vascular suppression techniques improve characterisation. Imaging around metallic hardware remains feasible at 3.0 T with appropriate techniques to reduce susceptibility artefacts.
Disease Patterns: Entrapments and Trauma
Entrapment neuropathies in the LSP commonly affect the sciatic and pudendal nerves, especially in the subgluteal region. Deep gluteal syndrome, often involving fibrovascular bands or piriformis muscle hypertrophy, may lead to sciatic nerve compression. Imaging may reveal nerve signal abnormalities or deviations in nerve course, although findings can sometimes be subtle. Ischiofemoral impingement and gemellus-obturator internus syndromes are less frequent but may also contribute to sciatica.
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Pudendal nerve entrapment, often seen within the Alcock canal, can lead to pelvic pain and functional disturbances. MRN findings in such cases may include scarring or distortion of fat planes. Meralgia paresthetica results from entrapment of the lateral femoral cutaneous nerve and presents with sensory disturbance of the anterolateral thigh. MRN typically shows nerve enlargement and hyperintensity at the entrapment site.
Traumatic LSP injuries, although rare, can occur due to pelvic fractures, especially those affecting the sacral or pubic bones. Such injuries may manifest with bladder dysfunction or sexual impairment, with MRN revealing nerve discontinuity, fibrosis or denervation changes. In post-traumatic scenarios such as hamstring avulsion or repair, the proximity of the sciatic nerve may lead to inflammation, scarring and chronic neuropathy.
Iatrogenic injuries also account for a significant proportion of LSP pathologies. Procedures such as pelvic lymphadenectomy, spinal fusion or total hip arthroplasty pose risks to nerves including the femoral, obturator and sciatic nerves. Imaging should focus on surgical sites, with skin markers aiding precise localisation of pathology.
Diffuse Neuropathies and Neoplastic Conditions
Beyond focal entrapments, MRN can detect diffuse polyneuropathies involving the LSP. Hereditary conditions like Charcot-Marie-Tooth disease show symmetrical nerve enlargement, while immune-mediated disorders such as chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) and multifocal acquired demyelinating sensory and motor neuropathy (MADSAM) display multifocal, asymmetric changes. Imaging findings include nerve thickening, hyperintensity and occasionally enhancement, with some conditions preferentially affecting the sciatic or femoral nerves.
Rare causes such as IgG4-related neuropathy and sarcoidosis also impact the LSP, though imaging features are often nonspecific. Retroperitoneal fibrosis, particularly when linked to IgG4, may show diffuse enhancement of fibrotic tissue encasing the nerves.
Neoplastic processes may be primary, such as schwannomas and neurofibromas, or secondary due to direct invasion or perineural spread. Benign peripheral nerve sheath tumours exhibit well-defined margins, hyperintensity on T2-weighted imaging and the classic “target” or “split fat” signs. Malignant tumours, including neurolymphomatosis and metastatic lesions, often show irregular thickening, intense enhancement and restricted diffusion. Advanced diffusion-weighted imaging helps differentiate benign from malignant lesions by evaluating apparent diffusion coefficient values.
Perineural spread of malignancy, particularly from pelvic tumours, is often seen in the sacral plexus and may be visualised as irregular nerve thickening and enhancement. Endometriosis may also secondarily affect the LSP through encasement or fibrosis, particularly in the sciatic or sacral regions.
Post-radiation plexitis is another differential consideration, typically manifesting as diffuse nerve thickening with preserved fascicular architecture and corresponding soft tissue changes. The enhancement pattern and absence of diffusion restriction help distinguish it from tumour recurrence.
MR neurography of the lumbosacral plexus is a powerful modality for evaluating a diverse range of neurologic conditions affecting the pelvis and lower limbs. Meticulous attention to imaging protocols, patient preparation and hardware selection significantly enhances diagnostic accuracy. Understanding the broad spectrum of disease manifestations—from entrapments and traumatic injuries to diffuse neuropathies and neoplasms—is essential for radiologists and clinicians alike. Continued technical innovation, particularly in sequence design and image reconstruction, promises to further refine MRN’s role in clinical practice.
Source: RadioGraphics
Image Credit: iStock
