Aetna considers magnetic resonance neurography experimental and investigational because the medical literature on the application of this technology in clinical situations remains in early stages of development.

Currently, diagnosis and management of nerve injury due to compression or trauma are generally undertaken without images of the nerves themselves.  Although nerves can sometimes be seen in standard magnetic resonance imaging (MRI), the method is so unreliable that nerve images have never played a significant role in diagnosis and clinical usefulness.  Magnetic resonance neurography is a new MRI technology requiring special software and hardware upgrades, which are not widely available; currently, this test is only available at a very limited number of medical centers.

Magnetic resonance neurography is capable of generating high resolution longitudinal and cross-sectional images of major peripheral nerves, and has been studied to supplement diagnostic evaluations by electromyography (EMG) and nerve conduction studies in patients with suspected peripheral nerve tumors, traumatic injury, post-irradiation neuritis, chronic compression, and pain syndromes where an anatomic lesion is suspected.  Although current evidence supports MR neurography as a promising technique, the outcome data that would determine the efficacy of this technology is limited to studies involving a small number of patients, making it premature to offer conclusions regarding its effectiveness for the general population.  Additionally, large-scale, well-conducted, controlled studies with this approach are warranted to determine its efficacy in imaging neurofibromas and distinguishing benign from malignant lesions.

In a prospective observational study of patients with sciatica, Zhang and colleagues (2009) investigated the effectiveness of three-dimensional (3-D) high-spatial resolution diffusion-weighted MR neurography based on steady state free precession (3-D diffusion-weighted steady-state free precession [DW-SSFP]) in the diagnosis of sciatica.  The 3-D DW-SSFP sequence was performed on 137 patients with sciatica and 32 patients in control group.  The post-processing techniques were used to generate images of lumbo-sacral plexus and sciatic nerve, and the images acquired were assessed based on the presence or absence of nerve abnormality.  The certainty of identifying the lumbo-sacral plexus and main branches from all cases was determined in each of the reconstruction planes for each case individually and assessed by using a 3-score scale.  The sciatic nerve and its main branches were differentiated and a clear picture was obtained in all subjects.  Compared with the control group, the presence of nerve root compression or increased T2 signal intensity changes can be observed in all patients.  The mean score of certainty of identifying the sciatic nerve and main branches was 1.76 +/- 0.4, which indicates that the sciatic nerve and main branches can be identified with certainty.  The authors concluded that the 3-D DW-SSFP MR neurography with high spatial and sufficient contrast is an excellent technique to define the nature of sciatica and assists in prognostication and possibly in management.

Du et al (2010) analyzed the role of MR neurography in the evaluation of spinal and peripheral nerve lesions.  Imaging studies, medical records, and EMG/nerve conduction studies (NCS) results were analyzed retrospectively in a consecutive series of 191 patients who underwent MR neurography for spinal and peripheral nerve disorders; 91 (47.6 %) of these patients also underwent EMG/NCS studies.  In those who underwent both MR neurography and EMG/NCS, MR neurography provided the same or additional diagnostic information in 32 % and 45 % of patients, respectively.  Magnetic resonance neurograms were obtained at a median of 12 months after the onset of symptoms.  The utility of MR neurography correlated with the interval between the onset of symptoms to MR neurography.  Twelve patients underwent repeated MR neurography for serial evaluation.  The decrease in abnormal signal detected on subsequent MR neurography correlated with time from onset of symptoms and the time interval between MR neurography, but not with resolution of symptoms.  Twenty-one patients underwent MR neurography post-operatively to assess persistent, recurrent, or new symptoms; of these 3 (14.3 %) required a subsequent surgery.  The authors concluded that MR neurography is a valuable adjunct to conventional MRI and EMG/NCS in the evaluation and localization of nerve root, brachial plexus, and peripheral nerve lesions.  They found that MR neurography is indicated in patients: (i) in whom EMG and traditional MR imaging are inconclusive; (ii) who present with brachial plexopathy who have previously received radiation therapy to the brachial plexus region; (iii) who present with brachial plexopathy and have systemic tumors; and (iv) in patients under consideration for surgery for peripheral nerve lesions or after trauma.  The authors also noted that MR neurography is limited by the size of the nerve trunk imaged and the timing of the study.

In a review on new approaches in imaging of the brachial plexus, Vargas et al (2010) stated that imaging plays an essential role for the detection and analysis of pathologic conditions of the brachial plexus.  Currently, several new techniques are used in addition to conventional 2-D MR sequences to study the brachial plexus: the 3-D STIR SPACE sequence, 3-D heavily T2w MR myelography sequences (balanced SSFP=CISS 3-D, True FISP 3-D, bFFE and FIESTA), and the diffusion-weighted (DW) neurography sequence with fiber tracking reconstruction (tractography).  The 3-D STIR sequence offers complete anatomical coverage of the brachial plexus and the ability to slice through the volume helps to analyze fiber course modification and structure alteration.  It allows precise assessment of distortion, compression and interruption of post-ganglionic nerve fibers thanks to the capability of performing maximum intensity projections (MIP) and multi-planar reconstructions (MPRs).  The CISS 3D, b-SSFP sequences allow good visualization of nerve roots within the spinal canal and may be used for MR myelography in traumatic plexus injuries.  The DW neurography sequence with tractography is still a work in progress, able to demonstrate nerves tracts, their structure alteration or deformation due to pathologic processes surrounding or located along the post-ganglionic brachial plexus.  It may become a precious tool for the understanding of the underlying molecular pathophysiologic mechanisms in diseases affecting the brachial plexus and may play a role for surgical planning procedures in the near future.

Eguchi et al (2011) stated that DW imaging (DWI) can provide valuable structural information that may be useful for evaluating pathological changes of the lumbar nerve root.  Diffusion-weighted MR neurography has recently been introduced as an alternative way to visualize nerves, but to date, quantitative DWI and MR neurography have not been applied to evaluate the pathology of lumbar nerve roots.  These researchers visualized lumbar nerve roots and analyzed their morphology by MR neurography, and measured the apparent diffusion coefficient (ADC) of lumbar nerve roots compressed by herniated disks using 1.5-T MR imaging.  A total of 10 consecutive patients (median age of  48.0 and range of 20 to 72 years) with mono-radicular symptoms caused by a lumbar herniated disk and 14 healthy volunteers were studied.  Regions of interests (ROIs) were placed on the lumbar roots at dorsal root ganglia (DRG) and distal spinal nerves on DWI to quantify mean ADC values.  The spinal nerve roots were also visualized by MR neurography.  In the patients, mean ADC values were significantly greater in the compressed DRG and distal spinal nerves than in intact nerves.  Magnetic resonance neurography also showed abnormalities such as nerve swelling at and below the compression in the symptomatic nerve root.  Increased ADC values were considered to be because of edema and Wallerian degeneration of compressed nerve roots.  The authors concluded that DWI is a potential tool for analysis of the pathophysiology of lumbar nerve roots compressed by herniated disks.

Merlini et al (2011) assessed the feasibility of MR neurography in children, and the potential roles of DWI and fiber-tracking (FT) techniques.  A total of 5 pediatric patients (age range of 6 to12 years) underwent MRI for various clinical indications: neurogenic bladder (case 1); persistent hand pain following minor trauma (case 2); progressive atrophy of the lower left extremity muscles (case 3); bilateral hip pain (case 4); and palpable left supraclavicular mass (case 5).  All studies were performed using a 1.5-T Avanto MRI scanner.  The protocol included 3D T2-weighted STIR and SPACE imaging, T1-weighted fat-saturation post-gadolinium imaging and diffusion tensor imaging (DTI) with tractography.  ADC (N×10(-3) mm(2)/s) and FA values were calculated from ROIs centered on the nerves.  Nerve-fiber tracks were calculated using a 4th-order Runge-Kutta algorithm (NeuroD software).  Magnetic resonance neurography allowed satisfactory visualization of all neural structures, and FA and ADC measurements were feasible.  The final diagnoses were Tarlov cysts, median-nerve compression, sciatic perineurioma, Charcot-Marie-Tooth disease and plexiform neurofibroma in a patient with NF-1.  The authors noted that measurements of FA and ADC are of little value because of the lack of normal reference values.  Nerve-fiber tractography (FT) may be of value in the characterization of tumor pathology, and is also helpful in the planning of surgical treatments.  They concluded that MR neurography is feasible in pediatric patients.  However, a considerable amount of work has yet to be done to establish its role in the clinical management of the wide range of peripheral nerve diseases.

The Work Loss Data Institute' clinical guideline on acute and chronic low back pain (2011) listed magnetic resonance neurography as one of the interventions/procedures that are under study and not specifically recommended. Guidelines on ulnar neuropathy from the Washington State Department of Labor and Industries (2010) considered but did not recommend magnetic resonance neurography. Guidelines on radial nerve entrapment from the Washington State Department of Labor and Industries (2010) recommended use of magnetic resonance neurography only in research settings. 

The American College of Radiology’s Appropriateness Criteria® on “Plexopathy” (2012) stated that “Magnetic resonance neurography, diffusion tensor imaging (DTI), and tractography are exciting developments currently under investigation”.

Chung et al (2014) noted that MRN has utility in the diagnosis of many focal peripheral nerve lesions.  The authors stated that when combined with history, examination, electrophysiology, and laboratory data, future advancements in high-field MRN may play an increasingly important role in the evaluation of patients with peripheral neuropathy.

Thawait et al (2014) stated that MRN is a specialized technique that is rapidly becoming part of the diagnostic algorithm of peripheral nerve pathology.  However, in order for this modality to be considered appropriate, its value compared with current methods of diagnosis should be established.  Therefore, radiologists involved in MRN research should use appropriate methodology to evaluate MRN's effectiveness with a multi-disciplinary approach.

Currently, the sensitivity, specificity, as well as PPV and NPV of MR neurography in the diagnosis and management of patients with peripheral nerve disorders remain unclear.  Thus, the accuracy and clinical value of MR neurography has yet to be established.