Elsevier

Clinical Imaging

Volume 40, Issue 6, November–December 2016, Pages 1118-1130
Clinical Imaging

Review Article
Magnetic resonance neurography in the diagnosis of neuropathies of the lumbosacral plexus: a pictorial review,☆☆

https://doi.org/10.1016/j.clinimag.2016.07.003Get rights and content

Abstract

Magnetic resonance neurography (MRN) is an important tool to detect abnormalities of peripheral nerves. This pictorial review demonstrates the MRN features of a variety of neuropathies affecting the lumbosacral plexus (LSP) and lower extremity nerves, drawn from over 1200 MRNs from our institution and supplemented by the literature. Abnormalities can be due to spinal compression, extraspinal compression, malignancy, musculoskeletal disease, iatrogenesis, inflammation, infection, and idiopathic disorders. We discuss indications and limitations of MRN in diagnosing LSP neuropathies. As MRN becomes more widely used, physicians must become familiar with the differential diagnosis of abnormalities detectable with MRN of the LSP.

Introduction

High-resolution magnetic resonance imaging (MRI) of peripheral nerves, or magnetic resonance neurography (MRN), is a technique for identifying anatomy and pathologic lesions of peripheral nerves [1], [2]. While the diagnosis of peripheral neuropathy remains electroclinical, MRN has emerged as a helpful technique for localizing lesions and elucidating the underlying etiology. Indeed, MRN can sometimes be more informative than electrodiagnostic investigations [3].

Technical aspects of MRN have been recently reviewed [4], [5], [6], [7]. Modern MRN uses high magnetic field strength (1.5 or 3 T) with high-resolution multiplanar structural sequences optimized for peripheral nerve visualization. MRN can help localize lesions by directly observing nerve signal abnormalities or by identifying myopathic changes in a particular nerve distribution; detect incidental lesions mimicking neuropathic symptoms; or exclude neuropathy by revealing completely normal imaging characteristics of both muscle and nerve [8]. Among its other uses, MRN is indicated to identify sites of entrapment [9]; help identify patients who would benefit from surgery [10]; evaluate the extent of nerve repair after surgery [11]; identify peripheral nerve tumors [12]; differentiate radiation damage from recurrent tumor [13]; and identify the extent of diffuse neuropathies [6].

In this pictorial essay, we review the utility of MRN in diagnosing lesions of the lumbosacral plexus (LSP) and contiguous neural elements, supplementing the existing literature with our own audit of almost 1300 MRNs of the LSP. We illustrate the variety of diagnoses that can be identified by MRN of the LSP and highlight information that MRN may yield when ordered for appropriate indications.

Of note, many lesions that affect the LSP are best diagnosed clinically without imaging, such as postpartum obturator mononeuropathies. Others are diagnosed easily with conventional imaging, such as retroperitoneal hematoma or pelvic abscesses. Although MRN can help visualize these abnormalities, these conditions will not be reviewed here.

The LSP encompasses the spinal roots and interconnections linking the lumbosacral spinal cord to the nerves of the lower extremities. The complex anatomy and relative inaccessibility to electrodiagnostic testing make the diagnosis of nerve disorders affecting the LSP challenging. Diagnosis normally relies on a combination of the clinical exam, electromyography/nerve conduction studies (EMG/NCS), and traditional MR or computed tomographic (CT) spinal imaging. However, electroclinical studies assess function, not structure, and they are often limited in interrogating the deeper nerves of the LSP. Traditional MRI and CT are limited by spatial resolution with regards to identifying individual nerve structure and pathology. Although MRN has already been shown to add clinically useful information beyond that provided by traditional MRI and EMG/NCS [3], MRN has not yet been widely adopted mostly due to insufficient awareness and technical expertise in the broader medical community.

MRN adapts conventional imaging techniques for optimal peripheral nerve visualization. This usually includes multiplanar high-resolution T1 and heavily T2-weighted fat-suppressed sequences [1]. Compared with muscle, normal peripheral nerves have isointense T1 and isointense to slightly hyperintense T2 signal. T1-weighted sequences with 2–4-mm slice thickness and high resolution (<1 mm2 pixel size) are excellent for demonstrating the fascicular pattern of the normal nerve, outlining the epineurial fat plane, and delineating the anatomic structures surrounding the nerve [14]. Fat-suppressed T2-weighted imaging enables sensitive detection of water content alterations in a variety of nerve pathologies. Because the intrinsic T2 hyperintense signal of fat surrounding a nerve may mask the T2 prolongation effect of nerve pathology, fat suppression techniques are routinely utilized with T2 imaging [14], [15].

Recent technological advances, including 3-T scanners and robust accelerated acquisition schemes, enable routine incorporation of three-dimensional (3D) sequences allowing for thinner slices (less than 1 mm), approximating in-plane matrix dimensions. The resulting isotropic voxel size also allows for maximum intensity projection, multiplanar, and curved-planar reformations, thereby increasing signal-to-noise ratio and better delineating complex LSP anatomy [16], [17].

Administration of intravenous MRI contrast agents may be helpful to supplement conventional structural imaging techniques, particularly when there is suspicion for pathology which may violate the integrity of the blood–nerve barrier, such as inflammatory or neoplastic neuropathies [18], [19]. As with T2 sequences, fat suppression is usually applied with postcontrast imaging so that enhancement is not masked by the intrinsic T1 shortening effect of adjacent fat.

Diffusion-weighted imaging (DWI) with high diffusion sensitizing gradient moments (i.e., b-value ≥500 s/mm2) takes advantage of the anisotropic diffusion of water within nerve fibers, preferentially paralleling the course of axonal bundles. DWI enhances nerve contrast, increasing sensitivity to pathologic alterations in nerve internal architecture [20], [21]. DWI with acquisition of at least six noncollinear diffusion gradient directions allows tensor modeling and diffusion tensor tractography [22], [23]. A hybrid 3D imaging technique combining a weaker diffusion sensitizing gradient moment that maintains T2 weighting but sufficiently nulls signal from rapidly moving protons in vessels enhances nerve contrast at the neurovascular bundle, where distinction between nerve and vessel can be difficult [24], [25]. Ongoing research is focusing on quantitative diffusion and T2 relaxometry techniques to provide more objective metrics of lumbosacral nerve pathology [26], [27], [28].

Of note, since many disease processes that affect the LSP also have a predilection for the lumbosacral roots and proximal nerves without regard for anatomical borders, in this paper, we refer to these three contiguous regions as LSP. For the sake of parsimony, we reluctantly sacrifice precision.

Section snippets

2.1 Methods

We reviewed the reports and findings of 1290 MRNs of the LSP on 1179 different patients completed at the University of California San Francisco (UCSF) from February 2000 through December 31, 2013. While the protocol changes based on indication, the “routine” lumbosacral MRN protocol used our institution can be found in Table 1. We collected data on the ordering physician, demographics, clinical indications, and radiographic findings from a radiology database. We then reviewed clinical

Pictorial essay

Low back pain is the leading cause of worldwide disability, with an estimated point prevalence of 9.3% [29]. Radiating sciatic type pain is among the most common and debilitating subtypes of low back pain [30]. Because degenerative disk and spine changes are so common radiographically, it can be difficult to distinguish symptomatic compression from another cause for sciatica with coincident degenerative spine disease. MRN's superior spatial resolution can often visualize nerve thickening and

Summary

In the last two decades, MRN has emerged as an important tool in the investigation of peripheral neuropathy due to a wide variety of etiologies, both compressive and noncompressive (see Table 2 for a summary of LSP neuropathies with select examples by category). At our institution, MRN of the LSP is widely used by both medical and surgical specialties to investigate pain, weakness, sensory loss, and tumor, as well as for pre- and postoperative planning. MRN can be an invaluable aid for lesion

Author contributions

  • 1.

    Guarantor of integrity of the entire study: N.M.R.

  • 2.

    Study concepts and design: N.M.R., N.B., C.T.C., V.J.D.

  • 3.

    Literature research: N.M.R., J.F.T.

  • 4.

    Clinical studies: N/A.

  • 5.

    Experimental studies/data analysis: N.M.R., N.B., V.J.D.

  • 6.

    Statistical analysis: N/A.

  • 7.

    Manuscript preparation: N.M.R., V.S., J.F.T., C.T.C., V.J.D.

  • 8.

    Manuscript editing: N.M.R., N.B., V.S., J.F.T., C.T.C., V.J.D.

Acknowledgments

The authors would like to thank Brian Scott for his clinical assistance.

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    Conflicts of interest: none.

    ☆☆

    Other disclosures: J.F.T. is a member of data monitoring committee for StemCells, Inc. This review was unfunded.

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