Magnetic resonance neurography (MRN) is a recent addition to the set of tools for the assessment of the lumbar plexus (LP) and peripheral nerves. The objective of this chapter is to review state-of-the-art MRN imaging techniques used to characterize disorders affecting the LP.
With the current techniques, MRN allows the examiner to distinguish mononeuropathies from polyneuropathies or tumors from inflammatory disease; to establish the exact location of a lesion; to quantify degrees of nerve injury related to trauma; and to localize nerve entrapment.
12.2 Anatomy
The LP is formed by the ventral rami arising between L1 and L4, in some cases with a small contribution from T12. The anterior divisions of these roots form anterior nerve branches including the iliohypogastric and ilioinguinal nerves (L1), the genitofemoral nerve (L1–L2), and the obturator nerve (L2–L4). The posterior divisions form posterior nerve branches, as the femoral nerve (L2–L4) and the lateral femoral cutaneous nerve (L2–L3, occasionally L4) (▶ Fig. 12.1).
Fig. 12.1 Coronal IDEAL T2-weighted image with fat suppression. MIP reconstruction identifying L1 to L4 roots, forming the lumbar plexus and the femoral nerve (FN); the figure also shows the L5 root and lumbosacral trunk.
Iliohypogastric, ilioinguinal, and genitofemoral nerves supply sensory innervation to the lower abdomen, genitals, and inner thigh. The lateral femoral cutaneous nerve does supplies the anterolateral thigh. The femoral and obturator nerves, on the other hand, provide motor innervation to pelvic and anterior and medial thigh muscles. 1, 2, 3
12.3 Lumbar Plexus Evaluation Protocol
For magnetic resonance imaging (MRI) studies of the LP, 3.0-tesla scanners are ideal since they provide better signal-to-noise ratio and contrast resolution, essentials for assessing the complexity of this structure. 4 Sequences for lumbosacral plexus study include T1- and T2-weighted images with and without fat suppression, and diffusion-weighted imaging (DWI). For homogeneous fat–water separation, special 2, 3, or multipoint Dixon protocols are used, such as IDEAL (Iterative Decomposition of water and fat with Echo Asymmetry and Least squares estimation) developed by Reeder for General Electric (GE) Healthcare. 48 Other options include Dixon TSE developed by Siemens Healthcare, or mDixon developed by Philips Healthcare. Fat suppression techniques such as short tau inversion recovery (STIR) or spectral adiabatic inversion recovery (SPAIR) are also used. 5 Special volumetric acquisitions with high spatial resolution can be applied, such as CUBE developed by GE Healthcare, SPACE (Sampling Perfection with Application Optimized Contrast) developed by Siemens Healthcare, or VISTA (Volume Isotropic Turbo spin echo Acquisition) developed by Philips Healthcare.
Volumetric sequences may be visualized through multiplanar reformations (MPRs), curved planar reformations, and maximum intensity projections (MIP). It allows one to follow the nerve across the organ and compares either size or signal intensity with the normal side in order to discriminate normal anatomy from pathology. 6
T1-weighted images are optimal for anatomical assessment, allowing perineural fat to be adequately outlined; if needed, fat suppression and intravenous contrast can be used to identify perineural enhancement. On fat-suppressed T2-weighted images, the contrast between background and water signal of the perineurium allows excellent MPR and MIP reconstructions along the nerve course, demonstrating changes in thickness or signal intensity that could be even measured. 7 DWIs with different b values are also useful for assessing nerves, allowing extensive MIP reconstructions without vessels overlapping. DWI is also valuable for studying tumors with a high degree of cellularity. Muscles affected by denervation may show hyperintensity in fat suppressed T2-weighted images during the acute phases due to edema, and hyperintensity in T1-weighted images during chronic phases because of fatty infiltration. 8
12.4 Pathological Conditions Affecting the Lumbar Plexus
LP could be involved in various disorders of the peripheral nerves. ▶ Table 12.1 lists the categories of peripheral nerve disorders that manifest with abnormalities in LP MRN.
Inflammatory disorders | Immune or vascular disorders | Neoplastic and infiltrative processes | Trauma or entrapment | Iatrogenic |
Diabetes, sarcoidosis, amyloidosis, vasculitis | Guillain–Barré syndrome, chronic demyelinating polyneuropathy, multifocal mononeuropathy |
| Trauma, injury, retroperitoneal abscesses or hematomas, posttraumatic neuroma | Hip or pelvic surgery, radiation neuropathy |
Abbreviations: MPNST, malignant peripheral nerve sheath tumor; PNST, peripheral nerve sheath tumor. | ||||
12.4.1 Lumbar Plexopathies
Trauma and Entrapment
Effects of direct trauma over the LP are rare due to the protective effect of the pelvic rim. Lumbar plexopathies caused by penetrating injuries, or root avulsions resulting from traffic accidents, are more common. 9
Traumatic injury of the sciatic nerve may occur during surgery over or in the proximities of the gluteal muscles.
LP entrapment is frequent in serious spine osteoarthritis, and when it is associated with scoliosis may cause radicular compression 3 (▶ Fig. 12.2).
Fig. 12.2 Nerve entrapment. Fifty-eight-year-old woman with left lumbar scoliosis and degenerative changes at L3–L4 level. (a, b) Coronal fat-saturated proton density weighted image. Thickening and high signal intensity of L4 nerve root, femoral nerve (FN), and lateral femoral cutaneous nerve (LFCN) is identified (arrows). (c) Coronal fat-saturated T1-weighted image after intravenous contrast shows enhancement of the same nerves.
Psoas muscle injuries such as hematomas or abscesses can generate LP or femoral nerve compression. 10
On MRN, fusiform enlargement of the nerve with high signal intensity on T2-weighted images, effacement of the perineural fat, and contrast enhancement are the imaging features of the formation of neuroma. In addition, signal changes in paraspinal and limb muscles innervated by the affected root or nerve trunk are common signs of denervation
Tumors
Intrinsic Tumors
The LP is a common site for the development of peripheral nerve sheath tumor (PNST). The most common neurogenic tumors are schwannomas and neurofibromas. There are three types of neurofibroma: localized, diffuse, and plexiform. 11, 12
In general terms, schwannomas are eccentric to the nerve and encapsulated within the perineurium. Neurofibromas can occur either sporadically or in the context of neurofibromatosis type 1 (NF1).
In PNST, MRN shows well-defined focal or fusiform masses. A dumbbell shape is typical for paraspinal lesions with neuroforaminal enlargement (▶ Fig. 12.3). Different classic imaging landmarks have also been described such as target nodule, contrast-enhancing tail, fascicular disarrangement of fibers, split perineural fat, and bag-of-worms appearance. It is almost impossible to distinguish between schwannomas and focal neurofibromas based solely in imaging features 11, 13 (▶ Fig. 12.4).
Fig. 12.3 Schwannoma. Fifty-year-old female patient. (a) Axial CUBE T2-weighted image. An expansive oval-shaped mass is identified in the right foramen at L5–S1 level (asterisk). (b) Coronal T1-weighted image after intravenous contrast shows L5 nerve root has expanded (arrow) surrounding the enhanced lesion.
Fig. 12.4 Plexiform neurofibroma. Eighteen-year-old male patient with neurofibromatosis type I. (a, b) Coronal STIR T2-weighted images. Multilobulated and coalescent hyperintense masses are observed along the nerve roots and branches of the LP.
Malignant peripheral nerve sheath tumors (MPNSTs) can occur in patients with NF1 or can develop de novo. Most MPNSTs are ill-defined masses larger than 5 cm in diameter and spread along large nerve trunks and may enlarge rapidly 14 (▶ Fig. 12.5).
Fig. 12.5 Malignant peripheral nerve sheath tumor in a 40-year-old female patient. (a) Coronal fat-saturated proton density–weighted image. A heterogeneous hyperintense lesion is observed enlarging L5 nerve root (arrow). (b) Coronal IDEAL T1-weighted image with fat suppression and intravenous contrast administration. Lesion enhancement is observed (arrow).
Matsumine et al 15 described MRI characteristics that distinguish between benign and malignant PNSTs. Large size, irregular tumor shape, peripheral enhancement, perilesional edema, and presence of T1-hyperintense areas are important features of MPNSTs.
“Neurolymphomatosis” is a term used to describe infiltration of roots and peripheral nerves by lymphoma. It is a rare extranodal manifestation of both B-cell and T-cell non-Hodgkin lymphoma (90%) or leukemia (10%) (neuroleukemiosis). 16 The diagnosis is based on clinical presentation, presence of lymphoma cells in spinal fluid, and nodular enlargement of plexus, roots, and peripheral nerves. 17 On MRN, nodular enlargement of dorsal roots and an LP with varying enhancement after contrast administration may be seen. 18
Extrinsic Tumors
Perineural spread of malignancy in peripheral nerves is less common, but is known to occur in rectal, prostate, and cervix cancers. Dissemination of prostate adenocarcinoma is associated with advanced disease. However, it has been recently established that approximately 15% of cases have perineural spread at the initial presentation, 19 making it crucial to differentiate spread from radiation-induced neuropathy.
MRN is very useful for depicting perineural tumor involvement. In addition to high signal intensity on T2-weighted images, nodular enhancement is typically present in perineural tumor infiltration 20 (▶ Fig. 12.6).
Fig. 12.6 Nerve entrapment by tumor. Forty-seven-year-old woman with lymphangioleiomyomatosis. MRI is performed due to left lumbar pain. Coronal IDEAL T1-weighted image with fat suppression (a) and axial T2-weighted images (b, c) show an oval-shaped lesion in the left retroperitoneum (star). The femoral nerve (FN) is thickened and hyperintense (arrow) on both sequences, entrapment between the lesion and the psoas muscle is evident.
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