The value of MR neurography for evaluating extraspinal neuropathic leg pain: a pictorial essay

Abstract

Fifteen patients with neuropathic leg pain referable to the lumbosacral plexus or sciatic nerve underwent high-resolution MR neurography. Thirteen of the patients also underwent routine MR imaging of the lumbar segments of the spinal cord before undergoing MR neurography. Using phased-array surface coils, we performed MR neurography with T1-weighted spin-echo and fat-saturated T2-weighted fast spin-echo or fast spin-echo inversion recovery sequences, which included coronal, oblique sagittal, and/or axial views. The lumbosacral plexus and/or sciatic nerve were identified using anatomic location, fascicular morphology, and signal intensity as discriminatory criteria. None of the routine MR imaging studies of the lumbar segments of the spinal cord established the cause of the reported symptoms. Conversely, MR neurography showed a causal abnormality accounting for the clinical findings in all 15 cases. Detected anatomic abnormalities included fibrous entrapment, muscular entrapment, vascular compression, posttraumatic injury, ischemic neuropathy, neoplastic infiltration, granulomatous infiltration, neural sheath tumor, postradiation scar tissue, and hypertrophic neuropathy.

fig 1.

Section orientation and coverage. Coronal T1-weighted localizer image (280/14/1) indicates the sagittal oblique plane and coverage area for imaging the LS plexus and proximal sciatic nerve

fig 2.

Normal sciatic nerve. A, Coronal T1-weighted spin-echo image (600/10/2) illustrates the normal longitudinal fascicular appearance of the pelvic sciatic nerve (arrows). B, T2-weighted fast spin-echo fat-saturated image (5200/102/3; echo train length, 8) shows normal sciatic nerve signal intensity (arrows). C, Sagittal oblique plane T1-weighted image (550/9/2) depicts the characteristic transverse fascicular morphology of normal sciatic nerve (arrow) at the right obturator internus muscle level (asterisk). D, Sagittal oblique plane T2-weighted fast spin-echo fat saturated image (5000/102/2; echo train length, 8), obtained at the same level as that show in C, portrays normal sciatic nerve signal intensity (arrow). The T2 signal of normal nerve will be less intense than that of adjacent vessels and slightly hyperintense (occasionally isointense) to that of adjacent muscle.

fig 3.

Compressive neuropathy. A, Sagittal oblique plane T1-weighted image (680/14/3) reveals marked flattening of the left sciatic nerve (arrow) between hypertrophied obturator internus and gluteus maximus muscles. Note, however, that the transverse fascicular morphology is preserved. B, Sagittal oblique plane fat-saturated T2-weighted fast spin-echo (6000/102/3; echo train length, 8) image, obtained at the same level as that shown in A, shows abnormally increased sciatic nerve T2 hyperintensity (arrow) that approaches regional vessel signal intensity.

fig 4.

Traumatic neuropathy. A, Sagittal oblique plane T1-weighted image (560/10/2), obtained at the level of the piriformis muscle (asterisk), reveals normal size and fascicular morphology of the left sciatic nerve (arrows). The adjacent adnexal cyst (cyst) approximates but does not distort or displace the normal sciatic nerve. B, Sagittal oblique plane fat-saturated T2-weighted fast spin-echo image (4000/108/2; echo train length, 8), obtained at the same level as that shown in A, confirms normal nerve signal intensity (arrows). C, Sagittal oblique plane T1-weighted image (560/10/2), obtained at the level of the obturator internus muscle, reveals flattening of the left sciatic nerve and disruption of the normal transverse fascicular pattern (arrow). D, Sagittal oblique plane fat-saturated T2-weighted fast spin-echo image (4000/108/2; echo train length, 8), obtained at the same level as that shown in C, shows abnormal sciatic nerve hyperintensity (arrow) equivalent to that of regional vessels. Unlike the case illustrated in figure 3, there is no identifiable mechanical compressive cause in this case.

fig 5.

Hypertrophic neuropathy. A, Axial T1-weighted image (600/20/2) shows the sciatic nerve in the left proximal thigh. The tibial division reveals a central mass (white arrow) that is isointense to muscle and that displaces several mildly swollen fascicles to the nerve periphery. The peroneal division (black arrow) appears normal at this level. B, Axial fast spin-echo inversion recovery image (4600/45/2; echo train length, 8), obtained at the same level as that shown in A, shows normal peroneal division morphology and signal intensity (black arrow). More distally (not shown), the peroneal division manifested abnormal T2 hyperintensity, reflecting a compressive neuropraxic injury that explained the foot drop. The central tibial nerve mass displaces several mildly swollen hyperintense fascicles to the periphery (white arrow). C, Axial contrast-enhanced T1-weighted image (550/14/2) shows modest enhancement of the central tibial division mass (white arrow). There is no abnormal enhancement of the adjacent peroneal division (black arrow).

fig 6.

Metastatic dermatofibrosarcoma. Axial contrast-enhanced T1-weighted fat-saturated image (600/9/2), obtained through the distal left thigh, reveals a 3-cm enhancing mass (arrows) centered within the sciatic nerve. Surgical resection revealed metastatic dermatofibrosarcoma.

fig 7.

Traumatic stump neuroma. A, Axial T1-weighted spin-echo image (600/9/2), obtained through the right distal thigh (stump), reveals a low signal intensity mass (arrow) within the distal sciatic nerve. B, Axial T1-weighted spin-echo contrast-enhanced fat-saturated image (550/14/2) shows mild enhancement of the mass (arrow). C, Axial T2-weighted fast spin-echo fat-saturated image (5000/90/2; echo train length, 8), obtained proximal to the mass, shows sciatic nerve enlargement with focal swollen T2-weighted hyperintense fascicles (arrows). D, Coronal T2-weighted fast spin-echo fat-saturated image (5000/102/2; echo train length, 8) confirms diffuse sciatic nerve enlargement and abnormal T2 hyperintensity (arrows) proximal to the mass. Final surgical pathologic analysis revealed traumatic neuroma.

fig 8.

Sarcoid. A, Axial T1-weighted spin-echo unenhanced image (600/9/2), obtained through the proximal right thigh, reveals an isointense (to muscle) mass (white arrow), centered within the sciatic nerve tibial division that distorts the adjacent peroneal division (black arrow). B, Contrast-enhanced axial-view T1-weighted spin-echo image (550/14/2) confirms moderate enhancement of the mass after the administration of contrast medium (white arrow). The adjacent peroneal division does not enhance (black arrow). There is homogeneous loss of normal fascicular architecture within the mass on both the unenhanced and contrast-enhanced images. C, Axial fast spin-echo inversion recovery image (5000/45/150; echo train length, 8) confirms a mildly hyperintense mass within the tibial division that enlarges the sciatic nerve and obscures normal fascicular architecture. There is also marked fascicular hyperintensity of the adjacent peroneal division (black arrow), reflecting the compressive neuropraxic injury.