Magnetic Resonance Neurography of the Sciatic Nerve 197

Radiología. 2013;55(3):195---202

www.elsevier.es/rx

UPDATE IN RADIOLOGY

High resolution (3 T) magnetic resonance neurography of the ଝ sciatic nerve

∗

C. Cejas , M. Aguilar, L. Falcón, N. Caneo, M.C. Acuna˜

Servicio de Resonancia Magnética, Departamento de Diagnóstico por Imágenes, Fundación para la Lucha de las Enfermedades

Neurológicas de la Infancia Dr. Raúl Carrea (FLENI), Buenos Aires, Argentina

Received 4 November 2011; accepted 12 April 2012

KEYWORDS Abstract Magnetic resonance (MR) neurography refers to a set of techniques that enable

Sciatic nerve; the structure of the peripheral nerves and nerve plexuses to be evaluated optimally. New

Sciatic neuropathy; two-dimensional and three-dimensional neurographic sequences, in particular in 3 T scanners,

Neurography; achieve excellent contrast between the nerve and perineural structures. MR neurography makes

Magnetic resonance it possible to distinguish between the normal fascicular pattern of the nerve and anomalies like

imaging inﬂammation, trauma, and tumor that can affect nerves. In this article, we describe the struc-

ture of the sciatic nerve, its characteristics on MR neurography, and the most common diseases

that affect it.

PALABRAS CLAVE Neurografía por resonancia magnética de alta resolución (3 Tesla) del nervio ciático

Nervio ciático;

Resumen La neurografía por resonancia magnética (RM) hace referencia a un conjunto de

Neuropatía ciática;

Neurografía; técnicas con capacidad para valorar óptimamente la estructura de los nervios periféricos y

de los plexos nerviosos. Las nuevas secuencias neurográﬁcas 2D y 3D, en particular en equipos

Imagen por

de 3 Tesla, consiguen un contraste excelente entre el nervio y las estructuras perineurales. La

resonancia magnética

neurografía por RM permite distinguir el patrón fascicular normal del nervio y diferenciarlo

de las anomalías que lo afectan, como inﬂamaciones, traumas y tumores. En este artículo

se describe la estructura del nervio ciático, sus características en la neurografía por RM y las

dolencias que lo afectan con mayor frecuencia.

Introduction

ଝ The study of peripheral neuropathies (PN) has been

Please cite this article as: Cejas C, et al. Neurografía por

historically the responsibility of neurophysiologists, who

resonancia magnética de alta resolución (3 Tesla) del nervio ciático.

contribute functional and qualitative-quantitative data of Radiología. 2013;55:195---202.

∗

Corresponding author. the lesion’s location, the conduction properties and neu-

E-mail address: ccejas@ﬂeni.org.ar (C. Cejas). ronal distribution. The most commonly used technique is

196 C. Cejas et al.

Epineuro the two divisions separate physically into tibial nerve and

common ﬁbular nerve.

Muscles innervated by the sciatic nerve

In the pelvis, the sciatic nerve innervates the piriform mus-

cle and the quadriceps femoris. In the thigh, the tibial

division of the sciatic nerve innervates the long head of the

biceps femoris muscle and the semitendinosus, semimem-

Vessels Fibers branosus muscle and the adductor magna. The ﬁbular

division innervates the short head of the biceps femoris Endoneuro

(Fig. 2).

Perineuro

Technical parameters of high resolution

neurography

Historically, the techniques used to evaluate peripheral

Figure 1 Diagram showing the structure of a peripheral

nerve. nerves prioritized the weighted sequences in T2. Nev-

ertheless, nowadays there are high resolution techniques

1,2 available which, by using 1---1.2 mm ﬁne cuts, without

the electromyogram (EMG). However, the EMG is a time-

spacing, and by combining weighted sequences in T1 for

consuming study, it is not comfortable for the patient, it

anatomical studies and weighted sequences in T2 with fat

does not locate the exact spot where the lesion is, or differ-

suppression for the pathological, make it possible for better

entiate perineural ﬁbrosis from compressive masses. Clinical

contrast among the fasciculi, the perifascicular and perineu-

electrophysiologic evaluation does not usually determine

3 ral fat to be obtained. 3 T equipment is not a requirement

the cause of sciatic neuropathy. The advent of MR neu-

to perform these studies, since other ﬁeld intensities also

rographic techniques has allowed for a more accurate

4 permit it. Byun et al., in a study with 24 patients, proved

diagnostic approach. At present, with 3 T MR equipment

that with a 3D sequence echo gradient with millimetric

and high resolution sequences, it is possible to study the

planes (Proset sequence) in a 1.5 T equipment and recons-

sciatic nerve structure in detail, its course, its relationship

tructions in the work station, it is possible to study with

with the adjacent structures which may contribute to trap

3,5 a high degree of accuracy the relation between forami-

it and compress it. This article reviews the sciatic nerve’s

nal and extraforaminal hernias with diseased nerve root.

anatomy, the RM technical parameters for its study and the

However, the advent of 3 T RM and high deﬁnition neuro-

most frequent processes that may affect it.

graphic sequences equipment made it possible to obtain

images with excellent signal/noise relation and to perform

Structure of the peripheral nerve

3D examinations. There are different ways to generate sat-

uration of a given tissue, among them, the most commonly

In order to understand better the characteristics of nor-

used is fat saturation. One of these saturation techniques is

mal and pathological nerves in RM, it is necessary to review

based on signal decomposition taking advantage of fat pro-

brieﬂy the structure of the peripheral nerve. The axon is

ton precession frequency. This method, described by Dixon,

the peripheral nerve’s functional unit, and it is contained in

is the basis for in-phase/out-of-phase images, which are

the sheath formed by the Schwann cells. A layer of connec- 10

broadly used in daily practice. At present, this sequence

tive tissue, the endoneuro, surrounds each axon. Multiple

allows us to work simultaneously with T1 and T2 images and

axons form a fasciculus that is covered by a ﬁbrous stroma

4 combinations of saturation pulses (water suppression, fat

called perineuro. Finally, a group of fasciculi is covered by

suppression and combined suppression of water and fat or in-

a layer of connective tissue, the epineuro, which contains

phase/out-of-phase images), by means of a modiﬁcation of

the vessels (Fig. 1).

Dixon’s technique (Table 1). This development corresponds

to the IDEAL sequence (iterative decomposition of water

Anatomy of the sciatic nerve and fat with echo asymmetry and least-squares estimation)

11,12

by General Electric (GE) Healthcare and to 3D T2 SPACE

Formed by the junction of roots L4---S3, the sciatic nerve is (sampling perfection with application optimized contrast),

13,14

the longest nerve of the body. It emerges through the greater by Siemens Healthcare. The sum of 3D T2 Cube images

sciatic notch where its two divisions can already be distin- (GE Healthcare) acquired in the coronal plane and proton

guished: the tibial and the ﬁbular divisions, but they are density with fat saturation allows for an optimal study of

both encased in a common nerve sheath. In its descent along the region and it is reserved for when there is suspicion

the pelvis, it presents anatomical variations in its relations of tumors and infections.

with the piriform muscle, usually going through underneath. The after process of the work station is an essential part

However, the nerve or one of its divisions, usually the ﬁbu- of the study. Multiple plane reconstruction (MPR), recons-

lar division, may go through the muscle. The nerve continues tructions with maximum projection intensity (MPI) and curve

to descend along the thigh, behind the adductor magna in reconstruction techniques are performed. The latter makes

front of the greater gluteus. In the distal third of the thigh, it possible for the nerve course to be unfolded and followed

3 Tesla magnetic resonance neurography of the sciatic nerve 197

Figure 2 Muscles innervated by the sciatic nerve. (A) Piriform muscle (arrow). (B) Quadriceps femoris (arrow). (C) Gracile muscles

(small asterisk), short head of the biceps femoris (arrow head), long head of the biceps (short arrow), semimembranous (large

asterisk) and semitendinous (long arrow).

along its length, thus providing information about the nerve approximately 1000---1200 s/mm , is essential for an optimal

with the adjacent structures. study of peripheral nerves.

In addition, the technique has been developed based Although it is necessary to protocolize the study of the

on water molecule diffusion, such as diffusion boosted sciatic nerve, on occasion, the selection of a technique for

sequences (DWI) and diffusion tensor (DTI) with tractog- an optimal examination must be agreed among the physician

raphy, to use them routinely in the evaluation of the referring the patient, the radiology doctor and technician

15,16

peripheral nervous system. It is predictable that, in that will perform it. Table 2 summarizes the protocol used

the future, these techniques might contribute information in our laboratory.

about nervous regeneration. DTI provides data about the

nerve’s microstructure and function, in addition to infor-

Normal and abnormal appearance of sciatic

mation about its trajectory. Diffusion along the nerve is

nerve by high resolution magnetic resonance

usually three times greater than through the nerve

(anisotropy), due to the restrictions generated by the myelin

The normal nerve shows a fascicular appearance, with sig-

sheath. Thus, DTI makes it possible to for a tractography

nal intensity from isointense to minimally hyperintense with

to be performed as well as the quantitative estimation

respect of the muscle in the sequences potentiated in T1

of parameters such as the apparent diffusion coefﬁcient

17---19

and T2. The perineural fat tissue has a homogeneous

(map ADC), which translates the degree of diffusion and

signal and a separation plane with the adjacent structures.

fractional anisotropy. The application of high b values, of

The perineural tissue around the sciatic nerve is especially

abundant, which makes it possible for it to be distinguished

clearly from the surrounding structures with the present MR

Table 1 Technical parameters. IDEAL sequence.

Technical parameters IDEAL sequence T1 and T2

Table 2 Sciatic nerve neurography protocol.

Coil: neurovascular phase array 8 elements

FOV (cm) Thickness RT/ET (ms) Frequency: 320 (mm) Phase: 256

3 NEX

Axial FSE T1: 30---40 3 780/10

ET: 90 ms (T2)/9.1 ms (T1)

Crown IDEAL T2 35 1---1.2 7160/90

RT: 7160 ms (T2)/575 ms (T1)

Sagittal IDEAL T1 35 1---1.2 575/9.1

Echo train: 20 (T2)/3 (T1)

Coronal 3D T2 Cube 30---40 1 1500/150

Thickness/spacing 1---1.2 mm/0---0.2 mm

All the sequences were performed with high resolution matrix

FOV: 35 cm

256 × 320.

FOV: Field of vision; IDEAL: iterative decomposition of water FOV: ﬁeld of vision; FSE: fast spin echo; IDEAL: iterative decom-

and fat with echo asymmetry and least-squares estimation; NEX: position of water and fat with echo asymmetry and least-squares

number of excitations; ET: Echo time; RT: repetition time. estimation; ET: echo time; RT: repetition time.

198 C. Cejas et al.

sequences. The nerve’s course is rather straight, without

acute angles. Thanks to high resolution sequences, nowa-

days it is possible to distinguish the sciatic nerve divisions,

the thicker, inner one is the tibial division and the thinner,

lateral one is the ﬁbular division (Fig. 3).

The abnormal nerve loses its fascicular pattern, it thick-

ens either focally or diffusely and it presents deviations,

interruptions or compressions in its course. In the sequences

weighted in T2, it is hyperintense with respect of its con-

tralateral and it usually enhances with the gadolinium

injection. In addition, it is accompanied by signal changes or

perineural fat stratiﬁcation, which is easier to see in poten-

tiated sequences in T1, it is also accompanied by adjacent

linear images and in contact with the nerve, characteristics 19---21

of ﬁbrosis.

Classiﬁcation of neuropathies

The major etiological forms of NPs were described by Seddon

in 1943, who highlighted those in which traction and/or

extrinsic compression damage occurs. The nerve undergoes

the effect of physical forces whether by friction, elongation

or compression, which generate three lesion mechanisms:

neuropraxia, axonotmesis and neurotmesis.

Neuropraxia is the smallest degree of lesion. The nerve

shows suffering without sheath or axon discontinuity. It is

generally a transitory disorder, one with complete restitu-

tion.

In axonotmesis the noxa generates axonal discontinu-

ity with integrity of the connective wrapping (perineuro,

endoneuro and epineuro). The complete rupture of the axon

produces a Wallerian degeneration of the second distal.

Prognosis in these lesions continues to be good, but recovery

is slow (axon regeneration of approximately 1 mm per day).

Neurotmesis is the greatest degree of lesion. Both the

axon and the perineural sheaths are affected. Functional

loss is complete and, unless it is surgically operated early,

granulation tissue and subsequently, ﬁbrosis appear produc-

ing posttraumatic neuromas and/or intraneural ﬁbrosis.

Microsurgery techniques have beneﬁtted surgery of

peripheral nerves in the last 20 years. Perineural ﬁbrous tis-

sue may be eliminated by neurolisis. In cases of severe

axonotmesis or neurotmesis surgical repair is necessary.

Direct suture of the nerve ends may generate tension. That

is why the present criterion is to use auto- or alografts.

Neuropathies speciﬁc of the sciatic nerve

Pyramidal syndrome

Described by Robinson in 1947, the pyramidal or piriform

syndrome results from sciatic nerve compression as it passes

through the piriform notch. The piriform muscle is the most

active in running athletes and it is inserted in the pedicles

of the third and fourth sacral vertebrae, it goes through

the greater sciatic foramen and it inserts into the greater

Figure 3 Normal appearance of the sciatic nerve in (A) the

trochanter by means of a thick sinew. Pyramidal syndrome

intercondylial notch (arrow), (B) upper third of thigh (arrow)

may result from modiﬁcations of the piriform muscle such

and (C) ﬁbular nerve division (long arrow) and tibial nerve divi-

as hypertrophies, contractures or spasms, direct traumas or

sion (arrowhead), in a FSE T1 sequence.

from anatomical anomalies in the nerve’s course as it goes

through the sciatic notch. Therefore, it is included within

3 Tesla magnetic resonance neurography of the sciatic nerve 199

the compression or entrapment neuropathies. Controversy

about the origin of distal lumbosacral neuropathy to the

neuroforamen has persisted for many years.

Images in pyramidal syndrome have not been very helpful

for years and the diagnosis has always been one of exclusion.

For Stewart, piriform syndrome has been underdiagnosed

for years, and he has suggested surgical examination for def-

inite diagnosis. During intervention, it is possible to identify

sciatic compression by the piriform muscle and/or associa-

tion with ﬁbrous bands. However, neurographic sequences

may show today the sciatic nerve along all of its course and

27,28

establish its relation with the piriform muscle. 320X256/3 NEX

06:00

Among the lower limb neuropathies, postural neu- 0,5 /1,5 sp

DFOV 26,0 cm

ropathies may occur in a variety of positions, are those HD cardiac

EC:1 / 1 62,5 kHz

of lithotomy and sitting. These causes for sciatic nerve TE:88,9 / Ef

TR:4100

lesions are preventable and they usually occur as a result SF/SK/Signa HDxt 3,0T

of intraoperation carelessness, whether because the patient

SPL

remains too long in the same posture or because of a posi- B

29---31

tioning error. This has had a high legal-medical impact.

In both positions, the same forces contribute to lesion by

stretching of the ischiotibial muscle group (for example,

biceps femoris), they may generate sciatic nerve stretch-

ing. Due to the fact that the position affects both limbs

at the same time, injury of sciatic nerve may be bilat-

eral. Piriform muscle trauma generates muscle spasm or

contracture, and secondarily, nerve lesion by compression

and/or stretching. the MR ﬁndings, taking into account

clinical antecedents, are very revealing. The images show

a fusiform thickening of the nerve as it passes through

the sciatic notch, a focal point of contusions in piri-

form, quadriceps femoris and gluteal muscles, as well

as in the adjacent fat planes. Contrast uptake by these

structures may be explained by the acute inﬂammatory

component of the vascular-nervous package of the neu- EC:1/1 62,5kHz

TE:94,2/Ef

ral sheath, along with the adjacent muscular compression TR:4120 SE/SK/Signa HDxt 3,0T

(Fig. 4). GEMS

IAR

Figure 4 13-year-old girl with an antecedent of surgery in a

Neuropathy by tumors and pseudotumors

sitting position for 8 h. (A) IDEAL sequence weighted in T2 with

fat saturation in the coronal plane. It is possible to observe a

The most common neoplasia described in the bibliography is

33---35 signal increase and diffuse thickening of both sciatic nerves,

the Peripheral Neural Sheath Tumor (PNST). The impor-

with prevalence on the left side (arrow). It is accompanied by

tance of being able to differentiate in images benign tumors

edema of the adjacent soft tissue on the left side (asterisk).

such as neuroﬁbroma and neurilemmoma or schwanoma lies

(B) MPR curve reconstruction in the nerve’s longitudinal axis.

in that the schwanoma may dry up without affecting the

nerve, because it is contained within the same capsule,

the epineuro, and it can usually be separated surgically. the nerve course and its branches, with a hyperintensity sim-

Contrariwise, neuroﬁbroma must be resected with part of ilar to water in the sequences weighted in T2. The large ones

the nerve because the tumor cannot be separated from its replace the adipose tissue creating a ‘‘hive-like’’ appear-

ﬁbers. Both tumors have been broadly studied and described ance. They manifest clinically by neuromatose elephantiasis

37---39

in RM works. In a study of 52 patients analyzing the differ- (Fig. 6).

ences in the images weighted in T2, the target sign (58 vs Neuroﬁbroma or malign schwanoma, also known as

15%), central highlight (75 vs 8%) and a combination of both malign PNST, account for 5---10% of the soft tissue sarco-

(63 vs 3%) were present mainly in the neuroﬁbromas. Con- mas and it is associated to Type I neuroﬁbromatosis in

trariwise, the ﬁndings suggesting neurilemmomas were the 25 to 70% of the cases. It has also been described 10---20

fascicular appearance, a thin hyperintense ring in T2, a com- years after it has been radiated. The sciatic nerve is the

33,40

bination of both (8% neuroﬁbromas vs 46% neurilemmomas) one most often affected by these tumors. Neurography

and diffuse highlight. Large neurilemmomas often undergo is the state of the art image study for diagnosis. It shows

degenerative changes that include cysts, hemorrhage and the characteristic fusiform morphology, sometimes areas of

ﬁbrosis (Fig. 5). necrosis or hemorrhage and, occasionally, cartilage or bone

The plexiform neuroﬁbroma has a pathognomonic heterotopic focuses. Local recurrence and metastasis are

27,41,42

appearance. It may be observed as a diffuse nodularity along frequent.

200 C. Cejas et al.

A S 260

EC:1 /1 31,2kHz TI:145,0 8Ch Body FullFOV FOV:4x44 WW: 768WL: 399

1,53

ET :2

B Figure 6 14 year-old boy, with Type I neuroﬁbromatosis

antecedents. SSFSE sequence weighted in T2 in the thigh’s coro-

nal plane, in which a voluminous formation is observed in the

soft part of the thigh, with high signal intensity, polylobulated,

with multiple septs. The biopsy showed a sciatic nerve plexiform neuroﬁbroma.

512X288/2 NEX 04 : 49 0,5/6,6sp DF0V 17,6 cm 8Ch Body Fu11FOV EC :1/1 41,7kHz TE :9,7/Ef TR :600 SE/SK/Signa HDxt 1,5T

FAST_GEMS\SAT_GEMS\TRF_GEMS\ACC_GEMS\FS I WW: 247WL: 18

Figure 5 A 4 year-old boy with polyneuritic gait of a year

of evolution, with EMG compatible with ﬁbular nerve lesion.

(A) SPGR sequence weighted in T1 with fat saturation and

gadolinium injection. It shows a nodule in the sciatic nerve’s

Figure 7 A 73 year-old Paciente presenting mild paresthesias

ﬁbular division (short arrow). Behind, it is possible to observe

in the ﬁbular area on both sides. The weighted T1 images show

the tibial division with a normal fascicular pattern (long arrow).

an increase in thickness of the sciatic nerve on both sides at the

(B) MPR curve reconstruction where the fusiform thickening in

expense of an intraneural fat component hypertrophy, a ﬁnding

the sciatic nerve distal third is observed (arrow).

compatible with sciatic nerve bilateral lipomatosis (arrow).

Sciatic nerve lipomatosis, also known as ﬁbromatose

hamartoma, is a rare pseudotumoral condition character- rarely lipomas, lymphangiomas, neuroﬁbrosarcomas and

ized by the fusiform thickening in a nerve by ﬁbroadipose desmoid tumor have been described.

tissue anomalous hypertrophy. The appearance of this entity Differential diagnosis between radiation and neoplastic

in the MR is pathognomonic. In the sequences weighted in T1 plexopathy may be complex. On these occasions, Positron

it shows the nerve’s fascicular pattern, hypointense, hyper- Emission Tomography (PET) with F-ﬂuorodeoxyglucose may

intense similar to the fat signal that is distributed evenly help differentiate them.

among the nerve’s ﬁbers. In the sequences weighted in T2,

and in particular in those with fat saturation or STIR (short

tau inversion recovery), the nerve appears homogeneously Conclusions

hypointense due to the suppression of the fat component

and the low signal of the normal fascicular pattern. The The current high resolution RM techniques improve visual-

43,44

amount of fat component is variable (Fig. 7). ization of normal and pathological peripheral nerves and

Moreover, perineural inﬁltration by lymphoma has provide complementary information for clinical and elec-

been described. Other processes simulate tumors such trophysiological trials. It is a complement for lumbosacral

46 5,47

as amyloidosis, the inﬂammatory pseudotumor. More spine MR in the study of lumbosciatalgias.

3 Tesla magnetic resonance neurography of the sciatic nerve 201

Ethical responsibilities with or without foraminal extension. Spine (Phila Pa 1976).

2012;3:840---4.

10. Dixon WT. Simple proton spectroscopic imaging. Radiology.

Animal and people protection. The authors declare that no

1984;153:189---94.

experiments were conducted either on people or on animals.

11. Fuller S, Reeder S, Shimakawa A, Yu H, Johnson J, Beaulieu

C, et al. Iterative decomposition of water and fat with echo

Data conﬁdentiality. The authors declare that they have asymmetry and least-squares estimation (IDEAL) fast spin-

followed the protocols of their work place about publication echo imaging of the ankle: initial clinical experience. Am J

of patients’ information and that all the patients included in Roentgenol. 2006;187:1442---7.

the study have been informed enough and they have given 12. Costa DN, Pedrosa I, McKenzie C, Reeder SB, Rofsky NM. Body

MRI using IDEAL. Am J Roentgenol. 2008;190:1076---84.

their consent in writing to participate in said study.

13. Vargas MI, Viallon M, Nguyen D, Beaulieu JY, Delavelle J, Becker

M. New approaches in imaging of the brachial plexus. Eur J

Right to privacy and informed consent. The authors

Radiol. 2010;74:403---10.

declare that no patient information appears in this article.

14. Zhang Z, Song L, Meng Q, Li Z, Pan B, Yang Z, et al.

Morphological analysis in patients with sciatica: a magnetic res-

Authors onance imaging study using three-dimensional high-resolution

diffusion-weighted magnetic resonance neurography tech-

niques. Spine (Phila Pa 1976). 2009;34:E245---50.

1. Person Responsible for the study’s integrity: CC. 15. McNab JA, Miller KL. Sensitivity of diffusion weighted steady

2. Conception of the study: CC and MA. state free precession to anisotropic diffusion. Magn Reson Med.

3. Design of the study: CC, MA, NC and LF. 2008;60:405---13.

16. Filler A. Magnetic resonance neurography and diffusion ten-

4. Data acquisition: MA, LF, NC and MCA.

sor imaging: origins, history, and clinical impact of the ﬁrst

5. Data analysis and presentation: LF, NC and MCA.

50,000 cases with an assessment of efﬁcacy and utility in a

6. Statistical treatment: Not applicable.

prospective 5000-patient study group. Neurosurgery. 2009;65 4

7. Bibliographic search: CC, MA and LF.

Suppl.:29---43.

8. Writing of the paper: CC, MA and NC.

17. Chhabra A, Andreisek G, Soldatos T, Wang KC, Flammang AJ,

9. Critical revision of the manuscript with relevant intel-

Belzberg AJ, et al. MR neurography: past, present, and future.

lectual contributions: CC, MA, LF, NC and MCA.

Am J Roentgenol. 2011;197:583---91.

10. Approval of the ﬁnal version: CC, MA, LF, NC and MCA. 18. Kim S, Choi JY, Huh YM, Song HT, Lee SA, Kim SM, et al. Role

of magnetic resonance imaging in entrapment and compres-

sive neuropathy: what, where, and how to see the peripherals

Conﬂict of interests

nerves on the musculoskeletal magnetic resonance image:

part I. Overview and lower extremity. Eur Radiol. 2007;17:

The authors declare that they have no conﬂict of interests. 139---49.

19. Kim S, Choi JY, Huh YM, Song HT, Lee SA, Kim SM, et al. Role

of magnetic resonance imaging in entrapment and compres-

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