Radiología. 2013;55(3):195---202

www.elsevier.es/rx

UPDATE IN RADIOLOGY

High resolution (3 T) magnetic resonance neurography of thesciatic 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 EnfermedadesNeurológicas de la Infancia Dr. Raúl Carrea (FLENI), Buenos Aires, Argentina

Received 4 November 2011; accepted 12 April 2012

KEYWORDSSciatic nerve;Sciatic neuropathy;Neurography;Magnetic resonanceimaging

Abstract Magnetic resonance (MR) neurography refers to a set of techniques that enablethe structure of the peripheral nerves and nerve plexuses to be evaluated optimally. Newtwo-dimensional and three-dimensional neurographic sequences, in particular in 3 T scanners,achieve excellent contrast between the nerve and perineural structures. MR neurography makesit possible to distinguish between the normal fascicular pattern of the nerve and anomalies likeinflammation, 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 diseasesthat affect it.© 2011 SERAM. Published by Elsevier España, S.L. All rights reserved.

PALABRAS CLAVENervio ciático;Neuropatía ciática;Neurografía;Imagen porresonancia magnética

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

Resumen La neurografía por resonancia magnética (RM) hace referencia a un conjunto detécnicas con capacidad para valorar óptimamente la estructura de los nervios periféricos yde los plexos nerviosos. Las nuevas secuencias neurográficas 2D y 3D, en particular en equiposde 3 Tesla, consiguen un contraste excelente entre el nervio y las estructuras perineurales. La

neurografía por RM permite distinguir el patrón fascicular normal del nervio y diferenciarlode las anomalías que lo afectan, como inflamaciones, traumas y tumores. En este artículose describe la estructura del nervio ciático, sus características en la neurografía por RM y lasdolencias que lo afectan con mayor frecuencia.

or El

© 2011 SERAM. Publicado p

� Please cite this article as: Cejas C, et al. Neurografía porresonancia magnética de alta resolución (3 Tesla) del nervio ciático.Radiología. 2013;55:195---202.

∗ Corresponding author.E-mail address: ccejas@fleni.org.ar (C. Cejas).

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2173-5107/$ – see front matter © 2011 SERAM. Published by Elsevier Esp

sevier España, S.L. Todos los derechos reservados.

ntroduction

he study of peripheral neuropathies (PN) has been

istorically the responsibility of neurophysiologists, whoontribute functional and qualitative-quantitative data ofhe lesion’s location, the conduction properties and neu-onal distribution. The most commonly used technique is

aña, S.L. All rights reserved.

196

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igure 1 Diagram showing the structure of a peripheralerve.

he electromyogram (EMG).1,2 However, the EMG is a time-onsuming study, it is not comfortable for the patient, itoes not locate the exact spot where the lesion is, or differ-ntiate perineural fibrosis from compressive masses. Clinicallectrophysiologic evaluation does not usually determinehe cause of sciatic neuropathy.3 The advent of MR neu-ographic techniques has allowed for a more accurateiagnostic approach.4 At present, with 3 T MR equipmentnd high resolution sequences, it is possible to study theciatic nerve structure in detail, its course, its relationshipith the adjacent structures which may contribute to trap

t and compress it.3,5 This article reviews the sciatic nerve’snatomy, the RM technical parameters for its study and theost frequent processes that may affect it.

tructure of the peripheral nerve

n order to understand better the characteristics of nor-al and pathological nerves in RM, it is necessary to reviewriefly the structure of the peripheral nerve. The axon ishe peripheral nerve’s functional unit, and it is contained inhe sheath formed by the Schwann cells. A layer of connec-ive tissue, the endoneuro, surrounds each axon. Multiplexons form a fasciculus that is covered by a fibrous stromaalled perineuro. Finally, a group of fasciculi is covered by

layer of connective tissue, the epineuro, which containshe vessels6 (Fig. 1).

natomy of the sciatic nerve

ormed by the junction of roots L4---S3, the sciatic nerve ishe longest nerve of the body. It emerges through the greaterciatic notch where its two divisions can already be distin-uished: the tibial and the fibular divisions, but they areoth encased in a common nerve sheath. In its descent alonghe pelvis, it presents anatomical variations in its relationsith the piriform muscle, usually going through underneath.

owever, the nerve or one of its divisions, usually the fibu-

ar division, may go through the muscle. The nerve continueso descend along the thigh, behind the adductor magna inront of the greater gluteus. In the distal third of the thigh,

otri

C. Cejas et al.

he two divisions separate physically into tibial nerve andommon fibular nerve.7

uscles innervated by the sciatic nerve

n the pelvis, the sciatic nerve innervates the piriform mus-le and the quadriceps femoris. In the thigh, the tibialivision of the sciatic nerve innervates the long head of theiceps femoris muscle and the semitendinosus, semimem-ranosus muscle and the adductor magna. The fibularivision innervates the short head of the biceps femoris8

Fig. 2).

echnical parameters of high resolutioneurography

istorically, the techniques used to evaluate peripheralerves prioritized the weighted sequences in T2.4 Nev-rtheless, nowadays there are high resolution techniquesvailable which, by using 1---1.2 mm fine cuts, withoutpacing, and by combining weighted sequences in T1 fornatomical studies and weighted sequences in T2 with fatuppression for the pathological, make it possible for betterontrast among the fasciculi, the perifascicular and perineu-al fat to be obtained. 3 T equipment is not a requiremento perform these studies, since other field intensities alsoermit it. Byun et al., in a study with 24 patients, provedhat with a 3D sequence echo gradient with millimetriclanes (Proset sequence) in a 1.5 T equipment and recons-ructions in the work station, it is possible to study with

high degree of accuracy the relation between forami-al and extraforaminal hernias with diseased nerve root.9

owever, the advent of 3 T RM and high definition neuro-raphic sequences equipment made it possible to obtainmages with excellent signal/noise relation and to performD examinations. There are different ways to generate sat-ration of a given tissue, among them, the most commonlysed is fat saturation. One of these saturation techniques isased on signal decomposition taking advantage of fat pro-on precession frequency. This method, described by Dixon,s the basis for in-phase/out-of-phase images, which areroadly used in daily practice.10 At present, this sequencellows us to work simultaneously with T1 and T2 images and

combinations of saturation pulses (water suppression, fatuppression and combined suppression of water and fat or in-hase/out-of-phase images), by means of a modification ofixon’s technique (Table 1). This development correspondso the IDEAL sequence (iterative decomposition of waternd fat with echo asymmetry and least-squares estimation)y General Electric (GE) Healthcare11,12 and to 3D T2 SPACEsampling perfection with application optimized contrast),y Siemens Healthcare.13,14 The sum of 3D T2 Cube imagesGE Healthcare) acquired in the coronal plane and protonensity with fat saturation allows for an optimal study ofhe region and it is reserved for when there is suspicionf tumors and infections.

The after process of the work station is an essential part

f the study. Multiple plane reconstruction (MPR), recons-ructions with maximum projection intensity (MPI) and curveeconstruction techniques are performed. The latter makest 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 (largeasterisk) and semitendinous (long arrow).

along its length, thus providing information about the nervewith the adjacent structures.

In addition, the technique has been developed basedon water molecule diffusion, such as diffusion boostedsequences (DWI) and diffusion tensor (DTI) with tractog-raphy, to use them routinely in the evaluation of theperipheral nervous system.15,16 It is predictable that, inthe future, these techniques might contribute informationabout nervous regeneration. DTI provides data about thenerve’s microstructure and function, in addition to infor-mation about its trajectory. Diffusion along the nerve isusually three times greater than through the nerve(anisotropy), due to the restrictions generated by the myelinsheath. Thus, DTI makes it possible to for a tractographyto be performed as well as the quantitative estimationof parameters such as the apparent diffusion coefficient(map ADC), which translates the degree of diffusion andfractional anisotropy. The application of high b values, of

Table 1 Technical parameters. IDEAL sequence.

Technical parameters IDEAL sequence T1 and T2Coil: neurovascular phase array 8 elementsFrequency: 320Phase: 2563 NEXET: 90 ms (T2)/9.1 ms (T1)RT: 7160 ms (T2)/575 ms (T1)Echo train: 20 (T2)/3 (T1)Thickness/spacing 1---1.2 mm/0---0.2 mmFOV: 35 cm

FOV: Field of vision; IDEAL: iterative decomposition of waterand fat with echo asymmetry and least-squares estimation; NEX:number of excitations; ET: Echo time; RT: repetition time.

approximately 1000---1200 s/mm2, is essential for an optimalstudy of peripheral nerves.17

Although it is necessary to protocolize the study of thesciatic nerve, on occasion, the selection of a technique foran optimal examination must be agreed among the physicianreferring the patient, the radiology doctor and technicianthat will perform it. Table 2 summarizes the protocol usedin our laboratory.

Normal and abnormal appearance of sciaticnerve by high resolution magnetic resonance

The normal nerve shows a fascicular appearance, with sig-nal intensity from isointense to minimally hyperintense withrespect of the muscle in the sequences potentiated in T1and T2.17---19 The perineural fat tissue has a homogeneoussignal and a separation plane with the adjacent structures.The perineural tissue around the sciatic nerve is especiallyabundant, which makes it possible for it to be distinguishedclearly from the surrounding structures with the present MR

Table 2 Sciatic nerve neurography protocol.

FOV (cm) Thickness(mm)

RT/ET (ms)

Axial FSE T1: 30---40 3 780/10Crown IDEAL T2 35 1---1.2 7160/90Sagittal IDEAL T1 35 1---1.2 575/9.1Coronal 3D T2 Cube 30---40 1 1500/150

All the sequences were performed with high resolution matrix256 × 320.FOV: field of vision; FSE: fast spin echo; IDEAL: iterative decom-position of water and fat with echo asymmetry and least-squaresestimation; ET: echo time; RT: repetition time.

1 C. Cejas et al.

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Figure 3 Normal appearance of the sciatic nerve in (A) theintercondylial notch (arrow), (B) upper third of thigh (arrow)

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equences. The nerve’s course is rather straight, withoutcute angles. Thanks to high resolution sequences, nowa-ays it is possible to distinguish the sciatic nerve divisions,he thicker, inner one is the tibial division and the thinner,ateral one is the fibular division (Fig. 3).

The abnormal nerve loses its fascicular pattern, it thick-ns either focally or diffusely and it presents deviations,nterruptions or compressions in its course. In the sequenceseighted in T2, it is hyperintense with respect of its con-

ralateral and it usually enhances with the gadoliniumnjection. In addition, it is accompanied by signal changes orerineural fat stratification, which is easier to see in poten-iated sequences in T1, it is also accompanied by adjacentinear images and in contact with the nerve, characteristicsf fibrosis.19---21

lassification of neuropathies

he major etiological forms of NPs were described by Seddonn 1943,22 who highlighted those in which traction and/orxtrinsic compression damage occurs. The nerve undergoeshe effect of physical forces whether by friction, elongationr compression, which generate three lesion mechanisms:europraxia, axonotmesis and neurotmesis.

Neuropraxia is the smallest degree of lesion. The nervehows suffering without sheath or axon discontinuity. It isenerally a transitory disorder, one with complete restitu-ion.

In axonotmesis the noxa generates axonal discontinu-ty with integrity of the connective wrapping (perineuro,ndoneuro and epineuro). The complete rupture of the axonroduces a Wallerian degeneration of the second distal.rognosis in these lesions continues to be good, but recoverys slow (axon regeneration of approximately 1 mm per day).

Neurotmesis is the greatest degree of lesion. Both thexon and the perineural sheaths are affected. Functionaloss is complete and, unless it is surgically operated early,ranulation tissue and subsequently, fibrosis appear produc-ng posttraumatic neuromas and/or intraneural fibrosis.22

Microsurgery techniques have benefitted surgery oferipheral nerves in the last 20 years. Perineural fibrous tis-ue may be eliminated by neurolisis.23 In cases of severexonotmesis or neurotmesis surgical repair is necessary.irect suture of the nerve ends may generate tension. That

s why the present criterion is to use auto- or alografts.20

europathies specific of the sciatic nerve

yramidal syndrome

escribed by Robinson in 1947,24 the pyramidal or piriformyndrome results from sciatic nerve compression as it passeshrough the piriform notch. The piriform muscle is the mostctive in running athletes and it is inserted in the pediclesf the third and fourth sacral vertebrae, it goes throughhe greater sciatic foramen and it inserts into the greaterrochanter by means of a thick sinew. Pyramidal syndrome

ay result from modifications of the piriform muscle such

s hypertrophies, contractures or spasms, direct traumas orrom anatomical anomalies in the nerve’s course as it goeshrough the sciatic notch. Therefore, it is included within

and (C) fibular nerve division (long arrow) and tibial nerve divi-sion (arrowhead), in a FSE T1 sequence.

199

320X256/3 NEX06:000,5 /1,5 spDFOV 26,0 cmHD cardiacEC:1 / 1 62,5 kHzTE:88,9 / EfTR:4100SF/SK/Signa HDxt 3,0T

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EC:1/1 62,5kHzTE:94,2/EfTR:4120SE/SK/Signa HDxt 3,0T

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Figure 4 13-year-old girl with an antecedent of surgery in asitting position for 8 h. (A) IDEAL sequence weighted in T2 withfat saturation in the coronal plane. It is possible to observe asignal increase and diffuse thickening of both sciatic nerves,with prevalence on the left side (arrow). It is accompanied bye(

tira3

mm2yoi

3 Tesla magnetic resonance neurography of the sciatic nerve

the compression or entrapment neuropathies. Controversyabout the origin of distal lumbosacral neuropathy to theneuroforamen has persisted for many years.25

Images in pyramidal syndrome have not been very helpfulfor years and the diagnosis has always been one of exclusion.For Stewart,26 piriform syndrome has been underdiagnosedfor years, and he has suggested surgical examination for def-inite diagnosis. During intervention, it is possible to identifysciatic compression by the piriform muscle and/or associa-tion with fibrous bands.26 However, neurographic sequencesmay show today the sciatic nerve along all of its course andestablish its relation with the piriform muscle.27,28

Among the lower limb neuropathies, postural neu-ropathies may occur in a variety of positions, are thoseof lithotomy and sitting. These causes for sciatic nervelesions are preventable and they usually occur as a resultof intraoperation carelessness, whether because the patientremains too long in the same posture or because of a posi-tioning error. This has had a high legal-medical impact.29---31

In both positions, the same forces contribute to lesion bystretching 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 limbsat the same time, injury of sciatic nerve may be bilat-eral. Piriform muscle trauma generates muscle spasm orcontracture, and secondarily, nerve lesion by compressionand/or stretching.32 the MR findings, taking into accountclinical antecedents, are very revealing. The images showa fusiform thickening of the nerve as it passes throughthe sciatic notch, a focal point of contusions in piri-form, quadriceps femoris and gluteal muscles, as wellas in the adjacent fat planes. Contrast uptake by thesestructures may be explained by the acute inflammatorycomponent of the vascular-nervous package of the neu-ral sheath, along with the adjacent muscular compression(Fig. 4).

Neuropathy by tumors and pseudotumors

The most common neoplasia described in the bibliography isthe Peripheral Neural Sheath Tumor (PNST).33---35 The impor-tance of being able to differentiate in images benign tumorssuch as neurofibroma and neurilemmoma or schwanoma liesin that the schwanoma may dry up without affecting thenerve, because it is contained within the same capsule,the epineuro, and it can usually be separated surgically.Contrariwise, neurofibroma must be resected with part ofthe nerve because the tumor cannot be separated from itsfibers. Both tumors have been broadly studied and describedin RM works. In a study of 52 patients analyzing the differ-ences in the images weighted in T2, the target sign (58 vs15%), central highlight (75 vs 8%) and a combination of both(63 vs 3%) were present mainly in the neurofibromas. Con-trariwise, the findings suggesting neurilemmomas were thefascicular appearance, a thin hyperintense ring in T2, a com-bination of both (8% neurofibromas vs 46% neurilemmomas)and diffuse highlight. Large neurilemmomas often undergo

degenerative changes that include cysts, hemorrhage andfibrosis36 (Fig. 5).

The plexiform neurofibroma has a pathognomonicappearance. It may be observed as a diffuse nodularity along

tnhf

dema of the adjacent soft tissue on the left side (asterisk).B) MPR curve reconstruction in the nerve’s longitudinal axis.

he nerve course and its branches, with a hyperintensity sim-lar to water in the sequences weighted in T2. The large oneseplace the adipose tissue creating a ‘‘hive-like’’ appear-nce. They manifest clinically by neuromatose elephantiasis7---39 (Fig. 6).

Neurofibroma or malign schwanoma, also known asalign PNST, account for 5---10% of the soft tissue sarco-as and it is associated to Type I neurofibromatosis in

5 to 70% of the cases. It has also been described 10---20ears after it has been radiated. The sciatic nerve is thene most often affected by these tumors.33,40 Neurographys the state of the art image study for diagnosis. It shows

he characteristic fusiform morphology, sometimes areas ofecrosis or hemorrhage and, occasionally, cartilage or boneeterotopic focuses. Local recurrence and metastasis arerequent.27,41,42

200 C. Cejas et al.

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A

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Figure 5 A 4 year-old boy with polyneuritic gait of a yearof evolution, with EMG compatible with fibular nerve lesion.(A) SPGR sequence weighted in T1 with fat saturation andgadolinium injection. It shows a nodule in the sciatic nerve’sfibular division (short arrow). Behind, it is possible to observethe tibial division with a normal fascicular pattern (long arrow).(t

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EC:1 /1 31,2kHzTI:145,08Ch Body FullFOVFOV:4x44

S 260

Figure 6 14 year-old boy, with Type I neurofibromatosisantecedents. SSFSE sequence weighted in T2 in the thigh’s coro-nal plane, in which a voluminous formation is observed in thesoft part of the thigh, with high signal intensity, polylobulated,with multiple septs. The biopsy showed a sciatic nerve plexiformneurofibroma.

Figure 7 A 73 year-old Paciente presenting mild paresthesiasin the fibular area on both sides. The weighted T1 images showan increase in thickness of the sciatic nerve on both sides at theec

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The current high resolution RM techniques improve visual-

B) MPR curve reconstruction where the fusiform thickening inhe sciatic nerve distal third is observed (arrow).

Sciatic nerve lipomatosis, also known as fibromatoseamartoma, is a rare pseudotumoral condition character-zed by the fusiform thickening in a nerve by fibroadiposeissue anomalous hypertrophy. The appearance of this entityn the MR is pathognomonic. In the sequences weighted in T1t shows the nerve’s fascicular pattern, hypointense, hyper-ntense similar to the fat signal that is distributed evenlymong the nerve’s fibers. In the sequences weighted in T2,nd in particular in those with fat saturation or STIR (shortau inversion recovery), the nerve appears homogeneouslyypointense due to the suppression of the fat componentnd the low signal of the normal fascicular pattern. The

43,44

mount of fat component is variable (Fig. 7).Moreover, perineural infiltration by lymphoma45 has

een described. Other processes simulate tumors suchs amyloidosis,46 the inflammatory pseudotumor.5,47 More

ipts

xpense of an intraneural fat component hypertrophy, a findingompatible with sciatic nerve bilateral lipomatosis (arrow).

arely lipomas,48 lymphangiomas, neurofibrosarcomas andesmoid tumor have been described.49

Differential diagnosis between radiation and neoplasticlexopathy may be complex. On these occasions, Positronmission Tomography (PET) with 18F-fluorodeoxyglucose mayelp differentiate them.50

onclusions

zation of normal and pathological peripheral nerves androvide complementary information for clinical and elec-rophysiological trials. It is a complement for lumbosacralpine MR in the study of lumbosciatalgias.

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3 Tesla magnetic resonance neurography of the sciatic nerve

Ethical responsibilities

Animal and people protection. The authors declare that noexperiments were conducted either on people or on animals.

Data confidentiality. The authors declare that they havefollowed the protocols of their work place about publicationof patients’ information and that all the patients included inthe study have been informed enough and they have giventheir consent in writing to participate in said study.

Right to privacy and informed consent. The authorsdeclare that no patient information appears in this article.

Authors

1. Person Responsible for the study’s integrity: CC.2. Conception of the study: CC and MA.3. Design of the study: CC, MA, NC and LF.4. Data acquisition: MA, LF, NC and MCA.5. Data analysis and presentation: LF, NC and MCA.6. Statistical treatment: Not applicable.7. Bibliographic search: CC, MA and LF.8. Writing of the paper: CC, MA and NC.9. Critical revision of the manuscript with relevant intel-

lectual contributions: CC, MA, LF, NC and MCA.10. Approval of the final version: CC, MA, LF, NC and MCA.

Conflict of interests

The authors declare that they have no conflict of interests.

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