magnetic resonance neurography of the sciatic nerve

Radiología. 2013;55(3):195---202
www.elsevier.es/rx
UPDATE IN RADIOLOGY
High resolution (3 T) magnetic resonance neurography of the
sciatic nerve夽
C. Cejas ∗ , M. Aguilar, L. Falcón, N. Caneo, M.C. Acuña
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
Sciatic nerve;
Sciatic neuropathy;
Neurography;
Magnetic resonance
imaging
PALABRAS CLAVE
Nervio ciático;
Neuropatía ciática;
Neurografía;
Imagen por
resonancia magnética
Abstract Magnetic resonance (MR) neurography refers to a set of techniques that enable
the structure of the peripheral nerves and nerve plexuses to be evaluated optimally. New
two-dimensional and three-dimensional neurographic sequences, in particular in 3 T scanners,
achieve excellent contrast between the nerve and perineural structures. MR neurography makes
it possible to distinguish between the normal fascicular pattern of the nerve and anomalies like
inflammation, trauma, and tumor that can affect nerves. In this article, we describe the structure of the sciatic nerve, its characteristics on MR neurography, and the most common diseases
that affect it.
© 2011 SERAM. Published by Elsevier España, S.L. All rights reserved.
Neurografía por resonancia magnética de alta resolución (3 Tesla) del nervio ciático
Resumen La neurografía por resonancia magnética (RM) hace referencia a un conjunto de
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
de 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 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.
© 2011 SERAM. Publicado por Elsevier España, S.L. Todos los derechos reservados.
Introduction
夽
Please cite this article as: Cejas C, et al. Neurografía por
resonancia 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).
The study of peripheral neuropathies (PN) has been
historically the responsibility of neurophysiologists, who
contribute functional and qualitative-quantitative data of
the lesion’s location, the conduction properties and neuronal distribution. The most commonly used technique is
2173-5107/$ – see front matter © 2011 SERAM. Published by Elsevier España, S.L. All rights reserved.
196
C. Cejas et al.
the two divisions separate physically into tibial nerve and
common fibular nerve.7
Epineuro
Muscles innervated by the sciatic nerve
Fibers
Vessels
Endoneuro
In the pelvis, the sciatic nerve innervates the piriform muscle 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, semimembranosus muscle and the adductor magna. The fibular
division innervates the short head of the biceps femoris8
(Fig. 2).
Perineuro
Technical parameters of high resolution
neurography
Figure 1
nerve.
Diagram showing the structure of a peripheral
the electromyogram (EMG).1,2 However, the EMG is a timeconsuming study, it is not comfortable for the patient, it
does not locate the exact spot where the lesion is, or differentiate perineural fibrosis from compressive masses. Clinical
electrophysiologic evaluation does not usually determine
the cause of sciatic neuropathy.3 The advent of MR neurographic techniques has allowed for a more accurate
diagnostic approach.4 At present, with 3 T MR equipment
and high resolution sequences, it is possible to study the
sciatic nerve structure in detail, its course, its relationship
with the adjacent structures which may contribute to trap
it and compress it.3,5 This article reviews the sciatic nerve’s
anatomy, the RM technical parameters for its study and the
most frequent processes that may affect it.
Structure of the peripheral nerve
In order to understand better the characteristics of normal and pathological nerves in RM, it is necessary to review
briefly the structure of the peripheral nerve. The axon is
the peripheral nerve’s functional unit, and it is contained in
the sheath formed by the Schwann cells. A layer of connective tissue, the endoneuro, surrounds each axon. Multiple
axons form a fasciculus that is covered by a fibrous stroma
called perineuro. Finally, a group of fasciculi is covered by
a layer of connective tissue, the epineuro, which contains
the vessels6 (Fig. 1).
Anatomy of the sciatic nerve
Formed by the junction of roots L4---S3, the sciatic nerve is
the longest nerve of the body. It emerges through the greater
sciatic notch where its two divisions can already be distinguished: the tibial and the fibular divisions, but they are
both encased in a common nerve sheath. In its descent along
the pelvis, it presents anatomical variations in its relations
with the piriform muscle, usually going through underneath.
However, the nerve or one of its divisions, usually the fibular division, may go through the muscle. The nerve continues
to descend along the thigh, behind the adductor magna in
front of the greater gluteus. In the distal third of the thigh,
Historically, the techniques used to evaluate peripheral
nerves prioritized the weighted sequences in T2.4 Nevertheless, nowadays there are high resolution techniques
available which, by using 1---1.2 mm fine cuts, without
spacing, and by combining weighted sequences in T1 for
anatomical studies and weighted sequences in T2 with fat
suppression for the pathological, make it possible for better
contrast among the fasciculi, the perifascicular and perineural fat to be obtained. 3 T equipment is not a requirement
to perform these studies, since other field intensities also
permit it. Byun et al., in a study with 24 patients, proved
that with a 3D sequence echo gradient with millimetric
planes (Proset sequence) in a 1.5 T equipment and reconstructions in the work station, it is possible to study with
a high degree of accuracy the relation between foraminal and extraforaminal hernias with diseased nerve root.9
However, the advent of 3 T RM and high definition neurographic sequences equipment made it possible to obtain
images with excellent signal/noise relation and to perform
3D examinations. There are different ways to generate saturation of a given tissue, among them, the most commonly
used is fat saturation. One of these saturation techniques is
based on signal decomposition taking advantage of fat proton precession frequency. This method, described by Dixon,
is the basis for in-phase/out-of-phase images, which are
broadly used in daily practice.10 At present, this sequence
allows us to work simultaneously with T1 and T2 images and
4 combinations of saturation pulses (water suppression, fat
suppression and combined suppression of water and fat or inphase/out-of-phase images), by means of a modification of
Dixon’s technique (Table 1). This development corresponds
to the IDEAL sequence (iterative decomposition of water
and fat with echo asymmetry and least-squares estimation)
by General Electric (GE) Healthcare11,12 and to 3D T2 SPACE
(sampling perfection with application optimized contrast),
by Siemens Healthcare.13,14 The sum of 3D T2 Cube images
(GE Healthcare) acquired in the coronal plane and proton
density with fat saturation allows for an optimal study of
the region and it is reserved for when there is suspicion
of tumors and infections.
The after process of the work station is an essential part
of the study. Multiple plane reconstruction (MPR), reconstructions with maximum projection intensity (MPI) and curve
reconstruction techniques are performed. The latter makes
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
with the adjacent structures.
In addition, the technique has been developed based
on water molecule diffusion, such as diffusion boosted
sequences (DWI) and diffusion tensor (DTI) with tractography, to use them routinely in the evaluation of the
peripheral nervous system.15,16 It is predictable that, in
the future, these techniques might contribute information
about nervous regeneration. DTI provides data about the
nerve’s microstructure and function, in addition to information about its trajectory. Diffusion along the nerve is
usually three times greater than through the nerve
(anisotropy), due to the restrictions generated by the myelin
sheath. Thus, DTI makes it possible to for a tractography
to be performed as well as the quantitative estimation
of parameters such as the apparent diffusion coefficient
(map ADC), which translates the degree of diffusion and
fractional anisotropy. The application of high b values, of
Table 1
approximately 1000---1200 s/mm2 , is essential for an optimal
study of peripheral nerves.17
Although it is necessary to protocolize the study of the
sciatic nerve, on occasion, the selection of a technique for
an optimal examination must be agreed among the physician
referring the patient, the radiology doctor and technician
that will perform it. Table 2 summarizes the protocol used
in our laboratory.
Normal and abnormal appearance of sciatic
nerve by high resolution magnetic resonance
The normal nerve shows a fascicular appearance, with signal intensity from isointense to minimally hyperintense with
respect of the muscle in the sequences potentiated in T1
and T2.17---19 The perineural fat tissue has a homogeneous
signal and a separation plane with the adjacent structures.
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
Technical parameters. IDEAL sequence.
Technical parameters IDEAL sequence T1 and T2
Coil: neurovascular phase array 8 elements
Frequency: 320
Phase: 256
3 NEX
ET: 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 mm
FOV: 35 cm
FOV: Field of vision; IDEAL: iterative decomposition of water
and fat with echo asymmetry and least-squares estimation; NEX:
number of excitations; ET: Echo time; RT: repetition time.
Table 2
Sciatic nerve neurography protocol.
Axial FSE T1:
Crown IDEAL T2
Sagittal IDEAL T1
Coronal 3D T2 Cube
FOV (cm)
Thickness
(mm)
RT/ET (ms)
30---40
35
35
30---40
3
1---1.2
1---1.2
1
780/10
7160/90
575/9.1
1500/150
All the sequences were performed with high resolution matrix
256 × 320.
FOV: field of vision; FSE: fast spin echo; IDEAL: iterative decomposition of water and fat with echo asymmetry and least-squares
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, nowadays 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 thickens 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 contralateral 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 potentiated sequences in T1, it is also accompanied by adjacent
linear images and in contact with the nerve, characteristics
of fibrosis.19---21
Classification of neuropathies
The major etiological forms of NPs were described by Seddon
in 1943,22 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 restitution.
In axonotmesis the noxa generates axonal discontinuity 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 producing posttraumatic neuromas and/or intraneural fibrosis.22
Microsurgery techniques have benefitted surgery of
peripheral nerves in the last 20 years. Perineural fibrous tissue may be eliminated by neurolisis.23 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.20
Neuropathies specific of the sciatic nerve
Pyramidal syndrome
Described by Robinson in 1947,24 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
trochanter by means of a thick sinew. Pyramidal syndrome
may result from modifications of the piriform muscle such
as hypertrophies, contractures or spasms, direct traumas or
from anatomical anomalies in the nerve’s course as it goes
through the sciatic notch. Therefore, it is included within
Figure 3 Normal appearance of the sciatic nerve in (A) the
intercondylial notch (arrow), (B) upper third of thigh (arrow)
and (C) fibular nerve division (long arrow) and tibial nerve division (arrowhead), in a FSE T1 sequence.
3 Tesla magnetic resonance neurography of the sciatic nerve
the compression or entrapment neuropathies. Controversy
about the origin of distal lumbosacral neuropathy to the
neuroforamen has persisted for many years.25
Images in pyramidal syndrome have not been very helpful
for years and the diagnosis has always been one of exclusion.
For Stewart,26 piriform syndrome has been underdiagnosed
for years, and he has suggested surgical examination for definite diagnosis. During intervention, it is possible to identify
sciatic compression by the piriform muscle and/or association with fibrous bands.26 However, neurographic sequences
may show today the sciatic nerve along all of its course and
establish its relation with the piriform muscle.27,28
Among the lower limb neuropathies, postural neuropathies may occur in a variety of positions, are those
of lithotomy and sitting. These causes for sciatic nerve
lesions are preventable and they usually occur as a result
of intraoperation carelessness, whether because the patient
remains too long in the same posture or because of a positioning error. This has had a high legal-medical impact.29---31
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 stretching. Due to the fact that the position affects both limbs
at the same time, injury of sciatic nerve may be bilateral. Piriform muscle trauma generates muscle spasm or
contracture, and secondarily, nerve lesion by compression
and/or stretching.32 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 piriform, 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 neural sheath, along with the adjacent muscular compression
(Fig. 4).
199
A
320X256/3 NEX
06:00
0,5 /1,5 sp
DFOV 26,0 cm
HD cardiac
EC:1 / 1 62,5 kHz
TE:88,9 / Ef
TR:4100
SF/SK/Signa HDxt 3,0T
B
SPL
EC:1/1 62,5kHz
TE:94,2/Ef
TR:4120
SE/SK/Signa HDxt 3,0T
GEMS
IAR
Neuropathy by tumors and pseudotumors
The most common neoplasia described in the bibliography is
the Peripheral Neural Sheath Tumor (PNST).33---35 The importance of being able to differentiate in images benign tumors
such as neurofibroma and neurilemmoma or schwanoma lies
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.
Contrariwise, neurofibroma must be resected with part of
the nerve because the tumor cannot be separated from its
fibers. Both tumors have been broadly studied and described
in RM works. In a study of 52 patients analyzing the differences in the images weighted in T2, the target sign (58 vs
15%), central highlight (75 vs 8%) and a combination of both
(63 vs 3%) were present mainly in the neurofibromas. Contrariwise, the findings suggesting neurilemmomas were the
fascicular appearance, a thin hyperintense ring in T2, a combination of both (8% neurofibromas vs 46% neurilemmomas)
and diffuse highlight. Large neurilemmomas often undergo
degenerative changes that include cysts, hemorrhage and
fibrosis36 (Fig. 5).
The plexiform neurofibroma has a pathognomonic
appearance. It may be observed as a diffuse nodularity along
Figure 4 13-year-old girl with an antecedent of surgery in a
sitting position for 8 h. (A) IDEAL sequence weighted in T2 with
fat saturation in the coronal plane. It is possible to observe a
signal increase and diffuse thickening of both sciatic nerves,
with prevalence on the left side (arrow). It is accompanied by
edema of the adjacent soft tissue on the left side (asterisk).
(B) MPR curve reconstruction in the nerve’s longitudinal axis.
the nerve course and its branches, with a hyperintensity similar to water in the sequences weighted in T2. The large ones
replace the adipose tissue creating a ‘‘hive-like’’ appearance. They manifest clinically by neuromatose elephantiasis
37---39
(Fig. 6).
Neurofibroma or malign schwanoma, also known as
malign PNST, account for 5---10% of the soft tissue sarcomas and it is associated to Type I neurofibromatosis in
25 to 70% of the cases. It has also been described 10---20
years after it has been radiated. The sciatic nerve is the
one most often affected by these tumors.33,40 Neurography
is the state of the art image study for diagnosis. It shows
the characteristic fusiform morphology, sometimes areas of
necrosis or hemorrhage and, occasionally, cartilage or bone
heterotopic focuses. Local recurrence and metastasis are
frequent.27,41,42
200
C. Cejas et al.
S 260
A
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 coronal 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.
A
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
fibular division (short arrow). Behind, it is possible to observe
the tibial division with a normal fascicular pattern (long arrow).
(B) MPR curve reconstruction where the fusiform thickening in
the sciatic nerve distal third is observed (arrow).
Sciatic nerve lipomatosis, also known as fibromatose
hamartoma, is a rare pseudotumoral condition characterized by the fusiform thickening in a nerve by fibroadipose
tissue anomalous hypertrophy. The appearance of this entity
in the MR is pathognomonic. In the sequences weighted in T1
it shows the nerve’s fascicular pattern, hypointense, hyperintense similar to the fat signal that is distributed evenly
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
hypointense due to the suppression of the fat component
and the low signal of the normal fascicular pattern. The
amount of fat component is variable43,44 (Fig. 7).
Moreover, perineural infiltration by lymphoma45 has
been described. Other processes simulate tumors such
as amyloidosis,46 the inflammatory pseudotumor.5,47 More
Figure 7 A 73 year-old Paciente presenting mild paresthesias
in the fibular area on both sides. The weighted T1 images show
an increase in thickness of the sciatic nerve on both sides at the
expense of an intraneural fat component hypertrophy, a finding
compatible with sciatic nerve bilateral lipomatosis (arrow).
rarely lipomas,48 lymphangiomas, neurofibrosarcomas and
desmoid tumor have been described.49
Differential diagnosis between radiation and neoplastic
plexopathy may be complex. On these occasions, Positron
Emission Tomography (PET) with 18 F-fluorodeoxyglucose may
help differentiate them.50
Conclusions
The current high resolution RM techniques improve visualization of normal and pathological peripheral nerves and
provide complementary information for clinical and electrophysiological trials. It is a complement for lumbosacral
spine MR in the study of lumbosciatalgias.
3 Tesla magnetic resonance neurography of the sciatic nerve
201
Ethical responsibilities
Animal and people protection. The authors declare that no
experiments were conducted either on people or on animals.
Data confidentiality. The authors declare that they have
followed the protocols of their work place about publication
of patients’ information and that all the patients included in
the study have been informed enough and they have given
their consent in writing to participate in said study.
Right to privacy and informed consent. The authors
declare that no patient information appears in this article.
10.
11.
12.
13.
14.
Authors
1.
2.
3.
4.
5.
6.
7.
8.
9.
Person Responsible for the study’s integrity: CC.
Conception of the study: CC and MA.
Design of the study: CC, MA, NC and LF.
Data acquisition: MA, LF, NC and MCA.
Data analysis and presentation: LF, NC and MCA.
Statistical treatment: Not applicable.
Bibliographic search: CC, MA and LF.
Writing of the paper: CC, MA and NC.
Critical revision of the manuscript with relevant intellectual contributions: CC, MA, LF, NC and MCA.
10. Approval of the final version: CC, MA, LF, NC and MCA.
15.
16.
17.
18.
Conflict of interests
The authors declare that they have no conflict of interests.
19.
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