Accurate assessment of the brachial plexus (BP) and sacral plexus (SP) requires a thorough understanding of this complex anatomy. The author presents a detailed discussion of the anatomy, current imaging techniques, and pathologic conditions of these two regions.
are in the Department of Radiology at the University of Miami
School of Medicine in Miami, FL.
ccurate assessment of the brachial plexus (BP) and sacral plexus
(SP) requires thorough understanding of the complex anatomy. What
follows is a discussion of the anatomy, current imaging techniques,
and pathologic conditions of these two regions.
--The brachial plexus is formed from the ventral rami of C5 to T1
spinal nerves, with variable contributions from C4 and T2. The
ventral rami are the five roots of the BP. At approximately the
lateral border of the anterior scalene muscle, the C5 and C6 roots
form the upper or superior trunk, the C7 root forms the middle
trunk, and the C8 and T1 roots form the lower trunk. The trunks are
located superior and posterior to the subclavian artery as the
plexus emerges from the interscalene triangle between the anterior
and middle scalene muscles and the first rib (figure 1). Unlike the
subclavian artery, which runs with the BP, the subclavian vein
courses anterior to the anterior scalene muscle (figure 2A). Beyond
the first rib, the subclavian vessels become the axillary vessels,
with the subclavian vein remaining anteroinferior to the artery.
The three trunks divide into three anterior and three posterior
divisions (figure 2B). These six divisions subsequently join to
form three cords in the medial aspect of the axilla. The plexus
enters the axilla by passing between the clavicle and first
The cords are named for their relationship to the axillary
artery. The anterior divisions of the upper and middle trunks form
the lateral cord, the anterior division of the lower trunk forms
the medial cord, and the posterior divisions of all three trunks
form the posterior cord. Near the lateral margin of the pectoralis
minor muscle, each cord ends in two major terminal branches: the
musculocutaneous nerve (from the posterior cord), axillary nerve
(from the lateral cord), radial nerve (from the posterior cord),
median nerve (from the lateral and medial cords), and ulnar nerve
(from the medial cord).
--The BP can be difficult to image due to its complex anatomy,
oblique orientation, and location in the neck and axilla where
there are often magnetic susceptibility effects. The goal of
imaging the brachial plexus is to produce high spatial resolution
with adequate signal-to-noise ratio (SNR) while keeping scan time
acceptable. To that end, it is important to strike a balance in the
number of sequences, matrix size, number of excitations (NEX), and
field of view (FOV) (Table).
Following an axial localizing scan, coronal and sagittal images
are obtained. Coronal images are approximately parallel to the long
axis of the plexus. Sagittal or oblique sagittal images, on the
other hand, are oriented approximately perpendicular to the long
axis of the plexus; these are useful to evaluate the
cross-sectional anatomy of the BP and adjacent landmarks. A small
FOV (14 to 26 cm) is used for both planes. Slice thickness ranges
from 4 to 5 mm with a 0.5 to 2 mm interslice gap in the sagittal
plane, and from 3 to 4 mm with a 0.5 mm gap in the coronal plane.
Matrix size is high: generally 256 (frequency) * 256 (phase) for a
14- to 16-cm FOV and 512 * 256 to 512 for an 18- to 26-cm FOV.
Standard, commercially available respiratory motion compensation is
used along with a spatial presaturation band over the heart and
aortic root to suppress flow artifacts. A phased-array
radiofrequency coil is preferred to a body or conventional surface
coil in order to obtain high SNR and adequate coverage.
T1-weighted spin-echo (SE) images are useful to identify
anatomic landmarks and evaluate regional anatomy. These are
acquired in both the coronal and oblique sagittal planes.
T2-weighted fast spin-echo (FSE) images with fat saturation are
used to detect abnormally increased signal in injured nerves. Fat
saturation is performed using either a frequency-selective
(chemical shift) or short-tau inversion recovery (STIR) technique.
While chemical shift fat saturation has a higher SNR and fewer
blood-flow related artifacts than the STIR technique, it suffers
from inhomogeneous fat suppression--a result of bulk susceptibility
effects in the neck and shoulder region. In our experience, the
STIR method provides more homogeneous fat suppression while
main-taining excellent T2 contrast.
Additional STIR images in the axial plane are excellent for the
evaluation of neural foramina and exiting nerve roots in cases of
suspected nerve root avulsion/pseudomeningocele. Postcontrast,
T1-weighted frequency-selective fat saturation images are obtained
for cases involving intraspinal/transforaminal spread of tumor or
infection, in postoperative evaluation, and in cases of stretch
injury or nerve entrapment.
The standard contrast dose of 0.1 mmol/kg is injected into the
antecubital vein on the contralateral side.
--In normal individuals, the major peripheral nerves are
bilaterally symmetric, linear (in-plane images) (figure 1), oval or
round (cross-sectional images) (figure 2) structures often
delineated by surrounding fat. They generally are isointense on
T1-weighted images and mildly hyperintense to adjacent muscle on
heavily T2-weighted images.
Using high resolution technique, the rod-like fascicles within the
nerve usually are visible on cross-sectional images. Enlargement of
the nerve, alteration in a fascicular pattern, or markedly
increased signal intensity on STIR and postcontrast images are
Abnormal hyperintense signal has been shown to correlate with
Pathologic changes result from traumatic or atraumatic
processes. Blunt or penetrating trauma with fracture, dislocation,
or hematoma formation may produce compression, stretching, or
avulsion of the BP with diffusely increased signal on T2-weighted
images and postcontrast enhancement (figure 3). Long, uniform
involvement of the nerve without eccentric masses favors the
diagnosis of stretch injury over neoplasia. Occasionally, a
posttraumatic neuroma may develop. These are usually well
circumscribed, mass-like areas of T2 hyperintensity within the
nerve. Nerve root avulsions are associated with a dural tear
resulting in the formation of a traumatic pseudo-meningocele, seen
as a focal CSF collection extending through the neural foramen.
Unlike stretch injuries, in which the degree of stretching usually
is insufficient to cause permanent paralysis, nerves completely
avulsed from the spinal cord produce fixed deficits.
Atraumatic processes include neoplasms--either primary tumors of
neural origin or other tumors--which secondarily involve the
plexus. For example, schwannoma and neurofibroma are benign primary
neoplasms of neural origin. Schwannoma is typically solitary and
causes focal enlargement of a nerve. The nerve is isointense or
slightly hyperintense to muscle on T1-weighted images and markedly
hyperintense on T2-weighted images (figure 4). Plexiform
neurofibromas usually are multiple and may diffusely enlarge many
components of the BP. Neurofibrosarcoma is a malignant nerve sheath
neoplasm seen in 3% to 13% of patients with neurofibromatosis.
Unfortunately, because it has similar signal characteristics to
benign nerve tumors, it cannot reliably be distinguished.
Tumors that may invade the brachial plexus, ribs, and vessels
include superior sulcus (Pancoast) carcinoma, breast carcinoma,
lymphoma, and spinal metastases. As with other pathologic
processes, these are typically isointense to muscle on T1-weighted
images and hyperintense on T2-weighted images.
Radiation plexopathy is unlikely to occur with less than 6000 Rads
of exposure, although other factors, including dose regimen and
previous regional surgery, may facilitate radiation injury. Unlike
metastases which are often painful and involve the lower trunk
(C8-T1), radiation plexopathy is usually painless and involves the
upper trunk (C5-6) or the entire plexus. On imaging, diffuse
symmetric thickening or irregularity of the BP without a focal mass
is noted. These changes usually are isointense to muscle on
T1-weighted images and can show postcontrast enhancement. On
T2-weighted images, nerves and surrounding connective tissue may be
hyperintense or isointense to muscle.
--The sacral plexus is formed from the ventral rami (roots) of L4
to S4. It innervates muscles of the buttocks and posterior thigh,
as well as muscles below the knee. The plexus roots are situated
anterior to the sacrum and the piriformis muscle. They split into
anterior and posterior divisions which subsequently give rise to
anterior and posterior branches of the plexus. Anterior branches
include the tibial part of the sciatic nerve and pudendal nerve,
and the medial part of the posterior femoral cutaneous nerve.
Posterior branches include the common peroneal part of the sciatic
nerve, the superior and inferior gluteal nerves, the lateral part
of the posterior femoral cutaneous nerve, and the nerve to the
Individual nerves may be difficult to distinguish on axial
T1-weighted images, as they may interdigitate with serrations in
the muscle. At the level of the greater sciatic foramen, the sacral
plexus and surrounding vessels generally have a dot-dash
configuration in the premuscular fat (figure 5).
The sciatic nerve is the continuation of the sacral plexus. It
is formed at the inferior margin of the piriformis muscle, from the
L4 to S3 roots. It exits the pelvis through the greater sciatic
foramen along with the piriformis muscle and the superior and
inferior gluteal vessels and nerves (figure 6). The sciatic nerve
courses laterally in the gluteal region, reaching the posterior
portion of the upper thigh. It then courses inferiorly, dividing
into the common peroneal and tibial nerves at the lower third of
--Using a phased-array RF coil, the sacral plexus is routinely
scanned in two planes (Table). First, a pad is placed beneath the
patient's knees to decrease lumbar lordosis. Spatial RF pulses are
placed superior and inferior to the imaging volume to decrease
signal intensity from flowing blood. Following a sagittal
localizing image, axial and coronal precontrast and fat suppressed
postcontrast T1-weighted images are obtained. Axial STIR or fat
suppressed, FSE T2-weighted images are usually obtained because
susceptibility effects are less pronounced in the pelvis than at
the cervicothoracic junction. Oblique "coronal" images, oriented
parallel to the plane of the sacrum, may be obtained to aid in
evaluation of the sacral foramina and proximal S1 to S4 roots.
Slice thickness ranges from 4 to 5 mm, with no interslice gap for
both cross-sectional (axial) images and in-plane (coronal) images.
The FOV is usually 22 to 26 cm with a 512 * 256 to 512 matrix.
--Sacral plexopathy may be due to a variety of etiologies including
vascular, degenerative, infectious, and neoplastic disease. Trauma
is less likely to produce a sacral rather than a brachial
Nervous compression may result from retroperitoneal or psoas
hematoma, aneurysm or pseudoaneurysm of the iliac or gluteal
arteries, lumbar disc herniation, or vertebral osteomyelitis with
associated psoas or iliacus abscess.
Neoplastic disease includes primary nerve sheath tumors
(schwannoma [figure 7] or neurofibroma), intraperitoneal tumors
(colon carcinoma), retroperitoneal tumors (renal cell carcinoma,
lymphoma), extraperitoneal tumors (uterine
leiomyoma/leiomyosarcoma, prostate carcinoma, cervical carcinoma,
rectal carcinoma), and distant metastases (lung and breast
carcinoma) (figure 8). Clinically, sarcomas tend to produce a
lumbar plexopathy (L1-L4), colorectal neoplasms a sacral plexopathy
(L5-S3), and genitourinary tumors a lumbosacral panplexopathy.
On imaging, T1-weighted sequences demonstrate obliteration or
replacement of fat planes around the nerves. T1-weighted,
postcontrast, fat suppressed images help to distinguish enhancing
tumor from nonenhancing muscle and to detect perineural spread.
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