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MRI imaging of peripheral nerves is useful in combination with clinical
evaluation and electrophysiological studies in the diagnosis of a broad
range of peripheral nerve disorders, including neoplasia, nerve root
compression and entrapment syndromes, trauma, and unexplained plexopathy.
MRI can define the specific location of nerve root pathology. MRI can also
detect secondary findings of muscle denervation.
Introduction. There have been dramatic improvements in the imaging of
disorders of the central nervous system over the last 2 decades. Neuroradiologists routinely use advanced MR applications, such as diffusion
and perfusion imaging for stroke assessment, high resolution imaging for
inner ear abnormalities, functional MRI and spectroscopy for brain tumor
evaluation.
In the past, patients with disorders of the peripheral nervous system were
diagnosed by history and physical findings in combination with
electrophysiological studies. Nerve conduction studies are an excellent and
sensitive test to evaluate patients with suspected peripheral neuropathy.
These techniques however may not be able to define the exact anatomic
location of the peripheral nerve problem.
Over the last decade, there have been dramatic improvements in MRI scanners,
pulse sequences, and high-resolution coil design. We can now image the
peripheral nervous system reliably and quickly. MRI is now commonly used in
combination with electrophysiological studies to anatomically localize and
diagnose the specific cause of peripheral nerve disorders.
The purpose of this article is to overview how to use MR to evaluate
patients with peripheral nerve problems. The technique for peripheral MR
neurography will also be discussed.
Technique. We use 2 kinds of sequences to evaluate the peripheral nervous
system. T1-weighted imaging is used to define the bony structures and tissue
planes surrounding the nerves. T1 imaging shows the detailed anatomy. T2
imaging is used to better characterize pathology.
MR Neurography is performed usually with a high-resolution fast spin echo T2
imaging technique. We also suppress the normal high signal intensity of fat
to make the nerves more conspicuous. The patient is imaged in at least 2
oblique planes.
Gadolinium enhanced T1 imaging is of limited value in most patients. It is
used primarily in the evaluation of peripheral nerve tumors, neoplastic
infiltration, and in idiopathic inflammatory disorders.
These studies are well tolerated and most can be completed in 30 minutes.
Indications. Malignancy or Peripheral Nerve Masses. MRI is now the technique
of choice in the evaluation of patients with suspected brachial and
lumbosacral plexus tumors. MRI shows the soft tissue extent of these lesions
and is useful both for diagnosis and for radiation and surgical treatment
planning.
 (Figure 1)
MRI is also used to evaluate nerve sheath tumors and soft tissue tumors that
secondarily involve peripheral nerves.
Figure 1. T2 weighted Coronal image of the Brachial Plexus Pancoast tumor of the left
lung invading the left brachial plexus. This patient presented with upper
extremity pain and weight loss. MRI revealed a mass in the left upper lobe
(solid arrow) invading the lower portion of the brachial plexus (dotted
arrow).
Nerve Root Compression or Entrapment. MRI is used to find the cause of
entrapment guided to the location of the compression by the history and
physical findings and nerve conduction studies. Common causes of peripheral
entrapment include fibrous adhesions or bands or compression by adjacent
bony structures. Bursa lesions, ganglion cysts, and inflammation of adjacent
fascial planes can also secondarily compress peripheral nerves. Nerves can
also be acutely compressed secondary to adjacent hematomas or edema due to
muscle trauma and rarely even by elevated pressure in an enclosed
compartment due to trauma or inflammation (compartment syndrome).
Median nerve compression can occur
at the level of the distal humerus or at the pronator teres origin. The most
common
site of median nerve compression is in the carpal tunnel. MR reliably
diagnosis carpal tunnel syndrome, however, imaging is not routinely needed.
The ulnar nerve may be compressed within the cubital tunnel, but less
commonly in the proximal forearm. (Figures 2a and 2b) MRI may define the
specific bony or fibrous cause of compression, but actually just shows the
swollen, bright nerve in the tunnel. Radial nerve compression syndromes are
much less common.
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Figure 2a. Normal ulnar nerve: T1 axial. The normal ulnar nerve (large arrow)
surrounded by bright fat in the cubital tunnel (small arrow). |
Figure 2b.
Cubital tunnel syndrome: T2 weighted image. This patient presented with
ulnar neuropathy. The ulnar nerve (white arrow) is enlarged and surrounded
by fluid in the cubital tunnel consistent with cubital tunnel syndrome. |
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Figure 3a and 3b. Left L5 nerve root compression. This patient presented with left leg
sciatica. The T1 weighted image (3a) shows left far lateral stenosis (white
arrow) at L5-S1. The compressed L5 nerve root (large arrow) is swollen and
hyper-intense on the T2 weighted image (3b) compared with the right normal
L5 nerve (small arrow). |
MR Neurography can also be used to evaluate patients with radiculopathy. The
compressed nerve root is enlarged and edematous. We are currently evaluating
this technique as a method to determine the specific nerve root involved in
patients suffering from multilevel disc disease.
T2 Axial Image. Masses, cysts, or endometriosis can compress the sciatic
nerve. Occasionally, the sciatic nerve can be compressed by ganglion cysts
around the sciatic notch or by a hypertrophied sacroiliac joint or piriformis muscle abnormalities. (Figure 4)
The peroneal nerve can be compressed adjacent to the fibular head by
osteophytes or cysts. Other lower extremity peripheral entrapment syndromes
are uncommon.
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Figure 4. T2 axial image just below the sciatic notch. This patient presented with
unexplained left leg radiculopathy. The left sciatic nerve (black arrow) is
hyper-intense and the left piriformis muscle (white arrow) is enlarged
consistent with piriformis syndrome. Symptoms resolved after 2 piriformis
blocks. |
Figure 5. T2 axial image. The patient presented with ulnar nerve palsy after a motor
vehicle accident. The ulnar nerve (arrow) is swollen and hyper-intense. |
Unexplained Plexopathy. Post viral or inflammatory processes can inflame the
brachial or lumbosacral plexus. Typical imaging findings include enlargement
of the nerves, hypertrophy, and abnormal contrast enhancement.
Trauma. MRI may also be useful in evaluating patients acute peripheral nerve
trauma or direct nerve lacerations pre-operatively to specifically localize
the site of the nerve injury. (Figure 5)
Conclusion. MRI imaging of peripheral nerves is useful in the evaluation of
a broad range of disorders. MRI can define the specific location of nerve
entrapment and compression and diagnose malignant infiltration and invasion.
MRI can also detect secondary findings of muscle denervation.
In the future, high-resolution imaging will continue to evolve. We will
likely be able to utilize diffusion techniques to evaluate peripheral
neuropathies before there is visible axonal injury and swelling. Muscle and
peripheral nerve metabolic evaluation has already been performed in
experimental studies. We may someday even be able to perform electro
physiology studies non-invasively using ultra-high field MR imaging
techniques.
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