DOI: 10.1148/rg.26si065511
RadioGraphics 2006;26:S133-S143
© RSNA, 2006
Brachial Plexus Injury: Clinical Manifestations, Conventional Imaging Findings, and the Latest Imaging Techniques1
Takeharu Yoshikawa, MD, PhD,
Naoto Hayashi, MD, PhD,
Shinichi Yamamoto, MD, PhD,
Yasuhito Tajiri, MD, PhD,
Naoki Yoshioka, MD, PhD,
Tomohiko Masumoto, MD, PhD,
Harushi Mori, MD,
Osamu Abe, MD, PhD,
Shigeki Aoki, MD, PhD and
Kuni Ohtomo, MD, PhD
1 From the Department of Computational Diagnostic Radiology and Preventive Medicine (T.Y., N.H.) and the Department of Radiology (T.M., H.M., O.A., S.A., K.O.), University of Tokyo Hospital, 7-3-1 Hongo, Bunkyo, Tokyo 113-8655, Japan; the Department of Rehabilitation for Movement Functions, Research Institute of National Rehabilitation Center for Persons with Disabilities, Tokorozawa City, Saitama, Japan (S.Y.); the Department of Orthopedics, Tokyo Metropolitan Hiroo Hospital, Tokyo, Japan (Y.T.); and the Department of Radiological Sciences, International University of Health and Welfare, Otawara City, Tochigi, Japan (N.Y.). Presented as an education exhibit at the 2005 RSNA Annual Meeting. Received March 13, 2006; revision requested April 24 and received May 25; accepted June 9. All authors have no financial relationships to disclose.
Address correspondence to T.Y. (e-mail: takeharu-yoshikawa{at}umin.ac.jp).
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Abstract
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Brachial plexus injury (BPI) is a severe neurologic injury that causes functional impairment of the affected upper limb. Imaging studies play an essential role in differentiating between preganglionic and postganglionic injuries, a distinction that is crucial for optimal treatment planning. Findings at standard myelography, computed tomographic (CT) myelography, and conventional magnetic resonance (MR) imaging help determine the location and severity of injuries. MR imaging sometimes demonstrates signal intensity changes in the spinal cord, and enhancement of nerve roots and paraspinal muscles at MR imaging indicates the presence of root avulsion injuries. New techniques including MR myelography, diffusion-weighted neurography, and Bezier surface reformation can also be useful in the evaluation and management of BPI. MR myelography with state-of-the-art technology yields remarkably high-quality images, although it cannot replace CT myelography entirely. Diffusion-weighted neurography is a cutting-edge technique for visualizing postganglionic nerve roots. Bezier surface reformation allows the depiction of entire intradural nerve roots on a single image. CT myelography appears to be the preferred initial imaging modality, with standard myelography and contrast material–enhanced MR imaging being recommended as additional studies. Work-up will vary depending on the equipment used, the management policy of peripheral nerve surgeons, and, most important, the individual patient.
© RSNA, 2006
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Introduction
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Brachial plexus injury (BPI) is the most severe nerve injury of the extremities, resulting in functional impairment of the upper limb (1). The therapeutic measures for BPI depend on the pathologic condition and the location of the injury. Clinical assessments, electrophysiologic examinations, and imaging studies are used for evaluation of the brachial plexus. Imaging studies consist of standard myelography, computed to-mographic (CT) myelography, and magnetic resonance (MR) imaging. After preoperative work-up, the diagnosis is confirmed with direct visualization and intraoperative electrophysiologic examinations if surgical exploration of the brachial plexus is indicated.
Imaging studies play an essential role in differentiating preganglionic injuries from postganglionic lesions, a differentiation that is crucial for determining the management of BPI (2). With respect to preganglionic injuries, functions of some denervated muscles are restored with nerve transfers. Postganglionic lesions are repaired with nerve grafting or followed up conservatively.
Most peripheral nerve surgeons as well as radiologists consider preoperative imaging studies useful (3,4). Belzberg et al (4) requested experienced peripheral nerve surgeons to answer a questionnaire on the management of BPI. For preoperative diagnosis of nerve root avulsion, 94% of the respondents indicated that they would use either CT myelography or MR imaging, with 41% using both.
In this article, we discuss and illustrate the clinical manifestations of BPI and its imaging appearances at standard myelography, CT myelography, and conventional MR imaging. In addition, we present new imaging techniques (MR myelography, diffusion-weighted neurography, Bezier surface reformation) and discuss their usefulness in the evaluation of BPI. We also discuss optimal imaging strategy for BPI evaluation.
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Clinical Manifestations
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BPI is caused by severe traction force exerted on the upper limb, resulting in complete or partial motor paralysis. An upper brachial plexus lesion involves spinal nerves C5 and C6 and leads to paralysis of the shoulder muscles and biceps. When the damage extends to spinal nerve C7, some of the wrist muscles are also impaired. A lower brachial plexus lesion involves spinal nerves C8 and T1 and incurs paralysis of the forearm flexor and the intrinsic muscles of the hand (5). Injuries to the stellate ganglion or cervical sympathetic trunk cause Horner syndrome.
The most common cause of BPI is traffic accidents, especially motorcycle accidents, with most of the victims being young males. The other common cause of BPI is birth palsy. The majority of obstetric BPI involves the upper brachial plexus and is referred to as Erb or Duchenne palsy. Lower type obstetric BPI (Klumpke palsy) is rare. Other traumatic causes include accidents at work, sports injuries, incised wounds, gunshot wounds, carrying a heavy rucksack, and patient malpositioning during surgery (1,5–7). Tumors, irradiation, and congenital abnormalities such as cervical ribs can be nontraumatic causes of brachial plexopathy.
BPI is classified into three categories: preganglionic lesions, postganglionic lesions, and a combination of the two. A preganglionic lesion signifies avulsion of nerve roots, whereas a postganglionic lesion involves the nerve structure distal to the sensory ganglion. Postganglionic lesions are further classified into nerve ruptures and lesions in continuity.
The management of BPI depends on the degree of damage; the site of injury; the type of involved roots; the time interval between the injury and the surgical procedure; and the patient’s age, sex, and occupation. The degree of damage and the site of injury are the most important factors (1). Treatment of BPI is either conservative or surgical. Representative surgical procedures include neurolysis, nerve grafting, nerve transfer, and other reconstructive procedures involving the transplantation of various structures.
Preganglionic injuries are not considered amenable to repair; consequently, the functions of some denervated muscles are restored with nerve transfers (1). In nerve transfer, donor nerves are attached to the ruptured distal stumps, sacrificing the original function of the nerve for more beneficial results in the upper limb (8). It is generally agreed that the top priority of nerve repair is restoration of biceps muscle function and the second goal is reanimation of shoulder function (4,5,9). Intercostal nerves are commonly used as the donor nerves transferred to the musculocutaneous nerve to regain elbow flexion. Functional recovery of the shoulder is largely achieved with spinal accessory nerve–suprascapular nerve transfer (4). Reconstructive surgery is indicated for infants with severe obstetric BPI, but management details are controversial even among professional peripheral nerve surgeons (4).
Postganglionic lesions with disruption of the nerve fiber are repaired with nerve grafting—that is, excision of the damaged segment and nerve autograft between two nerve ends. In cases in which (a) postganglionic lesions in continuity are nondegenerative or (b) fascicles are intact, spontaneous recovery is usually expected with conservative management. Lesions in continuity of degenerative type with damaged fascicles are treated with nerve grafting. Some surgeons doubt the benefit of neurolysis, although this technique has been reported to be effective (1,10).
Surgical nerve repair can provide a functional, useful limb in most patients with BPI (4). Nagano et al (11) reported that elbow flexion was restored in 82% of patients who were under 40 years of age and underwent intercostal nerve transfer within 6 months of injury. A recent report demonstrated good or excellent results in 75% of suprascapular nerve reconstructions, 40% of deltoid reconstructions, and 48% of biceps reconstructions (7). Some physicians still believe that there is rarely a need for surgery. However, patients with severe BPI should undergo an appropriate reconstructive procedure before denervated muscles become irreversibly atrophic and they are no longer good candidates for primary nerve repair (4).
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Standard Myelography and CT Myelography
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Standard myelography historically has been used to assess the level of BPI (12,13). Nowadays, standard myelography is almost always performed in combination with CT myelography. Standard myelography is a simple and economical modality that is available in most hospitals but involves radiation exposure and adverse effects from contrast material. In the evaluation of intradural nerve roots, standard myelography can be more sensitive than CT myelography in detecting root avulsion at the C8 and T1 levels (14,15). These nerve roots are sometimes difficult to evaluate with CT myelography owing to artifacts from the shoulders. This problem is significant in burly young males, who represent the majority of BPI patients. However, standard myelography is disadvantageous in that ventral and dorsal nerve roots cannot be evaluated separately.
Nagano et al (13) classified myelographic findings into six types; N, A1, A2, A3, D, and M (Figs 1–6). By definition, N is a normal shadow; A1 is a slightly abnormal root sleeve shadow in which shadows of roots and rootlets can be recognized but appear different from those on the unaffected side; A2 is obliteration of the tip of the root sleeve with the shadows of roots or rootlets visible; A3 is obliteration of the tip of the root sleeve with no shadows of roots or rootlets visible; D is a defect instead of a root sleeve shadow; and M is a traumatic meningocele. N is the sign of normality or a postganglionic lesion. A1 is observed in either a preganglionic or a postganglionic lesion; thus, detailed evaluation with CT myelography is necessary for this kind of finding. A2, A3, D, and M are indicative of a preganglionic lesion. A traumatic meningocele is a valuable sign of a preganglionic lesion, although it is not pathognomonic. Nerve root avulsion commonly occurs without a meningocele, and a meningocele occasionally exits without nerve root avulsion (16). Absence of roots is also an important sign in detecting a preganglionic lesion.
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Figure 1a. (1a) Drawing illustrates N type myelographic findings in BPI. (Modified, with permission, from reference 13.) (1b–1d) Standard myelogram (1b), coronal reformatted CT myelographic image (1c), and source CT myelogram (1d) demonstrate a normal root sleeve and nerve roots.
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Figure 1b. (1a) Drawing illustrates N type myelographic findings in BPI. (Modified, with permission, from reference 13.) (1b–1d) Standard myelogram (1b), coronal reformatted CT myelographic image (1c), and source CT myelogram (1d) demonstrate a normal root sleeve and nerve roots.
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Figure 1c. (1a) Drawing illustrates N type myelographic findings in BPI. (Modified, with permission, from reference 13.) (1b–1d) Standard myelogram (1b), coronal reformatted CT myelographic image (1c), and source CT myelogram (1d) demonstrate a normal root sleeve and nerve roots.
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Figure 1d. (1a) Drawing illustrates N type myelographic findings in BPI. (Modified, with permission, from reference 13.) (1b–1d) Standard myelogram (1b), coronal reformatted CT myelographic image (1c), and source CT myelogram (1d) demonstrate a normal root sleeve and nerve roots.
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Figure 2a. (2a) Drawing illustrates A1 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (2b–2d) Standard myelogram (2b), coronal reformatted CT myelographic image (2c), and source CT myelogram (2d) demonstrate a slightly deformed root sleeve and nerve roots. This lesion, like those in Figures 3–6, was clinically preganglionic.
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Figure 2b. (2a) Drawing illustrates A1 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (2b–2d) Standard myelogram (2b), coronal reformatted CT myelographic image (2c), and source CT myelogram (2d) demonstrate a slightly deformed root sleeve and nerve roots. This lesion, like those in Figures 3–6, was clinically preganglionic.
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Figure 2c. (2a) Drawing illustrates A1 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (2b–2d) Standard myelogram (2b), coronal reformatted CT myelographic image (2c), and source CT myelogram (2d) demonstrate a slightly deformed root sleeve and nerve roots. This lesion, like those in Figures 3–6, was clinically preganglionic.
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Figure 2d. (2a) Drawing illustrates A1 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (2b–2d) Standard myelogram (2b), coronal reformatted CT myelographic image (2c), and source CT myelogram (2d) demonstrate a slightly deformed root sleeve and nerve roots. This lesion, like those in Figures 3–6, was clinically preganglionic.
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Figure 3a. (3a) Drawing illustrates A2 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (3b, 3c) Standard myelogram (3b) and coronal reformatted CT myelographic image (3c) demonstrate obliteration of the tip of the root sleeve. Deformed nerve roots are also seen. (3d) Source CT myelogram depicts a deformed root sleeve and thickened nerve roots.
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Figure 3b. (3a) Drawing illustrates A2 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (3b, 3c) Standard myelogram (3b) and coronal reformatted CT myelographic image (3c) demonstrate obliteration of the tip of the root sleeve. Deformed nerve roots are also seen. (3d) Source CT myelogram depicts a deformed root sleeve and thickened nerve roots.
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Figure 3c. (3a) Drawing illustrates A2 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (3b, 3c) Standard myelogram (3b) and coronal reformatted CT myelographic image (3c) demonstrate obliteration of the tip of the root sleeve. Deformed nerve roots are also seen. (3d) Source CT myelogram depicts a deformed root sleeve and thickened nerve roots.
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Figure 3d. (3a) Drawing illustrates A2 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (3b, 3c) Standard myelogram (3b) and coronal reformatted CT myelographic image (3c) demonstrate obliteration of the tip of the root sleeve. Deformed nerve roots are also seen. (3d) Source CT myelogram depicts a deformed root sleeve and thickened nerve roots.
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Figure 4a. (4a) Drawing illustrates A3 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (4b, 4c) Standard myelogram (4b) and coronal reformatted CT myelographic image (4c) demonstrate obliteration of the tip of the root sleeve and the absence of nerve roots. (4d) Source CT myelogram also depicts the absence of nerve roots.
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Figure 4b. (4a) Drawing illustrates A3 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (4b, 4c) Standard myelogram (4b) and coronal reformatted CT myelographic image (4c) demonstrate obliteration of the tip of the root sleeve and the absence of nerve roots. (4d) Source CT myelogram also depicts the absence of nerve roots.
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Figure 4c. (4a) Drawing illustrates A3 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (4b, 4c) Standard myelogram (4b) and coronal reformatted CT myelographic image (4c) demonstrate obliteration of the tip of the root sleeve and the absence of nerve roots. (4d) Source CT myelogram also depicts the absence of nerve roots.
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Figure 4d. (4a) Drawing illustrates A3 type myelographic findings in BPI. (Modified, with permission, from reference 13.) (4b, 4c) Standard myelogram (4b) and coronal reformatted CT myelographic image (4c) demonstrate obliteration of the tip of the root sleeve and the absence of nerve roots. (4d) Source CT myelogram also depicts the absence of nerve roots.
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Figure 5a. (5a) Drawing illustrates D type myelographic findings in BPI. (Modified, with permission, from reference 13.) (5b) Standard myelogram demonstrates a root sleeve defect. (5c, 5d) Coronal reformatted CT myelographic image (5c) and source CT myelogram (5d) show a root sleeve defect. They also depict a traumatic meningocele, which was not visible at standard myelography (cf 5b). Intrathecal contrast material had not yet moved into the traumatic meningocele when standard myelography was performed but had filled the meningocele by the time CT myelography was performed.
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Figure 5b. (5a) Drawing illustrates D type myelographic findings in BPI. (Modified, with permission, from reference 13.) (5b) Standard myelogram demonstrates a root sleeve defect. (5c, 5d) Coronal reformatted CT myelographic image (5c) and source CT myelogram (5d) show a root sleeve defect. They also depict a traumatic meningocele, which was not visible at standard myelography (cf 5b). Intrathecal contrast material had not yet moved into the traumatic meningocele when standard myelography was performed but had filled the meningocele by the time CT myelography was performed.
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Figure 5c. (5a) Drawing illustrates D type myelographic findings in BPI. (Modified, with permission, from reference 13.) (5b) Standard myelogram demonstrates a root sleeve defect. (5c, 5d) Coronal reformatted CT myelographic image (5c) and source CT myelogram (5d) show a root sleeve defect. They also depict a traumatic meningocele, which was not visible at standard myelography (cf 5b). Intrathecal contrast material had not yet moved into the traumatic meningocele when standard myelography was performed but had filled the meningocele by the time CT myelography was performed.
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Figure 5d. (5a) Drawing illustrates D type myelographic findings in BPI. (Modified, with permission, from reference 13.) (5b) Standard myelogram demonstrates a root sleeve defect. (5c, 5d) Coronal reformatted CT myelographic image (5c) and source CT myelogram (5d) show a root sleeve defect. They also depict a traumatic meningocele, which was not visible at standard myelography (cf 5b). Intrathecal contrast material had not yet moved into the traumatic meningocele when standard myelography was performed but had filled the meningocele by the time CT myelography was performed.
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Figure 6a. (6a) Drawing illustrates M type myelographic findings in BPI. (Modified, with permission, from reference 13.) (6b–6d) Standard myelogram (6b), coronal reformatted CT myelographic image (6c), and source CT myelogram (6d) demonstrate a large traumatic meningocele.
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Figure 6b. (6a) Drawing illustrates M type myelographic findings in BPI. (Modified, with permission, from reference 13.) (6b–6d) Standard myelogram (6b), coronal reformatted CT myelographic image (6c), and source CT myelogram (6d) demonstrate a large traumatic meningocele.
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Figure 6c. (6a) Drawing illustrates M type myelographic findings in BPI. (Modified, with permission, from reference 13.) (6b–6d) Standard myelogram (6b), coronal reformatted CT myelographic image (6c), and source CT myelogram (6d) demonstrate a large traumatic meningocele.
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Figure 6d. (6a) Drawing illustrates M type myelographic findings in BPI. (Modified, with permission, from reference 13.) (6b–6d) Standard myelogram (6b), coronal reformatted CT myelographic image (6c), and source CT myelogram (6d) demonstrate a large traumatic meningocele.
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CT myelography is the most reliable imaging modality for discovering avulsion injuries (3,17). CT myelography allows separate evaluation of ventral and dorsal nerve roots and detection of intradural nerve defects. It is of equal or greater diagnostic accuracy compared with standard myelography and MR imaging, especially at the C5 and C6 levels (14,18,19), although skeletal artifacts from the shoulders are sometimes problematic at the C8 and T1 levels. The recent advent of multi–detector row CT has made it possible to obtain images over a broader area with better longitudinal spatial resolution. Sagittal and coronal reformatted images and curved reformatted images are easily available from three-dimensional data sets (20). Because coronal or oblique coronal CT myelographic images resemble standard myelograms, Nagano’s classification scheme is applicable to evaluation with a combination of reformatted CT myelographic images and source images (Figs 1–6). At CT myelography, the spinal cord is often displaced to the contralateral side of the avulsion, presumably due to the absence of normal traction from the intact roots or the presence of traumatic meningoceles.
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Conventional MR Imaging
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Findings at conventional MR imaging provide additional anatomic and physiologic information on injuries. Signal intensity changes in the spinal cord and enhancement of nerve roots and paraspinal muscles are discussed in this section (2,15,21–23).
Signal intensity changes are observed in the spinal cord in approximately 20% of patients with preganglionic injuries. Hyperintense areas on T2-weighted images suggest edema in the acute phase (Fig 7) and myelomalacia in the chronic phase. Hypointense lesions on T2-weighted images reflect hemosiderin deposition on account of hemorrhage (Fig 8). Signal intensity changes are either extensive in the affected side of the spinal cord or confined to the exit zone of the ventral nerve root. In rare cases, a defect is noted in the spinal cord; such a finding infers avulsion within the cord (Fig 9).
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Figure 7. BPI with edema in the spinal cord. Axial T2-weighted MR image demonstrates a hyperintense area (arrow) in the spinal cord, a finding that suggests edema in the acute phase.
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Figure 8. BPI with old hemorrhage in the spinal cord. Axial T2-weighted MR image demonstrates a hypointense area (arrow) in the spinal cord, a finding that indicates hemosiderin deposition on account of hemorrhage.
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Figure 9a. BPI with avulsion within the spinal cord. (a) T2-weighted MR image shows a lesion (arrow) in the exit zone of the right ventral nerve root. The lesion has a signal intensity similar to that of cerebrospinal fluid. (b) CT myelogram shows a defect (arrow) in the same area, indicating avulsion within the spinal cord.
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Figure 9b. BPI with avulsion within the spinal cord. (a) T2-weighted MR image shows a lesion (arrow) in the exit zone of the right ventral nerve root. The lesion has a signal intensity similar to that of cerebrospinal fluid. (b) CT myelogram shows a defect (arrow) in the same area, indicating avulsion within the spinal cord.
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Enhancement of intradural nerve roots and root stumps suggests functional impairment of nerve roots despite morphologic continuity (Figs 10, 11).
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Figure 10. Root avulsion injury. Axial contrast material–enhanced T1-weighted MR image demonstrates marked enhancement of the left dorsal root (arrow), a finding that indicates functional impairment.
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Figure 11. Root avulsion injury. Axial contrast-enhanced T1-weighted MR image demonstrates marked enhancement of the spinal cord surface at the right root exit zone (root stump) (arrow), a finding that is related to functional nerve root impairment.
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Hayashi et al (2) studied gadolinium-enhanced MR imaging of 27 BPI patients and compared the results with those of nerve root continuity studies using intraoperative somatosensory evoked potentials. Seven of 12 enhancing nerve roots and 38 of 42 enhancing root stumps were related to preganglionic injuries with functional discontinuity, even though they appeared normal morphologically. Breakdown of the blood-nerve barrier and dilatation of radicular veins are postulated as the mechanisms of intradural nerve root enhancement. Contrast-enhanced MR imaging is the only preoperative examination that can help detect functionally impaired nerve roots with anatomic normality and is helpful in avoiding abortive reconstructive procedures. Unfortunately, the enhancement of intradural nerve roots and root stumps is not a common finding.
Abnormal enhancement of paraspinal muscles is an accurate indirect sign of root avulsion injury (Figs 12, 13). Denervated muscles show enhancement as early as 24 hours after a nerve is injured (24). The presumed mechanisms for muscle enhancement are dilatation of the vascular bed and enlargement of the extracellular space. At unenhanced MR imaging, signal intensity changes and volume loss are observed in paraspinal muscles in patients with root avulsion injuries (25), but these findings have less accuracy and visibility than paraspinal muscle enhancement. Abnormal enhancement in the multifidus muscle is the most accurate of all paraspinal muscle findings, since the multifidus muscle is innervated by a single nerve root (23).
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Figure 12a. Root avulsion injury. (a) Axial T2-weighted MR image demonstrates areas of hyperintensity (arrowheads) in the right paraspinal muscles. (b) Axial contrast-enhanced T1-weighted MR image shows areas of marked enhancement (arrowheads) in the right paraspinal muscles, findings that are compatible with muscle denervation caused by root avulsion injury. (Fig 12 reprinted, with permission, from reference 23.)
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Figure 12b. Root avulsion injury. (a) Axial T2-weighted MR image demonstrates areas of hyperintensity (arrowheads) in the right paraspinal muscles. (b) Axial contrast-enhanced T1-weighted MR image shows areas of marked enhancement (arrowheads) in the right paraspinal muscles, findings that are compatible with muscle denervation caused by root avulsion injury. (Fig 12 reprinted, with permission, from reference 23.)
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Figure 13. Root avulsion injury. Coronal contrast-enhanced T1-weighted MR image shows marked enhancement in the left multifidus muscle (arrowheads). Abnormal enhancement in the multifidus muscle is the most accurate sign of root avulsion injury among paraspinal muscle findings. (Reprinted, with permission, from reference 23.)
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Another advantage of MR imaging is visualization of the postganglionic brachial plexus. Edema and fibrosis of the brachial plexus can manifest as thickening of the plexus (17,22,26,27). MR imaging is a good method for evaluating benign and malignant tumorous lesions (17,27–29) and, therefore, for evaluating stump neuroma derived after BPI (22). It is sometimes complicated to assess the brachial plexus when severe traumatic changes or surgical intervention distort normal anatomic structures.
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MR Myelography
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At conventional MR imaging, visualization of the intradural nerve roots is inadequate. Carvalho et al (19) evaluated the intradural integrity of 60 cervical roots using MR imaging and explored further by means of a hemilaminectomy. In their series, MR imaging provided an accurate diagnosis in only 52% of cases. The most common reasons for unreliable or inconsistent findings were partial root avulsion, intradural fibrosis, and traumatic meningoceles as well as technical pitfalls. Artifacts are caused by respiratory movements of the chest, swallowing movements of the hypopharynx and larynx, and flow in the cervical vessels. Thus, MR myelography should be performed in addition to conventional MR imaging for evaluation of nerve roots if preoperative preparation does not include CT myelography.
MR myelography has been applied to the cervical region despite little success in initial studies (30). Several researchers have used MR myelography to assess BPI, claiming that its diagnostic accuracy is nearly equivalent to that of CT myelography (31,32). Traumatic meningoceles were more clearly depicted at MR myelography than at standard myelography or CT myelography, since some of the traumatic meningoceles had little or no communication with the dural sac and intrathecal contrast material did not move into meningoceles promptly. Nevertheless, the spatial resolution of MR myelography is rather low for detailed visualization of intradural nerve roots.
MR myelography with state-of-the-art technology yields remarkably high-quality images but still cannot replace CT myelography completely. Fast imaging employing steady-state acquisition (FIESTA) is one of the steady-state coherent imaging sequences, and its signal is related to the ratio of T2 to T1. FIESTA achieves a high contrast-to-noise ratio with fewer flow artifacts; thus, this sequence is suitable for MR myelography (33). At MR myelography performed with FIESTA, visualization of nerve roots was excellent in healthy volunteers (Fig 14) and in 60% of patients with BPI (Fig 15). However, visualization of nerve roots was only fair in the remaining patients because of large traumatic meningoceles and artifacts (Fig 16). Therefore, even if MR myelography can partially replace CT myelography, CT myelography is still necessary in some patients.
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Figure 15a. Root avulsion injury. (a, b) Coronal MR myelographic image obtained with FIESTA (a) and axial reformatted MR image (b) demonstrate absence of the left C5 nerve roots (arrow) due to avulsion. The right nerve roots (arrowheads) are intact. (c) CT myelogram also shows absence of the affected nerve roots (arrow) and the intact nerve roots on the unaffected side (arrowheads).
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Figure 15b. Root avulsion injury. (a, b) Coronal MR myelographic image obtained with FIESTA (a) and axial reformatted MR image (b) demonstrate absence of the left C5 nerve roots (arrow) due to avulsion. The right nerve roots (arrowheads) are intact. (c) CT myelogram also shows absence of the affected nerve roots (arrow) and the intact nerve roots on the unaffected side (arrowheads).
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Figure 15c. Root avulsion injury. (a, b) Coronal MR myelographic image obtained with FIESTA (a) and axial reformatted MR image (b) demonstrate absence of the left C5 nerve roots (arrow) due to avulsion. The right nerve roots (arrowheads) are intact. (c) CT myelogram also shows absence of the affected nerve roots (arrow) and the intact nerve roots on the unaffected side (arrowheads).
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Figure 16a. Root avulsion injury. Axial reformatted MR image obtained with FIESTA (a) and CT myelogram (b) demonstrate large traumatic meningoceles (arrows) on the left side. The right nerve roots are intact on the CT myelogram (arrowheads in b) but are not clearly seen on the FIESTA image.
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Figure 16b. Root avulsion injury. Axial reformatted MR image obtained with FIESTA (a) and CT myelogram (b) demonstrate large traumatic meningoceles (arrows) on the left side. The right nerve roots are intact on the CT myelogram (arrowheads in b) but are not clearly seen on the FIESTA image.
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Diffusion-weighted Neurography
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Diffusion-weighted neurography is a cutting-edge technique for visualizing postganglionic nerve roots (34). Diffusion-weighted neurography is a kind of diffusion-weighted body imaging and makes use of the short tau inversion recovery technique for fat suppression and free breathing to secure relatively long scanning times (35). It makes use of a b value of about 1000 sec/mm2, which is larger than that of conventional diffusion-weighted body imaging, although such a b value is commonly used for diffusion-weighted imaging of the brain.
Diffusion-weighted neurography clearly depicts the postganglionic brachial plexus in healthy subjects (Fig 17), whereas it demonstrates loss of continuity in injured nerves (Fig 18). Because the brachial plexus has a complex anatomy and is sometimes difficult to distinguish from adjacent structures at conventional MR imaging, it is intriguing that nerves can be clearly differentiated from adjacent structures at diffusion-weighted neurography. Diffusion-weighted neurography can be a useful method for evaluating postganglionic brachial plexus lesions, although it will be necessary to optimize parameters for diffusion-weighted neurography and to study the clinical feasibility of this modality to determine its clinical applications.
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Figure 18. BPI with postganglionic lesions. On a diffusion-weighted neurogram, the left C4 and C5 nerve roots are poorly visualized (circled) compared with the corresponding contralateral nerve roots.
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Bezier Surface Reformation
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Bezier surface reformation is a technique for demonstrating entire intradural nerve roots on a single image. A Bezier surface is a parametric surface defined by mathematic formulas in terms of a set of control points. Yoshioka et al (36) developed interactive software to generate Bezier surface reformatted images from CT myelographic volume data. An operator plots several control points along nerve roots, and Bezier surface reformatted images are automatically obtained by depicting the corresponding pixel on the cubic Bezier patches. The obtained images are presented stereoscopically to illustrate the actual shape and position of the curved surface ("magic carpet" effect) (Fig 19). The entire reformation process takes only a few minutes.
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Figure 19. Root avulsion injury. Bezier surface reformatted images from CT myelographic volume data (stereographic view) demonstrate avulsion injuries of the left C6 and C7 nerve roots with traumatic meningoceles. This view illustrates the actual shape and position of the curved surface. Note that entire nerve roots are clearly depicted in the intact regions.
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Bezier surface reformation of CT myelograms can be a valuable tool in clinical practice. It has been difficult to evaluate nerve roots at several levels simultaneously because the cervical spine has some curvature and the relative courses of individual nerve roots differ. With Bezier surface reformation, entire nerve roots are demonstrated on a single image, and suspected injured roots are easily compared with roots on the contralateral side. This comprehensive image will help physicians grasp the location and extent of injuries and reduce the need for repeated observation of numerous axial images. Bezier surface reformation of CT myelograms is a promising technique for demonstrating BPI, although clinical applications are just beginning to appear.
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Optimal Imaging Strategy for BPI Evaluation
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We recommend CT myelography as the initial imaging modality, preferably with the addition of standard myelography and contrast-enhanced MR imaging. Of course, a single imaging strategy cannot be imposed on all institutions because imaging equipment and its capabilities vary, therapeutic measures used by peripheral nerve surgeons vary, and, most important, patients vary. As of this writing, however, CT myelography is the first choice for evaluating suspected preganglionic injuries because it is considered the most reliable imaging modality in detecting avulsion injuries. If CT myelography is not performed, MR myelography should be performed in addition to conventional MR imaging for the evaluation of nerve roots. Novel techniques may improve the evaluation and management of BPI, and it is desirable that the use of such techniques become widespread.
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Conclusions
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Abstract
Introduction
