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|NI FEATURE: CENTS (CONCEPTS, ERGONOMICS, NUANCES, THERBLIGS, SHORTCOMINGS) - COMMENTARY
|Year : 2019 | Volume
| Issue : 7 | Page : 118-124
Current status of magnetic resonance neurography in evaluating patients with brachial plexopathy
Vaishali Upadhyaya1, Divya N Upadhyaya2
1 Department of Radiodiagnosis, Vivekananda Polyclinic and Institute of Medical Sciences, Lucknow, Uttar Pradesh, India
2 Department of Plastic Surgery, King George Medical University, Lucknow, Uttar Pradesh, India
|Date of Web Publication||23-Jan-2019|
Dr. Divya N Upadhyaya
B-2/128, Sector - F, Janakipuram, Lucknow - 226 021, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
Magnetic resonance neurography (MRN) is recognized as the imaging modality of choice in the evaluation of patients with brachial plexopathy. It adds vital information to the results of the clinical evaluation and electrodiagnostic tests and facilitates patient management. Its indications include both trauma and non-traumatic forms of plexopathy such as inflammatory, neoplastic and compressive. This article will familiarize readers with the routine MRN protocol in clinical practice and discuss the utility of the different sequences. The timing of the scan is important, especially with reference to trauma and this has been discussed. Both the advantages and limitations of MRN have been elaborated upon.
Keywords: Brachial plexus, magnetic resonance imaging, neuritis, neurography, trauma, tumor
Key Message: This article discusses the routine magnetic resonance neurography protocol in clinical practice and also the utility of the different sequences.
|How to cite this article:|
Upadhyaya V, Upadhyaya DN. Current status of magnetic resonance neurography in evaluating patients with brachial plexopathy. Neurol India 2019;67, Suppl S1:118-24
|How to cite this URL:|
Upadhyaya V, Upadhyaya DN. Current status of magnetic resonance neurography in evaluating patients with brachial plexopathy. Neurol India [serial online] 2019 [cited 2020 Jul 9];67, Suppl S1:118-24. Available from: http://www.neurologyindia.com/text.asp?2019/67/7/118/250730
Magnetic resonance neurography (MRN) is recognized as the imaging modality of choice in the evaluation of patients with brachial plexopathy. It adds vital information to the results of the clinical evaluation and electrodiagnostic tests and facilitates patient management.,,,,,, MRN was introduced in 1992 when Howe et al., generated “neurograms” for the first time in which nerves appeared brighter than the surrounding tissues. Today, MRN has replaced all other forms of imaging of the brachial plexus due to its excellent soft tissue contrast, which enables visualization of the small nerves that constitute the plexus. Not only can the extraspinal plexus be visualized in intricate detail but intraspinal plexus can also be adequately delineated. It is multiplanar, noninvasive, and entails no exposure to ionizing radiation. Findings of the brachial plexus MRN enable the localization of the pathology as well as determine its extent and severity. If the findings are normal, they help the clinician to consider an alternative diagnosis that is causing the patient's symptoms.
This article will familiarize the reader with the set of sequences that are a part of the MRN protocol of the brachial plexus, the normal appearance of the plexus, as well as the spectrum of findings in various disorders that include trauma, inflammatory, and neoplastic conditions as well as thoracic outlet syndrome.
| » Anatomy of the Brachial Plexus|| |
The complex network of nerves comprising the brachial plexus innervates the skin, muscles, and joints of the shoulder and upper limb. It is formed by the ventral rami of C5, C6, C7, C8, and T1 spinal nerves. A prefixed plexus has a contribution from C4 and a postfixed plexus has a contribution from T2.
Each spinal nerve is formed by joining together of the ventral motor and dorsal sensory roots. The C5 and C6 spinal nerves join to form the upper trunk, C7 continues as the middle trunk, and C8 and T1 join to form the lower trunk. These trunks are formed at the lateral border of the scalene triangle, which lies between the anterior and middle scalene muscles. Just behind the clavicle, the trunks divide into anterior and posterior divisions. These divisions unite to form the cords posterior to the pectoralis minor. The anterior divisions of upper and middle trunks join to form the lateral cord, the anterior division of lower trunk forms the medial cord, and the posterior divisions of all the trunks form the posterior cord. The cords subsequently give rise to the terminal branches. The ulnar nerve, medial root of median nerve, and medial cutaneous nerves of arm and forearm arise from the medial cord. The lateral root of median nerve and musculocutaneous nerve arises from the lateral cord. The radial and axillary nerves arise from the posterior cord [Figure 1].,,
|Figure 1: Schematic diagram of the normal brachial plexus showing spinal nerves (Extra-spinal roots), trunks, divisions, cords and terminal branches|
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| » Magnetic Resonance Neurography Protocol|| |
The plexus can be imaged in both 1.5 and 3T scanners. Recent studies have suggested that 3T scanners may be better to image the plexus as they provide a better signal-to-noise ratio (SNR) and contrast., Others, however, still prefer 1.5T scanners as they decrease magnetic field inhomogeneity and susceptibility artifacts. However, imaging the brachial plexus with MR does seem to have a learning curve and consistently accurate results can only be achieved with a sound knowledge of anatomy along with dedicated imaging over several years, thus resulting in improved diagnostic accuracy.
Prior to imaging the plexus, all contraindications to MRI need to be ruled out. It is explained to the patient that scanning the plexus will take some time and he/she is requested to cooperate so that motion artifacts can be avoided. Some patients, especially children, may need sedation before the scan. Patients are scanned in the supine position. Arms are placed at the sides. Both cervical and body coils are used. The use of both coils enables generation of high-resolution images of the entire plexus.
The scan area extends from C3 to T3 level. The combination of sequences used to image the plexus includes both two-dimensional (2D) and three-dimensional (3D) sequences. The 2D sequences used include T1-weighted (T1W) and T2-weighted (T2W) fat suppressed sequences in the axial plane. The Dixon technique is used for fat suppression. In the coronal plane, T1W sequence is acquired, and in the sagittal plane, short tau inversion recovery (STIR) sequence is acquired on the side of the affected plexus. The 3D sequences include STIR SPACE sampling perfection with optimized contrasts application using varying flip angle evolutions in the coronal plane and sagittal T2 SPACE focused on the cervical spine. SPACE is a 3D sequence, which is available on Siemens scanners. Other vendors may, however, have different names for their 3D sequences such as Cube (General Electric – GE) and VISTA (Phillips). Such a combination of 2D and 3D sequences enables us to visualize nerves both along their long axis as well as obtain sections perpendicular to them.
The MRN protocol followed at the authors' institution on a 1.5T Siemens Magnetom Essenza scanner has been elaborated in [Table 1].
The T1W sequence helps to assess the normal anatomy of the plexus as well as adjacent muscles, bones, and vessels. T2W images help to detect the site and extent of the disease. The STIR sequence suppresses the hyperintense signal from fat. When obtained in the sagittal plane, it enables visualization of the cross-section of the nerves and their fascicular pattern. The relationship of the plexus to the subclavian and axillary vessels is nicely depicted on these images. The 3D STIR SPACE sequence that is obtained in the coronal plane allows visualization of the entire plexus. Here, the plexus stands out brightly as there is excellent fat suppression. Images can be reconstructed in multiple planes and their signal can be accentuated further by using maximum intensity projection. This facilitates optimal visualization of the location and extent of plexopathy. The 3D T2 SPACE sequence is acquired in the sagittal plane and it is focused on the cervical spine. Images can be reconstructed in the axial plane to see the intradural ventral and dorsal roots. Myelography-like images can be generated. Intravenous contrast is not routinely used in evaluating patients with plexopathy except when there is suspicion of neoplasm or infection. The blood–nerve barrier prevents enhancement of normal nerves.,,,,
A diffusion-weighted (DW) sequence can also be added to the routine MRN protocol. It accentuates the contrast between the plexus and surrounding structures. It enables visualization of the long course of the brachial plexus in the cervical region. The vascular signal can be suppressed so that nerves can be easily distinguished from vessels. The spinal cord, ganglia, and postganglionic nerve roots can be seen. Limitations of DW neurography include an inability to visualize small nerves above the level of C5 and the preganglionic plexus, poor visualization of the infraclavicular plexus as well as degradation of image quality by adjacent veins, bone marrow, and lymph nodes. It can assess the integrity of white matter but does not tell about the direction of diffusion of water molecules.,,,
Besides the anatomic evaluation of the plexus by MRN, now diffusion tensor imaging (DTI) provides a method to assess the microstructure and functional status of the nerves. It is based on the fact that there is anisotropic diffusion of protons along the long axis of the nerve. Evaluation of DTI data enables measurement of apparent diffusion coefficient, fractional anisotropy, and tensor calculation. Postprocessing of DTI data to generate nerve trajectories is called tractography. It can supplement important information to that obtained from conventional MRN in patients with brachial plexus trauma, neoplasms, and after surgery such as neurolysis or nerve transfers. It can evaluate the integrity of the nerves, grade the degree of injury, indicate whether fibers are being displaced or destroyed in neoplastic conditions as well as assess functional recovery after surgery. DTI is sensitive to artifacts and should be preferably done at 3T for good results. The increased signal-to noise ratio (SNR) at 3T helps to generate beautiful images of fiber tracts. With time, as DTI gets incorporated into the MRN protocol in routine clinical practice, radiologists will be able to convey important additional information about nerve function to referring clinicians.
| » Normal Magnetic Resonance Appearance of the Brachial Plexus|| |
A normal nerve shows a round or oval shape with well-defined smooth margins. It shows a signal intensity similar to muscle in T1W images and appears isointense to slightly hyperintense in T2W images. It appears hyperintense in fat-suppressed T2W and STIR images. This appearance is due to the presence of endoneurial fluid in nerve fascicles. This fluid has low protein and flows in a proximal to distal fashion. Changes in the endoneurial fluid can change the appearance of nerves. The size of the nerves appears same as that of adjacent arteries. The fascicular pattern can be appreciated in T1W and T2W images. It is based on the difference in signal intensity of the fascicles inside the perineurium and the interfascicular epineurium. Nerves have an expected course but they can change location with any change in position of the joint. Normally perineural fat planes are maintained and suppression of the signal intensity of perineural fat helps to see the nerve better.,,,
| » Applications|| |
All forms of plexopathy such as traumatic, inflammatory, neoplastic, and compressive can be evaluated by MRN [Table 2].,, Traumatic plexopathy is one of the most important indications for imaging the brachial plexus with MRN. The brachial plexus may be injured due to traction, crush, or penetrating injuries to the plexus. Most of the patients who sustain traction injuries are young adults, driving motorcycles who have been in a road traffic accident. Other forms of injury to the plexus include crush injury (fall of a heavy object) and penetrating injury by a sharp weapon (stab or gunshot injuries). Level of the injury of the brachial plexus can be classified as described by Chuang [Table 3].
MRN findings in trauma can be seen either in the spinal cord or the intraspinal and extraspinal plexus. These may include spinal cord edema or hemorrhage, myelomalacia, avulsion of roots from the spinal cord, pseudomeningoceles, nerve edema, transaction, and scarring. The spinal cord may show edema or hemorrhage in an acute injury and myelomalacia in chronic cases. Cord edema appears hyperintense in T2W images while blood degradation products in hemorrhage show a hypointense signal in T2W images. Traction injuries to the brachial plexus may lead to root avulsions, which may or may not be associated with pseudomeningoceles. The latter is seen as extradural fluid collections extending from the spinal canal into the neural foramina. These collections show the same signal intensity as cerebrospinal fluid. In the traumatized brachial plexus, the nerves can appear thickened with a hyperintense signal in T2W and STIR images due to edema. One may find nerve transections with discontinuity of nerve fibers and retraction. Fibrosis and scarring in chronic cases make the nerves appear clumped and distorted with a heterogeneous signal intensity. Neuroma-in-continuity and end neuromas may be seen in the injured plexus. The neuroma in continuity appears as a soft tissue mass, like a baseball, with nerves on either side of the lesion whereas the end neuroma shows a nerve with altered signal intensity ending in a similar bulbous mass. Changes in signal intensity of the paraspinal muscles due to denervation are also visible. Muscles show a hyperintense signal in T2 and STIR images due to edema in the initial stages. Later on, fatty infiltration leads to a hyperintense signal in T1W images followed by atrophy [Figure 2]a,[Figure 2]b,[Figure 2]c and [Figure 3].,,,
|Figure 2: (a) Coronal 3D STIR SPACE image of a young male patient with history of road traffic accident 3 months back showing pseudomeningocoeles at right C8 and D1 levels. (b) Coronal 3D STIR SPACE image of the same patient as in Figure 2A showing distortion and scarring of the trunks as well as scarring in the cords. (c) Axial T2W fat suppressed image of the same patient as in Figures 2a and 2B showing a pseudomeningocoele at right C8 level, denervation edema in subscapularis and infraspinatus muscles and right shoulder joint effusion|
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|Figure 3: Axial T2W image in a young male patient with history of road traffic accident 4 months back showing non-visualized ventral and dorsal nerve roots at right C6 level with pseudomeningocoele suggestive of root avulsion. The normal ventral and dorsal roots can be seen on the left side|
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Seddon has classified nerve injuries into three grades, which include neuropraxia, axonotmesis, and neurotmesis. These have been elaborated upon and correlated with nerve conduction study in [Table 4].
|Table 4: Correlation of grades of nerve injury with NCV and MRN findings|
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In cases of trauma, the timing of MRN is important and patients should be scanned 6 weeks or later after the injury so that findings are not obscured by soft tissue edema or hemorrhage. However, patients with suspected vascular injury need urgent evaluation/exploration. The advantages of MRN are that it can precisely localize the injury, describe its extent, and determine whether it is preganglionic or postganglionic, thus influencing patient management and surgical planning.,
Due to these advantages and also to the fact that there is no exposure to ionizing radiation, MRN is also the imaging modality of choice in the evaluation of children with neonatal brachial plexus palsy.
Inflammatory causes of brachial plexopathy include Parsonage-Turner syndrome (idiopathic brachial plexus neuritis), radiation-induced neuritis, and immune-mediated brachial plexopathy. MRN in these cases helps not only to identify the cause of the plexopathy but also to rule out other conditions, which may confound the diagnosis, such as cervical disc herniation, rotator cuff tear or impingement, tendinosis, and mass lesions.
In idiopathic brachial plexus neuritis, changes may be confined only to the supraspinatus and infraspinatus muscles or may extend to involve the deltoid, teres minor, and subscapularis. These muscles appear hyperintense in T2 images due to edema and later on may show fatty infiltration and atrophy. Sometimes, the plexus may show enlargement with T2 hyperintense signal [Figure 4] and [Figure 5]., In radiation-induced plexopathy, MRN findings include symmetric thickening, T2 hyperintense signal, and enhancement of the plexus in the radiation field without any focal enhancing mass lesion. In women with breast cancer who have had radiation therapy of the axilla, the infraclavicular plexus is mainly involved.
|Figure 4: Coronal 3D STIR SPACE images in a patient with history of fever 2 weeks back followed by weakness in right shoulder abduction. The right spinal nerves appear normal but hyperintense signal with interspersed foci of low signal intensity are seen in the upper trunk (block arrow on the left side) and lateral cord (block arrow on the right) of the right brachial plexus suggestive of neuritis|
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|Figure 5: (a) Coronal 3D STIR SPACE image in a patient with history of fever 1 month back followed by neck pain and weakness in bilateral shoulder abduction, more on right side. Bilaterally asymmetrical hyperintense signal is seen in the spinal nerves suggestive of neuritis. (b) Axial T2W fat suppressed image of the same patient as in Figure 5a showing altered signal intensity in the right subscapularis muscle due to denervation edema|
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Mass lesions involving the plexus can be either intrinsic or extrinsic. Intrinsic tumors include both neurogenic tumors, such as neurofibroma, schwannoma, plexiform neurofibroma, malignant peripheral nerve sheath tumor, and nonneurogenic tumors such as lipoma, desmoid, and lymphoma [Figure 6]. Common extrinsic tumors involving the brachial plexus include Pancoast tumor of the lung apex and metastatic lymph nodes [Figure 7]. These tumors can be imaged by MRN which indicates their location, size, and extent of involvement, compression of the plexus and reveals any other associated findings such as bone lesions. Schwannomas and neurofibromas are benign lesions, which show a signal intensity similar to a muscle in T1W images and appear heterogeneously hyperintense in T2W images. Both can show the target sign in T2W and postcontrast images due to a central fibrous component. Features suggestive of a malignant tumor on imaging include the large size of the mass (>5 cm), infiltration of adjacent structures, and presence of hemorrhage and necrosis within the mass.
|Figure 6: Coronal 3D STIR SPACE image in a patient with a mass lesion arising from the trunks of the left brachial plexus which was proven to be a schwannoma|
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|Figure 7: Coronal STIR image in an elderly male patient showing a large mass at the apex of left lung which is infiltrating the left brachial plexus. Biopsy revealed Pancoast tumour. The spinal nerves and trunks of right brachial plexus are normally visualized|
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Compression of the brachial plexus within the thoracic outlet leads to neurogenic thoracic outlet syndrome. This compression can occur in the interscalene triangle, costoclavicular space, and retro-pectoralis minor space. Causes of compression include cervical rib, enlarged transverse process of C7, fibrous bands, muscle hypertrophy, displaced clavicular or first rib fractures with callous, etc. T1W images need to be acquired in the sagittal plane with the arm by the side and with the arm elevated to look for compression of the plexus. MRN can show the deformed contour of the plexus with T2 and STIR hyperintense signal at the site of compression and adjacent to it. It can also show or suggest the cause of the compression [Figure 8] and [Figure 9].,,
|Figure 8: Coronal 3D STIR SPACE image in a twelve year old boy who accidentally shot himself. Callus from a left clavicular fracture is compressing the trunks of the left brachial plexus and there is altered signal intensity in the distal plexus- cords and terminal branches|
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|Figure 9: Coronal 3D STIR SPACE image in a child with a right cervical rib. There is altered signal intensity in the right C7 spinal nerve due to extrinsic compression by the cervical rib|
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In patients with cervical radiculopathy, findings in MRN are seen not only in the nerve roots but may also be seen in the distal part of the brachial plexus. These may be confused with posttraumatic changes or inflammation. In such cases, however, the correct diagnosis may be clinched by a combination of history, clinical examination, and MRN.
In traumatic brachial plexopathy, if the MRN study is done too soon after the injury, findings get masked by hemorrhage and edema. If the study is reported at this time, it may mislead the surgeon. Sometimes, scarring around an avulsed nerve root may make it look falsely normal.
Mild artifactual T2 hyperintense signal may be seen in nerves even in normal cases due to the magic angle artifact. An abnormal signal may persist for a long time in nerves despite clinical improvement. The reverse also holds true. Even with a decrease in the abnormal signal, the patient may not improve clinically.,,
| » Conclusion|| |
MRN has emerged as an incredibly good imaging modality to image the brachial plexus. It has surpassed other imaging modalities due to its excellent soft tissue resolution, noninvasiveness, multiplanar capability, and lack of exposure to ionizing radiation. In their several years of experience in imaging the brachial plexus using MRN, the authors have consistently demonstrated good results in both adult and pediatric age groups., Considering this, the authors can confidently recommend MRN as the imaging modality of choice for evaluating the brachial plexus. In the future, as advancements such as DTI get incorporated into the MRN protocol, it will become possible to assess the anatomical as well as the functional status of nerves simultaneously. This has the potential to markedly impact the prognostication and management of patients with brachial plexus lesions.
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Conflicts of interest
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
[Table 1], [Table 2], [Table 3], [Table 4]