Brachial Plexus Evaluation with MRI 

Evaluation of the brachial plexus is a clinical challenge. Physical examination has traditionally been a mainstay in evaluating and localizing pathology involving the brachial plexus. Physical examination is especially difficult in patients with scarring and fibrosis secondary to surgery or irradiation. Electrophysiologic studies can be used to detect abnormalities in nerve conduction, but they are poor for localizing a lesion.^{[1, 2]} (See the diagram below.)

Injury to the brachial plexus is associated with weakness and paresthesias of the upper extremity on the affected side. Thorough neurologic examination can be performed to localize the injury and to help the radiologist pinpoint the location of pathology.

As the technology and resolution have improved, magnetic resonance imaging (MRI) has become increasingly important in the evaluation of brachial plexus pathology. Correlation of imaging results with electrophysiologic findings increases overall specificity and sensitivity.^{[3, 4, 5]}

According to Nardin et al,^{[2, 6]} electromyelography (EMG) and MRI examinations are complementary. Their study demonstrated that the sensitivity of EMG and MRI were 72% and 60%, respectively. Plain radiography can depict large lesions affecting the brachial plexus. However, radiographs are far less sensitive than other studies. Computed tomography (CT) scanning has increased sensitivity for depicting extrinsic masses that compress the nerves; however, it offers poor soft tissue contrast for direct evaluation of the nerves.

With the advent of MRI, nerves that compose the brachial plexus can now be directly evaluated. Intrinsic and extrinsic pathology can be evaluated. Exact anatomic components of the brachial plexus, such as the roots, trunks, divisions, and cords, can be identified. MRI has the additional benefit of multiplanar imaging and increased soft tissue contrast. The tissue resolution of MRI is constantly improving with new pulse sequences and coil designs.^{[1, 6, 7, 8, 9, 10, 11]} (See the images below.)

With radiography and CT scanning, changes in the shape or position of the brachial plexus have been used to assess pathology.^[12] With MRI, the nerve can be directly visualized and evaluated for pathology. MRI sequences such as fat-saturated T2-weighted spin-echo, short-tau inversion recovery (STIR), and gadolinium-enhanced T1-weighted spin-echo sequences help in depicting subtle changes in the signal intensity of the nerves or enhancement and aid in refining the differential diagnosis. In addition, maximum intensity projections can make localization and visualization of the pathology most understandable for referring clinicians and surgeons. (See the images below.)

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see Medscape.

The brachial plexus can be easily identified on MRI by first identifying the anterior scalene muscle. The brachial plexus and subclavian artery (relationship outlined above) are deep to the anterior scalene. Normal components of the brachial plexus have low signal intensity on images obtained with all sequences and are surrounded by fat. The roots are best seen on axial images, whereas the remaining components are well seen on coronal and sagittal images.^{[7, 13, 14]} (See the images below.)

Radiofrequency coil

A surface coil provides resolution higher than that of a body coil, but it increases artifact due to respiratory motion. A combination of each may be used in sequences for the brachial plexus. As such, the surface coil is used for the spinal cord and exiting spinal nerve roots, whereas the body coil is used to image the plexus lateral to the interscalene triangle.

Field of view

Examination of the brachial plexus begins with the roots and trucks in the proximal aspects within the supraclavicular region and continues to the origin of the terminal branches at the lateral margin of the pectoralis minor muscle in the infraclavicular region.

The field of view (FOV) is 17-22 cm for the direct coronal orientation and 14-17 cm for sagittal or oblique sagittal orientations.

Matrix

A matrix of 512 X 256 or 512 X 512 is used.

Section thickness

The recommended section thickness is 4 mm with an intersection gap of 0-0.5 mm for direct coronal imaging and 4 mm with an intersection gap of 1-2 mm for sagittal or oblique sagittal imaging. If axial images are obtained, 4-mm thickness with a 1- to 1.5-mm intersection gap may be performed.

Orientation

Images should be obtained in 2 planes. Direct coronal plane imaging is preferred over oblique coronal imaging because the brachial plexus has a shallow obliquity relative to the true coronal plane and because it can be imaged on 1 or 2 coronal sections.

Cross-sectional imaging of the nerve components of the plexus may be performed by using either the true sagittal or the oblique sagittal plane on the side of interest. True sagittal imaging allows for comparison with the standard cross-sectional anatomy, which some find helpful in the recognition of appropriate anatomic landmarks. However, oblique sagittal imaging represents the true cross-section of the plexus more accurately than true sagittal imaging and thus allows for increased sensitivity to pathology, including changes in caliber, alteration in signal intensity, or presence of a fascicular pattern to the nerve components.

Some institutions include a contrast-enhanced T1-weighted axial sequence as part of their routine evaluation of the brachial plexus.

Pulse sequences

T1- and T2-weighted images are obtained with identical parameters in terms of FOV, matrix, section thickness, and imaging plane.

STIR or frequency-selective fat-saturation methods may be used for T2-weighted MRI to increase the conspicuity of abnormal signal intensity from the signal intensity of adjacent fat. Each has its advantages and disadvantages. STIR has been described as being more reliable than the other method because of its uniform and consistent fat suppression and excellent T2-like contrast when long repetition times are used.

The STIR method has several disadvantages. For example, it has a relatively low signal-to-noise ratio, it offers relatively low tissue contrast, and it is more susceptible to flow artifacts than other methods.

In contrast, frequency-selective fat-saturation methods have the advantage of an improved signal-to-noise ratio, T1-weighted imaging, and reduced flow-related artifacts. The major disadvantage of this type of fat suppression is the nonuniformity of fat suppression with the FOV primarily from inhomogeneity of B₀. This variability in fat suppression is exacerbated by the nonuniformity in B₁.

When radiation injury, neoplasm, infection, or inflammatory etiologies are present or suspected, contrast-enhanced fat-suppressed T1-weighted MRI should be performed. The suggested protocol is summarized in in the image below.