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Background
The brachial plexus originates from the nerve roots of C5-T1 and serves as the primary sensory and motor innervation of the upper extremity. Given its complex makeup, MRI is essential in visualizing its anatomic structure and any associated traumatic or nontraumatic pathologies.

Educational Goals / Teaching Points
- Discuss MRI techniques/sequences and protocols used for imaging of the brachial plexus. - Understand the anatomy and pathologies of the brachial plexus using MRI. - Recognize the MRI appearances of various traumatic and nontraumatic brachial plexopathies. - Develop a systematic approach on a case-by-case basis to formulate a limited differential diagnosis.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
- Imaging techniques used to visualize brachial plexus anatomy include ultrasound, conventional CT, CT myelography, and MRI. MRI is the modality of choice, owing to visualization of structural components, as well as microscopic changes. - On MRI, normal nerves are isointense to muscle on T1 and slightly hyperintense on T2, showing increased signal intensity around 55° orientation owing to the magic angle effect. Symmetrical bilateral signal intensity is expected. - Traumatic brachial plexopathies on MRI: pseudomeningoceles seen on T2-weighted images as fluid-intensity lesions at the site of nerve root avulsion; neuropraxic injury is seen as T2 hyperintense signal in the roots, trunks, or cords, with or without enlargement; nerve ruptures appear as discontinuity in the neural structures. Associated findings of denervation edema may be seen. Posttraumatic subclavian aneurysm may present with brachial plexus compression owing to the close proximity - Nontraumatic brachial plexopathies on MRI: radiation fibrosis is seen as diffuse thickening of the brachial plexus and iso- or hypointensity on T1- and T2-weighted images; absence of a focal mass differentiates it from metastatic disease. Brachial plexitis is seen on MRI as a focal or diffuse hyperintense signal in brachial plexus. Nerve sheath tumors appear as ovoid lesions isointense to muscle on T1-weighted images and show hyperintensity on T2-weighted images with target sign. Intense enhancement is revealed on administering gadolinium contrast.

Conclusion
Brachial plexus pathologies can be a challenging entity to diagnose based only on clinical findings. The use of imaging is an important addition to clinical examination and electrodiagnostic modalities. Among all, MR is the modality of choice to evaluate brachial plexopathies, owing to the high degree of precision and visualization of microscopic changes, although ultrasound has emerged as a vital tool to evaluate superficial structures. Knowledge and understanding of the structure and use of appropriate imaging techniques is essential for early diagnosis and management of brachial plexus pathologies.