The brachial plexus is a neuronal network providing sensory and motor innervation to the upper extremity, shoulder and upper chest. The brachial plexus is formed by lower cervical and upper thoracic ventral nerve roots, conventionally from C5-T1. Distally, the brachial plexus includes diverging and converging trunks, divisions, cords and terminal branches (Figure 1). Most commonly, the brachial plexus is comprised of five roots, three trunks (superior, middle, and inferior), six divisions (three anterior and three posterior), and three cords (lateral, medial and posterior). The major terminal branches include the median, musculocutaneous, radial, axillary and ulnar nerves. On imaging, surrounding muscles, vasculature, and bones can be used as landmarks to recognize the individual components of the brachial plexus.

Key landmarks to consider on imaging include the neural foramen, interscalene triangle, first rib (lateral border), coracoid process (medial border), and the lateral border of the pectoralis minor muscle. The neural foramen corresponds to the location of the dorsal root ganglion, an important structure in classification of preganglionic versus postganglionic injuries. At the lateral aspect of the interscalene triangle the nerve roots converge to form the three trunks; upper (C5-C6), middle (C7) and lower (C8-T1). These trunks each divide into anterior and posterior divisions near the lateral border of the first rib, with anterior divisions innervating the flexor muscles of the upper extremity and the posterior divisions innervating the extensor muscles. The six divisions form the lateral, posterior and medial cords at the medial border of the coracoid process, with the lateral and medial cords supplying the flexor muscles and the posterior cord supplying the extensors. Finally, the cords divide into the major terminal branches at the lateral border of the pectoralis minor muscle.

Variant anatomy can be seen in approximately 50% of the population, including: prefixed brachial plexus (C4-C8 roots) (Figure 2), postfixed brachial plexus (C6-T2 roots) (Figure 3), additional nerve roots from the C4 and/or T2 ventral rami, absent trunk/division/cord or variable branching patterns of the cords. The complex structure of the brachial plexus often challenges medical students and residents, leading to difficulties in identifying abnormalities on advanced imaging.