Normal anatomy [1-4]

Fig 1: Schematic representation of the anatomical components of the brachial plexus. Author: Marcos Sánchez. Department of Radiology, Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.

The brachial plexus originates from the ventral rami of the C5-T1 spinal nerves.

Fig 2: On the left, a sagittal T2 FSE image at the level of the neural foramen, shows the exit of the nerve roots. On the right, a schematic representation of the relationship between the nerve roots and the first rib (the T1 root is below the rib, while the C8 root is above the rib). Author: Marcos Sánchez. Department of Radiology, Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.

Within the interscalene triangle, we can detect three trunks (upper, middle and lower). The upper trunk forms by the union of the C5 and C6 roots, the middle trunk is a continuation of the C7 root, while the lower trunk represents the union of the C8 and T1 roots.

Fig 3: On the left, a sagittal T2 FSE imagen at the level of the interscalene triangle, with the trunks located within it (AS = anterior scalene; MS = medium scalene). On the right, an anatomical diagram of the components of the interscalene triangle. Author: Marcos Sánchez. Department of Radiology, Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.

At the lateral border of the first rib, each trunk separates into an anterior and posterior division. Together, the three anterior and posterior divisions form a cluster of six points.

Fig 4: On the left, a sagittal T2 FSE imagen at the level of the clavicle (CL), and its relationship with the divisions (AV = axillary vein; AA = axillary artery). On the right, a diagram of the anatomical components at this level. Author: Marcos Sánchez. Department of Radiology. Hospital Univesitario Virgen de la Arrixaca, Murcia, Spain.

The coracoid process serves as the anatomical reference for the cords. The divisions form three cords (lateral, posterior and medial), named for their relation to the axillary artery.

Fig 5: On the left, a sagittal T2 FSE imagen at the level of the coracoid process (CO), and its relationship with the cords (AA = axillary artery; AV = axillary vein). On the right, an anatomical diagram of the relationship between the cords, the coracoid process, and the axillary artery. Author: Marcos Sánchez. Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.

The last anatomic landmark is the pectoralis minor muscle, where the cords branch into five terminal nerves. The axillary nerve loops under the scapular neck. The remaining four branches are centered around the axillary artery. Their position can be approximated into quadrants:

• The median nerve is located in the antero-superior quadrant.
• The musculocutaneous in the posterior-superior quadrant.
• The radial nerve in the posterior-inferior quadrant.
• The ulnar nerve in the antero-inferior quadrant.
  

  Fig 6: On the left, a sagittal T1 FS image with IV contrast shows the clock-face relationship between the terminal branches of the brachial plexus and the axillary artery (Pm = pectoralis minor muscle; PM = pectoralis major; SC = subscapularis). On the right, the same relationship represented schematically. Author: Marcos Sánchez. Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.

This is a schematic summary of the 5 main anatomical landmarks and the brachial plexus nerve structures they contain.

Fig 7: Schematic combined representation of the five key anatomical levels of the brachial plexus and the neural elements they contain. Author: Marcos Sánchez. Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.

MRI imaging protocol

The MRI used for the study is the SIGNA™ Voyager model by GE, equipped with a head and neck coil.

Fig 8: 1.5T SIGNA Voyager™ MRI System. Author: Marcos Sánchez. Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.

The optimized protocol we recommend for brachial plexus studies is as follows:

• Sagittal 3D FIESTA: Allows MPR reconstructions and precise evaluation of pre/postganglionic injuries.
  

  Fig 9: 3D FIESTA sequence in saggital (A), axial (B), and coronal (C) planes. The imagen B highlights the ventral and dorsal rami of the spinal cord (yellow arrows). Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.
• Coronal T1 TSE: Assesses anatomy/muscle atrophy and abnormal structures (cervical ribs or large transverse processes).
  

  Fig 10: Coronal T1 TSE sequence. Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.
• Axial T1 and axial T2 TSE.
  

  Fig 11: Axial T1 and axial T2 TSE images at the level of the interscalene triangle (red brackets) (AS = anterior scalene; MS = medium scalene). Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.
• Sagittal T2 FSE of the pathological side or both sides if comparison is needed.
  

  Fig 12: Sagittal T2 FSE images at the five key anatomical reference points (neural foramen, interscalene triangle, clavicle, coracoid process, and pectoralis minor). Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.
• Coronal 3D Cube STIR with IV contrast: Essential sequence. Fat suppression techniques eliminate signals from fat and vessels, isolating nerve structure signals. Additionally, it allows for MPR and MIP reconstructions.
  

  Fig 13: MIP reconstruction of the brachial plexus using 3D CUBE STIR sequence with IV contrast. Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.
• Coronal 3D LAVA Flex: Primarily enables the evaluation of abnormal enhancements.
  

  Fig 14: Coronal 3D LAVA Flex sequence with three sets of images ("Fat", "Water" and "In Phase"). Department of Radiology. Hospital Universitario Virgen de la Arrixaca, Murcia, Spain.