MRI of the Brachial Plexus: Normal Anatomy, Pathology, and Imaging Findings

1. Normal Anatomy

Overview

Brachial plexus: roots (C5-T1), trunks, divisions, and cords, with sympathetic chain connections - Gray's Anatomy for Students

The "RTDC" Organization (Roots - Trunks - Divisions - Cords)

Key Anatomic Spaces

• Scalene triangle (interscalene): between anterior scalene, middle scalene, and first rib - contains roots and trunks
• Costoclavicular space: between clavicle and first rib - trunks/divisions pass here
• Retropectoralis minor (subcoracoid) space: cords and terminal branches here

2. Normal MRI Appearance

Imaging Protocol

Normal MRI Signal Characteristics

• T1W: Nerves appear as isointense to slightly hypointense cord-like structures surrounded by hyperintense fat. The nerve fascicles are uniform in caliber.
• T2W / STIR: Normal nerves show mild intermediate-to-slightly high signal - somewhat brighter than muscle but less bright than fluid. A faint internal fascicular pattern may be visible on high-resolution sequences.
• Post-contrast T1 FS: Normal nerve roots show minimal or no enhancement. Moderate enhancement of the nerve root sleeves at the level of the dorsal root ganglia is a normal finding.
• Caliber: Smooth and uniform. Normal root diameter at 3T is approximately 2-4 mm.

MRI Views: Coronal Orientation with C5-C8, T1

Coronal MRI (T2/STIR) with color overlay: C5-C8 nerve roots fan laterally. Superior trunk (C5+C6), middle trunk (C7), and inferior trunk (C8+T1) are identifiable in the scalene triangle.

Sagittal anatomical MRI: C5-T1 roots pass between the scalene muscles (K1 = scalene triangle), through the costoclavicular space (K2), and into the axilla (K3).

Axial MRI at the level of the scalene triangle: superior, middle, and inferior trunks cluster posterior to the subclavian artery (red dot), with the posterior scalene muscle laterally.

Key Landmarks on Normal MRI

• Coronal images: Roots exit foramina as linear structures, trunks run obliquely through the posterior triangle. The three trunks group tightly together before passing over rib I.
• Axial images: At the C5-C6 level, roots are seen between scalene muscles. At the supraclavicular level, three distinct trunks with uniform signal are grouped around the subclavian artery.
• Normal fat planes: Preserved fat planes between the nerves and surrounding structures are a key sign of normality.

3. Relevant Pathology and Imaging Findings

A. Traumatic Brachial Plexus Injury

Preganglionic (Root Avulsion) Injury

• Mechanism: Traction force pulling the head away from the shoulder tears the rootlets at the cord-root junction (intradural).
• MRI signs:
  • Traumatic pseudomeningocele: CSF-filled sac (T2 hyperintense, T1 hypointense) at the site of dural tear - pathognomonic for root avulsion
  • Absent nerve root sleeves on myelographic sequences
  • Cord edema / myelomalacia: Diffuse T2 hyperintensity of the cervical cord (do not mistake for direct cord injury)
  • Denervation edema of paraspinal muscles (multifidus, semispinalis) - T2 hyperintense, swollen acutely; fatty replaced chronically

Sagittal T2 MRI: Diffuse cervical cord edema (hyperintense signal) due to multi-level nerve root avulsions from brachial plexus injury. - Grainger & Allison's Diagnostic Radiology

Coronal T2 'myelogram' sequence: Left-sided brachial plexus avulsion at C8 and T1, with two traumatic meningoceles (white arrows) - bright CSF-filled pseudomeningoceles at the torn root sleeve sites. - Grainger & Allison's Diagnostic Radiology

Imaging Anatomy: (a) Sagittal T2W - dumbbell-shaped pseudomeningocele at right C6-C7 foramen (yellow arrow). (b) Axial T2W - atrophy of right multifidus (blue arrows), indicating preganglionic dorsal ramus injury.

(a,b) Coronal T2W and T1W: Large 4.5 × 1.6 cm traumatic pseudomeningocele (yellow arrows) from C6-C8 and T1 injury. Green arrow = cord atrophy. (c,d) Axial MR: Denervation atrophy of subscapularis, supraspinatus, infraspinatus, and serratus anterior. - Imaging Anatomy

Postganglionic (Distal) Injury

• Occurs distal to the dorsal root ganglion
• MRI signs:
  • No pseudomeningocele
  • Nerve root sleeves are intact on myelographic sequences
  • Nerve discontinuity, swelling, or T2 hyperintensity in the supraclavicular or infraclavicular region
  • Neuroma-in-continuity: fusiform nerve thickening, heterogeneous T2 signal

B. Thoracic Outlet Syndrome (TOS)

• Neurogenic TOS is the most common form, caused by compression of the inferior trunk (C8-T1) by a cervical rib, fibrous band, or anomalous scalene muscle.
• MRI findings:
  • Cervical rib or elongated C7 transverse process
  • Compression/displacement of the inferior trunk
  • In chronic cases: nerve thickening with T2 hyperintensity
  • Dynamic MRI (arms raised) can demonstrate positional vascular or neural compression
  • STIR may show reactive edema in compressed segments

Coronal STIR MRI of both brachial plexuses: Normal bilateral appearance. SA = subclavian artery, SV = subclavian vein. Applied Radiology

C. Neoplastic (Malignant) Plexopathy

• MRI findings:
  • Mass lesion adjacent to or encasing plexus elements - the key distinguishing feature from radiation plexopathy
  • T1: isointense to muscle; T2: heterogeneously hyperintense
  • Post-contrast: heterogeneous enhancement
  • Focal "mass-like" enhancement of plexus
  • Destruction of adjacent bone (vertebral body, ribs, clavicle)
  • Obliteration of normal fat planes around nerves
  • MRI sensitivity for tumor infiltration: 96%, specificity 95%, positive predictive value 95% (Bradley & Daroff's Neurology in Clinical Practice)
  • Pancoast tumor: apical lung mass with inferior trunk involvement; Horner syndrome from stellate ganglion invasion

D. Radiation-Induced Plexopathy

• MRI findings:
  • Diffuse, uniform swelling and thickening of plexus elements - no focal mass
  • T2/STIR: hyperintense signal of the BP within the radiation field
  • Low signal on T1 and T2 FS suggests fibrosis with tethering/downward traction
  • Mild, diffuse enhancement (vs. focal/asymmetric enhancement in tumor)
  • No bony destruction
  • Surrounding soft-tissue radiation change (skin thickening, subcutaneous edema/fibrosis)
• Key differentiator: Radiation plexopathy = no focal mass; metastatic plexopathy = focal enhancing mass

E. Neurogenic Tumors (Schwannoma, Neurofibroma)

• Schwannoma: Arises eccentrically from a single fascicle; the nerve can be displaced but remains largely intact.
  • T1: isointense; T2: markedly hyperintense ("target sign" - central hypointense with peripheral hyperintense rim)
  • Well-defined, smooth margins; homogeneous or slightly heterogeneous enhancement
• Neurofibroma: Diffusely involves the nerve fascicles; fusiform expansion.
  • Similar signal but nerve cannot be separated from mass
  • "Target sign" also possible
  • Multiple neurofibromas = neurofibromatosis type 1 (NF1)
• Malignant peripheral nerve sheath tumor (MPNST): Large, irregular, infiltrative; rapid growth; heterogeneous signal with necrosis; aggressive enhancement; associated with NF1 in ~50% of cases

F. Inflammatory / Immune Brachial Plexopathy (Parsonage-Turner Syndrome / Neuralgic Amyotrophy)

• Acute, often post-viral or post-vaccination immune-mediated plexitis
• MRI findings (often normal acutely):
  • T2/STIR: patchy hyperintensity of plexus elements and early denervation of innervated muscles
  • Nerve enlargement and increased signal on STIR
  • Denervation edema of shoulder girdle muscles (supraspinatus, infraspinatus, deltoid, serratus anterior)
  • No mass lesion
  • MRI neurography (3D STIR with MIP) best demonstrates the patchy distribution

G. Diabetic and Other Inflammatory Plexopathies

• CIDP / multifocal motor neuropathy: Diffuse thickening of roots, trunks, and peripheral nerves with increased T2 signal and marked post-contrast enhancement
• Sarcoidosis: Can involve brachial plexus - diffuse nerve thickening with T2 hyperintensity; may show enhancement

Summary Table: MRI Differential Diagnosis

References

• Gray's Anatomy for Students, Gray's Anatomy for Students, pp. 858-860
• Grainger & Allison's Diagnostic Radiology, Brachial Plexus Injury section, pp. 1338-1340
• Imaging Anatomy: Bones, Joints, Vessels and Nerves, Vol. 3, pp. 189-291
• Bradley & Daroff's Neurology in Clinical Practice, Brachial Plexopathy and Cancer chapters
• Neuroanatomy through Clinical Cases, 3rd ed.
• Applied Radiology: "MRI of the brachial plexus: A practical review" (appliedradiology.com)
• Radiopaedia: Brachial plexus MRI