MRI of the brachial plexus

Normal anatomy :

The brachial plexus is formed by the ventral roots of C5 to T1 nerve roots. These unite to form three trunks. The trunks split into three anterior and three posterior divisions. These unite to form the three cords that further divide into five peripheral nerves. The roots and trunks are supraclavicular in location while divisions are retroclavicular and the cords are infraclavicular.

Imaging technique :

• The T1-weighted images delineate the anatomy of nerves, muscles, and vessels as they are outlined by fat.
• The T2-weighted images reveal the signal abnormalities within the brachial plexus.
• Short-tau inversion recovery (STIR) images provide uniform and reliable fat suppression over curved surfaces and large field of view. The scan protocol consists of coronal STIR and T1-weighted images with large field of view (FOV), including both brachial plexi for comparison followed by sagittal T1-and T2-weighted images with small FOV for high spatial resolution.
• Intravenous Gadolinium is administered in patients with tumors or mass lesions. Gadolinium is not administered in patients with traumatic brachial plexopathy. In patients with traumatic brachial plexus injury, in addition to the previously described protocol, sagittal T2-weighted images are obtained through the cervical spine followed by axial T2-weighted images from C4 to T2 levels. In addition, a 3D gradient echo (GRE) sequence with thin slices is obtained to look for the nerve root avulsion.
• In patients suspected with thoracic outlet syndrome, in addition to coronal STIR and coronal T1-weighted images, sagittal T1-weighted images are obtained through the symptomatic side extending from midline to the axilla with arm in hyper-abducted position. These are compared with similar sagittal T1-weighted images obtained with arm in neutral position by the side of body.

Traumatic injuries to brachial plexus :

• The common causes of brachial plexus injuries are road traffic accidents and birth palsy.
• Brachial plexus injuries can be divided into pre- and post-ganglionic lesions.
• The pre-ganglionic lesions are avulsion of the nerve roots at their origin while post-ganglionic lesions may be lesions in continuity or nerve ruptures.
• The patient may have a combination of both pre- and postganglionic lesions.
• It is important to differentiate between pre and postganglionic lesions to determine the prognosis and plan further management.
• Pseudomeningoceles are formed due to extra-vasation of CSF through tear of the peri-neural sheath. These are seen on T2-weighted images as fluid-intensity lesions at the site of nerve root avulsion. However, presence of a pseudomeningocele is not always seen in nerve root avulsion and vice versa.
• Brachial plexus injuries may be associated with injuries to the subclavian artery due to their anatomical proximity to each other. Also post-traumatic pseudoaneurysm of subclavian artery may present with delayed brachial plexus paralysis due to compression of the brachial plexus.

Non-traumatic Brachial Plexus Pathologies :

1.Radiation fibrosis :

• Patients undergoing radiation therapy in axillary region, most commonly for breast carcinoma, may present with brachial plexopathy after several months to years.
• Radiation fibrosis is seen as diffuse thickening of the brachial plexus and iso- or hypointensity on T1- and T2-weighted images.
• The absence of a focal mass differentiates it from metastatic disease.
• Moreover, metastases appear hypointense on T1-weighted images and hyperintense on T2-weighted images.

2.Brachial plexus neuritis :

• Acute brachial plexitis presents with severe shoulder and upper arm pain lasting for few days to weeks followed by upper arm weakness.
• Idiopathic brachial neuritis is of unknown cause but an immune-mediated inflammatory reaction following viral infection, vaccination, surgery, pregnancy, etc., has been proposed as etiology.
• Bilateral brachial plexus neuritis in postpartum period.
• Brachial plexitis is seen on MRI as focal or diffuse hyperintense signal in brachial plexus.

3.Brachial plexus tumors :

• Nerve sheath tumors (schwannoma and neurofibroma) are seen as ovoid lesions isointense to muscle on T1-weighted images and hyperintense on T2-weighted images with ‘target’ sign.
• These reveal intense enhancement on administration of gadolinium contrast. Most common benign tumors that involve brachial plexus are lipomas and aggressive fibromatosis.
• Metastatic breast carcinoma , superior sulcus tumors (non-small-cell lung carcinoma arising from lung apex), and lymphoma involve the brachial plexus frequently.

4.Thoracic outlet syndrome :

• It is dynamically induced compression of neural and/or arterial structures crossing the cervico-thoraco-brachial junction.
• MRI plays an important role in demonstrating neurovascular compression, localizing it and identifying the structure causing the compression.
• The three spaces that are evaluated on sagittal T1-weighted images are the inter-scalene triangle, costo-clavicular, and retro-pectoralis minor spaces with arm in neutral as well as hyperabducted position to look for compression of the neurovascular structures. The costo-clavicular space is the most common site of compression followed by inter-scalene triangle.
• The lesions causing compression may be bony abnormalities (cervical rib, long transverse process of C7 vertebra, callus or osteochondroma of clavicle or first rib) or soft tissue pathologies (fibrous band, hypertrophy of scalenus anterior muscle, scalenus minimus muscle, and fibrous scarring).
• Bilateral hypoplastic first ribs fused to second ribs can also cause thoracic outlet syndrome.
• Contrast-enhanced MR angiography may be performed with the arm in elevated position to demonstrate narrowing of subclavian artery.