Thoracic Outlet Syndrome (TOS) is a condition defined by the compression of the nerves or blood vessels as they pass through the narrow passageway between the collarbone and the first rib, known as the thoracic outlet. Diagnosis can be challenging because symptoms often fluctuate, and the structural abnormalities causing the compression may not be apparent during a standard physical exam or imaging study. Specialized magnetic resonance imaging (MRI) protocols are needed for confirming the diagnosis and pinpointing the exact cause of the patient’s symptoms.
The approach to specialized imaging must be guided by which structure is being compressed, as there are three distinct types of the syndrome. Neurogenic TOS is the most common form, accounting for over 90% of cases, and involves the compression of the brachial plexus, the network of nerves that controls movement and sensation in the arm and hand. Venous TOS occurs when the subclavian vein is compressed, which can lead to swelling and blood clots in the upper extremity. Arterial TOS, the least common type, involves compression of the subclavian artery, potentially causing damage to the artery wall or reducing blood flow to the arm.
Limitations of Standard Structural MRI
A conventional, static MRI of the neck and shoulder is a useful starting point, as it can rule out other potential causes of arm and neck pain, such as disc herniation or tumors. This routine imaging, performed while the patient lies flat, can clearly visualize bony abnormalities like a cervical rib (an extra rib above the first rib) or scar tissue from a prior injury. It can also identify muscle atrophy that may result from chronic nerve compression.
However, the primary limitation of a standard MRI is its static nature, capturing only a single snapshot of the anatomy in a resting position. TOS is overwhelmingly a dynamic condition, meaning the compression often occurs only when the arm is moved into specific positions, such as reaching overhead. A negative static MRI does not exclude the diagnosis, as it misses the functional narrowing of the thoracic outlet that triggers the patient’s symptoms during movement. Specialized imaging is needed to capture this dynamic compression.
Dynamic and Positional MRI Protocols
The most appropriate MRI for suspected TOS incorporates dynamic or positional protocols. This specialized technique involves acquiring MRI sequences while the patient holds specific “stress positions” that provoke their symptoms. The most common stress position is arm abduction and external rotation, where the arm is raised and rotated, mimicking movements that narrow the costoclavicular space.
By scanning the patient in both a neutral and a provocative position, the radiologist can visually compare the size of the thoracic outlet space and determine the degree of compression. For example, in Neurogenic TOS, this comparison might demonstrate a significant reduction in the costoclavicular space (the area between the clavicle and the first rib), confirming the functional diagnosis. Dynamic MRI captures the intermittent compression of the neurovascular bundle often missed by a static scan, providing objective evidence of anatomical narrowing essential for guiding treatment decisions.
Specialized Imaging for Neural and Vascular Structures
Once the functional nature of the compression is established through positional imaging, specialized sequences focus on the specific structures involved, based on the suspected type of TOS.
Imaging for Neurogenic TOS
For Neurogenic TOS, Magnetic Resonance Neurography (MRN) is the preferred method for evaluating the brachial plexus. MRN uses specific sequences, often with fat suppression, to enhance nerve visibility, allowing for the detection of subtle signs of irritation or injury.
MRN sequences look for nerve swelling (edema) or signal hyperintensity on T2-weighted images, which suggests inflammation or injury related to chronic compression. This detailed neural imaging can also identify fibrous bands or other soft tissue anomalies that may be directly impinging on the nerve roots or trunks of the brachial plexus. These findings are combined with the positional protocols to demonstrate the mechanical distortion of the nerves in the symptomatic position.
Imaging for Vascular TOS
For Vascular TOS (venous and arterial forms), specialized sequences known as Magnetic Resonance Angiography (MRA) or Magnetic Resonance Venography (MRV) are ordered. These techniques visualize the subclavian artery and subclavian vein, respectively, often using an intravenous contrast agent to highlight the blood vessels. Contrast-enhanced 3D MRA or MRV with provocative arm positioning is the gold standard for non-invasive vascular assessment.
The vascular imaging sequences are performed in both the neutral and the stress position to check for narrowing (stenosis) or blockage of the vessels during movement. In Venous TOS, MRV confirms compression of the subclavian vein and looks for signs of a blood clot or post-stenotic dilation (a widening of the vessel just past the point of compression). Similarly, MRA in Arterial TOS demonstrates compression of the subclavian artery, which may be associated with an aneurysm or reduced blood flow to the arm in the elevated position.