Introduction
The thoracic inlet (also called the superior thoracic aperture) is the opening at the top of the thoracic cavity, through which major vascular, respiratory, lymphatic, and neural structures pass between the neck and thorax.
In imaging, understanding the anatomy of the thoracic inlet is important for evaluating thoracic outlet syndrome, mediastinal masses, vascular compression, and trauma.
CT and MRI provide excellent delineation of its bony, vascular, and soft tissue components.
Boundaries
Bony Boundaries
- Anterior: Superior border of manubrium sterni
- Posterior: Body of T1 vertebra
- Lateral: First pair of ribs and their costal cartilages
Planes and Dimensions
- The thoracic inlet is kidney-shaped in cross-section.
- Plane: Slants downwards anteroinferiorly from posterior to anterior.
- Average diameter:
- Transverse: ~10–12 cm
- AP: ~5–6 cm
Contents
1. Vascular Structures
- Arteries:
- Right & left subclavian arteries
- Right & left common carotid arteries (originates differently: brachiocephalic trunk on right, direct from aortic arch on left)
- Veins:
- Right & left brachiocephalic veins
- Internal jugular veins joining with subclavian veins
2. Airway & Digestive
- Trachea (central, slightly to right of midline)
- Esophagus (posterior to trachea, slightly leftward)
3. Nerves
- Vagus nerves (CN X) – anterior to subclavian arteries
- Phrenic nerves – anterior to anterior scalene muscle, lateral to vagus
- Sympathetic trunks – posterior, near vertebral bodies
- Recurrent laryngeal nerves – branch from vagus (right loops around subclavian, left around aortic arch)
4. Lymphatics
- Thoracic duct – ascends on left, posterior to esophagus
- Right lymphatic duct – drains right upper quadrant of body
Radiological Appearance
On CT
- Bony boundaries: Clearly seen on axial bone window (sternum anteriorly, ribs laterally, T1 posteriorly)
- Vascular structures:
- Arteries – opacify well with contrast; course in relation to scalene muscles
- Veins – larger caliber, anterior to arteries
- Airway: Trachea is low-attenuation air-filled structure; esophagus may be collapsed or contain air/fluid
- Soft tissues: Fat planes help delineate fascial compartments
On MRI
- Excellent for soft tissue contrast and neurovascular mapping
- Vessels – bright on T2-weighted, flow void on spin-echo sequences
- Nerves – intermediate signal intensity, better visualized with fat-suppressed sequences
- Useful for brachial plexus imaging and evaluation of thoracic outlet compression
Applied Anatomy in Imaging
- Thoracic Outlet Syndrome: Compression of brachial plexus or subclavian vessels – dynamic MRI or CT angiography used
- Mass lesions: Thyroid goiter, lymphadenopathy, mediastinal tumors can extend through inlet
- Trauma: Penetrating injuries may involve multiple vital structures
- Infections: Spread from neck spaces into superior mediastinum through thoracic inlet
Summary Table – Key Structures at Thoracic Inlet
| Structure Type | Examples | CT/MRI Landmark |
|---|---|---|
| Bones | T1 vertebra, 1st ribs, manubrium | High-attenuation cortical margins |
| Arteries | Subclavian, common carotid | Contrast-enhanced tubular structures |
| Veins | Brachiocephalic, internal jugular | Larger caliber, anterior to arteries |
| Airway | Trachea | Central low attenuation on CT |
| Digestive | Esophagus | Posterior to trachea, variable lumen |
| Nerves | Vagus, phrenic, sympathetic trunk | Seen as small linear structures in fat |
| Lymphatics | Thoracic duct, right lymphatic duct | Tiny tubular/serpiginous on MR |
