Overview
The thorax is an irregularly shaped cylinder bounded superiorly by the superior thoracic aperture (open, continuous with the neck) and inferiorly by the inferior thoracic aperture (closed by the diaphragm). Its principal components are the thoracic wall, two pleural cavities with contained lungs, and the centrally located mediastinum. Understanding the precise course of the phrenic nerves, their anatomical relationships, and the broader structural anatomy of the thorax is fundamental to interpreting chest imaging.
Thoracic Wall
Skeletal Framework
| Region | Components |
|---|---|
| Posterior | 12 thoracic vertebrae and intervertebral discs |
| Lateral | 12 pairs of ribs; three layers of flat intercostal muscles spanning intercostal spaces |
| Anterior | Sternum (manubrium, body, xiphoid process) joined to ribs via costal cartilages |
The sternal angle (manubriosternal junction / angle of Louis) corresponds to the T4/T5 intervertebral disc level and marks: - Superior/inferior mediastinal boundary - Origin and termination of the aortic arch - Tracheal bifurcation (carina) - Level at which the SVC penetrates the pericardium - Attachment of the second costal cartilage
Thoracic Wall Muscles
| Muscle Group | Location | Function | Innervation |
|---|---|---|---|
| External intercostals | Between ribs; fibres run inferomedially | Elevate ribs (inspiration) | Intercostal nerves (T1-T11) |
| Internal intercostals | Between ribs; fibres run inferolaterally | Depress ribs (expiration) | Intercostal nerves |
| Innermost intercostals | Deep to internal intercostals | Assist expiration | Intercostal nerves |
| Transversus thoracis | Internal anterior thoracic wall | Depress costal cartilages | Intercostal nerves |
| Serratus anterior | Lateral chest wall | Protracts/rotates scapula | Long thoracic nerve (C5-C7) |
| Pectoralis major/minor | Anterior chest wall | Arm movement; accessory inspiration | Medial/lateral pectoral nerves |
Posterior rami of thoracic spinal nerves supply erector spinae, transversospinalis, and levator costae muscles; all other thoracic wall muscles are supplied by anterior rami (intercostal nerves).
Neurovascular Supply of the Thoracic Wall
Intercostal neurovascular bundles run in the neurovascular plane between the internal and innermost intercostal muscle layers, in the costal groove along the inferior margin of each rib. Order from superior to inferior: vein, artery, nerve (VAN).
- Arterial supply: Posterior intercostal arteries from the thoracic aorta (spaces 3-11) and the supreme intercostal artery (spaces 1-2); anterior intercostal arteries from the internal thoracic and musculophrenic arteries
- Venous drainage: Posterior intercostal veins → azygos/hemiazygos systems
- Nerve supply: Anterior rami of T1-T11 (intercostal nerves); T12 = subcostal nerve. The intercostobrachial nerve (lateral cutaneous branch of T2) contributes to innervation of the medial upper arm. Lower intercostal nerves (T7-T11) extend beyond the costal margin to supply abdominal wall muscles, skin, and parietal peritoneum.
The internal thoracic arteries descend vertically ~1 cm lateral to the sternal edge, between transversus thoracis and costal cartilages, dividing at the level of the 6th cartilage into the superior epigastric and musculophrenic arteries. The superior epigastric vessels pass between the xiphisternal and costal (7th costal) fibres of the diaphragm.
Lymphatics of the Thoracic Wall
Superficial lymphatics drain predominantly to axillary nodes. Deeper drainage goes to intrathoracic nodes.
| Node Group | Location | Drainage Territory | Efferents |
|---|---|---|---|
| Parasternal (internal thoracic) | Anterior ends of upper 5 intercostal spaces, along internal thoracic artery | Anterior thoracic wall, medial breast, diaphragm | Bronchomediastinal trunks |
| Intercostal | Posterior ends of intercostal spaces | Posterior thoracic wall | Upper → thoracic duct or right lymphatic duct; lower → cisterna chyli |
| Diaphragmatic | Thoracic surface of diaphragm (anterior, lateral, posterior groups) | Diaphragm; right lateral group also receives hepatic lymph | Parasternal and posterior mediastinal nodes |
Mediastinum
Subdivisions
A transverse plane at the sternal angle / T4-T5 disc level divides the mediastinum into superior and inferior components. The inferior mediastinum is further partitioned by the pericardial sac:
| Compartment | Boundaries | Key Contents |
|---|---|---|
| Superior mediastinum | Superior thoracic aperture → sternal angle/T4-T5 plane | Trachea, oesophagus, aortic arch, brachiocephalic veins, SVC (upper part), thoracic duct, phrenic and vagus nerves, left recurrent laryngeal nerve, thymus (upper extent), brachiocephalic and paratracheal nodes |
| Anterior (inferior) mediastinum | Behind sternum, anterior to pericardial sac | Thymus (inferior extension), fat, lymph nodes, sternopericardial ligaments, mediastinal branches of internal thoracic vessels |
| Middle (inferior) mediastinum | Contains pericardial sac | Heart, pericardium, roots of great vessels, phrenic nerves, pericardiacophrenic vessels |
| Posterior (inferior) mediastinum | Behind pericardial sac, anterior to vertebral column | Oesophagus, descending thoracic aorta, azygos/hemiazygos veins, thoracic duct, vagus nerves, sympathetic trunks, posterior mediastinal lymph nodes |
CT-Based Three-Compartment Classification
On cross-sectional imaging, three compartments are commonly used for lesion localisation:
| CT Compartment | Contents |
|---|---|
| Anterior (prevascular) | Thymus, brachiocephalic veins, internal thoracic lymph nodes; all structures anterior to heart and great vessels |
| Middle (visceral) | Pericardium and contents, aortic arch and proximal great arteries, descending thoracic aorta, trachea, carina, main bronchi, oesophagus, tracheobronchial nodes |
| Posterior (paravertebral) | Paravertebral spaces, sympathetic chains, descending aorta (posterior portion) |
Phrenic Nerves - Detailed Course
The phrenic nerves are the sole motor supply to the diaphragm (including the crura) and provide somatic sensory innervation to the mediastinal pleura, fibrous pericardium, parietal layer of serous pericardium, and the diaphragmatic parietal peritoneum.
Origin
- Arise from anterior rami of C3, C4, and C5; C4 provides the dominant contribution
- Formed within the cervical plexus; descend on the anterior surface of scalenus anterior in the neck
Course Through the Thorax
| Segment | Right Phrenic Nerve | Left Phrenic Nerve |
|---|---|---|
| Entering superior thoracic aperture | Lateral to right vagus nerve; lateral and slightly posterior to right brachiocephalic vein | Lateral to left vagus nerve; lateral and slightly posterior to beginning of left brachiocephalic vein |
| Superior mediastinum | Descends along right lateral border of SVC | Descends lateral to left vagus nerve; crosses superficially over the arch of the aorta, passing anterior to the left superior intercostal vein |
| Middle mediastinum | Descends along right side of pericardial sac, within the fibrous pericardium, anterior to root of right lung; accompanied by right pericardiacophrenic vessels | Descends along left side of pericardial sac, within the fibrous pericardium, anterior to root of left lung; accompanied by left pericardiacophrenic vessels |
| Exit from thorax | Passes through the caval opening in the central tendon at approximately T8 (with IVC) | Pierces the muscle of the left dome near the cardiac apex |
Key Relationships
- Both phrenic nerves pass anterior to the roots of the lungs; vagus nerves pass posterior to the lung roots
- Phrenic nerves travel within the fibrous pericardium throughout most of their intrathoracic course - hence pericardial disease can refer pain to the shoulder tip (C4 dermatome)
- The right crus fibres that loop around the oesophageal opening receive innervation from the left phrenic nerve
- Spinal cord injuries below C5 preserve diaphragmatic function; injuries at or above C3-C4 require ventilatory support
Vascular Accompaniment
Pericardiacophrenic arteries (branches of the internal thoracic arteries) and their companion veins accompany the phrenic nerves throughout the thoracic course, supplying the pericardium. Additional contributions to the pericardium come from the musculophrenic and inferior phrenic arteries and the thoracic aorta.
Imaging of the Phrenic Nerves
The phrenic nerves are not directly visible on routine CT or MRI. Their course is inferred from: - Position relative to the pericardium, IVC, and diaphragm - Ipsilateral diaphragmatic elevation on plain radiograph or CT indicating paralysis (fluoroscopic "sniff test" - paradoxical ascent during inspiration) - MRI neurography for perineural tumour spread in select cases
Diaphragm
Attachments
| Region | Attachment |
|---|---|
| Anterior | Xiphoid process of sternum |
| Anterolateral | Inner surfaces of lower 6 costal cartilages and ribs (interdigitating with transversus abdominis) |
| Posterolateral | Ends of ribs XI and XII |
| Posterior (medial arcuate ligament) | Spans over psoas major; from upper lumbar vertebral body to transverse process |
| Posterior (lateral arcuate ligament) | Spans over quadratus lumborum; from transverse process to tip of rib XII |
| Posteromedial crura | Right crus: L1-L3 vertebral bodies; left crus: L1-L2 vertebral bodies |
Muscle fibres converge onto the central tendon, to which the pericardium is directly attached. The median sagittal plane of the diaphragm slopes inferiorly from its anterior xiphoid attachment (~T8/9) to the median arcuate ligament crossing anterior to the aorta at approximately T12.
Structures passing posterior to the diaphragm (behind arcuate ligaments): sympathetic trunks, splanchnic nerves, subcostal vessels and nerve (T12).
Hiatuses
| Hiatus | Vertebral Level | Contents |
|---|---|---|
| Caval opening | T8 | IVC, right phrenic nerve branches |
| Oesophageal hiatus | T10 | Oesophagus, vagus nerves (anterior and posterior trunks), oesophageal branches of left gastric vessels |
| Aortic hiatus | T12 | Descending aorta, thoracic duct, azygos vein |
Blood Supply
| Vessel | Source | Territory |
|---|---|---|
| Inferior phrenic arteries | Abdominal aorta | Main muscular bulk (crura and central mass); primary supply |
| Pericardiacophrenic arteries | Internal thoracic arteries | Superior diaphragmatic surface |
| Musculophrenic arteries | Internal thoracic arteries | Anterolateral diaphragm |
| Superior phrenic arteries | Thoracic aorta | Small contribution, superior surface |
| Lower 5 intercostal + subcostal arteries | Thoracic aorta | Costal margin of diaphragm |
Nerve Supply
- Motor: Solely from phrenic nerves (C3, C4, C5; predominantly C4). Each hemidiaphragm supplied by its ipsilateral phrenic nerve. Exception: right crus fibres looping around the oesophageal opening are supplied by the left phrenic nerve
- Sensory/proprioceptive: Central diaphragm (and central tendon) - phrenic nerves; peripheral diaphragm - lower intercostal nerves (T7-T11)
Diaphragmatic Lymphatics
Extraperitoneal lymph vessels on the abdominal surface pass through the diaphragm to lymph nodes on the thoracic surface (diaphragmatic nodes - anterior, lateral, and posterior groups), which drain to parasternal and posterior mediastinal nodes.
Pericardium and Pericardial Spaces
| Layer | Description |
|---|---|
| Fibrous pericardium | Tough outer layer; continuous with adventitia of great vessels superiorly; attached to central tendon of diaphragm inferiorly; connected to sternum via sternopericardial ligaments |
| Parietal serous pericardium | Lines the inner surface of fibrous pericardium |
| Visceral serous pericardium (epicardium) | Reflected onto heart surface at roots of great vessels |
| Pericardial cavity | Potential space between parietal and visceral layers; normally ≤50 mL serous fluid |
Two important recesses: - Transverse sinus: between the arterial reflections (aorta, pulmonary trunk) anteriorly and venous reflections (SVC, pulmonary veins) posteriorly; accessible anterior to SVC and posterior to ascending aorta - Oblique sinus: posterior to heart, bounded by reflections around pulmonary veins and IVC; a hand inserted under the cardiac apex and moved superiorly enters this sinus
Blood supply of pericardium: internal thoracic, pericardiacophrenic, musculophrenic, and inferior phrenic arteries, and the thoracic aorta. Veins drain to the azygos system and internal thoracic and superior phrenic veins.
On CT: normal pericardium appears as a thin curvilinear soft-tissue density (~1-2 mm) outlined by pericardial and epicardial fat. Pericardial effusion = homogeneous low attenuation surrounding the heart (~0-20 HU for transudates; higher for haemopericardium or exudates).
Tracheobronchial Tree and Lungs
Trachea and Bronchi
| Structure | Details |
|---|---|
| Trachea | Begins at C6 (inferior border of cricoid); bifurcates at carina at T4/T5; ~10-12 cm long, ~1.5-2 cm diameter |
| Right main bronchus | Shorter, wider, more vertical (~25° from vertical); inhaled foreign bodies more likely to enter right side |
| Left main bronchus | Longer, narrower, more horizontal (~45° from vertical); passes under aortic arch |
| Right upper lobe bronchus | Arises ~2 cm from carina; trifurcates into apical, posterior, anterior segmental bronchi |
| Bronchus intermedius | Continues distal to right upper lobe bronchus; gives rise to right middle and lower lobe bronchi |
| Left upper lobe bronchus | Upper division (apicoposterior, anterior segments) and lingular division (superior, inferior lingular segments) |
Lung Lobes and Fissures
| Feature | Right Lung | Left Lung |
|---|---|---|
| Lobes | Upper, Middle, Lower | Upper (with lingula), Lower |
| Fissures | Oblique + horizontal | Oblique only |
| Horizontal fissure | Runs at level of 4th rib anteriorly; T4 posteriorly | Absent |
| Oblique fissure | T4 posteriorly → 6th rib anteriorly | Same |
Pulmonary Vasculature
- Pulmonary arteries: deoxygenated blood; branch with bronchi. Right PA passes under the aortic arch; left PA arches over the left main bronchus
- Pulmonary veins: 4 veins (right superior, right inferior, left superior, left inferior) drain to the left atrium; travel independently of bronchi and arteries, running inferiorly in the lung parenchyma
- Bronchial arteries: systemic nutritive supply; typically 1 right (from right 3rd posterior intercostal artery) and 2 left (from descending thoracic aorta)
Autonomic Innervation of Lungs
- Pulmonary plexus at the lung root formed by sympathetic fibres and vagal branches
- Vagus (parasympathetic): bronchoconstriction, increased mucus secretion
- Sympathetic: bronchodilation
The visceral pleura has an autonomic nerve supply and is insensitive to pain.
Pulmonary Lymphatics
Two networks - superficial subpleural and deep peribronchial/perivascular - drain to tracheobronchial nodes (pulmonary → bronchopulmonary/hilar → subcarinal/inferior tracheobronchial → superior tracheobronchial → paratracheal). Efferents of paratracheal and parasternal nodes unite to form right and left bronchomediastinal trunks, which drain into the right lymphatic duct and thoracic duct respectively, or directly into the internal jugular-subclavian venous junction.
Pleura and Pleural Spaces
| Layer | Arterial Supply | Venous Drainage | Nerve Supply |
|---|---|---|---|
| Parietal costal | Intercostal, internal thoracic, musculophrenic arteries | Azygos system | Intercostal nerves (pain-sensitive) |
| Parietal diaphragmatic | As above | As above | Phrenic nerve (central dome); intercostal nerves (periphery) |
| Parietal mediastinal | As above | As above | Phrenic nerve |
| Visceral | Bronchial vessels | Bronchial veins | Autonomic only; insensitive to pain |
Pleural recesses: - Costodiaphragmatic recess: deepest recess; ~2 rib spaces below the inferior lung margin; site of fluid accumulation - Costomediastinal recess: anterior, where lung does not fully occupy the space between costal and mediastinal pleura
The parietal pleura projects up to 2.5 cm above the junction of the medial and middle thirds of the clavicle into the neck (cervical pleura), but does not extend above the neck of rib I.
On erect CXR: ~200-300 mL pleural fluid needed to blunt the costophrenic angle. Lateral decubitus view detects as little as ~50 mL.
Lymphatics and Lymph Nodes of the Thorax
| Node Group | Location | Drainage Territory | Efferents |
|---|---|---|---|
| Parasternal | Anterior ends of upper 5 intercostal spaces, along internal thoracic artery | Anterior chest wall, medial breast, diaphragm (via diaphragmatic nodes) | Bronchomediastinal trunks |
| Intercostal | Posterior intercostal spaces | Posterior thoracic wall | Upper → thoracic duct or right lymphatic duct; lower → cisterna chyli |
| Diaphragmatic | Thoracic surface of diaphragm | Diaphragm; right lateral receives hepatic lymph | Parasternal and posterior mediastinal nodes |
| Posterior mediastinal | Behind pericardium on oesophagus and descending aorta | Oesophagus, pericardium, diaphragm | Thoracic duct |
| Tracheobronchial (hilar and subcarinal) | Around main bronchi and carina (superior = tracheo-bronchial angles; inferior = subcarinal) | Lungs, heart | Paratracheal nodes |
| Paratracheal | Front and sides of trachea | Tracheobronchial nodes | Bronchomediastinal trunks |
| Brachiocephalic | Superior mediastinum, anterior to brachiocephalic veins | Thyroid, thymus, pericardium, heart | Bronchomediastinal trunks |
Thoracic Duct
Commences at the cisterna chyli at T12 (between aorta and azygos vein), ascends through the aortic hiatus, passes behind the oesophagus to lie against its right side, then inclines left at T5, ascending in the superior mediastinum to the left of the oesophagus. It lies posterior to the aortic arch and left subclavian artery, finally draining into the junction of the left internal jugular and left subclavian veins. Injury causes chylothorax (triglyceride-rich milky fluid; CT attenuation often near or slightly below water density).
Imaging Appearances - Key Points
| Structure | Plain Radiograph | CT | MRI |
|---|---|---|---|
| Trachea | Midline air column; slight right deviation at aortic arch | Air-filled; cartilaginous rings | Well-defined air column |
| Carina | Bifurcation angle normally <70° | Subcarinal angle <70°; subcarinal nodes visible | Clear on coronal/axial sequences |
| Phrenic nerves | Not visible directly | Not directly resolved; course inferred | MRI neurography for perineural spread |
| Diaphragm | Right dome at ~anterior 6th rib; left ~1.5 cm lower | Smooth dome; crura visible posteriorly | Well-defined muscle signal |
| Pericardium | Cardiac silhouette | 1-2 mm soft-tissue density line outlined by fat | Fibrous layer: low signal T1/T2; fluid: bright T2 |
| Mediastinal nodes | Hilar enlargement on PA view | Short axis >1 cm = abnormal threshold | PET-CT for metabolic activity |
Clinical and Radiological Pitfalls
- Phrenic nerve palsy: Unilateral diaphragmatic elevation on CXR; confirmed by fluoroscopic sniff test (paradoxical ascent on inspiration). Causes: malignancy, cardiac surgery, aortic aneurysm, neck dissection, birth trauma
- Left phrenic vs. left vagus: Phrenic passes anterior to the lung root; vagus passes posterior - both cross the aortic arch but on different planes
- Aortopulmonary window: Key nodal station; left recurrent laryngeal nerve loops here around the ligamentum arteriosum; enlarged nodes → left vocal cord palsy and hoarseness
- Right recurrent laryngeal nerve: Loops around the right subclavian artery in the root of the neck; right vocal cord palsy with apical mass suggests Pancoast tumour
- Referred pain: Phrenic nerve irritation (diaphragmatic pericarditis, subphrenic abscess, diaph