Anatomi Toraks

01Overview of Thoracic Anatomic Compartments

02Mediastinal Compartments: The ITMIG Classification

Dividing the mediastinum into defined compartments helps narrow differential diagnoses for masses detected on imaging, assists in biopsy and surgical planning, and improves multidisciplinary communication. Historically, mediastinal classification models have divided the mediastinum into three or four compartments depending on whether a separate superior compartment is included, and most older radiology schemes were based on arbitrary landmarks on the lateral chest radiograph.

The ITMIG (International Thymic Malignancy Interest Group) system, now accepted as the standard for cross-sectional imaging, is based on multidetector CT rather than the lateral radiograph. ITMIG established a cross-sectional imaging-derived, anatomy-based classification system that includes three compartments: prevascular (anterior), visceral (middle), and paravertebral (posterior).

Prevascular (Anterior) Compartment:
This compartment is bounded anteriorly by the posterior surface of the sternum and posteriorly by the anterior surface of the pericardium, the ascending aorta, and the brachiocephalic vessels. Contents include the thymus (or thymic remnant), fat, lymph nodes, and the left brachiocephalic vein. On board examinations, the "Four T's" mnemonic remains useful for anterior mediastinal masses: Thymoma, Teratoma (and other germ cell tumors), Terrible Lymphoma, and Thyroid (retrosternal goiter). The prevascular compartment is the most common location for thymomas, and the examinee must be able to identify it on axial CT to correctly localize a mass.

Visceral (Middle) Compartment:
Bounded anteriorly by the posterior boundary of the prevascular compartment and posteriorly by a vertical line 1 cm behind the anterior border of each thoracic vertebral body. Contents include the heart, pericardium, ascending and descending thoracic aorta (within the pericardial reflection), SVC and IVC, trachea, main bronchi, esophagus, thoracic duct, and vagus and phrenic nerves. Board-relevant masses in this compartment include lymphadenopathy, bronchogenic cysts, esophageal duplication cysts, and pericardial cysts. The esophagus lies posterior to the trachea and left atrium throughout most of its mediastinal course, a relationship that is a frequent source of exam questions regarding compression or invasion.

Paravertebral (Posterior) Compartment:
Defined as the region posterior to the visceral compartment boundary, this zone includes the thoracic spine, paravertebral soft tissues, neural foramina, and sympathetic chain. Neurogenic tumors (schwannomas, neurofibromas, ganglioneuromas) are the most common posterior mediastinal masses. On CT and MRI, the "dumbbell" configuration of a schwannoma extending through the neural foramen should trigger the recommendation for MRI with gadolinium to evaluate intraspinal extension.

03Tracheobronchial Tree and Bronchopulmonary Segments

Trachea and Central Airways

The trachea begins at the level of the cricoid cartilage (C6) and extends to the carina (approximately T4-T5 on CT, corresponding to the level of the aortic arch). The trachea measures approximately 10-12 cm in length and has an internal transverse diameter of about 15-25 mm in men and 10-21 mm in women. The posterior membranous wall is the soft-tissue density stripe that should be visible on CT; effacement or thickening of this stripe is abnormal and may indicate posterior tracheal invasion.

The carina is a high-yield anatomic landmark. The carinal angle normally measures less than 70-80 degrees. Widening of the carinal angle (splaying) suggests subcarinal lymphadenopathy, left atrial enlargement, or a subcarinal mass. On a board exam, if you are shown a widened carina, your search pattern should include Station 7 lymph nodes and the left atrium.

The right main bronchus is wider, shorter, and more vertically oriented than the left, which is why aspirated foreign bodies preferentially lodge on the right side. This anatomic relationship is frequently tested. The right main bronchus measures roughly 2.5 cm before branching into the right upper lobe bronchus and the bronchus intermedius. The left main bronchus is narrower and longer (approximately 5 cm) and courses beneath the aortic arch before dividing.

Bronchopulmonary Segments

Each lung is divided into 10 bronchopulmonary segments that serve as both anatomical and functional units, each supplied by its own segmental bronchus and artery and separated by connective tissue septa.

Right Lung (10 Segments, 3 Lobes):

Left Lung (8-10 Segments, 2 Lobes):

Note that the left lung has no true middle lobe. The lingula (segments B4 and B5) is the anatomic counterpart of the right middle lobe and is part of the left upper lobe. This distinction is tested because the lingula is supplied by the left upper lobe bronchus, not a separate lobar bronchus.

Clinical Correlations for Segmental Anatomy:
Tuberculosis preferentially involves the apical and posterior segments of the upper lobes owing to high ventilation-perfusion ratios in those regions, while aspiration pneumonia in the supine patient affects the apical segments of both lower lobes or the posterior segment of the right upper lobe because secretions gravitate to these dependent areas. Knowing these predilections allows the radiologist to narrow the differential when confronted with a segmental opacity.

04Pulmonary Hila: Cross-Sectional Anatomy

Understanding the normal hilum on CT is one of the most challenging yet highest-yield topics for board examinations. A hilar mass is one of the most common "miss" findings on the exam.

The hilar structures are enclosed in a sleeve of pleura that is continuous inferiorly with the inferior pulmonary ligament. Each hilum contains a main bronchus, a pulmonary artery, two pulmonary veins (superior and inferior), bronchial arteries, lymph nodes, and nerves.

Right Hilum:
The right hilum lies below the arch of the azygos vein, posterior to the superior vena cava and right atrium. The right pulmonary veins are located at the anterior part of the hilum, followed by the right pulmonary arteries, and lobar bronchi. On axial CT, the key relationship to remember is: the truncus anterior (the first major branch of the right pulmonary artery) lies anterior to the right upper lobe bronchus, while the right superior pulmonary vein lies anterolateral. The bronchus intermedius, which gives off the right middle lobe and right lower lobe bronchi, is easily identified as an air-filled tube. Its posterior wall should be pencil-thin; thickening of this wall suggests lymphadenopathy or tumor.

Left Hilum:
The left hilum lies below the aortic arch and anterior to the descending aorta. The left main bronchus occupies the most posterior aspect of the hilum, with the left superior pulmonary vein situated anteriorly and the left inferior pulmonary vein at the inferior portion. The left pulmonary artery is anterosuperior to the left main bronchus.

The left pulmonary artery has a characteristic hook-shaped course: it arises from the pulmonary trunk, passes over the left main bronchus, then descends posterolateral to the left upper lobe bronchus. On a PA chest radiograph, the left hilum is normally higher than the right because of this anatomic relationship of the left pulmonary artery arching over the left main bronchus. A right hilum that appears at the same level or higher than the left should raise concern for volume loss, a hilar mass, or pulmonary arterial enlargement.

Board Pearl: The Hilum Convergence Sign helps differentiate a true hilar mass from prominent pulmonary vasculature. If pulmonary vessels can be seen converging toward a dense hilum, the opacity is vascular. If they do not converge, or if they are displaced by the opacity, the finding represents a mass.

05Pulmonary Vasculature

Pulmonary Arteries

The pulmonary trunk arises from the right ventricle and bifurcates into the right and left main pulmonary arteries at approximately the level of T5. The normal diameter of the main pulmonary artery on CT should not exceed 29 mm (measured at its widest point, perpendicular to its long axis). A ratio of the main pulmonary artery diameter to the ascending aorta greater than 1:1 suggests pulmonary arterial hypertension.

Pulmonary arteries run alongside the bronchi in a bronchovascular bundle and are central within each bronchopulmonary segment. This is the opposite of pulmonary veins, which course along the periphery of the segments and through the interlobular septa. This distinction between the central course of arteries and the peripheral course of veins within the secondary pulmonary lobule is heavily tested.

Pulmonary Veins

Four pulmonary veins (two from each lung: a superior and an inferior) drain into the left atrium. The superior pulmonary veins lie anterior to the pulmonary arteries at the hila, while the inferior pulmonary veins take a more horizontal course inferiorly and can be traced entering the lower portion of the left atrium.

On CT pulmonary angiography, confusing a pulmonary vein for an artery is a recognized pitfall that can lead to a false-positive diagnosis of pulmonary embolism. Pulmonary veins do not accompany bronchi; instead, they course separately within the interlobular septa. This pattern is a board-tested anatomic fact.

Bronchial Arteries

Bronchial arteries supply the airways and mediastinal structures and are part of the systemic circulation. They arise most commonly from the descending thoracic aorta between the T5 and T6 vertebral body levels. The most common branching pattern is two left bronchial arteries and one right bronchial artery. The right bronchial artery typically shares a common trunk with an intercostal artery (the intercostobronchial trunk, or ICBT), which arises from the right posterolateral aorta. The ICBT anatomy is relevant for bronchial artery embolization in hemoptysis and is a common board question.

06Secondary Pulmonary Lobule: The Foundation of HRCT Interpretation

The secondary pulmonary lobule (SPL) is arguably the single most important anatomic concept for understanding diffuse lung disease on HRCT. The SPL is a fundamental unit of lung structure that reproduces the lung in miniature, with airways, pulmonary arteries, veins, lymphatics, and the lung interstitium all represented at its level.

The SPL is defined as the smallest portion of lung surrounded by connective tissue septa, measuring 10 to 25 mm in diameter. The bronchiole and branch of the pulmonary artery supplying the SPL are located centrally, whereas branches of the pulmonary veins are located peripherally within the interlobular septa.

Components of the SPL (from center to periphery):

• Centrilobular core (axial structures): Terminal bronchiole, centrilobular artery (a branch of the pulmonary artery), and centrilobular lymphatics. Normal centrilobular bronchioles measure less than 1 mm in diameter with wall thickness less than 0.1 mm, which is below the limit of visibility on HRCT, so normally the only visible centrilobular structure is the pulmonary artery branch.

• Lung parenchyma (acini): The lobule contains 3 to 12 pulmonary acini, each approximately 6-10 mm in diameter.

• Interlobular septa (peripheral structures): Thin connective tissue septa containing pulmonary veins and lymphatic channels. These septa define the polygonal boundaries of the lobule.

• Intralobular interstitium: A fine network of connective tissue fibers supporting the alveolar walls within the lobule, normally invisible on HRCT.

Disease Localization Within the SPL (Board Favorite):

Understanding where within the SPL a disease process localizes is often the single piece of information that clinches the diagnosis on HRCT. For instance, perilymphatic nodules follow the lymphatic routes: they cluster along fissures, along bronchovascular bundles, and within interlobular septa. This pattern immediately narrows the differential to sarcoidosis, lymphangitic carcinomatosis, or pneumoconiosis.

07Thoracic Aorta and Aortic Arch Variants

Normal Aortic Arch Anatomy

The normal (left-sided) aortic arch gives off three branches in sequence from proximal to distal: the brachiocephalic trunk (innominate artery), the left common carotid artery (LCC), and the left subclavian artery (LSA). This normal pattern was observed in approximately 60% of subjects in one large CTA study of 444 patients.

The aortic arch is conventionally described in segments on CT:

• Ascending aorta: From the aortic valve to the origin of the brachiocephalic trunk. Normal diameter in adults: approximately 3.5 cm or less.

• Aortic arch: From the brachiocephalic trunk origin to the attachment of the ligamentum arteriosum.

• Descending thoracic aorta: From the isthmus (just distal to the left subclavian artery) to the diaphragmatic hiatus at T12.

The aortic isthmus is the narrowed segment just distal to the left subclavian artery and proximal to the ligamentum arteriosum. This is the most common site of traumatic aortic injury because it represents a transition zone between the relatively mobile arch and the fixed descending aorta.

Aortic Arch Variants

A study of 1000 consecutive CT angiograms reported that 32.4% of subjects had branching variants, and cardiothoracic radiologists correctly identified anomalies more frequently than other radiologists (94% vs. 20%). This underscores the need to systematically evaluate arch branching on every thoracic CT.

The bovine arch is the single most common arch variant and is frequently seen as an incidental finding on thoracic CT. In one study, this pattern was found in 27.9% of subjects. Although often benign, it must be reported because it affects catheter selection during endovascular aortic interventions.

08Thoracic Lymph Node Stations: The IASLC Map

The IASLC lymph node map supersedes all previous schemas and reconciles discrepancies among older systems such as the Naruke classification and the Mountain-Dresler modified American Thoracic Society map. The intrathoracic lymph nodes are mapped into 14 stations according to their relationship to landmarks encountered during mediastinoscopy and thoracotomy for lung cancer, with stations 1 through 9 corresponding to mediastinal groups and stations 10 through 14 representing hilar and more peripheral nodal groups.

IASLC Lymph Node Stations (Summary Table):

Board Pearl: N-Stage Determination

The lymph node station number directly maps onto the TNM staging for lung cancer:

• N1: Stations 10-14 (ipsilateral hilar and intrapulmonary)

• N2: Stations 2-9 ipsilateral (ipsilateral mediastinal and subcarinal)

• N3: Contralateral mediastinal/hilar (any station on the contralateral side), or ipsilateral/contralateral scalene or supraclavicular (Station 1)

Stations 4 (paratracheal) and 7 (subcarinal) are the nodes most commonly involved in N2 disease for lung cancer staging. A lymph node with a short axis diameter exceeding 10 mm on CT is the generally accepted threshold for abnormality, though this criterion has limited sensitivity and is not the sole determinant.

Station 7 (Subcarinal) deserves special attention. It is the one station accessible by both EBUS (endobronchial ultrasound) and EUS (endoscopic ultrasound), making it a frequent biopsy target. Board questions often test whether a candidate can correctly identify a subcarinal node on CT and understand its staging implications.

09Pleural Anatomy and Fissures

Pleural Layers

The visceral pleura is adherent to the lung surface and extends into the fissures. The parietal pleura lines the inner surface of the chest wall, the diaphragm, and the mediastinum. Between them lies the pleural space, which normally contains only a few milliliters of fluid. The pleural layers come together at the hilum and extend inferiorly as the inferior pulmonary ligament, a structure that anchors the lower lobe to the mediastinum and creates a potential space for fluid to collect (the "sail sign" of a pulmonary ligament effusion).

Fissures

The normal lung has two types of fissures:

• Major (Oblique) Fissures: Present bilaterally, separating the lower lobe from the upper lobe (and, on the right, from the middle lobe). On a lateral chest radiograph, the major fissure extends from approximately T4-T5 posteriorly to the anterior costophrenic angle. On axial CT, the major fissure is obliquely oriented and best identified on sagittal reformats.

• Minor (Horizontal) Fissure: Present only on the right, separating the right upper lobe from the right middle lobe. It extends from the right hilum anterolaterally to the chest wall at approximately the level of the fourth anterior rib.

Incomplete Fissures are common anatomic variants (present in approximately 20-45% of subjects depending on the series). They are not pathologic but have clinical relevance: incomplete fissures allow collateral ventilation between adjacent lobes, which can cause the spread of pneumonia or air across lobes and can also predict reduced efficacy of endobronchial valve therapy for emphysema.

Azygos Fissure: This is a normal variant, not a true fissure. It is created by a laterally displaced azygos vein that indents the right upper lobe, carrying with it four layers of pleura (two visceral, two parietal). It appears as a curvilinear opacity extending from the right brachiocephalic vein to the azygos arch, demarcating the azygos lobe in approximately 0.4-1% of the population. It has no pathologic significance but is a commonly tested variant on board examinations.

Pleural Recesses

The posterior costophrenic recess is the deepest part of the pleural cavity and the most dependent in the upright patient. It is where small effusions first accumulate. On a lateral radiograph, a meniscus sign in the posterior costophrenic angle may represent as little as 200 mL of fluid, while blunting of the lateral costophrenic angle on the PA view requires roughly 200-300 mL. A lateral decubitus view can detect even smaller volumes (50-100 mL).

10Diaphragm Anatomy

The diaphragm is a musculotendinous structure that separates the thorax from the abdomen. It has three main openings:

The mnemonic "I Ate Ten Eggs At Twelve" (IVC at T8, Esophagus at T10, Aorta at T12) is a classic board review memory aid.

The diaphragm has two muscular crura that arise from the anterior surfaces of the lumbar vertebral bodies (right crus from L1-L3, left crus from L1-L2). The right crus is larger and longer than the left. On CT, the crura appear as soft tissue density structures anterior and lateral to the spine at the level of the diaphragmatic hiatus. They should not be confused with retrocrural lymphadenopathy, which is a board pitfall.

Diaphragmatic Hernias:

• Bochdalek hernia (posterolateral defect): The most common congenital diaphragmatic hernia, more frequent on the left side (~80-90%). In adults, it presents as an incidental finding of fat or abdominal viscera herniating through the posterolateral diaphragm.

• Morgagni hernia (anteromedial/retrosternal defect): Less common, more frequently right-sided, and typically contains omental fat or transverse colon. On a lateral chest radiograph, it appears as a soft tissue mass in the anterior cardiophrenic angle.

11Thoracic Inlet and Chest Wall Landmarks

The thoracic inlet (also called the thoracic outlet in surgical nomenclature, a source of deliberate confusion on examinations) is bounded by the first rib, the manubrium, and the first thoracic vertebra. Structures traversing the thoracic inlet include the trachea, esophagus, subclavian vessels, common carotid arteries, phrenic and vagus nerves, sympathetic trunks, thoracic duct (on the left), and the apices of the lungs.

The internal mammary arteries (IMA) course along the inner surface of the anterior chest wall, approximately 1 cm lateral to the sternal border. They are routinely visible on contrast-enhanced CT and must be identified to avoid confusion with a parasternal lymph node. The IMA is also relevant because it is used as a bypass conduit in coronary artery surgery, and post-surgical patients may have clips along this vessel.

The azygos vein arches over the right main bronchus to drain into the SVC. Its normal diameter should not exceed 10 mm in the upright position. Enlargement suggests elevated right-sided pressures, SVC obstruction, IVC obstruction with azygos continuation, or portal hypertension.

12Board Traps and Common Anatomic Pitfalls

Trap 1: The Left Brachiocephalic Vein Mimicking Lymphadenopathy
The left brachiocephalic (innominate) vein crosses the mediastinum from left to right, anterior to the great vessels. On a single axial CT slice without contrast, it can be mistaken for a prevascular lymph node. Solution: Trace the structure on consecutive slices; if it is tubular and connects to the SVC, it is a vein. Contrast-enhanced CT eliminates this ambiguity.

Trap 2: The Normal Thymus in Young Adults
In patients under 30, the thymus is often visible as a bilobed soft tissue structure in the prevascular mediastinum. It should have smooth, convex margins that become progressively more concave and fatty with age. A convex thymic contour in a patient over 40 is abnormal and should raise concern for thymoma or thymic hyperplasia.

Trap 3: Pericardial Recess Mimicking Lymphadenopathy
The superior aortic recess (a pericardial recess that extends behind the ascending aorta) and the transverse sinus (between the ascending aorta and left atrium) are fluid-density structures that can mimic lymph nodes or aortic dissection on unenhanced CT. Their characteristic water-density attenuation (0-20 HU) and location within the pericardial reflections distinguish them from solid masses.

Trap 4: Partial Volume Averaging at the Lung Bases
At the lung bases, especially near the costophrenic angles, partial volume averaging between the diaphragm and adjacent lung parenchyma can create pseudonodules. These pseudolesions are common on thick-section CT and resolve on thin-section images or multiplanar reformats.

Trap 5: The Azygos-Esophageal Recess
The azygos-esophageal recess is the interface between the lung and the right posterolateral mediastinal structures (azygos vein and esophagus). On a PA chest radiograph, this recess forms a concave interface with the right lung. Convexity of this line suggests subcarinal lymphadenopathy, esophageal mass, or hiatal hernia.

Trap 6: Confusing Pulmonary Arteries and Veins on CT Pulmonary Angiography
Pulmonary veins course within the interlobular septa and drain into the left atrium. Pulmonary arteries run with the bronchi. On CTPA, a vein should not be mistaken for an artery when evaluating for pulmonary embolism. The key distinguishing feature is that veins are traced to the left atrium while arteries are traced to the right ventricle/pulmonary trunk.

Trap 7: Cervical Ribs and the "Companion Shadow"
A cervical rib (arising from C7) is present in approximately 0.5-1% of the population and can cause thoracic outlet syndrome. On a chest radiograph, the normal companion shadow (a soft tissue stripe paralleling the inferior margin of the clavicle and the superior surface of the first rib) should not be confused with a pleural abnormality.

Trap 8: Incomplete Fissure vs. Contiguous Pneumonia
Because incomplete fissures allow parenchymal continuity between lobes, pneumonia may appear to cross a fissural boundary. This can mimic a mass or invasive process. Recognition of the fissure anatomy and knowledge that 20-45% of fissures are incomplete prevents overcalling this finding.

Summary: Core Competencies Tested

The following anatomic competencies are tested repeatedly across CORE, FRCR, and equivalent radiology board examinations:

1. ITMIG mediastinal compartments and the differential diagnosis framework they generate

2. Bronchopulmonary segmental anatomy and clinical correlations (aspiration, TB, carcinoma predilection)

3. Secondary pulmonary lobule components and disease distribution patterns (centrilobular, perilymphatic, random)

4. IASLC lymph node stations and their mapping to N-staging in lung cancer

5. Aortic arch branching and the recognition of variants (bovine arch, aberrant right subclavian artery)

6. Hilar anatomy and the ability to distinguish vascular structures from lymphadenopathy or masses

7. Pleural fissures and recesses including variants (azygos fissure, incomplete fissures)

8. Diaphragmatic openings and hernias (Bochdalek vs. Morgagni)

Mastery of these anatomic foundations is the prerequisite for accurate pattern recognition in thoracic imaging. Every pathologic finding is ultimately described by how it deviates from normal anatomy; the anatomy is the search pattern.

Key References:

• Carter BW et al. ITMIG Classification of Mediastinal Compartments and Multidisciplinary Approach to Mediastinal Masses. RadioGraphics. 2017;37(2):413-436.

• El-Sherief AH et al. IASLC Lymph Node Map: Radiologic Review with CT Illustration. RadioGraphics. 2014;34(6):1680-1691.

• Webb WR. Thin-Section CT of the Secondary Pulmonary Lobule: Anatomy and the Image (The 2004 Fleischner Lecture). Radiology. 2006;239(2):322-338.

• Bankier AA et al. Fleischner Society: Glossary of Terms for Thoracic Imaging. Radiology. 2024.

• Goodman LR. Felson's Principles of Chest Roentgenology. 6th ed. Elsevier; 2026.

• Naidich DP et al. Computed Tomography and Magnetic Resonance of the Thorax. 4th ed. Lippincott Williams & Wilkins.