Thoracic Anatomy in Section

In cross-section the thorax is organized around the central mediastinum, flanked by the two lungs and bounded by the bony chest wall. Axial CT in particular resolves the heart and great vessels, the airways, the lung parenchyma, and the pleural spaces, each appearing at characteristic levels from the thoracic inlet to the diaphragm.

Definition

Thoracic sectional anatomy is the slice-by-slice arrangement of the chest wall, pleura, lungs, mediastinum, heart, and great vessels as displayed on axial, coronal, and sagittal cross-sectional images.

Scope

The topic covers normal sectional anatomy of the chest: the mediastinal compartments and their contents (heart, great vessels, trachea and main bronchi, oesophagus), the lungs and their lobes and fissures, the pleura, and the chest wall, as displayed on standard imaging planes. It is reference and educational orientation to normal anatomy and not a guide to interpreting disease.

Core questions

• Which mediastinal and pulmonary structures appear at a given axial level from the thoracic inlet to the diaphragm?
• How are the lung lobes and fissures identified in section?
• How are the great vessels and cardiac chambers distinguished on cross-sectional images?

Key concepts

• Mediastinal compartments
• Heart chambers and pericardium
• Great vessels (aorta, pulmonary arteries, vena cavae)
• Trachea, carina, and main bronchi
• Lung lobes and interlobar fissures
• Pleura and pleural spaces
• Hila and pulmonary vasculature
• Chest wall and bony thorax

Mechanisms

On axial chest images structures recur at predictable levels: at the thoracic inlet the great vessels arise from the aortic arch; lower slices show the aortopulmonary window, the carina where the trachea bifurcates, and the pulmonary arteries; below this the cardiac chambers occupy the mediastinum, with the right-sided chambers anterior and the left atrium posterior. The lungs flank the mediastinum and are divided by fissures into lobes that can be inferred from the position of the fissures and the absence of large vessels along them. CT is well suited to the chest because the strong density differences between air-filled lung, soft tissue, blood, and bone produce high natural contrast, and viewing the same data at lung and soft-tissue display settings emphasizes parenchyma or mediastinum respectively.

Clinical relevance

Recognizing normal thoracic sectional anatomy is prerequisite to reading chest CT, which is among the most common cross-sectional examinations in medicine. This entry describes normal anatomy for educational orientation and is not guidance for diagnosis or treatment.

Evidence & guidelines

Sectional thoracic anatomy is documented in comprehensive anatomy texts, dedicated imaging atlases, and reference works on thoracic CT that describe the normal appearance of the mediastinum, lungs, and pleura on standard planes.

History

Cross-sectional display of the chest became routine with computed tomography, introduced by Hounsfield in 1973, which for the first time separated overlapping thoracic structures that plain radiography projects on top of one another. High-resolution CT later refined the depiction of fine lung anatomy.

Key figures

• Godfrey Hounsfield

Seminal works

• hounsfield-1973
• webb-2014
• weir-abrahams-2017

Frequently asked questions

Why is CT especially good for chest anatomy?

The chest contains air, soft tissue, blood, and bone in close proximity, and the large density differences between them give CT high natural contrast, letting overlapping structures be separated slice by slice.

How are the lung lobes identified on axial images?

The lobes are inferred from the position of the interlobar fissures, which appear as thin avascular lines, since the parenchyma on either side looks similar but the fissures and the distribution of vessels mark the boundaries.