Relationship of structures (syntopy)
The topography of the lung hila follows strict rules that must be considered in diagnostic imaging. The pulmonary arteries are closely adjacent to the bronchi and follow their branching, whereas the veins collect blood from the lung tissue and head toward the left atrium (atrium sinistrum).
Right lung root The elements are arranged from top to bottom in the following order:
- Main bronchus (bronchus principalis dexter) is located above all other structures.
- Pulmonary artery (a. pulmonalis dextra) lies slightly inferior and anterior to the main bronchus. The lower lobe branch descends from it along the lower lobe bronchus.
- Pulmonary veins occupy the lower part of the hilum: the superior vein (v. pulmonalis superior) is located anteriorly and inferiorly to the artery, and the inferior vein (v. pulmonalis inferior) is always situated at the very bottom of the root.
Left lung root Here, the relationship of structures is different:
- Pulmonary artery (a. pulmonalis sinistra) arches over the left main bronchus superiorly, occupying the uppermost pole of the hilum.
- Main bronchus (bronchus principalis sinister) is located directly below the pulmonary artery.
- Pulmonary veins, just like on the right, lie inferiorly: the superior vein is anterior and inferior to the bronchus, and the inferior vein is in the lowest position.
Evaluation of hila on conventional radiographs
On radiographs, the hilum shadow is formed by blood vessels; the bronchial walls are too thin and contribute negligibly. Normal lymph nodes are not visualized as separate structures.
- Posteroanterior (PA) projection: The so-called “hilum points” are evaluated — the intersection points of the superior pulmonary vein and the descending lower lobe artery. The transverse diameter of the lower lobe arteries can be measured on this view.
- Lateral projection: The relationship of vessels becomes more complex due to their superposition (overlapping). The central part of the right hilum is formed by the close apposition of the right pulmonary artery and the superior pulmonary vein. This combination can create an intense shadow that is sometimes mistaken for a tumor. It is important to know that the branching angles of large bronchi (e.g., between the middle lobe and right lower lobe bronchi) normally do not contain large vessels visible in the orthogonal projection. If a round shadow larger than 1 cm is identified in these angles, it is unlikely to be a normal vessel.
CT and MRI semiotics
Tomographic methods fundamentally change the visualization and detail of the structures of the root and mediastinum.
Computed tomography (CT): High-resolution contrast-enhanced CT details spatial anatomy. The posterior walls of the right main and intermediate bronchi (bronchus intermedius) are well outlined by air and appear as a thin strip. On axial slices below the level of the pulmonary veins, the basal segmental branches of the arteries normally have a diameter of less than 10 mm.
Magnetic resonance imaging (MRI): On standard spin-echo sequences, normal root structures produce a very weak signal. This occurs because the rapid blood flow inside large vessels and the air within the bronchial lumen do not generate an MR signal. The sources of signal in the hilum on MRI are limited to areas of slow-flowing blood, adipose tissue, bronchial walls, and lymph nodes.
Mediastinum: lymph nodes and thymus
The study of the lung hila is inextricably linked with the evaluation of the mediastinum on cross-sectional slices.
- Lymph nodes: About 95% of normal mediastinal lymph nodes have a diameter of less than 10 mm, and the rest do not exceed 15 mm. Small nodes (up to 6 mm) are typical for paraspinal zones, while larger ones (6–10 mm) are localized in the aortopulmonary window, pretracheal, and subcarinal spaces.
- Thymus gland: Its density and size depend on age. In young patients, it has a homogeneous soft tissue density. After puberty, fatty involution occurs, and in patients over 40 years of age, the density of the thymus on CT is often indistinguishable from the mediastinal adipose tissue.
Briefly: answers to topical questions
At what level relative to the thoracic vertebrae are the hila of the right and left lungs normally located?
The lung hila project posteriorly at the level of the V–VIII thoracic vertebrae. Anteriorly, their skeletotopy corresponds to the level of the II–IV ribs. Meanwhile, on a PA radiograph, the left hilum point is normally located at the same level as the right or slightly higher. The hila themselves (hilum pulmonis) represent an oval or rhombus-shaped depression located on the inner mediastinal surface of the lung.
What elements are part of the lung root besides the main bronchi, pulmonary arteries, veins, and lymph nodes?
In addition to the main bronchi, pulmonary arteries, pulmonary veins, and lymph nodes, the lung root (radix pulmonis) includes the following anatomical structures:
- Bronchial arteries and veins — vessels involved in the blood supply to lung structures.
- Lymphatic vessels — pathways for lymph drainage.
- Nerves — represented by branches of the pulmonary plexus and nerve plexuses.
Additionally, the root elements are surrounded by perivascular adipose tissue that communicates with the mediastinal fat, and they are subpleurally covered by extensions of the endothoracic fascia that form fascial sheaths.
For quick memorization of the lung root elements (from top to bottom), use the alphabetical rules. Right root: BAP (Bronchus, Artery, Vein). Left root: ABP (Artery, Bronchus, Vein).
Class and exam questions
Why does the center of the right root look like a mass lesion on a lateral radiograph?
This is an optical illusion caused by the superposition (overlapping) of the right pulmonary artery and the superior pulmonary vein, which are closely apposed to each other.
What are "hilum points" on a radiograph?
These are anatomical landmarks on a PA radiograph — the points where the superior pulmonary vein crosses the descending lower lobe artery.
Are normal lung root lymph nodes visible on imaging?
On conventional radiographs, normal lymph nodes are not visible. However, on modern multislice CT scans, nodes measuring a few millimeters are well differentiated.
Why does the lung root produce a weak signal on MRI?
In the standard spin-echo mode, the signal is absent due to the rapid blood flow in the major vessels and the presence of air in the bronchi.
What else to study on this topic
- Age-related involution and MRI semiotics of the thymus gland
- Detailed topography of intrathoracic lymph nodes
- Variant anatomy of the pulmonary veins (accessory veins)
- Axial CT anatomy of the lung hila at various levels
- Evaluation of bronchopulmonary angles on lateral radiographs
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