Radiation anatomy of the mediastinum: CT and MRI slices

Q: Which mediastinal lymph node groups are distinguished during a CT examination and where are they located?

- Supraclavicular zone — includes right and left supraclavicular nodes (1R, 1L); extends from the lower margin of the cricoid cartilage to the clavicles and, in the midline, to the upper margin of the manubrium sterni. - Upper zone — includes upper paratracheal nodes (2R, 2L), prevascular (3a), retrotracheal (3p), and lower paratracheal nodes (4R, 4L). The upper paratracheal nodes are located above the upper margin of the aortic arch or the crossing point of the brachiocephalic vein with the trachea; the lower paratracheal nodes are below the upper margin of the aortic arch. - Aortopulmonary zone — includes nodes of the aortopulmonary window: subaortic or paraaortic nodes (stations 5, 6). - Subcarinal zone — includes bifurcation or subcarinal nodes (station 7) located beneath the main bronchi within the mediastinal pleura, but above the upper margin of the left lower lobe bronchus or the right intermediate bronchus. - Lower zone — includes paraesophageal nodes (station 8) and pulmonary ligament nodes (station 9); extends downward to the diaphragm. - Root zone — includes root of the lung nodes, or hilar nodes (station 10). - Peripheral zone — includes peripheral lung nodes (stations 12–14).

Q: The fusion of which veins forms the superior vena cava and at what anatomical level does this occur?

- The superior vena cava is formed by the confluence of the right and left brachiocephalic veins. - The projection of the vessel's origin is determined at the right sternal border at the level of the first costal cartilage attachment.

Q: Which nerve trunks pass through the mediastinum and how are they positioned relative to the major vessels?

- Vagus nerves (nn. vagi) — pass through the superior and posterior mediastinum; the right vagus nerve crosses anterior to the right subclavian artery and descends along the right surface of the trachea and esophagus, while the left crosses the aortic arch, giving off the recurrent nerve, and descends along the left surface of the esophagus. - Phrenic nerves (nn. phrenici) — pass between the pericardium and mediastinal pleura anterior to the roots of the lungs, accompanying the pericardiacophrenic vessels. - Sympathetic trunks (trunci sympathici) — are located along the lateral surfaces of the spine in the superior and posterior mediastinum, with the thoracic ganglia situated on the heads of the ribs.

Imaging modalities

The primary tools for studying the cross-sectional structure of thoracic organs are MRI and КТ.

During MRI, scanning is performed in multiple projections. The baseline protocol includes axial (transverse) and coronal slices. T1-weighted sequences and gradient echo are used. For better tissue differentiation, especially of vascular structures, the examination is supplemented with intravenous administration of a gadolinium-based contrast agent.

CT anatomy is studied using a series of axial tomograms with a standard slice thickness of 1 cm. Depending on the clinical task and the contrast phase, the upper levels are frequently evaluated in thoracic aortography mode, while the lower levels (at the level of the heart chambers) are assessed using CT coronarography.

Axial levels: superior mediastinum

The analysis of transverse tomograms is performed strictly from top to bottom. Several key levels are distinguished in the upper floor of the thoracic cavity:

Slice above the aortic arch. The branches of the arch are visualized here. Directly behind the sternum, the venous bed is formed, represented by the right and left brachiocephalic veins. Posterior to them lie three major arteries: the brachiocephalic trunk (truncus brachiocephalicus), the left common carotid artery, and the left subclavian artery. Behind the vascular bundle lies the trachea.
Slice at the level of the aortic arch. The main landmark is the transverse part of the aortic arch (arcus aortae), which arches around the trachea from anterior to posterior and to the left.
Level of the aortopulmonary window. On this tomogram, the aorta is represented by two separate contours: the ascending part anteriorly and the descending part to the left of the spine. The left pulmonary artery also begins to be defined at this slice. Topographically, the esophagus is located posterior to the trachea.

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Axial levels: middle and inferior mediastinum

As we move further down, the structures of the pulmonary hroots and heart cavities are visualized.

Level of the pulmonary trunk bifurcation. The main pulmonary artery (truncus pulmonalis) divides into the right and left branches. The ascending aorta is located anteriorly and to the right of the pulmonary trunk, while the descending aorta maintains its position to the left of the spine.
Level of the pulmonary roots. The division of the airways into the right and left main bronchi, as well as the intermediate bronchus, is clearly traced. To the right of the ascending aorta, the superior vena cava is clearly visualized.
Levels of the atria and outflow tracts. In this zone, the right and left atria, as well as the left atrial appendage, are identified. The superior pulmonary vein empties into the left atrium. Anterior to the atria, the outflow tracts of the left and right ventricles are visible.
Level of the ventricles. This is a classic four-chamber slice on which both pairs of atria and ventricles are identified. It is important to note that the right ventricle is located most anteriorly, directly behind the sternum.

Coronal projections

Coronal (frontal) slices on MRI make it possible to evaluate the vertical extent of mediastinal structures. On T1-weighted images and contrast-enhanced gradient echo scans, long tubular structures are clearly visualized: the trachea, ascending aorta, and pulmonary trunk. In the lower part of the slice, the contours of the right atrium and left ventricle are determined.

International nomenclature and abbreviations

To standardize CT and MRI reports worldwide, English-language abbreviations are used. Knowledge of these abbreviations is essential for reading tomograms:

Arteries: A.Ao (ascending aorta), D.Ao (descending aorta), TA (aortic arch), MPA (main pulmonary artery), IA (brachiocephalic trunk). Pulmonary arteries are designated as RPA (right) and LPA (left).
Veins: SVC (superior vena cava), RIV and LIV (right and left brachiocephalic veins), SPV (superior pulmonary vein).
Heart: Chambers are designated as RA (right atrium), LA (left atrium), RV (right ventricle), LV (left ventricle). Outflow tracts are RVO and LVO, respectively.
Other: T (trachea), Oes (esophagus), RMB and LMB (main bronchi), BI (intermediate bronchus).

Briefly: answers to topical questions

Which mediastinal lymph node groups are distinguished during a CT examination and where are they located?

Supraclavicular zone — includes the right and left supraclavicular nodes (1R, 1L); extends from the lower margin of the cricoid cartilage to the clavicles and, in the midline, to the upper margin of the manubrium sterni.
Upper zone (upper zone) — includes upper paratracheal nodes (2R, 2L), prevascular (3a), retrotracheal (3p), and lower paratracheal nodes (4R, 4L). The upper paratracheal nodes are located above the upper margin of the aortic arch or the intersection of the innominate vein with the trachea; the lower paratracheal nodes are below the upper margin of the aortic arch.
Aortopulmonary zone — includes nodes of the aortopulmonary window: subaortic or para-aortic nodes (stations 5, 6).
Subcarinal zone — includes bifurcation or subcarinal nodes (station 7) located beneath the main bronchi within the mediastinal pleura, but above the upper margin of the left lower lobe bronchus or the intermediate bronchus on the right.
Lower zone — includes paraesophageal nodes (station 8) and pulmonary ligament nodes (station 9); extends downward to the diaphragm.
Root zone — includes root of the lung nodes, or hilar nodes (station 10).
Peripheral zone — includes peripheral lung nodes (stations 12–14).

The fusion of which veins forms the superior vena cava and at what anatomical level does this occur?

The superior vena cava is formed by the confluence of the right and left brachiocephalic veins.
The projection of the vessel formation is determined at the right border of the sternum at the level of attachment of the 1st rib cartilage.

Which nerve trunks pass through the mediastinum and how are they positioned relative to the major vessels?

Vagus nerves (nn. vagi) — pass through the superior and posterior mediastinum; the right vagus nerve loops around the right subclavian artery and descends along the right surface of the trachea and esophagus, while the left nerve crosses the aortic arch, giving off the recurrent nerve, and descends along the left surface of the esophagus.
Phrenic nerves (nn. phrenici) — pass between the pericardium and mediastinal pleura anterior to the lung roots, accompanying the pericardiacophrenic vessels.
Sympathetic trunks (trunci sympathici) — are located along the lateral surfaces of the spine in the superior and posterior mediastinum, with the thoracic ganglia lying on the heads of the ribs.

Class and exam questions

What scanning modes are used in mediastinal MRI?

T1-weighted sequences and gradient echo are predominantly used. The examination is frequently supplemented with intravenous contrast enhancement using gadolinium agents.

Where is the right ventricle located on axial CT slices?

On lower-level tomograms, the right ventricle occupies the most anterior position in the mediastinum, lying immediately behind the sternum.

Which vessels are visualized on the slice above the aortic arch?

At this level, the right and left brachiocephalic veins are defined anteriorly, and behind them (closer to the trachea) lie three arteries: the brachiocephalic trunk, the left common carotid, and the left subclavian.

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Radiation anatomy of the mediastinum