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Radiology · Chest Radiology

Chest X-ray segments

Segmenta bronchopulmonalia

On standard chest radiographs, the projections of pulmonary segments massively overlap, especially in the anteroposterior view. Due to collateral air drift, pathological processes rarely have strict segmental borders, which requires a precise understanding of the three-dimensional topography of the bronchial tree and interlobar fissures.

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Assessment difficultySegment projections significantly overlap on standard images.
Ring-shaped shadowsAre considered normal and represent an orthogonal projection of bronchi without edema.
Left bronchusIs almost twice as long as the right one before the origin of the first upper lobe branch.
T-shapeThis is how the septum separating segments 9 and 10 appears on CT on the right side.
3 min readFor medical studentsUpdated 10.10.2026
Material prepared by WhiteCoat Pro editorial team based on platform study guidesPending instructor review

Difficulties in X-ray assessment

Assessing an isolated segment on a planar overview radiograph is a non-trivial task. Due to the superposition phenomenon, the projections of segmental zones massively overlap each other, which is most pronounced on the direct radiograph.

Moreover, the pattern of pneumonia or atelectasis rarely stops precisely at the anatomical border of a segment. The reason lies in collateral air drift: respiratory gases flow freely across intersegmental barriers. If a bronchus is directed parallel to the X-ray beam (orthogonally), it is visualized on the image as a ring-shaped shadow. In the absence of interstitial edema, this is an absolute norm.

Nomenclature of pulmonary segments

Classical anatomy implies the division of lung tissue into zones ventilated by their own branch of the bronchial tree. Memorize the numbering according to the standard scheme:

Right lung:

  • Upper lobe: apical (1), posterior (2), anterior (3).
  • Middle lobe: lateral (4), medial (5).
  • Lower lobe: superior (10), medial basal (11), anterior basal (12), lateral basal (13), posterior basal (14).

Left lung:

  • Upper lobe: apicoposterior (6), anterior (7), superior lingular (8), inferior lingular (9).
  • Lower lobe: superior (15), anterior basal (16), lateral basal (17), posterior basal (18).

The anatomy of branching of segmental and subsegmental bronchi has many individual variants.

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Interlobar fissures as border markers

For a lobe boundary to appear on a standard radiograph, the X-ray beam must pass strictly tangentially to the interlobar fissure. The fissures themselves are often incomplete, creating additional pathways for the spread of infections.

  1. Main (oblique) fissures: Run symmetrically from the level of Th5 down and forward through the lung root, resembling a propeller blade. Usually not visible on a direct radiograph, and traced fragmentarily on a lateral one. Thickening of the lower part of the fissure can lead to local loss of the diaphragm silhouette — the "juxtaphrenic peak" sign.
  2. Accessory (horizontal) fissure: Present only on the right. Fans out laterally from the root and contacts the chest wall at the level of the axillary part of the 6th rib. It is this part that is most often visible in the anteroposterior view.

Visualization of septa on CT

Volumetric CT with thin collimation allows routine identification of airways down to the subsegmental level.

Bilateral intersegmental septa (intersegmental bilateral septa) are reliably visualized on axial slices. Deep underneath them lies the inferior pulmonary ligament (ligamentum pulmonale inferius). Its main task is to fix the lower lobe to the paraesophageal mediastinum. This anchoring function is clearly demonstrated in pneumothorax: the lung tissue collapses, but the lower lobe stubbornly remains attached to the mediastinum.

How to memorize

Rule for assessing the lateral radiograph: normally, the density of the shadow from the lungs projected onto the spine should gradually decrease (become lighter) from top to bottom.

Class and exam questions

Why does pneumonia rarely have clear boundaries limited to a single segment?

This is due to the phenomenon of collateral air drift across segmental borders and incomplete interlobar fissures.

Is it possible to symmetrically compare lung densities on a lateral radiograph?

No, on a lateral radiograph, symmetrical comparison is impossible, which greatly complicates the search for subtle areas of lung tissue consolidation.

What is the azygos lobe?

This is a developmental anomaly (found in 1% of people) in which the migration of the azygos vein is altered. At the same time, the true segmental architecture of the lung is not disturbed, so the term "lobe" is erroneous.

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What else to study on this topic

  • Roentgenoanatomy of pulmonary blood vessels and the physiological perfusion gradient.
  • Assessment of the density and structure of the lung roots (hila) on planar images, CT, and MRI.
  • Lymphatic system of the lungs: subpleural networks and Kerley lines (A and B).
  • Criteria for vascular dilatation: the ratio of the diameter of the artery to the adjacent bronchus.
  • Topography of accessory interlobar fissures and their significance in diagnostic radiology.

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Other topics in the «Chest Radiology» section

General anatomy of the lungsInterlobar fissures of the lungsBronchial tree and lung segmentsPulmonary angiographyX-ray anatomy of the lung rootsRadiation anatomy of the mediastinumMediastinal lymph nodesBoundaries and structures of the mediastinumExcursion of the diaphragmLung segments on X-ray and CTLung segments on X-rayRadiological picture of bronchitisAorta on radiographs
The entire «Chest Radiology» section

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