Interlobar fissures of the lungs

Visualization of pulmonary fissures on HRCT

The normal radiological appearance of interlobar fissures has several characteristic features. When analyzing computed tomography scans, it is important to pay attention to their spatial arrangement relative to the lung roots (hila).

• Major (oblique) fissures have a specific curvature depending on the scanning level. In the upper sections (above the lung hila), they curve posteriorly, whereas in the lower sections (below the hila), their course changes and they curve anteriorly.
• Minor (horizontal) fissure is localized anterior to the major fissure. On axial CT slices, it is identified as a distinct avascular zone, while its course in this plane may be wavy. The radiological appearance of the minor fissure features a wide variety of normal variants. In particular, on coronal reformations, it may also be visualized simply as an apparent avascular area.

For the most accurate assessment of the anatomical position of the interlobar fissures, it is recommended to use additional sagittal CT reformations. This is of decisive clinical importance: correct identification of the fissures makes it possible to promptly detect loss of lung volume, which is a key sign of cicatricial changes and atelectasis.

Inferior pulmonary ligament and septa

Inferior pulmonary ligament (ligamentum pulmonale) is a pleural fold. In the area where this ligament reaches the dome of the diaphragm, it may include a small accumulation of adipose tissue. Because of this, the contour of the diaphragm on images may be locally smoothed, forming a so-called juxtaphrenic peak.

It should be noted that an important diagnostic nuance is that during standard chest radiography, neither the inferior pulmonary ligament itself nor the intersegmental septa are normally visualized.

Anatomy of the central airways

In addition to the lungs themselves, diagnostic radiology requires a thorough analysis of the central airways, which are closely related to mediastinal structures.

Trachea (trachea) in children and young patients appears as a smooth, straight tube that descends and deviates posteriorly, located strictly in the midline. On cross-section, it has a round or oval shape; sometimes the posterior edge of the oval may be flattened. In aortic pathologies (such as its unfolding or ectasia), the trachea can be displaced—curving anteriorly and deviating to the right.

Tracheal dimensions in adults vary significantly depending on the imaging method (due to projection magnification on X-ray):

1. On plain radiography: maximum coronal and sagittal diameters in women are 21 and 23 mm, and in men 25 and 27 mm, respectively.
2. On CT (without magnification effect): the mean transverse diameter is 15.2 mm in women and 18.2 mm in men. The lower limit of normal is generally considered to be 12.3 mm for women and 15.9 mm for men.

Main bronchi originate from the division of the trachea—the carina (carina). In children, the takeoff angles of the right and left main bronchi are symmetrical. In adults, the anatomy changes: the right main bronchus courses inferiorly at a steeper angle than the left. Since the range of normal angles is extremely wide, any pathological deviations are assessed exclusively by comparing the right and left sides in a particular patient, rather than by any absolute degree values.

Azygos lobe and fissure

Azygos lobe and the azygos vein itself (vena azygos) are a well-known normal anatomical variant of development.

On computed tomography scans, this feature can be clearly traced in various planes:

• On axial reformations, both the azygos vein itself and the accompanying azygos fissure can be clearly distinguished.
• On coronal reformations, the appearance of the fissure and the adjacent vein are also perfectly visualized.
• Conventional radiography sometimes allows tracing the course of the azygos vein, starting from the mediastinal structures all the way to the lower edge of its corresponding fissure.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Assessment of lung volume loss by displacement of interlobar fissures
• Formation of the juxtaphrenic peak and anatomy of the inferior pulmonary ligament
• Measurement of tracheal diameter: difference between CT and radiography
• Visualization variants of the minor interlobar fissure on HRCT
• Radiological appearance of the azygos lobe in various planes

Other topics in the «Chest Radiology» section

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