Pulmonary fissures

Pulmonary fissures are invaginations of the visceral pleura that divide the lungs into lobes. There are three main (normal) fissures: the right oblique fissure, the right horizontal (minor) fissure, and the left oblique fissure.

Normal Fissures

• Right oblique fissure (ROF): Separates the right upper and middle lobes from the right lower lobe. It originates at the 5th rib posteriorly, crosses the 6th rib laterally, and extends to the anterior hemidiaphragm. It is complete in approximately 6477% of individuals.

• Right horizontal (minor) fissure (RHF): Separates the right upper lobe from the right middle lobe. It extends from the right oblique fissure posterolaterally to the anterior chest wall, most commonly at the level of the 4th rib. This is the most variable fissure complete in only 22-54%, incomplete in 35-67%, and absent in approximately 4-11% of individuals.

• Left oblique fissure (LOF): Separates the left upper lobe from the left lower lobe. It courses from the level of the 4th rib posteriorly, crosses the 6th rib in the midaxillary line, and reaches the anterior third of the hemidiaphragm inferiorly. It is complete in approximately 72-82% of individuals.

Fissure Incompleteness

Incomplete fissures are extremely common, found in nearly half of all lungs. Incompleteness is most frequent in the parahilar region for the oblique fissures and in the anteromedial portion for the horizontal fissure.

Accessory Fissures

Accessory fissures are present in approximately 14-42% of individuals. The most commonly encountered include:

• Inferior accessory fissure — separates the medial basal segment of the right lower lobe; most common accessory fissure on the right (~5–6%)

• Left minor fissure — separates the lingula from the rest of the left upper lobe, analogous to the right horizontal fissure; present in ~8–16%

• Superior accessory fissure — separates the superior segment from the basal segments of the lower lobe; ~6% on the right, ~2% on the left

• Azygos fissure — created by the azygos vein migrating through the right upper lobe apex; present in ~1.7%

Imaging Appearance

On chest radiography, fissures appear as hairline soft-tissue density lines. On standard CT, they typically appear as lucent, hypovascular bands, while on highresolution CT (HRCT), they most often appear as sharp thin lines. MDCT with thinsection volumetric imaging has a sensitivity for fissure detection equivalent to autopsy studies, with excellent interobserver agreement.