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Topic
The aortic knob is a radiographic and anatomical contour formed by the aortic arch as it curves posteriorly and to the left after arising from the ascending aorta. It is most commonly identified on frontal chest radiographs as a smooth convex shadow along the left upper mediastinal border and serves as an important landmark in thoracic imaging.
The appearance and position of the aortic knob reflect the size, course, and orientation of the aortic arch and are routinely assessed on chest imaging.
Synonyms
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Aortic arch contour
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Aortic knuckle
Location
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Located in the left superior mediastinum
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Formed by the left lateral contour of the aortic arch
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Typically seen at the level of the T3–T4 vertebrae
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Superior to the left pulmonary artery
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Posterior to the manubrium
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Anterior and left of the trachea and esophagus
Anatomical components
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Aortic arch segment between the ascending and descending thoracic aorta
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Branch origins (proximal):
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Brachiocephalic trunk
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Left common carotid artery
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Left subclavian artery
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Aortic wall:
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Intima
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Media
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Adventitia
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Relations
Anteriorly:
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Manubrium of the sternum
Posteriorly:
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Trachea
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Esophagus
Medially:
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Mediastinal structures
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Trachea
Laterally:
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Left lung apex
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Left pleura
Inferiorly:
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Left pulmonary artery
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Left main bronchus
X-ray appearance
Chest radiograph (PA view):
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Aortic knob: Smooth, rounded convexity along the left upper mediastinal border
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Margins: Well-defined and continuous
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Position: Above the left hilum
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Size and contour: Influenced by patient age, body habitus, and aortic orientation
CT appearance
Non-contrast CT:
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Aortic arch: Round to oval soft-tissue density structure
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Wall: Thin and well defined
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Relationship: Clearly separated from adjacent mediastinal fat
Post-contrast CT:
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Aortic lumen: Homogeneous contrast opacification
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Arch curvature: Well visualized in axial and multiplanar views
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Branch vessels: Clearly delineated at their origins
Lung window appearance:
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Aortic knob: Seen as a smooth soft-tissue contour adjacent to aerated left upper lobe
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Interface: Sharp contrast between vascular structure and lung parenchyma
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Utility: Helpful for assessing mediastinal–lung boundaries and excluding lung-related causes of contour alteration
MRI appearance
T1-weighted images:
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Aortic lumen: Flow void or low signal due to flowing blood
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Aortic wall: Thin, low-signal rim
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Surrounding mediastinal fat: High signal providing contrast
T2-weighted images:
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Aortic lumen: Flow-related signal void or low signal
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Adjacent soft tissues: Intermediate-to-high signal
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Clear delineation of the arch contour
STIR:
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Blood flow: Signal suppression with flow void appearance
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Mediastinal fat: Suppressed, improving visualization of vascular margins
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Arch outline: Well defined against suppressed background
CT image
X-Ray image
