Trigonum parietale (azygos lobe)

Last revised by Travis Fahrenhorst-Jones on 01 Jul 2021

The trigonum parietale refers to a triangular opacity seen on chest radiograph that correlates with a small piece of extrapleural areolar tissue that lies between the layers of pleura in the fissure of an azygos lobe 1-4. It may be seen at the most superior portion of the azygos fissure and should not be confused with the azygos vein that is usually present at the inferior end of the fissure as a radio-opaque tear shape 2

The location of the trigonum parietale determines the size of the azygos lobe, which would include a laterally placed trigonum parietale in a larger azygos lobe and a more medial placement in a smaller lobe 5

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Cases and figures

  • Figure 1: Annotated azygos fissure
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